"Magic-angle" phenomenon: a cause of increased signal in the normal lateral meniscus on short-TE MR images of the knee.

Abstract

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.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 o...