# Dual-Energy CT Applications in Urological Diseases
## 4 DECT Clinical Applications
### Characterization of Urinary Stones  
CT imaging is the gold standard for detecting nephrolithiasis, as nonenhanced scans have a sensitivity and specificity of \(>\)95% [23; 24]. The use of CT in the acute presentation of stones is increasing, and it is often the preferred imaging since it can help detect other pathologies responsible for acute abdominal pain. Uric-acid stones are made up of lower atomic number elements such as carbon, hydrogen, nitrogen, and oxygen, whereas non-uric-acid stones are made up of higher atomic number elements such as calcium, sulfur, and phosphorus [25]. CT imaging may provide insight into the composition of stones by measuring the attenuation in HU, as in general, uric-acid stones have HU between 200 and 400, while calcium oxalate stones have HU between 600 and 1200 [26; 27]. However, the ability to characterize stone composition with conventional CT scanning technology is limited, as there is a considerable overlap between the HU of different stone types. Because the diagnosis of uric-acid stones leads to medical dissolution therapy rather than surgical intervention, the characterization of stone composition is useful in clinical decision-making [28]. DECT can allow the differentiation of stone chemical composition by measuring the ratio of calculi attenuation at higher and lower energy. With dual-source DECT, the characterization is obtained by calculating the calculus ratio of HU at 80 kVp to HU at 140 kVp and plotting it against the attenuation profile of stones with known composition; the most common method employs a three-material decomposition algorithm that assumes every voxel contains a portion of either water, calcium, or uric acid. The quantity of each of these components is then taken into account when assigning a color to the voxels. This tool processes data quickly, taking only a few minutes, and displays acid uric stones with a red overlay and non-acid uric stones with a blue overlay [29]. With fast kilovoltage switching equipment, a plot of effective atomic number versus the attenuation on a VNC image allows the distinction of four categories of calculi: utilizing projection data, single-source DECT equipment estimates the atomic number of particular materials to distinguish between stones made mostly of uric acid, calcium oxalate, cystine, or struvite [30]. Specific changes in attenuation in HU between high- and low-energy spectra allow the differentiation between materials containing similar electron densities but differing photon absorption; through the application of post-processing techniques, such as material decomposition algorithms, the relative proportion of different materials within the stone can be analyzed, helping to identify mixed-composition stones or identify small amounts of certain components [31]. A common DECT protocol for nephrolithiasis begins with a low-dose conventional CT scan to identify potential urinary calculi, followed by a DECT acquisition restricted to the location of the stone. The images are then processed by software application on a three-material decomposition algorithm, which analyzes the stone composition based on the differences in attenuation at the two different energy levels. The software then color codes the stone based on the composition, with stones with behavior like calcium oxalate visualized as blue and stones with behavior similar to uric acid represented as red [32] (Figures 5 and 6). Using the above acquisition protocol, [PERSON] et al. calculated an effective radiation dose of 3.4-5.3 mSv [33]. When [PERSON] et al. compared the radiation exposure of such DECT protocols to that of low-dose and conventional CT scans, they found no significant difference in radiation exposure when compared to the low-dose CT scan alone; moreover, when compared to a conventional CT scan, DECT protocols at 80/140 kVp and 100/140 kVp resulted in a radiation dose reduction of 40% and 31%, respectively [34]. Different studies analyzed the accuracy of DECT in the characterization of urinary stones. [PERSON] et al.