Pyelonephritis

Pyelonephritis is infection of the renal parenchyma and collecting system, almost always ascending from the lower urinary tract. On imaging — most often contrast-enhanced CT — acute pyelonephritis appears as one or more wedge-shaped or striated zones of diminished enhancement radiating from the papilla to the cortical surface, reflecting tubular obstruction, interstitial edema, and vasospasm rather than true tissue necrosis. Imaging is not required to diagnose uncomplicated acute pyelonephritis in an otherwise healthy patient who responds to antibiotics, but it becomes essential when the patient is diabetic, immunocompromised, has an obstructing stone, fails to defervesce within 48–72 hours of therapy, or when abscess, emphysematous pyelonephritis, or xanthogranulomatous pyelonephritis (XGP) is a concern.

Quick Reference

• First-line imaging: contrast-enhanced CT (best sensitivity for parenchymal and perinephric findings); ultrasound as an initial, radiation-free screen, especially in pregnancy and children; MRI (including DWI) when iodinated contrast is contraindicated.
• Key US signs: raised echogenicity of the renal sinus fat (inflammatory infiltration), pelvicalyceal wall thickening, and focal/global loss of corticomedullary differentiation; look also for secondary ureteritis (thickened, echogenic ureteral wall) and cystitis (thickened, irregular bladder wall) as clues to the ascending source of infection.
• Classic CT sign: striated nephrogram — alternating bands of hyper- and hypoattenuating renal parenchyma on nephrographic-phase imaging.
• Core pattern: wedge-shaped, poorly marginated area(s) of decreased enhancement extending from papilla to capsule, with preserved renal contour (no mass effect) in uncomplicated disease.
• Complicated forms to actively exclude: renal/perinephric abscess, emphysematous pyelonephritis (gas-forming), xanthogranulomatous pyelonephritis (obstructing calculus + destroyed kidney), and pyonephrosis (infected obstructed system — a surgical emergency).
• Emphysematous pyelonephritis — Huang-Tseng CT classes: Class 1 = gas confined to collecting system only (this is actually emphysematous pyelitis, favorable); Class 2 = gas in renal parenchyma without extension; Class 3A = perinephric gas/abscess extension; Class 3B = extension beyond Gerota fascia; Class 4 = bilateral disease or solitary kidney with EPN. Higher class correlates with worse prognosis and higher likelihood of needing nephrectomy.
• DWI performance: diffusion-weighted MRI reported sensitivity ~95% for acute pyelonephritis versus ~67–88% for non-contrast/contrast-enhanced CT in one prospective series, and is useful when contrast is contraindicated (renal dysfunction, allergy).
• Extrarenal CT clues in severe disease: thickened gallbladder wall, periportal tracking/edema, dilated IVC, pleural effusion, thickened interlobular septa, ascites — reported in the majority of severe acute pyelonephritis cases in one CT series.
• Report impact: presence of abscess, gas, or obstructing calculus changes management from IV antibiotics alone to drainage, decompression (stent/nephrostomy), or nephrectomy — always state size/location of any collection and whether Gerota fascia is breached.

Background

Acute pyelonephritis is nearly always the result of ascending infection from the bladder, most commonly by uropathogenic Escherichia coli. Hematogenous seeding is less common and tends to produce multiple, bilateral, cortically based abscesses rather than the wedge-shaped pattern of ascending infection. Vesicoureteral reflux is an important predisposing factor, particularly in children, and CT-detected pyelonephritis correlates strongly with ipsilateral reflux on subsequent voiding cystourethrography, especially in boys.³ Risk factors for complicated disease include diabetes mellitus, obstruction (calculus, stricture, tumor), immunosuppression, pregnancy, and anatomic anomalies. Left untreated or unrecognized, acute pyelonephritis can progress to abscess formation, emphysematous pyelonephritis, or evolve into chronic/xanthogranulomatous pyelonephritis with irreversible parenchymal destruction.

Imaging Anatomy

The findings of pyelonephritis are best understood relative to the renal lobar architecture. Each renal lobe is a pyramid-and-cortex unit drained by its own collecting duct system converging on a papilla; ascending infection travels via the collecting ducts into the medullary rays and then the adjacent cortex, which is why the resulting inflammatory zone is wedge-shaped and oriented radially from papilla to capsule rather than following vascular territory. Understanding the perinephric compartments is equally important for staging complicated disease: the renal capsule; the perinephric space, bounded by Gerota (perirenal) fascia and containing perinephric fat; and the pararenal spaces outside Gerota fascia. Emphysematous and abscess-forming infections are staged largely by whether they remain confined to the renal parenchyma/collecting system or breach the capsule and Gerota fascia into the perinephric and pararenal spaces — the same anatomic framework used for renal trauma and tumor staging applies directly to infection staging.

Imaging Findings

Ultrasound

Ultrasound is often the first study obtained, particularly in children and pregnant patients, but it is insensitive for uncomplicated acute pyelonephritis — the kidney may appear entirely normal despite active infection. When present, findings include focal or global renal enlargement, loss of corticomedullary differentiation, and focal areas of altered (usually decreased) echogenicity. Ultrasound’s principal strength is detecting complications: hydronephrosis from an obstructing stone, a discrete hypoechoic/complex collection representing abscess, and gas within the collecting system or parenchyma (echogenic foci with dirty posterior shadowing/reverberation) suggesting emphysematous pyelonephritis. Doppler and contrast-enhanced ultrasound can improve detection of perfusion defects and are useful alternatives when iodinated contrast or radiation should be avoided.

Additional US signs worth actively looking for include raised echogenicity of the renal sinus fat, reflecting inflammatory infiltration of the peripelvic fat, and pelvicalyceal wall thickening, seen as mural thickening of the collecting system distinct from simple hydronephrosis. Because pyelonephritis is almost always ascending infection, it is also worth scanning for secondary features of ureteritis (a thickened, echogenic ureteral wall, sometimes with periureteral fat stranding) and cystitis (diffuse or focal bladder wall thickening with irregularity, occasionally with internal echoes from debris) — these findings support the ascending route of infection and help corroborate an equivocal renal finding.

CT

Contrast-enhanced CT is the imaging reference standard. On unenhanced images the kidney may look normal even with significant infection, though renal enlargement or subtle areas of decreased attenuation can be seen in more severe cases. After contrast, the nephrographic phase (roughly 80–180 seconds) is the most sensitive phase: acute pyelonephritis produces one or more wedge-shaped or geographic zones of diminished enhancement extending from the papilla to the renal capsule, often with a striated nephrogram — alternating linear bands of relatively preserved and diminished enhancement — caused by tubular obstruction from debris and interstitial edema slowing contrast transit through affected nephrons. Unlike a renal infarct or tumor, the affected parenchyma in uncomplicated pyelonephritis retains its overall renal contour without a discrete mass. Perinephric fat stranding, thickening of Gerota fascia, and mild renal enlargement are common associated findings and reflect inflammatory spread rather than necessarily indicating abscess. Delayed excretory-phase imaging can show a persistent nephrogram or striations and helps confirm that a hypoattenuating area follows the expected papilla-to-cortex distribution of pyelonephritis rather than the sharply demarcated, non-enhancing wedge of infarction. In more severe or bacteremic disease, extrarenal CT findings — thickened gallbladder wall, periportal edema/tracking, a dilated IVC, pleural effusion, interlobular septal thickening, and ascites — have been reported in a substantial majority of cases in dedicated case series and should prompt a search for associated sepsis physiology.²

Renal or perinephric abscess appears as a well- or ill-defined, thick-walled, fluid-attenuation collection with peripheral rim enhancement, distinguishing it from the non-enhancing but non-cavitary wedge of simple pyelonephritis; the distinction matters because abscesses ≥3 cm generally require percutaneous drainage in addition to antibiotics. Emphysematous pyelonephritis is diagnosed by gas within the renal parenchyma, collecting system, or perinephric space and is staged with the Huang-Tseng CT classification (Class 1: gas confined to the collecting system, essentially emphysematous pyelitis and the most favorable form; Class 2: gas within the renal parenchyma without extrarenal extension; Class 3A: extension of gas or abscess into the perinephric space; Class 3B: extension beyond Gerota fascia into the pararenal space; Class 4: bilateral emphysematous pyelonephritis or emphysematous pyelonephritis in a solitary functioning kidney).⁷ Higher-class disease correlates with a greater likelihood of requiring nephrectomy rather than percutaneous drainage or medical management alone. Xanthogranulomatous pyelonephritis, a chronic destructive process typically associated with an obstructing staghorn calculus, presents on CT as a diffusely or focally enlarged, poorly functioning kidney with multiple dilated, debris-filled calyces producing the classic “bear paw” appearance, surrounded by inflammatory fat stranding and often with a central staghorn calculus; the focal (tumefactive) form can closely mimic renal cell carcinoma and is an important pitfall.

MRI

MRI is reserved chiefly for patients in whom iodinated contrast is contraindicated (renal insufficiency, contrast allergy) or in equivocal cases requiring further characterization, and has been shown to be a reliable alternative in high-risk populations with renal dysfunction.⁵ Affected regions of pyelonephritis are typically T2 hyperintense (edema) and show wedge-shaped or patchy areas of diminished enhancement on post-gadolinium sequences, analogous to the CT nephrogram pattern. Diffusion-weighted imaging (DWI) is particularly useful: areas of pyelonephritis show restricted diffusion with correspondingly low apparent diffusion coefficient (ADC) values, and DWI has been reported to have sensitivity as high as ~95% for acute pyelonephritis, outperforming both non-contrast and contrast-enhanced CT in at least one prospective comparison, while also helping differentiate simple pyelonephritis from a frankly restricting, rim-enhancing abscess.^4,6 MRI is especially valuable in pediatric and transplant populations where minimizing radiation and contrast exposure is a priority.

Nuclear Medicine

Tc-99m DMSA renal cortical scintigraphy remains a highly sensitive method for detecting acute pyelonephritis (seen as focal or diffuse photopenic defects without volume loss) and is used particularly in children to assess for acute cortical involvement and, on follow-up scans months later, to detect renal scarring — a distinction that matters because scarring, not the acute infection itself, is what predicts long-term risk of hypertension and reduced renal function.

Differential Diagnosis and Pitfalls

• Renal infarction: also produces a wedge-shaped hypoenhancing defect, but infarction is sharply demarcated, spares a thin rim of enhancing cortex supplied by capsular collaterals (“cortical rim sign”), and does not show the striated/geographic pattern typical of pyelonephritis.
• Renal cell carcinoma (especially the focal/tumefactive form of XGP): a mass-like area of altered enhancement with an obstructing calculus and clinical infection favors XGP, but biopsy or close follow-up may be needed when the distinction is not clear-cut.
• Lymphoma or multifocal renal metastases: can produce multiple hypoenhancing renal lesions; clinical context (fever, pyuria, response to antibiotics) and lack of a discrete mass favor pyelonephritis.
• Normal renal lobulation or prominent columns of Bertin: should not be mistaken for a focal inflammatory or mass lesion — these follow expected anatomic locations and enhance normally.
• Missed complicated disease: the most consequential pitfall is failing to actively search for and exclude an obstructing stone, abscess, or gas once the diagnosis of pyelonephritis is suggested, since these findings change management from antibiotics alone to drainage, decompression, or surgery.

Clinical Impact and Reporting

The report should state whether findings are consistent with uncomplicated acute pyelonephritis versus a complicated form, and if complicated, specify the feature driving that distinction: presence, size, and location of any abscess or fluid collection (and whether it is amenable to percutaneous drainage); presence and extent of gas, with Huang-Tseng class if emphysematous pyelonephritis is present; presence of an obstructing calculus and degree of hydronephrosis; and whether Gerota fascia has been breached. These findings directly determine whether the patient is managed with antibiotics alone, antibiotics plus percutaneous drainage or ureteral stent/nephrostomy decompression, or emergent nephrectomy in fulminant emphysematous pyelonephritis or a non-functioning xanthogranulomatous kidney.

Frequently Asked Questions

What is the best imaging test for pyelonephritis?

Contrast-enhanced CT in the nephrographic phase is the most sensitive and widely used test for diagnosing and staging acute pyelonephritis and its complications; ultrasound is used as an initial radiation-free screen (especially in pregnancy and children), and MRI with diffusion-weighted imaging is preferred when iodinated contrast is contraindicated.

What does a striated nephrogram mean?

A striated nephrogram refers to alternating linear bands of increased and decreased attenuation (or signal) radiating through the renal medulla and cortex on contrast-enhanced imaging. It results from tubular obstruction by inflammatory debris and interstitial edema slowing contrast transit through affected nephrons, and is a characteristic though not entirely specific sign of acute pyelonephritis.

How is emphysematous pyelonephritis classified on CT?

The Huang-Tseng system classifies emphysematous pyelonephritis into four classes based on the extent of gas on CT: Class 1, gas confined to the collecting system only; Class 2, gas within the renal parenchyma; Class 3A, extension of gas or abscess into the perinephric space; Class 3B, extension beyond Gerota fascia; and Class 4, bilateral disease or involvement of a solitary functioning kidney. Higher classes are associated with worse prognosis and a greater likelihood of requiring nephrectomy.

Can pyelonephritis look like a renal mass?

Yes. The focal, tumefactive form of xanthogranulomatous pyelonephritis in particular can present as a mass-like area of altered renal enhancement and closely mimic renal cell carcinoma. An obstructing calculus, clinical signs of infection, and a destroyed, poorly functioning kidney favor xanthogranulomatous pyelonephritis, but biopsy is sometimes required when imaging is equivocal.

Is imaging necessary for every patient with suspected pyelonephritis?

No. Imaging is not routinely required for uncomplicated acute pyelonephritis in a patient who responds appropriately to antibiotics. It is indicated for patients with risk factors for complications (diabetes, immunosuppression, known obstruction), those who fail to improve within 48–72 hours of appropriate therapy, or when an alternative diagnosis is being considered.

What is the role of diffusion-weighted MRI in pyelonephritis?

Diffusion-weighted MRI detects areas of restricted diffusion and low ADC values corresponding to inflamed renal parenchyma, with reported sensitivity around 95% in at least one prospective series — higher than non-contrast or contrast-enhanced CT in that study. It is particularly useful in patients with renal dysfunction or contrast allergy who cannot receive iodinated or gadolinium-based contrast, and can help distinguish simple pyelonephritis from a discretely restricting abscess.

References

1. Craig WD, Wagner BJ, Travis MD. Pyelonephritis: radiologic-pathologic review. Radiographics. 2008;28(1):255-277. PMID: 18203942
2. Zissin R, Osadchy A, Gayer G, Kitay-Cohen Y. Extrarenal manifestations of severe acute pyelonephritis: CT findings in 21 cases. Emerg Radiol. 2006;13(2):73-77. PMID: 16941112
3. Dacher JN, Boillot B, Eurin D, Marguet C, Mitrofanoff P, Le Dosseur P. Rational use of CT in acute pyelonephritis: findings and relationships with reflux. Pediatr Radiol. 1993;23(4):281-285. PMID: 8414754
4. Rathod SB, Kumbhar SS, Nanivadekar A, Aman K. Role of diffusion-weighted MRI in acute pyelonephritis: a prospective study. Acta Radiol. 2015;56(2):244-249. PMID: 24443116
5. Pinto DS, George A, Johny J, Hoisala RV. Role of MRI in the evaluation of acute pyelonephritis in a high-risk population with renal dysfunction: a prospective study. Emerg Radiol. 2023;30(3):285-295. PMID: 36959518
6. Poustchi-Amin M, Leonidas JC, Palestro C, et al. Magnetic resonance imaging in acute pyelonephritis. Pediatr Nephrol. 1998;12(7):579-580. PMID: 9761359
7. Huang JJ, Tseng CC. Emphysematous pyelonephritis: clinicoradiological classification, management, prognosis, and pathogenesis. Arch Intern Med. 2000;160(6):797-805. PMID: 10737279
8. Verswijvel G, Oyen R, Van Poppel H, Roskams T. Xanthogranulomatous pyelonephritis: MRI findings in the diffuse and the focal type. Eur Radiol. 2000;10(4):586-589. PMID: 10795538