Vacuum phenomenon: a potential pitfall in the interpretation of gradient-recalled-echo MR images of the shoulder

Axial gradient-recalled-echo (GRE) MR images of the shoulder obtained with the arm in external rotation may show a curvilinear or circular low-signal-intensity focus in the superior portion of the glenohumeral joint space that is thought to represent gas within the joint (vacuum phenomenon). This study was performed to determine the prevalence of this finding on GRE MR images of the shoulder to confirm that it represents gas in the joint, and to establish criteria to distinguish it from a true abnormality.We reviewed all external-rotation GRE images of 45 consecutive patients and 10 volunteers for the presence of low-signal-intensity foci and joint effusion. These findings were correlated with findings on corresponding spin-echo and GRE images of the shoulder with the arm in neutral position. MR findings also were correlated with findings on plain radiographs (37 patients) and shoulder CT scans (three patients and five volunteers) and with results of arthroscopy (19 patients).In nine (20%) of 44 patients and in five (50%) of 10 volunteers, axial external-rotation GRE images showed curvilinear or circular low-signal-intensity foci interposed between the opposing glenohumeral articular surfaces at the superior aspect of the glenohumeral joint that were not seen on MR images obtained with the arm in neutral position. In two of three patients and in all five volunteer subjects, CT scans of the shoulder showed a small focus of gas within the joint, corresponding in location to the intraarticular low-signal-intensity focus seen on MR images. Joint effusions were seen on MR images of 15 patients, and none of these images showed intraarticular gas.Circular or linear areas of low signal intensity are frequently seen on GRE MR images of the shoulder obtained with external rotation of the arm and represent small foci of intraarticular gas (vacuum phenomenon). Awareness of this potential pitfall and an understanding of the characteristic appearance and location of this collection of intraarticular gas may help prevent misdiagnosis of intraarticular loose bodies or chondrocalcinosis.