Patten R M
Department of Radiology, University of Washington School of Medicine, Seattle 98195.
AJR Am J Roentgenol. 1994 Jun;162(6):1383-6. doi: 10.2214/ajr.162.6.8192004.
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.
在外旋位获取的肩部轴向梯度回波（GRE）磁共振成像（MR）可能显示盂肱关节间隙上部有曲线形或圆形低信号灶，被认为代表关节内气体（真空现象）。本研究旨在确定该表现在肩部GRE MR图像上的发生率，确认其代表关节内气体，并建立将其与真正异常相鉴别的标准。
我们回顾了45例连续患者和10名志愿者的所有外旋GRE图像，观察低信号灶和关节积液情况。将这些表现与手臂处于中立位时肩部相应的自旋回波和GRE图像表现进行对比。MR表现还与37例患者的平片及3例患者和5名志愿者的肩部CT扫描结果以及19例患者的关节镜检查结果进行对比。
在44例患者中的9例（20%）以及10名志愿者中的5例（50%），轴向外旋GRE图像显示在盂肱关节上方相对的盂肱关节面之间有曲线形或圆形低信号灶，而在手臂处于中立位时获取的MR图像上未见此表现。在3例患者中的2例以及所有5名志愿者中，肩部CT扫描显示关节内有小的气体灶，其位置与MR图像上所见的关节内低信号灶相对应。15例患者的MR图像上可见关节积液，且这些图像均未显示关节内气体。
在外旋手臂获取的肩部GRE MR图像上经常可见圆形或线性低信号区，代表关节内气体小灶（真空现象）。认识到这一潜在陷阱并了解关节内气体聚集的特征性表现和位置，可能有助于防止对关节内游离体或软骨钙质沉着症的误诊。
