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Background
Contrast-enhanced ultrasound (CEUS) utilizes IV microbubble contrast agents to identify vascularity and perfusion in sonographic targets. The use of microbubble contrast for evaluation of renal pathology is considered off-label but has been widely studied and utilized. This exhibit aims to familiarize the reader with the applications of CEUS in the evaluation of renal masses.

Educational Goals / Teaching Points
To recognize the advantages and limitations of CEUS compared to contrast-enhanced CT or MR. To understand the indications for CEUS in the workup of solid or cystic renal masses. To become familiar with the common CEUS characteristics of benign and malignant solid renal masses.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Microbubbles are composed of a low-solubility gas enclosed in a biomaterial shell. These bubbles are delicate, with a half-life on the order of several minutes. Once broken down, the gas is exhaled by the lungs and the shell is metabolized by the liver. As a result, microbubble contrast is not nephrotoxic and unlike CT and MR contrast agents, can be utilized independent of renal function. The dynamic nature of CEUS allows for continuous imaging though multiple phases and multiple doses can be administered during a single exam. CEUS faces many of the same challenges as grayscale US. Large body habitus, lesion depth > 10 cm, and small lesion size can limit evaluation. Lesions larger than the FOV and multiplicity of lesions (more than just a few focused targets) can also limit evaluation. Microbubble contrast is contraindicated in patients with a history of allergy to the contrast agent or any of its components. CEUS has the highest contrast resolution of any commonly used imaging modality and can detect minute flow in in hypoenhancing lesions, thin septa, or subcentimeter nodules and is well suited to the characterization and classification of renal cystic lesions. Characterization of renal cell carcinoma (RCC) subtypes is aided by comparing arterial phase lesion enhancement to renal cortex, and by comparing retention of contrast (or washout) to renal parenchyma. Clear cell RCC is classically hyperenhancing with early washout whereas papillary RCC is hypoenhancing on all phases of contrast. Chromophobe RCC has a more intermediate pattern of enhancement. Benign solid tumors such as oncocytoma and lipid-poor angiomyolipoma have extensive overlap in imaging findings with isoenhancing or hyperenhancing RCC and can be difficult to discriminate by perfusion alone.

Conclusion
CEUS is a safe option in patients who may not be able to undergo contrast CT or MRI evaluation of a renal mass and offers particular benefit to patients with chronic renal disease. CEUS can detect minimal enhancement with high sensitivity and specificity when discriminating between solid and cystic lesions. The high temporal and contrast resolution of CEUS enables accurate characterization, or surveillance, of complex cystic lesions. RCC subtypes can often be recognized by their CEUS perfusion characteristics, but overlapping patterns of enhancement can make it difficult to discriminate benign from malignant lesions by perfusion alone.