Noninvasive differentiation of uric acid versus non-uric acid kidney stones using dual-energy CT.

Symptomatic urinary stone disease affects approximately 900,000 persons in the United States each year, resulting in annual medical cost of $5.3 billion. Nephrolithiasis has traditionally been evaluated using plain film radiographic techniques with or without tomography or administration of intravenous contrast for excretory urography. Over recent years, however, computed tomography (CT) has supplanted these traditional techniques because of increased sensitivity, speed, and the lack of intravenous contrast (1). Furthermore, improved CT acquisition techniques can allow a lower radiation dose than was required for a traditional excretory urogram (2,3). Although state-of-the-art CT provides accurate submillimeter details of the size and location of renal stones (4,5), current routine clinical image analysis does not differentiate stone composition. This is particularly important in the case of uric acid (UA) stones (~10% of cases), because urinary alkalinization can be prescribed to dissolve UA stones and could thereby be initiated at presentation rather than following lengthy metabolic workup. Therefore simple and reliable differentiation of UA versus non-UA stone composition could potentially allow patients with UA stones to avoid invasive interventional urinary procedures for stone removal or external shock wave lithotripsy, both of which are expensive and might result in renal hemorrhage, fibrosis, or hypertension (6). Previous attempts to predict stone composition using spiral CT were based on the analysis of CT numbers. Several in vitro and in vivo studies have shown that this approach can discriminate UA from non-UA stones (7-11). Combining the analysis of CT numbers with visual assessment of stone morphology using a wide window setting (eg, bone window) substantially improves the accuracy of stone characterization (12). Bellin et al (13) reported in vitro prediction of stone composition with 64%- 81% accuracy, whereas Zarse et al (14) demonstrated that high-resolution spiral CT yields unique CT numbers for common types of stones if proper window settings are used to localize homogeneous regions within the stones. However, the CT number approach to predict stone composition is not yet robust or reliable enough to be used as a routine clinical application, and a dual-energy CT (DECT) technique could be considered as an alternative. Because UA stones are composed only of light chemical elements (H, C, N, O), their x-ray attenuation properties at high and low kVp are very different compared with those for other (non-UA) stone types (eg, calcium oxalate, calcium hydroxyapatite, cystine), whose composition includes heavy elements (P, Ca, S). As a consequence, UA stones have higher CT numbers at higher kVp than at lower kVp, whereas non-UA stones have higher CT numbers at lower kVp than at higher kVp. Indeed, some previous studies (8,13,15) have already exploited this fact and used the difference between CT numbers at high and low kVp to improve the prediction accuracy of the CT number approach. A recently introduced dual-source CT scanner (SOMATOM Definition, Siemens Medical Solutions, Forchheim, Germany) allows simultaneous dual-energy, high spatial resolution acquisition and immediate image postprocessing using a commercial material decomposition algorithm (SyngoDE, Siemens). Our previous study (16), which used both single-source and preliminary dualsource CT data analyzed with the beta version of the material decomposition software, demonstrated 75%-100% accuracy of the DECT technique in differentiating UA from non-UA stones. Several recent software and hardware upgrades have improved both the scanner performance and the material decomposition algorithm. Therefore the current study was undertaken to demonstrate the in vitro prediction accuracy of the DECT system to differentiate UA from non-UA stones under clinical conditions.