Erik Wennerström

Licentiate Thesis, Signals and Systems, Uppsala University, June 2007

The thesis is available from Signals and Systems, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden.

Abstract:

This thesis is concerned with reconstruction methods for ultrasonic imaging, primarily applied to non-destructive material inspection. In ultrasonic inspection, a specimen is insonified with pulses of high frequency ultrasound, which enters the material and propagates through it.

A region with different acoustic impedance that of the surroundings, such as a cavity, crack or other defect will reflect and scatter the ultrasonic pulse. The echoes from these reflectors are gathered and aggregated to infer information about the scatters position and relative size.

The work in this thesis is based on the Extended Synthetic Aperture Technique, a time domain method for estimating scattering strength of image elements in a discretized ultrasonic image. The method is further extended and improved in several ways in this thesis.

One part of the extension is computational, dividing the reconstruction problem into smaller subproblems and solving them sequentially. The sum of the computational load to compute all subproblems is less than that of solving the large problem, especially as the method utilizes symmetries in the related subproblems.

The next extension is also aimed at providing a less computationally demanding method. The reconstruction problem is limited to estimate only the most likely scatterer positions in the image. As long as the problem is sparse with only a few scatterers, most image elements can be discarded as their scattering strength is close to zero.

The final extension presented here is instead aimed at improving lateral resolution, iteratively solving the reconstruction problem to minimize the reconstruction error.