The cerebrospinal fluid secreted by the choroid plexuses moves through the ventricular system (Fig. 5-3), pushed along by newly formed CSF. It leaves the fourth ventricle through the lateral and median apertures, moves through subarachnoid space until it reaches the arachnoid villi (most of which protrude into the superior sagittal sinus), and finally joins the venous circulation (Fig. 5-4).

Figure 5-3 Circulation of CSF. CSF emerges from the fourth ventricle into the posterior fossa, so most of it has to flow through the tentorial notch, alongside the brainstem, to reach arachnoid villi in the superior sagittal and other sinuses.

(Thanks to Dr. John Sundsten.)

Figure 5-4 Movement of CSF through an arachnoid villus.

Movement across the arachnoid villi is passive, driven by the difference in hydrostatic pressure between the CSF in subarachnoid space and the venous blood in the superior sagittal sinus. The villi act like tiny flap valves so that reverse flow is prevented if venous pressure exceeds CSF pressure.

CSF Has Multiple Functions

CSF has two general kinds of functions—some physical and some chemical. The major physical functions are that (1) the CNS nearly floats in subarachnoid CSF, making it easier for the meninges to stabilize the CNS inside the head and spine, and (2) CSF is a spatial buffer and some can be squeezed out of the head to make way for things like arterial pulses (THB6 Figure 5-11, p. 109). The chemical functions are based on the free diffusion between CSF and the extracellular fluids around neurons; this means that CSF changes (e.g., changes in ionic concentrations or hormone levels) will reach neurons.

Imaging Techniques Allow Both CNS and CSF To Be Visualized

There are two big problems in trying to make pictures of the brains of live, unopened people. One is generating contrast (i.e., getting skull, blood, CSF, and brain to yield different amounts of some signal). The other is getting a 3D impression from a 2D picture. For a long time both problems were approached somewhat indirectly. Contrast was produced where it doesn’t usually exist, for example by getting air into the ventricles (pneumoencephalography) or x-ray dense liquids into blood vessels (angiography). Pictures were taken from multiple angles to create a 3D impression, but they still involved flattening all the contrast in a solid object into a 2D picture.

Tomography Produces Images of 2-Dimensional “Slices”

Constructing pictures of “slices” through the head can get around both problems: plotting x-ray density or some other parameter in a single plane prevents some areas from swamping out, or being superimposed on, others. There are several kinds of tomography, but all depend on basically similar computer-assisted calculations. Depending on the probe or the measuring device, the process may yield maps of x-ray density, water concentration, positron concentration, or something else. The key to interpreting tomographic images lies in understanding where the contrast comes from.

CT Produces Maps of X-Ray Density

Rotating x-ray sources and detectors around someone’s head and systematically varying the center of rotation allows the construction of maps of x-ray density (THB6 Figure 5-14, p. 112). (Even though all modern tomography is a variant of computed tomography, folks usually use the term CT specifically to mean x-ray CT.) Just as with plain skull x-rays, bone is white and air is black (Fig. 5-5). White matter, because of all the lipid, is less x-ray dense than gray matter and so is darker; CSF, which is mostly water, is darker still.

Figure 5-5 Using CT to map out the x-ray density of structures and areas in the head.

(Thanks to Dr. Ray Carmody.)

MRI Produces Maps of Water Concentration

Magnetic resonance imaging (MRI) is based on similar calculations but a fundamentally different signal—the re-emission of radio waves absorbed by atoms in a strong magnetic field. The most abundant source of such signals in the CNS is protons, mostly in water but also in other molecules. Because the free water concentration and the concentrations of other molecules varies in different parts of the CNS and adjoining areas, white matter, gray matter, and CSF all look different. Different time constants can be used to produce images with different appearances (Fig. 5-6), but areas with few protons, such as bone and air-filled cavities, give off little signal.

Figure 5-6 Using MRI to map out the distribution of water (and protons in other molecules) in structures and areas in the head. In T1-weighted images, areas with a lot of water, such as subarachnoid CSF (1), ventricular CSF (2), and the vitreous of the eye (3) are dark; in T2-weighted images they are bright. In T1-weighted images, gray matter such as cerebral cortex (4), the hippocampus (5), and cerebellar cortex (6) is darker than white matter such as the deep white matter of the cerebrum (7) and cerebellum (8); in T2-weighted images, gray matter is lighter than white matter. In both kinds of images, areas with few protons, such as the inner table of the skull (*), the temporal bone (9), and the maxillary sinus (10) produce little signal and are dark.

(Thanks to Drs. Elena Plante and Ray Carmody.)