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Figure 14b.  Emphysematous pyelonephritis. (a) Transverse US image through the middle of the left kidney shows high-amplitude echoes (long arrow) representing air along the nondependent aspect of a parenchymal fluid collection (*). The more posteriorly located perinephric air has a similar appearance (short arrows). (b) Contrast-enhanced CT scan (enteric contrast material) clearly depicts air within the kidney (*) and perinephric space (arrow).