Abstrakter - NORSK FORENING FOR ULTRALYD-DIAGNOSTIKK

A simulation study of SURF reverberation suppression in an aberrating
medium
Peter Näsholm, Halvard Kaupang, Svein-Erik Måsøy, Rune Hansen and Bjørn
Angelsen
Department of Circulation and Medical Imaging, Norwegian University of Science
and Technology, Trondheim, Norway
Introduction:
A numerical computer simulation study is presented in the form of a film showing
the shapes of transmit ultrasound pulses propagating through an heterogeneous body-
wall. The purpose is to evaluate the feasibility of SURF reverberation (multiple
scattering) suppression when a strongly heterogeneous body-wall is present within
the path of the wave-propagation. The study also compares the synthetic propagating
SURF wave to a standard fundamental imaging wave propagating within the same
medium. The study is important because the body-wall of a patient, where standard
fundamental ultrasound imaging is aggravated due to reverberation noise, also is
likely to produce propagation time-shifts due to aberration (spatially varying speed
of sound). Aberration causes a local defocusing of the beam, and is known to reduce
contrast and lateral resolution. SURF imaging reverberation suppression is based on
a synthetic beam generation where the synthetic beam is dependent on small time-
shifts arising from the non-linearity of the medium and interaction with a low-
frequency manipulation pulse transmitted simultaneously as the HF imaging pulse. If
the time-shifts introduced by aberration heavily distort the time-shifts needed for
adequate SURF synthetic beam generation, the reverberation suppression gain of the
SURF imaging method is no longer present.
Method:
The simulations performed has been for an annular array transducer with 14.2 mm
aperture focused at 82 mm. For the fundamental imaging scheme, the frequency was
3.5 MHz and for the SURF scheme the imaging frequency was 3.5 MHz with a 0.5
MHz manipulation pulse. The low frequent manipulation is transmitted from an
aperture with 20 mm diameter, but through the same surface as the high frequency.
The body wall model is 39 mm thick and is eqvivalent to a strongly aberrating
abdominal wall.
Results, discussion and conclusion:
Results show that both the generation of synthetic SURF reverberation suppression
imaging transmit beams is attainable, even though the modeled body-wall is
estimated to yield more severe aberration than estimated from real human body-wall
specimen. Reverberation noise is caused by scattering within the body-wall, and the
slow build-up of the SURF beam within the body-wall causes the beam to be less
sensitive to reverberation noise. The fundamental beam is on the other hand
generated from one single pulse, and maintains a high intensity through the body-
wall and is because of this more sensitive to reverberation noise generated by a first
scattering within the body-wall.
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