Analysis and Design of Ultrasound Phased Arrays for Hyperthermia Cancer Therapy

Abstract

Acoustic phased arrays, which can be focused and steered electronically, offer an attractive alternative to the mechanically scanned focused ultrasound systems currently in use in hyperthermia cancer therapy. This thesis introduces a new method which will allow the direct synthesis, without scanning, of the more diffuse heating patterns useful in hyperthermia. Two quite different phased arrays are theoretically evaluated as hyperthermia applicators: a concentric ring array (CRA), and an N x N square-element array.

The new synthesis method is based on the conjugate phase and amplitude matching techniques often used in optics. The field conjugation method (FCM), theoretically capable of tailoring the ultrasonic power deposition to virtually any tumor geometry without scanning, offers also the possibility of simultaneously focusing at different locations. The multiple focusing feature is combined with a new phasing technique involving angular phase rotation to eliminate hot spots that are often associated with the synthesis of annular patterns.

A concentric-ring array was chosen because of its ability to directly produce annular patterns with a minimum number of array elements. A new method, based on combining the multiple focusing feature and a simple mechanical movement of the applicator, is proposed as a means of heating different size tumors at various depths.

While a concentric-ring array is limited to the synthesis of annular and spot foci, an N x N square-element array is investigated as a means of synthesizing heating patterns with or without circular symmetry. Simulated heating patterns produced by the FCM are compared to those produced by electronic scanning. A method is also proposed to combine the FCM with electronic scanning to produce more complicated diffuse heating patterns. Moreover, an 8 x 8 square-element phased array prototype built and radiation patterns produced by single and multiple elements were measured, and agreed well with theoretically predicted patterns.

To evaluate the different applicators, the steady-state bioheat transfer equation was solved using a finite difference technique. The simulated temperature distributions associated with different power deposition patterns demonstrate the potential of the field conjugation technique for the design of diffuse heating patterns for hyperthemia cancer therapy. (Abstract shortened with permission of author.)