Pre-distorted amplitude modulated (PDAM) chirps for transducer compensation in harmonic imaging

Abstract

A pre-distortion by amplitude modulate (PDAM) method is evaluated for use in linear frequency modulated (LFM) coded harmonic imaging. LFM coded signals are known to boost the SNR of the low amplitude harmonics, but due to the nature of harmonic imaging, are located at inefficient regions of the transducer's bandwidth. As a consequence, the LFM signal is subjected to transducer-induced tapering which is particularly severe at the start or end of the signal. The motivation for this work is to investigate whether compensating for this tapering effect results in improved compression of the harmonic signal.PDAM signals are designed using an inverse filter method. Hydrophone measurements show successful transducer compensation of the fundamental signal. Fundamental and second harmonic images of a tissue mimicking phantom indicate minimal improvement when compared with non pre-distorted LFM signals (N-PDAM). A discussion is provided as to why this result can be attributed to the use of a sub-optimal compression filter.

The effect of amplitude modulation on subharmonic imaging with chirp excitation

“…The pressure calibration of the transducer was performed using a Polyvinylidene Difluoride (PVDF) 1 mm needle hydrophone (Precision Acoustics Ltd., Dorchester, UK). Transmit waveforms were pre-distorted according to the frequency response of the transducer [43], [44]. Before each microbubble scattering measurement the noise floor was determined for each pressure level, by performing a control measurement without microbubbles in the chamber.…”

“…Therefore, the UARP was programmed to generate three chirp waveforms with a center frequency of 7 MHz, duration of 20 µs, and bandwidth of 2 MHz for subharmonic imaging. To compensate for the transducer response and to keep the original window shapes, transmit waveforms were pre-distorted according to the frequency response of the medical probe [43], [44].…”

Performance of switched mode arbitrary excitation using Harmonic Reduction Pulse Width Modulation (HRPWM) in array imaging applications

“…Ultrasound systems require excitation amplitude control to adjust acoustic pressure, waveform shaping (temporal) and array apodisation (spatial), frequency control for coded excitation and reduced excitation harmonics for harmonic imaging. Control of excitation parameters allows the development of methods such as transmit pre-distortion [7], subharmonic imaging [8], superharmonic imaging [9] and microbubble manipulation [10]. This paper explores the application of Harmonic Reduction Pulse Width Modulation (HRPWM) [11]- [13] switched excitation using commercial transmit front end integrated circuits The HRPWM technique allows that control of excitation amplitude, and hence ultrasonic wave pressure, by defining two waveforms with varying switching angles δ 1 and δ 2 as illustrated in figures 1a and 1b [11], [12].…”