High frequency chirp coded excitation ultrasound imaging system

An increase of imaging frequency enhances the performance in spatial resolution of an ultrasound scanner while sacrificing echo signal to noise ratio (eSNR) because of a rise in the amount of attenuation. As solutions to raise the eSNR level while maintaining the enhanced spatial resolution, both fundamental and harmonic coded excitation imaging methods have been proposed. For this purpose, ultrasound bio-microscopes (UBM) capable of chirp coded excitations were implemented on both a linear and a sector mechanical scanning system. A custom-designed broadband (5 MHz – 140 MHz) linear power amplifier was designed as a part of a low noise arbitrary pulse generator to replace a commercial bench-top power amplifier in the linear UBM. The modified imaging system exhibited a better eSNR of 7 dB than the system with a commercial power amplifier while maintaining the spatial resolution in a wire target measurement. In-vivo imaging was carried on pig eye and zebrafish embryo using a 40 MHz and a 100 MHz single element transducer respectively. For high frame rate coded excitation imaging, a standalone frontend system for a sector UBM was implemented. The custom-designed system consisting of a transmitter, a receiver, a motor controller and a motor position interpreter achieved an eSNR of 66 dB utilizing chirp coded excitation for wire target imaging. Moreover, contrast to noise ratio (CNR) was improved from 2.92 to 3.18 with coded excitation. This modified scanner allowed contraction of cardiac chambers of both zebrafish and mouse hearts to be clearly visualized. As an effort to enhance the spatial resolution, pulse inversion chirp coded tissue harmonic imaging (PI-CTHI) was programmed into the custom-designed frontend system. In a wire target study using a 40 MHz single element transducer, lateral resolution was found to be 110 μm, which was narrower than fundamental imaging at153 μm. CNR was found to improve by 34 % when chirp coded excitation was applied to harmonic imaging. This imaging mode was shown to better delineate the clot formation in adult zebrafish heart imaging when the heart was amputated and allowed to regenerate.