Joseph Junewick, MD FACR
over 6 years ago
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Joseph Junewick, MD FACR
|Diagnostic Category: Infectious-Inflammatory
|Created: over 3 years ago
|Updated: 8 months ago
Female infant with urinary tract infection with break through fevers.
US – Multiple round thick-walled complex hypovascular renal lesions.
CT – Multiple lamellated round hypodense renal lesions
Pyelonephritis, defined as a tubulointerstitial inflammatory response involving the renal pelvis and parenchyma, is most commonly caused by bacteria (usually Escherichia coli) ascending from the urinary bladder or seeding the kidney hematogenously. Generally, while not required for diagnosis or treatment, radiologic imaging may further reveal the nature and extent of disease, as well as abscesses, obstructions, or other complications.
On CT imaging, acute bacterial pyelonephritis most commonly presents as one or more wedge-shaped areas or streaky zones of reduced enhancement that extends from the renal papilla to the cortex. This pattern of reduced enhancement during the nephrographic phase is explained by the reduced flow of contrast material due to the underlying inflammatory reaction. For 3-6 hours after the administration of contrast, hyperattenuating wedge-shaped defects are observed in the same areas that previously displayed hypoattenuation, again due to the decreased flow of contrast material. Renal enlargement is also observable. Abscess cavities may be either intra- or extra- parenchymal, and appear on CT as round low-attenuation collections, with or without a ring of enhancement that surrounds a non-enhancing central region. When CT imaging reveals peripheral low-attenuation renal lesions, hematogenous seeding should be considered.
Ultrasonography is occasionally used to evaluate patients with clinical features of pyelonephritis but is not well characterized in adults on gray-scale examination. US findings may include hydronephrosis, renal enlargement, loss of renal sinus fat due to edema, changes in echogenicity (hypoechoic and hyperechoic regions), loss of corticomedullary differentiation, abscess formation, and areas of hypoperfusion. Occasionally, in acute bacterial pyelonephritis, areas of abnormal echogenicity may take on a mass-like appearance. When evaluating for pyelonephritis using US, the urinary bladder should always be imaged in order to estimate outflow obstruction. Tissue harmonic imaging may show patchy, hypoechoic lessons that are either focal or segmental, and extend from the medulla to the renal capsule. Abscesses typically appear as hypoechoic masses with through transmission that lack internal flow on color Doppler flow imaging.
Craig, WD, Wagner, BF, and Travis, MD. Pyelonephritis: Radiologic-Pathologic Review. RadioGraphics (2008);28: 255-276.