Determination of renal stone composition with dual-energy CT: in vivo analysis and comparison with x-ray diffraction.

PURPOSE To preoperatively assess the composition of urinary stones by using dual-energy computed tomography (CT), with postoperative in vitro x-ray diffraction analysis as the reference standard. MATERIALS AND METHODS Institutional review board approval was obtained, and all participants provided written informed consent. Twenty-seven patients aged 50-64 years with renal stones, who were scheduled for stone extraction with percutaneous nephrolithotomy (PCNL), preoperatively underwent nonenhanced single-source dual-energy multidetector CT with 2-mm section thickness, 1-mm increments, 140 kVp, and 250 mAs. Regions of interest were drawn on low- and high-energy images, and low- and high-energy attenuation ratios were calculated for each stone scanned in vivo. The attenuation ratios for the patients were compared with those for an in vitro stone library phantom model of 37 stones with known chemical compositions. After surgery, the extracted stones were analyzed by using x-ray diffraction. The results of in vivo multidetector CT and ex vivo chemical analysis were compared. RESULTS Dual-energy low- and high-energy attenuation ratios measured with the phantom were less than 1.1 for uric acid, 1.1-1.24 for cystine, and greater than 1.24 for calcified stones. Struvite stones had attenuation ratios that overlapped with calcified stone ratios and thus could not be assessed reliably. Four patients had mixed stones (<75% of a single component), and one patient had a struvite stone. Of 27 patients, 22 (82%) (exact confidence interval [CI]: 68%, 92%) received a correct diagnosis with dual-energy CT: all six (100%; exact CI: 54%, 100%) patients with uric acid stones, 15 (79%; exact CI: 62%, 95%) of the 19 patients with calcium stones, and the one (100%) patient with a cystine stone. The patient with a struvite stone did not receive a correct dual-energy CT-based diagnosis. CONCLUSION Dual-energy multidetector CT may enable accurate in vivo characterization of kidney stone composition.

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