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Abstract
The theory for modeling nonlinear acoustic propagation is addressed in the
dissertation. The solutions to both the linear and nonlinear wave equations
have been found by an angular spectrum approach (ASA), in which an analytical
expression can be derived. This makes the calculation complete without
iteration steps. The ASA is implemented in combination with Field II
and extended to simulate the pulsed ultrasound fields. The simulated results
from a linear array transducer are made by the ASA based on Field II, and
by a released nonlinear simulation program Abersim, respectively. The calculation
speed of the ASA is increased approximately by a factor of 140. For
the second harmonic point spread function the error of the full width is 1.5%
at 6 dB and 6.4% at 12 dB compared to Abersim. To further investigate the
linear and nonlinear ultrasound fields, hydrophone measurements are performed
under water by two geometrical focused piston transducers. It can
be seen that the time pulses measured from a 0.5 inch diameter transducer
and linearly simulated using the ASA are fairly comparable. The root mean
square (RMS) error for the second harmonic field simulated by the ASA is
10.3% relative to the measurement from a 1 inch diameter transducer.
A preliminary study for harmonic imaging using synthetic aperture sequential
beamforming (SASB) has been demonstrated. A wire phantom underwater
measurement is made by an experimental synthetic aperture realtime
ultrasound scanner (SARUS) with a linear array transducer. The second harmonic
imaging is obtained by a pulse inversion technique. The received data
is beamformed by the SASB using a Beamformation Toolbox. In the measurements
the lateral resolution at 6 dB is improved by 66% compared to the
conventional imaging algorithm. There is also a 35% improvement for the
lateral resolution at 6 dB compared with the sole harmonic imaging and a
46% improvement compared with merely using the SASB.
Original language  English 

Place of Publication  Kgs. Lyngby, Denmark 

Publisher  Technical University of Denmark 
Number of pages  106 
Publication status  Published  2011 
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Projects
 1 Finished

Nonlinear ultrasound imaging
Du, Y., Jensen, J. A., FerkinghoffBorg, J., Torp, H., Willatzen, M. & Jensen, H.
01/06/2008 → 28/09/2011
Project: PhD