Multidetector CT

Recent improvements in MDCT technology allow thinner collimation and faster scanning that provides multiphasic, high‐resolution images of the kidneys. These hardware developments, coupled with advances in three‐dimensional imaging software and the availability of cheaper data‐storage capacity, have provided new opportunities for imaging renal and urinary tract pathology. Isotropic imaging of the kidneys, ureters, and bladder is now possible, providing two‐dimensional multiplanar reformations (MPR), CT urography (CTU), and three‐dimensional imaging formats, with minimal artifacts, all derived from a single data acquisition [15, 16]. The kidneys can be scanned in less than 5 seconds, allowing for acquisition of thin‐section images during the corticomedullary, nephrographic, and excretory phases. This multiphasic evaluation permits high‐resolution imaging of not only the renal parenchyma but also the renal vasculature and collecting system.

Evaluation of a renal mass by CT requires a dedicated multiphasic CT protocol which may include: a precontrast scan; arterial phase (15–25 seconds delay) scan; corticomedullary phase (35–80 seconds delay) scan; nephrographic phase (85–180 seconds delay) scan; and excretory phase (>3 minutes delay) scan. The radiation dose in the CT urography protocol is approximately 10–12 mSv. In order to reduce the patient radiation dose, the precontrast, nephrographic phase, and excretory phase scans are the ones most commonly obtained [15–17].

Noncontrast scans of the kidneys are performed initially to evaluate for the presence of fat or calcification in the mass and to detect nephrolithiasis. This scan also provides a baseline from which to quantitate lesion enhancement on postcontrast scans. The scans performed during the arterial or corticomedullary phase, while limited for mass visualization, nicely display the renal vasculature, which is critical for surgical planning, particularly when partial nephrectomy is being considered. Arterial phase scans best show small hypervascular neoplasms and the renal arteries. The nephrographic phase optimizes mass visualization. The delayed phase scans are helpful in assessing the relationship of the renal mass to the renal collecting system and also visualizing the remainder of the urinary tract. The data from these various series can then be reconstructed at thinner intervals and transferred to a workstation where multiplanar reformatted images, maximum intensity projection (MIP) images, and 3D volume rendered images can all be created [15–17].

Contrast enhancement and subsequent de‐enhancement are characteristic features of renal malignancies. A renal lesion is considered to be enhanced when its density increases more than 20 Hounsfield units (HU). If the density of the lesions increases between 10 and 20 HU, it is considered indeterminate for enhancement and requires further testing. While these criteria seem straightforward, the accuracy of determining enhancement varies based on the type of CT scanner used as well as technical factors including the kilovoltage and amperage of the X‐ray beam, pixel size, image noise, slice thickness, beam‐hardening artifacts, and artifacts related to large body habitus. The size of the lesion is also important because volume averaging commonly occurring in lesions smaller than 10 mm in size.

Pseudoenhancement occurs when a cyst, usually <2 cm, appears to artifactually enhance because it is surrounded by strongly enhancing parenchyma. This typically occurs during peak renal enhancement and should be suspected when a lesion is homogeneous and measures less than 10 HU on an unenhanced CT scan [18–21].

Dual‐energy CT (DECT) is a recent development that involves the simultaneous acquisition of CT data at two different peak tube voltages and has the potential to reduce radiation dose. By exploiting the differences in energy‐related attenuation of tissues within the body, DECT potentially can characterize tissue composition to a greater extent than that possible with single‐energy techniques. When this technique is applied to remove the iodine contribution for a postcontrast image, a virtual noncontrast image can be produced. This technique exploits the fact that soft tissue and iodine demonstrate unique attenuation differences at different tube voltages, allowing for material decomposition. This technique also makes it possible to measure iodine concentration (instead of attenuation values) within the lesion. Enhancing renal masses demonstrate the presence of iodine, whereas benign, nonenhancing lesions have an absence of intralesional iodine.

Magnetic resonance imaging

Significant advances in MR imaging hardware and software with routine use of phased‐array coils and parallel imaging permit fast and high spatial resolution imaging of the kidneys. Short breath hold scans can produce superb multiphase renal images without significant peristaltic or respiratory motion artifacts [22–25]. The following imaging sequences should be obtained when evaluating a renal mass with MR: coronal T2‐weighted single‐shot turbo spin‐echo sequence to serve as a localizing scan; axial T2‐weighted gradient and spin‐echo sequence with fat suppression to search for renal mass invasion of the adjacent fat; a dual‐echo axial T1‐weighted gradient‐echo sequence with in‐phase and opposed‐phase images to search for fat within the tumor; and an axial T1‐weighted fat‐suppressed gradient‐echo sequence for dynamic imaging using 20 ml of intravenous gadolinium with precontrast and postcontrast images obtained during the arterial, corticomedullary, and nephrographic phases to assess tumor vascularity. The physiologic principles behind the various dynamic postcontrast phases described in the MDCT section also hold true for dynamic MR imaging [26].

MR‐derived functional imaging in the form of diffusion‐weighted imaging (DWI) is now part of the routine MR evaluation of the kidneys and has been found to be very useful in assessing the likely malignant potential of renal parenchymal masses and transitional cell neoplasms. The apparent diffusion coefficient (ADC) values of these neoplasms is significantly higher than those of normal renal parenchyma and normal urothelial mucosa. The ability of DWI to identify changes in the molecular level has dramatically changed the diagnostic approach of radiologists, which was solely heretofore based on morphological criteria. It can improve the diagnostic accuracy of conventional MRI, assist the assessment of tumor response to treatment regimens and detect tumor recurrence with better spatial resolution, negative radiation, and diagnostic accuracy compared to PET‐CT. The ability to quantify the diffusion has also led to potential prediction of tumor aggressiveness and grade which directly correlate with patient prognosis and management.

MRI is generally employed as a secondary imaging test when better characterization of a mass found on MDCT or ultrasound is needed. MRI is used as a primary imaging test in patients who have a severe allergy to iodinated contrast material or who are pregnant. Because of the risk of nephrogenic systemic fibrosis, MRI with gadolinium‐based agents should be performed with caution in patients with poor renal function. Gadolinium‐based contrast agents have been classified, based on the chemical structure of the ligand, as linear versus macrocyclic. The rates of dissociation of gadolinium from macrocyclic ligand agents are slower than dissociation from linear ligands and are thus considered to be more “stable.” Accordingly, these macrocyclic agents (e.g. gadoterate meglumine, gadobutrol, and gadoteridol) are preferable to linear agents when assessing urinary tract pathology in these patients.

MRI may become more commonly employed for following renal lesions given the increased concerns regarding cumulative radiation dose. This is particularly applicable in young patients.

The chemical‐specific tissue characterization capabilities of MRI are particularly helpful in characterizing certain types of renal lesion. Confident characterization of hemorrhagic cysts can be problematic on CT because the lack of enhancement of a small, dense renal mass may be difficult to confirm. Hemorrhagic cysts usually have high signal on T1‐weighted images on MRI. The signal intensity of hemorrhagic or proteinaceous cysts on T2‐weighted images is variable, ranging from low to mildly increased signal compared with renal parenchyma, but usually lower signal than seen in adjacent cerebrospinal fluid or other simple cysts. The combination of high signal on T1‐weighted images and the lack of enhancement on MR imaging are diagnostic of a hemorrhagic renal cyst [22–26].

MR imaging is also useful in characterizing complex renal cysts. Septations within complex renal cysts that are vague on CT are readily depicted with MRI. Because fluid signal on T1‐weighted images is usually darker than the low attenuation seen by CT, contrast enhancement within the septations is usually more apparent on MR than on CT, especially with subtraction images [22–26].

MR is more sensitive than CT in the detection of macroscopic fat within lesions such as angiomyolipomas (AMLs). Although most AMLs can be characterized with CT, it may be difficult to obtain accurate CT density measurements on small lesions. MR can confidently characterize AMLs in two ways. First, frequency‐selective chemical fat suppression will result in subjective signal loss within fat‐containing regions of the lesion. Second, chemical shift artifact will show a sharp dark interface between the AML and normal renal parenchyma [22–26].

Ultrasound

Although CT is the premier imaging test in patients with suspected renal pathology, ultrasound is often the first imaging test ordered for renal evaluation. Despite its technical limitations, a large number of renal tumors can be correctly characterized sonographically. Recent advances in gray‐scale and color flow Doppler techniques, as well as tissue harmonics, have enhanced the ability of ultrasound to distinguish solid from cystic lesions, and to make the diagnosis of a simple renal cyst [27–30]. Sonography is employed on a daily basis for evaluating lesions detected incidentally on CT which are not clearly simple cysts.

Lesions that are felt to represent hyperdense cysts either contain hemorrhage or proteinacious fluid. Those containing proteinacious fluid are typically simple on ultrasound, whereas those containing blood can appear heterogeneous and partly solid.

Ultrasound is useful in evaluating complex cystic lesions and detecting septations or minimal mural nodularity. This technique, however, is operator dependent and can be extremely limited in obese patients or when there is a large amount of adjacent bowel gas. Sonography can be quite useful for assessing the presence of renal vein thrombus with 75% sensitivity and 96% specificity, and 100% accuracy for detecting thrombus in the inferior vena cava [29, 30]. Intraoperative ultrasound has become a useful tool in guiding the surgeon during nephron‐sparing surgery of small renal cell neoplasms.

Positron emission tomography–computed tomography

Most malignancies exhibit increased metabolic activity, leading to increased utilization of glucose. PET imaging exploits the fact that the glucose analog ¹⁸F‐2‐fluoro‐2‐deoxy‐glucose (FDG) shows increased intracellular accumulation in malignant tissue. PET‐CT is a fixed combination of PET and CT scanners in a combined imaging system. The nearly simultaneous data acquisitions lead to minimization of spatial and temporal mismatches between modalities by eliminating the need to move the patient during the exam. The result is a fused image that provides biological and anatomic information. Imaging metabolic information of tumor tissue provides often more sensitive and specific information concerning the extent of malignancy than anatomic information alone [31–34].

Although FDG‐PET is a well accepted method for the detection and staging of a number of malignancies, including lung, breast, colorectal, and esophageal cancer, it currently has a limited role in evaluating renal cell carcinoma (RCC). Several studies have shown that FDG‐PET has a high specificity, but its sensitivity is inferior to that of CT and MR imaging in the evaluation of suspicious primary or metastatic RCC. FDG‐PET surpasses the 90% sensitivity mark only for lesions at least 2 cm in diameter [31, 32]. Although there are studies that report sensitivity of FDG‐PET to be as high as 94%, it is generally accepted that malignancy cannot be ruled out with a negative study. RCCs have inconsistent FDG uptake, which may be due to hypo‐ or isometabolism relative to background tissues, lack of accessibility of FDG, or heterogeneity of glucose transporter expression [33, 34].

Positron emission tomography–magnetic resonance imaging

PET‐MRI combines the unique tissue characterization of MRI – achieved through an array of conventional and emerging pulse sequences – with the quantifiable functional and molecular information provided by PET, thereby providing distinct potential clinical advantages over other imaging modalities. PET‐MRI has many potential advantages over PET‐CT. The improved lesion detection within the kidneys of MRI compared with that of CT is expected to achieve an overall diagnostic advantage for PET‐MRI over PET‐CT. In addition, lesion margins may be better defined by MRI than by CT in certain locations. Preliminary data also suggest that lesion registration is improved through hybrid PET‐MRI, an advantage that will potentially improve tissue segmentation, attenuation correction, and ultimately PET quantification.

PET‐MRI offers practical advantages for patients requiring both PET and MRI examinations for oncologic assessment by providing both modalities within a single appointment and imaging session. Radiation exposure is also reduced because CT is not used in the imaging protocol.

Finally, PET‐MRI allows accurate, temporally and spatially aligned multiparametric imaging that combines high‐contrast anatomic MR images with the quantitative power of molecular imaging of both PET and MRI, including DWI, perfusion MRI, and MR spectroscopy. This potential creates numerous opportunities for characterizing tumor biology across all of the dimensions of imaging offered by PET and MRI.

Category I

These lesions are benign simple cysts requiring no further diagnostic imaging or follow‐up (Figure 123.1). On CT, these lesions must have a paper‐thin wall, have entirely fluid attenuation measuring between 0 and 20 HU, and they must demonstrate no enhancement following the intravenous administration of contrast medium. These lesions do not contain septations or calcifications.

Category II

These lesions are also benign, but are minimally complicated (Figures 123.2–123.4). They may contain a few paper‐thin septations. They may also contain fine curvilinear mural or septal calcification. This category also includes hyperattenuating cysts which measure less than 3 cm, are round and sharply marginated with at least one‐quarter of the lesions extending outside the renal parenchyma. Most importantly, they must demonstrate uniform high attenuation and no enhancement with intravenous contrast. Cysts that measure between 20 and 40 HU are usually proteinacious cysts and have a density of greater than 40–50 HU are likely to be hemorrhagic. The latter will appear complex on ultrasound. They may appear solid or semisolid due to clot retraction and could be miscategorized by ultrasound.

Dual‐energy CT is most helpful in depicting hemorrhagic renal cysts (see Figure 123.3).

Kang et al. attempted to differentiate high‐attenuation renal cysts from RCCs on unenhanced CT scans [16]. They retrospectively evaluated the attenuation values and degree of uniformity/heterogeneity in 56 hyperdense renal cysts and 54 RCCs. The mean attenuation for hyperdense cysts was 53 HU and for RCCs was 38 HU, but there was significant overlap when evaluating attenuation values alone. However, when evaluating uniformity versus heterogeneity of the lesion, the authors concluded that a homogeneous, hyperattenuating renal masses with an attenuation of 70 HU or greater, had a 99.9% likelihood of being a benign, high‐attenuation renal cyst. Most RCCs in their study had attenuation values lower than those of renal parenchyma (20–30 HU) and were often heterogeneous in attenuation. High‐attenuation renal cysts can in many cases be differentiated from RCC on unenhanced CT [16, 41, 42].

Category IIF

These lesions are likely benign but require a period of observation or follow‐up (the F stands for follow‐up) to prove their benignity. These lesions may contain an increased number of septa or have minimal septal or mural thickening. Category IIF lesions may contain thick, irregular, or nodular calicifications. When a high‐attenuation lesion is completely intrarenal, the smoothness of its wall cannot be assessed. Accordingly, completely intrarenal high‐attenuation lesions as well as lesions greater than 3 cm in diameter are included in this category. These lesions may also have a slightly indistinct interface with the adjacent renal parenchyma [33–37].

These lesions should be reimaged with CT or MRI in six months and then annually or every other year for 5 years. Stability over time suggests these lesions are benign, recognizing that some malignant lesions can grow slowly and that cysts can enlarge. Therefore on follow‐up imaging their morphologic characteristics must be evaluated in addition to their size. If there is any increase in the thickness or in the irregularity of the wall or septa, surgical exploration is indicated. MRI can be particularly useful in evaluating hemorrhagic cystic renal masses and evaluating for the presence of septal or mural enhancement. The introduction of the IIF category in 1993 has prevented unnecessary surgery in large numbers of patients [33–37].

Category III

These are truly indeterminate lesions which cannot be diagnosed accurately as benign or malignant based on their imaging appearance (Figure 123.5). They contain thick walls or thick septa which demonstrate measurable enhancement. They may contain more extensive, thickened, and irregular calcifications. At pathology, these lesions are multilocular cysts, infected or hemorrhagic cysts, multilocular cystic nephromas, or cystic RCCs. The risk of malignancy in this group has been reported to range up to 59%. Since a significant number of patients with Bosniak type III lesions undergo surgery for ultimately benign disease, some have advocated biopsy in this patient cohort [1, 4, 20].

Category IV

These lesions are clearly malignant and require surgical removal (Figure 123.6). They may have findings similar to category III lesions but also have enhancing soft tissue components adjacent to the wall or septa. These lesions are usually cystic clear cell carcinomas (accounting for 4–15% of all RCCs), multilocular cystic RCCs, cystic nephromas, and mixed epithelial and stromal tumors [35–38].