Characterization of urinary stones with dual-energy CT: improved differentiation using a tin filter.

Purpose: To assess image quality and capability of stone differentiation between UA-containing and non-UA-containing uroliths with the latest dual-energy (DE) computed tomography (CT) system equipped with a tin filter (TF) using various data acquisition parameters in a work bench model.

Methods And Materials: One hundred ten urinary stones (4.2 +/- 3.0 mm, 0.4-12 mm) of 15 compositions were examined in an ex vivo phantom, using DE dual-source CT (Definition Flash, Siemens Healthcare) equipped with a TF. Phantom was scanned in a water tank and contained stones in acrylic elliptic spheres filled with a parenchyma substitute. Scans were performed at 3 different settings: at 80 and 140 kVp without TF, at 80 and 140 kVp with TF, and at 100 and 140 kVp with TF. Tube current time products were adapted to yield constancy in CT dose indices (CTDIvol = 18.84 mGy, 18.95 mGy, and 18.90 mGy, respectively). CT numbers of urinary stones and image noise were electronically measured by placing regions of interest. DE indices (DEI) were calculated and compared using analysis of variances for repeated measures and paired t tests; image noise (IN) using the Friedman test. The stones were classified as UA-containing or non-UA-containing on color-coded images based on the DEI. Diagnostic accuracy was calculated using crystallographic analysis as standard of reference.

Results: Of the 110 stones (60%), 65 contained UA; 45 stones (40%) contained no UA. DEI was greatest at 80 and 140 kVp when using the TF (DEI80 kVp/TF140 kVp = 0.038 vs. DEI80 kVp/140 kVp = 0.028, DEI100 kVp/TF140 kVp = 0.025; P < 0.01). IN of high kVp acquisitions were similar (P = 0.15), whereas IN of low kVp acquisitions were significantly (P < 0.001) different being lowest at 100 kVp. The semiautomated DE software correctly classified all stones at all settings with a diagnostic accuracy of 100% (95% confidence interval: 97%-100%).

Conclusion: DECT with TF and 80-140 kVp tube voltage settings significantly improves the discrimination between UA-containing and non-UA containing urinary stones as compared with DECT without using the TF on the basis of DEI. The 100/140 kVp setting with TF is associated with lower IN but demonstrates similar discrimination abilities as compared with 80/140 kVp setting without the use of the TF.