INTERVERTEBRAL DISKS ON MR-IMAGES - VARIATION IN SIGNAL INTENSITY WITH THE DISK-TO-MAGNETIC FIELD ORIENTATION

PURPOSE: To demonstrate and quantitate the magic angle effect in the i ntervertebral disk. MATERIALS AND METHODS: Magnetic resonance (MR) ima ges of a lumbar intervertebral disk in a healthy volunteer were obtain ed with MR imagers with horizontal and vertical magnetic fields. Thin- section spin-echo images of an excised intervertebral disk were obtain ed with a horizontal field machine at orientations with respect to the main magnetic field between +90 degrees and -90 degrees. T1 and T2 we re measured independently in this disk at two orientations. RESULTS: I mages of the in vivo disk demonstrated a variation in the signal inten sity of the anulus fibrosus that was different for the two MR units. T he images obtained in the excised intervertebral disk demonstrated a s ignal intensity variation with disk orientation that was most pronounc ed in the anterior portion of the anulus. Relaxation time measurements showed the signal intensity reduction to arise from a reduction in T2 with oblique orientations. CONCLUSION: The observed signal intensity variation with disk orientation arises from an anisotropy in T2 caused by the restriction of water associated with collagen in the anulus. T he magic angle effect in intervertebral disks will be observed with ve rtical magnetic field MR imagers.