The components of the intervertebral disk serve two purposes. The hydrophilic matrix is contained by circumferential rings of fibrocartilage (outer anulus) and Sharpey's fibers. The gellike hydrophilic matrix acts as a cushion between the vertebral body endplates by providing an even distribution of axial and rotational compressive forces. The construction and integrity of the intervertebral disk are assessed well by MR. The purpose of this pictorial essay is to show the in vitro and in vivo MR appearances of the normal and degenerated disk, and to correlate these MR changes with their respective histopathologic findings.

The in vivo and ex vivo microanatomic appearance of early disc degeneration were identified by magnetic resonance imaging and correlated with their respective histopathologic findings. Five cadaver spines (18 discs) and 25 patient studies (122 discs) all imaged at 1.5 Tesla were studied. Two signs of early degenerative disc disease were found: infolding and the central dot. Infolding of the central fibers of the outer annulus coalesced into a central dot of low signal intensity that was seen on both the ex vivo and in vivo images. Infolding was seen 29 of 122 times, and the central dot was observed 15 to 122 times on the in vivo images. A later form of degenerative disc disease was identified as a separation of the nucleus pulposus from the hyaline cartilage end-plate. This separation was seen as a linear area of either low or high signal intensity on the ex vivo images but only as a band of high signal intensity on the in vivo spin-echo 2,500-msec/80-msec images. Only 7 of 122 in vivo discs showed this separation. Internal herniation of nucleus pulposus into the outer annulus was seen only on the ex vivo images. Early degenerative disc disease may exist before there is loss of disc height or signal intensity on the long time-to-repetition (TR)/time-to-echo (TE) magnetic resonance images.

The appearance on magnetic resonance (MR) images of the normal pars interarticularis in 13 patients was reviewed and contrasted with that of the pars in eight patients with spondylolysis. The pars defect usually had an intermediate signal intensity with all pulse sequences; however, this intensity was somewhat variable depending on the exact ratio of cartilage, fat, and fluid within each bone defect. The pars defect was best seen with spin-echo 600/20 (repetition time msec/echo time msec) images. In three cases, out-of-phase images showed the spondylolysis best, because of extension of fat to the borders of the defect. The sagittal view allowed one to separate spondylolysis from the joint space of posterior facets since the orientation of the defects is perpendicular to the facets; thus, a common pitfall encountered with cross-sectional axial imaging techniques is avoided. MR imaging poorly delineated bone fragments around the defect, which may produce nerve root impingement, but revealed other numerous complications that occur with spondylolysis, including spondylolisthesis and herniation of the disk above.

A retrospective study of the magnetic resonance (MR) images of the lumbar spines of 13 healthy subjects and 30 patients with degenerative changes was done. In the healthy subjects, the vertebral facets, thickness of the cartilage and ligamentum flavum, signal characteristics of the bone marrow, and size of the spinal canal were studied. In the patients with degenerative changes in one of these structures, MR images in the sagittal plane were useful in demonstrating hypertrophy of the ligamentum flavum or the vertebral facets, in grading the degree of foraminal stenosis, and in measuring the sagittal diameter of the spinal cord. MR images in the axial plane facilitated detailed analysis of the facet joint and more accurate measurements of the thickness of the ligamentum flavum and spinal canal diameter. MR images were compared with computed tomography scans in 12 patients.

Magnetic resonance (MR) images of the lumbar spine from 150 patients were retrospectively reviewed. In 14 of these patients, at 18 disk levels, a vacuum phenomenon (VP) had been identified on plain radiographs and/or computed tomographic scans. The MR imaging appearance of these gas collections in 17 disks was an area without signal, best seen on spin-echo sequences with short repetition time and echo time in the sagittal view. MR imaging precisely located the VP in the anulus fibrosus, the nucleus pulposus, and Schmorl nodes. In all but one case, degeneration of the disk was complete and associated with adjacent changes in vertebral bone. Pitfalls of MR imaging detection of VP included chemical shift artifact, calcifications, and tears without gas in the disk.