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FIG 7. Phase-contrast image (left) and graph of velocities (right) show velocity aliasing during caudad flow of CSF in the foramen magnum of a patient. In the image a region of bright white signal intensity anterior and to the right of midline is surrounded by dark black signal intensity. The graph indicates that flow for the region of interest appears to turn negative (dark) at 30-ms trigger delay and then unexpectedly reverts to positive from approximately 60-ms trigger delay to approximately 260-ms trigger delay. The apparent reversal of sign in the graph and in the image result from velocity aliasing.
