图 1　仰卧位扫描时 ,双侧肺外围与后胸壁间出现密度均匀的新月状增高影。　图 2　俯卧位扫描时 ,在同一层面的肺外围与前胸壁间出现密度均匀的新月状高密度影 ,其后胸壁与肺外围间的病灶随即消失。游离性胸腔积液在常规Ｘ线诊断中是不困难的。ＣＴ检查对于反映有无游离性胸腔 Figure 1 supine position scan, the bilateral lung periphery and posterior chest wall appear uniform density of crescent-shaped shadow. Figure 2 prone position scan, in the same level of the outer wall of the lung and the anterior chest wall with uniform density of crescent-shaped high density, then the chest wall and peripheral lung lesions immediately disappear. Free pleural effusion in the conventional X-ray diagnosis is not difficult. CT examination for the presence or absence of response to the chest