HIFU Drive System Miniaturization Using Harmonic Reduced Pulsewidth Modulation

Chris Adams,James R Mclaughlan,Thomas M Carpenter,David Cowell,Steven Freear

doi:10.1109/tuffc.2018.2878464

Chris Adams, James R Mclaughlan + Show 3 more

Open Access

https://doi.org/10.1109/tuffc.2018.2878464

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Abstract

Switched excitation has the potential to improve on the cost, efficiency, and size of the linear amplifier circuitry currently used in high-intensity focused ultrasound (HIFU) systems. Existing switching schemes are impaired by high harmonic distortion or lack array apodisation capability, so require adjustable supplies and/or large power filters to be useful. A multilevel pulsewidth modulation (PWM) topology could address both of these issues but the switching-speed limitations of transistors mean that there are a limited number of pulses available in each waveform cycle. In this study, harmonic reduction PWM (HRPWM) is proposed as an algorithmic solution to the design of switched waveforms. Its appropriateness for HIFU was assessed by design of a high power five-level unfiltered amplifier and subsequent thermal-only lesioning of ex vivo chicken breast. Three switched waveforms of different electrical powers (16, 26, 35 W) were generated using the HRPWM algorithm. Lesion sizes were measured and compared with those made at the same electrical power using a linear amplifier and bi-level excitation. HRPWM produced symmetric, thermal-only lesions that were the same size as their linear amplifier equivalents (n}{}p > 0.05n). At 16 W, bi-level excitation produced smaller lesions but at higher power levels large transients in the acoustic waveform nucleated undesired cavitation. These results demonstrate that HRPWM can minimize HIFU drive circuity size without the need for filters to remove harmonics or adjustable power supplies to achieve array apodisation.

Highlights

H IGH-INTENSITY focused ultrasound (HIFU) is a noninvasive surgical technique that is used to generate coagulative necrosis in tissue through localized thermal ablation [1], [2] and other mechanical effects [3], [4].The main application areas of high-intensity focused ultrasound (HIFU) are the treatment of soft tissue tumors [5], [6] in liver [7], kidney [8], prostate [9], [10], breast [11], and in the brain [12], [13]
When harmonic reduction PWM (HRPWM) excitation was used, the lesion crosssection area was only slightly smaller (−12%) than the linear amplifier lesion. This is likely due to HRPWM having energy at higher order harmonics that are attenuated by the tissue
HRPWM was compared with a linear amplifier and bi-level excitations numerically and experimentally

Summary

Introduction

H IGH-INTENSITY focused ultrasound (HIFU) is a noninvasive surgical technique that is used to generate coagulative necrosis in tissue through localized thermal ablation [1], [2] and other mechanical effects [3], [4].The main application areas of HIFU are the treatment of soft tissue tumors [5], [6] in liver [7], kidney [8], prostate [9], [10], breast [11], and in the brain [12], [13].

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