4. Basal CSF cisterns

The **subarachnoid cisterns** are areas within the subarachnoid space where the pia mater and arachnoid membrane are not in close approximation. The subarachnoid tissue is not as abundant here as in the normal subarachnoid space and cerebrospinal fluid (CSF) gathers to form pools or cisterns.

* Numerous trabeculae, septae, membranes cross SAS → create smaller compartments termed cisterns
* All cranial nerves, major arteries/veins traverse cisterns
* All structures within cisterns invested with thin pial-like layer of cells
* All SAS cisterns communicate with each other and with ventricular system (through foramina of Magendie and Luschka)
* SAS cisterns divided into supra- and peritentorial, infratentorial groups

Supratentorial/peritentorial cisterns

* Suprasellar cistern: Superior to pituitary gland
* Interpeduncular cistern: Between cerebral peduncles, Liliequist membrane
* Ambient (perimesencephalic) cisterns: Wrap around midbrain, connect suprasellar, quadrigeminal cisterns
* Quadrigeminal cistern: Under corpus callosum splenium, behind pineal gland, tectum; continuous anteriorly with velum interpositum
* Cistern of velum interpositum: Formed by double layers of tela choroidea (pia), lies above third ventricle; communicates posteriorly with quadrigeminal cistern

Infratentorial (posterior fossa) cisterns

* Midline (unpaired)
	+ Prepontine cistern: Between upper clivus, anterior pons
	+ Premedullary cistern: From pontomedullary junction above to foramen magnum below; between lower clivus and medulla
	+ Superior cerebellar cistern: Between upper vermis, straight sinus
	+ Cisterna magna: Between medulla (anterior) and occiput (posterior), below/behind inferior vermis
* Lateral (paired)
	+ Cerebellopontine cistern: Between anterolateral pons/cerebellum, petrous temporal bone
	+ Cerebellomedullary cistern (sometimes included as lower cerebellopontine cistern): From dorsal margin of inferior olive laterally around medulla

Fissures

* Interhemispheric fissure: Longitudinal cerebral fissure separates hemispheres
	+ Inferior part contains cistern of the lamina terminalis; upper part contains pericallosal cistern
* Sylvian (lateral) fissure: Separates frontal, temporal lobes anteriorly, courses laterally to cover insula

Internal Contents

* **Supratentorial/peritentorial cisterns**
	+ Suprasellar cistern: Infundibulum, optic chiasm, circle of Willis
	+ Interpeduncular cistern: Oculomotor nerves (CN3), basilar artery (BA) bifurcation, posterior thalamoperforating arteries
	+ Ambient cisterns: Trochlear nerves (CN4), P2 posterior cerebral artery (PCA) segments and branches, superior cerebellar arteries (SCAs), basal veins of Rosenthal
	+ Quadrigeminal cistern: Pineal gland, trochlear nerves (CN4), P3 PCA segments, medial & lateral posterior choroidal arteries, vein of Galen (VofG) + tributaries
	+ Cistern of velum interpositum: Internal cerebral veins (ICVs), MPChAs
* **Infratentorial cisterns**
	+ Prepontine cistern: BA, anterior inferior cerebellar artery (AICA), CN5 and 6
	+ Premedullary cistern: Vertebral arteries (VAs), anterior spinal artery, posterior inferior cerebellar artery (PICAs), CN12
	+ Superior cerebellar cistern: SCA branches, superior vermian and precentral cerebellar veins
	+ Cisterna magna: Cerebellar tonsils (often have dense trabecular attachments), tonsillohemispheric PICA branches
	+ Cerebellopontine cistern: CN5, 7 & 8; AICA; petrosal vein
	+ Cerebellomedullary cistern: CN9, 10 & 11
* **Fissures**
	+ Interhemispheric fissure: Falx cerebri with inferior sagittal sinus, anterior cerebral artery (ACA) and branches
	+ Lateral fissure: Middle cerebral artery (M1-3 segments) & vein

Normal variant

* Hexagonal (71.5%) or tetragonal (28.5%) configuration of suprasellar cistern
* Mega cisterna magna: focal enlargement of the subrachnoid space in the inferior and posterior portion of the fossa
* Empty sella: deficient diaphragma sellae with herniation of cistern space around the pituitary
* Arachnoid cyst: cisterna magna, quadrigeminal cistern, ventricles, suprasellar cistern.