MR Imaging and T2 mapping of femoral cartilage: In vivo determination of the magic angle efect.

OBJECTIVE. The purpose of this study was to perform a quantitative evaluati on of the effect of static magnetic field orientation on cartilage transver se (T2) relaxation time in the intact living joint and to determine the mag nitude of the magic angle effect on in vivo femoral cartilage. MATERIALS AND METHODS. Quantitative T2 maps of the femoral-tibial joint wer e obtained in eight asymptomatic male volunteers using a 3-T magnet. Cartil age T2 profiles (T2 vs normalized distance from subchondral bone) were eval uated as a function of orientation of the radial zone of cartilage with the applied static magnetic field (B-0). RESULTS. At a normalized distance of 0.3 from bone, cartilage T2 is 8.6% lo nger in cartilage oriented 55 degrees to B-0 compared with cartilage orient ed parallel with B-0. Greater orientation variation is observed in more sup erficial cartilage. At a normalized distance of 0.6, cartilage T2 is 18.3% longer. The greatest orientation effect is observed near the articular surf ace where T2 is 29.1% longer at 55 degrees. CONCLUSION. The effect of orientation on cartilage T2 is substantially less than that predicted from prior ex vivo studies. The greatest variation in cartilage T2 is observed in the superficial 20% of cartilage. Given the sma ll orientation effect, it is unlikely that the "magic angle effect" account s for regional differences in cartilage signal intensity observed in clinic al imaging. We hypothesize that regional differences in the degree of carti lage compression. are primarily responsible for the observed regional diffe rences in cartilage T2.