Bounded by quadrigeminal plate, splenium, vermis, & tentorial margin

Extends between layers of 3rd ventricle tela choroidea

Contents: Caudal internal cerebral veins → vein of Galen, distal parts of quadrigeminal artery, PCA P4 segment, & CN9 exit

Synonyms: Cisterna quadrigeminalis, cistern of great cerebral vein, cisterna venae magnae cerebri, Bichat canal, cisternal quadrigeminalis, & superior cistern

Homogeneous fluid-filled mass above & clearly distinct from tectum

55-60% slightly T1 hyperintense to CSF; FLAIR doesn’t suppress; 60% enhance (partial/complete rim, nodular)

Cystic expansion of pineal in some females begins in adolescence, decreases with age

Can’t distinguish from pineocytoma on basis of imaging studies alone

Axial MR/CT shows triangular-shaped CSF space between bodies of lateral ventricles

FLAIR suppresses completely; no enhancement

Dilatation of velum interpositum, precise etiology unknown

Common in early infancy, rare in adults

Avidly enhancing mass, trapped pools of CSF common, focal calcification may represent displaced pineal

Arise from posterior portion of the velum interpositum, falx, or tentorium

Velum interpositum meningiomas: M = F, in both pediatric & adult populations

May be symptomatic from compression of quadrigeminal plate

Enhancing, circumscribed pineal mass which “explodes” pineal Ca++

May mimic pineal cyst or pineoblastoma

May compress but does not invade adjacent structures

˜ 45% of pineal parenchymal tumors

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

Quadrigeminal arachnoid cysts (AC) are 3rd most common infratentorial AC

Symptoms depend on compression of brain stem, cerebellum, & aqueduct

Elevated ICP & sudden death have been reported

Tectal distortion or thickening by localized mass

T1 hypointense, T2 hyperintense, ± enhancement

Onset aqueductal stenosis often without associated brain stem signs

Reported as indolent lesions often remaining stable in size for many years

May involve cisterns > parenchyma > ventricles

Basal cistern cysts may be racemose

Cysts variable, typically 1 cm, range from 5-20 mm and contain a 1-4 mm scolex

Cystic lesion isointense to CSF, may see discrete, eccentric scolex

Well-delineated lobulated extra-axial mass with fat attenuation/intensity

40-50% interhemispheric fissure (over corpus callosum)

Ca++ varies from none to extensive

Fat-suppressed MR is diagnostic

Corpus callosal descent can efface QC

Sagittal shows brain descent in 40-50%

Diffusely, intensely enhancing dura in 85%

Bilateral subdural fluid collections in 15%

Mechanism

Imaging

Pineal region mass that “engulfs” the pineal gland

T1/T2 iso- or hyperintense to gray matter

Strong uniform enhancement, ± CSF seeding

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 CA

Fat appearance: Use fat suppression sequence to confirm

Rupture → fat droplets in subarachnoid spaces with extensive enhancement possible from chemical meningitis

< 0.5% of primary intracranial tumors

Rare malignant degeneration into squamous cell carcinoma

Dilated arteries feeding into large midline venous pouch

Thin sagittal images define anatomy & relationship to cerebral aqueduct

< 1% cerebral vascular malformations at any age

Neonatal > infant presentation most common; rare adult presentation