Magic Angle Effects in MR Neurography

BACKGROUND AND PURPOSE: Magic angle effects are well recognized in MR imaging of tendons and ligaments, but have received virtually no attention in MR neurography. We investigated the hypothesis that signal intensity from peripheral nerves is increased when the nerve’s orientation to the constant magnetic induction field (B0) approaches 55° (the magic angle). METHODS: Ten volunteers were examined with their peripheral nerves at different orientations to B0 to detect any changes in signal intensity and provide data to estimate T2. Two patients with rheumatoid arthritis also had their median nerves examined at 0° and 55°. RESULTS: When examined with a short TI inversion-recovery sequence with different TEs, the median nerve showed a 46 –175% increase in signal intensity between 0° and 55° and an increase in mean T2 from 47.2 to 65.8 msec. When examined in 5° to 10° increments from 0° to 90°, the median nerve signal intensity changed in a manner consistent with the magic angle effect. No significant change was observed in skeletal muscle. Ulnar and sciatic nerves also showed changes in signal intensity depending on their orientation to B0. Components of the brachial plexus were orientated at about 55° to B0 and showed a higher signal intensity than that of nerves in the upper arm that were nearly parallel to B0. A reduction in the change in signal intensity in the median nerve with orientation was observed in the two patients with rheumatoid arthritis. CONCLUSION: Signal intensity of peripheral nerves changes with orientation to B0. This is probably the result of the magic angle effect from the highly ordered, linearly orientated collagen within them. Differences in signal intensity with orientation may simulate disease and be a source of diagnostic confusion. Neurography is an expanding application of MR imaging (1, 2). The technique typically involves the use of surface coils and a high-spatial-resolution fat-suppressed T2-weighted or short TI inversion-recovery (STIR) fast spin-echo pulse sequence. Nerves are identified in transverse section by their fascicular pattern. Abnormalities are recognized by differences in shape, size, and contour of the nerves, as well as by the changes in signal intensity. Common applications include the brachial plexus (3–9), the median nerve in