Coded Excitation Methods for Ultrasound Harmonic Imaging

Coded excitation methods offer the potential for improving the SNR without increasing the peak transmitted power and without sacrificing resolution. Our study examines the potential application of coded waveforms, specifically FM chirps, in harmonic imaging. Such a system, in which nonlinear echoes from tissue are used to form the image, requires the extraction and compression of the second harmonic portion of the echo signal. Our objective is to obtain the second harmonic using just one transmission, thereby avoiding problems of frame rate reduction and movement artifacts associated with multiple transmission schemes. With the help of an efficient method for predicting the transient nonlinear field from a focused transducer, design issues such as waveform and bandwidth selection, as well as filters for second harmonic extraction and compression are examined. Simulations reveal the presence of axial sidelobes in the compressed echo waveform as the bandwidth of the transmitted chirp is increased. These sidelobes, resulting from the overlap of the fundamental and third harmonic bands with the second harmonic, cannot be removed using conventional Fourier filtering. Alternative filtering techniques which utilize the separation of the harmonic bands of a backscattered chirp in the joint time-frequency domain are suggested.