Use of chirps in medical ultrasound imaging

A "chirp" is a frequency modulated signal widely used in ultrasound imaging to increase the signal-to-noise ratio and penetration depth. In medical ultrasound imaging, resolution and penetration are two major criteria that are inversely proportional. Because of this inverse relation, short duration pulses cannot achieve a high resolution with good penetration. The reasons for this trade-off are the decrease in signal energy due to shorter pulse duration and the attenuation in tissue, which increases with the excitation frequency. The chirp coded excitation however can increase the total transmitted energy using longer pulse durations, while the resolution can be recovered by decoding on receive. Therefore, chirp signals offer potential advantages over single carrier short duration pulses for medical imaging. This work addresses the possible problems encountered in medical ultrasound imaging with chirps and offers new solutions to these problems in terms of signal processing. These proposed solutions are then applied to three major categories of medical ultrasound imaging; hard-tissue ultrasound imaging, soft-tissue ultrasound imaging and contrast-enhanced ultrasound imaging. The application of coded excitation in medical ultrasound imaging is the main motivation behind this work. Therefore, the concepts of frequency modulation and matched filtering are introduced first, and ultrasound specicific problems for pulse compression of chirps are discussed. Examples are given on specific applications and circumstances, where the performance of the traditional pulse compression techniques drops significantly. Alternate methods of pulse compression and filtering of frequency modulated chirps using the Fractional Fourier transform (FrFT) and the Fan Chirp transform (FChT) are presented. Rather than restricting the chirp analysis in the time or frequency domain; these proposed methods transform the signal of interest into a new domain, which is more suitable to analyse frequency modulated chirps.