American Journal of Neuroradiology 25:431-440, March 2004
© 2004 American Society of Neuroradiology
SPINE
Magic Angle Effects in MR Neurography
a Department of Imaging, Hammersmith Hospital NHS Trust, London, England
b Department of Histopathology, Hammersmith Hospital NHS Trust, London, England
c Oxford University Centre for Clinical Magnetic Resonance Research, MRS Unit, John Radcliffe Hospital, Oxford, England
d The Robert Steiner Magnetic Resonance Unit, Imaging Sciences Department, MRC Clinical Sciences Centre, Imperial College School of Medicine, London, England and University of San Diego, CA
Address reprint requests to Graeme M. Bydder, MB, ChB, Department of Radiology, UCSD Medical Center, 200 West Arbor Drive, San Diego, CA 92103-8756
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.