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PCMR images of diastolic flow (left column) and systolic flow (right column) in successively lower levels of the upper cervical spinal canal in a patient with Chiari I. CSF velocities increase from region 1 near the tonsils to region 4 near the C4 level. At C4, aliasing produces negative flow voxels (black) scattered among the voxels with positive flow (white voxels). Modified from Shah et al.8
