Computed tomography of the post-operative lumbar spine: the need for, and optimal dose of, intravenous contrast medium.

Forty-five patients who had undergone previous lumbar spine surgery with symptoms and signs warranting further investigation were examined by computed tomography (CT). The findings in three separate subgroups of 15 patients were compared. The first randomly selected group underwent CT without the use of intravenous contrast medium; the second and third groups were examined both before and after the administration of contrast medium, using 16.25 g and 32.5 g iodine, respectively. In the latter two groups the CT studies were reported both before and after enhancement. In the 15 patients in the first group (no contrast medium), uncertainty as to whether disc material or fibrosis was responsible for the radiological abnormalities was expressed in the final report at six of the 20 post-surgical sites. Among the 30 patients who received contrast medium, 40 post-surgical sites were examined; at six of these sites there was no radiological cause for concern; at 16 sites there were unequivocal radiological appearances of either disc material or fibrosis. Enhancement resolved the radiological uncertainty at 13 of the remaining 18 sites, almost always by reinforcing the initial radiological opinion; this left five sites where uncertainty persisted in the final report. The larger dose of contrast medium caused the more marked enhancement of those abnormalities considered to be due to fibrosis, although enhancement was readily recognised using the lower dose. While intravenous enhancement assists radiological interpretation at those sites where diagnostic uncertainty persists after routine CT, it would seem that it is not essential in patients in whom routine CT studies show no serious abnormality or the characteristic appearances of disc material or fibrosis.