Two-Dimensional Ultrasonic Phased Arrays in an Application for Noninvasive Surgery: Random or Regular?

Abstract

High power 2‐D ultrasonic phased arrays are employed in noninvasive surgery and high‐intensity short‐duration hyperthermia. Advantages include easier design of wide‐aperture HIFU sources, compensation of amplitude and phase aberrations for optimum beam formation, simultaneous production of several foci for treating larger volumes, and electronic steering of foci. However, their discrete structure results in unwanted grating lobes and other secondary intensity maxima in the acoustic field that can lead to overheating of non‐targeted volumes of tissue, especially when steering the focus or a set of foci of the array along or off its geometric axis. In this work a simple but effective method to improve the quality of acoustic fields generated by arrays is discussed. Two types of arrays, consisting of 256 circular elements of 5 mm diameter and 1.5 MHz operating frequency distributed either in a random manner or in a square pattern on a spherical shell of 13 cm aperture and 12 cm radius of curvature, are compared. Generation and steering of 9 or 25 simultaneous foci along and off the array axis, and irradiation of larger volumes by switching between sets of four or five foci are investigated. Results are presented for spatial distributions of ultrasound intensity, temperature, and thermal dose in tissue. It is shown that a random distribution of elements significantly improves the array performance by suppressing the levels of grating lobes and other secondary maxima both in the intensity and temperature fields as compared to those associated with the regular array.