Figure 312

Sagittal T2WI of a degenerative spine. Note the anteriorly directed discal herniation (arrow).

MRI examination will show the herniated disc to be isointense to the parent disc on T1-weighted images and isointense or hyperintense on T2-weighted films. On sagittal views the herniated disc may be seen lifting the anterior longitudinal ligament away from the anterior vertebral wall (Figure 3-12).

The direction of herniation can be easily determined on axial cuts. First one has to look for the facet joint in the particular axial image to ascertain that the cut is actually passing through the disc. The most common herniations, those that are directed posterolater-ally, are fairly easily detected on CT and MRI studies. A herniated disc may be seen continuous with the "mother disc" or separated altogether from it (sequestered herniation) while retaining the same density and signal characteristic of the "mother disc" on CT and MRI studies (Figure 3-13). The herniated disc may deform the thecal sac, spinal cord, or roots. Following contrast administration the disc does not enhance.

Less frequently a "true" lateral disc herniation may occur. In a true lateral disc herniation, a piece of a disc herniates into the neural foramen, impinging on the exiting nerve root and compressing the dorsal root ganglion, which is located within the nerve root canal (Figure 3-14). This occurs more commonly at the L3-L4 level. Normally, the nerve roots are surrounded by fat within the neural foramen. This is clearly seen especially on MRI scans, in which the fat has a bright signal on T1-weighted images whereas the nerve root is darker (Figure

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