A filter-based approach to real-time three-dimensional pulse-echo imaging using ultrasound arrays.

A filter-based coded-excitation pulse-echo imaging modality capable of parallel image line reconstruction is presented for real-time 3D ultrasound imaging applications. The proposed approach is based on a discretized linear model of the received sampled data vector formed from the coherent sum of echoes from all scatterers in the region of interest (ROI). The discretization of the model leads to a regularized pseudoinverse imaging operator (PIO) for scatterer reconstruction. Under simplifying (but realistic) conditions, the PIO can be implemented by a transversal filter bank with each filter designed to extract echoes from a specific direction from the ROI. The number of filters in the filter bank determines the number of image lines that can be acquired simultaneously to meet real-time requirements on frame rates. One advantage of this filter bank is that it not only provides pulse compression along the range direction, but also decouples echoes from different directions. Hence, correlation artifacts caused by nonideal spatial coding are eliminated or significantly reduced. This removes the major drawback of current coded-excitation systems implemented based on matched filters. Another important feature of the new approach is that the image reconstruction is not based on the conventional delay-and-sum beamforming technique. This means that, first, the imaging array may not have to be Nyquist sampled. The difficulty of 2D array fabrication, which is also considered to be a major technical barrier for realizing 3D imaging systems, may thus be reduced significantly using sparse arrays. Second, the lateral (also elevational) resolution of the system is not subject to the Rayleigh diffraction limit. In fact, the proposed filter bank provides Wiener inverse filtering in both temporal and spatial frequency domains. The resulting equalization in the temporal frequency domain gives pulse compression in the range direction. The resulting equalization in the spatial frequency domain, on the other hand, may lead to lateral (or elevational) resolution which exceeds the diffraction limit, depending on system signal to noise ratio and modeling errors. The new system may be realized by modifying existing imaging systems. On the transmit side, the transmitters are modified to transmit pseudo-random binary sequences rather than single pulses. On the receive side, a modified conventional delay-and-sum beamformer can be used to form a spatial mask for blocking the echoes from outside the ROI and controlling data flow into the receiver. The only major modification for the system is the filter bank. This, however, can be implemented compactly using currently available VLSI technology. Preliminary results indicate that the proposed approach can be used to reconstruct speckle-generating phantom images. Such coded-excitation approach based images, to the best of our knowledge, have not been reported previously. With the capability of parallel processing multiple image lines, the new imaging modality may find many applications in medical ultrasound imaging, NDE and SONAR.PhDApplied SciencesBiomedical engineeringElectrical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130583/2/9732181.pd