Monitoring of microwave ablation in porcine liver using ultrasonic Nakagami imaging

It is highly desirable to monitor microwave ablation (MWA) treatments noninvasively for substantially improving the clinical implementation of MWA procedures and progress toward full clinical uses worldwide. This paper explored the feasibility of using ultrasonic Nakagami imaging to evaluate the thermal lesion induced by microwave ablation (MWA) in porcine liver. 2-D RF data backscattered from the ablated region were captured by a modified diagnostic ultrasound scanner to estimate ultrasonic Nakagami parameters of the thermal lesions, and to reconstruct the ultrasonic B-mode and Nakagami images during HIFU and MWA. A term contrast-to-noise ratio (CNR) between the thermal lesions and the surrounding normal tissue is used to estimate the contrast resolution of the ultrasonic B-mode and ultrasonic parameter images. After thermal ablation, a bright hyper-echoic region appeared in ultrasonic Nakagami parameter images as an indicator of the thermal lesion and this bright spot enlarged with lesion development during MWA exposure. The mean values of the Nakagami parameter in the region of thermal lesion were increased to 0.58, 0.71 and 0.91 after MWA with duration of 1, 3 and 5 min. The mean value of Nakagami parameter increased from approximately 0.47 to 0.82 during the MWA exposure. At the end of the postablation stage, the mean value of Nakagami parameter decreased to 0.55 and was higher than that before the MWA exposure. The values of CNR calculated for the Nakagami parameter images increased from 0.13 to approximately 0.61 during MWA and it decreased to 0.26 at the end of the postablation stage, and the corresponding values of CNR calculated for the ultrasonic B-mode images were 0.24, 0.42 and 0.17. This study on a porcine model suggested that the Nakagami parameter may have the potential use to evaluate the formation of thermal lesions for monitoring MWA treatment. The ultrasonic Nakagami imaging may be utilized as an alternative modality in the development of real-time monitoring systems for the image-guided MWA treatment.