R. M. Arthur, D Basu, Y Guo, JW Trobaugh, WL Straube and EG Moros, "Temperature Imaging during NonUniform Tissue Heating with Ultrasonic Backscatter Energy using Self-Calibration", Ultrasonic Imaging, vol. 32, pp. 281-282, 2010.
Background: Hyperthermia alone or in conjunction with chemotherapy and radiation
is used for cancer treatment. One of its limitations is lack of detailed
temperature monitoring. Ultrasound is an inexpensive, non-ionizing and readily
available method with potential for non-invasive temperature imaging. Previously
we predicted monotonic changes in backscattered energy (CBE) of ultrasound with
temperature. Measured CBE values from bovine liver, turkey breast, and pork
muscle in 1D and 2D matched our prediction. In this study, the volumetric (3D)
change in ultrasonic backscattered energy (CBE) was calibrated and used to
estimate temperature during non-uniform heating .
Methods: For accurate temperature validation, a grid of thermocouples was calibrated using a NIST- traceable thermometer. 3D ultrasonic data sets were obtained by moving a 7.5 MHz linear, phased-array transducer in 0.6 mm steps in elevation. CBE was computed from a ratio of motion-compensated, envelope-detected images and a reference ultrasonic image, typically taken at 37oC. CBE curves obtained from turkey breast muscle were well matched by a linear regression that had a slope of 0.3dB/oC. To evaluate the effects of noise, scatterer distribution, and spatial resolution on estimation errors, thermal modeling was performed for non-uniform heating using finite element methods. Specimens of turkey breast muscle were heated non-uniformly from a central 65oC source so that the spatial temperature pattern decreased radially. Temperature images were computed from CBE maps using a fixed CBE sensitivity of 0.3dB/oC, as well as from self-calibration from one indwelling thermocouple.
Results: Estimated temperature maps with a spatial resolution of 0.5 cm2 were validated using thermocouple readings at locations distributed throughout the specimens. Estimation errors during non-uniform heating with 0.3dB/oC sensitivity were 0.3 ± 1.9oC. Even though the CBE temperature images were qualitatively similar with self calibration, error was reduced to 0.07± 1.0oC.
Conclusion: This work, which validated the use of CBE as a non-invasive thermometer during non-uniform heating, was the first of its kind. It also helped clarify sources of estimation errors, such as the size of the temperature image pixel. Validation of CBE thermometry in vitro during non-uniform heating is an important step in making the transition from the laboratory to the clinical application of CBE temperature imaging for hyperthermia and other thermal therapies.
1) D Basu, Doctoral Dissertation, Washington University in St. Louis, May 2010.
R01-CA107558 and the Wilkinson Trust at