In addition to direct visualization of the ureteral stone, secondary signs of ureteral obstruction on noncontrast CT scans include unilateral nephromegaly, perinephric stranding, hydronephrosis, and periureteral stranding. The combination of perinephric stranding and unilateral hydronephrosis has a positive predictive value (PPV) of 96% for the presence of stone disease. The absence of both of these signs has a negative predictive value (NPV) of 93% for excluding stone disease. CT also gives information that determines therapy. Stones that are ≤5 mm, smooth shape, and located within the distal one third of the ureter are likely to pass spontaneously [11].

The major pitfall in noncontrast helical CT when evaluating the urinary tract for stone disease is difficulty in distinguishing pelvic phleboliths from ureteral calculi. The presence of a tissue rim sign usually indicates that the calcification is a stone rather than a phlebolith, with the rim representing edema of the ureteral wall. Alternatively, absence of the tissue rim sign or presence of a comet-tail sign strongly suggests that the calcification is a phlebolith rather than a stone. In practice, the presence of two or more secondary signs of obstruction, even without clear visualization of a calcification within the ureter, indicates obstruction. If there is no history of recent stone passage and the CT scan demonstrates findings suggestive of obstruction, a contrast-enhanced study of the upper tracts may be needed to exclude a urothelial neoplasm, with additional cystoscopy to optimally evaluate the bladder.

Excretory urography (EU) is a less desirable study than unenhanced CT in an acute pain setting and has been relegated to historical significance only in evaluating patients presenting with acute colic. EU was once considered to have a potential role in evaluating pregnant patients with acute flank pain when results of US examination were negative or equivocal. However, even in this patient population, low-dose CT may be the technique of choice, as it provides a definitive and rapid diagnosis. These advantages outweigh the slightly greater radiation exposure [18, 19]. Acute colic in pregnant patients is discussed below.

US, usually combined with a plain film, is an alternative method for evaluating the obstructed or dilated urinary tract. It is often used as the first imaging procedure in patients who should avoid radiation, such as pregnant women, and children. Calculi as small as 0.5 mm may be detectable in optimal conditions. When stones are >5 mm, US has a sensitivity and specificity of 96% and nearly 100%, respectively [20]. Although US allows for excellent evaluation of the renal parenchyma and collecting system to the ureteropelvic junction, it is limited in evaluating the ureter and of soft-tissue lesions within the collecting system. The use of renal US in evaluating suspected acute ureteral obstruction is also limited because dilatation often does not develop for hours, or even days. In these cases, US findings are normal in up to 50% of patients. US alone or combined with conventional radiography has been compared with unenhanced CT. US has a much lower sensitivity, varying from 24% to 77% [5, 21–23], compared with 92–96% for CT. In Sheafor et al.’s study that compared CT and US [21], CT depicted 22 of 23 ureteral calculi (sensitivity 96%); US depicted 14 of 23 (sensitivity 61%), which was statistically significant (P=0.02). Specificity for each technique was 100%. CT can give a rapid and definitive diagnosis of urinary calculus disease, as well as other abdominal disorders with the same presentation. Identifying ureteral jets within the urinary bladder lumen on US is helpful for assessing the presence of obstruction. One study showed an absent ureteral jet in 11 of 12 patients with high-grade obstruction and in three of 11 patients with low-grade obstruction [23]. Identification of jets at the ureterovesical junctions indicates that obstruction is incomplete, and thus may be used to guide therapy. It is well recognized that renal calculi <5–7 mm, particularly in large or obese patients, may not be detected on US [25, 26]. However, US may overestimate stone size in urolithiasis by as much as 1.9 mm compared with CT, particularly stones <5 mm [27]. This is an important limitation, as it has implications for stone management.

In diuresis renography, radionuclides are injected to evaluate the urinary tract for obstruction. Because considerably less anatomic detail is available with this test than with other radiographic examinations, it is less useful in the acute setting than for follow-up or for evaluating chronic urinary tract obstruction. Diuresis renography does have the advantage of yielding objective data regarding the significance of hydronephrosis and also allows for evaluation of the function of each kidney. Administration of a diuretic, usually furosemide, augments the standard renogram and is useful in evaluating dilated urinary systems to determine whether there is physiologically significant obstruction.

Magnetic resonance urography (MRU) using rapid scanning techniques, such as half-acquisition turbo spinecho or single-shot fast spin-echo and 3D gradient-echo contrast-enhanced sequences are used for evaluating the urinary tract. Following administration of 250 ml of normal saline and 10 mg of furosemide, kidneys and dilated ureters are very bright on T2-weighted imaging, and their stable position allows for clear imaging of the level of obstruction (Table 3). However, stones appear as signal voids and can be difficult to identify and measure on MRU. Small calculi (the majority of symptomatic stones) are difficult to detect on MR imaging (MRI). Urothelial lesions, blood clots, and debris can also mimic calculi. Noncontrast-enhanced functional MRI (fMRI) techniques, such as blood-oxygen-level-dependent (BOLD) MRI or diffusion-weighted MRI (DWI), can detect urinary obstruction in patients with renal calculi, even when the collecting system is not dilated. These sequences might be particularly useful in patients with renal impairment, children, or pregnant women. Although these techniques do not allow the detection of ureteral calculus, they provide information on the presence or absence of acute ureteral obstruction, as evidenced in a small number of patients [28, 29]. Furthermore, these techniques might be helpful in transplanted kidneys to differentiate obstruction from dilatation due to denervation. However, larger-scale studies are yet needed in order to confirm these initially promising results.