Figure 1b. Drawings show the measurements made on (a) supine and (b) prone MR images. The vertical black structure at the top of the drawing is the spinal cord and medullary cone. Ap = distance between the medullary cone and the ventral dura at MR imaging with the patient in the prone position, As = distance between the medullary cone and the ventral dura at MR imaging with the patient in the supine position, W = width of dural sac at the level of the tip of the medullary cone. Percentage of movement = (As - Ap)/(W x 100).