US Color Doppler Kidneys and Bladder Retroperitoneal US color doppler of the kidneys, bladder, and retroperitoneum is not beneficial in the initial imaging evaluation for the first-time presentation of suspected APN in an uncomplicated patient [15]. Variant 2 : Suspected acute pyelonephritis. Complicated patient (eg, recurrent pyelonephritis, diabetes, immune compromise, adv anced ag e, vesicoureteral reflux, or lack of response to initial therapy). Initial imaging. The goal of imaging in a complicated patient with suspected APN is to identify the presence or absence of APN and identify associated complications. Patients with a history of renal calculi are separately discussed in variant 3. CT Abdomen and Pelvis There is widespread agreement that CT of the abdomen and pelvis with IV contrast is a useful study to diagnose APN in a complicated patient without a prior history of stone disease [1,8,14-21]. CT imaging should include the pelvis for multiple reasons. For example, unsuspected urolithiasis can be in the distal ureters or urinary bladder, congenital abnormalities of the distal ureters and abnormal insertion sites can be identified, and abnormalities of the urinary bladder can be detected, among other potential sources of APN. Contrast-enhanced CT can be used to detect signs of APN including focal or multifocal decreased parenchymal enhancement, complications of APN including a renal or perirenal abscess or emphysematous pyelonephritis, and underlying problems including hydronephrosis, obstructing stones, or congenital abnormalities [1,8,14,17-19]. In a retrospective study of patients with suspected APN that underwent both unenhanced and contrast-enhanced CT (n = 183), contrast-enhanced CT detected parenchymal inv olvement in 62.5% of patients, whereas unenhanced CT detected parenchymal involvement in only 1.4% of cases, and 4.6% of patients had renal abscesses that were missed on unenhanced CT and on ly detected on contrast-e nhanced CT [19] . Furthermore, un enhanced CT missed the
Acute Pyelonephritis. US Color Doppler Kidneys and Bladder Retroperitoneal US color doppler of the kidneys, bladder, and retroperitoneum is not beneficial in the initial imaging evaluation for the first-time presentation of suspected APN in an uncomplicated patient [15]. Variant 2 : Suspected acute pyelonephritis. Complicated patient (eg, recurrent pyelonephritis, diabetes, immune compromise, adv anced ag e, vesicoureteral reflux, or lack of response to initial therapy). Initial imaging. The goal of imaging in a complicated patient with suspected APN is to identify the presence or absence of APN and identify associated complications. Patients with a history of renal calculi are separately discussed in variant 3. CT Abdomen and Pelvis There is widespread agreement that CT of the abdomen and pelvis with IV contrast is a useful study to diagnose APN in a complicated patient without a prior history of stone disease [1,8,14-21]. CT imaging should include the pelvis for multiple reasons. For example, unsuspected urolithiasis can be in the distal ureters or urinary bladder, congenital abnormalities of the distal ureters and abnormal insertion sites can be identified, and abnormalities of the urinary bladder can be detected, among other potential sources of APN. Contrast-enhanced CT can be used to detect signs of APN including focal or multifocal decreased parenchymal enhancement, complications of APN including a renal or perirenal abscess or emphysematous pyelonephritis, and underlying problems including hydronephrosis, obstructing stones, or congenital abnormalities [1,8,14,17-19]. In a retrospective study of patients with suspected APN that underwent both unenhanced and contrast-enhanced CT (n = 183), contrast-enhanced CT detected parenchymal inv olvement in 62.5% of patients, whereas unenhanced CT detected parenchymal involvement in only 1.4% of cases, and 4.6% of patients had renal abscesses that were missed on unenhanced CT and on ly detected on contrast-e nhanced CT [19] . Furthermore, un enhanced CT missed the