What is acute pyelonephritis, and what are its CT and MR imaging features?
Acute pyelonephritis is an infection of the kidney and renal pelvis. Patients often present with fever, chills, and flank pain and tenderness. It is most often due to ascending retrograde infection via the bladder, ureter, and collecting system with subsequent infection of the renal parenchyma.
CT and MRI findings may include normal-appearing kidneys; focal striated or wedge-shaped areas in the kidneys with increased T2-weighted signal intensity, restricted diffusion, or hypoattenuation/hypointensity on nephrographic phase contrast-enhanced images, resulting in a heterogeneous “striated nephrogram” pattern; a reversal of the striated nephrogram pattern on delayed phase contrast-enhanced images related to stasis of contrast material within edematous tubules; enlargement of the kidneys; perinephric inflammatory fat stranding; renal fascial thickening; and renal pelvic and ureteral thickening with hyperenhancement ( Figure 32-7 ). Sometimes, associated hydroureteronephrosis (dilation of the collecting system and ureter), renal infarction, or renal or perinephric abscess formation may be seen.