Sidelobe Degradation of Complementary Golay Codes by Nonlinear Effects in Ultrasonic Imaging

Advances in electronics miniaturization have led to the development of low-power, low-cost ultrasound scanners which have found popularity in the large farm animal veterinary market. When imaging the bovine reproductive tract, veterinarians seek to obtain high resolution images with appropriate imaging penetration to diagnose pregnancy, allowing the farmer to make informed herd management decisions. Signal to Noise Ratio of an ultrasound system can be improved through the use of pulse compression imaging, however the interaction of complementary Golay coded excitations with harmonic distortion introduced by nonlinear phenomena, such as propagation through large areas of fluid or fat in the body, has not been fully investigated. This work is the first use simulated and experimental work to investigate the impact of harmonic distortion on complementary Golay codes. Sidelobes degraded by 80dB on average when nonlinear acoustics theory was introduced to simulations of 96 different Golay codes. Experimental results using commercial ultrasound scanners for veterinary imaging showed that an increase to the Spectral Index, the percentage of waveform energy above the fundamental transmit frequency, of 8.2% led to the Peak Sidelobe Level degrading by 19dB. Three mitigations are investigated herein with the following outcomes: simulations of 96 complementary codes show that some more robust to harmonic distortion than others, though the reasons for this remains unsolved; conventional receive bandpass filters centred around the fundamental transmit frequency can reduce, but not eliminate, sidelobe levels; and introducing spacing between code symbols can give improved sidelobe cancellation performance, albeit unpredictably. Ultimately, however, the best course of action is to reduce the formation of harmonic distortion as much as possible.