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FIG 4. Representative phase contrast images of the foramen magnum in a patient (A) with the same vbgor color scale as in the normal subject. Flow velocities differ markedly in different regions in the subarachnoid space. Velocities anterior to the cord exceed those posterior to the cord. Velocities in the anterolateral subarachnoid space, exceed those elsewhere in the subarachnoid space, especially in diastole. A single frame (B) from late diastole is shown with a cursor placed to illustrate the region in which flow was measured (white oval) and the vertebral arteries (arrows). Note that flow in the subarachnoid space reverses while flow in the vertebral artery has continuous flow.
