Abdominal pain & fever.

Acute right pyelonephritis. CT when performed before, immediately after, and at delayed intervals from contrast material injection, is the preferred modality for evaluating acute bacterial pyelonephritis. After administration of contrast material, acute bacterial nephritis most commonly manifests as one or more wedge-shaped areas or streaky zones of lesser enhancement that extend from the papilla to the renal cortex. This pattern of differential enhancement reflects the underlying pathophysiology of tubular obstruction caused by inflammatory debris within the lumen, interstitial edema, and vasospasm. All three of these pathophysiologic disturbances tend to decrease the flow of contrast agent through the tubule, which also helps explain the pattern of delayed and persistent enhancement seen 3–6 hours after administration of contrast material. The sites that originally demonstrated reduced attenuation during the nephrographic phase transmute from hypoattenuation to hyperattenuation wedge-shaped defects because of the prolonged accumulation of contrast agent that slowly transits the compromised tubules.

From the Archives of the AFIP: Pyelonephritis: Radiologic-Pathologic Review. RadioGraphics. 2008;28(1):255-276.