Renal emergencies are typically classified into four major categories: infectious, hemorrhagic, vascular, and traumatic causes. 

Infectious Etiologies: The majority of urinary tract infections (UTIs) are effectively managed with antibiotics and do not necessitate imaging studies. However, in certain instances, UTIs can progress to more complex conditions, such as pyelonephritis, renal or perinephric abscesses, pyonephrosis, or emphysematous pyelonephritis/pyelitis. These complications often require additional diagnostic imaging to guide treatment and management.

Emphysematous Pyelonephritis and Emphysematous Pyelitis: Emphysematous pyelonephritis is a severe and potentially life-threatening infection of the renal parenchyma, characterized by the presence of gas-forming bacteria. The condition is most commonly caused by Escherichia coli, followed by Klebsiella species. This rare but serious infection requires prompt diagnosis and intervention to prevent life-threatening complications.

Computed tomography (CT) is the most effective modality for assessing the extent of gas formation and tissue destruction associated with emphysematous pyelonephritis. Key imaging findings typically include areas of gas within the renal parenchyma or collecting system, renal enlargement with associated destruction, fluid collections that may exhibit gas-fluid levels, and evidence of tissue necrosis. These features help to guide clinical decision-making and management.

Renal Abscess: Renal and perinephric abscesses are localized collections of inflammatory cells that typically result from ascending urinary tract infections, or, less commonly, from hematogenous spread. On computed tomography (CT), renal abscesses often present as well-defined, round masses, sometimes causing a bulge or focal distortion of the renal contour. Associated perinephric inflammation and thickening of Gerota’s fascia are commonly observed. A perirenal abscess typically appears as a fluid collection within the perinephric space, which may contain gas. On magnetic resonance imaging (MRI), renal abscesses are characterized by a central area of T2 hyperintensity and T1 hypointensity, with a thickened wall that demonstrates variable enhancement on T1-weighted postcontrast images. Additionally, perinephric inflammatory stranding and significant diffusion restriction may also be noted, aiding in diagnosis and management.

Pyelonephritis: On contrast-enhanced computed tomography (CT), pyelonephritis typically presents with wedge-shaped or streaky areas of reduced cortical enhancement. This can lead to a characteristic "striated nephrogram" appearance, most prominent during the nephrographic and early pyelographic phases. The pattern consists of alternating linear bands of high and low attenuation, aligned parallel to the axis of the renal tubules and collecting ducts . Delayed and persistent enhancement of these initially hypoattenuating areas may also be observed, reflecting slow contrast transit and prolonged contrast accumulation within the affected regions of the kidney.

Hemorrhagic Etiologies: Perinephric, renal, and retroperitoneal hemorrhages often present with nonspecific and variable clinical symptoms, which are influenced by the volume and duration of the bleeding. These hemorrhages can arise from various conditions, including spontaneous hemorrhage (Wunderlich syndrome), trauma, neoplasms, and vascular abnormalities such as pseudoaneurysms, aneurysms, and arteriovenous malformations (AVMs). The clinical manifestations and severity of these hemorrhages depend on the underlying cause and the extent of the blood loss.

Vascular :

Renal Infarct: On computed tomography (CT), a renal infarct typically appears as a wedge-shaped area of hypodensity on postcontrast images. The renal cortex may also exhibit a patchy hypodense pattern. A thin peripheral rim of enhancement, known as the cortical or nephrographic rim sign, is often observed during the nephrographic phase. This sign indicates a viable band of renal cortical tissue, sustained by a distinct capsular vascular supply, and is typically first noticeable several days after the infarct occurs. Over time, chronic renal infarction can lead to scarring and a loss of renal cortical tissue.

Renal Vein Thrombosis: Renal vein thrombosis can result from various etiologies, including hypercoagulable states, neoplastic invasion, trauma, thrombus extension from the left ovarian vein, and, in infants, dehydration. It is more commonly observed on the left side of the renal vein. Clinical presentation often includes flank pain and hematuria. On contrast-enhanced computed tomography (CT), the thrombus appears as a filling defect within the affected renal vein. Tumor thrombus is characterized by enhancement of the clot. Additionally, renal vein thrombosis is frequently associated with kidney enlargement and perinephric edema. 

Renal Artery Aneurysm: Renal artery aneurysms (RAAs) are typically asymptomatic. CT angiography offers superior reproducibility and enhanced anatomical detail of the renal vasculature with minimal operator dependency. On CT imaging, an RAA is characterized by saccular, non-calcified dilation of the renal artery, most commonly located at the bifurcation of the main renal artery. Conventional angiography remains the gold standard for guiding endovascular treatment interventions.

Renal Pseudoaneurysm: Renal pseudoaneurysms result from disruptions in the arterial wall, leading to contained hemorrhage within the adjacent renal parenchyma or capsule. On unenhanced CT, a pseudoaneurysm appears as a low-attenuation, rounded mass, typically located at an arterial anastomosis or within the renal parenchyma. Depending on the degree of thrombosis, the lesion may exhibit partial enhancement on contrast-enhanced CT (CECT). Conventional angiography is usually reserved for cases with ambiguous anatomy or when endovascular intervention is required. 

Renal Trauma: When evaluating renal trauma with CT, contrast administration is essential for optimal assessment. Imaging should be performed during both the corticomedullary phase (60-70 seconds after contrast administration) and the excretory phase (3-5 minutes post-contrast) to comprehensively evaluate the extent of the injury. Several grading systems have been developed to classify renal trauma, with the American Association for the Surgery of Trauma (AAST) grading system, based on surgical findings, being the most widely utilized.

Grade I injuries are the most common and may present with no imaging findings or show mild contusions, such as focal areas of decreased enhancement, or small subcapsular hematomas. Grade II injuries involve perinephric hematomas and superficial cortical lacerations of less than 1 cm, often appearing as linear low-density areas in the renal parenchyma. In Grade III injuries, the cortical laceration extends beyond 1 cm without involvement of the collecting system. Grade IV injuries are characterized by lacerations that extend into the collecting system or involve the main renal artery or vein. Vascular involvement, such as active extravasation of enhanced blood during the initial or delayed imaging phases, and collecting system involvement, indicated by the extravasation of enhanced urine in the delayed phase, can help confirm the severity of the injury. Grade V injuries represent the most severe form, including a shattered kidney, avulsion or thrombosis of the main renal artery or vein, or avulsion of the ureteropelvic junction.