R. M. Arthur, J. W. Trobaugh, Y. Guo, W. L. Straube  and E. G. Moros, "Change in Ultrasonic Backscattered Energy for Temperature Imaging:  Factors Affecting Temperature Accuracy and Spatial Resolution" 32nd International Symposium on Ultrasonic Imaging and Tissue Characterization, Arlington, Virginia, 16-18 May  2007.

Abstract

Ultrasound is an attractive modality for non-invasive temperature imaging to enhance the ability to target tumor heating at therapeutic levels.
Previously, we measured monotonic changes in ultrasonic backscattered energy (CBE) in vitro in 2D and in 3D for multiple porcine, bovine and turkey tissues and in vivo in living normal murine tissue with implanted tumors (HT29 colon cancer line) on nude-mouse preparations. Measured changes were consistent with predictions for certain sub-wavelength scatterers and with results from simulations for images of populations of temperature-dependent scatterers. Application of temperature imaging to monitoring of tumor heating requires accurate temperature estimation over small volumes, e.g., to within 0.5^oC for tissue volumes of 1cm³, requiring consistent and predictable measurements of the temperature dependence of CBE. Using simulation studies, we have studied the impact of various factors on the variation of measurable CBE with temperature. In these simulations, we represented the imaging system by its point-spread function and the tissue medium by discrete scatterers with a variable range of temperature dependence. Images were simulated to represent temperatures from 37 to 50^oC by changing the scatterer amplitudes according to curves predicted previously for single scatterers, and CBE was computed for regions based on the means and standard deviation for CBE over all pixels in the region. CBE computed from simulation showed the same monotonic increase and decrease as in experimental results and covered ranges similar to both prediction and experiment. We used multiple simulations to characterize temperature-dependent CBE statistically and found that the region size, type and distribution of scatterers in a population, and image signal-to-noise ratio (SNR) all had a significant impact, i.e., each could change the mean and/or standard deviation of the temperature-dependent CBE by 0.5-1dB resulting in potential temperature estimation errors of 1-2^oC. With a consistent scatterer population and image SNR, however, a region size equivalent to a 1cm3 tissue volume produced estimation error less than 0.5^oC. We are currently conducting 3D in vitro heating experiments with turkey tissue to apply these simulation findings to experimental measurements and investigate temperature accuracy and spatial resolution. 2D images are acquired with a Terason 3000 (Teratech Corp., Burlington, MA), laptop-based, phased-array system using a 7-MHz linear probe (model 12L5). 3D images are generated by translating the probe in the elevation dimension using a stepper motor system and are acquired at temperatures from 37.0 to 50.0^oC in 0.5^oC steps. Apparent motion in the images is compensated using a 3D non-rigid motion-compensation algorithm. The 3D motion field is approximated as varying linearly over the tissue volume and is estimated by maximizing the 3D normalized cross-correlation using standard optimization techniques available in MatlabŪ. Initial CBE measurements are consistent with previous results. We expect that by using a consistent region size and accounting for SNR, CBE can be calibrated in this tissue to provide estimation of temperature to 0.5^oC for a 1cm³ volume.

Support:  R21-CA90531, R01-CA107558 and the Wilkinson Trust at Washington University, St. Louis.