How do radiographic tests help to evaluate patients with renal stones?
A plain radiograph of the abdomen (kidney–ureters–bladder [KUB]) should be obtained in all stone formers and shows stones with the following features: calcium (small, dense, and circumscribed); cystine (faint, soft, and waxy); and struvite (irregular and dense). Uric acid, xanthine, and indinavir stones are radiolucent and not seen. The progress of the stone can be easily monitored with the KUB. Phleboliths that can be confused with ureteral stones show a lucent center on KUB. Intravenous pyelography (IVP) localizes stones in the urinary tract and shows the degree of obstruction. A radiolucent obstruction on IVP suggests a uric acid stone. Ultrasonography reveals the size and location of larger stones, is sensitive for diagnosing obstruction, and may be best when radiation should be avoided, as in pregnancy. However, the initial radiographic procedure of choice for stone evaluation requires no patient preparation and is easy, sensitive, specific, and accurate. It should be ordered as follows: noncontrast helical (spiral) computed tomography (CT) with renal stone protocol using thin collimation of 2 to 3 mm. A newer technique that may replace noncontrast helical CT, but is not widely available, is dual-energy CT (DECT) with advanced postacquisition processing. DECT assesses stone attenuation at two different peak kilovolt (kVp) levels and can discriminate among several subtypes of urinary calculi without a formal stone analysis. Indinavir stones are not seen on KUB or CT scan and may be missed on IVP. Indinavir, atazanavir, and darunavir stones, which are diagnosed after suspicion is raised by history, physical examination, and signs of obstruction, may require contrast-enhanced CT scanning or IVP.