M.Sc Student | Nedjar Samuel |
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Subject | Advanced Beamforming Techniques for Medical Ultrasound Imaging |
Department | Department of Electrical Engineering | Supervisors | Professor Emeritus Arie Feuer |
Professor Emeritus Dan Adam | |
Full Thesis text | ![]() |
Ultrasound is a popular imaging modality for investigating soft-tissue structures within the body. Unlike other medical imaging modalities, e.g. CT and SPECT, ultrasound does not utilize ionizing radiation and is therefore a preferable diagnostic modality. In addition, it provides the user with imaging and quantitative information concerning the blood flow. Ultrasound is the only non-invasive medical imaging modality that acquires true real-time images, thus allowing imaging of dynamic structure like the heart, and allows monitoring during treatment.
In order to generate an image, ultrasound systems must interrogate the tissue
by emitting narrow beams to different directions. In order to transmit and
receive narrow beams and orient them, multiple electro-acoustic transducers are
used. The data acquired by the electro-acoustic array, has traditionally been
processed by means of spatial filtering, also called beamforming, to obtain an
image of the scattering objects. The classical time-domain Delay-And-Sum (DAS)
method is usually used, which is analogous to the operation of an acoustical
lens and can be performed efficiently in real time using delay-line operations,
or using post-processing as in synthetic aperture systems. Conventional
beamforming, though, has several drawbacks. Mainly, DAS beamformer has
relatively high side-lobe level that translates to poor contrast. By using
aperture shading, the side-lobe level of the beam profile can be significantly
reduced but the main-lobe gets enlarged, resulting in increased contrast at the
expense of resolution. Moreover, off-axis scatterers can introduce clutter,
reducing the overall quality of the obtained data.
In contrast to the predetermined shading in DAS, adaptive beamformers use the
recorded wavefield to compute the aperture weights. By suppressing interfering
signals from off-axis directions and allowing large side-lobes in directions
from which no signal energy is received, the adaptive beamformers can increase
resolution. Adaptive beamforming algorithms have been extensively studied and
implemented for applications such as passive SONAR, RADAR, or seismology.
However, few studies have directly addressed the application of adaptive beamforming
to ultrasound imaging.
In this work, we survey and compare state-of-the-art adaptive beamforming
methods that specifically address issues related to diagnostic ultrasound
imaging. As a background, this work includes a survey of general
beamforming techniques such as synthetic aperture and multi-line acquisition
methods; but the main purpose of this work is to study ultrasound adaptive
beamforming techniques.