Below/behind inferior vermis

Large medullary cistern masses may extend laterally, posteriorly into CM

Indirect (secondary effect on tonsil) > direct (lesion in tonsil)

Most often 2° to posterior fossa (PF) mass effect

Tonsils pushed down into CM

“Peg-like” configuration of tonsils

Tonsil folia usually oriented horizontally → become vertically oriented when herniated

4th ventricle may obstruct, cause obstructive hydrocephalus

Pointed cerebellar tonsils > 5 mm below foramen magnum

Posterior fossa (PF) usually normal size

Age-related tonsil descent below “opisthion-basion line” common

Treatment aim = restore normal CSF flow at foramen magnum (FM)

Small PF → contents shift ↓

“Cascade” of tissue (vermis, not tonsil) herniates ↓ through FM

˜ 100% associated myelomeningocele

DWC a broad spectrum of cystic posterior fossa (PF) malformations

DW malformation: Large posterior fossa and large CSF cyst, normal 4th ventricle absent, lambdoid-torcular inversion

DW variant: Failure of “closure” of 4th Ventricle, vermian hypoplasia

Mega cisterna magna: Communicates freely with 4th ventricle, basal subarachnoid spaces

2/3 have associated CNS and/or extracranial anomalies

Sharply demarcated extra-axial cyst that follows CSF attenuation/signal

FLAIR suppresses; no diffusion restriction

Size varies from a few mms to giant

Often asymptomatic, found incidentally

CPA location > CM

Cellular ependymomas more common in children

Soft or “plastic” tumor squeezes out of 4th ventricle foramina into cisterns

Ca++ common (50%); ± cysts, hemorrhage

Sagittal imaging can distinguish origin as floor vs. roof of 4th ventricle

Heterogeneous T1/T2 signal with mild to moderate enhancement

CM rare PF location (CPA, medullary cisterns more common)

CM meningiomas usually arise from occipital squamosa

Well-demarcated, lobulated/rounded enhancing mass with dural attachment

Hyperostosis, tumoral calcifications, ↑ vascular markings

Linear or nodular meningeal enhancement

MR CSF flow may be helpful establishing location and degree of CSF obstruction

Primary tumors include breast, lung, melanoma, prostate

Lymphoproliferative malignancy = lymphoma and leukemia

Primary CNS tumor seed basal cisterns (drop metastases)

Image entire neuraxis!

Sagittal shows brain descent in 40-50%

Caudal displacement of tonsils in 25-75%

Diffusely intensely enhancing dura in 85%

Bilateral subdural fluid collections in 15%

Frequently misdiagnosed syndrome of headache caused by ↓ intracranial CSF pressure from spontaneous spinal CSF leak

T2 hyperintense lobular, nonenhancing intraventricular mass

Arises from 4th ventricle floor, may extend posteroinferiorly into cisterna magna

More common in middle-aged, older adults

0.7% of intracranial neoplasms

Lobulated, irregular, CSF-like mass with “fronds” insinuates cistern

FLAIR usually doesn’t completely null; diffusion yields high signal restriction

0.2-1.8% of all primary intracranial tumors

Congenital inclusion cysts; rare malignant degeneration into squamous cell carcinoma

Fat appearance: Use fat suppression sequence to confirm

With rupture find fat droplets in cisterns, sulci, ventricles with extensive MR enhancement possible from chemical meningitis

Rare: < 0.5% of primary intracranial tumors

Rupture can cause significant morbidity/mortality

Rare malignant degeneration into squamous cell carcinoma

Intra-axial posterior fossa mass with cyst, enhancing mural nodule abutting pia

Classified as meningeal tumor of uncertain histogenesis

Familial = von Hippel-Lindau

7-10% of posterior fossa tumors

Round/lobulated nonenhancing, slightly hyperintense to CSF mass

Most intracranial NECs found in posterior fossa

Benign malformative endodermal CNS cyst

Part of split spinal cord malformation spectrum; persistent neurenteric canal

Location