Acute Non-Traumatic Urinary Tract Emergencies: The Central Role of CT Imaging and the Emerging Role of Photon-Counting CT

Ultrasound (US)

Ultrasound is frequently the first-line imaging modality in patients with suspected urinary tract emergencies. Its widespread availability, rapid bedside applicability, and lack of ionizing radiation make it particularly suitable for vulnerable populations, including pregnant women, children, and patients with renal impairment. US is especially useful for the initial assessment of hydronephrosis, bladder distension, urinary retention, and renal size, as well as for the detection of large renal calculi and perinephric fluid collections.^{9,21,26,30,47} Gray-scale US provides evaluation of renal parenchymal echotexture, collecting system dilatation, cortical thickness, and perinephric fluid or hematoma. Color and power Doppler imaging add functional information, allowing assessment of renal perfusion, arterial and venous flow, and vascular patency. This is particularly valuable in suspected renal infarction or renal vein thrombosis, where absent or altered flow may serve as an early diagnostic clue.^{9,16,21,26,30,47-50} Despite its advantages, US is operator-dependent and can be limited by patient body habitus, overlying bowel gas, and difficulty visualizing the mid and distal ureters. Small calculi, early infectious changes, and subtle obstruction may be missed. Nevertheless, US is often used as a screening tool, guiding subsequent cross-sectional imaging with CT when findings are equivocal or complications are suspected.

Computed Tomography (CT)

CT remains the cornerstone imaging modality for adult patients with acute non-traumatic urinary tract emergencies due to its high spatial resolution, rapid acquisition, and ability to comprehensively evaluate the kidneys, ureters, bladder, and surrounding retroperitoneum.^{1-12,19-31,40,51} Non-contrast CT is the gold standard for detecting urinary calculi. It accurately determines stone size, composition (via attenuation), location, and secondary signs of obstruction, such as hydronephrosis, perinephric stranding, or ureteral wall thickening. Non-contrast CT is rapid, highly sensitive, and not limited by patient body habitus, making it the preferred first-line modality in adults presenting with acute flank pain. Contrast-enhanced CT, including “multiphasic protocols,” is essential for evaluating infectious, vascular, hemorrhagic, and neoplastic conditions. Typical phases include:

To optimize diagnostic yield while reducing radiation exposure, “split-bolus techniques” may be employed. In this approach, contrast material is administered in 2 sequential injections separated by a timed delay, allowing simultaneous visualization of the nephrographic and excretory phases in a single acquisition. Split-bolus CT is particularly useful in the emergency setting, as it provides combined evaluation of renal parenchyma and the urinary collecting system while minimizing scan time and cumulative radiation dose. This technique is well suited for suspected urinary tract obstruction, urothelial pathology, or urinary leaks when comprehensive assessment is required but full multiphasic imaging is undesirable. CT also identifies perinephric collections, renal infarctions, gas-forming infections such as emphysematous pyelonephritis, and spontaneous hemorrhage. Low-dose CT protocols are increasingly utilized to minimize radiation exposure, particularly in patients with recurrent stones or young adults. However, contrast administration must be carefully considered in patients with renal impairment or history of contrast allergy.^{1-12,19-31,40,51}

Photon-Counting CT (PCCT)

PCCT represents a major advancement in CT technology, offering improved spatial and contrast resolution with the potential for radiation dose reduction. In genitourinary emergencies, PCCT may allow better differentiation of urinary stone composition, improved detection of subtle renal parenchymal abnormalities, and enhanced visualization of small urinary leaks or vascular abnormalities. Beyond improved spatial resolution, its added value lies in detector-based spectral imaging and quantitative analysis, which may support the assessment of infectious, inflammatory, vascular, and oncological conditions presenting as GU emergencies. Early clinical studies suggest that PCCT may increase diagnostic confidence while reducing radiation burden, a particularly relevant advantage in emergency imaging, where rapid and high-quality diagnosis is essential.^31-45,51-54

Magnetic Resonance Imaging (MRI)

MRI plays a complementary but limited role in the acute emergency setting. Its principal advantages are excellent soft tissue contrast and absence of ionizing radiation, making it valuable when CT is contraindicated, such as in pregnancy or in patients with severe contrast allergy.^{16,28,44,55-59} Magnetic Resonance urography can delineate urinary obstruction, evaluate collecting system integrity, and detect urinary leaks. Diffusion-weighted imaging (DWI) provides functional information, highlighting areas of restricted diffusion in renal infections or infarctions. MRI is particularly useful for characterizing renal masses, vascular abnormalities, and complex infections without exposing patients to radiation.^{16,28,44,55-59} Limitations of MRI include longer acquisition times, susceptibility to motion artifacts, limited availability in emergency settings, and difficulties in unstable patients. Nevertheless, in select scenarios, MRI can prevent radiation exposure, clarify equivocal CT findings, or serve as a problem-solving tool for complicated renal and urinary tract conditions.

Urolithiasis

Urolithiasis is the most prevalent cause of acute non-traumatic urinary tract obstruction, accounting for a substantial proportion of emergency presentations for flank pain. The lifetime incidence of urinary stone disease is approximately 10% to 15%, with a higher prevalence in men and a peak occurrence in the 30 to 50-year. ⁷ Predisposing factors include dehydration, metabolic disorders, dietary influences, obesity, urinary tract infections, and genetic susceptibility. Stones can form throughout the urinary tract, from the renal calyces and pelvis to the ureters and urinary bladder.

Imaging Evaluation

Stone size, density, and location inform management decisions. Stones smaller than 5 mm often pass spontaneously, whereas stones larger than 10 mm or those impacted at anatomical narrowing points frequently require intervention, including ureteroscopy, extracorporeal shockwave lithotripsy, or percutaneous extraction. Dual-Energy CT (DECT) provides additional functional and compositional information by acquiring datasets at 2 different X-ray energy levels. DECT enables reliable differentiation between uric acid and non–uric acid stones based on their material-specific attenuation properties, which has direct therapeutic implications, as uric acid calculi may be managed conservatively with urine alkalinization. DECT may also improve stone conspicuity in challenging locations and assist in predicting response to shockwave lithotripsy. With modern protocols, radiation dose is generally comparable to standard NCCT. Photon-Counting CT (PCCT) is an emerging CT technology that directly converts individual X-ray photons into electrical signals and sorts them by energy. Compared with conventional energy-integrating CT systems, PCCT offers higher spatial resolution, reduced image noise, and enhanced spectral separation. In urolithiasis, PCCT improves detection of very small calculi, better delineates stones adjacent to dense structures, and allows more accurate characterization of stone composition. Intrinsic spectral information may enable material differentiation similar to, or exceeding, that of dual-energy CT, potentially at equal or lower radiation dose.^32-46,60-62 Ultrasound (US) is a radiation-free option, particularly in pregnant patients and children. Stones appear as echogenic foci with posterior acoustic shadowing, and the color Doppler “twinkling artifact” may help identify small stones. Hydronephrosis is an important indirect sign of obstruction. However, US is less sensitive for distal ureteral calculi, small stones (less than 3 mm), and is limited by patient body habitus and bowel gas.^{9,21,26,30,47,63,64} Magnetic Resonance Imaging (MRI) is rarely used for direct stone detection, as calculi are typically hypointense and may be overlooked. However, MR urography can reveal secondary signs of obstruction, including collecting system dilatation, renal edema, and impaired excretory function. MRI is mainly reserved for problem-solving in patients with contraindications to CT or in radiation-sensitive populations.^{16,28,44,55-59,65}

Pitfalls

Small distal ureteral stones may be missed on ultrasound.

Radiolucent stones, particularly uric acid calculi, may appear subtle on conventional NCCT; dual-energy CT and photon-counting CT improve detection and compositional assessment.

Hydronephrosis may persist after spontaneous stone passage, potentially mimicking ongoing obstruction and requiring careful clinical correlation.

Acute Hydronephrosis

Acute hydronephrosis results from sudden obstruction of urinary outflow, causing retrograde dilatation of the renal pelvis and calyces. If unrecognized, prolonged obstruction can lead to acute kidney injury and permanent parenchymal damage. Causes include calculi, blood clots, strictures, tumors, and extrinsic compression from adjacent structures such as lymphadenopathy or retroperitoneal masses.

Imaging Features

Ultrasound is highly effective in detecting pelvicalyceal dilatation, renal cortical thinning, and perinephric fluid. Doppler assessment can reveal alterations in renal perfusion in cases of severe obstruction. CT provides superior anatomic detail, allowing precise localization of the obstruction, characterization of the underlying cause, and detection of associated complications such as infection or hemorrhage. Delayed contrast-enhanced images may demonstrate impaired or absent excretion in the affected kidney.^{1-12,19-31,40,51} MRI may be used in select cases to evaluate obstruction in radiation-sensitive populations, showing collecting system dilatation, parenchymal edema, and functional excretory impairment.^{16,28,44,55-57}

Reporting Recommendations

Radiology reports should systematically describe:

Non-Calculous Obstruction

Non-calculous obstruction can arise from malignancy, retroperitoneal fibrosis, or intraluminal blood clots. Though less common than calculous obstruction, prompt recognition is crucial, as these conditions may progress rapidly or be associated with significant morbidity.

Imaging Features

CT is the preferred modality, providing excellent evaluation of intrinsic and extrinsic causes of obstruction.^{1-12,19-31,40,51} It identifies: (1) Hyperattenuating filling defects (blood clots) within the collecting system. (2) Soft tissue masses causing intrinsic obstruction (tumors). (3) Extrinsic compression from retroperitoneal fibrosis, lymphadenopathy, or adjacent masses. Blood clots appear as hyperattenuating intraluminal material on NCCT and may be associated with hematuria or recent instrumentation. Differentiation from tumors is critical, as the management and prognosis differ significantly. In ambiguous cases, follow-up imaging, cystoscopy, or contrast-enhanced evaluation may be necessary.

Pitfalls

Intraluminal blood clots can mimic calculi or neoplasms. Retroperitoneal fibrosis may be subtle early, requiring careful review of perinephric and ureteral soft tissues. Bilateral obstruction from non-calculous causes may present subtly but has high risk of acute renal failure, necessitating urgent recognition.

Acute Pyelonephritis

Acute pyelonephritis is a bacterial infection of the renal parenchyma and collecting system, commonly presenting with fever, flank pain, and urinary symptoms. Imaging is not routinely required in uncomplicated cases but is indicated in patients with severe illness, immunocompromised population, or patients with poor response to therapy. Contrast-enhanced CT typically demonstrates wedge-shaped areas of decreased enhancement, renal enlargement, and perinephric fat stranding.^{1-12,19-31,40,51} MRI with DWI may show restricted diffusion in affected areas.^{16,28,44,55-57} Photon-counting CT (PCCT) may improve the evaluation of acute pyelonephritis (Figure 1) by enabling earlier detection of renal parenchymal abnormalities and a more accurate delineation of the extent of infection. Owing to its higher spatial resolution and improved contrast-to-noise ratio, PCCT can better depict subtle inflammatory changes, such as wedge-shaped areas of decreased enhancement and striated nephrogram patterns. In addition, the enhanced image quality may facilitate earlier recognition of complications, including renal or perinephric abscesses and collecting system involvement. These features may ultimately increase diagnostic confidence, particularly in atypical or complicated presentations of acute pyelonephritis, and may contribute to a more precise assessment of disease severity and extent (Table 3). Complicated pyelonephritis includes abscess formation, gas production, or obstruction. Emphysematous pyelonephritis is a life-threatening condition characterized by gas within the renal parenchyma, collecting system, or perinephric space, most commonly in diabetic patients. CT is the modality of choice, allowing classification of disease extent and guiding management decisions, including percutaneous drainage or nephrectomy.

OPEN IN VIEWER

Figure 1.

A 60-year-old woman presenting with fever, left flank pain, and dysuria. (A-C) Coronal MPR reconstructions from contrast-enhanced Photon-Counting CT obtained during the arterial, venous, and excretory phases demonstrate an enlarged left kidney with multiple wedge-shaped areas of reduced perfusion, most prominent in the upper-mid third (arrow), consistent with acute pyelonephritis. (D) Volume-rendered reconstruction (posterior view) from the portal venous phase shows surface irregularity of the left kidney corresponding to the previously described areas of decreased vascularity.

OPEN IN VIEWER

Table 3.

Infectious, Ischemic, and Vascular Emergencies.

Condition	Characteristic imaging features	Modality of choice
Acute pyelonephritis	Wedge-shaped hypoenhancing areas	Contrast CT/PCCT/MRI
Emphysematous pyelonephritis	Gas in parenchyma or collecting system	CT/PCCT
Renal abscess	Rim-enhancing fluid collection	CT/PCCT
Renal infarction	Non-enhancing wedge-shaped defects, cortical rim sign	Contrast CT/PCCT
Renal vein thrombosis	Enlarged kidney, venous filling defect	Contrast CT/PCCT

Renal and Perinephric Abscess

Renal and perinephric abscesses may arise as complications of untreated pyelonephritis. CT findings include rim-enhancing fluid collections with surrounding inflammatory changes. Prompt diagnosis is critical, as management often requires drainage in addition to antibiotics.

Renal Infarction

Renal infarction occurs when renal arterial blood flow is acutely interrupted, leading to ischemia and necrosis of the renal parenchyma. The condition is uncommon but frequently misdiagnosed as urolithiasis or acute pyelonephritis due to overlapping clinical presentations.

Etiologies include:

Spontaneous Renal Hemorrhage (Wunderlich Syndrome)

Spontaneous renal hemorrhage is defined as non-traumatic bleeding into the subcapsular or perinephric space (Figures 3 and 4). It is most often associated with:

OPEN IN VIEWER

Figure 3.

A 76-year-old woman. Axial unenhanced (A), contrast-enhanced arterial (B), and venous (C) phase CT images, together with a coronal venous-phase reconstruction (D), show a fat-containing mass arising from the inferior pole of the left kidney (asterisk in A-D), associated with extensive perirenal, anterior pararenal, and posterior pararenal hematoma (arrow in A-D), consistent with Wunderlich syndrome secondary to rupture of a renal angiomyolipoma. Digital subtraction angiography (DSA) (E) confirms a hypervascular lesion in the lower third of the left kidney corresponding to the angiomyolipoma (dotted arrow in E), with a pseudoaneurysm arising from a feeding vessel originating from an arcuate branch of the inferior segmental renal artery (arrowhead in E). Superselective coil embolization of the feeding arcuate artery was successfully performed (F).

OPEN IN VIEWER

Figure 4.

A 24-year-old woman presenting with acute flank pain and hematuria. Unenhanced CT (A) shows hyperdense material within the right renal pelvis (thin arrow in A). Contrast-enhanced CT in the corticomedullary phase (B) demonstrates an enhanced vascular tangle in the renal sinus (arrowhead in B) with early opacification of the right renal vein (dotted arrow in B), consistent with a renal arteriovenous malformation. A delayed nephrogram with absence of iodinated urine excretion up to 20 minutes is observed (C, D). Follow-up CT performed 6 hours later shows a striated nephrogram and intraparenchymal arteriovenous malformations (arrowhead in E) (E). Digital subtraction angiography (DSA) confirms a high-flow renal arteriovenous malformation (arrowhead in F) supplied by arcuate arteries arising from both anterior and posterior segmental branches (F). Selective coil embolization of the feeding interlobar arteries was performed (G).

Clinical Presentation

Patients present acutely with flank or abdominal pain, palpable mass, hematuria, and sometimes hypotension or hemorrhagic shock. The classic Lenk triad includes pain, mass, and hypotension.

Imaging Features

Pitfalls

Management Implications

OPEN IN VIEWER

Table 4.

Hemorrhage and Urinary Leaks.

Condition	Imaging findings	Clinical relevance
Wunderlich syndrome	Perinephric/subcapsular hematoma	Possible life-threatening
Spontaneous rupture	Contrast extravasation, urinoma	Urgent management
Urinoma	Encapsulated fluid collection	Risk of infection

Urinary Tract Rupture and Urinoma Formation

Urinary tract rupture occurs when there is a breach in the integrity of the renal collecting system, ureter, or bladder. This leads to extravasation of urine into the perinephric, retroperitoneal, or even intraperitoneal space, forming a urinoma.

Etiology

Pathophysiology

Obstruction increases intraluminal pressure, which initially dilates the collecting system. If pressure exceeds the tensile strength of the urothelium, urine leaks into surrounding spaces, causing local inflammation, fibrosis, and potentially infection. Persistent urinomas can compress adjacent structures and impair renal function.

Clinical Presentation

Symptoms are variable and nonspecific, often including:

Because symptoms overlap with renal colic, pyelonephritis, or gastrointestinal conditions, imaging plays a critical role in accurate diagnosis.

Imaging Evaluation

CT urography (CTU) is the gold standard for detecting urinary leaks and urinomas. Key imaging findings include^{1-12,19-31,40,51}:

Protocol Considerations

Ultrasound may identify perinephric fluid collections, but small leaks or retroperitoneal urinomas may be missed. Ultrasound can be used for follow-up or drainage guidance.

MRI with MR urography can demonstrate urine leaks in patients where CT is contraindicated, particularly in children or those with impaired renal function. T2-weighted sequences and heavily weighted MR urography highlight fluid collections and continuity with the collecting system.

Pitfalls and Mimics

Management Considerations

Prompt recognition is essential to prevent complications such as infection, sepsis, or loss of renal function. Management depends on the size of the leak, hemodynamic stability, and underlying cause: