| Accurate monitoring of high intensity focused ultrasound therapy is critical for widespread clinical use. Past research has established that, given physical registration between tissue and an ultrasound array, relative changes in the time-of-flight to scatterers in the tissue, recorded before and after application of high intensity focused ultrasound therapy, are directly related to the temperature changes in the material. These time-of-flight changes appear as relative displacements, and the proper analysis of these displacements offers the potential to attain the limits of precision for diagnostic ultrasound (DU) therapy monitoring. To obtain these limits, and use the ultrasound data in an efficient manner, the physics of heating and diagnostic ultrasound measurement must be modeled correctly.;Temperature changes alter DU backscatter in a coherent manner, and it is this coherence which is modeled and exploited to formulate estimators, allowing for inferences about therapy. This dissertation shows these estimators are efficient; they attain the identified lower bounds on monitoring accuracy. Conversely, given knowledge of the therapy, these same estimators allow inferences to be made about the modeling, and whether the observed displacements adhere to the modeling employed. Thus, a method for analyzing backscatter is presented, which can be tested on both existing and proposed models to assess their validity. A key result is that existing models describing the influence of temperature change on backscatter measurements are shown to be inconsistent with backscatter data collected from carefully controlled thermal experiments. From a careful analysis of this data, a new modeling scheme is proposed, one which attributes the time-of-flight changes to both scattering from temperature induced heterogeneities, and an apparent diffraction effect, the combined result of spherical radiation from an array element and spherical backscattering from the heterogeneities. This modeling is evaluated using the proposed analysis technique, indicating a much stronger adherence to the ultrasound measurements than conventional approaches. |
