From the archives of the AFIP - Pyelonephritis: Radiologic-pathologic review

Urinary tract infections are the most common urologic disease in the United States and annually account for over 7 million office and I million emergency department visits. In adults, diagnosis of urinary tract infection is typically based on characteristic clinical features and abnormal laboratory values. Imaging is usually reserved for patients who do not respond to therapy and for those whose clinical presentation is either atypical or potentially life threatening. Urinary tract infection typically originates in the urinary bladder; when it migrates to the kidney or is seeded there hematogenously, a tubulointerstitial inflammatory reaction ensues, involving the renal pelvis and parenchyma. The condition is characterized as pyelonephritis. Complicated and uncomplicated pyelonephritis, xanthogranulomatous pyelonephritis, and tuberculosis are all urinary tract infections for which imaging evaluation adds diagnostic information important for patient care. Computed tomography (CT), when performed before, immediately after, and at delayed intervals from contrast material injection, is the preferred modality for evaluating acute bacterial pyelonephritis. CT is also preferred over conventional radiography and ultrasonography (US) for assessing emphysematous pyelonephritis. Xanthogranulomatous pyelonephritis is a chronic granulomatous process, induced by recurrent bacterial urinary tract infection. Although US is useful in the diagnosis of this condition, CT is the main imaging tool, as it provides highly specific findings and accurate assessment of the extrarenal extent of disease, which is essential for surgical planning. The increasing prevalence of tuberculosis and continued emergence of antibiotic-resistant strains have significance for genitourinary radiologists, as the urinary tract is the most common extrapulmonary site of tuberculosis. Familiarity with the renal manifestations of the disease-pelvoinfundibular strictures, papillary necrosis, cortical low-attenuation masses, scarring, and calcification-will help in the diagnosis, even in the absence of documented pulmonary disease.