Two segments of the supraspinous muscle: cause of high signal intensity at MR imaging?

A zone of increased signal intensity seen in asymptomatic subjects near the insertion of the supraspinous tendon onto the greater tuberosity ("pseudogap") leads to difficulty in the interpretation of magnetic resonance (MR) images of the shoulder. To better understand this pseudogap, the authors dissected and sectioned the shoulders of four cadavers and analyzed 20 MR images of patients' shoulders. They found that the supraspinous muscle consists of two distinct portions: an anterior fusiform portion that contains the dominant tendon and a straplike posterior portion. The orientation of the dominant tendon differs from that of the main muscle by approximately 10 degrees. In addition, oblique-coronal MR images were obtained parallel to the supraspinous main muscle and the central tendon in six volunteers; the appearance of the pseudogap was similar with both projections. Fat-saturated and gradient-echo images of the shoulder were obtained with increasing echo times in four additional volunteers. There was no evidence of fat within the pseudogap. Because the anatomic composition of the pseudogap could not be explained with partial volume averaging with adjacent muscle or fat, the authors conclude that it may represent a manifestation of unique focal tissue relaxation parameters or depend on the tendon orientation in the main magnetic field.