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Background and Purpose: Ablation by cold (cryoablation) or radiofrequency energy (RFA), has been popularized for the treatment of small renal tumors. Regrettably, there currently is no reliable method of radiologically monitoring the propagation of RF lesions in real time. Ultrasonography enhanced by gas-filled microbubble contrast agents allows depiction of regions of tissue perfusion and has been described as a useful adjunct in diagnosing renal pseudotumors, improving prostate biopsy results, and confirming successful ablation of liver tumors. We hypothesized that contrast-enhanced ultrasonography (CEUS) would allow us to define, in real time, areas of cell death secondary to RFA and thus determine successful treatment.

Materials and Methods: Five female swine underwent initial laparoscopic exploration and creation of ipsilateral upper- and lower-pole renal RFA lesions. Lesion size was measured with standard gray-scale, Doppler, and microbubble CEUS. After 2 weeks, an identical procedure was performed on the contralateral kidney, including repeat sonographic measurements on the first kidney. All swine were then immediately sacrificed, and both kidneys (20 lesions) were harvested for pathologic analysis (hematoxylin-eosin and nicotinamide adenine dinucleotide stains). Radiographic lesion size was then compared with the gross and microscopic findings.

Results: The RFA lesions could not be imaged accurately in real time with standard gray-scale or Doppler sonography. However, microbubble CEUS was able to monitor parenchymal blood flow and, thus, the lesions (no blood flow) in real time. Hypoechoic lesions (no bubble enhancement) imaged during contrast sonography corresponded with regions of cell death as demonstrated on pathologic analysis.

Conclusions: Microbubble CEUS is can monitor RFA lesions in real time. This novel imaging modality should allow more effective renal tumor ablation.