Indications for imaging
In patients with uncomplicated urinary tract infection, if therapy is effective within the first 72 hours, diagnostic imaging is not indicated. The choice of imaging modality in complicated cases depends on medical history, availability of equipment, and the patient's condition. According to the Society of Uroradiology guidelines, the report must describe the nature of the lesion (focal or diffuse, unilateral or bilateral) and the presence of organ enlargement.
Ultrasound examination (ultrasound)
Ultrasound is frequently ordered as a first-line method because it is safe and accessible. However, in 75–80% of patients with typical uncomplicated infection, the ultrasound appearance is completely normal. The method is highly operator-dependent and often underestimates the extent of the lesion.
Main sonographic features:
- Organ enlargement: length exceeds 15 cm or the difference with the healthy kidney is more than 1.5 cm (occurs due to generalized edema).
- Alteration in echogenicity: may decrease with pronounced edema or increase with hemorrhage.
- Loss of differentiation: blurring of the corticomedullary junction.
- Pyonephrosis: visualization of internal echoes (purulent debris) within the pelvicalyceal system.
Contrast-enhanced ultrasound (CEUS) enables the detection of hypoperfusion zones and established abscesses with an accuracy comparable to CT.
Computed tomography (CT)
Computed tomography is the most accurate evaluation method in complicated cases. The protocol should include unenhanced, early nephrographic, and pyelographic phases.
| Method | Sensory function | Specificity |
|---|---|---|
| CT | 86,8% | 87,5% |
| ultrasound | 74,3% | 56,7% |
Important: a normal finding on the unenhanced series does not rule out pyelonephritis; contrast administration is mandatory. In the nephrographic phase, a classic sign is the «striated nephrogram»—the appearance of wedge-shaped or band-like areas of decreased attenuation. They extend from the papillae to the capsule and result from vasospasm, inflammatory detritus in the tubules, and interstitial edema.
CT allows differentiation of the route of infection: in hematogenous infection, the lesions are rounded and located peripherally, whereas in ascending infection, they are wedge-shaped.
Magnetic resonance imaging (MRI) and scintigraphy
MRI is used in patients with iodine contrast allergies, pregnant women, and in pediatric practice. Inflamed areas on T1-weighted images show a heterogeneous low signal, and on T2-WI, a high signal. A key diagnostic marker is diffusion restriction on DWI (high signal at b-factor > 600). Following gadolinium administration, areas of nephritis remain hypointense compared to healthy tissue.
Scintigraphy with 99mTc-DMSA is used not for acute diagnosis, but for assessing sequelae. The study is performed no earlier than 6 months after the episode to detect scarring zones (appearing as areas of photopenia—reduced radiopharmaceutical uptake).
Severe complications: abscesses and emphysematous involvement
Lack of response to antibiotic therapy is the main marker of suppurative complications.
- Renal abscess: Pathogenesis includes vasospasm, necrosis, and the coalescence of microabscesses into a single cavity. On CT, a mature abscess appears as a fluid collection that does not enhance internally but is surrounded by an enhancing pseudocapsule (halo sign).
- Emphysematous pyelonephritis: A fulminant, life-threatening infection (most commonly caused by E. coli or Klebsiella) characterized by the accumulation of gas within or around the organ. In 90% of cases, it develops in the setting of uncontrolled diabetes mellitus. CT most accurately determines the localization of gas: confined to the collecting system (emphysematous pyelitis) or extending into the perinephric space.
For differential diagnosis on CT, remember the routes of infection: ascending infection forms "wedges" (from the renal pelvis to the capsule), while hematogenous infection forms "circles" (peripheral lesions).
Class and exam questions
Can the diagnosis of pyelonephritis be made based on ultrasound alone?
No, in 75–80% of patients with uncomplicated infection, the ultrasound appearance is completely normal. The method frequently underestimates the severity of inflammation.
How to differentiate renal infarction from focal pyelonephritis?
Both conditions present with wedge-shaped defects. However, an infarction zone does not deform the renal contour and shows absolute lack of contrast enhancement, whereas in pyelonephritis, the contour may bulge and the tissue retains some ability to enhance.
Why is the unenhanced CT phase necessary when infection is suspected?
It allows for ruling out causes of infravesical obstruction (e.g., radiopaque calculi), as well as detecting areas of hemorrhage and gas that may be masked following contrast administration.
What else to study on this topic
- Criteria for the differential diagnosis of acute pyelonephritis and renal cell carcinoma on CT
- Contrast-enhanced ultrasound (CEUS) in nephrology
- Excretory urography: historical role and reasons for abandoning routine use
- Diffusion restriction (DWI) on MRI in the differentiation of fluid collections
- The role of DMSA scintigraphy and voiding cystourethrography in pediatric practice
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