MAGIC-ANGLE PHENOMENON - A CAUSE OF INCREASED SIGNAL IN THE NORMAL LATERAL MENISCUS ON SHORT-TE MR-IMAGES OF THE KNEE

OBJECTIVE. Increased signal intensity is frequently present in the ups loping, medial segment of the posterior horn of the normal lateral men iscus on routine short-TE MR images of the knee. This attribute can mi mic or obscure abnormalities in this portion of the meniscus, In the p resent study, we examined whether this appearance could be accounted f or by the ''magic-angle'' phenomenon resulting from the angular orient ation of this meniscal segment relative to the static magnetic field. SUBJECTS AND METHODS. Fifty-eight consecutive knee MR examinations wer e studied. Sixteen were excluded because of frank evidence of preexist ing abnormality of the lateral meniscus. In the remaining 42 examinati ons, the signal intensity in the medial segment of the posterior horn of the lateral meniscus on short-TE (<20 msec) sequences was correlate d with the angular alignment of this meniscal segment relative to the static magnetic field. In five asymptomatic volunteers, meniscal signa l intensities were monitored as the leg was positioned in various degr ees of abduction. RESULTS. Increased signal was present in the medial segment of the posterior horn of the lateral meniscus in 31 (74%) of t he 42 patients. In 25 (81%) of these, this meniscal segment was orient ed at 55-60 degrees. Increased signal intensity was also present in th is meniscal segment in three (60%) of five asymptomatic knees imaged i n the neutral position. In each of these, abduction of the leg decreas ed the meniscal signal by 52-80%. Pathologic evaluation of two menisci showed numerous concentrically arranged collagen fibers. CONCLUSION. Increased signal in the upsloping portion of the posterior horn of the lateral meniscus on short-TE images often is due to the magic-angle p henomenon rather than to meniscal degeneration or tear.