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heads, lunate, capitellum, tarsal navicular and the dome of the talus. Osteonecrosis can be a complication of frostbite (Fig. 8) and high voltage burns.

In AVN imaging with radiophosphate, a sequence of events are predicted, beginning with an initial photopenia that corresponds to the avascular focus and followed in time by an intense circumferential uptake of the reactive reparative phase. Upon completion of revascularization and healing, the uptake returns to normal. Because of the limitations of spatial resolution, the photopenia is not commonly seen in small bones, but may be detected in the femoral capital epiphyses and about the knees. To demonstrate this photopenia, which is characteristic of AVN, imaging must take place early in the evolution of bone necrosis with high resolution SPECT or pinhole collimation. In a comparison of imaging modalities, there were 14 patients with proven AVN but normal plain radiographs, in whom 54% had positive CT scans, 71% had characteristic radiophosphate bone scans and 86% had positive MRIs. In 55 patients with various stages of AVN, positive results were obtained in 78% of the plain radiographs, 87% of the radiophosphate scans, 96% of the CTs and 96% of the MRIs.

Where MRI is readily available it has largely replaced radiophosphate bone imaging in the initial work-up of suspected AVN or infarction because it is more sensitive and specific. AVN can be asymptomatic and be detected incidentally in bone scans ordered for other reasons. Some investigators suggest both MRI and

Figure 8. Frostbite. This patient sustained frostbite osteonecrosis of the hands and feet as a result of being stranded overnight in the mountains in sub-zero temperatures. Radiophosphate bone scans show complete absence of blood flow to the hands and feet, which were subsequently amputated. Arrows point to the demarcation between viable and nonviable bone at the wrist and ankle.

Figure 8. Frostbite. This patient sustained frostbite osteonecrosis of the hands and feet as a result of being stranded overnight in the mountains in sub-zero temperatures. Radiophosphate bone scans show complete absence of blood flow to the hands and feet, which were subsequently amputated. Arrows point to the demarcation between viable and nonviable bone at the wrist and ankle.

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