Phase-Aberration Correction for HIFU Therapy Using a Multielement Array and Backscattering of Nonlinear Pulses

Abstract

Phase aberrations induced by heterogeneities in body wall tissues introduce a shift and broadening of the high-intensity focused ultrasound (HIFU) focus, associated with decreased focal intensity. This effect is particularly detrimental for HIFU therapies that rely on shock front formation at the focus, such as boiling histotripsy (BH). In this article, an aberration correction method based on the backscattering of nonlinear ultrasound pulses from the focus is proposed and evaluated in tissue-mimicking phantoms. A custom BH system comprising a 1.5-MHz 256-element array connected to a Verasonics V1 engine was used as a pulse/echo probe. Pulse inversion imaging was implemented to visualize the second harmonic of the backscattered signal from the focus inside a phantom when propagating through an aberrating layer. Phase correction for each array element was derived from an aberration-correction method for ultrasound imaging that combines both the beamsum and the nearest neighbor correlation method and adapted it to the unique configuration of the array. The results were confirmed by replacing the target tissue with a fiber-optic hydrophone. Comparing the shock amplitude before and after phase-aberration correction showed that the majority of losses due to tissue heterogeneity were compensated, enabling fully developed shocks to be generated while focusing through aberrating layers. The feasibility of using a HIFU phased-array transducer as a pulse-echo probe in harmonic imaging mode to correct for phase aberrations was demonstrated.

Fig. 1.

(a) Photograph of the 256-element HIFU array. (b) Layout of the array elements with one of the 16 spiral arms shown in green.

Fig. 2.

Path of the loop across a 2D projection of the array, generated using the travelling salesman problem optimization method. Element number 0 is represented in red while element 255 is in magenta.

Fig. 3.

(a) Focal pressure waveform produced by the HIFU array in the pulse/echo mode. (b)-(d) Simulated HIFU pressure distributions along the propagation axis z (b) and the two transverse axes (c) and (d) in the focal plane at the fundamental frequency of 1.5 MHz , second harmonic and third harmonic at a system drive voltage of 8V (68 W acoustic power) [46].

Fig. 4.

(a) Pressure pulses constituting a pulse inversion sequence produced by the HIFU array and measured with FOPH at the array focus during a pulse inversion sequence for the system driving voltage of 8V. (b) Sum of the pressure pulses. (c) Sum of the pressure pulses reflected from the FOPH tip positioned at the HIFU focus and acquired by the HIFU array elements shown for all array elements.

Fig. 5.

Experimental setup.

Fig. 6.

Top: RF signal on one element. (a) Proximal face of the body wall phantom, (b) interface between the layers in the body wall phantom, (c) proximal face of the blood clot phantom, (d) focus of the transducer, where represents the length of the focal lobe of the second harmonic. Bottom: beamsum around the focus.

Fig. 7.

Example of the output of step 2 of the aberration correction algorithm, where the threshold Γ is represented as . The area between the two vertical dashed lines illustrates a cluster of HIFU array elements with low correlation coefficients to the beamsum signal, and a corresponding high variance in phase error estimates.

Fig. 8.

Representative example of the output of Step 3 (nearest neighbor method) , plotted together with the output of Step 2 (beamsum method) . High phase error variance from the beamsum method was successfully removed by the “anchored” nearest neighbor method.

Fig. 9.

RF signal segments corresponding to the HIFU focus obtained with the soft tissue phantom (a) without the aberration-inducing phantom; (b)with the aberration-inducing phantom, before aberration correction. The transmit focus time T_f is outlined as .

Fig. 10.

(a) Normalized value of the function Φ for each iteration. (b) Phase correction applied to each element of the array. (c) RF signals of the corrected beam with the transmit focus time T_f is outlined as .

Fig. 11.

Legend: measurements with the aberration-inducing phantom and no correction: , with the aberration-inducing phantom and correction: , in water: . (a) Waveforms at the focus for the same derated voltage of 15V in water. (b) Peak positive pressure and (c) peak negative pressure at the focus as functions of the input voltage (derated if the phantom is present). (d)-(g) Transverse beam profiles in the focal plane for the same derated voltage as 15V in water.