A coded excitation system for pulsed Doppler ultrasound based on the fractional Fourier transform

A novel coded excitation scheme is presented and evaluated for pulsed Doppler ultrasound systems. Its purpose is to increase the velocity aliasing limit by allowing operation at pulse repetition frequencies (PRF) beyond the Nyquist limit. Prior research has shown that operation at increased PRF results in unwanted echoes that contribute to the received signal from outside the sample volume. The proposed scheme uses the fractional Fourier transform (a powerful generalized Fourier transform) to create a linear-time-variant filter for echo separation. Single and multi-scatterer simulations results for 1-D and 3-D for a new class of bi-lobed chirp signals demonstrates significant reduction of the ghost echo problem. The challenge of completely eliminating unwanted echoes in high-PRF schemes is addressed from a time-frequency perspective, and methods of overcoming this difficulty are suggested.