In the early postoperative period, interpretation of the MR images is extremely difficult. The presence of fat graft, hematoma, gas, and inflammation complicates the observed signal intensities. Moreover, recurrent disk and epidural scar exhibit similar topographical and signal characteristics. Endnote After about one month, the acute postoperative changes resolve, making it easier to distinguish scar from disk. As before surgery, recurrent disk is often in continuity with the parent disk. Discontinuity in the anulus fibrosus is not entirely reliable because it can result from the surgical incision as well as from disk rupture. Unless the disk material has become separated as a free fragment or sequestration, it remains similar in signal characteristics to the parent disk on both T1- and T2-weighted images. In general, herniated disks are relatively well-defined and, in some cases, have a hypointense rim.

      On the other hand, epidural scar has poorly defined margins and is either isointense or hypointense on short TR/TE sequences compared to the adjacent disk. With more T2-weighting, scar generally increases in signal, but to a lesser degree many months or years after surgery. In addition, if the soft-tissue abnormality can be followed posteriorly along the lateral margin of the spinal canal to the region of the laminectomy, it is probably scar. Retraction of the thecal sac to the side of the soft tissue is another sign favoring postoperative scar.

      Gadolinium should be used routinely in the postoperative back because it is a valuable aid for differentiating the various postoperative tissues. Epidural scar enhances to a much greater degree on MR than on contrast-enhanced CT. The enhancing scar clearly identifies nerve roots trapped within the scar and outlines any retained or recurrent disk fragments. A disk fragment induces a local inflammatory reaction, and vascular granulation tissue often forms about its perimeter. As a result, the perimeter of a herniated disk may enhance with gadolinium, but the central part will not, thus distinguishing it from epidural scar. Endnote  

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