Sequential radiographic and magnetic resonance (MR) imaging examinations were performed in nine patients with an intravertebral vacuum cleft indicative of avascular necrosis. Progressive changes in the content of the cleft occurred within an hour after the patients were placed in a supine position. Initially, the cleft showed a gaslike pattern during extension of the spine, with a radiolucent band on radiographs and a signal void on MR images. Later, the vacuum phenomenon disappeared on radiographs, and a fluidlike high-signal-intensity pattern appeared on T2- or T2*-weighted MR images, suggestive of a slow fluid inflow within the intravertebral cleft. Because the recognition of a vacuum cleft in a collapsed vertebral body helps avoid confusion with malignancy or infection, it is important to search for this in examinations performed immediately after supine positioning.
In 15 hips with typical signs of avascular necrosis of the femoral head on plain radiographs and magnetic resonance (MR) images, gadolinium-enhanced spin-echo and fat-suppressed MR images were obtained and compared with nonenhanced T1- and T2-weighted images. Both enhanced and nonenhanced areas were consistently detected in the abnormal femoral heads. Enhanced areas showed a low signal intensity (SI) on T1-weighted MR images obtained before contrast material was administered and an intermediate to high SI on T2-weighted images. Nonenhanced areas showed an SI either identical (pattern 1) or hypointense (pattern 2) to that of fat on both sequences. Histologic correlation (six resected femoral heads) helped confirm that enhanced and nonenhanced areas corresponded respectively to viable and necrotic tissue. In most cases, SI analysis of nonenhanced T1- and T2-weighted images allows the differentiation of hypervascularized viable tissue from hypovascularized necrotic tissue of the sequestrum.
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