Quantitative imaging of temperature elevations in tissues due to thermal therapies

Thermal ablation and hyperthermia remain as potent treatment options for cancer. However, the inability to closely monitor temperature elevations from thermal therapies in real time continues to limit clinical applicability. Therefore, the development of new imaging techniques capable of providing feedback and temperature monitoring is highly medically significant. In this study, quantitative ultrasound imaging techniques based on spectral estimates were examined for their ability to monitor and map temperature elevations induced in tissues using either microwave ablation or high intensity focused ultrasound (HIFU). Ex vivo liver samples were treated with microwave ablation while ultrasound image frames were recorded using a SonixTouch system and linear array (14L5). In vivo tumors in rats (MAT) were treated using a custom-built HIFU system while concurrently imaged using a SonixRP scanner and linear array (14L5). The ultrasound scanners provided raw radio frequency (RF) data. From the RF data, the backscatter coefficient was calculated using the reference phantom technique. The backscatter coefficient was parameterized by estimating an effective scatterer diameter (ESD) and effective acoustic concentration (EAC) assuming a spherical Gaussian model. Maps of the ESD and EAC were created for each acquired frame. Temperature was measured by placing a needle thermocouple in the samples during treatment and temperature changes were correlated with changes in the scattering parameters. In the ex vivo liver samples, the ESD was observed to increase with temperature elevation while the EAC was observed to decrease with temperature elevation. Specifically, the mean ESD increased by 7 μm and EAC decreased by 1.5 dB as the temperature increased from 18 to 42 °C. Conversely, in the in vivo tumor samples treated with HIFU, the EAC was observed to increase with increasing temperature, i.e., the EAC increased by 20 to 30% as the temperature increased from 37 °C to a range of 50 to 60 °C. When the HIFU was turned off, the EAC continued to track the decrease in temperature of the tumor. In the in vivo studies, the tumors were grown on the chest wall of the rats and, therefore, large out of plane tissue motion occurred due to the breathing of the animal. In spite of this, the EAC parameter was capable of tracking temperature in the presence of large tissue motion.