"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 upsloping, medial segment of the posterior horn of the normal lateral meniscus on routine short-TE MR images of the knee. This attribute can mimic or obscure abnormalities in this portion of the meniscus. In the present study, we examined whether this appearance could be accounted for by the "magic-angle" phenomenon resulting from the angular orientation of this meniscal segment relative to the static magnetic field.

SUBJECTS AND METHODS

Fifty-eight consecutive knee MR examinations were studied. Sixteen were excluded because of frank evidence of preexisting abnormality of the lateral meniscus. In the remaining 42 examinations, the signal intensity in the medial segment of the posterior horn of the lateral meniscus on short-TE (< 20 msec) sequences was correlated with the angular alignment of this meniscal segment relative to the static magnetic field. In five asymptomatic volunteers, meniscal signal intensities were monitored as the leg was positioned in various degrees of abduction.

RESULTS

Increased signal was present in the medial segment of the posterior horn of the lateral meniscus in 31 (74%) of the 42 patients. In 25 (81%) of these, this meniscal segment was oriented at 55-60 degrees. Increased signal intensity was also present in this meniscal segment in three (60%) of five asymptomatic knees imaged in the neutral position. In each of these, abduction of the leg decreased 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 phenomenon rather than to meniscal degeneration or tear.