Decoding acute pyelonephritis: imaging signatures and patterns of renal involvement across modalities

Abstract

Aims: Acute pyelonephritis (APN) and acute focal bacterial nephritis (AFBN) are severe forms of urinary tract infection with overlapping clinical features. Differentiating these entities and assessing disease extent by imaging are critical for guiding management and preventing complications. Each modality has unique strengths and limitations. This study aimed to evaluate imaging-based differences between APN and AFBN and to assess the diagnostic performance of commonly used modalities. Methods: This retrospective study included 87 adult patients with renal parenchymal infection evaluated between 2018 and 2024. Imaging modalities comprised computed tomography (CT, n=87; 20 non-contrast, 67 contrast-enhanced), magnetic resonance imaging (MRI, n=20), and ultrasound (USG, n=53). Imaging features were compared between APN and AFBN. The diagnostic performance of non-contrast CT (NCCT), USG, and MRI was assessed using contrast-enhanced CT (CECT) as the reference standard. Agreement between CECT and contrast-enhanced MRI (CE-MRI) was analyzed using Cramer’s V. Results: APN was diagnosed in 40 patients (46%) and AFBN in 47 (54%). Complication rates, causative microorganisms, and associated findings did not differ significantly between groups. CECT was the most reliable modality, whereas USG (sensitivity 26.4%) often failed to detect the infection, and NCCT frequently failed to delineate the extent of disease. CE-MRI showed excellent agreement with CECT (Cramer’s V=0.93, p<0.001) but offered no additional diagnostic value. DWI was more sensitive in defining infection extent, revealing additional foci in AFBN or more localized disease in APN. All lesions visible on CE-T1WI were also detected on T2WI, and the combined use of DWI and T2WI improved diagnostic confidence, with T2WI contributing additional specificity. Conclusion: While APN and AFBN share similar clinical and microbiological characteristics, radiological patterns suggest differences in inflammatory pathogenesis. CECT remains the reference standard, but USG and NCCT have limited diagnostic value. MRI—particularly diffusion-weighted imaging (DWI) combined with T2-weighted image (T2WI) —demonstrated superior sensitivity and may serve as a promising contrast-free alternative, especially in children, pregnant patients, and those with impaired renal function.

Keywords

• Acute pyelonephritis
• acute focal bacterial nephritis
• magnetic resonance imaging
• diffusion-weighted imaging
• contrast-enhanced CT

Supporting Institution

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Ethical Statement

This study was approved by the Institutional Review Board of Istanbul Medipol University (Approval No. 819/03.07.2025).

Akut piyelonefritin çözümlenmesi: modaliteler arasında görüntüleme bulguları ve renal tutulum paternleri

Abstract

Arka Plan: Akut piyelonefrit (APN) ve akut fokal bakteriyel nefrit (AFBN), üriner sistem enfeksiyonlarının ağır formlarıdır. Bu iki tablonun ayırt edilmesi ve hastalık yaygınlığının görüntüleme ile değerlendirilmesi, hasta yönetimi ve komplikasyonları önlemek açısından kritik öneme sahiptir. Her görüntüleme modalitesinin kendine özgü güçlü ve sınırlı yönleri bulunmaktadır. Bu çalışmanın amacı, APN ve AFBN arasındaki görüntüleme temelli farklılıkları ortaya koymak ve sık kullanılan modalitelerin tanısal performansını değerlendirmektir. Yöntem: Bu retrospektif çalışmaya 2018–2024 yılları arasında renal parankim enfeksiyonu tanısı almış 87 erişkin hasta dahil edildi. Hastaların BT (n = 87; 20 kontrastsız, 67 kontrastlı), MRG (n = 20) ve ultrasonografi (USG, n = 53) görüntülemeleri değerlendirildi. Görüntüleme bulguları APN ve AFBN olguları arasında karşılaştırıldı. Kontrastsız BT (NCCT), USG ve MRG’nin kontrastlı BT (CECT)’ye göre tanısal performansları değerlendirildi. CECT ile kontrastlı MRG (CE-MRI) arasındaki uyum Cramer’s V ile analiz edildi. Bulgular: APN 40 hastada (%46), AFBN ise 47 hastada (%54) saptandı. Komplikasyon oranları, etken mikroorganizmalar ve ilişkili bulgular gruplar arasında anlamlı fark göstermemesi nedeni ile radyolojik farklılıkların farklı enflamatuvar patogenezlerden kaynaklandığı düşünüldü. CECT ile karşılaştırıldığında USG (duyarlılık %26,4) sıklıkla enfeksiyonu saptayamadı; NCCT ise hastalık yaygınlığını ortaya koymakta yetersiz kaldı. CE-MRI, CECT ile mükemmel uyum göstermesine rağmen (Cramer’s V = 0,93; p < 0,001), ek tanısal katkısı görülmedi. DWI’nun enfeksiyon yayılımını tanımlamada daha duyarlı olduğu gözlemlendi; AFBN’de ek odakları, APN’de ise daha lokalize hastalığı ortaya koydu. CE-T1WI’de görülen tüm lezyonlar T2WI’de de saptandı ve DWI ile T2WI’nin birlikte kullanımı tanısal güvenirliliği sağladı. Sonuç: APN ve AFBN benzer klinik ve mikrobiyolojik özellikler gösterse de, radyolojik paternler farklı enflamatuvar patogenezlere işaret etmektedir. CECT referans standart görüntüleme olarak alındığında, USG ve NCCT’nin tanısal değeri sınırlı kalmış, MRG—özellikle DWI ile T2WI’nin birlikte kullanımı—yüksek duyarlılık göstermiştir ve özellikle çocuklar, hamile hastalar ve böbrek fonksiyonları bozulmuş bireylerde kontrastsız umut verici bir alternatif olabileceği düşünülmüştür.

Keywords

• Akut piyelonefrit
• Akut fokal bakteriyel nefrit
• Difüzyon ağırlıklı görüntüleme
• Manyetik rezonans görüntüleme
• Kontrastlı BT

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