Broadband transskull multi-focus wavefront synthesis

Abstract

Multiple-focus pattern synthesis using ultrasound arrays has been previously demonstrated. Furthermore, an optimal synthesis method was shown to provide well-behaved solutions. This method creates predefined focus patterns at a single frequency for an ultrasonic array operating in continuous wave. In this paper, we extend the approach to the broadband case using orthogonal frequency-division multiplexing (OFDM). A broadband multiple-focus pattern is formed by summing the results of the synthesis algorithm at a set of discrete frequencies within the transducer bandwidth. The OFDM approach allows for achieving a predictable focusing gain at the target point(s) due to the waveform orthogonality. Outside the target region, the different frequency components can be expected to provide a larger degree of destructive interference. Hydrophone scans of multiple-focus patterns in plain water as well as transskull were performed using a phased array with a multichannel arbitrary waveform generator. The array has a 3.5 MHz center frequency with a 6-dB bandwidth in the range 2.2 MHz–4.8 MHz. The skull samples were obtained from 300 to 350 g rats that were used in other in vivo experiments. The results show greater focus resolution than for any single frequency pattern while reducing the interference patterns outside the target region.Multiple-focus pattern synthesis using ultrasound arrays has been previously demonstrated. Furthermore, an optimal synthesis method was shown to provide well-behaved solutions. This method creates predefined focus patterns at a single frequency for an ultrasonic array operating in continuous wave. In this paper, we extend the approach to the broadband case using orthogonal frequency-division multiplexing (OFDM). A broadband multiple-focus pattern is formed by summing the results of the synthesis algorithm at a set of discrete frequencies within the transducer bandwidth. The OFDM approach allows for achieving a predictable focusing gain at the target point(s) due to the waveform orthogonality. Outside the target region, the different frequency components can be expected to provide a larger degree of destructive interference. Hydrophone scans of multiple-focus patterns in plain water as well as transskull were performed using a phased array with a multichannel arbitrary waveform generator. The array has a 3...