Ultrasonic Spatial and Temporal Determination of Heat Deposition in Three Dimensions

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed sub-optimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging has been recently proposed as a means to measure non-invasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional nature of traditional ultrasound imaging. In this study, we present methodology, results, and validation of a three-dimensional spatial and temporal ultrasound temperature estimation technique to track the evolution of heat deposition over a treatment volume. Ultrasonic backscattered radio-frequency data were captured using a commercial clinical imaging system to monitor heat generated by a complex, three-dimensional geometrical construct of resistive wires in an ultrasonic phantom. Thermocouples were embedded at various locations in the phantom to independently measure the temperature evolution during 90 seconds of heating (peak temperature increase of 8.5 degrees C) and the subsequent cool-down. X-ray computed tomography was used to ensure accurate placement of thermocouples. Good agreement was observed between the ultrasound derived temperature estimates and the invasive thermocouple measurements throughout the heating and cool-down phase. The mean difference between the ultrasound and thermocouple data was calculated to be 0.06 degrees C, RMS difference: 0.12 degrees C, and maximum difference: 0.24 degrees C. Time varying, three-dimensional ultrasonic measurements of temperature yielded parametric maps of thermal deposition within the phantom, and illustrated the complex three-dimensional geometry of the heat source. The results demonstrate potential for the applicability of the technique in monitoring and guidance of thermal therapy. To the best of the knowledge of the authors, this work represents the first demonstration of three-dimensional spatial and temporal ultrasound thermometry.