MRI Flashcards
White matter characteristics on T1W & T2W
Hypointense on T2 & Hyperintense on T1
Myelin contains membrane phospholipids which have rapid T2 relaxation –> “MR invisible”
NO lipid peak on spectroscopy therefore signal intensity determined by other factors (e.g - 12% less H20 than grey matter)
Demyelination lesions are T2 hyper intense compared to WM b/c of increased H20 protons in lesions
Exit of ophthalmic branch of trigeminal?
Orbital fissure
Exit of mandibular branch of trigeminal?
Oval foramen
Exit of maxillary branch of trigeminal?
Round foramen
Where is the rostral alar foramen located?
Level of hamuli of pterygoids
Where are the caudal alar foramen & oval foramen located?
Level of TMJ
What is contained within the oval foramen?
Mandibular branch of CrN 5
What is contained within the foramen lacerum?
Internal carotid artery
Where is the foramen lacerum located?
Rostral part of basioccipital & flanked by small bony processes from tympanic bulla
What is contained within the tympani-occipital fissure?
CrN 9, CrN 10, CrN 11
What is the internal opening in dogs of the tympani-occipital fissure?
Jugular foramen
What is contained within the stylomastoid foramen?
CrN 7
True or false, leptomeningeal enhancement extends into the cerebral sulci?
True!
But, pachymeningeal enhancement does not.
Contrast enhancement of the pachymeninges involves what structures?
Dura mater and adjacent periosteum, ‘dural’ enhancement
Contrast enhancement of the leptomeninges involves what structures?
Arachnoid & pia mater, ‘pial’ enhancement
True or false, meningoencephalitis is more likely to involve only supratentorial portions of the brain in comparison to neoplasia?
False! More likely to involve BOTH supratentorial and infratentorial portions.
Yorkies and French Bulldogs have been reported to get which type of disease process causing cerebral & brainstem subcortical white matter & cortical gray matter T2 hyperintense & T1 hypointense lesions with possible ringlike enhancement?
Necrotizing leukoencephalitis
What are the MR characteristics of chronic distemper meningoencephalitis?
- Bilaterally symmetric T2 hyperintense white matter lesions at junction of parietal & frontal lobes
- T2 hyperintensity of the arbor vitae –> loss of cerebellar gray-white matter junction
- T2 hyperintensity of caudal brainstem
- Pachymeningeal contrast enhancement
- Lesions NOT visible on T1 pre & post
What is the most common MRI finding of FIP?
Contrast enhancement of the meninges & ventricular lining.
(Histopath lesions = meningitis, ventriculitis, choroiditis, & periventricular vasculitis)
What are two differences between MRI findings of FIP versus other feline meningoencephalitis?
1) Meningeal, ependymal, or periventricular mild to moderate contrast enhancement
2) If present, parenchymal lesions have distinct margins
What locations are most commonly affected by cryptococcosis fungal meningoencephalitis?
Frontal & olfactory lobes
What are the two categories of MRI lesions of Blastomycosis?
1) Mass lesions
2) Ependymal & ventricular changes
What is unique about focal mass like lesions from Blastomycosis?
Some have been reported to be T2 iso- or hypointense, although most are T2 hyperintense/T1 hypo- or isointense
What are 2 diseases with lesions that may be T2 iso- or hypointense & T1 hyperintense?
1) Blastomycosis
2) Cryptococcus
T2*W signal voids on gradient echo images indicate what?
Intralesional hemorrhage
Contrast enhancement of cochlear fluid can occur with what disease process?
Aspergillosis
Monocytotropic ehrlichiosis affects what portions of the brain?
Caudate nuclei & thalamus
Does cerebral grey or white matter have lower blood supply?
White matter
Abscess lesions that span both grey and white matter have what characteristics?
Can have relatively thinner medial rims (due to decreased blood supply of white matter) –> may predispose to rupture into adjacent ventricle
What is a distinguishing feature of brain abscessation on MRI?
T2W hypointense peripheral rim
- caused by paramagnetic free radicals w/in phagocytic macrophages
- concurrent susceptibility artifact in the rim on T2*W gradient echo images
If a brain abscess communicates with the ventricle, what will be the MRI findings?
Abnormal T1W intensity & incomplete T2 FLAIR suppression of ventricular CSF
What are the MRI changes associated with neosporosis?
1) Cerebellum atrophy, 2) Decreased grey/white matter distinction , 3) Widening of sulci between the folia, 4) T2 hyper, T1 hypo to iso variable contrast-enhancing lesions in thalamus, brainstem, internal capsule, or cerebral cortex
Which infectious disease can result in an extra-axial mass resembling a meningioma?
Toxoplasma-associated granuloma
What are the 3 types of brain herniation and explain what is herniated.
1) Foramen magnum herniation = herniation of caudal portion of cerebellum into & through foramen magnum
2) Subfalcine herniation = herniation of portion of the cerebral cortex across midline (causes ipsilateral cingulate gyrus to be pushed ventrally & under midline falx –> contralateral cingulate gyrus compression & depression of ipsilateral corpus callosum)
3) Caudal transtentorial herniation = displacement of portions of the cerebral cortex ventral to tentorium cerebelli (–> displacement of brainstem & rostral aspect of cerebellum away from the mass)
Which type of herniation can lead to obstructive hydrocephalus?
Caudal transtentorial herniation
What is the most common brain tumor in dogs & cats?
Meningiomas (D: 45-51.5%; C: 73%)
What is the origin of meningiomas?
Meningeal lining of brain
Name two dog breeds predisposed to meningiomas?
Golden Retrievers & Boxers
(DSH are also predisposed)
What is the most common location for meningiomas?
Rostrotentorial (especially fronto-olfactory)
- Association with caudal fossa or ventricular system also possible
Which tumor is associated with a dural tail sign?
Meningiomas
Where is the predominant location for cystic meningiomas?
Rostral fossa
What are 5 intra-axial tumors that may have meningeal involvement?
1) Meningiomas
2) DIsseminated histiocytic sarcoma
3) Lymphoma
4) Granular cell tumor
5) Metastatic disease (meningeal carcinomatosis)
What dog breeds are predisposed to glial tumors?
Boxers & Boston Terriers
What are the two most common glial tumors?
Oligodendrogliomas & astrocytomas
What are the common locations of glial tumors?
Cerebrum or thalamus. Cerebellum & caudal brainstem are LESS common.
What are some features that may be useful to differentiate an oligodendroglioma from an astrocytoma?
1) Location: caudal fossa more common for astrocytomas & oligodendrogliomas more likely to contact brain surface
2) Oligodendrogliomas more likely to cause ventricular distortion
3) Oligodendrogliomas can occasionally be intraventricular in origin & lead to drop metastasis (similar to more common intraventricular tumors such as choroid plexus tumors)
What are 4 ventricular tumors?
1) Choroid plexus tumors (dogs)
2) Ependymomas
3) Meningiomas
4) Neurocytomas
What is the most common ventricular tumor and who is predisposed to it?
Choroid plexus tumors (carcinomas > papillomas)
Golden Retrievers
What is the order from most common to least common in location of choroid plexus tumors?
4th ventricle > 3rd ventricle > lateral ventricles
________ spread/extension of a primary choroid plexus carcinoma may lead to multifocal T2 hyper/T1 hypo intense lesions.
Leptomeningeal
True or false, Ependymomas can have a periventricular origin?
True
What type of ventricular tumor accounts for the majority in cats?
Meningiomas
What is the histopathologic finding of meningioangiomatosis?
Leptomeningeal and meningovascular proliferation
This benign lesion has a predilection brain stem site…
Meningioangiomatosis
Describe the characteristics of a meningioangiomatosis lesion.
Leptomeningeal plaque extending along the perivascular spaces into adjacent parenchyma plaque. Usually superficial lesion extending from the subarachnoid space along the perivascular spaces into the adjacent parenchyma
Benign brainstem lesion that is T1 hyper to iso intense & T2, T2*, and T2 FLAIR hyperintense
Meningioangiomatosis
MRI findings in feline acromegaly?
1) Thickened frontal bone
2) Abnormal ST fluid accumulation in nasal cavities, sinuses, and pharynx
What pre-contrast feature is characteristic of pituitary microtumors?
T1W hyperintense neurohypophysis (due to vasopressin displacement)
What are characteristic MR signal intensities of melanoma metastases?
T1 hyperintense, T2 hypointense, with T2* signal void ( due to paramagnetic effects of melanin & hemorrhagic changes)
What is the second most common intracranial neoplasia in cats?
Lymphoma (14% of cases)
Is histiocytic sarcoma more likely intra- or extra-axial?
Extra-axial
What two intra-cranial, extra-axial masses share common imaging features with eachother? And how can you tell them apart?
Histiocytic sarcoma & Meningiomas
1) Transtentorial herniation & syringomyelia more prevalent with histiocytic sarcoma
2) Leptomeningeal involvement with contrast enhancement in Histiocytic sarcoma
Most granular cell tumor are reported in what location?
Meningeal/extra-axial
Often plaque-like & extensive along the convexity of the cerebrum
What are the signal characteristics of granular cell tumors?
Iso- to hyperintense to gray matter on T2W, variably intensity on T1W although spontaneous T1 pre-contrast hyperintensity is common. Strong contrast enhancement
What disease results in near complete destruction and/or lack of development of the neocortex?
Hydranencephaly (typically caused by in utero viral infection)
In cases of hydranencephaly, the fluid-filled cavity is contiguous with which ventricle?
Lateral ventricle
What brain lobes are affected by hydranencephaly?
Total loss of parietal & temporal lobes with partial loss of frontal & occipital lobes on the affected side.
How is porencephaly different from hydranencephaly?
Porencephaly - cystic cavities present in cerebrum due to cell destruction or failure to develop.
Hydranencephaly - in utero viral infection —> loss of neocortex
Describe the MRI findings associated with porencephaly?
- Unilateral or bilateral lesions
-Commonly wedge shaped & variable size - Cavities may communicate with ventricles or subarachnoid space
What is the origin of intracranial intra-arachnoid diverticula?
Arise from splitting/duplication of arachnoidae & occur in close association with arachnoid cisterns.
(cisterns are focal expansions of subarachnoid space w/ increased separation between pia mater & arachnoid, pooling of CSF, & decreased # of trabeculae
Where is the quadrigeminal cistern located?
Between the splenium of the corpus callosum & rostral portion of the cerebellum. Separated from 3rd ventricle by thin membrane
Which dog & cat breeds are predisposed to intracranial intra-arachnoid diverticula?
Small breeds, especially Shih Tzus & other brachycephalics as well as Persian cats
How does an intracranial epidermoid cyst originate?
Failure of neural tube closure when epithelial ectoderm becomes entrapped w/in nervous tissue & forms a cystic mass
What are the two most locations for intracranial epidermoid cysts?
Cerebellopontine angle or 4th ventricle
What MRI sequence can be used to differentiate intracranial intra-arachnoid diverticula & intracranial epidermoid cysts? How so?
T2-FLAIR. Epidermoid cysts will remain hyperintense while diverticula suppress (because fluid is iso intense to CSF)
True or false, ependymal cysts are associated with the ventricular system?
False
What are the characteristics of Dandy-Walker syndrome?
Partial or complete absence of cerebellar vermis & cystic dilation of the 4th ventricle
What syndrome causes failure of forebrain bifurcation? And what are 3 other characteristics associated with this condition?
Holoprosencephaly
- Absence or reduction in size of midline prosencephalic structures
- Incomplete separation of normal paired forebrain structures
-Hydrocephalus
Which is more common, ischemic or hemorrhagic stroke?
Ischemic
Does grey or white matter have more cerebral blood flow?
Grey
-some coupling of CBF with metabolism (e.g. thalamus has greater perfusion)
When does vasogenic edema start & when does it peak?
Begins w/in 24 hours (after breakdown of cell memor & BBB)
-Peaks at 3-4 days
Which lobes of the brain have the greatest perfusion?
Occipital & parietal
What regions of the brain are particularly susceptible to ischemia?
- Grey matter > white
- Occipital & parietal lobes
- Certain regions of the hippocampus :: Cornu ammonis 1, cerebellum, & caudate nucleus
The size of ischemic penumbra can be estimated using what sequences?
Combo of DWI & PWI (perfusion-weighted imaging)
-DWI shows infarct core
-PWI shows area of ischemi a
What are 3 predisposing factors for stroke?
-PLN
- Hypertension
- Hypercoagulability
What 3 dog breeds are predisposed to ischemic infarcts?
Sighthounds, Greyhounds (hypertension) & spaniels
What is the appearance of acute infarction on DWI & ADC maps, respectively.
Restricted diffusion leads to DWI hyperintensity & low signal on ADC
What is T2 shine through?
A mimic of restricted diffusion that occurs as high signal on DWI images (remember DWI images usually have some T2 weighting)
What is the most common site of territorial infarcts in dogs?
Cerebellum
What is the vascular territory covered by the rostral cerebral artery?
Rostromedial & dorsal surfaces of cerebral cortex along both sides of medial longitudinal fissure
What is the vascular territory covered by the middle cerebral artery?
Lateral cerebral cortex
What is the vascular territory covered by the caudal cerebral artery?
Caudomedial and dorsal surfaces of cerebral cortex along both sides of medial longitudinal fissure
What is the vascular territory covered by the rostral cerebellar artery?
Rostral part of cerebellar hemisphere, vermis, & dorsolateral brainstem
What is the vascular territory covered by the caudal cerebellar artery?
Caudal & ventral cerebellum
Lateral medulla
What is the primary arterial supply to the brain in dogs?
Internal carotid & basilar arteries.
(Form an arterial ring on ventral surface of brain adjacent to pituitary)
What vessels form the circle of Willis?
- Internal carotid
-Basilar arteries - Rostral & caudal communicating
What structure gives rise to the rostral cerebellar arteries?
Caudal communicating artery
What is the main arterial blood supply to the brain in cats?
Maxillary & pharyngeal arteries
What are the differences between the Circle of Willis in cats versus dogs?
- Circle is incomplete
- External carotid supples almost the entire brain except caudal brainstem
- Caudal brainstem supplied by vertebral arteries
What are the regions most commonly affected by territorial infarcts in dogs?
Regions supplied by rostral cerebellar & middle cerebral arteries.
Multiple lacunar infarcts in differing vascular territories is suggestive of what?
Embolic cause
What are 4 broad categories of differentials for bright lesions on DWI?
1) T2 shine through artifact
2) Lesions containing viscous/proteinaceous fluid
3) Cytotoxic edema
4) Densely cellular masses
On T2W images, hemorrhage is hypointense in all stages except which?
Hyperacute & subacute.
Name the magnetic properties and T1 signal of oxyhemoglobin, deoxyhemoglobin, methhemoglobin, ferritin, & hemosiderin.
Oxyhemoglobin: Diamagnetic, Isointense
Deoxyhemoglobin: Paramagnetic, Isotense
Methemoglobin: Paramagnetic, Hyperintense
Ferritin: Superparamagnetic, Isointense to slightly hyper
Hemosiderin: Paramagnetic, Isointense to slightly hyper
What sequence is more sensitive to detection of cerebral microbleeds compared to T2*W sequences?
SWI
What are other differentials for T1W hyperintensity on brain images?
Melanin, high protein, flow artifacts, or paramagnetic effects (e.g. manganese)
Discuss the pneumonic for T1 & T2W changes of hemorrhage.
“ I Bleed, I Die, Bleed Die, Bleed Bleed, Die Die”
Hyperacute: T1 iso, T2 bright
Acute: T1 Isointense, T2 dark
Early subacute: T1 bright, T2 dark
Late subacute: T1 bright, T2 bright
Chronic: T1 dark, T2 dark
Name 7 things that are T1 hyperintense that can mimic intracranial hemorrhage.
1) Melanin
2) Vasopressin within pituitary gland
3) Flow artifacts
4) Lipid (dermoid cyst, lipoma)
5) Protein effects ( colloid cysts, Rathke’s cleft cyst, epidermoid, laminar cortical necrosis)
6) Manganese/iron/copper/gadolinium deposition
7) Calcification
What are the two most common causes of multifocal, non-traumatic large (> 5mm) intracranial hemorrhages?
Metastatic neoplasia or coagulopathy.
What is pituitary apoplexy?
Acute bleeding within pituitary masses
How are subdural and epidural hemorrhage differentiated from eachother?
Subdural hemorrhage
Epidural can cross suture lines but not midline unlike epidural hemorrhage. Epidural hemorrhage May cross dural folds (e.g. falx) but NOT suture lines where the dura is tightly adhered to overlying calvarium.
What MR sequence is most helpful in identification of extracranial soft tissue trauma?
STIR
What are two intracranial structures that are considered dural folds?
Falx cerebri & osseous tentorium cerebelli.
Where do epidural hematomas occur?
In the potential space between the inner surface of the skull & the dura mater.
Where do subdural hematomas occur?
In the potential space between the pia-arachnoid & dura mater.
What causes diffuse cerebral swelling?
Secondary to increased cerebral blood volume or vasogenicf & cytotoxic edema.
What are the MRI findings of diffuse cerebral swelling?
1) Effacement of cerebral sulci & ventricular compression
2) Loss of grey-white matter distinction in cytotoxic edema.
3) Diffuse T2 hyperintensity along cerebral white matter tracts in vasogenic edema.
What occurs in rostral transtentorial herniation?
Portions of the cerebellum are displaced dorsal to the tentorium cerebelli.
What is the best MRI sequence for assessing CrN?
Transverse T2W (2mm slice thickness)
Which CrN are consistently visualized on MRI in dogs?
CrN 2, 3, 5 & it’s divisions, & 8
Inconsistent - 4, 7, 9, 10, & 11
Very difficult or not seen - 6 & 12
Which CrN are consistently identified in cats?
CrN 2, 5 & it’s divisions, 7, & 8
Not individually visible - 4 & 6
Poorly visible - 3
What are the relative intensities of CrN on T1 & T2W images?
CrN typically isointense to grey matter on both sequences. However in cats, CrN 9,10, 11 & 12 are hyperintense on T2W images
Where is the hypoglossal nerve located & where does it emerge?
Location: medulla, adjacent to midline on floor of 4th ventricle
Emerges brainstem, lateral to pyramids (ventral surface of medulla)
What is the exit of CrN 2?
Optic canal
Which CrN exit through the orbital fissure?
CrN 3, 4, 6, & ophthalmic branch of 5 (via trigeminal canal)
What is the course of the maxillary branch of CrN 5?
Trigeminal canal -> Round foramen -> Alar canal (not found in cats) -> Rostral alar foramen
What is the course of the mandibular branch of CrN 5?
Trigeminal canal -> Oval foramen
The mandibular branch of CrN is motor to what muscles?
Tensor veli palatini
Masticatory muscles - temporalis, masseter,
medial/lateral pterygoid, & rostral portion of digastricus
Where do CrN 9,10, &11 emerge from?
Lateral to myelencephalon at the level of the lateral recess of 4th ventricle & caudal cerebellar peduncles
What is unique about the intensities of CrN 9,10, & 11 in cats & dogs?
Dogs - T2 hypointense
Cats - T1 hyperintense
Where is the facial nerve located & what is its MRI landmark?
Emerges from pons, Immediately rostral to the vestibule, dorsal to the cochlea.
- cochlea is spiral shaped with high internal T2 signal (fluid)
Where does the facial nerve emerge from?
Internal acoustic meatus
What is the first branch of the trigeminal nerve & where does it exit?
Maxillary branch / round foramen
Which nerves exit through the orbital fissure?
Abducens, Trochlear, Oculomotor, & ophthalmic branch of trigeminal
Where is the orbital fissure located?
Ventrolateral to the optic chiasm
Which CrN has a thin T2 hyperintense rim due to extension of CSF through the meninges?
Optic nerve
The inner & outer layers of the optic nerve sheath are continuations of what?
Inner sheath = pia mater
Outer sheath = dura mater
What are the 3 presentations of GME?
Disseminated, focal mass, or ocular.
What are the 4 most common neoplasias of the optic nerve?
In no particular order:
- Retrobulbar Meningioma
- Peripheral nerve sheath tumor
- Lymphoma
-Glial cell tumors
What travels through the cavernous sinus in dogs?
-Internal carotid artery
- CrN 3,4, ophthalmic branch of 5, & 6
(all exit rostrally through orbital fissure)
-portion of maxillary branch of CrN 5 also travels with the above structures until it exits via round foramen
What two CrN exit through the internal acoustic meatus?
Facial & vestibulocochlear
The facial nerve can be secondarily affected by space-occupying lesions in what region of the brain?
Cerebellopontine angle (e.g. Meningiomas)
Which nasal sinus cavity is the largest?
Rostral frontal sinus ( occupies much of the frontal bone, including zygomatic process)
What is the term for herniation of brain cavity content into the nasal cavity?
Meningoencephalocele (usually through cribriform proteins late defect)
-meningocele = only meninges herniated
What are the origins of nasal inflammatory polyps and nasopharyngeal polyps, respectively?
Nasal inflammatory polyps arise in nasal passages & occasionally extend into nasopharynx.
- Nasopharyngeal polyps originate from Eustachian tube & can grow into both middle ear & nasopharynx.
As protein concentration increases, what is the effect on T1 & T2 intensity?
T1 changes from hypo- to hyperintense.
T2 changes from hyper- to hypointense.
Once protein concentration exceeds 28%, secretions are inspissated & are hypointense on T1 & T2
Where do sinonasal tumors usually arise from?
Caudal 2/3rds of nasal cavity
What are some differences in T1/T2 signal intensity that may help to differentiate between sinonasal carcinoma & sarcoma?
Carcinoma tend to be T1 isointense & T2 hyperintense versus sarcomas which tend to be mildly T1 hyperintense & T2 iso or hypointense.
What are the 6 bones that form the orbit?
1) Frontal
2) Maxillary
3) Palatine
4) Sphenoid
5) Lacrimal
6) Zygomatic
Describe the MRI findings associated with ocular GME.
- T1 & T2 isointense mass-like enlargement of optic chiasm
- Strong optic nerve contrast enhancement
- T2 hyperintensity (edema) of optic pathway, ventral thalamus, & forebrain
Intraocular tumors arise mostly from where?
Iris & Ciliary body (most commonly melanoma & carcinoma)
What are the characteristic signal intensities of ocular melanoma?
T1 hyperintense & T2 hypointense (due to paramagnetic properties of melanin b/c of its high affinity for metal ions)
Describe MRI changes that may occur with ocular Meningiomas.
- Expansile, partially mineralized soft tissue mass caudal to eye
- Mass can be tracked to optic nerve
- Marked atrophy of orbital wall & optic canal
What is the MRI appearance of orbital myxosarcoma?
- Exophthalmos
- Pterygopalatine fossa swelling
- lesions lie mainly within fascial planes
-often caudal extension between zygomatic salivary gland & pterygoid muscles medially as well as mandibular coronoid orocess & temporal muscle laterally - communicating, sometimes multiloculated fluid-filled cavities (T2 hyper, T1 hypo, non enhancing)
-Peripheral lesion enhancement on T1 post - +- osteolysis of mandibular coronoid process & TMJ
What MRI findings can be seen with feline restrictive orbital myofibroplastic sarcoma (FROMS)?
- Mild thickening of episclera, sclera, & adjacent orbital structures
- Decreased $ partially effaced orbital fat
Dental structures are _______ on all MRI pulse sequences.
Hypointense
What are the 5 muscles of mastication?
1) Temporal
2) Masseter
3) Medial pterygoid (close jaw)
4) Lateral pterygoid
5) Digastric muscles (open jaw)
What is the origin & insertion of the temporal muscle?
Origin = Sagittal crest
Insertion = Mandibular coronoid process
What muscle lies on the lateral surface of the mandibular ramus?
Masseter
Canine masticatory myositis leads to immune-mediated auto antibodies against what?
Type 2M muscle fibers (exclusively found in masticatory muscles)
What is the difference between dogs & cat salivary glands?
Cats have an additional molar salivary gland.
(Both share zygomatic, mandibular, parotid, sublingual & ventral buccal glands)
Where is the zygomatic salivary gland located?
In the pterygopalatine fossa, lateral to origin of pterygoid muscle
Zygomatic salivary gland disorders commonly result in what?
3rd eyelid protrusion, exophthalmos, decreased/absent retropulsion, pain when opening mouth, & regional swelling
Zygomatic sialadenitis is often associated with what?
Inflammation of adjacent masticatory mm
Which gland(s) are most commonly affected by sialocele?
Sublingual glands
The afferent & efferent lymphatic vessels drain to which salivary glands, respectively?
- Afferent lymphatics to mandibular lymph nodes (drain entire head except tongue/pharynx/larynx/ear).
- Efferent lymphatics to MRPLN
What structures are drained by the MRPLN?
-Parotid & mandibular lymph nodes
- Cranial aspect of esophagus & trachea (including thyroid)
- most of neck musculature
Where is the lateral retropharyngeal lymph node located and whom can it be found in?
Only present in ~ 30% of dogs & ALL cats.
-Superficially located between war base & wing of C1
What are the most common oropharyngeal neoplasias in dogs?
malignant melanoma, SCC, & fibrosarcoma
More than ____% of thyroid masses in dogs are ______. And of these they are ___________in 90% of cases.
70%; thyroid carcinomas; non-functional
Discuss MRI findings associated with thyroid carcinomas.
- T1 (95%) and T2 hyperintense
- Marked heterogeneous T1 post contrast enhancement
- Cystic ( T1 hypo, T2 hyper) or dystrophic mineralized (T1 & T2 hypo) regions
- Capsuksr disruption in 2/3rds of cases
Although carotid body tumors share similar MRI findings with thyroid carcinomas, what is the difference in location?
Carotid body tumors located dorsolateral to larynx at level of bifurcation of common carotid artery
What are imaging features of carotid body paragangliomas?
- Mass centered at carotid burfutcation, often entrapping carotid artery
- Heterogenously hyperintense to muscles on T1 & T2
- Salt & pepper T2 heterogeneity ( salt = hyperintense/high signal regions corresponding to slow flow or hemorrhage; pepper = vessel signal void on T1 & T2)
What structure is present that abuts the dorsal part of the annulus fibrosus and helps to prevent dorsal disc herniation? Additionally where does the structure span between?
Intercapital ligaments between T2-T11.
Where is the conus medullaris located in small dogs, cats, and large dogs respectively?
Caudal to L6 in cats & small dogs.
Cranial to L6 in large dogs.
On transverse spinal cord images, what are the differences between grey & white matter? How does this differ from intracranial appearance?
Central butterfly-shaped grey matter is hyperintense to peripheral white matter in spinal cord.
In the brain, white matter is hyperintense to grey matter (which may be due to longer T1 relaxation times of spinal cord white matter & longer T2 relaxation times of spinal cord grey matter)
Patchy T1 & t2 hyperintensities of vertebral bone marrow can occasionallybe seen and represent what normal process?
Normal red hematopoietic bone marrow conversion into yellow fatty marrow.
The nucleus pulposus is the surviving structure of what?
Embryologic notochord
Describe Hansen Type 1 IVDD.
- Aka chondroid degeneration
- Most common in young chondrodystrophic dogs
- Early notochordal & chondrocyte - like cell senescence within nucleus pulposus
- Water & proteoglycan content decreased & increased disc mineralization
- Ultimately nucleus pulposus extrudes through annulus
Describe Hansen Type 2 IVDD.
- Aka fibroid degeneration
- Most common in older, non- chondrodystrophic dogs
- Progressive increase in fibrous content in nucleus
- Ultimately causes rupture of inner layers of annulus & partial displacement of nucleus pulposus into disrupted annulus (protrusion)
Describe features of acute hydrated nucleus pulposus extrusion.
- Well hydrated nucleus pulposus placed under excessive stress –> dorsal annulus fibrosus rupture
-hydrated nuclear material can diffuse through epidural space –> only secondary changes attributable to acute cord contusion - both compressive & non compressive forms
Describe MRI features of extradural spinal cord compression at the level of an intervertebral disc.
- Loss of hyperintense epidural fat signal
- Altered spinal cord shape
- Altered shape of disc (normally ovoid)
- Extradural material causing mass effect w/ compression/displacement of hyperintense T2 subarachnoid space
- Displaced degenerated disc material normally T1 & T2 hypointense
- Narrowed intervertebral disc space
- Loss of T2 hyperintense signal from disc
What features may be helpful in differentiating between intervertebral disc extrusion versus protrusion?
Protrusion: Midline disc herniation & partial (versus complete) intervertebral disc degeneration
Extrusion: Single (versus multiple) disc herniations & dispersion of disc material beyond borders of dis space
What technical considerations can be made to decrease metallic susceptibility artifact on MRI scans?
1) Multiple 180 degree pulses compensate in part for fixed inhomogenities of the magnetic field associated with the debris.
2) If spin echo sequences used, a short TE should be preferred to decrease influence of dephasing induced by debris
3) increasing receiver bandwidth (this in turn decreases SNR & may require increasing # of excitations to maintain good signal
4) swap phase & frequency encoding directions
5) decrease voxel size by increasing matrix at constant FOV or decreasing FOV while keeping same matrix; alternatively may decrease slice thickness (this decreases SNR, but can be compensated for by increasing NEX)
Cervical intervertebral foraminal disc extrusion most commonly affects what sites?
C5-6 and C6-7
What is the most common location for feline Meningiomas?
3rd ventricle
What MRI lesion should be highly considered in patients presenting with ophthalmoplegia?
Cavernous sinus syndrome.
Remember::CrN 3,4,6, and ophthalmic branch of 5 run through the nearby orbital fissure associated w/ this disease.
Why is compressive disease cranial to T10 uncommon?
Intercapital ligaments run between corresponding rib heads (T2-T11) & lie ventral to dorsal longitudinal ligament, effectively shielding the cord in these regions.
Cranial thoracic disc herniation is more common in what large breed dog and at which sites does it tend to occur?
GSD; T2-T5
In cases of acute TL disc extrusion, T2 areas of hyperintensity can be identified. What are possible etiologies & how does this correlate to clinical/prognostic outcome?
DDX::
1) Necrosis
2) Myelomalacia
3) Intramedullary hemorrhage
4) Inflammation
5) Edema
- Associated w/ more severe neuro deficits at presentation
- If length of T2 hyperintensity exceeds length of L2 associated w/ poor outcome in 55% of cases
What is the most common site for non-compressive acute hydrated nucleus pulposus extrusion?
T3-L3 segment (67-72% of cases)
[C1-C5 = 14-27% of cases]
In general terms, describe what occurs with non-compressive AHNPE?
- Typically occur during strenuous exercise or following trauma
- Results in spinal cord contusion immediately dorsal to affected disc
- No significant spinal cord compression b/c gel-like hydrated disc material diffuses in epidural space
- Acute/hyperacute, often lateralizing paresis or plegia w/ initial short period of deterioration ( <24 hrs) followed by static or improving clinical course
In general terms, describe what occurs with compressive AHNPE?
- Usually not associated w/ trauma or exercise
- Extruded hydrated disc material accumulates ventral to spinal cord, centered on midline, immediately dorsal to affected disc
- Almost exclusively reported in cervical spine (C4-5 most commonly affected, followed by C3-4 & C5-6)
- Acute onset of non-painful cervical myelopathy w/ tetraplegia or non-ambulatory tetraparesis & progressive deterioration w/in 24-48 hrs
- Respiratory dysfunction can occur
What are the MRI features of non-compressive AHNPE?
- Focal T2 hyperintensity overlying intervertebral disc (maximal hyperintensity usually just dorsal to affected disc space), often lateralized & typically affects both grey & white matter
- Extraneous material or signal changes within epidural space dorsal to affected disc space w/ absence of, or minimal cord compression
- Communicating T2 hyperintense tract can sometimes be seen traversing the dorsal annulus between residual nucleus pulposus & vertebral canal
- T2* hypointensity occasionally seen (associated w/ parenchymal hemorrhage)
- Post contrast enhancement uncommmon
What is proposed/considered to be a likely mechanism in the development of cervical spondylomylopathy?
Shape of vertebral canal - height of vertebral canal is significantly smaller in large breed dogs versus small
What are the 2 pathophysiologic mechanisms of spinal cord compression in CSM?
1) Disc-associated compression : middle age large breed, particularly Dobermans
2) Osseous- associated compression: young giant breed, such as Great Danes
What are 3 factors that act to explain pathophys of disc-associated CSM?
1) Relative vertebral canal stenosis
2) More pronounced torsion in caudal cervical region (leads to IVDD)
3) Protrusion of larger volume discs in caudal cervical spine
List some common intramedullary tumors.
- Ependymomas
-Metastatic neoplasia (TCC, HSA) - less commonly oligodendroglioma, nephroblastoma; astrocytoma , chordoma
List some common intradural -extramedullary tumors.
-Nerve sheath tumors
- Meningiomas
-Nephroblastoma
List some common extradural tumors?
- Peripheral nerve sheath
-Osteosarcoma - Synovial myxoma/myxosarcoma
- Plasma cell tumor
- Lymphoma
- Metastatic neoplasia (prostatic carcinoma, TCC)
-Histiocytic sarcoma
What is the most common spinal neoplasia in cats?
Lymphoma
What is/are the most common locations of spinal lymphoma in cats?
Extradural & mixed extradural/intradural
What is the 2nd most common spinal neoplasia in cats?
Osteosarcoma (commonly Extradural)
Describe the basic location & features of osteochondromas?
-Cartilage capped exostosis with a cortex & medulla
- Generally arises prior to skeletal maturity
- Located at the metaphysis or juxta-epiphyseal regions of axial & appendicular skeleton
-can be mono or polyostotic
- malignant transformation into osteosarcoma or chondrosarcoma can occur
What are the MRI features of osteochondromas?
-T2 hyper, T1 iso to hypo curvilinear structure conforming to mass (cartilage cap)
- Mass has similar signal intensity to adjacent normal bone
Herniated disc material is typically what intensity?
-T2 hypointense
(Can be hyperintense in cases of concurrent hemorrhage & depending on degree of pre-existing disc degeneration)
MRI features of infiltrative lipomas?
- paraspinal location
- T1 & T2 hyperintense (similar signal intensity to normal SQ fat)
- local invasiveness into adjacent muscles +- vertebral canal (via intervertebral foramina or direction vertebral invasion)
- bone lysis areas typically well-marginated
- typically no contrast enhancement (adjacent muscles may enhance due to regional myositis)
What are the two most common intradural-extramedullary tumors in dogs?
Meningiomas & peripheral nerve sheath tumors
What is the most common location for an intradural-extramedullary Meningioma in a dog?
Cranial cervical spine, typically cranial to or at the level of C3
What is the consistent location for a spinal nephroblastoma?
Intramedullary-Extradural between T9 and L3
Leptomeningeal Meningiomas may appear as ____ lesions as opposed to _______.
What feature can be used as a clue that the lesion is meningeal in origin?
Intramedullary; intradural -extramedullary; spontaneous pre- contrast T1 hyperintensity
Most intramedullary tumors are of what origin?
Primary neural tumors of glial cell origin
Incomplete suppression of central canal lesions on T2 flair series indicates what?
That the material is not made of pure CSF
Name the 5 canine breeds predisposed to histiocytic sarcoma.
1) Bernese mountain dog
2) Golden Retrievers
3) Flat-coated retrievers
4) Labrador Retrievers
5) Rottweilers
What are the predisposed sites for ischemic myelopathy in dogs?
L4-S3 and C6-T2 (in histo confirmed cases)
T3-L3 (antemortem diagnosis)
What are the predisposed sites for ischemic myelopathy in cats?
Cervical spinal cord
What are MRI suggestive of ischemic myelopathy?
- Intramedullary lesion extending over several vertebral bodies
- Often present near degenerative disc
- Cord changes often NOT centered directly over disc space
- Lesion usually focal & well-demarcated
- Lesion predominantly affects gray matter
- Lesions most commonly lateralized or asymmetric
- ## In acute stages cord edema often present
What are the 5 distinct stages of spinal cord hemorrhage?
1) Hyperacute :: intracellular oxyhemoglobin –> long T1 & T2
2) Acute :: intracellular deoxyhemoglobin —> long T1 & short T2
3) Early subacute :: intracellular methemoglobin –> short T1 & T2
4) Late subacute :: extracellular methemoglobin –> short T1 & long T2
5) Chronic :: ferritin & hemosiderin –> short T2
What are the most common locations for arachnoid diverticula to occur in dogs & cats, respectively?
Cervical (C2-3, Less commonly C5-6) & thoracolumbar regions (T9-13, more commonly T13-L1)
What locations are more common in larger & small/medium dogs for arachnoid diverticula? Any breed predispositions?
Cervical diverticula = larger breeds, Rotties predisposed.
Thoracolumbar diverticula in small/medium breeds; Frenchies & Pugs predisposed
What is the difference between hydromyelia and syringomyelia?
Hydromyelia = pathologic dilation of the central canal
Syringomyelia = fluid dissects through ependymal lining of central canal which creates a focal fluid collection within the cord outside of central canal
Spin determines the __________ __________ and is independent of what?
Gyromagnetic ratio (unique for each element)
- magnetic field
During a standard spin echo sequence, what is applied simultaneously with the RF pulses?
Slice select gradient
During a standard spin echo sequence, what is applied between the 90 and 180 degree RF pulses?
PE Gradient
During a standard spin echo sequence, when is the FE gradient applied?
At the same time as the echo is formed during signal readout
The MR signal is not directly recorded in the _______ ________, but instead in the ______ _______.
spatial domain
Time domain
After an MR signal is recorded in undergoes _______ _______ and the data is collected in the _______ _________.
Fourier transform
Frequency domain
What is another name for the frequency domain?
K-space
What does the inverse Fourier transform do?
It decodes the spatial domain & assigns each pixel a signal intensity that corresponds to the actual spins belonging in each particular voxel
If a FE gradient is applied along the x axis, the relationship between frequencies & spatial location on that axis depends on what?
The slope (strength) of the gradient
The phase acquired in each line in the direction of the PE gradient is a function of what?
1) the Gyromagnetic ratio (gamma)
2) strength of the PE gradient
3) the time the
gradient is applied for
The center of K space contains what?
Higher amplitude, lower frequency —> contrast & general shape
The periphery of K space contains what?
Lower amplitude, higher frequency —> spatial resolution & details/edges
Describe the relationship between dimensions of the FOV & spacing between successive sampling points in K space?
Inverse relationship , aka the more space there is between lines & columns of K space, the smaller the FOV
What are the 3 parameters that define the spatial resolution of an MR image?
1) Slice thickness
2) FOV dimensions
3) Image matrix size
For a given FOV, increasing/decreasing matrix size will decrease the pixel dimension & therefore does what to resolution?
Increasing
Increases resolution
What determines the size of the pixels?
Maximum frequencies encoded in K space in both the FE & PE direction (k^xmax, k^ymax)
Higher freq = smaller pixels
Increasing the slope of the gradient will have what effect on the FOV?
Decrease it - the spacing (delta k) between the lines of k space is inversely related to FOV
Phase wrap artifact occurs in what direction?
Phase encoding direction
The FOV in the FE direction is related to what 2 parameters?
1) strength of FE gradient
2) receiver bandwidth
The maximum frequency contained in an MR echo that can be accurately sampled is what? (Describe the equation)
f (FE^max) = Sampling freq / 2
[aka = rBW / 2]
What is the equation that describes the FOV in the FE direction?
FOV^FE = rBW / Gyromagnetic ratio x G^FE
What is the relationship between SNR and spatial resolution ?
Inverse relationship, Increased resolution —> decreased SNR
What are 2 benefits to using a larger rBW?
1) Shorter readout time, so that shorter TEs can be used
2) Reduced chemical shift artifact between fat & water
In order to maintain the size of the FOV(FE) when rBW is doubled, what should one do to the frequency encoding gradient?
Double it [remember the equation : FOV (FE) = rBW / Gyromagnetic ratio x G(FE) ]
What is the equation for overall acquisition time?
T(acq) = N(PE) x NEX x TR
Describe why T1 images have good anatomic detail?
High SNR because short TE —> more residual transverse magnetization to record
Nulling fat signal is more efficient with which sequence, STIR or fat saturation technique, and why?
STIR -b/c it is NOT sensitive to magnetic field inhomogenities
