INCREASED SIGNAL IN THE NORMAL SUPRASPINATUS TENDON ON MR-IMAGING - DIAGNOSTIC PITFALL CAUSED BY THE MAGIC-ANGLE EFFECT

OBJECTIVE. Increased signal intensity within the distal portion of the supraspinatus tendon during MR imaging is a frequent observation even in healthy subjects. This finding has been variously attributed to th e presence of fat, muscle, connective tissue, abnormal vascularity, or degenerative changes. More recently, the effect of tendon orientation in the static magnetic field (B-o) has been implicated. It has been s hown that tendons at the magic angle of 55 degrees to B-o show markedl y increased signal. This study was designed to determine the contribut ion of the magic-angle effect to the NIR signal in the distal portion of the supraspinatus tendon. SUBJECTS AND METHODS. Five healthy Volunt eers were imaged in a 1.5-T unit using short TR/TE sequences in standa rd supine position; they were then reimaged laterally flexed at the wa ist to reorient the plane of the distal portion of the supraspinatus t endon by approximately 20 degrees relative to B-o. In the second part of the study, three cadaveric shoulders were similarly imaged, first i n standard position and then reoriented approximately 35 degrees by si mple rotation of the specimen. The supraspinatus tendon was evaluated in each subject by noting the length of the segment with increased sig nal and the position of this segment relative to the insertion of the tendon on the greater tuberosity. Comparisons were made for each live and cadaveric subject between neutral and rotated positions. RESULTS. Segments of increased signal changed in length and position for each l ive and cadaveric subject from the neutral to the reoriented position. CONCLUSION. Our study suggests that tendon orientation contributes si gnificantly to the presence of increased signal within the supraspinat us tendon, as caused by the magic-angle effect. Failure to recognize t his effect may lead to diagnostic inaccuracy when evaluating the rotat or cuff on short TR/TE sequences.