What is acute pyelonephritis, and what are its CT and MRI features?
Acute pyelonephritis is inflammation of the renal parenchyma and renal pelvis and is most often due to an ascending urinary tract infection and less commonly a hematogenous infection. The diagnosis of acute pyelonephritis should be made clinically, with imaging reserved for patients in whom there is no clinical improvement despite appropriate antibiotic therapy; imaging is then helpful in detecting the development of complications such as a renal abscess.
Heterogeneity of the renal parenchyma during the nephrographic phase of enhancement may be seen as linear or wedge-shaped areas of decreased enhancement alternating with areas of normal enhancement (known as the “striated nephrogram” pattern); this pattern is secondary to tubular obstruction caused by inflammatory debris, interstitial edema, and vasospasm. During the delayed phase of enhancement, a reversal of the striated nephrogram pattern may be seen, where contrast retention is seen in the linear or wedge-shaped areas that were hypoenhancing in the nephrographic phase. On MRI, areas of pyelonephritis appear hypointense on T1-weighted images, appear hyperintense on T2-weighted images, and restrict diffusion on diffusion-weighted images (DWI) relative to renal parenchyma. Other associated findings with acute pyelonephritis may include renal enlargement, inflammatory perinephric fat stranding, and thickening of the renal fascia. When pyelonephritis occurs focally, it may mimic the appearance of a renal mass.