# Dual-Energy CT Applications in Urological Diseases
## 4 DECT Clinical Applications
### Characterization of Urinary Stones
analyzed 213 stones extracted from 64 patients and discovered that UAC-containing stones had a different dual-energy index (\(p\) = 0.001) compared to pure calcium stones (0.073) and mixed calcium stones (0.077), with a sensitivity of 98.4% and specificity of 98.1% for the differentiation of uric-acid from non-uric-acid stones [35]. [PERSON] et al., analyzed DECT images from 33 patients with 62 stones that were spontaneously excreted or treated and found that they had 100% sensitivity and specificity in predicting 15/15 UA stones and 47/47 non-UA stones [36]. [PERSON] et al., evaluated nephrolithiasis patients scanned with a low-dose CT followed by a DECT scan: conventional CT correctly detected the composition of 52% (26/50) stones, while DECT correctly detected 90% (45/50) stones; moreover, the conventional CT scan was able to differentiate the uric-acid stones from non-uric-acid stones in 80% (40/50) cases, compared to 96% (48/50) in the DECT scan [37]. In a prospective study, [PERSON] et al., examined 84 patients who underwent DECT followed by a stone procedure and demonstrated 98.6% consistent results between the DECT and infrared spectrometry. They were able to predict urinary-acid stones from non-urinary-acid stones with a sensitivity of 84.6%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 98.5%. In their model, the combination  
Figure 5: (**a**) Left kidney stone segmented. The composition was revealed to be uric acid (**b**). HU = Hounsfield Units. Figure 6: Stone composition analysis. (**a**) Intraurethral hyperdense stone; (**b**) automatic segmentation of the stone; (**c**) in the graphic, the stone is placed next to the line of calcium oxide. HU = Hounsfield Units. of urine pH with DECT resulted in an area under the curve of 0.97 compared to 0.92 for DECT alone, which suggests that combining urine pH with DECT can even enhance the accuracy of DECT [38]. [PERSON] et al., retrospectively included patients who received a low-dose unenhanced DECT for the detection or follow-up of urolithiasis and stone extraction with stone composition analysis. A total of 227 stones in 203 patients were analyzed using X-ray diffraction as the reference standard and identified as 15 uric-acid and 212 non-uric-acid stones with mean size and volume of 4.7 mm \(\times\) 2.8 mm and 114 mm\({}^{3}\), respectively. Size, volume, CT attenuation, and stone characterization were assessed using DECT post-processing software, and 225 of 227 kidney stones were correctly classified by DECT, with pooled sensitivity, specificity, and accuracy of 1.0 (95% CI: 0.97, 1.00), 0.93 (95% CI: 0.68, 1.00), and 0.99 (95% CI: 0.97, 1.00), respectively. Finally, 82 of 84 stones with a diameter of \(\leq\)3 mm were correctly classified [39]. Although still effective, the accuracy of DECT may be slightly diminished in patients with a large body habitus and when the stone size is <3 mm [40,41], this may not be a clinically significant finding, as most stones <3 mm do not warrant intervention and spontaneously pass on their own. The detection rate of stones is highly dependent of the concentration of iodine in urine; in fact, in a phantom study, >95% of 2-4 mm stones in solution with diluted iodinated contrast medium were identifiable on VNC images, whereas the detection rate was markedly reduced when stones were surrounded by extremely highly concentrated iodine solution. Highly iodinated urine may result in failed iodine subtraction in VNC when the attenuation value of the iodine solution reaches a maximal CT value on low-kV images, thus reducing the sensitivity in stone identification [42]. VNC images created from both nephrographic and pyeographic phases were compared for stone detection [43], with a sensitivity of VNC images from nephrographic phase of 77.5% and from urographic phase of 73.8% (59/80).