Real-time Monitoring of High Intensity Focused Ultrasound (HIFU) Ablation of <em>In Vitro</em> Canine Livers Using Harmonic Motion Imaging for Focused Ultrasound (HMIFU)

Abstract

Harmonic Motion Imaging for Focused Ultrasound (HMIFU) is a technique that can perform and monitor high-intensity focused ultrasound (HIFU) ablation. An oscillatory motion is generated at the focus of a 93—element and 4.5 MHz center frequency HIFU transducer by applying a 25 Hz amplitude-modulated signal using a function generator. A 64-element and 2.5 MHz imaging transducer with 68kPa peak pressure is confocally placed at the center of the HIFU transducer to acquire the radio-frequency (RF) channel data. In this protocol, real-time monitoring of thermal ablation using HIFU with an acoustic power of 7W on canine livers in vitro is described. HIFU treatment is applied on the tissue during 2 min and the ablated region is imaged in real-time using diverging or plane wave imaging up to 1000 frames/second. The matrix of RF channel data is multiplied by a sparse matrix for image reconstruction. The reconstructed field of view is of 90° for diverging wave and 20 mm for plane wave imaging and the data are sampled at 80 MHz. The reconstruction is performed on a Graphical Processing Unit (GPU) in order to image in real-time at a 4.5 display frame rate. 1-D normalized cross-correlation of the reconstructed RF data is used to estimate axial displacements in the focal region. The magnitude of the peak-to-peak displacement at the focal depth decreases during the thermal ablation which denotes stiffening of the tissue due to the formation of a lesion. The displacement signal-to-noise ratio (SNRd) at the focal area for plane wave was 1.4 times higher than for diverging wave showing that plane wave imaging appears to produce better displacement maps quality for HMIFU than diverging wave imaging.

Real-Time Spatiotemporal Control of High-Intensity Focused Ultrasound Thermal Ablation Using Echo Decorrelation Imaging in ex Vivo Bovine Liver

“…Various monitoring algorithms have been developed to detect HIFU-induced thermal lesioning based on ultrasound pulse-echo imaging, including B-mode (Vaezy et al, 2001; Gudur et al, 2012), M-mode (Kumon et al, 2012), backscatter (Seip et al, 2002; Chen et al, 2011; Shishitani et al, 2013), echo strain imaging (Souchon et al, 2005), harmonic motion imaging (HMI) (Han et al, 2015; Grondin et al, 2015; Yang et al, 2017), elastography (Iwasaki et al, 2016), and acoustic radiation force imaging (Lizzi et al, 2003). All these methods aim to provide feedback on the progress of thermal treatment and to confirm the completeness of ablation.…”

Fast lesion mapping during HIFU treatment using harmonic motion imaging guided focused ultrasound (HMIgFUS)in vitroandin vivo

“…In order to calculate the peak-to-peak HMI displacement without sacrificing the display frame rate, only 10 consecutive frames within a single HMI vibration were obtained at each acquisition and transferred to a host computer. The RF channel data matrix was placed into a GPU matrix within Matlab and multiplied by a beamforming sparse matrix to get the beamformed RF data (Hou et al 2014a, Grondin et al 2015). A 1D cross-correlation method (Luo and Konofagou 2010) was used to estimate the incremental axial displacement between 2 consecutive RF frames.…”

“…A fast imaging and processing algorithm is essential to ensure real-time lesion mapping. Recently, HMI was shown capable of streaming HMI displacement in real time at 15 frames per second (Grondin et al 2015). Although the oscillatory HMI displacement contains important information on tissue stiffness change, it does not provide information on the lesion location or lesion size itself in real time.…”