Ultrasonography is helpful for initial screening of renal lesions as well as to discriminate cystic lesions from solid lesions and monitoring growth of previously determined lesion [6]. Renal cancers usually present as a focal, expansile mass with heterogeneous echogenicity different from adjacent hypoechoic renal parenchyma. Heterogeneous echogenicity and expansile nature of renal cancers are helpful in distinguishing renal cancers from pseudotumoral renal lesions such as column of Bertin and dromedary hump of the kidney . However detection of small renal cancers (<3 cm) confined in renal parenchyma without expansile appearance may be difficult with US especially if these cancers have isoechoic appearance similar to renal parenchyma. Renal cell carcinoma (RCC) as being most frequently encountered renal tumor usually manifests as hypo-, iso-, or hyperechoic expansile mass on US (Figs. 1.1 and 1.2).

Small renal masses (<3 cm) may more likely present with hyperechoic appearance than larger tumors [7]. Papillary RCCs usually appear as hypoechoic or isoechoic and rarely hyperechoic solid masses (Fig. 1.3) [8]. However it is nearly impossible to differentiate subtypes of RCCs such as clear cell RCC , papillary RCC, and chromophobe RCC by US due to similar sonographic features of these tumors on grayscale US and CDUS. Generally, papillary cell types of RCCs have less vascularity compared to other types of RCCs on CDUS (Fig. 1.3). Renal lymphomas and metastases may manifest as multifocal infiltrative masses (Fig. 1.4). Renal pelvis tumors, most frequently as transitional cell carcinomas (TCCs), present with a hypoechoic appearance within the hyperechoic renal sinus (Fig. 1.5). However TCCs or other tumors localized in renal pelvis are usually more susceptible to be missed on US compared to renal parenchymal tumors.

Cystic renal masses detected on US should be elaborated in terms of malignancy. Sonographic features that increase the likelihood of malignancy in complex renal cysts include thickened cyst wall, numerous or thickened or nodular septations within the cyst, presence of irregular or central calcifications, and the presence of blood flow in the septations or cyst wall (Fig. 1.6) [9]. US can be an important complementary method by revealing cystic nature of hyperdense, solid-appearing renal lesions on CT and solid nature of lesions which appear as cystic mass on CT [10].

Contrast-enhanced US seems to be a promising imaging technique for distinguishing benign and malignant renal tumors. A meta-analysis study including 11 studies reported a pooled sensitivity of 88% and specificity of 80% in differentiation between benign and malignant renal tumors by CEUS [11].

Ultrasound elastography is an emerging technique based on measuring elasticity of biological tissues by calculating their response to manually applied force by US probe or propagating sound waves. Since malignant renal tumors are assumed to be stiffer than benign tumors, it has been suggested that US elastography can be used to differentiate benign and malignant renal tumors (Fig. 1.7). Although successful results imply the utility of US elastography in differentiation between benign and malignant renal tumors, determination of subtypes of renal tumors seems to be unpredictable by this technique [12].

Assessment of renal vein involvement is mandatory in case of renal cancers. Renal veins should be visualized from renal hilum to inferior vena cava (IVC) on CDUS to detect hypo- or hyperechoic filling defect with solid appearance in renal vein representing tumoral thrombus. CDUS is comparable to MRI for detecting tumoral extension to renal veins and inferior vena cava with a sensitivity of 86% and specificity of 94% [13–15].

Main limitation of US in assessment of renal tumors is difficulty to detect and characterize small renal masses. One study showed that 42% of renal lesions between 15 and 20 mm were not detected on US while CT detected 100% of lesions [16]. User dependency which may cause interobserver variability in follow-up of lesions is another limitation of US.

Intraoperative US yields high-resolution images in partial nephrectomies and enucleation of tumors. Intraoperative US can demonstrate new findings which were not detected on preoperative imaging in 10.6% of cases and alters the surgical management in 71.4% of patients with renal cancers [17].

Computed tomography is the decision-making imaging technique in assessment of renal tumors. Awareness of sonographic imaging features of renal mass may be helpful for planning CT protocol since renal parenchymal or pelvis tumors should be scanned with different CT protocols. The use of intravenous (IV) iodinated contrast material and ionizing radiation in CT examination mandates appropriate planning of CT protocol. Multiphasic CT protocols used in the evaluation of renal mass include precontrast scanning and corticomedullary (scan delay 35–40 s after IV contrast injection), nephrographic (scan delay 70–90 s after IV contrast injection), and delayed excretory (scan delay 5–10 min after IV contrast injection) phases [18]. Precontrast images demonstrate calcifications in the renal masses and yield a baseline density measurement to compare enhancement degree and pattern of the tumors on contrast-enhanced images. Precontrast CT is also critical in depicting hypovascular hemorrhagic cysts which may be misdiagnosed on contrast-enhanced images as hypovascular papillary RCC [19]. In these cases, unenhanced CT demonstrates hyperdense appearance of hemorrhagic cysts secondary to high attenuation of the blood on CT and helps to realize pseudoenhancement of hemorrhagic cysts on contrast-enhanced CT images (Fig. 1.8). Corticomedullary images demonstrate lesion vascularity and renal vascular anatomy. Renal cortex enhances more than renal medulla in corticomedullary phase. Images acquired on this phase may help to distinguish hypervascular clear cell carcinoma from hypovascular papillary cell carcinoma [19]. However renal masses localized in renal medulla may be missed on corticomedullary phase images. In nephrographic phase renal parenchyma enhances homogeneously with similar enhancement in renal cortex and medulla. Renal tumors manifest as less enhancing solid or semisolid lesions compared to renal parenchyma (Fig. 1.9). This phase is the most helpful imaging phase for detection and characterization of renal masses [20]. Nephrographic phase images have superiority in detection especially small (<3 cm) renal masses in the renal parenchyma [18]. Excretory images are helpful to delineate renal collecting systems, ureters, and bladder with tumoral involvement of these structures (Fig. 1.9).

Malignant potential of a renal mass increases with presence of significant enhancement which is defined as an attenuation increase of at least 15–20 HU on postcontrast images with respect to the precontrast image [20]. Enhancement of a lesion up to 10 HU is defined as pseudoenhancement which may be encountered in some renal cysts. Enhancement of 10–20 HU in a renal mass on CT refers to indeterminate mass that necessitates assessment with MRI. Other scanning phases give additional valuable information for presurgical planning. Contrast enhancement characteristics of renal masses can be a distinguishing feature in prediction of subtypes of RCCs. Conventional type or clear cell type of RCC as being most frequently detected RCC subtype presents usually as well-circumscribed, heterogeneous mass containing usually two components as solid hypervascular portion and necrotic or hemorrhagic necrotic, avascular portion [21]. Typical clear cell RCC manifests with intense enhancement in the corticomedullary phase and less enhancement compared to renal parenchyma at nephrographic phase. Papillary RCCs were reported as homogeneously enhancing renal mass in comparison to renal parenchyma and other subtypes of RCC [22, 23]. A hypovascular solid renal mass without fat content usually suggests papillary RCC as 82% of the cases manifest with less than or equal to 40 HU enhancement [24].

Small renal lesions which are smaller than 10 mm constitute a challenge for both urooncologists and radiologists. Characterization of renal masses less than 1 centimeter is frequently difficult on CT [21]. If a renal parenchymal lesion appears hypodense compared with the renal cortex on precontrast CT images with the density values of <10 HU or <−20 HU regardless of density values after IV contrast administration, these lesions can be assumed to be a benign renal parenchymal lesion mostly renal cortical cyst and small angiomyolipoma, respectively [21]. When density measurement of small renal parenchymal lesions does not yield any informative value, these lesions can be defined as “indeterminate microlesion, with no suspect characteristics” and can be followed up with imaging [21].

Cystic renal masses detected on CT should be interrogated in terms of malignancy. Bosniak classification system is widely accepted as a reliable tool to define complicated cystic renal masses for likelihood of malignancy. Although Bosniak classification was firstly introduced as CT classification system, the classification scheme may also be applied to MRI [25]. According to Bosniak classification system, category I lesions refer to simple cysts. Category II lesions have smooth septa and minimal wall thickening. Category I and II lesions are benign lesions requiring no further workup. Category IIF lesions include well-marginated cysts with enhancing or nonenhancing multiple hairline-thin septa and nonenhancing high-attenuation renal lesions. These lesions are indeterminate moderately complicated cystic renal masses that require follow-up to demonstrate stability. Category III lesions have thickened wall or septa and include some imaging features suspicious of malignancy that may be managed surgically. The presence of solid component in cystic renal mass refers to Bosniak category IV lesion and indicates high suspicion for malignancy (Fig. 1.10). Category IV lesions are managed surgically. Pseudoenhancement which is characterized as increased density in the cyst wall or septa after IV contrast administration is a pitfall that can cause misdiagnosis of cystic renal malignancy. Pseudoenhancement of cystic renal masses results from volume averaging and beam-hardening effects on CT [22]. Smaller renal cysts tend to be more amenable to pseudoenhancement [26]. Hemorrhagic cysts can present with pseudoenhancement; however hyperdense appearance of hemorrhagic cysts on precontrast CT is characteristic for hemorrhagic cysts.

CT can easily identify macroscopic fat in renal masses. The diagnosis of angiomyolipoma can be established safely on CT when the density of a mass measured as <−20 HU with no content of calcification or necrosis [21]. However RCCs may rarely present with fat component. Fat content in a RCC mostly occurs in papillary cell type [27]. In the setting of fat-containing renal mass, the possibility of malignancy should be thought if a large, solid, infiltrating, and heterogeneous lesion is detected on CT. Calcifications may be encountered in 30% of RCCs which are typically central and irregular [21]. Invasion of the renal vein and inferior vena cava (IVC) occurs in 23% and 7% of RCCs, respectively [28] (Fig. 1.11).

RENAL nephrometry scoring system is a numerical scoring system of imaging features of renal mass on CT or MRI including maximal tumor radius, exophytic versus endophytic nature of the tumor, relationship of the tumor to the collecting system or sinus, location relative to polar lines, and anterior or posterior tumor location [20]. It was reported that RENAL nephrometry scoring system can be used as a predictor of surgical outcomes of laparoscopic partial nephrectomy and histology and grade of RCCs [29].

Dual-energy CT (DECT) is an evolving CT technology, which is characterized by simultaneous acquisition of CT data with two different energies or peak tube voltages [30]. In this technology different tissues in the organs can be separated by attenuation difference behavior at two different tube voltage levels. Virtual unenhanced CT images can be acquired which contributes to decreasing ionizing radiation dose up to 47% compared to multiphasic CT examination [31]. Iodine content of the renal masses instead of attenuation values (HU) after IV contrast administration can be measured with this technique (Fig. 1.12). DECT can also be helpful to demonstrate pseudoenhancement of renal masses [32].

Magnetic resonance imaging (MRI) is a problem-solving tool in characterizing renal tumors with its high soft tissue contrast and multiplanar imaging capabilities [33]. MRI can depict water and fat content of renal tumors. Benign and malignant renal tumors may be more accurately differentiated by MRI due to capability of obtaining various sequences which enable to determine fat and water content of renal masses. MRI was shown to be better in evaluating renal lesions which were previously deemed indeterminate on CT [34].

Renal mass evaluation with MRI should include T1-W axial in- and out-of-phase gradient-echo sequence to identify macroscopic and microscopic fat, T2-W axial and coronal sequences to evaluate overall anatomy, renal collecting system, and complexity of cystic renal lesions and dynamic contrast-enhanced (DCE) T1-W fat-suppressed sequences consisting of corticomedullary, nephrographic, and excretory phases. Renal tumors usually appear hypointense on T1-W and hyperintense on T2-W images, while papillary cell RCCs manifest as hypointense lesions on T2-W images (Fig. 1.13). Cystic renal masses can be more easily and accurately characterized by MRI compared to CT. The presence and thickness of septa, wall thickness, and contrast enhancement patterns of renal cystic lesions can be depicted on DCE-MR images. Coronal T1-W images at excretory phase with administration of diuretics can delineate collecting system and ureters and may be helpful in the diagnosis of TCCs.

DWI technique is increasingly used in the assessment of renal tumors. Solid renal tumors demonstrate increased signal intensity on DW images and decreased signal intensity on ADC maps secondary to restricted diffusion of water molecules in renal tumors (Fig. 1.14). DWI has potential to discriminate malignant renal tumors from benign tumors with ADC measurements . It was shown that malignant renal masses have lower ADC values than benign renal masses (Fig. 1.15) [19]. The ADC values of clear cell RCC were shown to be significantly higher than chromophobe and papillary cell RCC which may be helpful to differentiate these subtypes of RCC [35, 36].

Superiority of MRI over other imaging techniques is most remarkable on renal cystic masses with high protein content and hemorrhage. Since these lesions demonstrate high density on precontrast images and may show pseudoenhancement on contrast-enhanced CT images, their diagnosis may be difficult on MDCT. MRI provides a solution for this problem with subtraction technique. With MRI subtraction technique, precontrast MR image of a T1-W hyperintense lesion can be subtracted from contrast-enhanced image of same lesion (Fig. 1.16). MRI was shown to be superior than CT on depicting additional septa, thickening of the wall or septa, or enhancement of the complex renal cysts [25]. Application of Bosniak criteria to cystic lesions on MRI may lead to upstaging of lesions in 10% of cases which were previously categorized on CT [25].

MRI is also a key imaging tool for differentiation between fat-poor angiomyolipomas (AMLs) from RCC. A study using combination of MR sequences reported sensitivity, specificity, and accuracy values of 73%, 99%, and 96%, respectively, in distinguishing AML from RCC [37].

Multiparametric MRI (mp-MRI) of the kidney refers to acquisition of DCE-MRI , DWI, and perfusion MRI for evaluation of renal tumors. Perfusion MRI techniques including arterial spin labeling (ASL) and blood-oxygen-level-dependent (BOLD) MRI were reported to be helpful in distinguishing between benign and malignant renal masses with the capability of obtaining high-temporal-resolution images compared to conventional dynamic MRI. ASL is characterized by using the endogenous contrast properties of arterial blood and noninvasively labeling inflowing spins without exogenous contrast material administration [38]. ASL was shown to be helpful in distinguishing between RCC and oncocytomas as well as between papillary RCCs from other subtypes of RCC [38, 39]. BOLD MRI may be helpful for distinguishing RCCs from AMLs at 3 T MRI and for differentiation between benign cystic lesions from RCCs [40].

Although gadolinium-based contrast agents that are used in MRI were thought as safe contrast agents before, it is well known that these patients especially ones with impaired kidney function are at the risk of nephrogenic systemic fibrosis. Therefore, the use of gadolinium contrast in patients with low glomerular filtration rate (<30 mL/min/1.73 m²) is not recommended according to guidelines of American College of Radiology unless risk-benefit assessment favors the use of gadolinium contrast agent [20].

Adequate distention of the bladder is essential for sonographic assessment of patients with suspicion of bladder cancer. Bladder cancers present as hypo- or isoechoic polypoid solid lesions protruding into the bladder lumen or sessile asymmetric thickening of bladder wall. CDUS usually demonstrates blood flow within the bladder cancer (Fig. 1.21). US has a sensitivity of 61–84% for polypoid bladder cancer larger than 5 mm [62]. Small polypoid tumors (<5 mm) can be easily missed on US. Hematomas can be differentiated from bladder tumors by visualizing mobility of hematomas with patient movement, absence of blood flow on CDUS, and fragmentation of solid-appearing mass by applying pressure by US transducer. US can reveal the number, size, and appearance (sessile or pedicled) of bladder cancer and their topography relative to the prostatic urethra and to the ureteral meatus [63]. US can be more helpful in bladder cancer localized in a bladder diverticulum where cystoscopy may be insufficient for evaluation [64]. Three-dimensional (3D) virtual sonography combined with 2D sonography has a sensitivity of 96.4%, specificity of 88.8%, a positive predictive value (PPV) of 97.6%, and a negative predictive value (NPV) of 84.2% for detection of bladder cancer [65]. CEUS has higher sensitivity (88.5%) and specificity (88.9%) values for detection of bladder cancer compared to grayscale US [66].

CT is one of the most frequently used imaging techniques in detection, staging, and surveillance of the bladder cancer. American College of Radiology (ACR) appropriateness criteria refer CTU as the best initial imaging examination technique for the evaluation of hematuria [67]. CTU series generally include precontrast images and enhanced images at nephrographic phase and excretory phase. 3D reformatted images of the excretory phase of CTU demonstrate the entire urinary tract by delineating the tract with iodinated contrast agent. In order to reduce radiation dose during CTU, some institutions use split-bolus technique that refers to acquisition of nephrographic phase images and excretory phase images at same scanning time to demonstrate contrast-enhanced renal parenchyma and contrast agent-filled renal collecting system, ureters, and bladder in the same image series. This technique reduces the radiation dose of CTU; however contrast resolution of images may be diminished secondary to dilution of contrast agent in the urinary tract. Split-bolus technique reduces radiation dose and is comparable to that of intravenous urography (IVU) [65].

Stage T1 bladder cancers appear as pedunculated polypoid lesions or asymmetric thickening of the bladder wall on CT (Fig. 1.22). T2 tumors usually present as sessile lesions (Fig. 1.23). MDCT including excretory phase has 96% sensitivity and 99% specificity for detection of polypoid bladder cancers between 5 and 10 mm, 89% sensitivity for polypoid lesions smaller than 5 mm, and 40% sensitivity for polypoid lesions smaller than 3 mm [68]. The specificity and NPV of CTU for detection of bladder cancer are higher in patients with hematuria than in patients without hematuria [69].

It should be kept in mind that the diagnostic efficiency of CT for detection of bladder cancer may be decreased when CT examination is performed shortly after bladder interventions such as biopsy or transurethral resection of the bladder (TURB). A study which compared the frequency of concordance between CT findings and histologic examinations showed that CT examinations performed 7 days after the bladder intervention were more in concordance with the histopathological results than CT examinations performed in the following 7 days after TURB [70].

CTU is frequently used in NMIBC patients to demonstrate involvement of upper urinary tract. CTU overweighs MRI in the evaluation of tumoral involvement in upper urinary tract owing to its higher spatial resolution [64]. Occult intramural or transmural extension of tumor and measurable pelvic lymphadenopathy suggestive of local spread of bladder cancer can be diagnosed with CT. Perivesical fat infiltration can be detected on CT with 89% sensitivity and 95% specificity values [68, 70]. Bladder tumors arising in the bladder diverticulum should be rigorously assessed in terms of perivesical fat invasion since muscularis mucosa layer is absent in the bladder diverticula (Fig. 1.24). Infiltration of prostate and seminal vesicles by bladder cancer can be detected on CT only when these organs were massively infiltrated. CT is helpful to assess possible invasion of digestive tract such as sigmoid colon or loops of the small intestine [68]. Distant metastasis including peritoneal carcinomatosis can also be detected with CT [71]. Multiplanar imaging capability of CT with thin slices provides 3D visualization of the entire urinary tract resembling IVU images. These 3D images may be easy to evaluate the urinary tract; however abnormalities visualized in 3D reconstructed images should be confirmed on axial images. In one study, 21 of 27 upper urinary tract neoplasms were missed using the 3D reconstructed images alone [72].

For monitoring NIMBC, CT urography is recommended every 2 years; however CT urography can be performed in the presence of positive cytology or any symptom indicating upper urinary tract involvement [68]. In monitoring patients with MIBC, CT urography should be performed every 6 months for 2 years after radical cystectomy followed by annual CT scanning [73].