# Dual-Energy CT Applications in Urological Diseases
## 4 DECT Clinical Applications
### Study of Renal Lesions
Iodine-containing renal lesions can be distinguished from non-iodine-containing cysts because iodine attenuation increases at lower energies (\"upward-curve-type\"); in contrast, cyst attenuation levels remain constant across the whole monochromatic multi-energy range (\"flat-curve-type\") [9] (Figure 7). In a study by [PERSON] et al., 78 renal incidental lesions (24 enhancing; 54 nonenhancing) measuring 1-4 cm were studied with a rapid kilovoltage-switching DECT and the results were compared with histology: the analysis of the monochromatic images and of the iodine curves significantly differentiated the enhancing and nonenhancing lesions (\(p<0.001\)) [63]. Proper preoperative pathological classification of RCC is essential for evaluating the malignancy of the tumor and selecting the appropriate surgical approach. DECT can be used to determine the tumor blood supply of a lesion. In fact, tumor blood supply types are correlated with pathological grade, with well-differentiated clear-cell renal-cell carcinoma (ccRCC) exhibiting mature arteries and enhanced vascular permeability, and poorly differentiated ccRCC exhibiting hemodynamic abnormalities and arteriovenous shunts [64]. Differentiating between papillary (p) RCC and cRCC can be challenging in imaging [65], but a recent study by [PERSON] et al., indicated that ccRCC had a significantly higher iodine concentration than pRCC, and a positive correlation was found between the iodine concentration and microvascular density on DECT, and a cut-off value of \(\leq\)3.1 mg/mL at iodine concentration analysis allowed the diagnosis of pRCC with an accuracy of 86.8% [56]. DECT can also be used in the characterization of AML. AML is typically characterized by macroscopic fat, which can be confirmed by visual analysis and attenuation measurements of \(<\)\(-\)10 to \(-\)15 Hounsfield units (HU) on CT. However, misclassification of AML can occur in cases of fat-poor AML or when imaging with only contrast-enhanced CT due to elevated attenuation measurements. This can happen in small AMLs due to volume averaging of adjacent renal parenchyma or pseudo enhancement, as well as due to the presence of internal enhancing vessels within the AML (Figure 8). Up to one-third of AML may be indeterminate by HU measurements when evaluating only contrast-enhanced CT, leading to recall for further imaging such as dedicated thin-section unenhanced CT or MRI for confirmation. Figure 7: Example of a spectral attenuation curve of a small left renal cyst. (**a**) Monochromatic image at 40 keV; (**b**) monochromatic image at 80 keV; (**c**) iodine map showing no contrast enhancement within the cyst, confirmed by the flat-curve-type of the cyst in blue (**d**), compared with the upward-curve-type of the aorta in yellow that shows a rise in Hounsfield units for low energy levels. HU = Hounsfield Units. However, since the introduction of DECT, other analysis systems such as iodine maps, VMI with spectral curves, atomic-number maps, and fat maps have been developed to improve the signal fat and increase the sensitivity of CT in the recognition of intralesional adipose tissue. Material-specific fat images derived from DECT seem to be highly accurate in detecting areas of macroscopic fat, with fat concentration measurements being useful to confirm the presence of macroscopic fat in renal masses and establish the diagnosis of renal AML when non-contrast-enhanced CT is unavailable. In cases where DECT was used, the nodule could have been definitively characterized as containing macroscopic fat without recalling the patient for additional imaging [66]. In a prospective study including 95 patients with renal tumors, the authors analyzed the absolute enhancement, the enhancement ratio, and iodine content of each lesion on nephrographic phase images and were able to distinguish fat-poor AML from renal malignant lesions [67]. In Table 2, we summarize the results of the above studies.