Compounded Plane-Wave Imaging (CPWI) has the ability to provide ultrafast imaging for many applications like colour flow imaging, microbubble imaging and elastography. The compounding operation improves the imaging quality at the expense of reducing the frame rate. Due to the importance of frame rate in ultrafast imaging, selecting the number and value of the compounded angles is a critical step to achieve the best possible imaging quality using the minimum number of angles whilst preserving the frame rate. This paper produces a new method for selecting the angular range and the number of angles in CPWI depending on the characteristics of the transducer and medium using Field II program. Experiments were performed on a wire phantom to show the efficiency of the produced method. The results show a comparative imaging quality of CPWI at the selected parameters when compared with linear imaging.

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Selecting the number and values of the CPWI steering angles and the effect of that on imaging quality

In ultrasound array imaging, the beamforming operation is performed by aligning and processing the received echo signals from each individual array element to form a complete image. This operation can be performed in many different ways, where adaptive and non-adaptive beamformers are considered as the main categories. Adaptive beamformers exploit the statistical correlation between the received data to find a weighting value at the focal point, instead of using a fixed weighting window in non-adaptive beamforming. This results in a significant improvement in the image quality in terms of resolution and sidelobes reduction. This improvement is necessary for ultrafast imaging because of the lack of focusing in Plane Wave Imaging (PWI) that results in lowering the SNR, and thus the produced imaging quality is reduced. This thesis analyses different adaptive beamforming techniques for ultrafast imaging. For accurate medical diagnosis, the frame rate, the imaging resolution, contrast and speckle homogeneity are all considered as important parameters that contribute to the final imaging result. To be able to evaluate each technique by minimizing the effect of external parameters, two different analysis were performed. First an empirical expression for PWI lateral resolution is produced after studying the effect of the imaging parameters on this imaging method. Then a method for selecting the suitable steering angles in Compound PlaneWave Imaging (CPWI) is introduced, with a detailed explanation for the effect of the compound angles on resolution and sidelobes level. In order to add the contrast improvement to the properties of adaptive beamformers, some techniques like the coherence-based factors and Eigenspace-Based Minimum Variance (ESBMV) are produced in the literature. After demonstrating the principle of Minimum Variance adaptive beamformer, a detailed comparison for the types of coherence-based factors is given. In addition, a new technique of Partial-ESBMV is introduced to modify reference ESBMV so that no Black Box Region artefacts nor dark spots appear when using this method in medical imaging. After explaining its background and properties using cystic and wire phantoms, the proposed method is applied to the real RF data of carotid artery, as an application to clarify the efficiency of this method in medical ultrasound imaging.

Plane Wave Imaging Beamforming Techniques for Medical Ultrasound Imaging

During manual palpation of breast masses, the perception of its stiffness and slipperiness are the two commonly used information by the physician. In order to reliably and quantitatively obtain this information several non-invasive elastography techniques have been developed that seek to provide an image of the underlying mechanical properties, mostly stiffness-related. Very few approaches have visualized the "slip" at the lesion-background boundary that only occurs for a loosely-bonded benign lesion. It has been shown that axial-shear strain distribution provides information about underlying slip. One such feature, referred to as "fill-in" was interpreted as a surrogate of the rotation undergone by an asymmetrically-oriented-loosely bonded-benign-lesion under quasi-static compression. However, imaging and direct visualization of the rotation itself has not been addressed yet. In order to accomplish this, the quality of lateral displacement estimation needs to be improved. In this simulation study, we utilize spatial compounding approach and assess the feasibility to obtain good quality rotation elastogram. The angular axial and lateral displacement estimates were obtained at different insonification angles from a phantom containing an elliptical inclusion oriented at 45°, subjected to 1% compression from the top. A multilevel 2D-block matching algorithm was used for displacement tracking and 2D-least square compounding of angular axial and lateral displacement estimates was employed. By varying the maximum steering angle and incremental angle, the improvement in the lateral motion tracking accuracy and its effects on the quality of rotational elastogram were evaluated. Results demonstrate significantly-improved rotation elastogram using this technique.

Rotation elastogram: a novel method to visualize local rigid body rotation under quasi-static compression

The health of the carotid artery is an important
indicator of cardiovascular
 disease (CVD). The advent of CVD
results in reduced elasticity and flexibility of the arterial wall.
These changes in elasticity can be measured using
 non-invasive
vascular elastography (NIVE). Elastography is performed by
subjecting the tissue under investigation to a form of mechanical
excitation. The
 resultant tissue displacement is measured and used
to compute the spatial variation of strain within the tissue. These
strain maps, known as elastograms,
 serve as surrogates for tissue
elasticity. While elastography can be performed
 with any imaging
modality, ultrasound is portable, inexpensive and has high
 frame
rates, making it ideal for diagnostic screening purposes. However,
linear array based elastography can accurately estimate
displacement only in the
 axial direction. Consequently, NIVE
cannot characterize strain across artery
 cross-sections,
restricting the diagnostic value of this technique.
 The objective
of this thesis was to investigate the feasibility of using
synthetic aperture (SA) imaging to accurately estimate the 2D
displacement vector, thereby enhancing the performance of NIVE. We
demonstrated, through
 simulation and experiment, that SA imaging
can accurately measure both axial and lateral displacements. These
displacements generate high quality radial
 and circumferential
strain elastograms of the arterial cross-section. Compared
 to
conventional ultrasound elastography, SA elastography improved the
error
 in lateral displacements and the resultant strain
elastograms by an order of
 magnitude. However, the low frame rates
and large data volumes required by
 SA imaging render this approach
clinically inviable. To overcome this limitation, we developed a
sparse array based elastography system. We demonstrated
 that as
few as eight transmit elements can generate strain elastograms with
a
 16x improvement in frame rate and data volume, at a minimal loss
of image
 quality. The efficacy of sparse array elastography was
compared to that of
 a compounded plane wave imaging system. It was
demonstrated that sparse
 array imaging displayed higher lateral
sensitivity thereby producing strain elastograms with 20% improved
image quality.
 From the success of this novel approach to
elastography, it was concluded
 that further development and
integration on a commercial ultrasound system
 would greatly
enhance clinical use.

Sanghamithra Korukonda

Papers

Application of Synthetic Aperture Imaging to Non-Invasive Vascular Elastography