Focal pyelonephritis (acute focal bacterial nephritis, acute lobar nephronia) is an acute bacterial infection affecting a lobe or segment of the kidney. Develops from ascending urinary infection or hematogenous spread. It is an early-stage inflammatory process that has not yet progressed to abscess formation. Striated nephrogram and wedge-shaped hypodense area on contrast-enhanced CT are typical. Clinically presents with fever, flank pain, pyuria, and leukocytosis. Usually resolves completely with antibiotic therapy; may progress to abscess if untreated. Much more common in women than men. Increased risk in diabetics, immunosuppressed patients, and those with urinary obstruction.
Age Range
20-70
Peak Age
40
Gender
Female predominant
Prevalence
Common
Focal pyelonephritis develops when bacteria reach the renal parenchyma and create a localized inflammatory response. Most commonly caused by ascending E. coli infection (bladder → ureter → kidney). When bacteria reach collecting ducts, interstitial edema, neutrophil infiltration, and tubular damage begin. Inflammation is limited to one lobe or segment — necrosis and liquefaction (abscess) have not yet developed. The striated nephrogram finding on imaging results from tubular flow disruption due to inflammatory edema: some tubules are blocked by edema (non-enhancing striations) and some remain patent (enhancing striations), creating the alternating pattern of striated appearance. Perinephric fat stranding reflects perirenal extension of inflammation. Gerota fascia thickening indicates capsular inflammatory reaction.
Alternating enhancing and non-enhancing parallel striations in the affected kidney segment on nephrographic phase. Some tubules are blocked by inflammatory edema while adjacent tubules remain patent, reflecting tubular-level heterogeneity. Characteristic of acute pyelonephritis.
Alternating enhancing and non-enhancing parallel striations are observed in the affected region on nephrographic phase — striated nephrogram. Striations extend from medulla to cortex, following the tubular flow direction. The affected segment appears generally hypodense compared to normal parenchyma. This pattern affects a single lobe or segment, not the entire kidney.
Report Sentence
Striated nephrogram with alternating enhancing and non-enhancing parallel striations in the upper/lower pole of the left/right kidney on nephrographic phase, consistent with acute focal pyelonephritis.
Wedge-shaped or wedge-like area of decreased enhancement on corticomedullary phase. Base faces the renal capsule, apex points toward the hilum. Borders may be more ill-defined and irregular than renal infarct. Density difference exists with surrounding normal parenchyma, but unlike infarction, it shows decreased rather than absent enhancement.
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Wedge-shaped area of decreased enhancement in the upper/lower pole of the kidney on corticomedullary phase, consistent with focal pyelonephritis.
Gerota fascia thickening and perinephric fat stranding around the affected kidney. Thickened Gerota fascia may enhance. Perinephric stranding indicates spread of inflammatory fluid into the perinephric space. This finding is more prominent in diffuse pyelonephritis but frequently accompanies focal pyelonephritis as well.
Report Sentence
Gerota fascia thickening and inflammatory stranding in the perinephric fat around the affected kidney are observed.
Focal restricted diffusion in the affected kidney segment on DWI — high signal on DWI, low signal on ADC map. The area of restricted diffusion corresponds to the hypoenhancing area on CT. MRI provides radiation-free assessment as an alternative to CT in pyelonephritis, especially in pregnant women and children. DWI may be more sensitive than CT for early detection of focal pyelonephritis.
Report Sentence
Focal area of restricted diffusion in the upper/lower pole of the kidney on DWI, consistent with acute focal pyelonephritis.
Focal hypoechoic or hyperechoic area may be observed in the affected region on B-mode US — however, US can frequently appear normal in focal pyelonephritis. Decreased perfusion in the affected segment may be visible on color Doppler. Power Doppler is more sensitive. US is used for initial assessment and complication monitoring (abscess development) in pyelonephritis but is not as sensitive as CT.
Report Sentence
Focal hypoechoic area in the kidney with decreased perfusion on Doppler at this region, suggestive of focal pyelonephritis with clinical correlation.
Criteria
Acute bacterial infection affecting a single renal lobe. Pre-abscess stage. Focal mass-like lesion with decreased enhancement and ill-defined borders.
Distinct Features
May show mass effect and can be confused with tumor. Heterogeneous hypodense lesion with decreased enhancement on CT. Resolves in 2-3 weeks with antibiotic therapy.
Criteria
Acute bacterial infection affecting multiple renal lobes. Multiple hypodense areas and extensive striated nephrogram.
Distinct Features
More aggressive infection. Multiple wedge-shaped hypodense areas observed. Kidney may appear diffusely swollen. Requires longer antibiotic therapy.
Criteria
Progression of focal pyelonephritis to abscess formation due to untreated or inadequately treated infection. Liquefactive necrosis has developed with rim-enhancing collection.
Distinct Features
Rim-enhancing collection, marked restricted diffusion on DWI (pus), gas bubbles (rare). Requires drainage + antibiotic therapy.
Distinguishing Feature
Renal infarct shows a completely non-enhancing wedge area while focal pyelonephritis shows decreased enhancement and striated nephrogram. Cortical rim sign is positive in infarct but not expected in pyelonephritis. Clinically, infarct presents with elevated LDH + AF while pyelonephritis presents with fever + pyuria.
Distinguishing Feature
Abscess appears as a rim-enhancing collection containing liquefied fluid while focal pyelonephritis shows striated nephrogram and hypodense parenchymal area. Abscess demonstrates marked restricted diffusion on DWI. Pyelonephritis has not yet progressed to abscess formation in the early stage.
Distinguishing Feature
RCC shows mass-like enhancement while focal pyelonephritis demonstrates wedge-shaped decreased enhancement with striated nephrogram. Fever/pyuria are not expected in RCC. Pyelonephritis resolves on follow-up CT after antibiotic therapy — RCC persists. Clinical correlation is critical.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
6-monthFocal pyelonephritis is treated with intravenous antibiotic therapy (typically 14-21 days). Empiric treatment is started with broad-spectrum antibiotics and narrowed based on culture results. Biopsy is not needed — clinical + imaging findings are diagnostic. Complication monitoring is important: follow-up CT is recommended if abscess development is suspected. Underlying predisposing factors (urinary obstruction, stones, vesicoureteral reflux) should be investigated. Post-treatment imaging at 4-6 weeks should confirm complete resolution.
Focal pyelonephritis is treated with antibiotics and usually resolves completely. It may progress to abscess formation if untreated. Since it can mimic malignancy, resolution should be confirmed with follow-up imaging after treatment.