Fig. 1. (  A –  I ) Magnetic resonance images before (  left ;  A ,  D , and  G ) and after (  center ;  B ,  E , and  H ) epidural saline (15 ml) injection in a 40-yr-old man, whose cerebrospinal fluid volume decreased from 40.4 ml to 31.2 ml. Images taken before the epidural saline injection were subtracted from those taken afterward (  center –  left ) are also presented (  right ;  C ,  F , and  I ). In these axial images, anterior is toward the top of the figure and anatomic right is to the left in the figure. (  A ) Sagittal magnetic resonance image of lower thoracic lumbosacral column before epidural saline injection. (  B ) After injection, saline accumulated posteriorly in the epidural space, compressing the dura (  outlined arrow ). Conus medullaris and cauda equina dislocated anteriorly (  solid arrow ). (  C ) Subtracting the image taken before injection from that after injection, both anterior and posterior epidural saline is represented as a bright signal (  black arrow ). (  D ) Axial magnetic resonance image at the L2–L3 disk level. The dural area at this level was 1.88 cm². (  E ) After injection, saline accumulated posteriorly in the epidural space, significantly compressing the dura, as indicated by the dark appearance of the cerebrospinal fluid. Dural area at this level decreased to 1.12 cm². (  F ) The observations in  E were verified in this subtracted image. (  G –  I ) The epidural catheter can be seen (  solid arrows ) between the right ligamentum flavum and posterolateral dura. (  G ) Axial magnetic resonance image at the L3–L4 disk level. Dural area at this level was 1.55 cm². (  H ) After injection, saline accumulated posteriorly in the epidural space, compressing the dura. Saline passed predominantly to the left of the dura, which was the opposite side of the catheter, and accumulated anterior to the dura. The dural area at this level decreased to 0.91 cm². (  I ) The observations in  H were verified in this subtracted image.