HIFU Drive System Miniaturization Using Harmonic Reduced Pulsewidth Modulation

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 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$p > 0.05$ \end{document}). 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.

Gallium Nitride Based High-Power Switched HIFU Pulser with Real-Time Current/Voltage Monitoring

“…The use of a Harmonic-Reduction Pulse-Width Modulation (HRPWM) technique [12] [13] [14] has been previously shown to allow partial cancellation of the third and fifth harmonics across multiple amplitudes. This has been demonstrated previously for HIFU applications [15].…”

“…The previously demonstrated University of Leeds (UoL) High-Intensity Focussed Ultrasound Array Research Platform (HIFUARP) system [15] makes use of a P-Channel MOS-FETs (PMOS)/N-Channel MOSFETs (NMOS) topology that is capable of switching at around 1.1 MHz albeit with limited potential of amplitude control due to the rise/fall times of the transistors. A method to improve the amplitude control using a Modified Harmonic-Reduction Pulse-Width Modulation (mHRPWM) scheme have been shown [16] that attempts to maximise control performance.…”

HIFU Power Monitoring Using Combined Instantaneous Current and Voltage Measurement

“…This amplifier was chosen over the one used in the previous study due to the continuous power ratings being higher. For the lesions made with a switched circuit, the High Intensity Focused Ultrasound Array Research Platform (HIFUARP) was used [25]. The switched system had voltage rails that were set to asymmetric values to compensate for the different resistances of the P and N channel MOSFETs, such that the maximum and minimum possible voltage values were ±68 V. The 5level HRPWM modulation algorithm was used to design the switched waveforms [49].…”

“…Three measures were taken to ensure that mechanical damage was reduced. Firstly, a threshold-based passive cavitation detector (PCD) system was used [25], [50]. Secondly, exposure times and intensities within the limits of previously published lesioning work (in chicken breast) were used [51].…”

“…For ultrasound imaging systems, the focus of electronics research has largely been on improving computational power and element count [22]. However, for HIFU systems where the continuous power ratings must be significantly higher [23], the focus has been on reducing the cost, size and complexity [24], [25] whilst maintaining MRI compatibility [26], [27]. The exception is transcranial systems where the element count continues to increase due to improved steering capabilities [16], [28]- [30] When making thermally-formed lesions using HIFU, the total acoustic power is arguably the most critical value as it dictates the spatial-peak pulse-average intensity (I SPPA ) and spatial-peak temporal-average (I SP T A ), which both correlate with lesion volume (the latter being a better metric of heating for low duty cycle exposures [31]).…”