Tissue Harmonic Imaging with Coded Excitation

Abstract

It becomes more important to obtain medical ultrasound images with higher signal-to-noise ratio (SNR) and higher spatial resolution. In the last decade, tissue harmonic imaging (THI) and coded excitation to medical ultrasound imaging have been investigated. Coded excitation can overcome the trade-off between spatial resolution and penetration, which occurs when using the conventional pulse (Chiao, 2005), (Hu et al., 2001), (Tanabe et al., 2008). It is said that chirp signal is the most robust code for medical ultrasound image (Misaridis & Jensen, 2005). THI can acquire higher spatial resolution image and has been used in the commercial medical ultrasound system. A combination of coded excitation and THI (coded THI) has been investigated (Arshadi et al., 2007), (Hu et al., 2001), (Song et al., 2010), (Tanabe et al., 2010). As a problem of THI, the frequency bandwidths of the fundamental and the harmonic components often overlap. The spectral overlap causes degradation of spatial resolution and the undesirable artifact. For the solution to the problem, only the harmonic component is extracted by pulse inversion (PI) method. However, if reflectors in the region of interest (ROI) move even a little, the fundamental components of echos are not cancelled completely. The intensity of the extracted harmonic components is still much smaller than the intensity of the residual fundamental component. Hence, the residual fundamental component has a bad effect on THI. In sections II through IV, THI, coded excitation, and the combination of them are explained. In section V, we propose a new method which can extract broader bandwidth of the harmonic component for coded THI. The feasibility of the method is evaluated with experiments and simulations, and the expected performance in term of axial resolution and SNR has been verified.