An Investigation of Brightness Changes in Ultrasound Images due to Temperature Variations in Liver Tissue during Radiofrequency Ablation

Introduction: one of the minimally invasive methods of treatmenting liver malignancies is Radiofrequency Ablation (RFA) which can be applied to primary or secondary tumors. Nevertheless, the disease recurrence is probable after RFA application partially due to the inadequate capability of temperature monitoring of the target tissue and visualizing the thermal damage. The goal of this work was to study the possibility of real-time temperature monitoring of liver tissue undergoing RFA. For this purpose, variations of brightness in DICOM ultrasonic images were investigated.  Materials and Methods: 10pieces of fresh bovine liver underwent RFA in ex vivo experiments. Active and passive needle electrodes were inserted into the tissue and a micro-thermometer was placed between them using ultrasonography guidance to invasively monitor temperature changes. Corresponding to each degree of temperature up to 70 ˚C, ultrasonic images were acquired followed by turning the RF generator off and taking the images down to 30 ˚C. Ultrasonic images were delivered to a PC via PACS and processed in order to investigate the subsequent changes of brightness corresponding to temperature changes. Correlation analysis was performed between the changes of brightness and temperature variations with a 95% confidence level. Results: this study showed that during tissue warm-up between 20 ˚C and 50 ˚C, tissue brightness of the ultrasound images increases linearly (r = 0.99). As temperature rises between 50 ˚ and 70 ˚C, the variation of brightness shows a non-linear behavior due to microbubble formation. During the cool-down process, the brightness once again behaves linearly (r = 0.98) and the variation rate of brightness is faster in the cool-down than that of the warm-up. So that at the end of RFA, when the temperature reaches the initial level the tissue is approximately 60% more hypoechoic. The results are the same for both frequencies employed. Discussion and Conclusion: it seems that the changes of brightness in the ultrasound images can be used for real-time thermal monitoring during radiofrequency ablation.