Findings of acute pyelonephritis (AP) are almost always asymmetric

Renal enlargement with loss of corticomedullary differentiation

Geographic areas of altered echogenicity

Urothelial thickening

In general, ultrasound is much more sensitive for causes (obstruction) and complications (abscess) of AP than for AP itself, which is clinical diagnosis

Many kidneys with pyelonephritis will be sonographically normal

Foci of gas in parenchyma (rare) could indicate emphysematous pyelonephritis; treat as urologic emergency

Altered nephrogram on CT, classically striated, best seen in excretory phase

Microabscesses or areas of necrosis can emerge after 1-2 weeks of infection

Most common organism: Escherichia coli

Route of spread of infection: Ascending (85%) > hematogenous (15%)

Risk factors include obstruction, ureteric reflux, diabetes, pregnancy, lower UTI

Positive urine culture for bacilli is typical

Remember, especially in children, absence of lower UTI does not exclude pyelonephritis

Pyelonephritis usually asymmetric; sonographic changes may be subtle in acute setting

Focused US evaluation for ureteral stones if AP is suspected, including transvaginal images for distal ureter stones, because presence of stones would alter management

Higher frequency linear transducers, especially in thin patients, may help identify subtle areas of involvement

Best acoustic windows are often through liver or spleen but evaluation with different acoustic windows important for full evaluation