# Dual-Energy CT Applications in Urological Diseases
## 4 DECT Clinical Applications
### Study of the Bladder and Ureters  
Urothelial tumors (UC) are divided into upper-tract urothelial carcinoma (UTUC), involving calico-pyelic cavities and ureters, and lower-tract urothelial carcinoma (LTUC), involving bladder (90-95%) and proximal urethra. Compared to cystoscopy, which represents the gold standard in bladder cancer diagnosis, URO-CT is not a useful method in the identification of LTUC but is useful in follow-up and in muscle-invasive and metastatic bladder cancer to evaluate its extension [1; 69], multiple high-risk tumors, or in the trigone vesica localization. On the other hand, CT urography is the most sensitive method for the identification of UTUC [70]. DECT appears to be better both in the diagnosis and in the follow-up of UC because it has many benefits in the study of them, such as less exposure to ionizing radiation and iodinated contrast agents [71; 72]. UC can be visible on CT as small filling defects, mass lesions, vegetations, or wall thickening [71]. The detection of UC relies on the contrast between the tumor and an adjacent non-pathological urothelial wall or surrounding urine. During the parenchymal phase of contrast enhancement, UC shows increased contrast enhancement, while the urine is unenhanced and the surrounding normal urothel shows minimal enhancement; in the urographic phase instead, UC reduces the enhancement and becomes hypodense, and iodinated urine shows high density. The higher contrast is, however, shown in the corticomedullary phase, related to their extensive vascularity. In this phase, any focal area of urothelial hypereenhancement should be considered suspicious for malignancy [73; 74]. DECT offers the possibility of reducing radiation exposure due to VNC, avoiding baseline scans starting from postcontrast acquisition and removing iodine from soft tissues [7; 75]. The combination of VNC images and the split-bolus protocol have been proposed to obtain a further reduction in radiation exposure [70]. The nephrographic and excretory phases are acquired at the same time in the split-bolus protocol, and the contrast medium is administered in two separate injections: the first part (usually one-third or half) is injected; the second part (usually a dose greater than or equal to the first one) is then injected about 5-10 min later, and the combined nephrographic-excretory-phase images are acquired at 2-5 min when the kidneys show enhancement of the renal parenchyma [12]. The ratio between the first and second contrast portions of the split bolus, as well as the ideal delay time, vary widely among different studies [12]. This protocol allows evaluating renal masses and UC correctly thanks to the concurrent visualization of the cortico-medullary, nephrographic, and excretory phases with a single scan [76; 77]. The possibility of reducing the amount of contrast material administration is another advantage of DECT, especially in the case of multiple follow-ups or patients with kidney failure or older patients with multiple comorbidities [78]. This is possible because of the use of low-energy images which allow for obtaining higher contrast images, due to the higher attenuation coefficients at lower energy, and increased tumor conspicuity [7]. Low-dose VMI allows a better quantitative rating of primary UC than conventional venous-phase imaging with similar qualitative ratings for both primary UC and metastases [79]. Another issue that DECT can help with is beam-hardening artifacts caused by metal prostheses, which falsify pelvic evaluations [80]. These artifacts can be reduced due to VMI with doses between 105 keV and 150 keV [81; 82; 83] at the expense of contrast and, consequently, the worst evaluation of soft tissues, albeit with noise reduction. Reconstruction algorithms that use datasets at different energy levels can help to overcome this obstacle [84; 85]. The evaluation of the contrast produced between the tumor and the surrounding normal urothelial wall or overlying urine is the basis for the detection of urothelial tumors (Figures 9 and 10).