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The Magic-angle Effect of the Supraspinatus Tendon

Context Rotator cuff injuries are common and result in either a tear or tendinopathy within the distal portion of the supraspinatus tendon. Magnetic resonance (MR) imaging shows the injury as a high signal region within the distal tendon. Similarly, a magic-angle effect also appears within healthy patients in this same region on T1- or proton density (PD)-weighted MR sequences.

Objective This study was conducted to evaluate the prevalence of the magic-angle effect found with MR imaging within the supraspinatus tendon.

Methods In this study, 300 consecutive patients were selected from those symptomatic patients referred for MR evaluation of the shoulder using a 1.5T unit and both PD and T2-weighted sequences. To eliminate patients with a real injury, the written reports were reviewed; those with positive findings for injury to the supraspinatus tendon were eliminated from the sample group, leaving 164 patients. Two radiographers independently examined the oblique coronal images nearest the center of the supraspinatus tendon and compared the signal on PD and T2-weighted images. Images found to have a higher signal with the PD sequence were classified as having the magic-angle effect since a real injury would be more likely to have a strong signal on T2-weighted images.

Results Of the 164 patients evaluated, both reviewers found the same 8 patients to have the magic-angle effect.

Summary Based on these results, our study suggests the artifact will appear in 5% of healthy patients and may lead to false-positive results on oblique coronal PD-weighted images through the supraspinatus tendon. In comparison with previous studies that have shown a much greater incidence rate, our findings also suggest that external rotation of the arm greatly reduces the incidence of the magic-angle effect.