This is a 67-year-old patient with severe comorbidities in whom on evaluation for cardiac transplant, noncontrast computerized tomography (CT) of abdomen and pelvis showed an indeterminate small left renal mass measured to be 1.5 cm (fig. 1). His past medical history is significant for chronic renal insufficiency (CKD IV), congestive heart failure (chronic systolic, ejection fraction 24% on milrinone pump), atrial fibrillation (on apixaban), insulin-dependent diabetes mellitus (IDDM), hyperlipidemia (HLD). Of note, he was overall asymptomatic from the incidentally discovered renal mass without hematuria or flank pain. Given his consideration for cardiac transplantation, further workup and diagnosis was required; however, the patient’s glomerular filtration rate/renal insufficiency made contrast-enhanced CT or magnetic resonance imaging a suboptimal option.

Figure 1. Noncontrast CT abdomen and pelvis showing posterior left 1.5 cm exophytic nodule (red arrow) arising from left lower pole of kidney. Indeterminate lesion on noncontrast evaluation.

After consultation regarding options for proceeding as a cardiac transplant candidate, the decision was made to undergo contrast-enhanced renal ultrasonography. Radiology’s protocol involved 4.8 cc of Lumason^® microbubble administration intravenously. Grayscale imaging confirmed a 1.7 × 1.6 cm lesion arising exophytically from the lower pole of the left kidney (fig. 2). The target lesion demonstrated low-level internal echogenicity and mild increased through transmission of sound. Following contrast administration, there was a centripetal pattern of enhancement with early washout. The lesion enhanced to a lesser degree than the adjacent normal renal cortex. On impression, this enhancement pattern was suspicious for papillary renal cell carcinoma. The diagnostic step was a percutaneous renal biopsy, which confirmed a pathologic diagnosis of papillary renal cell carcinoma (low grade).

Figure 2. Left, grayscale ultrasound with hypoechoic lesion arising posteriorly from left lower kidney (red). Right, contrast-enhanced dynamic image with Lumason microbubble administration. 1.7 × 1.6 cm exophytic lesion with low-level internal echogenicity. Centripetal pattern of enhancement. Lesion (red) shows lesser enhancement compared to normal renal cortex (purple).

For end-stage renal disease, patients are often able to undergo contrast-enhanced cross-sectional imaging when coupled with their dialysis session. However, in the setting of a patient who still has some viable parenchyma left, the decision to administer contrast comes with the risk of injuring any remaining nephrons. The development of stable and reproducible intravenous contrast agents has led to the use of contrast-enhanced ultrasonography. Initially introduced to monitor hepatic lesions, widespread adaptation has led to diverse applications such as monitoring activity of bowel inflammation in Crohn disease to even characterizing tumor response to antivascular drug therapies.

Ultrasound affords the benefits of real-time imaging with high spatial resolution in which thin septae or small masses can be identified in a radiation-free manner.¹ In 2019, the U.S. Food and Drug Administration released a white paper highlighting their initiative to reduce unnecessary radiation exposure from medical imaging and underlined the use of contrast-enhanced ultrasonography.² Ultrasound contrast agents are synthesized using microbubbles of heavy gas stabilized with a lipid, protein or lipoprotein shell. Their unique size (∼size of a red blood cell) prevents extravasation to the interstitium and stays entirely intravascular. Elimination occurs with exhalation via the lungs, with lipoprotein shell metabolic breakdown, and therefore ultrasound contrast agents have no hepatic or renal toxicity profiles.¹

While the idea of contrast-enhanced ultrasonography represents a novel imaging modality, multiple studies have highlighted the use of contrast-enhanced ultrasound to identify and evaluate renal masses/abnormalities.^3–5 Inherent in the ultrasound contrast agent’s properties, vascular enhancement of renal neoplasms differs from the surrounding renal cortex in at least 1 vascular phase.¹ Characteristic patterns of enhancement allow further differentiation of benign vs malignant tumors. In a study looking at evaluation of indeterminate renal masses, Barr et al found a negative predictive value of 100% in diagnosing benign lesions.⁵ While most practicing clinicians routinely use contrast-enhanced CT or magnetic resonance imaging as the gold standard to characterize renal masses, advances in ultrasonography may offer a portable, safe and cheaper diagnostic modality to replace current standards in the near future.