MR imaging of lumbar arachnoiditis.

To assess the usefulness of MR in defining the changes of lumbar arachnoiditis, we reviewed retrospectively the MR, plain-film myelographic, and CT myelographic findings in 100 patients referred for evaluation of failed-back-surgery syndrome. In 11 of 12 cases of arachnoiditis demonstrated by plain-film and CT myelography, an abnormal configuration of nerve roots was seen by MR. The correlated MR and CT and plain-film myelographic changes were divided into three anatomic groups: group 1 showed conglomerations of adherent roots residing centrally within the thecal sac, group 2 demonstrated roots adherent peripherally to the meninges giving rise to an "empty-sac" appearance, and group 3 demonstrated a soft-tissue mass replacing the subarachnoid space. There was one false-negative MR study. For the diagnosis of moderate to severe arachnoiditis, we found MR to correlate excellently with CT myelographic and plain-film myelographic findings.