Quantitative Ultrasound for Monitoring High-Intensity Focused Ultrasound Treatment <italic>In Vivo</italic>

Abstract

The success of any minimally invasive treatment procedure can be enhanced significantly if combined with a robust noninvasive imaging modality that can monitor therapy in real time. Quantitative ultrasound (QUS) imaging has been widely investigated for monitoring various treatment responses such as chemotherapy, radiation, and thermal therapy. Previously, we demonstrated the feasibility of using spectral-based QUS parameters to monitor high-intensity focused ultrasound (HIFU) treatment of in situ tumors in euthanized rats [Ultrasonic Imaging 36(4), 239–255, 2014]. In the present study, we examined the use of spectral-based QUS parameters to monitor HIFU treatment of in vivo rat mammary adenocarcinoma tumors (MAT) where significant tissue motion was present. HIFU was applied to tumors in rats using a single-element transducer. During the off part of the HIFU duty cycle, ultrasound backscatter was recorded from the tumors using a linear array co-aligned with the HIFU focus. A total of 10 rats were treated with HIFU in this study with an additional sham-treated rat. Spectral parameters from the backscatter coefficient, i.e., effective scatterer diameter (ESD) and effective acoustic concentration (EAC), were estimated. The changes of each parameter during treatment were compared with a temperature profile recorded by a fine-needle thermocouple inserted into the tumor a few millimeters behind the focus of the HIFU transducer. The mean ESD changed from 121 ± 6 to 81 ± 8 μm(p-value = 0.0002), and the EAC changed from 33 ± 2 to 46 ± 3 dB/cm3 (p-value = 0.0002) during HIFU exposure as the temperature increased on average from 38.7 ± 1.0° C to 64.2 ± 2.7° C. The changes in ESD and EAC were linearly correlated with the changes in tissue temperature during the treatment. When HIFU was turned off, the ESD increased from 81 ± 8 to 121 ± 7 μm and the EAC dropped from 46 ± 3 to 36 ± 2 dB/cm3 as the temperature decreased from 64.2 ± 2.7° C to 45 ± 2.7° C. QUS was demonstrated in vivo to track temperature elevations caused by HIFU exposure.

A Review of Quantitative Ultrasound-Based Approaches to Thermometry and Ablation Zone Identification Over the Past Decade

“…In terms of thermometry interval, only the temperature range measured in 23,25,36,96 exceeded 56°C, while the approaches reviewed in this paper were mainly applied to the scenarios of mild hyperthermia therapy. Among these approaches, echo time shift and CBE methods calculate the relative temperature elevation; the other methods establish a correlation between the corresponding parameters and the reference temperatures.…”

“…In terms of thermometry interval, only the temperature range measured in 23,25,36,96 exceeded 56°C, while the approaches reviewed in this paper were mainly applied to the scenarios of mild hyperthermia therapy.…”

“…Moreover, the variance of EACs of sample tissue was larger than that of ESDs, so ESD could be used as a more reliable indicator for assessing the size of thermal lesions. Ghoshal et al 95,96 found that changes in ESD and EAC provided good correlation with temperature variations during HIFU ablation of rat tumor in vivo. It was observed that the EAC parameter had a positive linear relationship with temperature variations, while the ESD parameter had a negative linear relationship with temperature variations.…”

Imaging of Thermal Effects during High-Intensity Ultrasound Treatment in Liver by Passive Elastography: A Preliminary Feasibility in Vitro Study

“…Ultrasound thermometry has also been investigated using various ultrasound characteristics, such as the speed of sound (Bayat et al , 2015), the nonlinearity coefficient B/A (Choi et al , 2011), and the backscattered coefficient (Ghoshal et al , 2016). However, to date, techniques based on changes in the speed of sound have been limited to temperatures below 55°C.…”

Characterization of High Intensity Ultrasound Exposures by Quantitative Ultrasound

“…Another class of theoretical scattering model is the concentrated Structure Factor Model (SFM) that describes tissue as an ensemble of discrete scatterers and considers interference effects caused by coherent scattering using a structure function [5][6]. Changes in ESD and EAC QUS parameters have been correlated with the temperature changes during HIU exposure [7][8]. The aim of this study is to quantify the changes in QUS parameters due to temperature elevation after HIU exposure of ex vivo porcine livers.…”