"Acute pyelonephritis (APN) is an infection of the upper urinary tract, specifically the renal parenchyma and renal pelvis. It accounts for approximately 300,000 emergency department (ED) visits per year (10% of total ED urinary tract infection (UTI) visits) and has a mortality rate of up to 9.8%.1–3 Its direct and indirect costs amount to $2.14 billion dollars a year.1–3 APN has an incidence of 15-17 cases per 10,000 females and 3-4 cases per 10,000 males.4 There is a higher risk among young sexually active women, elderly, infants, and pregnant women.4 It most commonly manifests with sudden onset of symptoms of systemic inflammation (fever, chills, malaise) along with symptoms of lower urinary tract inflammation (increased frequency, dysuria, and urgency).4"
Take a look at article published in emDocs " CT Imaging in Pyelonephritis: Pearls & Pitfalls"