Digital subtraction in contrast-enhanced MR imaging of the postoperative lumbar spine

The objective of this study was to evaluate the routine use of digital image subtraction as an adjunct to standard MR sequences in patients with failed lumbar spine surgery.

Unenhanced and contrast-enhanced T1-weighted MR images of 112 consecutive patients with failed back surgery were reviewed, and corresponding axial images at the level of previous surgery were digitally subtracted by using standard software. The technique was successful in 96 patients and was abandoned in 16 owing to patient motion. Two radiologists independently assessed the subtracted images for areas of enhancement not seen on T1-weighted images, and suggested a diagnosis based on a combination of the findings on standard and subtracted images. This diagnosis was compared with the original MR diagnosis, and surgical findings were sought in cases of conflict.

Enhancement was best seen on the subtracted images in muscle fascial planes (100%), anterior epidural fibrosis (55%), spinal canal fat (31%), posterior epidural fibrosis (27%), types I and II marrow change (24%), and facet joints (17%). Subtraction improved the homogeneity of contrast enhancement, thereby improving the visualization of nerve roots lying in scar tissue (16%), and better defined the extent of epidural fibrosis, particularly when the fibrosis was contiguous with fat. The subtracted image increased diagnostic confidence in 25% of cases, but altered final diagnosis in only two patients.

Although digital subtraction revealed areas of enhancement not seen on standard spin-echo MR images, it rarely altered final diagnosis and does not appear to be useful for routine imaging of patients with failed lumbar spine surgery. It might, however, be useful for increasing diagnostic confidence or as a problem-solving technique in selected patients.