Model Based Reconstruction of Ultrasonic Images
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.
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
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.
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.
NDE research at Signals andSystems
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