Cerebrospinal fluid (CSF) is an ultrafiltrate of plasma, produced by the vascular choroid plexus within the ventricles. CSF is in constant flux, as detailed in section V. A couple important related concepts are the Monro-Kellie hypothesis and an understanding of the inner arachnoid membranes. The Monro-Kellie hypothesis (again an oversimplification) states that all intracranial contents in a mature skull can be categories as either blood, brain, or CSF and that the sum volume of these three substances is constant. We will see that this concept facilitates understanding of several topics: why the inflow of blood during intracranial systole propels CSF; the imaging findings of some pathologic states such as CSF hypotension; and solid search pattern approaches for interpretation of intracranial imaging examinations (ensuring brain, vascular structures, and CSF are all evaluated).

While an arachnoid membrane covers the external surface of brain, there is also a network of inner arachnoid membranes within the basilar cisterns. These are, with rare exception, not directly visualized with routine imaging, but do divide the cisterns into named compartments. In addition the CSF, the intracranial compartment contains interstitial fluid within the brain. In fact, there is about twice as much interstitial fluid as there is cSF. These concepts and a couple other related ones are further discussed in the full atlas and the companion cases.

The ventricles are a series of lakes of CSF in the center of the brain, connected by thin channels (foramen of Monro and aqueduct of Sylvius). The ventricles are where CSF is formed by the choroid plexus. The thin connecting channels are prone to obstruction, although other areas (in particular the temporal horns of the lateral ventricle) can also become 'trapped.'

Thus, ventricular size is important in the assessment of cross-sectional imaging of the brain. Enlarged ventricles could indicate obstructive hydrocephalus, non-obstructive (communicating) hydrocephalus, shunt malfunction, brain atrophy, or underdeveloped adjacent white matter. Small ventricles could be related to mass effect or shunt over drainage. Knowledge of key anatomy and clinical findings can help distinguish these possibilities.

The ventricles are lined by ependyma, which is relevant for several pathologic processes, including subependymal nodules in the setting of tuberous sclerosis, subependymal spread of glioma, and a specific tumors such ependymoma.

Several other unique tumors may arise within the ventricles and certain tumors have a predilection for specific areas. For example, the central neurocytoma typically arises at the septum pellucidum, a membrane that divides the bodies of the lateral ventricles.

Xanthogranulomas of the choroid plexus are commonly seen and may demonstrate restricted diffusion.

The third ventricle has a shape that appears somewhat like that of Lake Superior, with the anteroinferior tip (suprachiasmatic recess) pointing to Duluth MN. The infundibular recess is just posterior to it, extending to the root of the pituitary infundibulum at the superior pituitary stalk. Note that these recesses in the normal state are sharp.

The thalami form the lateral borders. Other important adjacent structures seen here are the anterior and posterior commissures and the pineal gland. The paired fornices are split by the anterior commissure, with the majority of fibers extending posterior to the commissure adjacent to the ventricle.

The dorsal 'roof of the 4th ventricle is lined by strips of white matter called the superior medullary velum and the inferior medullary velum (which have ependymal linings). The 'floor' of the 4th ventricle is formed by eminences of the surface of the pons and rostral medulla.

The nodulus of the cerebellar vermis forms a prominent focal impression upon the 4th ventricle. The middle cerebellar peduncles are important lateral boundaries of the 4th ventricle.

The 4th ventricle has 4 outflow tracts. These include the paired foramen of Luschka (lateral apertures) arise from the lateral recesses and extend between the medulla and flocculus. These are important as they contain choroid plexus which can be seen as enhancement or calcification on imaging. 4th ventricle ependymomas also have a tendency to extend through these foramen. The other outflow tracts include the foramen of Magendie (median aperture), a single posterior midline tract to the cisterna magna, and an inferior outflow tract via the obex to the central spinal canal.