|
Time-Domain Reconstruction Methods for Ultrasonic Array Imaging
A Statistical Approach
Fredrik Lingvall
PhD Thesis, Uppsala University,
ISBN 91-506-1772-9,
Sept. 2004, 177 pp.
Dissertation in
Electrical Engineering with specialization in Signal Processing,
to be publicly examined
in room K23, Magistern, Dag Hammarskjölds väg 31,
Uppsala on October 15, 2004 at 10.15 a.m.
Faculty Opponent:
Professor Karl-Jörg Langenberg, Elektrotechnik, Kassel, Germany
The thesis available
In Pdf
(9.27M).
Paper copies of the thesis can be obtained from
Ylva Johansson,
Signals and Systems Group, Uppsala University,
Box 534, SE-75121 Uppsala, Sweden.
- Outline:
-
-
Abstract:
-
This thesis is concerned with reconstruction techniques for ultrasonic
array imaging based on a statistical approach. The reconstruction
problem is posed as the estimation of an image consisting of scattering
strengths.
To solve the estimation problem, a time-domain
spatially-variant model of the imaging system is developed based on the
spatial impulse response method. The image reconstruction is formulated
as a linearized inverse-scattering problem in which the time and space
discrete natures of the imaging setup as well as measurement
uncertainties are taken into account. The uncertainties are modeled in
terms of a Gaussian distribution.
The scattering strengths are estimated
using two prior probability density functions (PDF's), Gaussian and
exponential PDF's. For the Gaussian PDF,
the maximum a posteriori (MAP)
approach results in an analytical solution in the form of a linear
spatio-temporal filter which deconvolves the diffraction distortion due
to the finite-sized transducer. The exponential distribution leads to a
positivity constrained quadratic programming (PCQP) problem that is
solved using efficient optimization algorithms. In contrast to
traditional beamforming methods (based on delay-and-summation), the
reconstruction approach proposed here accounts both for diffraction
effects and for the transducer's electro-mechanical characteristics.
The
simulation and experimental results presented
show that the performances
of the linear MAP and nonlinear PCQP estimators are superior to
classical beamforming in terms of resolution and sidelobe level, and
that the proposed methods can effectively reduce spatial aliasing errors
present in the conventional beamforming methods.
-
Keywords:
-
Ultrasonic array imaging, Synthetic aperture imaging,
amximul a posteriori estimation,
linear minimum mean square error estimation,
SAFT, Bayesian image reconstruction.
|
Main entry in publ. lists
|
This material is presented to ensure timely dissemination
of scholarly and technical work. Copyright and all rights
therein are retained by authors.
All persons copying this information are expected to
adhere to the terms and constraints invoked by each authors
copyright. This work may not be reposted
without the explicit permission of the copyright holders.
|