TOMOGRAPHIC IMAGING WITH DIFFRACTING SOURCES (SCATTERING, DOPPLER, ULTRASOUND)
Abstract
The problem of cross sectional (tomograhic) imaging of objects with diffracting sources is addressed. Specifically the two areas of investigation are: the diffraction effects in Doppler ultrasonic imaging and the effect of multiple scattering and attenuation phenomena in diffraction imaging. The past theoretical contributions in Doppler ultrasonic imaging have borrowed heavily from the electromagnetic case. In these contributions most points of departure between the ultrasonic and electromagnetic cases were taken care of by heuristic incorporation of factors in the derived formulas. A theory is presented that is more complete in the sense that it specifically accounts for the diffracted fields of the transducer aperture (assumed to be a source of a Gaussian focussed beam), the interaction of these fields with the scattering sites, and the interaction of the transducer aperture with the back-scattered fields. The theoretical formulations are utilized in the implementation of a series of computer simulations of a typical Doppler ultrasound system to examine the role of different parameters of the system. The validity of either the Born or the Rytov approximations is the basic assumption behind all the inverse scattering techniques in diffraction tomography. To test these techniques when these assumptions are not satisfied, we have developed a computational procedure for the calculation of the "true" scattered fields from a multi-component object. Using this procedure, the performance of two available diffraction reconstruction techniques is examined in the presence of multiple scattering effects. The simulation results show the superiority of the Synthetic Aperture technique. We have also studied the role of attenuation in the reconstruction techniques. To calculate the scattered fields from an object in the presence of attenuation, new computer simulation programs are developed. These codes are used in a simulation study of the effect of the attenuation parameter on the object reconstructions.
Degree
Ph.D.
Subject Area
Electrical engineering
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