Added value of restricted diffusion in acute pyelonephritis: Two cases with different clinical presentations

Abstract

Acute pyelonephritis (APN) is characterized as infection of the renal parenchyma and collecting system generally evaluated by integrating clinical assessment, blood tests, urinalysis, and urine culture. If complications are suspected, computed tomography (CT) is the preferred modality; however, if inconclusive or contraindicated, magnetic resonance imaging (MRI) can provide valuable diagnostic information, though less well-described in literature. Herein, we report 2 cases in which MRI demonstrated restricted diffusion in the kidneys, corresponding to APN. In the first case, MRI confirmed findings established by CT (acute lobar nephronia); in the other, MRI provided primary diagnostic evidence of APN after indeterminate CT and US in a patient with prior history of acute pyelonephritis.

Introduction

Acute pyelonephritis (APN) is characterized by infection of the renal parenchyma and collecting system and is primarily diagnosed clinically with supportive laboratory findings, urinalysis and urine culture. Imaging is typically reserved for patients with diabetes, immunocompromised states, a history of urolithiasis or renal surgery, suspected urinary obstruction or emphysematous changes, or those who fail to respond to antibiotic response [1]. Computed tomography (CT) is the preferred modality for detecting renal and extrarenal complications associated with complicated APN and is critical for guiding disease management [2,3].

Magnetic resonance imaging (MRI), particularly diffusion-weighted imaging (DWI), can detect areas of restricted diffusion corresponding to renal inflammation. In cases with contraindications to radiation or contrast administration, or when CT findings are indeterminate, MRI can reliably aid in diagnosis of APN and help differentiate it from mimics such as renal infarction, abscess, or neoplasm [1]. The study by De Pascale et al. reported a high sensitivity, specificity and accuracy (each at 95%) with DWI in uncomplicated pyelonephritis [4]. DWI also helps in differentiating between pyonephrosis and hydronephrosis because of lower apparent diffuse coefficient of debris in pyonephrosis [5].

Although ultrasound and CT imaging characteristics of pyelonephritis have been well described, reports specifically addressing the diagnostic utility and imaging features of restricted diffusion MRI in acute pyelonephritis remain limited in the literature. We report 2 cases of acute pyelonephritis in which MRI demonstrated focal restricted diffusion in kidneys. The first case highlights the diagnosis of APN following an equivocal CT scan, while the second demonstrates the diagnosis of APN following an indeterminate CT scan in a patient with prior episode of APN. Together these cases emphasize the added value of DWI/MRI in the diagnostic evaluation of APN.

Case presentations

Case #1

The first case describes a 37-year-old female with a history of cholecystectomy who initially presented with fever, dysuria and flank pain to urgent care and showed no response to oral antibiotics for 2 days. She was subsequently referred to the emergency department for evaluation of a complicated urinary tract infection. She was febrile (102.3°F) and tachycardic with WBC count of 12.01 × 10⁹ cells/L (normal: 4.3-11.1 × 10⁹ cells/L), lactic acid 2.0 mmol/L (normal: 0.5-2.2 mmol/L), bilirubin of 2.8 mg/dL (normal: 0.1-1.1 mg/dL), AST 90 U/L (normal: 13-40 U/L) and ALT 69 U/L (normal: 5-35 U/L). Urinalysis revealed positive leukocyte esterase and elevated WBCs.

CT abdomen pelvis with contrast revealed diffuse mural thickening of the urinary bladder and a hypodense lesion in right kidney (Fig. 1A), along with mild intrahepatic and extrahepatic biliary dilation. There was no kidney enlargement, calculi, hydronephrosis or perinephric collection. Given the mild hepatic dysfunction and biliary dilation without an identifiable cause and the need to rule out focal APN from renal infarction, MRI of abdomen was recommended alongside empirical treatment. The patient was given intravenous (IV) fluids, IV ceftriaxone (2 gm/d) and admitted to hospital for further management.

Fig. 1.

(A) Contrast enhanced CT abdomen and pelvis demonstrates focal hypoenhancement (white arrow) in the anterior aspect of the right renal midpole. (B) Contrast-enhanced T1-weighted MRI abdomen image shows the corresponding region of hypoenhancement in the right renal midpole (white arrow). (C and D) Diffusion-weighted imaging (DWI) (C) and ADC (D) images demonstrate focal bright signal on DWI (white arrow) and corresponding dark signal on ADC (white arrow), consistent with diffusion restriction. These findings are compatible with acute pyelonephritis in the context of this patient’s presentation.

One day second of admission, subsequent MRI of the abdomen with and without contrast demonstrated patchy cortical hypoenhancement on post-contrast sequences with corresponding areas of restricted diffusion on DWI and low signal on ADC (Apparent Diffusion Coefficient) maps, suggestive of acute pyelonephritis (Figs. 1B, C and D). The biliary tree appeared unremarkable without evidence of choledocholithiasis. One day fifth of admission, she improved gradually with medical management. The hospital course was uneventful, and the patient was discharged home in stable condition.

Case #2

The second patient was a 38-year-old female with a history of prior acute pyelonephritis, who presented to the emergency department with fever, headache, shortness of breath and abdominal pain for 2 days. She was tachypneic (RR: 24 breaths/min) and tachycardic (HR: 112 bpm) with a temperature of 102.4°F. Oxygen saturation and blood pressure were within normal limits. Physical examination revealed mild right flank and suprapubic tenderness.

Laboratory evaluation showed WBC count of 18.01 × 10⁹ cells/L (normal: 4.3-11.1 × 10⁹ cells/L), lactic acid of 1.6 mmol/L (normal: 0.5-2.2 mmol/L), bilirubin of 1.0 mg/dL (normal: 0.1-1.1 mg/dL), AST of 13 U/L (normal: 13-40 U/L) and ALT of 28 U/L (normal: 5-35 U/L). Urinalysis revealed positive leukocyte esterase and elevated WBCs (22/hpf). She was admitted from the emergency department for further management of sepsis. Broad-spectrum antibiotics (doxycycline 100 mg PO q12h and ceftriaxone 2 gm IV q24h) were started. CT abdomen and pelvis demonstrated common bile duct dilation with mild intra- and extrahepatic biliary ductal dilation and mildly thickened gallbladder without other acute abdominal or pelvic abnormalities. Bilateral cortical thinning was presumed to represent sequelae of prior pyelonephritis (Fig. 1A).

On day second of admission, follow-up ultrasound revealed no evidence of cholecystitis or choledocholithiasis. On day third of admission, urine and blood culture came negative but WBC count went further up from 24.6 × 10⁹ cells/L to 28.6 × 10⁹ cells/L. Fever persisted. Therefore, MRI abdomen and pelvis with and without contrast was planned.

On day fourth of admission, the MRI abdomen and pelvis with and without contrast was done. The report revealed multifocal patchy hypoenhancement bilaterally with bilateral cortical thinning. DWI/ADC demonstrated multifocal areas of restricted diffusion in the left kidney, consistent with acute pyelonephritis (Figs. 1B, C, and D). The WBC dropped to 12.1 × 10⁹ cells/L and she was symptomatically better. Doxycycline was discontinued.

On day fifth of admission, she was transitioned to amoxicillin-clavulanate (825-125 mg q12h) to complete total of 7 days of therapy as recommended by the infectious disease specialist for complicated urinary tract infection. The patient was discharged in stable condition (Fig. 2).

Fig. 2.

(A) Contrast-enhanced CT abdomen and pelvis shows focal cortical thinning in the posterior aspect of the left renal midpole (white arrow), suggesting renal scarring; no abnormal enhancement is identified. (B) Contrast-enhanced T1-weighted MRI abdomen subtraction sequence illustrates cortical thinning like the CT, in the left renal midpole (white arrow). This suggests renal scarring from prior history of pyelonephritis. (C and D) DWI (C) and ADC (D) sequences demonstrate bright signal on DWI (white arrow) and corresponding dark signal on ADC (white arrow), consistent with diffusion restriction. These findings are compatible with acute pyelonephritis, which was not apparent on the initial contrast-enhanced CT.

Discussion

Imaging studies are not usually required for uncomplicated cases of acute pyelonephritis (APN) in patients who respond appropriately to antibiotic therapy within 48-72 hours; however, imaging should be considered in patients with unusual or complicated presentations including persistent fever or symptoms beyond 48-72 hours, clinical deterioration, or high-risk features such as diabetes, immunocompromised status, urinary tract anomalies or obstruction, sepsis, pregnancy, or transplant status [1]. Both patients in our report had complicated presentations that warranted further imaging evaluation. Ultrasound is not a reliable modality to exclude APN due to limited sensitivity for parenchymal inflammation and its inability to fully evaluate perinephric complications [6]. Contrast-enhanced CT remains the standard imaging modality for detecting renal parenchymal inflammation, perinephric fluid collections, or abscesses; APN typically appears as focal, wedge-shaped areas of decreased enhancement with associated striated nephrogram and perinephric stranding on CT [2].

However, chronic/prior acute pyelonephritis can alter the typical imaging appearance of acute pyelonephritis on CT, making interpretation more challenging. Chronic changes such as focal polar scars, cortical thinning, global atrophy and compensatory hypertrophy of residual parenchyma can mimic other pathologies including renal infarcts or mass-like lesions and therefore complicate differentiation from active infection [6,7]. Several imaging studies have demonstrated that renal scarring is a common sequela of pyelonephritis, particularly in patients with recurrent or severe infections and may persist long after the acute episode has resolved [8,9]. In some cohorts, a substantial proportion of patients especially those with more extensive initial involvement develop chronic cortical defects, which must be recognized to avoid confusing residual scar with active inflammation that requires therapy [9]. In the presence of significant chronic scarring, CT findings for acute pyelonephritis may be inconclusive or have reduced specificity due to altered baseline anatomy and overlapping appearances. In such cases, MRI offers added diagnostic value, particularly with its superior soft-tissue contrast and ability to distinguish active inflammation from fibrosis or scar without ionizing radiation [7].

Diffusion-weighted imaging (DWI) detects subtle tissue changes, aiding in the diagnosis of acute pyelonephritis and other infections, and serving as a safer alternative when contrast-enhanced CT cannot be performed or is non-diagnostic [10,11]. It can also identify renal scarring, congenital anomalies, abscesses, and hydronephrosis. However, a known limitation of DWI is its low sensitivity for detecting gas, which restricts its role in emphysematous pyelonephritis [12]. DWI which reflects the Brownian motion of water molecules within tissues, becomes restricted in inflammatory states, edema, or increased cellularity, producing high signal intensity on DWI and low apparent diffusion coefficient (ADC) values [13,14]. In acute pyelonephritis, bacterial infection causes tubulointerstitial inflammation and edema, restricting water movement and appearing as segmental high DWI signal with low ADC in the affected parenchyma [15]. In contrast, chronic pyelonephritis is characterized by tubular atrophy and interstitial fibrosis with extracellular matrix deposition, where diffusion restriction results from structural distortion rather than acute cellular swelling. These areas often demonstrate variable DWI signal with relatively modest ADC reduction in a patchy or diffuse pattern reflecting chronic architectural change rather than acute edema. When acute infection is superimposed on chronic pyelonephritis, the combined effects of fibrosis (low ADC from restricted extracellular diffusion) and acute inflammation (high DWI signal and low ADC from edema and cellular infiltration) may overlap, emphasizing the need to interpret DWI and ADC findings in conjunction with clinical and conventional imaging [16,17].

The major limitation of our cases was that we did not do follow up imaging and it was also not required clinically. In both scenarios, DWI and ADC contribute significantly to confirming a suspected diagnosis and detecting APN when conventional findings are equivocal or indeterminate. They also assist in distinguishing APN from chronic renal changes. The table below summarizes the important features of common differential diagnoses (Table 1).

Table 1.

Summary of differential diagnoses and important imaging features [[6], [7], [8], [9], [10]].

Differentials	CECT	T1 Post	DWI	ADC	Additional features
Acute pyelonephritis	Wedge-shaped or patchy hypoenhancing areas; renal enlargement; perinephric fat stranding; Striated nephrogram;	Decreased or delayed enhancement	↑	↓	Perinephric fluid, loss of corticomedullary differentiation; may have abscess (non-enhancing center, enhancing rim)
Acute on prior history of pyelonephritis	Cortical thinning; hypo enhancement	Decreased/patchy enhancement in scarred segment	↑ (if active inflammation)	↓	Cortical scar
Renal infarction	Wedge shaped hypodensity; delayed cortical rim sign	No enhancement	↑	↑	Possible renal artery thrombus/ embolus
Renal metastasis	Usually multiple, bilateral, hypovascular solid lesions; hypoenhancing	Minimal or heterogeneous enhancement	↑	↑	Other primary cancer
RCC	Variable enhancement; Increased in clear cell	↑ arterial enhancement	↑↑	↓↓	Hypointense rim d/t tumor pseudocapsule. Renal vein and IVC tumor thrombus, calcification, necrosis

Conclusion

Imaging is indicated in patients with suspected acute pyelonephritis (APN) who are at risk of complications, including those with diabetes, immunocompromised status, pregnancy, urinary obstruction or calculi, or lack of response to initial therapy within 48 to 72 hours. Because clinical symptoms and laboratory findings cannot reliably determine the underlying pathological process, imaging becomes essential for confirming the diagnosis, assessing disease severity, and guiding medical management. Contrast-enhanced CT of the abdomen and pelvis is usually the preferred initial imaging modality. When CT findings are equivocal or indeterminate, MRI particularly diffusion-weighted imaging (DWI) with ADC mapping provides additional diagnostic value. DWI offers a radiation-free technique with enhanced sensitivity for detecting acute pyelonephritis, especially in vulnerable groups such as women of reproductive age, pediatric patients, and individuals with renal impairment. These 2 cases highlight the importance of DWI as a complementary tool in the evaluation of complicated APN.

Authorship

The authors declare that this is their original work, and they all approve the content of this manuscript. They confirm that this manuscript has not been published previously, in any language, in whole or in part, and is not currently under consideration elsewhere.

Ethical clearance

This project did not involve any research, and no ethical clearance was required.

Patient consent

A written informed consent was obtained from the patient for the publication of this case report.