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FIG 1. Phase contrast images of the foramen magnum in a normal volunteer. The images are displayed in vbgor color scale, with violet and blue showing flow in the caudad (negative) direction and orange and red showing flow in the craniad (positive) direction. The scale is set to +3 to -3 cm/s. The 14 consecutive images through the cardiac cycle from top left to lower right (A) show fairly uniform flow velocities within the foramen magnum. In one image from the series (B), the subarachnoid space is highlighted by means of an oval placed by an illustrator and the vertebral arteries are designated by arrows.
