Brain Flashcards

Question

Non-hemorrhagic infarcts

Answer 1

_CT imaging_ features of nonhemorrhagic infarction may be **subtle** and include **focal or regional hypoattenuation from edema and variable, but often minimal, mass effect.** Nonhemorrhagic infarction may appear **mildly T1 hypointense** and **T2 hyperintense** with **variable mass effect** involving both gray and white matter on _unenhanced MR images_. **Due to restricted water diffusion,** ischemic regions of the brain will appear **hyperintense on diffusion‐weighted images** and **hypointense** on corresponding **apparent diffusion coefficient (ADC) maps**. Perfusion images may define specific regions of perfusion deficit, and magnetic resonance angiographic (MRA) images can reveal relative or absolute flow deficits in affected vessels. **Gradient echo T2\*** images will display relatively **little or no susceptibility effect.**

Answer 2

13y FS Corgi with a 1‐day history of tonic/clonic seizures and a 2‐year history of systemic hypertension. * A **well‐delineated**, **T1 isointense** (a), **T2 hypointense** (b) **intraaxial cerebral mass** is present in the region of the **right piriform lobe**. * There is **moderate edema surrounding** the mass (b) as well as a **thin peripheral rim of contrast enhancemen**t (c). * There is **uniform susceptibility effect** within the lesion (d). Edema will appear T1 hypointense and T2 hyperintense on MR images with hemorrhagic regions having a T2\* signal void and an otherwise variable appearance depending on duration since trauma. Edematous regions will appear hypoattenuating and focal areas of hemorrhage will appear hyperattenuating on unenhanced CT images

Answer 3

7y MC Greyhound with central nervous system deficits associated with an anesthetic recovery complication following routine dental prophylaxis performed 5 days previously. * There is **marked T1 hyperintensity** (a) and **mixed T2 intensity** (b) involving the **left cerebral hemisphere**. * The lesion involves **primarily the cerebral cortex** based on distribution on the T1 image, but more **extensive edema involving gray and white matter is appreciated on T2** and **FLAIR images** (b,c), resulting in a **midline shift**. * The 5‐day history of intracranial signs and imaging characteristics are consistent with subacute hemorrhage.

Answer 4

12y FS Poodle with 5‐day history of right forebrain deficits. The initial MR examination (a–c) was acquired 5 days following the onset of clinical signs. The follow‐up MR examination (d–f) was acquired approximately 6 weeks later, at which time the dog had clinically improved. * On the initial examination, there is a **large mass** within the **right frontal lobe** that is **T1 iso- to hyperintense** (a) and of **mixed T2 intensity** (b). A **prominent susceptibility effect** is present on the **T2\* image** (c). This constellation of imaging features is **consistent with an acute to subacute intracranial hemorrhage**, supported by the 5‐day duration of clinical signs. * The **marked reduction in lesion volume** and the **uniform hypointensity** evident on all imaging sequences on the second examination acquired 6 weeks later are consistent with a **resolving chronic hematoma** (d–f). A postmortem examination performed 1 year later for an unrelated cause of death revealed extensive neuropil loss and other chronic degenerative changes of the right frontal lobe consistent with residual effects of a healed infarct.

Answer 5

10y FS Labrador Retriever with lymphoplasmacytic encephalitis. The CT image was acquired immediately following CT‐guided brain biopsy. * **Epidural hemorrhage** is evident involving the **left parietal region**. * The **biconvex shape** is consistent with an epidural hemorrhage. Although acute hemorrhage is hyperattenuating on unenhanced CT images, the increased attenuation in this patient is due, in part, to enhancement following intravenous contrast medium administration.

Answer 6

5y MC Mixed Breed with head trauma 4 days previously. * A **crescent‐shaped**, **T1 hyperintense**, **right‐sided subdural hematoma** is present (a: arrow). * The hemorrhage has **central hypointensity** and **peripheral hyperintensity on the T2** image (b: arrow), consistent with **subacute hemorrhage** and the 4‐day history of head trauma. * A **focal, nonenhancing, T1 hyperintense lesion is also present** in the **left pyriform lobe** (a,b: arrowhead), associated with **regional edema** and consistent with **subacute parenchymal hemorrhage**. * Given the location of this second lesion in relation to the subdural hematoma, it is thought to represent a contrecoup brain contusion. Necropsy confirmed both the subdural hematoma and the brain hemorrhage.

Answer 7

13y FS Pug with central vestibular signs of 2‐day duration. * There is a **well‐delineated** region of **T1 hypointensity** (a: arrow) and **T2 hyperintensity** (b,c: arrow) involving the **right side of the cerebellum**. * The lesion has **minimal mass effect** and is not associated with other intracranial lesions. * The focal area of hyperintensity on the B1000 diffusion image (d: arrow) and the corresponding region of **hypointensity on the apparent diffusion coefficient map** (e: arrow) are attributable to **diffusion restriction** in the **ischemic tissue**. MR findings and clinical signs are consistent with nonhemorrhagic cerebellar infarction caused by right rostral cerebellar artery thrombosis.

Answer 8

_Lysosomal storage disorders_ Lysosomal storage disorders are a group of **more than 50 rare inherited diseases** characterized by **failure of lysosomes to metabolize lipids or glycoproteins**. Most are autosomal recessive disorders that cause a single enzyme deficiency. Although the clinical presentations associated with this spectrum of diseases vary depending on the specific defect, most include central nervous system pathology and consequent neurologic clinical signs. Comprehensive coverage of these disorders is beyond the scope of this text, but we will highlight two representative examples. * **Neuronal ceroid lipofuscinosis** has been reported in many canine breeds, including Cocker Spaniels, Border Collies, American Bulldogs, Chihuahuas, Schnauzers, English Setters, Tibetan Terriers, and Polish Lowland Sheepdogs. The underlying pathology includes **generalized neuronal loss with diffuse astrogliosis**. Remaining neurons contain an intracytoplasmic accumulation of **yellow lipopigments**. Retinal cells can be similarly affected, and other organ involvement can occur. In a single case report of CT features of ceroid lipofuscinosis in a Border Collie, there was **generalized cortical atrophy and ventricular dilation**. MR descriptions also include cortical atrophy and ventriculomegaly. One citation describes **intense enhancement of thickened meninges** in a group of affected Chihuahuas, but this has not been reported elsewhere. In our experience, there is also a **loss of definition of the gray–white matter interface** on proton density images and **possible cerebellar atrophy**. * **Galactosialidosis** is caused by a cathepsin A mutation that results in β‐galactosidase and neuraminidase deficiency. The disorder has three clinical variants and affects multiple organs, but the central nervous system is consistently involved. In people, neuropathologic features include **atrophy of the optic nerve, thalamus, globus pallidus, lateral geniculate bodies, brainstem, and cerebellum**. Microscopic findings include neuronal loss, gliosis, and abnormal lysosomal storage in remaining neurons. A lysosomal storage disorder similar to galactosialidosis has been reported in a 5‐year‐old Schipperke dog with progressive cerebellar and central vestibular signs. Enlarged and vacuolated neurons were seen on postmortem examination, which were documented to be due to the presence of glycolipid‐laden intracytoplasmic lysosomes. Imaging reports in people are lacking, but the MR features of the canine patient with a presumptive diagnosis of galactosialidosis include **cerebellar atrophy and ventriculomegaly**. There was diffuse cerebellar purkinje cell and granular cell loss and extensive neuronal cytoplasmic lysosomal storage in the cerebellum and hippocampus on postmortem microscopic examination. _Case findings:_ 2y MC Border Collie cross with 4‐month history of progressive behavior changes, ataxia, and incoordination. There is generalized reduction in cerebral and cerebellar volume with concomitant generalized ventriculomegaly and prominence of the subarachnoid space due to gyral atrophy and sulcal widening (a,c–e). There is also loss of white and gray matter definition on the proton density image (b). Postmortem examination revealed neuronal accumulation of eosinophilic granular intracytoplasmic material. The material was autofluorescent and stained positive with PAS, LFB, and Sudan black B, all of which supported the diagnosis of neuronal ceroid lipofuscinosis.

Answer 9

_Thiamine deficiency_ Thiamine deficiency is rare and usually results from animals being fed commercial or noncommercial **diets deficient** in thiamine. The disorder has also been documented in dogs and cats fed commercial pet food formulations containing sulfur dioxide as a preservative. Certain species of fish contain high levels of thiaminase, and some drugs can also induce thiamine deficiency. Clinical manifestations include: * Neurologic * Ocular * Gastrointestinal * Cardiac signs As with other metabolic/toxic central nervous system disorders, specific regions of the brain are vulnerable, leading to **symmetrical multifocal lesions** on imaging studies. The **lateral geniculate, dorsal cochlear, oculomotor, mammillary, and red nuclei and the caudal colliculi** are most often involved. The cerebral cortex, cerebellar vermis, basal ganglia, and hippocampus can also be variably affected. Microscopic pathology includes neuronal degeneration and necrosis, myelin degeneration, and secondary vascular changes. Lesions are usually: * **Well defined** and **bilaterally symmetric** * Have **minimal mass‐effect** * Appear **T2 and FLAIR hyperintense** * Variably **T1 hypointense** * **Do not typically contrast enhance** * Not all vulnerable regions are affected in every patient. _Hepatic encephalopathy_ MR features of the brains of dogs and cats with portosystemic shunts include: * **T1 hyperintensity** of the **lentiform nuclei**. * These lesions are **T2 isointense** * **Do not contrast enhance** * Subside following correction of portosystemic shunt Comparable MR lesions are described in people with chronic hepatic encephalopathy and are **thought to be due to a focal accumulation of manganese**, which has also been documented in dogs with the same condition. More fulminant clinical signs and MR imaging features occur with **acute hepatic encephalopathy**. MR findings in people include **diffusion restriction, diffuse cortical T2 and FLAIR hyperintensity**, and **focal T2 and FLAIR hyperintensity of thalamic nuclei without contrast enhancement**. Intensity changes are due primarily to _cytotoxic edema_, but **cortical laminar necrosis** has also been described. Similar MR features have been described in a dog with fulminant hepatic encephalopathy. _Osmotic demyelination syndrome_ Myelinolysis due to **hypernatremia** and **from aggressive correction of hyponatremia** has been reported in people. Myelinolysis occurs due to a high gradient between intracellular and extracellular osmolarity, which injures cells as a result of abnormal transmembrane water transit. The general term osmotic demyelination syndrome defines the underlying pathology in both clinical conditions. Hypernatremic osmotic demyelination, however, occurs both in the **pons and in extrapontine locations**, including **white matter, corpus callosum, basal ganglia, hippocampus, cerebellum, and cortex**. Clinical signs vary depending on the vulnerable tissues affected but include neurocognitive changes and motor dysfunction. MR features of acute disease include: * Focal or multifocal, **T1 hypointense** * **T2 and FLAIR hyperintense** lesions in the anatomic locations listed above * **Diffusion restriction** on diffusion‐weighted imaging. * Lesions typically **do not enhance** _Peri‐ictal encephalopathy_ In people, MR imaging performed within a few days of generalized seizures reveals **transient diffusion restriction, swelling, and T2 hyperintensity of cortical gray matter, subcortical white matter, and hippocampus**. These changes are ascribed to **seizure‐induced transient vasogenic and cytotoxic edema**. MR features of peri‐ictal encephalopathy in dogs have also been reported. MR imaging features **within 14 days** of seizuring included variable **T1 hypointensity** and **T2 hyperintensity** of the **piriform and temporal lobes**. Contrast enhancement of the lesions occurred in only one dog, and lesions **resolved within 10–16 weeks** after the initial MR examinations. Microscopic features included edema, neovascularization, reactive astrocytosis, and acute neuronal necrosis and were similar to those reported in people. In our experience, lesion distribution may extend beyond the piriform and temporal lobes. _Case findings:_ 14y MC Domestic Shorthair with 2‐day history of inappetence, vomiting, and ataxia. Neurologic examination revealed multifocal neurological deficits involving the cerebellum, brainstem, and cerebrum. There are focal regions of T2 and FLAIR hyperintensity involving the lateral geniculate nuclei (a,g: arrowheads), the caudal colliculi (b,d,h: arrowheads), and vestibular nuclei (c: arrowheads). Other thalamic nuclei were similarly affected (not shown). There is also ill‐defined T2 and FLAIR hyperintensity of the axial regions of the parietal and occipital cortex (a,b,d: arrow), which enhance following contrast administration (f: arrow). These MR features are characteristic of multifocal polioencephalopathy due to thiamine deficiency. Further questioning of the owner revealed the cat had been fed an almost exclusively meat diet. Clinical signs resolved with dietary change and thiamine supplementation.

Answer 10

12y MC Shiba Inu with left‐sided vestibular signs. * There is **prominence** of the **subarachnoid space** and **lateral ventricles** due to cortical atrophy (a,b). * The **cortical mantle** is **thin, and gyri are smaller** than expected. * The h**ippocampus also appears small** with **central foci of increased T2 intensity** (b: arrows). * No other abnormalities were detected on the MR examination, and clinical signs resolved after approximately 2 weeks.

Answer 11

Although a number of noninfectious inflammatory disorders of the brain of the dog and cat have been reported, _two entities_, **granulomatous meningoencephalitis** and **necrotizing encephalitis**, are the most common and best described. Necrotizing encephalitis can be further subdivided into: * Necrotizing meningoencephalitis * Necrotizing leukoencephalitis * It is possible that these represent different manifestations of the same disease. All three of these conditions are thought to be **autoimmune disorders.** _Granulomatous meningoencephalitis_ Granulomatous meningoencephalitis (GME) is an idiopathic inflammatory disorder of the central nervous system, characterized by **perivascular mononuclear cell infiltrates**. Young to middle‐aged (4–5 years), female, small‐ and **toy‐breed** dogs are predisposed, while the disorder is less common in large‐breed dogs and rare in cats. Lesion distribution may be f**ocal, disseminated (multifocal), or ocular**, with the focal and disseminated forms predominating. Lesions **primarily involve** **white matter**, but gray matter and meninges may also be affected. This disorder most often affects the **forebrain, brainstem, or spinal cord,** with the **cerebellum and optic nerves less frequently involved.** Depending on the extent of associated edema, lesions may have ill‐defined **hypoattenuation** on _unenhanced CT_ images and will **variably contrast enhance**. Enhancement can be absent, heterogeneous and ill-defined, or may sometimes reveal a well‐delineated mass. Lesions are typically **T1 iso‐ to hypointense** and **T2 hyperintense** and have similar contrast enhancement characteristics as described for CT. **Meningeal involvement** is documented in many patients with GME, and abnormal imaging findings are sometimes limited to **prominent meningeal enhancement**. In a minority of patients, imaging **may be normal** or lesions may not contrast enhance. _Necrotizing encephalitis_ * **Necrotizing meningoencephalitis**, sometimes referred to as **Pug dog** encephalitis, is a **nonsuppurative, necrotizing, inflammatory** brain disorder. Small‐ and toy‐breed dogs are predisposed, and Pug, Maltese, and Chihuahua breeds are highly overrepresented. Median age of onset is 1.5–3 years, and females are affected more commonly than males. Lesions may be **focal or asymmetrically multifocal and involve both gray and white matter** of the **cerebral hemispheres and overlying meninges**. Cerebellar and brainstem involvement, though uncommon, has been reported. Grossly, lesions are frequently cavitary and associated with significant brain swelling from inflammation and edema. Lesions may be **hypoattenuating** on _unenhanced CT_ images when cavitary or when associated with sig- nificant brain edema. **Edema may also induce midline shift, brain herniation, and other features of mass effect**. **Contrast enhancement is variable** on CT, ranging from absent to moderate, but when present, the enhance- ment pattern is heterogeneous and margins may be poorly delineated. _On MR images_, lesions are **T1 iso‐ or hypointense** and **T2 hyperintense**, involve the cerebral gray and white matter, and typically have indistinct margins. About half to two thirds of lesions contrast enhance on MR, but **enhancement is minimal to moderate** and **nonuniform**, when present. Meningeal enhancement is evident in about 50% of patients * **Necrotizing leukoencephalitis** is also a **nonsuppurative, necrotizing, inflammatory** brain disorder affecting both **gray and white matter**. Grossly, there are subcortical regions of liquefaction and cavitation. Microscopically, lesions are characterized by mononuclear infiltrates, gitter cells, and frank necrosis. Descriptions of the anatomic distribution of this disorder are sparse, but lesions can be **focal, asymmetrically multifocal, or regionally diffuse** with a predilection for the **cerebral hemispheres,** although **brainstem** lesions have also been reported. Lesions are **iso‐ to hypoattenuating** on _unenhanced CT_ images and **may appear contiguous with the ventricles**. **Contrast enhancement is absent to moderate** and nonuniform and ill defined, if present. _On MR images_, brain lesions are **T1 hypointense** and **T2 hyperintense** and **minimally to moderately contrast enhance.** When enhancement is present, it is typically nonuniform and sometimes peripheral

Answer 12

3y M Pomeranian with a recent history of seizures and circling to the left. * There are **multiple foci** of **marked T2 and FLAIR hyperintensity** (a,d: arrowheads) and **T1 hypointensity** (b: arrowheads) involving the **cerebral cortex** and **regional T2 hyperintensity of the white matter and caudate nucleus** on the right (a,d: arrow). * There is **mild amorphous contrast enhancement** of the right caudate nucleus (e,f: arrow) and of the meninges overlying the cerebral cortical lesions (e,f: arrowheads). Postmortem examination revealed extensive areas of cortical necrosis and encephalomalacia, consistent with necrotizing meningoencephalitis.

Answer 13

2y MC Yorkshire Terrier with acute onset of ataxia, hypermetria, and obtundation. * There is **diffuse T2 and FLAIR white matter hyperintensity** in both cerebral hemispheres, consistent with **vasogenic edema** (b,c). * There is focal right hemispheric **subcortical T2 hyperintensity**, **FLAIR mixed intensity**, and **T1 hypointensity**, consistent with a **focal fluid collection** (b–d: arrow). * An additional **T2 hyperintense mass** is present within the **pons** (a: arrow). * Following contrast administration, **multiple contrast‐enhancing foci are seen** in the parenchyma of the **cerebrum and thalamus** (e,f: arrows). * **Diffuse meningeal enhancement** is also evident (e,f: arrowhead). Postmortem examination revealed widespread multifocal lymphohistiocytic leukoencephalitis with cystic malacia and necrosis, consistent with necrotizing leukoencephalitis.

Answer 14

Infectious causes of encephalitis and meningoencephalitis include **viral, bacterial, mycotic, protozoal, and parasitic** agents. Detailed imaging descriptions are sporadic, but features of the more common entities are included here. _Viral Encephalitis_ * Canine distemper encephalitis * Feline infectious peritonitis _Bacterial Meningoencephalopathy:_ * Intracranial abscess * Bacterial meningoencephalitis _Mycotic Meningoencephalitis:_ * Cryptococcus * Cladophilidophora * Coccidiodes * Prototheca * Aspergillus _Protozoal Meningoencephalitis:_ * Neospora * Leishmania * Acanthamoeba _Helminth-induced meningoencephalopathy:_ * Angiostrongylus vasorum * Neurocysticercosis

Answer 15

The feline infectious peritonitis (FIP) coronavirus causes systemic illness in domestic cats, with central nervous system involvement being a common component, particularly in cats with the dry or pyogranulomatous form of the disease. FIP infection causes an **immune‐complex pyogranulomatous vasculitis**, and in the central nervous system (CNS), it targets the: * Leptomeninges * Choroid plexus * Ependymal cells * Brain parenchyma * Eyes **Generalized or regional obstructive hydrocephalus** may be present because of ependymal and choroid inflammation. Cerebrospinal fluid **(CSF)** may have **variable T1** and **FLAIR intensity** depending on cellular and macromolecular content. The brain parenchyma may appear unremarkable on unenhanced T1 MR images, although **cerebellar herniation may be evident** in cats with obstructive hydrocephalus. Focal or multifocal regions of **parenchymal hyperintensity** may be evident on **T2** images, and the **meninges may appear thickened** and **T2 hyperintense**. **Choroidal, ependymal, an**d meningeal enhancement may be marked on contrast‐enhanced T1 images _Case findings:_ 9mo MC Domestic Shorthair with 2‐week history of lethargy, inappetence, and reduced motor function in the pelvic limbs. The ventricular system is markedly and uniformly enlarged, indicative of obstructive hydrocephalus (a–d). The choroid plexus of the floor of the lateral ventricles is enlarged (b: arrowheads). The cerebrospinal fluid (CSF) appears **abnormally hyperintense on the FLAIR image, indicating a high cellularity or macromolecular content** (c: arrowheads). There is nonuniform T2 and FLAIR hyperintensity of the white matter in both hemispheres consistent with the presence of vasogenic edema. Contrast‐enhanced T1 images reveal uniform and intense enhancement of a thickened ependymal lining but no parenchymal enhancement (e–h). MR imaging findings characteristic of this disorder can be quite shocking (f,i). Based on the combination of clinical signs, MR findings, and CSF analysis, a clinical diagnosis of feline infectious peritonitis associated encephalitis and ventriculitis was made.

Answer 16

Intracranial abscesses are caused by: * Penetrating injuries, such as migrating foreign bodies and bite wounds * Extension of ear and nasal infections * From bacteremia or septic emboli. Their location within or adjacent to the brain is dependent on the initiating cause, with the **brainstem most often** affected by extension of **otitis media/interna**. Because of the inflammatory nature of the lesion and the often significant **mass effect, surrounding vasogenic edema is usually pronounced.** **Epidural or subdural fluid collections may be present**, particularly when the abscess is caused by penetrating injury. **Obstructive hydrocephalus may occur**, depending on abscess location. Intracranial abscesses usually appear as **solitary space‐occupying masses** of variable size and with a **hypoattenuating center** on unenhanced CT images. Depending on the thickness of the abscess capsule, this may appear as a distinct **iso‐ or mildly hyperattenuating lesion rim surrounded by relatively hypoattenuating parenchymal edema.** Abscesses are generally **intensely peripherally enhancing**, and **regional meningeal enhancement** may be present in some patients. On MR images, abscesses are **centrally T1 hypointense** (but higher intensity than normal CSF) and **T2 hyperintense** because of the presence of abscess fluid and **T1 hypointense and T2 hyperintense peripheral to the mass** because of **vasogenic edema.** Abscess **contents appear hyperintense on FLAIR sequences**. A distinct **T1 iso‐ to hyperintense abscess capsule** may also be evident. Abscesses **intensely peripherally contrast enhance** and **regional meningeal enhancement may be seen**. Diffusion weighted imaging **(DWI)** typically reveals **high signal intensity** on the DWI map and **low signal on the apparent diffusion coefficient (ADC)** map, indicative of *restricted water diffusion* (Figure 2.7.5). This may be useful for differentiating abscesses from necrotic brain lesions and mucinous intra‐axial tumors, such as oligodendroglioma. _Case findings:_ 16y FS Abyssinian with bilateral ceruminous gland cystadenomas and otitis media. Recent onset of intracranial neurologic signs. Nonuniformly contrast‐enhancing, soft‐tissue attenuating material is present within both tympanic bullae (a). There is a peripherally contrast‐enhancing intracranial mass, consistent with an abscess, adjacent to the internal margin of the petrous portion of the right temporal bone (a,b: arrowhead). The internal acoustic meatus can be seen (a: black arrow).

Answer 17

2.5y FS Domestic Shorthair with acute onset ataxia and obtundation. * **T2 and T1 intensity** within the **left tympanic bulla** is consistent with **effusion** (a,b). * There is a **T2 hyperintense, T1 hypointense mass** adjacent to or within the **left side of the brainstem** (a,b: arrowhead), **surrounded by a halo of T2 hyperintense edema** (a). * The mass **intensely peripherally enhances** following contrast administration, indicative of an **abscess** (c). * **Adjacent meninges also enhance** and appear thickened, indicative of **regional meningitis** (c: arrowhead). * The **lining of the left tympanic cavity intensely enhances,** consistent with **otitis media** (c). Biopsy and culture of the lining of the left middle ear confirmed suppurative otitis media. The intracranial abscess and meningitis was thought to have occurred by ascending infection through the internal acoustic meatus.

Answer 18

9mo F Dachshund with 2‐week history of progressive weakness and lethargy. Cerebrospinal fluid analysis revealed marked suppurative inflammation and intracellular rod‐shaped bacteria. * **Meningeal and periventricular hyperintensity** is evident on the **FLAIR image** (c). * The **cerebellum appears enlarged** with **loss of cerebellar folia definition** (c) and **ventral foraminal herniation** (d: arrowhead). * There is **marked thickening and regional enhancement of the meninges** in the **caudal aspect of the cranial vault** (d–f: arrows). * Cerebrospinal fluid analysis also revealed large numbers of neutrophils. A craniotomy was performed to reduce intracranial pressure. Meningeal biopsy confirmed severe suppurative meningitis.

Answer 19

6y FS Labrador Retriever with progressive lethargy and cervical pain. * There is a **mass** in the **left dorsal thalamic region** causing a **midline shift and compression of the third and left lateral ventricles**. * The mass is characterized by **T1 hypointensity** (a: arrow) with **heterogeneous T2 intensity** (b: arrow), and there is evidence of **extensive perilesional edema** (b: arrowheads). * A **similar‐appearing lesion** (not shown) was present in the **left occipital lobe**. * The mass **intensely and heterogeneously enhances** following contrast administration, and mass **margins are poorly defined and irregular** (c: arrow). Postmortem examination confirmed granulomatous encephalitis from systemic aspergillosis. The disease was widely disseminated with multiple organ systems affected.

Answer 20

4y FS Bengal cross with pelvic limb weakness. * **Multiple ill‐defined** and **variably sized T1 hypointense** and **T2 hyperintense foci** are seen in the **piriform lobes and parietal cortex** (a,b,d,e: arrows; additional lesions not shown). * These focal lesions **heterogeneously enhance** and have **ill‐defined margins** following contrast administration (c,f,g: arrows). * **Prominent multifocal meningeal enhancement** is also seen (f,h: arrowheads). Postmortem examination confirmed multisystemic inflammatory disease caused by Cryptococcus gattii. The chronic inflammatory response in the meninges and brain was histiocytic and lymphoplasmacytic with intralesional fungal elements.

Answer 21

_Neoplasia of the meninges:_ * Menigioma * Granular Cell Tumor _Neoplasia of the neuroepithelial origin:_ * Astrocytoma * Oligodendroglioma * Mixed glial cell tumors * Ependymal tumors * Choroid plexus tumors _Lymphoma and hematopoietic tumors:_ * Lymphoma * Hystiocyctic tumors * Metastatic neoplasms

Answer 22

Meningiomas are the most common of the primary **intracranial, extraaxial** neoplasms in dogs and cats. German Shepherd Dogs, Collies, Golden Retrievers, and Boxers are overrepresented. Meningiomas are derived from meningothelial cells and are divided into three grades: * WHO grade I, benign * WHO grade II (atypical), which have intermediate histologic features * WHO grade III, malignant. In a report of 112 canine meningiomas, 56% were grade I, 43% were grade II, and less than 1% were grade III. Meningiomas in dogs most frequently impinge on the **olfactory bulbs and frontal lobes** and are often of a macrocystic histological subtype. Other common sites include the cerebral or cerebellar convexity and **cerebellopontine, basilar, tentorial, falcine, foraminal, or intraventricular locations**. **Multiple meningiomas** may be present simultaneously, particularly in **older cats**, and it is unclear whether these represent multicentric disease or metastasis from a single primary site. Meningiomas **may occasionally have a mineralized, cystic, or hemorrhagic component.** Meningiomas are typically **iso‐ to mildly hyperattenuating to cortical gray matter** on _unenhanced CT_ images and can produce considerable mass effect. Cystic components appear hypoattenuating, as does **surrounding peritumoral edema**, which **can be extensive**. In some patients, **hyperostosis** of calvarial bone adjacent to a meningioma will appear **thickened and hyperattenuating.** Solid meningiomas are **usually** **uniformly T1 isointense** on _unenhanced MR images_ but are **occasionally hypo‐ or hyperintense.** Approximately **70%** of meningi omas are **T2 hyperintense**, with the remainder being isointense. Despite the relatively benign biological behavior of the majority of meningiomas, about **95% are accompanied by edema**, which may be peritumoral (40%) or diffuse (50%). **Edema in T2 or FLAIR images** often clearly **delineates the meningioma margin,** confirming its extraaxial origin. **Signal void of adjacent calvarial bone due to reactive hyperostosis** can sometimes be seen. On both CT and MR images, approximately **60–70%** of meningiomas show **marked, uniform contrast enhancement**, with the remainder being heterogeneous and often associated with cystic, hemorrhagic, or mineralized com ponents. Contrast enhancement usually reveals **well‐defined tumor margins**; a **globoid, plaque‐ like, or irregular shape**; and a **broad‐based superficial margin conforming to the meningeal plane**. On contrast‐enhanced MR images, thickening and intense enhancement of meninges adjacent to the tumor, often referred to as a **dural tail sign**, is a common imaging feature of meningiomas, although not pathognomonic for the disorder. This feature may be difficult to recognize on contrast‐enhanced CT images because of the hyperattenuation of adjacent bone. _Case findings:_ 13y MC West Highland White Terrier with central vestibular signs. A soft‐tissue mass, which is slightly hyperattenuating compared to occipital lobe cortex, is identified in the right caudal fossa adjacent to the ventral margin of the os tentorium (a: arrow). The left side of the cerebellum is hypoattenuating, suggesting the presence of perilesional edema (a: arrowhead). The mass intensely and uniformly enhances following contrast administration and is well margined and broad based, indicating an extraaxial origin (b: arrow). Adjacent cerebellum and brainstem are displaced and compressed (b: arrowheads). Postmortem examination confirmed a diagnosis of meningioma.

Answer 23

Granular cell tumors have many imaging features they have in common with meningiomas. GCTs are usually **well defined** and **extraaxial** with a **plaque‐like, sessile distribution involving the meninges**. They are preferentially located along the convexity of the cerebrum, the falx cerebri, or the floor of the cranial vault, and those involving the cerebrum can be quite extensive. **Peritumoral edema and mass effect** associated with these tumors can be seen on both CT and MR images. Granular cell tumors are **mildly hyperattenuating** on _unenhanced CT_ images and **mildly T1 hyperintense** and **T2 iso‐ to hyperintense** _on MR images._ Granular cell tumors **intensely and uniformly enhance** on both CT and MR images following contrast administration, and tumor margins are usually well defined _Case findings:_ 12y FS Miniature Poodle with vestibular signs. There is a thin, plaque‐like, T1 and T2 hyperintense mass involving the entire right cerebral cortex, causing a pronounced midline shift (a,b: arrowheads).The mass intensely and uniformly enhances following contrast administration (c–e: arrowheads). A gross cross‐sectional image, which correlates with MR image e, confirms the extraaxial origin of the tumor (e,f: arrowheads). The mass was histologically confirmed to be a granular cell tumor. Anwer et al 2013.19 Reproduced with permission from Wiley.

Answer 24

Astrocytomas are one of the **most common of the intraaxial** central nervous system (CNS) neoplasms. **Boxers and some other brachycephalic breeds** are highly predisposed. Older dogs are most frequently affected, but astrocytomas also occur in young animals. Although astrocytomas can originate from either white or gray matter, those that occur within the cerebrum appear to arise predominantly from white matter. The **frontal, piriform, and temporal lobes** are the most common sites. The current human WHO classification scheme grades astrocytomas based on cytological characteristics. * **Grade I and II** astrocytomas (diffuse astrocytomas) are considered the least biologically aggressive forms and consist of a **uniform, well‐differentiated infiltrative cell population without mitotic activity**. * **Grade III** (anaplastic) astrocytomas have more nuclear atypia, a much higher cell density, and mitotic activity. * **Grade IV** astrocytomas (glioblastoma multiforme) are the most malignant and infiltrative, frequently having **regions of necrosis, microvascular proliferation, and sometimes intratumoral hemorrhage**, which contribute **heterogeneity** to their **MR imaging** appearance in both humans and animals. Astrocytomas **may be globoid or irregularly shaped**, and **peritumoral edema is variable** but usually minimal to moderate. **Intratumoral hemorrhage may also occur** in high‐grade tumors. MR features of gliomas and presumed cerebrovascular accidents can be similar, although gliomas tend to be distributed primarily in the cerebrum, whereas vascular lesions are more likely to be located in the cerebellum, thalamus, midbrain, and brainstem. **Diffusion‐weighted imaging** can be used to discriminate between these two disorders, with vascular lesions more likely to result in diffusion restriction. Astrocytomas are generally **hypoattenuating** on _unenhanced CT_ images, and **mass margins may be ill defined, particularly when surrounded by peritumoral edema or when biological grade is low**. Astrocytomas typically appear **mildly to moderately T1 hypointense** and **moderately and heterogeneously T2 hyperintense**. Surrounding edema may mask tumor margins on both T1 and T2 images. With both CT and MR, the **intensity of tumor enhancement** following contrast administration reflects **microvascular proliferation** and **blood–brain barrier disruption** and **tends to increase with astrocytoma grade**. Low‐grade astrocytomas typically do not enhance, or enhance minimally, whereas high‐grade astrocytomas are more likely to show moderate or marked, nonuni form or peripheral contrast enhancement, although the degree of enhancement is not a reliable indicator of tumor grade Case findings: 9y MC Toy Poodle with a 3‐week history of seizures. A large, T1 hypointense, T2 and FLAIR hyperintense ovoid mass is seen in the ventral aspect of the left frontal lobe (a–c: arrow). There is mild enhancement in part of the mass following contrast administration (d,e: arrow). A more rostral contrast‐enhanced CT image reveals a hypoattenuating mass effect in the left frontal lobe inducing midline shift (f: arrow). Biopsy revealed the mass to be a grade II astrocytoma.

Answer 25

Oligodendrogliomas occur with similar frequency to astrocytomas and affect **older dogs**, particularly **Boxers and other brachycephalic breeds.** The appearance of oligodendrogliomas on _unenhanced CT_ images is **similar to that of astrocytomas**. They are typically **hypoattenuating, and mass margins may be ill defined or lacking**, particularly when peritumoral edema is present. **Marked central hypoattenuation** of **some** oligodendrogliomas, caused by the **high water content of the mucinous core**, may increase the index of suspicion for this tumor. Oligodendrogliomas are **moderately T1 hypointense** and **markedly T2 hyperintense**, specifically when there is significant central mucinous content. Peritumoral edema ranges from minimal to moderate, although even large oligodendrogliomas may induce little edema formation. **Contrast enhancement** of oligodendrogliomas on both CT and MR images is **highly variable**, ranging from none to marked, and when present it is often peripheral or non uniform. Focal or regional contrast enhancement is often distributed centrally or eccentrically within the greater tumor volume and may have a serpentine shape. Although **high‐grade oligodendrogliomas tend to contrast enhance to a greater degree** than low‐grade tumors, as with astrocytoma, this imaging feature is not a reliable indicator of biological grade

Answer 26

Canine mixed glial tumors are usually comprised of tumor cells with both **astrocytic and oligodendrocytic features**, or they may contain a combination of astrocytic and oligodendrocytic subpopulations. These tumors have MR imaging features similar to those of astrocyto mas and oligodendrogliomas.

Answer 27

Ependymomas are uncommon tumors that arise from the **ependymal lining cells of the ventricular** system and thus may occur within the **ventricular system of the brain and spinal cord.** Ependymomas usually affect older dogs and cats without breed predisposition. These tumors are predominantly **intraventricular, although some invade the adjacent brain parenchyma**, and they **expand to fill the ventricular cavity** in which they arise, causing distortion of the ventricle and **obstructive hydrocephalus**, depending on their size and location. Ependymomas may be well differentiated (WHO grade II) or anaplastic and aggressive (WHO grade III). Grossly, the tumors can be soft, lobular (papillary type), or solid (cellular subtype) and may contain cysts and/or hemorrhage. Ependymomas **may be accommodated by gradual ventricular dilation, hence edema is usually absent or minimal,** unless the tumor invades the periventricular brain parenchyma or hydrocephalus causes periventricular interstitial edema. Ependymomas are typically **isoattenuating** on _unenhanced CT images_, although they can have a heterogeneous appearance. Ependymomas appear **slightly T1 hypointense** **to slightly hyperintense** on unenhanced images and **moderately to markedly T2 hyperintense.** **Contrast enhancement is usually marked** and may be heterogeneous, which reflects the coarse texture of the tumor parenchyma, on both CT and MR images. Heterogeneity may be even more pronounced when cysts or hemorrhage are present. Tumor margins are typically distinct, because the majority of the mass extends into a ventricular lumen

Answer 28

Choroid plexus tumors (CPT) are relatively common neoplasms that arise from the **choroid plexus epithelium within the lateral, third, and fourth ventricles and the lateral recesses**. About 50% originate in the fourth ventricle or lateral recesses. The average age of dogs at diagnosis is **6 years,** which is **earlier than most other intracranial tumors. Golden Retrievers** appear to be highly overrep resented. Classification of canine CPT distinguishes choroid plexus papillomas (CPP), comparable to WHO grade I and morphologically benign, from choroid plexus carcinomas (CPC), comparable to WHO grade III and histologically more abnormal and more likely to invade the brain or give rise to intraventricular or intrathecal metastases. **Mild to moderate edema** is present in about 45% of CPP and about 70% of CPC. Choroid plexus tumors **share many CT and MR imaging features with ependymomas.** _Initially_, they **may conform to the shape of the ventricle** in which they grow, but enlargement **may lead to hydrocephalus** because of ventricular **obstruction** or, possibly, **overproduction of CSF**. Tumors have **variable attenuation** _on unenhanced CT images_ and may be **T1 hypo‐, iso‐, or hyperintense** and **T2 hyperintense** _on MR images._ Choroid plexus tumors often appear **heterogeneous, particularly when there is intratumoral hemorrhage.** Choroid plexus tumors usually show m**arked, uniform enhancement** on both CT and MR images following contrast administration, which reflects the underlying papillary vascular architecture of these tumors. **_Intraventricular and intrathecal “drop metastases”_** may appear as **intensely contrast‐enhancing foci** in the **ventricles or subarachnoid space**. Choroid plexus papillomas and carcinomas cannot be reliably distinguished by CT or MR imaging, although presence of drop metastases suggests CPC.

Answer 29

Characteristic CT features of histiocytic sarcoma have not been documented, but these tumors are **T1 iso‐ to hypointense** and **T2 iso‐ to hyperintense** on unenhanced MR images and can cause a mass effect accompanied by regional or diffuse peritumoral edema. **Contrast enhancement** on both CT and MR images **is moderate to marked** and can be either u**niform or heterogeneous.** As observed with meningiomas, the margins of **extraaxial histiocytic sarcomas** may appear **well defined on T2 and enhanced T1 images, although the degree of contrast enhancement** may sometimes be less than that of meningiomas and have a fine granular pattern (Figure 2.8.18). **Dural tail signs** have also been reported inconsistently with these tumors.

Answer 30

On MR images, metastatic hemangiosarcoma often appears as multiple mass lesions, although metastasis should still be considered when a solitary lesion is found. Hemangiosarcomas typically have a **marked mass effect, mixed signal intensity on T1 and T2 sequences** and **heterogeneous intensity with susceptibility effects on T2\*** images **because of intratumoral hemorrhage**. **Peritumoral edema may be marked. Contrast enhancement is variable and often peripheral** (Figure 2.8.19).

Answer 31

Adult dog of unknown gender, age, or breed. * A **mildly T1 hyperintense, T2 hyperintense broad‐ based mass** is present in the **right temporoparietal** region causing a **midline shift and compression of the right lateral ventricle** (a,b: arrow). * Adjacenct to the calvarium and the **thin T2 hyperintense rim suggest an extraaxial origin** of the mass. * Diffuse **surrounding T1 hypointensity and T2 hyperintensity** are consistent with v**asogenic edema** (a,b: arrow- heads). * The mass **intensely and uniformly enhances** following contrast administration, and mass margins are reasonably well defined (c,d). * **Dural tails,** seen best on the dorsal plane image, confirm the mass is extraaxial (d: arrowheads).

Answer 32

8y MC Golden Retriever with recent onset of seizures. * There is a **sessile T1 and T2 hyperintense mass** involving the l**eft olfactory bulb/ frontal lobe** region (a,b: arrow) with an **adjacent T1 hypointense**, **T2 hyperintense, and FLAIR‐nulling cystic component** (a–c: arrowhead). * The **solid component** of the mass i**ntensely and uniformly enhances** following contrast administration (d–f). * There is a thin **rim of enhancement in the cystic** part, but it is otherwise unchanged.

Answer 33

10y MC Australian Shepherd with recent onset of seizures. * There is a **large ovoid mass** with mixed T**1, T2, and FLAIR intensity** in the **left cerebrum**. * Mass margins are **well defined on T2 and FLAIR** images, and the **halo of hyperintensity surrounding the mass** on these sequences is indicative of **vasogenic edema**. * The mass **nonuniformly enhances** following contrast administration, with **more intense enhancement peripherally**. Postmortem examination confirmed glioblastoma multiforme. There was extensive intratumoral hemorrhage consistent with the complex, mixed signal intensity seen on unenhanced images.

Answer 34

5y FS French Bulldog with inappetence, lethargy, and head tilt. * There is a **large, irregularly shaped T1 hypointense** and **T2 hyperintense mass** in the **right temporal–piriform region**, **resulting in midline shift** and **obscuration of the ventral aspect of the right lateral ventricle** (a,b,d). * Perilesional edema is conspicuously absent. * The mass **enhances nonuniformly** following contrast administration, with a **thin contrast rim defining the mass margins** (c,e). Postmortem examination confirmed the mass to be a grade III oligodendroglioma with extension into the right lateral ventricle (f: arrow). The _high water content of the mucinous center explains the T2 hyperintensity of the tumor._

Answer 35

7y MC German Shorthair Pointer with 2‐week history of behavioral changes and neurologic deficits. * There is a **well‐demarcated ovoid mass** within the **third ventricle**, which is predominantly **T1, T2, and FLAIR hyperintense** (a–d). The mass **deforms the lateral ventricles**, but there is **minimal hydrocephalus** and **no peritumoral edema.** * The mass **intensely and nonuniformly enhances** following contrast administration (e,f). Postmortem examination confirmed an ependymoma within the third ventricle (g). The granular appearance on cut surface correlates with the irregular cobblestone appearance of the tumor on MR images. An unrelated diagnosis of lymphoplasmacytic meningitis explains the meningeal contrast enhancement seen in image f.

Answer 36

8y MC dog of unknown breed with a recent onset of depression progressing to obtundation. * There is a **mildly T1 hypointense, FLAIR hyperintense spherical mass** in the **third ventricle**, causing **displacement of the axial margins of the lateral ventricles** (a,b). * There is **focal periventricular edema dorsal** to the mass (a: arrows) and **nonuniform ventriculomegaly**, with the **mesencephalic duct and fourth ventricle disproportionately enlarged** (e: arrows). * The mass **intensely and uniformly enhances** following contrast administration (c,f). Postmortem examination confirmed a choroid plexus carcinoma of the third ventricle (d). The irregular cut surface of the tumor correlates with the pattern of enhancement on MR images. The nonuniform hydrocephalus may be a result of either partial obstruction or overproduction of cerebrospinal fluid by the tumor. Westworth et al (2008).32 Reproduced with permission from Wiley.

Answer 37

7y MC Chow with tetraparesis. * There is a **large, T1 and T2 hyperintense ovoid mass** in the **left lateral recess** (a,b,e: large arrow) that **encroaches on the fourth ventricle**, distends the contralateral lateral recess (b: arrowheads), and **displaces and compresses the brainstem** (a,b: small arrow) **and cerebellum** (a: arrowhead). * A **smaller mass** is seen **ventral to the brainstem** (e: arrowhead). * Both masses **intensely and uniformly enhance** following contrast administration (c,f: arrows). * Distension of the third ventricle and the infundibular recess (e: small arrow) is indicative of **obstructive hydrocephalus.** ****The large mass in the left lateral recess was confirmed to be a choroid plexus carcinoma (d: large arrow). The small mass was determined to be a local metastatic lesion (d: small arrow). The fourth ventricle and right lateral aperture were somewhat distended (d: arrowheads).

Answer 38

11y MC Shetland Sheepdog with recent onset of vestibular signs. * There is a **large T1 hypointense** and **T2 isointense spherical mass** in the **left** side of the **caudal fossa** (a,b: arrow), causing **displacement and compression of the cerebellum and brainstem**. * The **T2 and FLAIR hyperintense rim** surrounding the mass lends evidence that this is an e**xtraaxial lesion** (b,c: arrowheads). * The mass **enhances** following contrast administration and has a subtle nonuniform **“ground glass” enhancement** pattern (d–f). The mass was confirmed to be a histiocytic sarcoma on postmortem examination. Microscopically, the mass was well demarcated, but unencapsulated, and invaded adjacent brain parenchyma from a broad meningeal base.

Answer 39

11y MC Pit Bull Terrier with acute onset obtundation and tetraparesis. * There is a large **left thalamic mass** of **T1 and T2 mixed intensity**, causing **ventricular compression and distortion** (a,b: large arrow). * A second, smaller mass is seen in the l**eft endomarginal gyrus**, which also has mixed T2 intensity (b: small arrow). * There is **marked susceptibility effect within both masses** on the gradient echo **T2\*** image, **indicative of hemorrhage** (c: arrows). * Both masses **nonuniformly and peripherally enhance** following contrast administration (d,e: arrows). Widespread hemangiosarcoma metastasis was confirmed by postmortem examination. Cut surfaces of the brain masses reveal extensive intratumoral hemorrhage, consistent with the complex intensity patterns seen on MR images (f: arrows). Anwer et al 2013.19 Reproduced with permission from Wiley.

Answer 40

_Normal pituitary gland_ The **_sella turcica_** is the **osseous boundary of the pituitary gland**. _It is comprised of the:_ * **Pituitary fossa** ventrally * The **rostral and caudal clinoid processes** dorsally. * The pituitary gland is located in the pituitary fossa of the **basisphenoid bone.** _The pituitary gland is formed of two parts:_ * The vascular, **glandular adenohypophysis** * The **neurohypophysis** * It is suspended from the hypothalamus by the **infundibulum**, which courses through an incomplete dural septum covering the dorsal aspect of the fossa. The ventral aspect of the **third ventricle** extends centrally through the infundibulum to the proximal neurohypophysis. The **pituitary gland** is **perfused by** branches of the **internal carotid** and **communicating arteries of the circle of Willis**, with **venous drainage into the cavernous and intercavernous sinuses**. The **optic chiasm** is located **immediately rostral** to the origin of the **infundibulum,** and the **third cranial nerves arise caudal to**, and course lateral to, the **pituitary fossa.** _On unenhanced CT images,_ the pituitary gland is **isoattenuating** to **deep gray matter** and contiguous with the adjacent hypothalamus, with ventral margins well delineated by the basisphenoid bone. When the third ventricle is prominent, a relatively hypoattenuating infundibular recess may be visible extending to the proximal neurohypophysis. _On contrast‐enhanced CT and MR images_, the **pituitary gland markedly contrast enhances** compared to brain tissue because of a **rich vascular suppl**. _Dynamic contrast imaging_ studies of the normal pituitary describe an **initial central contrast blush**, attributable to **early enhancement of the neurohypophysis***, followed by a slightly delayed peripheral enhancement of the adenohypophysis with diminished central enhancement.* _On MR images_, the normal pituitary gland usually has a **focal T1 hyperintensity**, which is thought to represent either **vasopressin‐containing neurosecretory granules or glial cell lipid droplets** in the **neurohypophysis.** T2 intensity is similar to cortical gray matter. The lipid‐rich marrow of the basisphenoid bone is hyperintense on both T1 and T2 images. **Normal canine pituitary** size on CT images is approxi mately **4.5mm in height and 6mm in width**. On MR images, it is reported to be 5.1±.9mm in height and 6.4±1.0mm in width, with little correlation to brain measurements or body weight. **Normal feline pituitary** size has been estimated to be approximately **5mm in height and 3.5mm** in width on both CT and MR imaging. While these dimensions may be useful as general guidelines, they are not particularly useful for diagnosis of microadenomas. With both imaging modalities, **the presence of a prominent convexity to the dorsal pituitary margin and elevation of the dorsal margin above the sella turcica on a sagittal plane** image are additional qualitative imaging features that are suggestive of a pituitary disorder. _Dynamic CT and MR protocols_ Dynamic contrast CT protocols have been **used to detect small (micro) hypophyseal masses**. However, the current trend in human and veterinary medicine has been toward the use of MR for diagnosis and characterization of pituitary disorders. Dynamic CT and MR procedures described in the veterinary literature involve **dynamic thinly collimated image acquisition** through the pituitary gland **every few seconds for 2–5 minutes** following intra venous bolus contrast administration **to detect neurohypophysis displacement by adenohypophyseal tumors**

Answer 41

Although rare, acute pituitary **hemorrhage** may occur either **spontaneously or secondary to infarction of a pituitary tumor** or other underlying pathology. When the hemorrhage is associated with *acute clinical signs* of obtundation, nausea, vomiting, or visual and other cranial nerve deficits resulting from increased parasellar and intracranial pressure, the disorder is _referred to as *pituitary apoplexy.*_ _CT and MR features include:_ * A **suprasellar mass or mass effect,** particularly when an underlying pituitary tumor or a substantial hematoma is present * **Evidence of acute hemorrhage** within the pituitary gland and/or suprasellar mass. On CT images, this may appear as amorphous hyperattenuation on unenhanced images and variable T1 and T2 intensity on MR images, depending on the specific age of the hemorrhage. With both modalities, **contrast enhancement may occur if residual viable pituitary parenchyma remains, an underlying pituitary tumor is present, or if hemorrhage is active.**

Answer 42

Primary tumors arising from the **adenohypophysis are common** and **frequently associated with endocrinopathies**, such as feline acromegaly, whereas those arising from the n**eurohypophysis are rare**. _Adenohypophyseal neoplasms_ are histologically classified as **adenomas or adenocarcinomas**. * Small tumors that do not appreciably alter total pituitary volume, generally those that are l**ess than 10 mm in total pituitary height**, are categorized as **microtumors**, whereas those **greater than or equal to 10 mm** in height are classi fied as **macrotumors**. * Macroadenomas can be further differentiated as either noninvasive or biologically more aggressive and invasive into adjacent bone. Pituitary microtumors are challenging to diagnose based on imaging features alone. On MR images, the **focal T1 hyperintensity** within the **neurohypophysis may be displaced caudally, dorsally, and laterally** because of expansion of the adenohypophysis. **Marked convexity of the dorsal pituitary margin** and **elevation of the dorsal pituitary margin** above the dorsal rim of the sella turcica may lend additional support for diagnosis of microtumor. **Dynamic contrast‐assisted CT and MRI may** be used to more clearly identify pituitary microtumors by temporal differences in enhancement of the neurohypophysis and adenohypophyseal mass. Imaging features of adenomas, invasive adenomas, and adenocarcinomas are not sufficiently different to reliably differentiate these entities. * Pituitary macroadenomas and adenocarcinomas are greater than 10 mm in height and arise from the sellar region. * Although invasive adenomas are on average larger than noninvasive adenomas (1.9 cm vs. 1.2 cm mean height in one study), this is not a reliable criterion for differentiat ing the two. * Both macroadenomas and adenocarcinomas can have smooth or irregular margins, can contain cysts or hemorrhage, and can occasionally be mineralized. _On CT images_, **macrotumors** can be isoattenuating or slightly hypo‐ or hyperattenuating to adjacent brain parenchyma. Intratumoral cysts may be present as hypoattenuating foci, and mineralization is hyperattenuating. When present, paratumoral edema can appear hypoattenuating to normal brain parenchyma. _On MR images,_ pituitary **macrotumors** are **typically T1 isointense**, **variably T2 hyperintense**, and may be accompanied by **surrounding T2 hyperintense hypothalamic and thalamic edema**. Macrotumors are generally **intensely and uniformly contrast enhancing** on both CT and MR images because of the rich vascular supply of the gland.

Answer 43

10y FS Beagle. Images a–c are representative transverse images at the level of the pituitary fossa. Images d–e are comparable sagittal images, and image f is a magnified view of image e. * The pituitary gland is positioned within the **pituitary fossa** (height=4mm, width = 6 mm) and has a relatively **flat dorsal margin** that **does not extend above the dorsal limits of the sella turcica** (d–f). * The pituitary gland is **partly T1 hyperintense** because of the presence of secretory granules in the **neurohyphysis** that result in T1 shortening (a,d: arrow). * The normal pituitary gland is **markedly contrast enhancing** as a result of the high vascular density of the gland (e,f), and the **pituitary stalk** is also evident (e,f: large arrow). * Cerebrospinal fluid in the chiasmatic cistern is evident dorsolateral to the pituitary gland and is T1 hypointense and T2 hyperintense (b,c: large arrows). * The **cavernous sinus enhances** on the contrast‐enhanced T1 image (c: small arrows), and circular hypointense signal voids can be seen within the sinus, representing flow voids in the **internal carotid** and/or communicating arteries. * The **optic chiasm is located rostral** to the pituitary gland (e,f: small arrow) and can be encroached upon by expansile pituitary masses. * Part of the **mandibular branch of the left trigeminal nerve** can also be seen (c: arrowhead).

Answer 44

_Empty Sella Syndrome (Canine)_ 10y FS Pekinese acutely nonambulatory with neurologic deficits referable to a cerebrothalamic lesion. The dog had no clinical or clinical chemistry evidence of central endocrinopathy. Images a and b are representative transverse images at the level of the pituitary fossa. Images c–e are comparable images oriented in the sagittal plane, and image e is a magnified view of image d. * There is **diffuse brain atrophy** resulting in **enlargement of the ventricular system** and **subarachnoid space** (a–e). * The **pituitary fossa is fluid filled** and appears **T1 hypointense and T2 hyperintense** (a–d: arrow). This fluid collection appears to communicate with the third ventricle (e: asterisk) by way of the infundibular recess (e: large arrow). * The interpeduncular (e: small arrow) and chiasmatic (e: arrowhead) cisterns are also prominent.

Answer 45

_Pituitary Cyst (Canine)_ 10y FS Labrador Retriever with acute obtundation and disorientation. * The dog had multiple meningiomas involving the right cerebrum and pons, causing the clinical signs, and the **cystic pituitary gland** was an **incidental** finding. * Image b represents a magnified view of image a. Images c–e are representative close‐up views of the pituitary fossa in the transverse plane. The pituitary fossa contains a **predominantly T1 hypointense** and **T2 hyperintense cyst** (a–d) with a **thin rim that is isointense to adjacent deep gray matte**r (c: arrows). * The cyst **enhances peripherally** (e: arrows). * Ill‐defined T2 hyperintense edema is present in the hypothalamus (d) because of the meningiomas.

Answer 46

_Pituitary Hemorrhage (Apoplexy) (Canine)_ 10y MC Chow cross with neurologic signs consistent with increased intracranial pressure. The dog also had concurrent thrombocytopenia. * A **large, T1 and T2 hyperintense, heterogeneous mass** is present involving the pituitary, hypothalamic, and thalamic regions (a,b). * The **mixed‐signal pattern** is consistent with **intraparenchymal hemorrhage**. * The mass **moderately and heterogeneously contrast enhances** (c). Postmortem examination found that the pituitary was expanded, and in some regions obliterated, by lakes of free red blood cells and a mass composed of red blood cells, fibrin, and degenerate cells (hematoma).

Answer 47

12y FS Border Terrier with confirmed **pituitary‐ dependent hyperadrenocorticism**. Images a and b are representative transverse images at the level of the pituitary fossa. Image d is a magnified view of image c. * A **uniformly contrast‐enhancing** and **symmetrical pituitary gland** is evident (b–d: arrow). * The gland is considered within normal limits for size (height = 4 mm, width = 6 mm), but the **dorsal margin is convex** and **extends beyond the dorsal extent of the sella turcica**.

Answer 48

3y MC Corgi with confirmed pituitary‐dependent hyperadrenocorticism. Images a–c are representative transverse images at the level of the pituitary fossa. Image f is a magnified view of image e. * The pituitary gland has **nonuniform mixed T1 intensity** and T2 signal that is isointense with that of deep gray matter. * Although the gland measures within normal limits for size (height = 5 mm, width = 5 mm), **the adenohypophysis appears prominent**, **displaces the more hyperintense neurohypophysis dorsally, and extends beyond the dorsal extent of the sella turcica** (d: arrow). * The adenohypophysis enhances uniformly following contrast administration (c,f: arrow). History, clinical signs, and results of a low‐dose dexamethasone suppression test were indicative of pituitary‐dependent hyperadrenocorticism.

Answer 49

The optic nerve, or cranial nerve II, is a tract of the brain and is unique among the cranial nerves in that it has a **meningeal covering** and a **subarachnoid space**. Axons arising from retinal ganglion cells form the optic nerve after collecting and exiting at the optic disc of the eye. The nerve extends caudally in the retrobulbar space and enters the cranium through the **optic canal**. The paired nerves partially **decussate at the optic chiasm**, with the resulting optic **tracts terminating in the lateral geniculate nucleus** and other nuclei with vision functions. The normal canine optic nerve has been reported to be between 1.2 and 2.4 mm in diameter. The normal optic nerve is **isoattenuating to brain** parenchyma on unenhanced CT images and is **T1 and T2 isointense to normal white matter** on MR images. The margins of the nerve are usually well delineated because of orbital fat within the retrobulbar space on both modalities and as a result of surrounding CSF on MR images. The normal optic nerve can be followed from the optic chiasm, through the optic canal, and into the retrobulbar space. Thin‐collimation CT imaging and volume‐acquisition MR techniques can be used to define reformatted imaging planes that parallel the path of a nerve. The normal optic nerve **may have a striated “tram‐ track” appearance** following **contrast medium administration** on both imaging modalities as a result of the **relatively greater enhancement of the surrounding dural sheath compared to the nerve**. Fat‐suppression techniques are particularly useful for increasing the conspicuity of the optic nerve on contrast‐enhanced MR sequences.

Answer 50

Cranial nerve V The trigeminal nerves **arise from either side of the pons** and **exit the cranium** through the **trigeminal canal of the temporal bone**. Within the temporal bone, sensory components of the nerve form the large trigeminal ganglion. The nerve divides to form **three major peripheral branches, the ophthalmic, maxillary, and mandibular nerves**, that e**xit through the orbital fissure, round foramen, and oval foramen**, respectively. In a review of MR contrast enhancement patterns of cranial nerve V in 42 dogs without clinical signs referable to trigeminal nerve dysfunction, the **entire nerve enhanced in over 90% of dogs**, and enhancement was limited to the region of the trigeminal ganglion in the remaining dogs. Intensity of enhancement was subjectively determined to be less than that of the pituitary gland.

Answer 51

_Cranial nerve VII_, the facial nerve, **arises in the medulla oblongata** and **emerges from the trapezoid body**. The nerve **exits the cranial cavity** through the i**nternal acoustic meatus**, courses through the **facial canal in the temporal bone**, and **exits through the stylomastoid foramen**. The origin and intracranial path of _cranial nerve VIII_, the vestibulocochlear nerve, is similar to that of the facial nerve, **emerging from the trapezoid** body adjacent and **dorsal to the emergence of the facial nerve**. The nerve also **exits the cranial cavity** through the **internal acoustic meatus**. Because of the close proximity of intracranial parts of these nerves, disorders affecting one can often also affect the other.

Answer 52

_Noninfectious disorders_ **Idiopathic cranial neuropathy** * _Idiopathic trigeminal neuropathy_ is **peripheral, often bilateral,** and is the most common cause of **masticatory muscle paralysis** in the dog. This disorder causes dropped jaw from dysfunction of the **mandibular branch** motor innervation of the masticatory muscles. **Variable facial sensory deficits** may also be present. * Idiopathic trigeminal neuropathy imaging characteristics include **diffuse nerve enlargement** that is **T1 isointense** and **T2 iso‐ to hyperintense** on MR images. Affected nerves **consistently enhance following contrast administration**. * _Idiopathic facial paralysis_ is the most common cause of **acute** **facial nerve neuropathy** in the dog and is also seen in cats. This disorder is **peripheral** in origin and is **most often unilateral** but can be bilateral. Clinical signs include: palsy of the external ear, lips, and cheek; a lack of palpebral closure; and ptyalism. * Imaging diagnosis of idiopathic facial neuropathy may be more challenging. The nerve is visible on unenhanced images, but features may be unremarkable. Affected nerves **variably enhance** following contrast administration. Contrast‐enhanced ultrafast gradient echo sequences have been reported to increase sensitivity of detection of nerve enhancement in dogs with facial nerve neuropathy. **Ocular granulomatous meningoencephalitis** * The ocular form of granulomatous meningoencephalitis (GME) is **uncommon** compared to disseminated and focal forms. Features of GME are more fully described in Chapter 2.6, but the ocular manifestation is characterized clinically by **blindness and optic neuritis** and may occur as a component of more widespread disease. Ocular and optic nerve involvement is **most often bilateral**. Magnetic resonance imaging features have been reported to include **T1 and T2 isointensity** with **enhancement following contrast** administration. In our experience, ocular granulomatous meningoen- cephalitis can often have subtle MR imaging features. _Infectious inflammatory disorders_ Infectious cranial neuritis can be **viral, bacterial, mycotic, or protozoal**. Imaging descriptions of infectious inflammatory cranial neuropathies are sparse, but expected features would include: * nerve **enlargement** * **variable T1 and T2 intensity** on unenhanced MR images * **some degree of enhancement** following contrast administra-tion. * **Mass lesion**s may also be seen when a **suppurative or granulomatous inflammatory response** is present. * **Seventh and eighth cranial neuropathy** often occurs **from intracranial extension of bacterial otitis media/ interna, and neuritis** may be accompanied by **regional** **meningitis and abscess formation**.

Answer 53

_Optic nerve meningioma_ * Because the meninges cover the **optic nerves, retrobulbar meningiomas** can occur either in situ or by expansion of an intracranial meningioma through the optic canal. Imaging features of meningiomas are described in Chapter 2.8. Optic nerve meningiomas **can have both extracranial and intracranial components and cause exophthalmos**. Owing to location, dural tails are not a feature of extracranial meningiomas. _Peripheral nerve sheath tumor_ * Peripheral nerve sheath tumors most commonly affect the origin and branches of the **trigeminal nerve**. These tumors may be benign or malignant. Clinical signs associated with cranial nerve V nerve sheath tumor are **unilateral** and include **atrophy of the temporalis and masseter muscles**. * Trigeminal nerve sheath tumors appear as an **isoattenuating extraaxial mass**, usually in the region of the origin of the nerve **lateral to the pons**. The ophthalmic, maxillary, and mandibular branches of the nerve can all be involved. Trigeminal nerve sheath tumors appear **T1 isointense and T2 iso‐ or hyperintense** on MR images. These tumors **generally intensely, and uniformly contrast enhance on both CT and MR** images. Affected cranial nerve V branches are enlarged and have a similar enhancement pattern as the central tumor mass. The **trigeminal canal, the orbital fissure, and the round and oval foramina** are often **enlarged** as a result of **bone resorption** resulting from expansion of the nerve branches. Marked **unilateral temporalis and masseter muscle atrophy** is most often also present. On MR images, the **affected muscle is T1 and T2 hyperintense**, because of fatty infiltration from denervation, and mildly to moderately contrast enhances. _Lymphoma_ * Lymphoma can occasionally involve the cranial nerves, either locally or as part of a more widespread central nervous system or systemic distribution. One or more cranial nerves can be affected, and **nerve involvement is often bilateral**. Affected nerves are **generally enlarged** and **T1 iso‐ to hypointense and T2 iso‐ to hyperintense**. Uniform **moderate to marked enhancement** is seen following contrast administration.

Answer 54

The **cavernous sinuses** are **located on either side of the sella turcica** and contain the: * **Internal carotid arteries** and their associated sympathetic plexuses * The third, fourth, and sixth cranial nerves; and branches of the fifth cranial nerve. **CN III, CN IV, CN V, CN VI** Mass lesions that encroach on or invade the cavernous sinuses will therefore often cause a **cranial polyneuropathy** with clinical signs referable to the functions of these cranial nerves. Both **neoplastic and inflammatory causes** have been reported. Imaging features will depend on the inciting lesion but often include the presence of a **space‐occupying mass within or near the pituitary fossa** with evidence of invasion or compression of the sinuses

Answer 55

_Idiopathic Trigeminal Neuropathy (Canine)_ 3y F Pug with a 6‐day history of dropped jaw. Neurologic deficits were limited to cranial nerve V motor function bilaterally. * The **mandibular branches of the fifth cranial nerves** are **symmetrically enlarged** and are **T1 and T2 isointense** on unenhanced images (a,b: arrows). * Both nerves **uniformly and intensely contrast enhance**, defining enlarged **intracranial and distal components that exit the base of the skull through the oval foramina** (c,d: arrows). * Image d is a parasagittal view of one of the mandibular nerves. A diagnosis of idiopathic trigeminal neuropathy was made in this patient based on the lack of any identifiable cause for the neuropathy and gradual improvement with supportive care.

Answer 56

_Cranial Nerve II Granulomatous Meningoencephalitis (Canine)_ 4y MC Golden Retriever with ataxia and weakness. * The **right** optic nerve is **mildly enlarged** and is T1 isointense and T2 **hyperintense** (a,b,d: arrow). * There is also a **slight bulge of the optic disc**, best seen on the dorsal plane T2 image (a: arrowhead). * The nerve **moderately enhances** following contrast administration (c,e: arrow). Postmortem examination confirmed granulomatous meningoencephalitis involving the brain and the right optic nerve. The MR features of the optic nerve lesion are subtle in this patient.

Answer 57

_Cranial Nerve II Chronic Suppurative Neuritis (Feline)_ 12y MC Domestic Shorthair with progressive left‐sided exophthalmos. Images a and b are to the left of midline in the parasagittal plane at the level of the optic canal. Image c is somewhat more lateral in the same plane. * A **large left retrobulbar mass** (a–d: asterisk) results in rostral displacement of the ipsilateral globe. * The mass **extends caudally through the left optic canal** (a,b: large arrow) and incorporates the **optic chiasm** (a,b: small arrow). * **Regional meningeal enhancement** is also evident (c: arrow). * The left optic nerve is not well delineated, but neural enlargement is assumed from the parasagittal images (a,b: arrowhead). Postmortem examination revealed severe chronic suppurative optic neuritis and meningitis.

Answer 58

_Cranial Nerve II Meningioma (Canine)_ 10y FS Kelpie with right exophthalmia and absent menace response of 3 week duration. * The **right optic nerve is enlarged** and has a **nonuniform diameter** (a–c: arrow). * The **meningeal sheath intensely enhances**. * Oblique plane images were generated to approximate the path of the right (c) and left (d) optic nerves through their respective optic canals (c,d: arrow). A right orbital exenteration was performed, and a right optic nerve meningioma was confirmed histologically. The dog returned 2 years later with clinical signs referable to intracranial neurologic disease. * The **proximal remnant of the right optic nerve is enlarged, irregularly margined, and nonuniformly contrast enhances** (e: arrow). * The mass now includes both **retrobulbar and intracranial components** (e–g: arrowhead).

Answer 59

_Cranial Nerve V Benign Peripheral Nerve Sheath Tumor (Canine)_ 7y MC Shetland Sheepdog with left‐sided masticatory muscle atrophy and other signs of left trigeminal nerve dysfunction. * MR images reveal a **large, well‐delineated, T1 hypointense, T2 hyperintense left‐sided extraaxial mass** causing **compression of the brainstem and cerebellum** (a–d: arrow). * The mass **intensely and uniformly contrast enhances** and extends rostrally through the **trigeminal canal** (d: arrowhead). * There is also **marked atrophy and T1 and T2 hyperintensity of the left masseter muscle**, consistent with **fatty infiltration from chronic denervation** (a,b: arrowheads). * A CT examination was performed for radiation treatment planning. Images e and f are unenhanced and enhanced images acquired at the level of the trigeminal canal. Images h and i (next page) are more caudal, centered at the level of the mass. * There is **destruction of the temporal bone pyramid** that forms the **trigeminal canal** (e: arrowhead). * Note that the mass is not seen on the unenhanced CT images (h). * The previously identified mass **intensely but nonuniformly enhances** (f,g,i: arrow). * The **globoid part of the mass** is located in the **caudal fossa** (g,i: arrow) but extends **rostrally through the trigeminal canal** (g: arrowhead). Postmortem examination confirmed the presence and location of the extraaxial mass (j: arrowheads). Microscopic diagnosis was left trigeminal nerve neurofibroma.

Answer 60

_Cranial Nerve V Malignant Peripheral Nerve Sheath Tumor (Canine)_ 7y FS Labrador Retriever with neurologic deficits localized to the mandibular branch of the left trigeminal nerve. * There is a **large, T1 hypointense, T2 hyperintense tubular mass** that arises at the **origin of the left trigeminal nerve** and **extends rostrally through the trigeminal canal of the temporal bone** (a–e: arrow). * There is **marked left temporal muscle atrophy** consistent with mandibular nerve motor dysfunction, and the muscle is **mildly T1 and T2 hyperintense compared to the contralateral temporal muscle** as a result of fatty infiltration from chronic denervation (a,b: arrowheads). * The mass **intensely and uniformly enhances** following contrast administration (c–e: arrow). Postmortem examination confirmed a peripheral nerve sheath tumor of the origin and mandibular branch of the left trigeminal nerve (f: arrowheads).

Answer 61

_Cranial Nerve V Lymphoma (Canine)_ 7y MC German Shepherd Dog with multiple cranial nerve neuropathy, including pronounced bilateral trigeminal nerve involvement. * Prominent, **well‐delineated, T1 hypointense, T2 hyperintense extraaxial masses** are present on **either side of the brainstem**, causing **axial compression bilaterally** (a,b: arrows). * The masses **moderately and uniformly contrast enhance** (c: arrows). Cerebrospinal fluid analysis revealed the presence of abnormal lymphocytes. The diagnosis of lymphoma was confirmed on postmortem examination performed 6 months following the MRI examination.

Answer 62

_Cavernous Sinus Syndrome (Canine)_ 8y FS Shepherd cross with cranial nerve III, IV, V, VI, and ocular sympathetic innervation deficits consistent with cavernous sinus syndrome. * There is a **large, well‐defined, T1 isointense, T2 hyperintense ovoid mass** arising from the **pituitary fossa** (a,b: arrow). * The **carotid arteries** are identified in cross‐section and appear to be **surrounded by the mass** (a,b: arrowheads). * Following contrast administration, the mass **uniformly and intensely enhances**, and the basilar part of the mass can be seen to invade the cavernous sinus bilaterally. Carotid arteries are confirmed to be incorporated into the mass (c: arrowheads). The cavernous sinus contains the carotid arteries, its sympathetic plexus, and the third, fourth, sixth, and some branches of the fifth cranial nerves. The invasion of the mass within the sinus explains the clinical signs.