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4D MRI, Alzheimer’s, amyloid, brain, cerebral aqueduct, cerebral blood volume, choroid plexus, CSF, exercise, fMRI, fourth ventricle, glymphatic, hydrocephalus, Monro-Kellie, oscillations, pulsations, sleep, superior sagittal sinus, subarachnoid space, Virchow-Robbins


Specifying the exact mechanism driving cyclic, oscillatory flows of cerebrospinal fluid (CSF) through the cerebral ventricles and subarachnoid spaces remains an open problem. Here is a new quantitative hypothesis describing how cyclic expansion and contraction of the cerebral blood volume moves CSF within the closed box of the cranium. As whole brain tissue expands with added blood, volume compensation is provided by partial collapse of the superior sagittal and transverse venous sinuses. The magnitude of the expansion and contraction of whole brain tissue volume can be estimated from the changes in intravascular volume, which in turn can be divided into arterial, venous, and capillary fractions. Each of these volume fractions changes cyclically on a separate time scale. Arterial volume changes at the frequency of the heartbeat. Venous volume changes at the frequency of breathing in response to changes in intrathoracic pressure. Capillary volume changes according to temporal patterns of autoregulation, driven by chemical signals resulting from neural activity, especially during sleep. These volume changes can be predicted from principles of classical anatomy and physiology. In turn, using approximate geometric models of CSF filled spaces, local volumetric flows and flow velocities can be calculated. The calculations imply the existence of cyclic changes in CSF flow at three different frequencies and having three different magnitudes, which are in agreement with observational data in normal humans. Such models can be individualized using routine clinical data and imaging. Further, the predicted tidal movements of CSF over the cerebral convexities would continually refresh fluid drawn into and out of the Virchow-Robbins spaces, providing enhanced clearance of waste products during sleep and during strenuous exercise.