Abstract
Theoretical and experimental evidence continues to suggest that backscattered ultrasonic energy changes predictably with temperature. Previously, theoretical results for a single scatterer showed that backscattered energy increases or decreases monotonically, depending on the lipid or aqueous nature of the scatterer. Recent experimental results showed that the same trend holds for measurements over an image region subtending multiple scatterers. To extend our theory to a more realistic tissue composition, we have developed methods for simulating ultrasonic images of multiple randomly distributed scatterers. In the simulations, the imaging system was described by its point-spread function, and the tissue medium was represented by discrete aqueous and lipid scatterers. Images were simulated to represent temperatures from 37 to 50oC by changing the scatterer amplitudes according to curves predicted previously for single scatterers. Change in backscattered energy (CBE) was computed for each image pixel, referenced to the initial image. To characterize CBE for a region, the means of the positive- and negative-changing pixels and the standard deviation of all pixels were computed. The region CBE showed the same monotonic increase and decrease as in experimental results and covered ranges similar to both prediction and experiment. Subsequent simulations included additive noise and showed striking agreement with experimental CBE measurements, replicating both an initial jump and noise throughout the range. These results support the use of CBE for noninvasive temperature estimation, showing that our model for the temperature dependence of CBE can be successfully applied to measurements from multiple scatterers. These simulation methods also provide a means for exploring limits on temperature accuracy and spatial resolution with varying imaging systems and tissue types.
Acknowledgement: Supported in part by NIH grant R21-CA90531 from the National Cancer Institute and the Wilkinson Trust at Washington University in St. Louis.