High-resolution MR imaging of sequestered lumbar intervertebral disks.

We retrospectively reviewed the MR examinations of 20 patients with surgically documented sequestered lumbar disks (free fragments). Sixteen of 20 cases demonstrated extradural masses that were distinct from the interspace of origin and had intermediate signal on T1-weighted images but increased signal on T2-weighted images. In 12 cases there was migration of the sequestered fragment. Sequestered disks that migrated away from the interspace had an irregular, oblong appearance, while those near the interspace were round in configuration. Additionally, the interspace of origin consistently demonstrated loss of signal on T2-weighted images when compared with intact lumbar disks. Sagittal 12-weighted images best depicted the absence of a high-signal pedicle contiguous with the interspace of origin in sequestered disks. These findings were applied to a prospective group of 20 patients undergoing lumbar diskectomy. There was an 89% sensitivity, 82% specificity, and 85% accuracy for MR in distinguishing sequestered disks from other forms of lumbar disk herniation. We conclude that high-resolution MR imaging is sensitive in detecting disk disease and specific in characterizing various subtypes of extradural defects. MR uses morphology as well as changes in signal intensity to make these distinctions.