Deep localized hyperthermia with ultrasound-phased arrays using the pseudoinverse pattern synthesis method

One of the major limitations of hyperthermia as a cancer treatment modality is the lack of heating equipment and techniques capable of consistent therapeutic heating of deepseated tumors. This thesis introduces a new pattern synthesis method capable of precisely controlling the power deposition level at a set of control •points in the treatment volume using ultrasound phased arrays. This method, called the pseudoinverse pattern synthesis method, reduces the pattern synthesis problem to one of estimating the minimum-norm least-square solution to a matrix equation of the form, H u = p , where u is the arrayexcitation vector, p is the desired complex pressure at the control points, and H is a matrix propagation operator from the surface of the array to the control points. A useful solution to this problem is obtained when the number of control points is less than the number of elements of the array and the matrix H is full rank. This solution, called the minimum-norm solution, allows the array to be focused at several points simultaneously. This multiple-focus approach is important when ultrasound is used as a heating agent as it reduces the spatial-peak temporal-peak intensity required to generate a specified heating pattern. Furthermore, the minimum-norm solution allows the optimization of the array excitation efficiency and the intensity gain at the control points. These quantities are very significant for achieving deep localized heating with phased arrays. In fact, optimization of the intensity gain at the control points generally results in removal of high intensity interference patterns from the synthesized field. The removal of high intensity interference patterns eliminates one of the major disadvantages of multiple focusing. The pseudoinverse pattern synthesis method is introduced and discussed in detail. Simulation results are used to demonstrate its powerful capabilities as a pattern synthesis method. Its generality is demonstrated by the use of several different array structures to synthesize different multiple-focus patterns. Simulation results indicate that direct synthesis of multiple-focus patterns can provide an alternative to single-focus scanning. Finally, measured intensity profiles using a prototype cylindrical-section array agree well with theoretically predicted profiles.