Multidetector computed tomography (MDCT) is the gold standard for imaging patients with suspected obstructive uropathy. Stone/lesion size, location, and degree of obstruction can be quantified. Non-contrast CT (NCCT) has a sensitivity of 95–98 % and specificity of 96–100 % for detecting urinary stones [3–5]. NCCT is relatively quick, without the use of intravenous or oral contrast, making it particularly useful in the emergency setting. Most stones appear radiodense on CT. Rarely, in HIV patients who are poorly hydrated and treated with indinavir, crystalline stones develop which are radiolucent on NCCT. When findings are negative or equivocal, intravenous contrast can be administered to assess for additional renal or abdominal processes. Besides urinary stones, other etiologies of intraluminal obstruction include blood clots, which are dense but lower in attenuation than calculus; masses, which measure soft tissue attenuation and enhance following contrast administration; and ureteral spasm, strictures, or injury. Extraluminal compression may be caused by retroperitoneal masses, abscess, inflammation, or posttraumatic hematoma [4, 5].

At CT evaluation for ureteral stone, one should focus at the most common locations; narrowing of the ureter occurs naturally at the ureterovesicular junction (UVJ), the pelvic brim as the ureter crosses iliac vessels, and at the ureteropelvic junction (UPJ) (Fig. 7.1b, c) [3]. Frequently, a transition point is visible, with urinary decompression more distally. Secondary CT findings in obstructive uropathy include hydroureter, hydronephrosis, perinephric stranding, and possible enlargement of the unilateral kidney (Fig. 7.1d) (Table 7.1). Periureteral wall thickening and perinephric/periureteral fat stranding reflect acute inflammation. The “soft tissue rim sign” is due to edema of the ureteral wall at the site of stone impaction and may help differentiate a ureteral stone from a phlebolith in an adjacent vein (Fig. 7.1c) [6]. Renal edema may be appreciated, with decrease in Hounsfield attenuation on NCCT (“pale kidney” sign). Following contrast administration, there may be delayed excretion of contrast into the collecting system/ureter or beyond the level of obstruction (“delayed nephrogram/urogram”), which may be partial or complete (Fig. 7.2). Pyelosinus or forniceal rupture manifests as a perinephric fluid (Fig. 7.3). In cases of extrinsic compression, the ureter appears abnormally deviated due to mass effect [4, 5, 7].

Ultrasonography (US) is a useful screening examination for pregnant and pediatric patients, in whom radiation ­exposure is a concern. The sensitivity for urinary obstruction is 60–70 %, but this is highly operator and patient dependent and involves limited ureteral assessment [8]. Renal stones are echogenic foci with or without shadowing, depending on size, composition, and technique (Fig. 7.4). Smaller stones may blend into the echogenic renal sinus and be missed. Ureteropelvic and ureterovesical junction stones, which lie superficially within good acoustic windows, may be seen. However, mid-ureteral stones are extremely difficult to detect. The finding of hydronephrosis is suggestive but may be absent in early obstruction (false negative) or present due to alternate causes such as pregnancy and reflux (false positive) (Fig. 7.5a). One must distinguish true hydronephrosis from mimics such as parapelvic cysts, extrarenal pelvis, and pelvicaliectasis. Additional signs include absent or decreased ureteral jets and a Doppler arterial resistive index greater than 0.70 (or difference between kidneys greater than 0.10). However, these findings are neither sensitive nor specific [8].

Magnetic resonance imaging (MRI) is an alternative to CT, particularly in pediatric and pregnant patients for whom radiation exposure is a concern. MRI offers superior soft tissue contrast, albeit with increased costs and longer scan times. Ultrafast scanning protocols have been developed to image the urinary system with good spatial resolution in multiple planes. Fluid-sensitive T2-weighted sequences, such as single-shot fast spin echo (SSFSE) and balanced steady-state free precession (SSFP), utilize urine as an intrinsic contrast agent [9]. Fat-suppressed sequences are also acquired to enable differentiation from intraperitoneal/retroperitoneal fat. Urinary calculi, which are proton poor, may appear as hypointense filling defects (Fig. 7.5b, c). Stones smaller than 1 cm are frequently not seen – in which case diagnosis relies on signs of urinary obstruction, such as a ureteral transition point [9]. Acute obstruction frequently presents with soft tissue wall thickening, stranding, and edema, manifesting as high T2 signal and loss of renal corticomedullary differentiation. Multiphasic gadolinium-enhanced imaging can be performed using a T1-weighted spoiled gradient echo sequence. This allows assessment of the renal parenchyma, improves urinary-calculi contrast, and quantifies collecting system transit time [9].