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.
Abstract
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 [1].
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.
Support: R21-CA90531,
R01-CA107558 and the Wilkinson Trust at