Detection and separation of chirp echoes in coded excitation ultrasound imaging using fractional Fourier transform

The use of coded excitation in ultrasound imaging leads to increase the penetration depth in ultrasound imaging, so long duration excitation signals instead of short pulses are applied to the transducers. Linear chirps are the most robust excitation signal which have been used in attenuating medium. The long duration of coded excitation, results in overlapping different echoes from adjacent reflectors. In many applications such as acoustic microscopy, Phase imaging and non destructive tests it is desired to separate the individual mixed echoes in time domain but the ordinary time or frequency filtering is unable to separate. In this paper we use fractional Fourier transform to separate the individual overlapped chirp echoes. Finding the position of chirp echoes in fractional domain in noisy received signal is the most important step in separation process. In order to identify the presence of chirp echoes and their position in the received signal, a constant false alarm chirp detector are proposed in fractional domain. The results of applying the separation method to the synthetic mixed signal shows that the individual echoes are recovered with maximum amplitude error of 0.3%. Using Field II program and coded excitation, the proposed method are applied to noisy received echo signal of four point scatterers. Simulation results show that the individual echo signals, corresponds to each point scatterer, are detected and separated successfully.