Imaging Approaches

Renal Mass Evaluation

CT : Although rapid improvements in scanner technology have resulted in dramatic increases in spatial and temporal resolution, the imaging marker for potential neoplasia (enhancement of soft tissue components of renal masses after contrast administration) has not changed over several generations of CT scanners. Typical renal mass protocols consist of NECT of the kidneys followed by contrast-enhanced images obtained during nephrographic and excretory phases (roughly defined as 85-120 seconds and 3-5 minutes, respectively, postcontrast injection). Corticomedullary-phase images (25-70 seconds post contrast) may also be obtained, though small, central, endophytic tumors may not be perceived given the lack of medullary enhancement during this phase. An increase in soft tissue attenuation of soft tissue components postcontrast administration of > 10-20 Hounsfield units (HU) is classically considered to be indicative of true enhancement, though "pseudoenhancement" of small lesions may result in false-positive examinations, and slowly enhancing lesions (e.g., papillary carcinomas) may be missed if delayed images are not acquired. Despite these caveats, if such a protocol is employed, most renal masses can be accurately characterized as potential surgical lesions (i.e., enhancing: Benign/malignant neoplasia) or leave-alone entities (either fat-containing angiomyolipomas or simple-/high-attenuation cysts). Assessment of cyst complexity at CT, utilizing features described by Bosniak a generation ago, also aids in the triage of this particularly problematic category of renal masses. Suffice to say that increasing levels of complexity (septa thickness, wall/nodularity enhancement, and calcification) increase the likelihood of malignancy.

Optimized protocols should utilize the advantages of multidetector technology. Thin (.625- to 1.250-mm) source images are acquired and thicker (e.g., 2.5-mm) axial coronal images are reconstructed. Such protocols allow for diagnostic coronal reconstructed image sets with isotropic or near isotropic resolution; these are particularly helpful for staging of renal cell carcinoma. The potential utility of dual energy for the characterization of renal masses is a topic of current intense investigation. These platforms may permit assessment of enhancement by calculating iodine concentration in the mass, though criteria for meaningful changes have not been universally accepted. The reproducibility of postprocessed HU calculations will also need to be validated.

MR : Lesion enhancement is the feature that is also primarily assessed with tailored renal MR protocols. MR offers several unique advantages: Numerous fat-saturation T1-weighted dynamic enhanced phases can be obtained directly in a coronal plane, and dedicated Gadolinium (Gd)-enhanced delayed coronal imaging is the standard of care for identification and characterization of renal vein-caval thrombus. Chemical shift imaging and frequency selective fat-saturation technique may be utilized to identify the fat that is characteristic of angiomyelolipomas. Assessment of T2 and T1 signal intensity improves diagnostic accuracy: Enhancing T2-hyperintense lesions are likely renal cell carcinomas, whereas enhancing T2-hypointense lesions are typically either nonfat-containing acute myeloid leukemias or papillary renal cell carcinoma. Diffusion-weighted imaging assessment may also be employed, though the apparent diffusion coefficient value overlap between benign and malignant renal lesions has limited the utility of this technique. A final caveat regarding MR assessment of renal masses is that Gd enhancement of renal masses on MR remains a largely qualitative biomarker of neoplasia (though one that is better assessed by evaluating subtracted images).

Ultrasound : Despite marked improvements in ultrasound platforms over the past 40 years, the role of ultrasound in the renal mass imaging algorithm has not changed: Ultrasound is typically performed to determine if a lesion is cystic or solid. Technique should be optimized: Compound and harmonic imaging should be routinely employed to reduce artifacts and to obtain sufficient penetration. Color Doppler may be utilized to help identify pseudolesions (e.g., column of Bertin, renal scars) and, rarely, to identify flow within solid or complex cystic masses. Microbubble contrast agents have been employed in several centers to increase the accuracy of ultrasound assessment of renal masses: Unlike CT and MR, enhancement of masses may be observed at continuous real-time examination using low mechanical index protocols, although standards for enhancement are not uniform, and FDA approval for this indication is still forthcoming.