Head and Neck Flashcards

Question

Answer 1

**Subchondral bone cysts** Are occasionally seen in the **condyloid process** and are often clinically silent. * Some cysts appear to be **closed** * While others **may communicate with the joint** space at the caudal aspect of the process. _On CT images_, cysts appear as **spherical defects** with **well‐ demarcated dense bone margins**. _On MR images_, cysts are typically **T2 hyperintense** and **T1 hypointense** centrally with a well‐defined signal void peripher- ally because of the dense bone margin. Case findings: 3y M Rottweiler. A CT scan of the head was performed as part of a diagnostic evaluation for chronic otitis. A well‐delineated circular subchondral bone cyst is seen in the left mandibular condylar process (a). Contents are fluid‐dense and surrounded by a thin rim of compact bone. The left condyle (b) is unremarkable and included in this figure in the same orientation for comparison. The cyst was clinically silent and identified as an incidental finding on this study.

Answer 2

Temporomandibular dysplasia has been reported in several canine breeds, including **Dachshunds, Cocker Spaniels, Cavalier King Charles Spaniels, and Irish Setters.** The disorder is clinically characterized by: * Temporomandibular joint laxity, resulting in subluxation or luxation, * Inability to close the mouth. _CT imaging features include:_ * **Flattening** of the condyloid process and mandibular fossa * **Hypoplasia** of the **retroarticular process.** * Although overt luxation is uncommon, the joint frequently appears **incongruent or subluxated.** As with other forms of dysplasia, the phenotypic expression of this disorder is variable, and imaging findings may be subtle in some patients. _Case findings:_ 3y MC Lhasa Apso presented to the emergency service with temporomandibular luxation. The representative transverse images (a,b) are ordered from caudal to rostral. The **left condyloid process** is l**uxated rostrodorsally** (b,d,e: arrow), and the **right condyloid process** is **subluxated** (a,c). The **condyloid processes** are **misshapen**, and the **mandibular fossae are flattened** with **hypoplastic retroarticular processes** (c: arrowheads).

Answer 3

Craniomandibular osteopathy is an autosomal recessive developmental disease primarily affecting young **West Highland White** and other **Terriers** but also reported in a number of other breeds. _Clinical signs include:_ * **Swelling** of the jaw due to **bilaterally symmetrical new bone production,** which can involve the mandibular body, ramus, and articular parts of the mandible. * With **severe** manifestations, proliferative new bone **encases the temporomandibular joints** and **extends to the temporal regions of the calvarium**. Although radiographic evaluation usually suffices for diagnosis of the disorder, CT imaging may be useful to more accurately characterize the extent of temporomandibular joint involvement. _CT imaging features include:_ * **Symmetrically** distributed uniformly dense **proliferative medullary and external woven bone** formation **involving the mandible** and possibly the **temporomandibular joints.** _Case findings:_ * 1y MC Golden Retriever with a history of mandibular swelling and pain. The representative transverse images (a–c) are ordered from rostral to caudal. There is **marked, irregular, periosteal** productive response that is **symmetrically affecting the caudal mandible** and **temporomandibular joints**. This productive response has **extended to the temporomandibular** joints (a: black arrows) and involves the temporal bones (b–d: arrows).

Answer 4

Septic arthritis and osteomyelitis of the temporomandibular joint are occasionally encountered as a result of **extension of otitis externa/media** or a d**irect penetrating injury** and may include **articular cartilage and subchondral bone destruction, joint distension, and surrounding cellulitis**. _Case findings:_ 8y FS Rhodesian Ridgeback with regional cellulitis associated with otitis media/interna. **T2 hyperintensity** is seen adjacent to the medial margin of the **right temporomandibular joint, the right lateral pterygoid muscle, and the dorsal aspect of the pharynx** (a: arrows). The same region **contrast enhances** (b: large arrows), and additional **meningeal enhancement** is evident (b: small arrows). There is periarticular contrast enhancement involving the right temporoman- dibular joint, with associated intraarticular enhancement, and a diminished subchondral signal void (c: arrow). The left temporomandibular joint is normal by comparison (d).

Answer 5

Although **uncommon**, neoplasia involving the temporomandibular joint may arise from intrinsic structures of the joint or from encroachment from adjacent neoplasms. * **Benign bone tumors**, such as osteomas that arise from the mandible or temporal bone, **may impinge** on the temporomandibular joint and will typically appear as a dense, well‐delineated mass on CT and as a low or no signal intensity mass on all MR sequences. * _CT features_ of sarcomas and carcinomas in this region may include **osteolysis and soft‐tissue mass** with **nonuniform contrast enhancement**. * _MR features_ are similar and may also include **replacement** of T1 and T2 hyperintense **medullary fat with lower intensity tumor** _Case findings:_ 6y M Golden Retriever with recent onset of oral pain. A large, **aggressive bone‐destructive mass** is centered on the caudal aspect of the **left side of the mandible**. **Osteolysis** of the left **mandibular ramus** (a: arrows) and **condyloid process** (b: arrow) is evident. Bone destruction **extends to and includes the subchondral bone** of the process, implying an **intraarticular component** to the mass. On comparable contrast‐enhanced images, the mass has a **complex, lobular appearance** (c,d). Aspiration biopsy revealed the mass to be a _fibrosarcoma._

Answer 6

Although commonly performed in people because of the high incidence of debilitating degenerative temporomandibular joint disorders, there are few reports on the use of high‐resolution CT and MR imaging for diagnosis of this disorder in veterinary medicine. Although **articular cartilage and the articular disc should be well visualized by MR** using appropriate coils and pulse sequences, MR features of degenerative temporomandibular joint disease have not been fully described in dogs and cats. _CT imaging features_ include: * **Narrowing of the joint space** (best seen on sagittal plane reformatted images) * Condyloid process **remodeling** * Subchondral bone **sclerosis** * Periarticular new bone formation Similarly, MR imaging findings may include joint space narrowing and subchondral bone and periarticular new bone signal void. _Case findings:_ 6y MC Miniature Schnauzer with a history of difficulty opening its mouth. Tests to assess for the presence of immune‐mediated joint disease were negative. In the sagittal image, rostral is oriented to the left and caudal is to the right. **Marked narrowing** of the temporomandibular **joint space** is evident on both the transverse and sagittal images (a,b), implying a **loss of articular cartilage and meniscal degeneration**. Imaging findings are consistent with temporomandibular osteoarthrosis.

Answer 7

Occasionally, periarticular productive remodeling may be exuberant enough to restrict temporomandibular joint range of motion. This can be due either to primary temporomandibular degenerative joint disease or an adjacent proliferative response of the temporal bone associated with chronic otitis. **True ankylosis** is defined as **bone fusion or synostosis**. **Most patients** with reduced range of motion, in fact, **have extracapsular or fibrous ankylosis.** _CT imaging findings:_ * Osteoarthrosis features in addition to **more pronounced periarticular** new bone formation. Comparable MR features would be expected in the form of ill‐defined and nonuniform periarticular signal void on all sequences.

Answer 8

_Mandibular Condylar Dysplasia (Canine)_ 10mo MC Bassett Hound with a history of **pain when opening the mouth** and periodic episodes of inability to close the mouth. The sagittal reformatted image is oriented rostral to the left and caudal to the right. The left **manibular condyle is misshapen** (a,b: asterisk), and there is evidence of **subluxation of the temporomandibular joint** (a–c: arrow). The sagittal image reveals **abnormal flattening of the articulating surfaces** and striking **hypoplasia of the retroarticular process** resulting in **ventral subluxation** (b: arrowhead). Temporomandibular joint findings were bilaterally symmetrical in this dog.

Answer 9

_Inflammatory Mandibular Mass with Temporomandibular Subluxation (Feline)_ 12y FS Domestic Shorthair with iatrogenic open wound in the oropharyngeal region following a traumatic pill administration. Clinical signs included malocclusion, oral pain, and inability to close the mouth. The cat had a previous enucleation that is unrelated to the current presenting complaint. The transverse images are comparable unenhanced and contrast‐enhanced images. The two images reformatted in the sagittal plane are oriented in the same direction for easier comparison. In both images, rostral is to the left and caudal is to the right. A **heterogenously contrast‐enhancing mass** is evident **surrounding the body of the mandible** on the left (b: arrowhead). The left **temporo-mandibular joint is subluxated** as a result of extraarticular encroachment by the mass (c: arrow). The right temporomandibular joint is normal by comparison (d). Biopsy of the oro-pharyngeal region confirmed the presence of suppurative abscess and cellulitis.

Answer 10

_Condylar Fossa Fracture (Canine)_ 1y German Shepherd Dog hit by a car 24 hours previously. The representative transverse images are ordered from rostral to caudal. A **transverse fracture** is seen in the **rostral part of the right zygomatic bone** near its articulation with the maxilla (a: arrow). A **second, mildly displaced** comminuted articular fracture is present near the origin of the **zygomatic process** of the right temporal bone (b–d: arrow). Another fracture line is evident coursing **parallel to the subchondral bone margin of the fossa** (b: arrowheads).

Answer 11

_Temporomandibular Septic Arthritis (Canine)_ 8y FS Rhodesian Ridgeback with regional cellulitis associated with otitis media/interna. **T2 hyperintensity** is seen adjacent to the **medial margin of the right temporomandibular joint**, t**he right lateral pterygoid muscle**, and the **dorsal aspect of the pharynx** (a: arrows). The same region **contrast enhances** (b: large arrows), and additional **meningeal enhancement** is evident (b: small arrows). There is **periarticular contrast enhancement** involving the **right temporomandibular joint**, with associated **intraarticular enhancement, and a diminished subchondral signal void** (c: arrow). The left temporomandibular joint is normal by comparison (d).

Answer 12

_Temporal Bone Chondrosarcoma (Canine)_ 8y FS German Shepherd Dog with neurologic signs relating to left cerebral and thalamic disease. A **large, locally invasive complex mass arises from the temporal bone**, with components **extending intracranially** and into the **adjacent temporal musculature**. Left temporal bone medullary signal **intensity is reduced on unenhanced T1 images** (a: arrow) as a result of marrow displacement by the mass, and cortical margins are attenuated. **Multiple high intensity foci** suggest the mass is **multicameral and cystic** (b). The mass is **nonuniformly contrast enhancing** (c,d). Biopsy revealed a highly _anaplastic chondrosarcoma._

Answer 13

_Temporomandibular Sarcoma (Canine)_ 8y MC Rottweiler with progressive right temporal and masseter muscle atrophy and pain upon opening the mouth. A poorly margined lobular mass arises in the region of the right mandibular process, resulting in mandibular cortical bone destruction and diminished marrow signal intensity on the unenhanced T1 image (b: arrows). The mass is moderately and uniformly contrast enhancing (c–e). Replacement of the normal right condyloid process architecture by the mass with extension into the right temporomandibular joint space is best seen on the right sagittal image (c: arrow). The left temporomandibular joint is normal by comparison (f). Right temporal and masseter muscle atrophy is seen associated with increased T2 and T1 signal intensity (a,d: asterisk), consistent with dysfunction of the mandibular branch of the right trigeminal nerve. Aspiration biopsy of abnormal spindle cells was consistent with sarcoma.

Answer 14

Developmental disorders of the skull: * Occipitoatlantoaxial malfomrations * Atlantooccipital overlappin * Benign calvarial hyperostosis * Trauma Inflammatory disorders: * Masticatory muslce myositis * Abscess * Osteomyelitis Neoplasia: * Ostoemas * Osteosarcomas * Multilobular osteochondrosarcomas * Meningioma * Pituatary adenoma

Answer 15

Congenital occipitoatlantoaxial malformations are rare in dogs; however, hypermotility or stenosis can cause severe neurologic compromise secondary to **compression of the spinal cord.** The **occipital bone, foramen magnum, atlas, and ligamentous structures** make up this region. The spectrum of abnormalities includes: * **Hypoplasia** of the **occipital condyles** * **Fusion** of the **atlas to the occiput** * Multiple separate **centers of ossification** * **Malformation** of the **dens** * The abnormally fused cranial segments may result in **atlantoaxial instability or subluxation** causing spinal cord compression. CT and MR imaging allow 3D visualization of the malformation itself as well as the effects on the spinal cord. Dogs should be positioned with care if instability is suspected.

Answer 16

Atlantooccipital overlapping is **rostral malposition of the atlas** and axis resulting in **compression of the cerebellum** and **kinking of the medulla oblongata**. Since it is seen with other congenital anomalies, such as **Chiari‐like malformation** and **dens hypoplasia** resulting in **atlantoaxial instability**, it may be a consequence of other anomalies; however, it can also be seen as a sole abnormality. **Syringomyelia,** seen as a **continuous or intermittent T2 hyperintense fluid collection in the spinal cord parenchyma**, is associated with the chronic compression. **Fibrous bands** dorsal to the atlantoaxial or atlantooccipital junctions can also be seen with many of these disorders and contribute to the spinal cord compression.

Answer 17

Benign calvarial hyperostosis has been described in young **Bull Mastiffs** as a **diffuse thickening** of the bones of the calvarium, with **some similarities to craniomandibular osteopathy**. _On MR images_ of one patient, the frontal bones were markedly thickened with **hypointense T1 and T2** signal due to l*oss of normal marrow* signal and T2 hyperintensity of the surrounding tissues. T**2\* GRE images accentuated** the signal from bone and provided good image quality for evaluating hyperostosis. **Contrast‐enhanced T1 images** with fat saturation were recommended to reveal tissue **enhancement**. _CT imaging_ also demonstrates the i**ncreased bone attenuation** in this syndrome (Figure 1.4.2).

Answer 18

Common regions of trauma to the calvarium include: * Sphenoid and pterygoid bones * Frontal bone * Temporal bone Fractures of the temporomandibular joint and maxilla/mandible are discussed in Chapters 1.3 and 1.9. * **Gas may enter the calvarium** as a result of **open trauma** to the skull and is identified as **signal void on MR images** and **hypoattenuating regions on CT** images. * Associated **hemorrhage** may be seen in the **dural tissues or brain** * 3D reformations of CT images of the skull may be helpful in depicting the spatial location of fragments. However, small fractures are often best seen in the two‐dimensional images.

Answer 19

Masticatory myositis is an **autoimmune i**nflammatory condition of the **masseter, temporal,** and **pterygoid muscles** in which autoantibodies are directed against myosin. * Affected dogs have pain opening the mouth and atrophy of the muscles of mastication. The atrophy can be seen on both CT and MR images. The **affected muscles** are **hypoattenuating on CT** on unenhanced images and have **diffuse or peripheral enhancement on contrast**-enhanced images. * Regions of myositis appear **hyperintense on MR T2** sequences and, similar to CT, are contrast enhancing. * **Nonenhancing** regions represent areas of **necrosis** _Case findings:_ 1.5y MC Rottweiler with rapidly progressive inability to open mouth. Representative CT images were acquired immediately following contrast medium administration. There is **moderate, diffuse contrast enhancement of the left masseter and temporal muscles** (a,b: arrows). Pterygoid muscles appear relatively unaffected (a,b: arrowhead). Muscle biopsy revealed diffuse, chronic, lymphoplasmacytic myositis with muscle atrophy and fibrosis.

Answer 20

Abscesses can occur in the musculature of the head secondary to **penetrating wounds** from the skin, oral cavity, and pharynx or secondary to **otitis media**. Areas of abscessation appear **hypoattenuating on CT** and **hyper-intense on T2** MR images. On both modalities, c**ontrast enhancement tends to be peripheral.** A contrast‐enhancing **tract may help to localize any foreign material or to trace the origin** of the wound. _Case findings:_ 9y FS Chow with pain when opening mouth. A focal **draining lesion** was seen in the **caudal oral cavity**. The contrast‐enhanced image shows a **poorly delineated cavitary lesion within the left temporal muscle, consistent with an intramuscular abscess** (b: arrow). **Peripheral contrast enhancement** extends to the **medial surface of the coronoid process** of the left mandible and to the **external surface of the left parietal bone**, but overt **bone reactivity is not appreciated**. Fascial and muscle contrast enhancement is **also evident ventrally** (b: arrowheads), indicative of more diffusely distributed **cellulitis**. Biopsy revealed chronic suppurative cellulitis.

Answer 21

Osteomas are **benign** tumors of unknown etiology, comprised of compact or cancellous bone, that occasionally occur in the skull. * **Periosteal osteomas** arise from the surface of the bone * **Endosteal osteomas** develop in the center of the bone These tumors have been reported in cats and dogs in the region of the skull. These may appear on CT images as: * **Primarily compact peripheral** types, with **uniform, hyperattenuating centers** and **smooth margins** * **Central cancellous** types, with **slightly lower attenuation** and more **irregular margins** with invasion into adjacent bone These masses may affect the skull, oral cavity, or orbit. _Case findings:_ 2y MC Golden Retriever with cranial mass. There is a **smooth, dense production** of bone centered on the **parietal bone** and **expanding both intracranially and extracranially**. The mass is **hyperattenuating** and **uniform on CT images** (a). On MR images, the **mass effect** is evident **with compression of the brain and lateral ventricle** next to the mass (b,c), as well as **displacement of the falx cerebri to the right** (d). There is **T2 hyperintensity** of the white matter next to the mass, (b) **indicating edema.**

Answer 22

_Osteosarcoma_ occurs **most commonly** in the **maxilla and mandible** in the axial skeleton and also occurs in the **bones of the calvarium** _Chondrosarcoma_ occurs in the **flat bones** of the skull, most commonly in the **nasal cavity**. _Imaging characteristics of primary bone tumors_ * are similar on CT and MR images * expansile irregular new bone production * cortical lysis * associated soft‐tissue masses with heterogeneous contrast enhancement Other primary bone tumors, such as fibrosarcoma, hemangiosarcoma, as well as metastatic neoplasia, are infrequently encountered. _Case findings:_ 4y F Bull Mastiff with a 2‐month history of a right facial mass. Representative images are unenhanced (a,b) and contrast enhanced (c,d) at comparable anatomic levels. Images are ordered from rostral to caudal. A **variably attenuating expansile mass** appears to arise from within the **right zygomatic bone** (a,b) and is **heterogeneously contrast enhancing** (c,d: arrowhead). Cortical remnants of the zygomatic bone are still evident (b: arrowheads). The mass **fills the orbital space, displacing the right globe dorsally** (c: arrow). Biopsy revealed osteosarcoma with minimal osteoid, which reflects the predominantly destructive appearance of the mass on CT.

Answer 23

Multilobular osteochondrosarcoma occurs in the **flat bones** of the skull of dogs and occasionally in cats. It is comprised of **multiple lobules of bone or cartilage separated by fibrous septae,** which give it a _characteristic_ **stippled appearance on CT images**. These tumors tend to be **round and well circumscribed to irregular** in shape. They **often expand into the calvarium or orbit**, causing a significant mass effect. * **Brain edema** can be seen as **T2 hyperintensity** * **O****bstructive hydrocephalus** may result _On CT images,_ the masses are **mildly contrast enhancing**. _MR imaging characteristics_ of these masses include **T1 and T2 hypointensity with regions of hyperintensity**. Contrast enhancement is heterogeneous to uniform. _Case findings:_ 12y FS Dachshund Terrier cross with large craniofacial mass. Two representative images at the level of rostral extent (a) and middle (b) of the **frontal sinus** are included here. A **partially and diffusely mineralized mass** **arises from the right frontal bone and extends around the right zygomatic arch**. The mineralized component of the mass has a **coarse, granular appearance** characteristic of multilobular osteochondrosarcoma. The mass is osteodestructive (a,b: arrow) and displaces normal soft‐tissue structures (b: arrowhead, right globe) but has **virtually no soft‐tissue component** beyond the osseous margins. The 3D rendering reveals the full surface extent of the mass (c). Excisional biopsy confirmed multilobular osteochondrosarcoma.

Answer 24

Rarely, intracranial tumors, such as meningioma, **can expand outside the calvarium**. * Meningioma in **cats** can also cause **hyperostosis** of the adjacent calvarium * **Hyperostosis with bone lysis** has been reported in the dog. Tumors of the soft tissues surrounding the head, such as adenocarcinoma or squamous cell carcinoma, can also involve the bones of the skull.

Answer 25

Lipomas or liposarcomas have a characteristic **fat attenuation (–100 HU)** within the musculature or soft tissues _Case findings:_ 8y FS Doberman Pinscher with a left‐sided facial mass. Representative CT images are ordered from rostral to caudal. A **well‐defined fat‐attenuating mass** is present within the **left masseter muscle**. The **striated pattern** within the mass is due to **adipose infiltration** between muscle fasciculi A s**ubchondral bone cyst** is noted in the left condyloid process incidentally. Biopsy confirmed infiltrative lipoma.

Answer 26

**Cats with pituitary adenomas** may develop **acromegaly** secondary to secretion of growth hormone and insulin‐like growth factor. They tend to develop **increased frontal bone thickness** and **excess soft tissue** in the **nasal cavity, sinuses, and pharynx**, which can be seen on CT images.

Answer 27

_Occipitoatlantoaxial Dysplasia (Canine)_ 8mo MC Yorkshire Terrier with **atlantoaxial instability**. An imaging diagnosis of occipital dysplasia was made as a component of a more complex anomaly of the atlantoaxial–occipital region. The transverse image is of the caudal aspect of the occipital bone at the level of the foramen magnum. The **foramen magnum is larger** than normal and elongated in the dorsal–ventral axis (a,b: two‐headed arrow). The rostral margin of the **dorsal arch of the atlas extends into** the dorsal part of the **foramen resulting in atlantooccipital overlapping** (a,b: arrowhead). The **occipital condyles** (c: arrows) are **hypoplastic** but appear to articulate well with the articular fovea of the atlas (c: arrow- heads). Marked **rotational subluxation** of the atlantoaxial joint is evident, and the odontoid process of the axis is hypoplastic (b).

Answer 28

_Benign Calvarial Hyperostosis (Canine)_ 1y M Bernese Mountain Dog with prominent midline cranial mass. An **irregular but well‐defined osseous mass** arises from the **dorsal calvarium**. The proliferative mass is dense and highly organized and has **no appreciable overlying soft‐tissue component.** Bone biopsy revealed essentially normal bone tissue with considerable woven bone embedded in dense fibrous tissue overlying lamellar bone. This entity has previously been described in young _Bull Mastiffs._

Answer 29

5y MC Domestic Shorthair that sustained trauma of unknown cause within the past 48 hours. This cat sustained a number of skull fractures commonly associated with high‐impact trauma. Representative images are ordered from rostral to caudal. Injuries include: * **fracture–luxation** involving the **nasal and maxillary bones** (a: arrow) * **mandibular symphyseal** separation (b: arrow) * **fractures** of the **perpendicular processes of the palatine bones** (c: arrows) * **separation** of the **palatine symphysis** (c: arrowhead) * **fractures** of the **pterygoid bones** (d: arrows) * **caudal luxation** of the **right condyloid process** (e: arrowhead) * **fracture** through the **zygomatic process of the left temporal bone** (e: arrow).

Answer 30

_Masticatory Myositis (Canine)_ 9mo Miniature Pinscher with recent onset of left temporal muscle atrophy. The unenhanced T1 and T2 images are at the same anatomic level. The contrast‐enhanced T1 image is more caudal. **Marked atrophy** of the **left temporal muscle** and **moderate atrophy** of the **left masseter muscle** are evident on all sequences. There is a pronounced **increase in signal** intensity of affected temporal (a: large arrows), masseter (a: small arrow), and pterygoid (a: arrowheads) muscles **on the T2** image that corresponds to regions of **mild hyperintensity present on the T1 image** (b). The same regions **markedly contrast enhance** (c). Serum creatinine kinase was significantly elevated and an antibody test confirmed the diagnosis of masticatory myositis.

Answer 31

_Masticatory Myositis (Canine)_ 8y FS Golden Retriever with a 2‐week history of weight loss, stridor, cranial nerve deficits, and temporal muscle atrophy. **Bilaterally symmetrical temporal muscle** atrophy is evident on all sequences. There is **marked, diffuse, and symmetrical hyperintensity of temporal** (c: large arrows), **masseter** (c: small arrows), **and pterygoid** (c: arrowheads) muscles on the **T2 and STIR images** corresponding to regional enhancement on the **contrastenhanced T1 image** (b). A similar diffuse T2 hyperintensity and contrast enhancement pattern of the **laryngeal tissues** is evident (b: arrow). Necropsy revealed severe, bilateral, chronic, and diffuse lymphoplasmacytic myositis with myonecrosis and myodegeneration. This dog also had *laryngeal cellulitis.*

Answer 32

_Chondrosarcoma (Canine)_ 8y MC Golden Retriever with a progressively enlarging rostral cranial mass. Representative CT images are ordered from rostral to caudal. **Osteolysis** and **unorganized osteoproliferation** of the caudal aspect of the **right maxilla** (a: arrow) and **both frontal bones** (b,c) is seen. The **right frontal sinus** is also **filled with soft‐tissue attenuating material** that subsequently **nonuniformly contrast enhances**. There is also evidence of **osteolysis of the internal margin of the right frontal bone** (c: arrowhead). **Slight meningeal enhancement** at this site was noted on the contrast‐enhanced images, but there was no evidence of further tumor extension. Biopsy revealed chondrosarcoma.

Answer 33

_Multilobular Osteochondrosarcoma (Canine)_ 7y FS Golden Retriever with a recent history of progressive incoordination. Neurologic examination localized neurologic deficits to the brainstem. A **spherical mass** arises from the **occipital bone**, causing **extensive occipital osteolysis,** and **extends both intracranially and extracranially**. The mass is **hypointense** on the unenhanced **T1** image (b), has **heterogeneous intensity and perilesional edem**a on the **T2 image** (a), and **intensely and nonuniformly contrast enhances** (c,d,g). The intracranial component of the mass is predominantly within the **caudal fossa, causing rostral cerebellar displacement** (d: large arrow), **cerebellar and brainstem compression** (d: small arrows), and **obstructive hydrocephalus** (d,g). The rostrodorsal aspect of the mass also encroaches on the caudal aspect of the rostral fossa, compressing the occipital lobes (d,g: arrowheads). Occipital osteolysis is also appreciated on the CT images (e). The **coarse, granular, and diffuse mineralization** (e,f) is characteristic of multilobular osteochondrosarcoma. Excisional biopsy confirmed the diagnosis of multilobular osteochondrosarcoma.

Answer 34

_Adenocarcinoma (Feline)_ 17y FS Domestic Shorthair with a 2‐month history of pain on opening its mouth. Representative images include unenhanced and contrast‐enhanced images at the level of the zygomatic process (a,b) of the frontal bone and a contrast‐enhanced image near the level of the temporomandibular joint (c). A **pronounced productive periosteal response** is seen involving both the **medial and lateral cortical surfaces of the left mandible**. Contrast‐enhanced images reveal a **poorly defined, heterogeneously enhancing mass** extending the l**ength of the left mandible and extending into the left orbit**. A biopsy of the mass was interpreted as adenocarcinoma of possible salivary origin.

Answer 35

_Aggressive Epithelial Neoplasia (Canine)_ 11y Dalmatian with recent onset right‐sided exophthalmia. A **poorly defined right orbital mass** is seen associated with **aggressive osteolysis** of the **right frontal** (a: arrowheads) and **palatine** (a: arrow) bones. The mass **extends into the right frontal sinus and the rostral cranial vault** and is in intimate contact with the **contrast‐enhancing meninges of the right olfactory bulb and frontal lobe** (b,c: arrows). Fine‐needle aspiration biopsy yielded a diagnosis of malignant epithelial neoplasia of possible basal cell origin.

Answer 36

_Squamous Cell Carcinoma (Canine)_ 11y MC Shetland Sheepdog with left‐sided epistaxis. There is a **highly invasive** and **poorly defined soft‐tissue mass** centered on the l**eft retropharyngeal region**, causing **profound osteolysis** of the **basisphenoid bone** (a: large arrow) and **obliterating the left pterygoid muscles** (b: arrow). **Periosteal reaction and sclerosis of the zygomatic process of the left temporal bone i**s also seen (a,c: arrowhead), and the **left digastricus and masseter muscles are atrophied** (c: arrows). Mass margins extend **intracranially**, and pronounced **meningeal enhancement** is evident ventrally and adjacent to the **left temporal lobe (**d: arrows). The oval foramina remain intact (a: small arrows), but the **mandibular branch of the left trigeminal nerve cannot be delineated within the mass** (d). The _location and aggressive imaging features_ of this lesion are characteristic of _squamous cell carcinoma_. Biopsy of the mass confirmed squamous cell carcinoma.

Answer 37

_The orbit is bounded by:_ * The **frontal bones medially** * The **zygomatic arch caudolaterally** * The **orbital ligament dorsally** Contains the globe and associated vascular and glandular structures. The **orbital ligament:** * Spans the **frontal process of the zygomatic bone** and the **zygomatic process of the frontal bone** * Visible as a **hyperattenuating** structure on **CT** images and a **hypointense structure on T1** a**nd T2** images on MRI. * In **cats**, the dorsal orbit is **mainly bone** as these processes are close together. * **Mineralization** of the orbital ligament is **common in dogs.** ****Extrinsic ocular muscles, the zygomatic salivary gland (dogs), vasculature, the lacrimal gland and gland of the third eyelid, the globe, and the optic nerve fill the orbit. * The **ocular muscles enhance to a greater degree than surrounding musculature on MR** images in normal dogs. * The **nasolacrimal duct** travels through the **lacrimal canal in the lacrimal bone and maxilla** to enter the **nasal cavity rostrally and ventrally**. * The **optic nerve** passes through the o**ptic canal, formed by the pterygoid bone**, to enter the calvarium. It is best visualized on **MR** images in a **dorsal oblique plane**, parallel to the nerves. Several sequences may be used for visualization but 3D T1 weighted images with a 1–2 mm slice thickness are optimal. _Developmental Disorders:_ * Head conformation, particularly in **brachycephalic** breeds of cats, can **alter** the path of the normal **nasolacrimal duct**. The dorsal rotation of the facial bones and canine teeth causes the nasolacrimal duct to pass under the canine teeth and results in some **obstruction to drainage**. Similar anatomic changes have been reported in brachycephalic dogs. **CT** is an excellent approach for performing **dacryorhinocystography** to evaluate the patency of the lacrimal duct and identify causes of obstruction. * **Dacryops,** developmental **cysts of the lacrimal system**, can **obstruct** the lacrimal canal and d**eform the surrounding bones** as they expand over time and can be imaged with CT or MR. They may contain sedimenting debris, best seen on T2 weighted MR images. The **walls** of such cysts exhibit **mild contrast enhancement.** _Trauma_ Trauma to the skull often affects the orbit, either by fracturing the bones forming the orbital boundaries or by damaging the soft tissues within. Orbital fractures can be appreciated best on transverse images, but 3D images can also help to describe the displacement of the bones and alteration to orbital shape. * **Acute trauma** results in **sharply marginated fracture lines** that **may extend to the nasal cavity and calvarium.** * **Chronic fractures** may heal with malunion and deform the shape of the orbit by proliferative change or by areas of disruption of the skull. * The **globe** may be **acutely displaced** from the orbit **or shrunken (phthisis bulbi)** as a result of **previous trauma.** _Inflammatory disorders:_ * **Inflammation** can affect the soft tissues within the orbit, often manifesting as e**xophthalmos or periorbital swelling**. Penetrating trauma, foreign bodies, and infections of the eye may be initiating causes. **Cellulitis or myositis** results in **increased soft‐tissue attenuating material** or **T2 hyperintensity of the orbital tissues**. The volume of the tissues appears greater, and there is **loss of definition of the normal fat and extraocular muscles**. The eyelids and surrounding tissues may also be affected. On contrast‐enhanced images, there is **diffuse enhancement** of the tissues surrounding the globe. * **Abscesses** may also form in the tissues of the orbit, resulting in **fluid‐attenuating, or T2 hyperintense and T1 hypointense, collections.** These lesions are **peripherally contrast enhancing**. In addition to the extraocular muscles within the orbit, the pterygoid muscle is located medial to the zygomatic salivary gland and may also be affected by inflammatory disease. * Rarely, the inflammation within the orbit **may extend intracranially**. MR imaging features of intracranial extension include **T2, STIR, and FLAIR hyperintensity** of the tissues in the **skull foramina and orbital fissure,** which do not extend into the brain or meninges. * **Zygomatic sialadenitis**, often **with sialocele formation**, can be a cause of inflammation and exophthalmos in the orbit. _Neoplasia:_ Neoplastic disease may arise from the soft tissues or osseous structures surrounding the orbit. **On CT images**, neoplasia usually has **more clearly defined margins compared to inflammatory disease**. Common tumors include: * Carcinomas (adenocarcinoma, squamous cell carcinoma) * Sarcoma (fibrosarcoma, liposarcoma, rhabdomyosarcoma, osteosarcoma) * Round‐cell neoplasia (lymphoma, mast cell tumor) * Meningioma. These tumors may primarily involve the tissues of the orbit, extend from the nasal cavity and maxilla, or represent metastasis. _Imaging features_ include l**ocal bone destruction, irregular bone production, and increased soft‐tissue mass within the orbit.** Tumors are **heterogeneously to intensely contrast enhancing**. The surrounding structures should be evaluated to determine the involvement of bone, nasal cavity, optic nerve, and cranium. * **Myxosarcoma** has a predilection to the orbit in dogs, with CT and MR imaging characteristics of **extensive, fluid‐filled cavities** within the orbit and surrounding fascial planes. They can extend to the temporomandibular joints and mimic a salivary mucocele.14 * **Osteoma and multilobular tumor** of bone also occur in this location. Osteoma has a characteristic smooth, uniform attenuation on CT images and may enlarge to affect adjacent structures by mass effect. * **Restrictive orbital myofibroblastic sarcoma** of **cats**, which was previously named *idiopathic sclerosing orbital pseudotumor*, is an invasive, low grade neoplasm affecting the orbital tissues. CT and MR images show **diffuse thickening of the orbital tissues, sclera, and eyelids** with i**ntense contrast enhancement**. The disease often affects both **eyes and/or the oral cavity.**

Answer 38

* Images b–e are ordered from rostral to caudal. The orbit is represented by the shaded circle (a) and is mainly comprised of the **frontal bone and zygomatic arch**. * The **extraocular muscles** (b: white arrowhead) are surrounded by fat. * The **zygomatic salivary gland** is strongly contrast enhancing in the ventrolateral orbit. * The **pterygoid muscle** (b: asterisk) lies medial to the gland. * The **orbital ligament** (c: double‐ended arrow) joins the zygomatic process of the frontal bone (a: #) to the zygomatic arch. Focal mineralization is common (c: small arrowhead). * The **lacrimal gland** is ventral to the orbital ligament (b: thin white arrow) and is contrast enhancing. * The **ramus of the mandible** is medial to the zygomatic arch. * The **optic nerve** (d: black arrowhead) is hypoattenuating and travels toward the optic canal. * The **optic chiasm** (f: open arrow) is visible within the calvarium. * F (frontal sinus), M (mandible), S (zygomatic salivary gland), Z (zygomatic arch).

Answer 39

This CT study was acquired following instillation of **iodinated contrast medium** through the lacrimal punctum. Representative images are ordered from caudal to rostral, mimicking the order in which contrast medium flows through the duct. The normal nasolacrimal duct **originates from the lacrimal sac**, enters the **lacrimal canal** through the **lacrimal bone** (a: arrow), and continues rostrally within the **maxillary bone** (b,c: arrow). The duct **exits** the maxillary bone and continues to course **rostrally** (d,e: arrow), terminating in the **nasal cavity** (f: arrow). Contrast medium in the ventral aspect of the nasal cavity has exited the duct and is distributed in the dependent interstices of the turbinates (b–f: arrowhead).

Answer 40

3y MC Golden Retriever with right‐sided epiphora of 2‐month duration. Images b–f are at approximately the same anatomic level near the proximal extent of the lacrimal canal. Similarly, images a and g–i are at the same anatomic level in the dorsal plane. * A **large expansile mass** arises from the **region of the lacrimal bone**, causing **resorptive deformation** of the adjacent **lacrimal, maxillary, and frontal bones** (a,b,e,h: arrows). * The mass also **extends medially into the nasal cavity** and causes **deformation of the dorsal aspect of the maxillary sinus** (c: arrow). * **Centrally**, the mass is **fluid attenuating on CT** and is **T2 hyperintense and T1 hypointense**. * A **sedimentary layer** is also appreciated on the transverse T2 image (d: arrow), further documenting a **cystic character**. * The **thin cystic wall minimally contrast enhances** on both CT and MR studies (c,f,i). * The normal appearance of the caudal extent of the contralateral lacrimal canal (b: large arrowhead) and the infraorbital canal (b: small arrowhead) is seen on the left. * Excisional biopsy confirmed this to be a ductular cyst that arose near the proximal origin of the nasolacrimal duct.

Answer 41

2y MC Domestic Shorthair with acute head trauma of unknown cause. There is a **moderately displaced fracture** of the **right ventromedial orbit** (a–c: arrow). The fracture line is in the region of the **convergence of the maxillary, lacrimal, and zygomatic bones**. The cat also sustained a displaced fracture of the hard palate and separations of the incisive and mandibular symphyses (c: arrowheads).

Answer 42

16mo M Chihuahua bitten in the face 1 year prior. The representative transverse images a–c are ordered from rostral to caudal. Images e and f are contrast‐enhanced images comparable to b and c, respectively. There is **marked deformation of the right maxilla and frontal bone** (a,b: arrowheads) and **multiple fenestrations** within the right maxilla and left frontal bone (b,c: arrows), causing both **orbits to be misshapen**. The 3D rendering (d) shows the extent of trauma and remodeling of the dorsal margin of the right orbit. **Right‐sided phthisis bulbi** (e: arrow) and an **intracranial cyst** (f: asterisk) resulting from the previous trauma are also evident.

Answer 43

11y FS Pointer with 24‐hour history of left‐sided exophthalmos. Images a–c depict the same anatomy. Image d is slightly more ventral. * There is a **mild diffuse increase in extraocular musculature** and a**dipose volume** in the **left retrobulbar space** (a: arrow). * There is also a **loss of definition of muscle, scleral, conjunctival, and intraocular margins** seen on all sequences. * **Diffuse retrobulbar and conjunctival enhancement** is evident on the left (c,d). Conjunctival biopsy revealed neutrophilic and plasmocytic conjunctivitis. Clinical signs rapidly resolved with systemic and topical antibiotic therapy.

Answer 44

1y MC Labrador Retriever with bilateral blepharospasm and reduced response to retropulsion. Images d–e are at the level of the globe and images g–i are further caudal within the retrobulbar space. * There is an **increase in volume** and **loss of definition of the extraocular muscles** (a–c,g–i). This is associated with an increase in extraocular muscle signal **intensity on T2** images (b,h). * There is **diffuse contrast enhancement** of the **extraaxial muscles** (c,i) and **ocular adnexa** (f), bilaterally. * **Focal meningeal enhancement** is also present in the **frontal lobar** region bilaterally (c: arrowheads) and *may represent an extension of the inflammatory response through the optic canal or orbital fissure*. Extraocular muscle biopsy revealed chronic lymphohistiocytic myositis and myocyte atrophy, which is consistent with the microscopic description reported for extraocular myositis. Conjunctival biopsy revealed marked subacute fibrinosuppurative conjunctivitis.

Answer 45

10y MC Newfoundland with acute onset left‐sided conjunctival hyperemia. Representative dorsal plane images all depict the same anatomy. These images are ventral to the globes and depict the ventral aspect of the orbits. * A **fusiform abscess** that has **T1 hypointensity** and **T2 hyperintensity** (a–d: arrows) is evident centrally. * The **core does not contrast enhance**, but the **periphery intensely enhances** (c,d). * Distribution of **contrast enhancement** is best seen on the **fat‐suppressed contrast‐enhanced T1 image** (d). * The **left zygomatic salivary gland** is also **mildly enlarged** and **contrast enhances** more than the contralateral gland as a result of secondary **sialadenitis** (a,d: arrowhead). Aspiration cytology revealed marked septic suppurative inflam- mation with a mixed bacterial population.

Answer 46

3y M Golden Retriever with right‐sided buphthalmos and pain opening the mouth. * There is an **irregularly shaped, hypoattenuating region** in the **right pterygoid muscle** (a,c). * The pterygoid muscle is located **medial to the hyperattenuating zygomatic salivary gland** on the transverse images (a) and on ultrasound (b). * The region of inflammation is hypoechoic and irregular on ultrasound images (b). * There is additional **soft‐tissue opacity in the left nasal cavity** indicating **rhinitis.** * No migrating foreign bodies were observed, but this remains a primary differential as a cause of the abscess.

Answer 47

8y MC German Shepherd cross with resistance to retropulsion of the right eye. Images a–c and d–g depict the same anatomy in dorsal and transverse planes, respectively. * A **well‐demarcated lobular mass** is seen in the **ventral** aspect of the **right retrobulbar space** (a–g). * The mass is **highly contrast enhancing** and has a **complex internal architecture consistent with salivary glandular tissue** (b,f,g). * Images c and g show the value of **fat suppression** for increasing conspicuity of **contrast‐enhancing lesions in the retrobulbar space**. Aspiration biopsy of the mass revealed salivary tissue and chronic granulomatous inflammation.

Answer 48

Mature cat of unknown age with left‐sided exophthalmos. Representative transverse images are ordered from rostral to caudal. * A **large uniformly attenuating mass** (a,b: asterisk) within the left retrobulbar space causes marked **displacement of the globe**. * The mass has **eroded through the perpendicular** (a,b: arrowhead) and **horizontal laminae of the palatine bone** (a: large arrow) and fills the **nasopharynx and the left sphenoidal sinus**. * The mass has also **eroded through endoturbinates** and the left ventral part of the **cribriform plate** (b: small arrow). * Both **frontal sinuses are fluid filled** (b) as a result of **obstructive sinusitis** from the intranasal component of the mass (not seen). Aspiration biopsy confirmed lymphoma.

Answer 49

13y FS Golden Retriever with right‐sided exophthalmos. Representative transverse CT images (a–c) are ordered from rostral to caudal. The gross pathology image e is approximately comparable to CT image a. * A **well‐delineated sessile soft‐tissue mass** is seen within the **medial aspect** of the **right orbit** (a–d: asterisk), causing **compression and lateral displacement** of the **adjacent globe** (a: arrowhead). * The mass is only **minimally contrast enhancing**. Tissue biopsy revealed melanotic melanoma. A highly pigmented mass is seen in the dorsomedial aspect of the right orbit on the gross pathology image (e: arrow).

Answer 50

17y FS Domestic Shorthair with swelling of the left side of the head and diminished retropulsion of the left eye. A **right middle lung lobe mass** was seen on thoracic radiographs. Representative transverse CT images are through the caudal aspect of the retrobulbar spaces and are ordered from rostral to caudal. * An **ill‐defined, peripherally contrast‐enhancing mass** is present within the **caudal extent** of the **left retrobulbar space** (b). * **Contrast enhancement** also extends to **surrounding musculature and fascial planes** (a). * A **large well‐defined mass** is present in the **ventral part of the right middle lung lobe** (c: asterisk). Tissue biopsies from necropsy revealed a primary bronchogenic carcinoma of the right middle lung lobe and an aggressive, highly invasive metastatic mass of the left retrobulbar space.

Answer 51

7y FS Retriever cross with right‐sided exophthalmos and intracranial neurologic signs. Representative unenhanced (a–c) and comparable contrast‐enhanced (d–f) images are ordered from rostral to caudal. * An **ill‐defined partially mineralized soft‐tissue mass** is present in the **right retrobulbar space** (a–c: arrows), causing **dorsal and rostral displacement** of the **right globe** (d: arrowhead). * There is **osteolysis and medial displacement of the right frontal bone,** **perpendicular lamina of the right palatine bone, and ethmoturbinates** (a,b: arrowheads). * The mass **extends into the olfactory and frontal regions of the cranial vault** (d–f: asterisk). * The **expansile intracranial component** of the mass also results in **attenuation and lateral displacement of the left frontal and palatine bones** (c,f: arrowheads). * The mass has a **heterogeneous contrast‐enhancement** pattern in the retrobulbar space (d,e) and a **peripheral enhancement** pattern intracranially (f: asterisk). Histologic diagnosis following necropsy was highly anaplastic sarcoma consistent with fibrosarcoma. The intracranial component was centrally necrotic, consistent with the pattern of contrast enhancement seen on CT images.

Answer 52

3y MC Tibetan Terrier with a 2‐week history of left‐sided exophthalmos. Representative dorsal plane images are at (a–c) and ventral to (d–f) the level of the eyes. * There is a **large irregularly shaped mass** within the retrobulbar space (a–c,g,h: asterisk), which is **isointense to gray matter** on **both T1 and T2 images**. * The mass **invades the left temporal muscle** (a–c: arrow) and appears to cause **osteolysis of the left zygomatic bone** (d,h: arrowheads). * The mass is **nonuniformly contrast enhancing**. Histologic diagnosis following necropsy was rhabdomyosarcoma.

Answer 53

7y MC Cavalier King Charles Spaniel with a 4‐month history of progressive right‐sided exophthalmos. The first three images (a–c) depict the same anatomy and are at the level of the caudal aspect of the retrobulbar spaces. * There is a **well‐defined mass** in the caudal aspect of the right retrobulbar space that is **hyperintense to gray matter on both T1 and T2 images** (a,b: arrow). * The mass is **moderately and uniformly contrast enhancing** (c,d: arrowhead). * The **caudal extent** of the mass is **adjacent to the region of the orbital fissure and optic canal** (d–e,f: arrow). These structures are not clearly seen on the images, but the arrows indicate their approximate location. Upon surgical exploration, the mass was found to arise from the ophthalmic branch of the right trigeminal nerve (cranial nerve V) as it exited the orbital fissure. Excisional biopsy revealed meningioma. Although a specific preoperative diagnosis could not be made from MR images, the caudal and central location of the mass within the retrobulbar space and the uniform contrast‐enhancement pattern is suggestive of a neoplasm arising from one of the nerves arising from the orbital fissure or optic canal.

Answer 54

16y FS Siamese with bilateral conjunctival and episcleral hyperemia, corneal ulcerations, and reduced extraocular muscle function. Images a–d are at the same anatomic level. Image e is somewhat more ventral and highlights the caudoventral recesses of the orbits. * There is **pronounced thickening of the sclera and episcleral tissues**, which is best seen on the contrast‐enhanced images (d–f: arrows). * **Periorbital edema** is also evident on the **STIR image** (c: arrows). * There is a **loss of thickening of the extraocular muscles** and **loss of muscle margin definition** associated with **retrobulbar contrast enhancement** (e: arrowheads). Biopsy confirmed a diagnosis of restrictive orbital myofibroblastic sarcoma involving both orbits. Thomasy et al (2013).16 Reproduced with permission from Wiley.

Answer 55

_CT_ be used for imaging the major structures of the globe, including the: * Anterior chamber * Vitreous chambers * Lens _MRI_ is an excellent modality to image the structures of the globe and optic nerve: * Cornea * Anterior chamber * Posterior chambers * Ciliary body * Lens * Vitreous chamber * Retina * The optic nerve can also be evaluated and followed to the optic chiasm, both in transverse images and when dorsal or sagittal plane sequences are oriented obliquely along the long axis of the nerve. The **optic nerve is surrounded by cerebrospinal fluid (CSF)**, which appears **hyperintense on T2 and hypointense on T1 images**. **Fat nullifying sequences, such as STIR,** as well as thin collimation can help to suppress the bright fat signal and allow visualization of the CSF and nerve. _Trauma_ * Trauma to the eye commonly presents as **proptosis,** with the possibility of additional trauma to the surround ing bones and soft tissues of the orbit. * **Penetrating trauma** to the eye is not commonly evaluated with CT or MRI, but **hemorrhage, inflammation, and altered anatomic structures** would be expected findings. * **Foreign bodies and associated inflammation** in the sclera can be seen as a **mass lesion that deforms the scleral** shape with **contrast enhancement**. * Traumatic **ruptures** of the lens and globe have been reported to be better seen on MR than on CT images. _Inflammatory disorders_ * **Optic neuritis** can be seen on MR images as **hyperintensity** of one or both **optic nerves** in watersensitive imaging sequences. The nerve may be **hyperintense within the orbit and/or at the level of the optic chiasm.** * **Granulomatous meningoencephalitis** has been reported to involve the optic nerves. On MR images, it appears as **isointense regions on T1 and T2** weighted images, with **intense contrast enhancement**. * **Anterior uveitis** is defined as inflammation of the anterior chamber of the eye. On MR images, **increased signal intensity** can be seen on **T1 images**, and **contrast enhancement may also** be observed. * **Episcleritis** involves inflammation of the **tissues surrounding the sclera**. The **periocular tissues** are **thickened and T1 and T2 hyperintense** and are **intensely contrast enhancing**. _Neoplasia_ * **Melanoma** arising from the uvea has been described on MR imaging in dogs. The mass was **T1 hyperintense** and **T2 hypointense** with **contrast enhancement.** *T1 hyperintensity* is a property of *melanin* that has been described in other regions of the body. A case of melanoma arising from the choroid and surrounding the optic nerve was T1 and T2 hypointense. On CT images, melanoma may appear **hyperattenuating to the vitreous**. * **Round cell neoplasia** such as lymphoma can also **primarily involve the eye**. Metastatic disease due to other neoplasms can also occur. * Neoplasia **should be considered in patients with ocular hemorrhage** where there is a clear fluid–fluid interface on MR image. _Degenerative disorders_ * **Retinal detachment** is **uncommon** in cats but may be caused by bullous or effusive mechanisms, a tear with vitreous filling the space between the retinal and choroid, or contraction of postinflammatory fibrous strands in the vitreous pulling the retina anteriorly. The CT appearance of retinal detachment is a **V‐shaped linear structure** (shaped *like seagull wings)* with the **apex centered at the optic disc.** **Hyperattenuating material between the retina and the choroid** may indicate **proteinaceous fluid or hemorrhage**. Bullous retinal detachment due to fluid accumulation has been reported in cats with hypertension. On MR images, the fluid external to the retina is hyperintense on T1 and T2 images. * **Cataracts** are a degenerative disease of the lens that results in **increased density and opacification**. On CT images, cataracts appear as **hyperattenuating strands within the lens**, which can progress to involve the entire lens. **On MR i**mages, the lens becomes **decreased in signal intensity**. The **lens is normally hyperattenuating to the vitreous on CT** images and **hypointense on MR images**. **Deformation of the shape** of the lens can also occur with cataract formation. * **Glaucoma** can result in **luxation of the lens** into the vitreous chamber. * Some patients are fitted with ocular **prostheses following enucleation**. If CT or MR imaging is performed, characteristic artifacts may occur. **Silicone‐based prostheses** may contain pigments to mimic brown or black eye color. In one study, the **brown‐pigmented prosthesis** contained iron oxide or titanium dioxide, producing **susceptibility artifact.** A black prosthesis was pigmented with carbon black and did not produce ferromagnetic artifact, with no signal on any sequence. On **CT** images, the prostheses tend to be **hyperattenuating.**

Answer 56

* The **lens** is **hyperattenuating** (a: asterisk) and maintains its position through **circumferential attachment to** the **ciliary body** (b: white arrowhead). * The **anterior chamber** (a: black arrowhead) and **vitreous chamber** (a: open arrow) are **fluid attenuating**. * The **sclera** is **hyperattenuating** and **surrounds the posterior portion** of the eye (a: white arrow).

Answer 57

* The **lens** is visible as a **hypointense** structure **with a hyperintense capsule** (b: asterisk). * The **ciliary body** supports the lens (c: arrowhead) and is **contrast enhancing** (d). * The **anterior** (b: small white arrow), **posterior** (d: \<), and vitreous chambers (b: open arrow) are **hypointense on T1** and **hyperintense on T2** images. * The **cornea** (d: arrowhead) is visible anteriorly. * The **enhancing retina** (b: large solid white arrow) is present posterior to the vitreous.

Answer 58

3y FS Brittany Spaniel with acute head trauma of unknown cause. Transverse images are ordered from rostral to caudal. The globe is located cranial and lateral to the orbit (a). * There is **soft‐tissue swelling** **within the orbit** (b: asterisk), **likely due to hemorrhage and edema**. * The **optic nerve appears enlarged** and **surrounded by hypoattenuating tissue** as it crosses this region (b: arrow).

Answer 59

1y M Labrador Retriever with progressive swelling and cloudiness of the right eye. Glaucoma and uveitis were diagnosed following a complete ophthalmic examination. * There is **increased signal intensity** of the **right anterior chamber** on T1 images (a: arrow) compared to the normal eye. There is a **similar but more subtle** increase **in vitreous attenuation** in the same eye. * On contrast‐enhanced images, there is **marked enhancement** of the fluid in the anterior chamber (c). The eye was enucleated after a corneal rupture, and plant material was found within the lens capsule.

Answer 60

9y F Border Collie mix that was presented for eye infection. * There is **thickening of the tissues surrounding the eye** as well as the **sclera itself**, best seen on **contrast‐enhanced images** (c,f: arrow). * The sclera is contrast enhancing and thickened compared to the normal eye (b,c,f).

Answer 61

11y M Golden Retriever with red‐eye of 3-week duration. This was a melanotic melanoma of the anterior uvea, explaining the **pronounced T1 hyperintensity** on the **unenhanced image.** The diagnosis was confirmed histologically following enucleation.

Answer 62

11y MC German Shepherd Dog with aspergillosis and an incidentally discovered ocular mass. * The mass (a: arrows) is visible as a **lobular, soft‐tissue attenuating** structure **caudal to the lens**. * There is **interruption of the frontal bon**e and **absent nasal turbinates** (c: asterisk) due to **previous rhinosinusitis**. Vitreous humor aspirate revealed melanotic melanoma.

Answer 63

12y MC Domestic Longhair who is FIV positive, with blindness and decreased mentation. * There is **nuclear sclerosis** of the **right lens** (a). * **Retinal detachment** is present (d: arrow) with thickening of the retinal tissue on contrast‐enhanced images (c: arrow). The **material posterior to the retina** is **T2 hyperintense** and **T1 hypointense** (a,b: arrowhead). * The **anterior chamber** is filled with **contrast‐enhancing material** (f: asterisk). * The **optic nerve is enlarged** and **contrast enhancing**, extending **through the orbital fissure** to a **hyperintense, contrast‐enhancing mass within the brain** (g: open arrows). Lymphoma was diagnosed after enucleation of the eye.

Answer 64

11y MC Border Collie with a history of ocular hemorrhage. * There is **increased intensity** within the **vitreous chamber** of the left globe on T1 (a) and T2 (b) weighted sequences. * The **retina is poorly defined and appears detached**. * There is a **clearly demarcated line** (a: arrow) separating **two different fluid signals** in the vitreous chamber. **Neither component nulls on the FLAIR** image (c), and the lesion **does not enhance** on contrast‐enhanced images (d). * The normal right globe (e) can be compared with the abnormal globe (f) on sagittal images. A hemangiosarcoma metastasis was found in the iris on histopathology and was the source of intraocular hemorrhage, which was confirmed on postmortem examination. The partitioning was thought to be due to separation of serum and cellular components.

Answer 65

8y MC Domestic Shorthair with hypertension, acute onset blindness, and obtundation. The CT study was performed on an emergent basis to evaluate for intracranial causes of obtundation. * There is a **broad‐based, hyperattenuating structure** (arrow) on the **ventral** aspect of the globe. * The most caudal aspect of the structure is in the **region of the optic disc.** * The **hyperattenuation indicates hemorrhage or proteinaceous fluid.** * **Nuclear sclerosis** was also present. Imaging findings were confirmed by full ophthalmic examination.

Answer 66

18y MC Domestic Shorthair with progressive central nervous system disease, diabetes mellitus, and chronic renal failure. * There is **bilateral retinal detachment** with a **classic “V” shape centered at the optic disc**. * The **material posterior to the retina** is **hyperintense on T2** images (a) and **slightly hyperintense on T1 images** (b) and represents **retroretinal exudate**. Bullous serous retinal detachment, confirmed by complete ophthalmic examination, was thought to be secondary to systemic hypertension.

Answer 67

11y Poodle/Maltese cross. A cataract had been developing in the right eye for the previous 4 years. * In the **bone window** (a) and **soft‐tissue window** (b), **mineral attenuating regions** are visible in the **peripheral and central regions of the right lens**. The cataract was brunescent and resulted in loss of vision.

Answer 68

* The left lens is in a normal position caudal to the iris (a: white arrow). * The **right lens is misshapen** and **reduced in size** as a result of a **hypermature cataract** (b,c: asterisk) and has **been displaced caudal to the iris** (b: \>) **into the vitreous** (b–e). * The **sclera is slightly irregular** in the right eye (e: black arrow), and the **right globe is larger** than the left (d,e). * There is a **thin hypointense linear structure** (b: arrowheads) medial to the luxated lens that represents the detached retina. These findings, confirmed on postmortem examination, were attributed to glaucoma and to systemic hypertension.

Answer 69

The salivary glands include the: * Mandibular glands * Zygomatic glands * Parotid glands * Lingual glands The **mandibular salivary gland** is a **large, oval, uniform** structure located **caudal to the mandible**. On CT images it has a **uniform texture**. The **parotid salivary gland** is t**hin, elongated**, and has a **finely textured lobular** structure. It is located **lateral to the vertical ear canal** and cranial and dorsal to the mandibular salivary gland. The **parotid and mandibular salivary glands** are **moderately hyperintense to muscle on T1** images, but the **mandibular salivary gland is hyperintense to the parotid on T2** images. The **zygomatic salivary gland** is **variable in size and shape**, and it is **located in the orbit, lateral to the pterygoid muscles** and **ventral to the globe**. **Contrast enhancement** on CT is **slightly heterogeneous** because of the glandular architecture. The major **sublingual salivary gland** is **fused to the cranial capsule of the mandibular salivary gland**. It appears **triangular** in shape in the sagittal plane and may be more difficult to visualize on MR images. * Glands are **isointense to hyperintense to adjacent musculature on T1** and **hyperintense on T2**. * The salivary glands **moderately to intensely contrast enhance** on both CT and MR images. * Sialography of the salivary glands using CT has been performed in cadavers by placing a cannula and extension set in the oral salivary duct. Diluted nonionic contrast medium mixed with methylcellulose was used to fill the ducts. The parotid duct travels from the rostral and ventral border of the gland, lateral to the masseter muscle, to the level of the 4th premolar. The mandibular duct travels medial and parallel to the mandible and enters the oral cavity at the level of the sublingual caruncle. The zygomatic duct enters the oral cavity caudal to the parotid duct at the level of the first upper molar and often has several diverticula.2 _Inflammatory disorders_ * **Zygomatic sialadenitis** is an inflammatory condition of the zygomatic salivary gland. The position of the gland in the ventrolateral orbit causes **secondary exophthalmo**s when enlarged and inflamed. CT and MR imaging show an **enlarged, hypoattenuating (CT)** and **T1 hypointense, T2** and **FLAIR hyperintense** (MR) gland with **surrounding loss of detail due to inflammation.** The disease is usually unilateral but can also be bilateral. Formation of **fluid‐attenuating** or **T2 hyperintense sialoceles i**s common. The mandibular and parotid salivary glands are occasionally affected by sialadenitis. This appears on CT images as enlargement of the gland lateral and ventral to the ear canal. Affected glands are **intensely contrast enhancing** and **often retain their glandular structures**, including ducts, despite the inflammatory change. * The structure **may become disrupted** with **sialocele or abscess** formation. _Neoplasia_ * Tumors of the salivary glands are **uncommon.** * These lesions produce **mass effect** in the regional tissues of the head and irregular enlargement of the gland of origin. * **Contrast enhancement** may be **strong** to **heterogeneous if there is fluid present or regions of necrosis**. * Tumors can be differentiated from sialadenitis by the **disruption of the normal architecture of the gland**. * Examples include **adenocarcinoma** of the mandibular and parotid salivary glands, as well as zygomatic basal cell adenocarcinoma _Sialolithiasis and sialocele_ * When the salivary ducts are **blocked by mucus** concretions **or sialoliths**, the production of saliva causes **expansion of cavities within the salivary gland**, which extend beyond the normal borders. These fluctuant masses tend to **contain fluid in large thin‐walled chambers** in the ventral mandibular area, sublingual region, or orbit (see zygomatic sialadenitis earlier in this chapter). * The **communication** with the salivary gland **may be difficult** to localize on CT and MR images. * Other imaging characteristics include fluid that is **hypoattenuating on CT (0 HU)** with **no contrast** **enhancement**, and MR **hyperintensity on T2 i**mages with **hypointensity on T1** images. * The **fluid compartments** may exhibit **peripheral contrast enhancement** on CT and MR images. * The **location of the fluid** in relation to the salivary glands **may suggest a gland of origin**. For example, fluid in the sublingual region is likely associated with obstruction to the sublingual salivary duct. * Sialoceles may also occur bilaterally, appearing as roughly symmetric fluid‐filled masses on both sides of the mandible. * If sialoliths are responsible for the ductal obstruction, they may be seen on CT images within the fluid of the mass, within the ipsilateral or contralateral gland, or within the duct itself. Sialoceles may also form in other salivary glands, including minor glands that are less commonly seen on imaging examinations.

Answer 70

Representative images were acquired immediately following contrast medium administration and are ordered from rostral to caudal. * The **normal mandibular salivary glands** are characteristically **oval** in shape and **smoothly** margined (a–d: arrows). * The **normal parotid salivary glands** are more **elongated** and have **lobular** margins (a–c: arrowheads). * Both glands are **highly and uniformly contrast enhancing**. * The **major sublingual salivary gland** is a small **triangular** structure fused to the **cranial margin of the mandibular salivary gland** (d: open arrow).

Answer 71

The normal mandibular salivary glands are characteristically oval in shape and smoothly margined (a–c: arrows). The normal parotid salivary glands are more elongated and have a mildly heterogeneous appearance (a–c: arrowheads). Although the glands have **similar hyperintensity to muscle on unenhanced T1** images, the **mandibular salivary glands are hyperintense compared to the parotid** glands on **T2 images**. Both glands are intensely and uniformly contrast enhancing (c).

Answer 72

Normal zygomatic salivary glands **moderately contrast enhance** (large arrows). The **nonuniformity of enhancement** is consistent with the glandular architecture. The medial pterygoid muscle is located adjacent and medial to the zygomatic salivary gland at this level (small arrows).

Answer 73

Zygomatic salivary glands (a–d: arrows) are extremely **variable in both size and shape.** Glands are **hyperintense to adjacent musculatur**e on both **T1 and T2** images (a,b) and are **moderately and uniformly enhancing** on contrast‐enhanced **T1** images (c,d).

Answer 74

2y MC Australian Shepherd with left‐sided exophthalmos. * A **multicameral cystic mass** arises from the **left zygomatic salivary gland** (a–c: arrow), causing **dorsal displacement of the left globe**. * The right zygomatic salivary gland is normal in appearance. Aspiration cytology of the mass revealed neutrophilic inflammation consistent with zygomatic sialoadenitis.

Answer 75

9y FS Cocker Spaniel with left‐sided exophthalmos. * The **left zygomatic salivary gland** (a–f: large arrow) is **markedly enlarged** and is **T1 hypointense** and **T2 hyperintense** to the contralateral gland (a–c: small arrow). * The contrast‐enhanced images reveal that the **glandular architecture is retained**, including the arborizing ductal pattern, suggesting diffuse inflammation rather than neoplasia. Clinical signs resolved with antibiotic and anti‐inflammatory therapy.

Answer 76

12y MC Rottweiler with a ventral cervical mass. Representative contrast‐enhanced images are ordered from rostral to caudal. * There is a **large predominantly fluid attenuating cavitary mass** **contiguous with the lateral margin of the right mandibular salivary gland** (a: asterisk). * The lateral **contour of the gland is altered,** and the **fluid center extends into the glandular parenchyma** (a). * The mass is **thick walled, peripherally enhancing, and poorly margined**. * The **center** consists predominantly of **fluid** but also includes a small volume of **fragmented gas** (b). A mixed bacterial population was cultured from a fluid aspirate.

Answer 77

8y FS Siamese with a ventral cervical mass. Representative images are at the level of the mandibular salivary glands. * There is a **large, spherical, low‐attenuation mandibular salivary mass** in the right ventral cervical region (a,b: large arrow). * The mass **contrast enhances nonuniformly centrally** and has a t**hin but prominent peripheral rim of enhancement** (b). * The normal left mandibular salivary gland is also evident (a,b: small arrow). Biopsy confirmed mandibular salivary adenocarcinoma.

Answer 78

10y MC Afghan Hound. The CT study was performed as part of a diagnostic evaluation of suspected pituitary‐dependent Cushing’s syndrome. * An **oval fluid‐dense mass** (a: asterisk) is located **medial to the left digastricus muscle** (a: arrow). * The **attenuation of the fluid was 0 HU** and was similar on **unenhanced** and contrast‐enhanced images. The clinical diagnosis was sialocele arising from the mandibular salivary gland (not seen in this image).

Answer 79

10y MC Maltese with a fluctuant ventral cervical mass. The CT image is at a level just rostral to the mandibular salivary gland. * An **ill‐defined and diffusely hypoattenuating mass** (arrow) is present **adjacent to the right external ear canal and tympanic bulla**. * **Multiple focal mineral opacities** are distributed within the mass. * Smaller numbers are seen on the left side. The mass was thought to represent a sialocele containing multiple sialoliths.

Answer 80

The lymph nodes of the head include: * Facial ymph nodes * Parotid ymph nodes * Mandibular ymph nodes * Lateral and medial retropharyngeal lymph nodes Lymph from the **rostral lymph nodes** passes through the lymph node chain sequentially and **may mix and cross** to the contralateral side **before** reaching the **medial retropharyngeal** lymph nodes. * The **mandibular** lymph node group consists of **three to four** lymph nodes surrounding the **facial vein** on the ventral neck and are, in aggregate, 10–25 mm in length. * The **parotid** lymph nodes (**one or two** nodes) are located **lateral to the temporomandibular joint**, **medial to the parotid salivary gland** and are detected infrequently on CT and MR images. * The **medial retropharyngeal** nodes are located **between the mandibular salivary gland and common carotid artery** and are 30–70mm in length in dogs and average 20.7 × 4.2 × 13.1 mm in cats. * The **lateral retropharyngeal lymph** nodes are less frequently seen in normal animals. Lymph nodes are **isoattenuating to muscle on CT** and **strongly contrast enhance**. They are **hypointense to fat** and **isointense to muscle on T1** images and **hypointense to fat and hyperintense to muscle on T2** images. Lymph nodes are **isointense to fat on contrast‐enhanced T1** images. The lymph nodes of the head are **generally less than 5 mm in width**. * In the caudal part of the neck, the s**uperficial cervical lymph nodes** are located **lateral to the serratus ventralis and scalenus muscles**. _Inflammatory disorders_ * Lymph nodes affected by regional disease, such as **abscesses, myositis, otitis externa, and other inflammatory disorders**, undergo hyperplasia as part of the immune response. * _On CT and MR_ images, the lymph nodes appear **mildly to moderately enlarged**. * _On CT_, reactive lymph nodes are **normally iso attenuating to hypoattenuating on unenhanced images** and moderately to s**trongly contrast enhancing** with a **uniform or central pattern.** * Lymph nodes are similarly mildly to moderately enlarged _on MR_ images with **homogeneous to heterogeneous contrast enhancement**. * The parotid and lateral retropharyngeal lymph nodes may be visible when enlarged. * In severe cases, lymph nodes may become **abscessed** with **central hypoattenuating regions** and **peripheral contrast enhancement** * Reactivity may also cause **poor margination and soft‐ tissue stranding** in surrounding fat. _Neoplasia_ * Neoplasia of the head and oral cavity may metastasize to the regional lymph nodes. The mandibular and medial retropharyngeal lymph nodes should be evaluated for **enlargement, heterogeneity, and change of shape** to detect metastatic disease. * **Metastatic deposits tend to lodge in** the **lymphatic sinuses** of affected nodes, and when macrometastases are present, **filling defects** can be identified on **contrast‐ enhanced images**. * _On CT images_, these **need to be distinguished from fat** within the lymph node hilus, which can mimic a parenchymal filling defect. * _On MR images_ of dogs with _mast cell tumors,_ affected lymph nodes were **larger and more heterogeneous on T2 and contrast-enhanced** images than normal lymph nodes. * Lymph nodes _in cats with metastatic disease from squamous cell carcinoma_ were **not significantly large**r than normal lymph nodes. Therefore, affected lymph nodes **cannot always be detected on CT and MR images**, and **fine‐ needle aspiration cytology is necessary for diagnosis.** * _Lymphoma_ may also affect the lymph nodes of the head and neck. Diffuse large‐cell B‐cell lymphoma results in **marked enlargement** of the retropharyngeal and/or mandibular lymph node groups. The **contrast enhancement** in these lymph nodes is **uniform with a slightly foamy appearance**. Small lymph nodes that are not normally identified, such as the parotid lymph node, may become visible with increased size. T‐cell lymphoma may affect a single lymph node in the head with similar imaging characteristics.

Answer 81

* Mandibular lymph node aggregates are seen ventrally (a: arrows). * Normal mandibular nodes are variable in both size and number. * **Normal** lymph nodes are **highly and uniformly contrast enhancing** (b). * The **facial vein** (b: arrows) courses next to the lymph nodes and should be distinguished from them by viewing serial contiguous images (not shown).

Answer 82

* Mandibular lymph node aggregates are seen ventrally, appearing **isointense to muscle on T1** images (a: arrows) and **hyperintense on T2** images (b: arrows). * Normal mandibular lymph nodes are variable in both size and number. * Normal lymph nodes are **highly and uniformly contrast enhancing** (c). * The **facial vein** (b: arrowheads) courses next to the lymph nodes and can be distinguished from the lymph nodes on MR by the **flow void artifact.**

Answer 83

* On transverse images (a), the medial medial retropharyngeal lymph nodes (a: arrow) appear as oval, **isoattenuating** structures medial to the **mandibular salivary gland** (a: M) **and lateral to the carotid artery**. * A **hypoattenuating linear** structure representing **fat in the hilus** is visible in the rostral portion of the lymph node (a: open arrow). * Sagittal and dorsal plane images (b,c) show the elongated, oval shape of the lymph node (b,c: arrows). * **Strong, mildly heterogeneous contrast enhancement** is visible on all images.

Answer 84

7y MC Weimaraner with recent‐onset difficulty swallowing and pain on manipulation of the head and neck. Representative contrast‐ enhanced CT images are at the level of the mandibular (a) and retropharyngeal (b) lymph nodes. * Mandibular lymph nodes are **enlarged** and have **extensive nodal and ill‐defined perinodal contrast enhancement** (a: arrows). * Similar findings are seen associated with the medial retropharyngeal lymph nodes (b: arrows). * In addition, the **left medial retropharyngeal lymph node is greatly enlarged** and **contains a fluid component** ventrally consistent with frank **abscessation**. Aspiration cytology from the medial retropharyngeal lymph node confirmed suppurative inflammation and necrosis.

Answer 85

8y MC Labrador Retriever with previously excised right tonsillar squamous‐cell carcinoma. Representative CT images are at the level of the medial retropharyngeal lymph nodes and are ordered from rostral to caudal. * The **left** medial retropharyngeal lymph node (a: arrow) is normal in size, shape, and contrast enhancement. The central linear filling defect represents the **normal fat‐filled lymph** node hilus. * The **right medial retropharyngeal lymph** node is **markedly enlarged** and has **irregular margins** (b: arrow). Multiple **parenchymal contrast filling defects** are **characteristic of lymph node metastatic deposits** Aspiration cytology of the right medial retropharyngeal node confirmed metastatic squamous cell carcinoma.

Answer 86

9y M Fox Terrier. CT was performed for lymphoma staging. Representative unenhanced (a,b) and contrast‐enhanced (c,d) images are at the level of the mandibular (a,c: arrows) and medial retropharyngeal (b,d: arrows) lymph nodes. * The lymph nodes are markedly **enlarged** but **retain an oval shape and smooth contours**. * **Contrast enhancement is characteristically uniform with a slightly foamy appearance**. * The right parotid lymph node, often not easily recognized on CT in normal dogs, is prominent in this patient (a,c: arrowheads). Mandibular lymph node biopsy documented T‐cell lymphoma.

Answer 87

_Developmental disorders:_ * Congenital anomalies of the teeth are occasionally seen on CT images. * **Supernumerary teeth** appear as complete dental structures adjacent to the normal tooth at an abnormal angulation because of displacement. These have been reported to occur with high frequency in **Greyhounds** at the level of **maxillary P1.** * The **maxillary second premolar** and the **maxillary first molar** frequently have **fused roots** in cats and are occasionally absent. * **Brachycephalic syndrome** has been assessed using CT, with **increased thickness of the soft palate** being the main associated finding in severely affected animals. _Trauma:_ * **Developing teeth may be damaged** in traumatic events, which causes altered development of the tooth and occasionally of the adjacent structures _Inflammatory disorders:_ * Dental disease can result in **abscess formation** around affected **tooth roots** and may be observed on CT images as **contoured or rounded regions of alveolar bone osteolysis surrounding the tooth roots** * Occasionally, the **medial wall of the alveolus** will be **eroded** by the infectious process and cause **regional rhinitis** * Dental disease may also progress to frank **osteomyelitis**, producing **regional osteolysis and irregular to smooth periosteal reaction** surrounding the affected bone * Soft tissues of the oral cavity, such as the tongue, can be affected by inflammation and abscessation due to trauma or penetrating foreign bodies. On CT and MR images, **abscessation** appears as an **encapsulated lesion** **with central fluid intensity** or **attenuation and circumferential contrast enhancement**. * Oral infection can spread to other neighboring tissues of the head, such as the pharynx and brain _Odontogenic neoplasia_ * Odontogenic neoplasia is challenging to classify, and recent reports indicate that further study is required to accurately diagnose these tumors. _Cysts_ * **Dentigerous cysts** are rare and form from **squamous epithelium surrounding tooth remnants** * **Radicular cysts** are lined with squamous epithelium and occur **adjacent to the tooth root** These lesions cause **expansile bone destruction surrounding the tooth of origin** and an associated **fluid‐ attenuating mass.** _Tumors of the periodontal ligament_ * **Fibromatous epulis** is a common lesion of **soft‐tissue proliferation in the oral cavity**, **some** of which undergo **mineralization.** * **Fibromatous epulides** and **ossifying fibromatous epulides** are subdivided histologically into focal fibrous hyperplasia, which is inflammatory and benign, and peripheral odontogenic fibroma, having dental epithelium and a neoplastic behavior. CT imaging features of these lesions have not been described. _Tumors of odontogenic epithelium without odontogenic mesenchyme_ * Canine **acanthomatous ameloblastoma** (acanthomatous epulis) is an **aggressive** tumor that can affect the underlying **bone of the maxilla or mandible**. This lesion occurs mainly in **medium to large‐breed** dogs of middle age, most frequently in the **rostral mandible**. The majority of dogs had **osteolysis of the apical border of the alveolus** with an **expansile pattern.** The **soft‐tissue component** of the mass shows **intense, uniform contrast enhancemen**t. * **Amyloid‐producing odontogenic tumors** are **benign, mineralizing masses without encapsulation** and are relatively rare (Figure 1.9.12). _Tumors of odontogenic epithelium with odontogenic mesenchyme_ * **Ameloblastic fibroma** is a rare tumor that has a **benign** behavior and does not recur after excision. Imaging characteristics of a single case include an **expansile soft‐tissue mass with expansion of the surrounding mandible**. * **Ameloblastic fibro‐odontoma** is similar to ameloblastic fibroma with the **addition of enamel and dentin**. * Feline inductive odontogenic tumor is unique to cats and is rare, with no imaging features described. * **Complex odontoma** is comprised of **dental tissue that does not form recognizable tooth‐like structures**. These masses are **disorganized and highly attenuating** with **expansion of the surrounding bone**. * **Compound odontoma** is a **malformation of dental tissues** that are not organized into a normal tooth structure. _Oral cavity neoplasia_ * The most common types of neoplasia encountered in the oral cavity are **squamous cell carcinoma, melanoma, and fibrosarcoma.** More rarely encountered tumors, such as liposarcoma, may arise as a soft‐tissue attenuating mass in the fatty tissues of the tongue with intense contrast enhancement. * **Squamous cell carcinoma** is an **aggressive** tumor affecting cats and dogs. In cats, it may cause a mass effect in the soft palate, sublingual or lingual region, lip, buccal mucosa, maxilla, or mandible. When adjacent to bone, squamous cell carcinoma frequently results in **osteolysis**, with a **primarily lytic appearance and peripheral expansion of mineralized tissue with heterogeneous contrast enhancement**. When the soft palate is thickened in cats, adjacent otitis media or bulla effusion may occur concurrently. **Metastasis** to the mandibular and retropharyngeal lymph nodes is common and results in lymph node enlargement with **heterogeneous enhancement** or **well‐defined parenchymal filling defects**. * **Oral melanoma** may also affect the soft tissues of the mouth and can cause **osteolysis** of adjacent bone. The **soft tissue portion** of the mass is **expansile and heterogeneously contrast enhancing**. **Metastasi**s to local lymph nodes is also **common,** causing enlargement and **peripheral contrast enhancement with central nonenhancing regions** * **Fibrosarcoma** is the third most common oral tumor in dogs and is generally **locally destructive with osteolytic lesions**. CT scanning can improve outcomes by *defining the tumor margins for surgical planning*. * **Primary bone tumors**, including osteosarcoma, also occur in the oral cavity. They form **destructive and productive lesions** centered on the bone of origin and **expand peripherally**

Answer 88

* The **central lucent pulp cavity** (small black arrow) is surrounded by a **dense cementum** layer (large black arrow). * The **thin lucent** layer of the **periodontal ligament** (small white arrow) is in turn **surrounded by the dense lamina dura** of the alveolus (large white arrow). * The **outer layers of the crown** (black arrowhead) are composed of dense **enamel and dentin.** Pulp \> cementum \> dense enamel and dentin \> periodontal ligament \> lamina of the alveolus

Answer 89

11y FS Australian Cattle Dog. The CT scan was performed for an unrelated disorder. Representative sequential images acquired at the level of the fourth maxillary premolar are shown and ordered from rostral to caudal. * There is f**ocal destruction of periapical alveolar bone** of the rostral (a: arrows) and caudal (b: arrow) roots, and of the left maxillary fourth premolar. Imaging findings are characteristic of periapical abscess.

Answer 90

* The left maxillary canine tooth is **fractured at the tip** (a: arrow), and the **pulp chamber is widened** compared to the right side (a: arrowhead). * The adjacent **nasal cavity is filled with soft‐tissue opacity material** (b: open arrow). * The **alveolus is lytic** and open to the adjacent nasal cavity (c: arrow).

Answer 91

10y MC Labrador Retriever with a facial mass. The right maxillary fourth premolar was previously removed on suspicion of tooth root abscess. Unenhanced (a,b) and comparable contrastenhanced transverse images (c,d) are ordered from rostral to caudal. * On bone windowed images, a **cystic expansile lesion** is present in the right maxilla (b: solid arrow), **extending** from the third premolar to the right mandibular first molar tooth roots (b: open arrow) and **into the orbit.** * There is **peripheral enhancement** of the lesion on contrast enhanced images (c: arrows), and **fluid‐attenuating** material is present **centrally.** * The **circular‐shaped mandibular lysis** can be seen on the 3D image (e). * The **fourth maxillary premolar is absent**. Exisional biopsy confirmed the mass to be a radicular cyst.

Answer 92

8y FS Border Collie with possible infected tooth. * There is **marked osteolysis** and **irregular bone production** surrounding the caudal left mandible (a,b: open arrows). * **Multiple teeth are absent.** * There is a **large soft‐tissue mass** associated with the osseous lesion, which is **intensely, heterogeneously contrast enhancing** (d: arrows). * The 3D image shows the extent of the osseous component of mandibular lesion (e). Biopsy confirmed squamous cell carcinoma.

Answer 93

10y MC Labrador Retriever with difficulty eating. * There is a **mass** in the region of the **soft palate and tonsils** that is **multilobular** in shape (b: arrows). * On contrast‐enhanced images, the mass is **peripherally and heterogeneously enhancing** (e: arrows). * The **left mandibular lymph node** is enlarged with a **central, hypoattenuating, nonenhancing region** (c,f: open arrow) and **peripheral contrast enhancement.** Primary melanotic melanoma with regional lymph node metastasis was confirmed by excisional biopsy.

Answer 94

11y Standard Poodle with enlarging left maxillary mass. Representative CT images are at the level of the maxillary canine teeth. * There is a **left‐sided soft‐tissue mass** with an **intranasal component**, the latter of which has resulted in adjacent **ectoturbinate destruction** (a: asterisk). * The **left maxillary canine tooth is missing**, and **osteolysis** of the alveolus and lateral maxillary cortex is evident (a: arrows). * The mass **moderately and heterogeneously contrast enhances** (b). Biopsy revealed fibrosarcoma.

Answer 95

12y MC Miniature Schnauzer with left‐sided facial swelling and an infraorbital draining tract. * There is a **mixed productive and destructive mass** originating from the left maxilla (a: arrows). * The mass **extends into the nasal cavity, obliterating the ventral nasal turbinates.** * The mass is **mildly enhancing** on contrast‐enhanced images. Biopsy confirmed osteosarcoma.

Answer 96

_Developmental disorders_ * A **laryngeal cyst** causing **upper airway obstruction** has been reported in a dog. CT imaging characteristics were a **fluid‐attenuating mass with a thin rim of contrast enhancement** that did not communicate with the laryngeal lumen. _Trauma_ * Trauma due to bite wounds or other direct insult to the neck and laryngeal region may cause **disruption of the hyoid apparatus** at its **attachment to the skull** or its **intrinsic joints**. *Symmetry* of the hyoid apparatus is helpful in determining whether disruption has occurred, both on transverse and 3D images * Trauma may also result in **hematoma** formation that causes a **mass effec**t with characteristics of **blood or proteinaceous fluid on MR images** _Inflammatory Disorders_ * **Nasopharyngeal polyps** arise from the **tympanic bulla or auditory tube**, are inflammatory in origin, and are most often seen in cats. On CT images, they have **poorly defined margins** and are **hypoattenuating on unenhanced images**. Following contrast administration, masses remain **centrally hypoattenuating with an intensely contrast‐enhancing margin**. In most cats, a **stalk connecting** the polyp to the **widened auditory tube** can be seen. Ipsilateral or bilateral **otitis media** is usually present as **soft‐tissue or fluid attenuating material within the bulla**, which may be **thickened, expanded, and occasionally lytic**. * Inflammation of the pharyngeal region often occurs secondary to **foreign bodies** within the nasopharynx or penetrating foreign bodies from the skin or pharynx that localize in the neck or retropharyngeal region. Unless they are of sufficient size, plant material foreign bodies are **often not directly visualized** on CT or MR images. Larger foreign bodies, such as sticks, may have a definite shape and exhibit internal architecture. _On MR_ images, the **foreign bodies** may be **isointense on T1** and **hypointense or hyperintense on T2 images**. _On CT images_, the foreign material may be **hyperattenuating**. There is **surrounding tissue and edema** that is **hypoattenuating on CT** or **hyperintense on T2** on MR, with **strong peripheral contrast enhancement**. Fistulography may be considered to define any draining tracts and to attempt to outline the foreign body. _Neoplasia_ * **Ectopic thyroid neoplasia** has been recognized to occur in the **laryngeal region**, with possible invasion of the laryngeal lumen or ventral musculature. Masses are **oval or bilobed** and are **centered on the basihyoid bone with osteolysis**, and normal thyroid glands are present. **Metastasis** to local **lymph nodes or the lungs** may occur. * The musculature of the larynx may also give rise to neoplasia, such as **rhabdomyoma or rhabdomyosarcoma** * **Neuroendocrine tumors**, such as **carotid body tumors**, also occur in the region of the larynx and may be mistaken for thyroid carcinoma _Idiopathic and other disorders_ * **Upper airway obstruction** may occur secondary to **laryngeal paralysis or laryngeal collapse**. On CT images, the imaging findings of laryngeal paralysis include **failure to abduct the arytenoid cartilages, air‐filled lateral ventricles, and a narrowed rima glottis**. * **Everted laryngeal saccules,** collapse of the cuneiform and corniculate processes, and narrowed rima glottis were seen in sedated dogs with laryngeal collapse. These features may be difficult to assess in animals that are intubated under general anesthesia.

Answer 97

5y FS Jack Russell Terrier with a 3‐month history of coughing, gagging, and nasal discharge. Images a and b are the same image with and without annotation. These are 5 mm collimated transverse images that include the basihyoid bone (b: arrowhead), caudal ends of the ceratohyoid bones (b: small arrows), portions of the epihyoid bones (b: large arrows), and the distal end of the left stylohyoid bone (b: black arrow). The caudal end of the angular process of the right mandible is also seen (b: black arrowhead). The **right epihyoid bone is displaced laterally, indicative of trauma.**

Answer 98

8y MC Golden Retriever. Previous endoscopy (1 week prior) with laryngeal biopsy resulted in a hematoma. * A **large, well‐circumscribed mass** that is **hypointense to muscle on T1** sequences and **hyperintense to muscle on T2, PD, and FLAIR** sequences is present in the right cranioventral cervical region adjacent to the larynx (a: arrows). * There is a **thin, peripheral rim of enhancement** on contrast‐enhanced images (d: arrowhead). * The mass partially **suppresses on the FLAIR** image (e).

Answer 99

4y FS Domestic Shorthair with upper respira- tory noise and open‐mouth breathing. * There is s**oft‐tissue attenuating material filling the nasopharynx and choana** on transverse images (a,b: arrows). * The mass is **well circumscribed** and **peripherally contrast enhancing** (c: arrowhead). * The **left tympanic bulla** is filled with a combination of **mineral and soft‐ tissue attenuating material** (d: open arrow).

Answer 100

4y FS Pit Bull Terrier with progressive dyspnea. Representative contrast-enhanced images are at the cranial cervical level and are ordered from cranial to caudal. * A **contrast‐enhancing perilaryngeal** and **retropharyngeal mass** (a–d: arrow) is evident, **bounded by the longus capitus muscles dorsally** (a–c: asterisks). * The **medial retropharyngeal lymph nodes (**a,b: arrowheads) are moderately **enlarged** and have a **nonuniform contrast‐enhancement** pattern. Biopsy of retropharyngeal tissue confirmed chronic neutrophilic and plasmacytic cellulitis with extensive fibrosis.

Answer 101

11y MC English Setter with a left‐sided laryngeal mass. Images a and b are at the level of the larynx. Images c and d are at the level of the mandibular and medial retropharyngeal lymph nodes, respectively. * There is a **centrally hypoattenuating** **left laryngeal mass** (a,b: arrow) that **peripherally contrast enhances** and causes **rotational displacement** of the cranial border **thyroid cartilage** (b: arrowhead). * **Ipsilateral mandibular** (c: arrows) and **medial retropharyngeal** (d: arrow) **lymph nodes** appear normal. Biopsy revealed granular cell rhabdomyosarcoma. The dog was alive, and there was no evidence of mass recurrence 4 years following mass excision and permanent tracheostomy.

Answer 102

14y FS Golden Retriever cross with a right ventral cervical mass. Images d–f are at the level of the larynx. * An **irregularly margined,** **ovoid mass** is seen **adjacent to the larynx** in the region of the **right retropharyngeal lymph node**. * The mass has a **solid but heterogeneous center** (a–f) and **markedly contrast enhances** (c,f). * The **right carotid artery is displaced laterally** (d: arrow) compared to the left carotid artery (d: open arrow). * Both thyroid lobes were identified and appeared normal (not shown). Biopsy revealed the mass to be a neuroendocrine tumor of indeterminate origin.

Answer 103

10y FS Siamese with intermittent dyspnea of 1‐month duration. * A **large, predominantly right‐sided contrast‐enhancing mass** (a,b: asterisk) arises from the **right laryngeal wall**, **displacing and partially occluding the intralaryngeal ostium and caudal nasopharynx** (a: arrow). Aspiration cytology revealed this mass to be a malignant neuroendocrine tumor.

Answer 104

2y MC Labrador Retriever with progressive dyspnea. * A **well‐defined, uniformly contrast‐enhancing** mass **arises from the dorsal nasopharyngeal wall** and **nearly completely obstructs the nasopharyngeal lumen** (a,b: arrow). Endoscopically acquired biopsy (c) revealed the mass to be a primitive undifferentiated round‐cell tumor.

Answer 105

The normal thyroid gland is composed of **two flattened ellipsoid lobes** that lie adjacent and **dorsolateral** to the cranial segment of the **cervical trachea**. _In dogs_ the right lobe is located slightly cranial to the left lobe. Canine thyroid gland size is variable and may be conjoined by an isthmus. _Feline thyroid_ lobes are approximately 2 cm in length by **0.5cm in maximum width**. The thyroid gland is supplied by the **cranial and caudal thyroid arteries** and is **highly perfused**. _Parathyroid glands_ average **four** in number, although there is individual variability. These glands are **ovoid and less than 5mm in longest diameter**. Parathyroid glands are located toward the cranial and caudal poles of each thyroid lobe, with cranial glands tending to be located superficially and caudal glands embedded in thyroid parenchyma. _On transverse CT_ images, thyroid lobes are usually easily detected as **small oval or triangular paratracheal structures** that are **denser than surrounding tissues** as a result of the presence of **iodine** within the gland parenchyma. On long‐axis reformatted CT images, thyroid lobes have a characteristic **elongated ovoid shape**. **Unenhanced** attenuation of canine and feline thyroid tissue is approximately **110 HU and 125 HU**, respectively, and glands **uniformly contrast enhance**. _On MR images_, thyroid glands may have a **homogeneous or heterogeneous appearance** and are **T1 isointense or mildly hyperintense** and **T2 hyperintense** compared to adjacent cervical muscle. Normal thyroid glands are **markedly and uniformly contrast enhancing.** Normal parathyroid glands are not routinely recognized on either CT or MR images, although larger diameter glands may appear as focal hyperintensities in T2‐weighted MR images. Because high‐resolution ultrasound and scintigraphy studies are highly effective for evaluation of primary thyroid and parathyroid disorders, CT and MRI may be best employed as adjunct imaging techniques to assess extent and operability of aggressive thyroid neoplasia or for detection of ectopic thyroid and parathyroid masses. _Hypothyroidism_ * Approximately **half of dogs** with functional hypothyroidism have **lymphocytic thyroiditis**, while the **majority** of the remainder suffer from **idiopathic** thyroid atrophy. Although descriptions of CT and MR features of hypothyroidism are lacking in the veterinary literature, those patients with **thyroiditis** would be expected to have **thyroid enlargement**, whereas **idiopathic thyroid atrophy** would result in **reduced thyroid size**. _Feline functional thyroid nodular hyperplasia and adenoma_ * Functional **benign adenomatous** neoplasms and **hyperplastic** masses are common in the older cat. Because these are usually adequately characterized using other methods, descriptions of the CT and MR appearance of these lesions may have little clinical utility. On both imaging modalities, thyroid glands are **unilaterally or bilaterally enlarged** and m**ay include discrete mass lesions or diffuse lobar enlargement**. Affected thyroid glands **may have irregular margins and cystic components** that appear **hypoattenuating on CT** images and hypointense and hyperintense on T1 and T2 MR images, respectively. The thyroid glands of affected cats are **moderately to markedly contrast enhancing** and may be nonuniform in appearance. _Thyroid adenocarcinoma_ * Canine thyroid carcinomas are **most commonly unilateral**, are usually **poorly encapsulated**, and **aggressively invade adjacent tissues and vessels.** A tentative diagnosis is often made before CT or MR imaging is performed, and these studies are most useful for confirming thyroid origin, determining operability, and for specific surgical planning. On both CT and MR imaging, thyroid carcinomas are **often large and may displace or invade adjacent cervical musculature, blood vessels, trachea, larynx, and esopha- gus**. Some tumors appear to be well encapsulated, but others are unconstrained and highly invasive to adjacent tissues. Malignant tumors are typically **highly vascular,** and the parenchyma often appears **heterogeneous** and may have **cystic and mineralized components**. **Regional lymph node metastasis** is common. _On CT images_, thyroid adenocarcinomas are generally **isoattenuating** to adjacent ventral cervical musculature on unenhanced images, with **hypoattenuating and hyperattenuating regions** within the parenchyma corresponding to **cavitary lesions or mineralization**, respectively, when present. Malignant neoplasms are **markedly and nonuniformly contrast enhancing**, and tumor thrombi may be evident in neoplasms with vascular invasion. _On MR images_, tumors are generally **T1 hyperintense** on unenhanced images and of mixed **hyperintensity on T2** images. Contrast enhancement on MR images parallels that seen on CT, with tumor parenchyma **markedly and heterogeneously contrast enhancing**. * Ectopic thyroid tumors may occasionally be encountered in the **ventral cervical region** or **cranial mediastinum**. Ectopic thyroid tumors involving the hyoid apparatus. CT and MR appearances of ectopic thyroid carcinomas are the same as described for in situ masses. * Other ventral cervical masses that *can mimic* the imaging appearance of thyroid carcinomas include c*arotid body tumors, hemangiosarcoma, undifferentiated carcinoma, granulomatous lymphadenitis, and paraesophageal abscess.* _Parathyroid nodules_ * Disorders of the parathyroid glands resulting in **hypercalcemia** are categorized as **primary or secondary hyperparathyroidism**. _Primary hyperparathyroidism_ is usually due to the presence of a solitary autonomously **functional parathyroid adenoma or carcinoma**, while **secondary hyperparathyroidism** is caused by **hypocalcemia** that leads to **parathyroid hyperplasia** of multiple glands. Both entities lead to parathyroid gland enlargement; however, neoplastic glands tend to be solitary and, on average, are larger than hyperplastic glands, although there is considerable overlap. * _CT_ would be expected to be an **insensitive modality** for detecting parathyroid lesions because of the **relatively small size** of parathyroid nodules and similarity in density to surrounding thyroid tissue. * _On MR images_ parathyroid nodules may appear as **well delineated T1 hypointense** and **T2 hyperintense** lesions within or adjacent to thyroid parenchyma

Answer 106

7y Himalayan with recent history of dysphagia and anorexia. * An **ovoid hypoattenuating mass** is present in the left ventral cervical region on ultrasound examination (a). On CT images, the mass has attenuation less than adjacent soft tissues but significantly more than fat (b: arrow). * The mass **contrast enhances nonuniformly and margins are well defined** (c: large arrow). * The **left common carotid** artery is **displaced dorsally** (c: small arrowhead), and the left jugular vein is displaced laterally (c: large arrowhead). * The right thyroid gland appears normal (c: small arrow). Excisional biopsy confirmed a diagnosis of thyroid adenoma.

Answer 107

12y FS Labrador Retriever with previously diagnosed pulmonary and cervical masses. * The mass is **mildly hypoattenuating** on an unenhanced CT image (b: arrow). * The left thyroid lobe appears normal in size and is hyperattenuating (b: arrowhead). * The mass **moderately contrast enhances** but less so than surrounding normal thyroid tissue (c: arrow) and the contralateral thyroid lobe. * On dorsal plane maximum‐intensity projections (MIP) of contrast‐enhanced imaging data, a thick‐slab MIP reveals the course of the two common carotid arteries dorsal to the thyroid lobes (d: arrowheads). A thinner‐ slab MIP excluding the carotid arteries reveals the thyroid lobes (e: arrows) and the specific location of the mass within the right lobe (e: arrowhead). Excisional biopsy revealed solid and follicular thyroid carcinoma with vascular and capsular invasion.

Answer 108

12y FS Australian Shepherd with a right‐sided ventral cervical mass. Ultrasound examination revealed a large, solid vascular mass in the region of the right thyroid lobe (a). A smaller hypoechoic mass was seen within the body of the left thyroid lobe (b). Unenhanced and contrast‐enhanced transverse CT images (c–f) are paired and ordered from cranial to caudal. * A **large right‐sided, uniformly contrast‐enhancing** mass is present (c–f: large arrow). The mass **margin is poorly defined ventrally**, and there appears to be **extracapsular extension** of the mass and diffuse **enhancement of adjacent tissues** (e,f: arrowhead). * A normal‐appearing left thyroid lobe is seen on the more cranial CT image (c,e: small arrow). On the more **caudal image, the left thyroid lobe is larger and has lower attenuation characteristics than expected**, suggesting the presence of a **second smaller left thyroid mass** (d,f: small arrow). Imaging findings were corroborated at the time of surgical excision (i,j: arrows). Excisional biopsy revealed right‐ sided thyroid carcinoma with extracapsular inva- sion and left‐sided thyroid adenoma.

Answer 109

10y MC Labrador cross with a 3‐month history of dysphagia and ventral cervical mass. Representative CT images are paired unenhanced (a–c) and contrast‐enhanced (d–f) images ordered from cranial to caudal. * An **extensive soft‐tissue mass** with **heterogeneous attenuation** is seen in the right cervical region, **extending from the hyoid apparatus** rostrally to the mid‐cervical region caudally. * The **mass displaces the larynx to the left, crosses midline, and extends into the dorsal and left lateral laryngeal and retropharyngeal regions**. * The mass is **highly and heterogeneously contrast enhancing**. * The mass displaces the larynx to the left, invades the laryngeal soft tissues (e: small arrow), and **incorporates the carotid artery and internal jugular vein** on the right (e: black arrow). There are **filling defects and distension of these vessels caudally** (f: arrowheads) and of the **right facial vein** (d: arrowhead), indicating **tumor invasion and the presence of tumor thrombus**. * The **left retropharyngeal lymph nodes** are enlarged and have a **heterogeneous pattern of enhancement** (e: arrowhead), suggesting contralateral regional lymph node metastasis. Postmortem examination confirmed a diagnosis of thyroid carcinoma involving both thyroid lobes with extensive infiltration into the adjacent soft tissues and metastasis to regional lymph nodes and lung. The mass extended into and expanded the oropharyngeal wall with marked compression of the pharynx and laryngeal opening.

Answer 110

9y FS Boston terrier with a cervical mass. Carotid body tumors (Chemodectoma) are similar in location to thyroid masses. * However, the normal, high‐attenuating thyroid lobes are easily identified adjacent to the tracheal wall on the unenhanced image in this patient (a: arrows). * The carotid body tumor is **highly vascular and intensely contrast enhancing.** * In comparison to thyroid tumors where the carotid artery is usually displaced laterally, the **carotid artery is contained within the mass** (b: open arrow). Regional lymph node metastasis (not shown) was also confirmed from surgical excisional biopsy.