Pyelonephritis Overview:

• Pyelonephritis - the inflammation of the renal pelvis and parenchyma, usually secondary to an ascending urinary tract infection (UTI) caused by gram-negative bacteria. While uncomplicated UTIs typically require no imaging, contrast-enhanced CT (CECT) is the gold standard for assessing pyelonephritis severity in high-risk patients (i.e., diabetes, immunocompromised, history of stones or prior renal surgery) or in patients lacking response to appropriate therapy and in evaluating for complications of pyelonephritis. [1]
• Acute Pyelonephritis - a sudden and severe kidney infection that can be focal or diffuse. The infection triggers inflammation, vasospasm, tubular obstruction and interstitial oedema, leading to areas of poorly or nonfunctioning parenchyma.
• Chronic Pyelonephritis - a persistent condition caused by recurrent infections. It leads to renal scarring, fibrosis and progressive loss of function, potentially resulting in end-stage kidney disease. [1, 2]

Complications of Pyelonephritis:

• Intrarenal abscess - if left untreated or inadequately managed, pyelonephritis can progress to tissue necrosis and liquefaction, leading to abscess formation. Small collections may merge into a larger cavity and in the absence of obstruction, abscess drainage typically occurs through the calyces and ureter. [3, 1]
• Perinephric, paranephric, subdiaphragmatic or pelvic abscesses - organised collections of purulent material that can result from the rupture of a renal abscess or the extrarenal extension of acute pyelonephritis. Confined within Gerota’s fascia, perirenal abscesses are more common in patients with risk factors such as diabetes and septic emboli. [1] Abscesses may further breach through the anterior or posterior renal fasciae and involve other retroperitoneal compartments, leading to extensive abscess collections. [4]
• Pyonephrosis - is characterised by purulent accumulation in the obstructed, hydronephrotic kidney, requiring urgent drainage. It is linked to compromised host defences, as well as local urinary tract abnormalities and pregnancy. CT helps identify hydronephrosis and its causes; however, distinguishing pyonephrosis from noninfected hydronephrosis can be challenging, especially in patients who cannot undergo CECT. [5, 3]
• Emphysematous pyelonephritis - is a rare form of acute necrotising pyelonephritis, typically associated with diabetes mellitus, with a high mortality rate. The disease is characterised by gas accumulation in the renal parenchyma, collecting system, and perinephric tissue (not associated with interventional procedures). [2]
• XGP - a rare, aggressive variant of chronic granulomatous pyelonephritis associated with long-term urinary tract obstruction, infection and staghorn calculi. [1] XGP is characterised by an atypical immune response, leading to the deposition of lipid-laden macrophages at the infection site, resulting in irreversible renal parenchymal damage. Two forms of XGP have been described - the diffuse form may extend to perirenal tissues and the retroperitoneum. The less common localised form appears as a circumscribed swelling and is sometimes referred to as pseudotumoral, as it closely resembles conditions like renal cell carcinoma. [2, 3]

We retrospectively reviewed CECT imaging using a multiphasic protocol, including non-contrast, corticomedullary, nephrogenic and excretory phases. Among the multiple cases seen in our institution, we selected those showcasing relevant radiological patterns for acute, chronic and complicated pyelonephritis.