Elastic modulus imaging (EMI) for visualizing thermal ablation zone: Initial experience in a porcine model

Radiofrequency (RF) ablation is the most common minimally invasive therapy used in the United States to treat primary (hepatocellular carcinoma) and metastatic liver cancers. The ability to accurately evaluate the thermal ablation zone while a patient is undergoing radiofrequency ablation may help reduce the high recurrence rates (as high as 55%) following radiofrequency ablation therapy. In this paper we demonstrate the feasibility of performing ultrasound-based elastic modulus imaging (EMI) to evaluate thermal ablation zones in a porcine model. A total of 14 in vivo RF and microwave ablation zones (1-3 cm in diameter) created in 5 porcine animals with normal livers were studied. After open-abdominal ablation, RF ultrasound echo data were acquired under the guidance of a real-time strain imaging system. Imaging results of ablation areas obtained by the EMI and strain imaging methods were then compared with gross pathology section obtained in the corresponding imaging planes. When comparing ablation area measurements, EMI had higher correlation with gross pathology measurements than strain imaging (EMI Pearson coefficient = 0.95, p<0.0001; strain Pearson coefficient = 0.85, p<0.0001). Furthermore, the EMI method improved contrast-to-noise ratios by approximately a factor of 2 compared to strain images. These preliminary results support the hypothesis that EMI might potentially enhance the ability to visualize thermal ablation zones, thereby improving assessment of ablative therapies. Our future work will be directed to test the EMI method in tumor-bearing animal models and to rigorously validate our results with histology in these models.