Quantitative Ultrasound for Monitoring High-Intensity Focused Ultrasound Treatment In Vivo

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/cm 3 (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/cm 3 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.