Low-frequency parametric ultrasound imaging using pulse compression with maximum-length sequence excitation

In the present study, to realize low-frequency ultrasound (US) imaging, the application of maximum-length sequence (M-sequence) to the compressed parametric US is attempted to reduce time sidelobes, and the performance is experimentally discussed. A modulated primary US at a carrier frequency of 2.8 MHz was radiated in water to generate a 400-kHz parametric US coded by an M-sequence with a chip length equal to one cycle of a 400-kHz signal, and brass rods were used as targets. Experimental results showed that the pulse compression of parametric US with M-sequence reduced artifacts in image. Moreover, the lateral resolution obtained by the proposed method was improved in comparison with that obtained by the compressed-chirped parametric US with the bandwidth from 100 to 500 kHz. These results suggest that the application of M-sequence is well-suited for the parametric US imaging with improved artifacts and lateral resolution.