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The Use of Focused Beams with Unique Biomedical Ultrasonic Imaging Systems

David Duncan

Professor Robert Waag

CSB 426

Abstract

This thesis investigates the use of focused beams in the unique ultrasonic transducer systems and software in the Ultrasound Research Laboratory at the University of Rochester. Three projects involving focused beams were studied to help improve the diagnostic capabilities of biomedical ultrasonic imaging.

In the first project, pressure scattered by cylindrical and spherical objects with elevation-focused illumination and reception has been analytically calculated and corresponding cross sections have been reconstructed with a two-dimensional algorithm. Elevation focusing was used to elucidate constraints on quantitative imaging of three-dimensional objects with two-dimensional algorithms. Focused illumination and reception are represented by angular spectra of plane waves that were efficiently computed using a Fourier interpolation method to maintain the same angles for all temporal frequencies. Reconstructions were formed using an eigenfunction method with multiple frequencies, phase compensation, and iteration. The results show the scattered pressure reduces to a two-dimensional expression and two-dimensional algorithms are applicable when the region of a three-dimensional object within an elevation-focused beam is approximately constant in elevation. The results also show that energy scattered out of the reception aperture by objects contained within the focused beam can result in the reconstructed values of attenuation slope being greater than true values at the boundary of the object. Reconstructed sound speed images, however, appear to be relatively unaffected by the loss in scattered energy. The broad conclusion that can be drawn from these results is that two-dimensional reconstructions require compensation to account for uncaptured three-dimensional scattering.

In the second project, the k-space forward solver codes were reorganized into an object-oriented framework and converted to solve for the scattered pressure. This was done to generalize the codes and allow for easy modification of experimental acoustic parameters. The modified codes were used to compute the scattered pressure from a cylinder and were compared to its analytic solution for verification. The codes were also used to compute the retransmitted eigenfunctions of the far-field scattering operator which are useful in the eigenfunction reconstruction methods. The results indicate that the modified k-space codes are a good tool for performing focusing experiments, such as the computation of the F-fields.

In the last project, the use of a spatio-temporal inverse filter in b-mode ultrasonic imaging is more closely analyzed. Results from previous research studies are reviewed, and the use of a spatial inverse filter in a homogeneous medium is presented. Imaging artifacts in both simulated and measured data show that more consideration is required when using a STIF-focusing method.