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 intervertebral disk. MATERIALS AND METHODS: Magnetic resonance (MR) images of a lumbar intervertebral disk in a healthy volunteer were obtained with MR imagers with horizontal and vertical magnetic fields. Thin-section spin-echo images of an excised intervertebral disk were obtained with a horizontal field machine at orientations with respect to the main magnetic field between +90 degrees and -90 degrees. T1 and T2 were measured independently in this disk at two orientations. RESULTS: Images of the in vivo disk demonstrated a variation in the signal intensity of the anulus fibrosus that was different for the two MR units. The images obtained in the excised intervertebral disk demonstrated a signal intensity variation with disk orientation that was most pronounced 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. The magic angle effect in intervertebral disks will be observed with vertical magnetic field MR imagers.