Computed Ultrasonic Reflection Tomography

A digital image reconstruction technique was developed and tested to assess the feasibility of imaging the ultrasonic reflectivity in tissue cross sections using backscattered energy obtained by fanbeam and plane-wave pulse-echo interrogation. In the fan-beam case, circular-arc wavefronts are produced, while in the plane-wave case the wavefronts consist of parallel straight lines. In both cases the wavefronts were wide enough to cover the entire width of the object cross section but were focused perpendicular to the image plane to effectively confine the wave interactions to a thin tomographic slice. The tomographic scanning geometry consists of recording backscattered signals for multiple source/receiver positions located at uniformly spaced angular increments on a circle surrounding the image plane. At each angular location a complete projection is obtained from the time-sampled reflected ultrasonic signal. From these projections a tomographic image of reflectivity is computed based on the mathematics of image reconstruction from projections. Experimental reconstructions of wire test targets, natural sponges, natural sponges with embedded calcification-simulating reflectors, and beef liver are presented to illustrate the performance of the plane-wave and fan-beam refleclivity imaging methods. Results using wire targets demonstrate that a spatially uniform resolution of better than one wavelength (0.3 mm at 5 MHz) can be obtained under ideal experimental conditions. While good reconstructions are obtained using both the fan-beam and plane-wave sources, the plane-wave approach results in simpler transducer fabrication, a more ideal match to efficient reconstruction algorithms, and requires only a 180" view angle range as compared to the 360" range needed for the fan-beam case.