Browsing by Subject "Coherence"
- Results Per Page
- Sort Options
Item Open Access Beamforming of Ultrasound Signals from 1-D and 2-D Arrays under Challenging Imaging Conditions(2015) Jakovljevic, MarkoBeamforming of ultrasound signals in the presence of clutter, or partial aperture blockage by an acoustic obstacle can lead to reduced visibility of the structures of interest and diminished diagnostic value of the resulting image. We propose new beamforming methods to recover the quality of ultrasound images under such challenging conditions. Of special interest are the signals from large apertures, which are more susceptible to partial blockage, and from commercial matrix arrays that suffer from low sensitivity due to inherent design/hardware limitations. A coherence-based beamforming method designed for suppressing the in vivo clutter, namely Short-lag Spatial Coherence (SLSC) Imaging, is first implemented on a 1-D array to enhance visualization of liver vasculature in 17 human subjects. The SLSC images show statistically significant improvements in vessel contrast and contrast-to-noise ratio over the matched B-mode images. The concept of SLSC imaging is then extended to matrix arrays, and the first in vivo demonstration of volumetric SLSC imaging on a clinical ultrasound system is presented. The effective suppression of clutter via volumetric SLSC imaging indicates it could potentially compensate for the low sensitivity associated with most commercial matrix arrays. The rest of the dissertation assesses image degradation due to elements blocked by ribs in a transthoracic scan. A method to detect the blocked elements is demonstrated using simulated, ex vivo, and in vivo data from the fully-sampled 2-D apertures. The results show that turning off the blocked elements both reduces the near-field clutter and improves visibility of anechoic/hypoechoic targets. Most importantly, the ex vivo data from large synthetic apertures indicates that the adaptive weighing of the non-blocked elements can recover the loss of focus quality due to periodic rib structure, allowing large apertures to realize their full resolution potential in transthoracic ultrasound.
Item Open Access Charge Transfer and Energy Transfer: Methods Development and Applications in Bio-molecular Systems(2017) Liu, ChaorenSystem-environment interactions are essential in determining charge-transfer (CT) rates and mechanisms. We developed a computationally accessible
method, suitable to simulate CT in flexible molecules (i.e., DNA) with hundreds of
sites, where the system-environment interactions are explicitly treated with numerical
noise modeling of time-dependent site energies and couplings. The properties of the
noise are tunable, providing us a flexible tool to investigate the detailed effects of
correlated thermal fluctuations on CT mechanisms. The noise is parameterizable by
molecular simulation and quantum calculation results of specific molecular systems,
giving us better molecular resolution in simulating the system-environment interactions than sampling fluctuations from generic spectral density functions. The spatially correlated thermal fluctuations among different sites are naturally built-in in our method but are hard to be incorporated by approximate spectral densities. Our method has quantitative accuracy in systems with small redox potential differences ($
With the method of incorporating spatially and temporally correlated thermal fluctuations into charge transfer process, we study and engineer
coherence in guanine-rich DNA sequences.
Electronic delocalization in redox-active polymers may be disrupted by the heterogeneity of the environment that surrounds each monomer. When the differences in monomer redox-potential induced by the environment are small (as compared with the monomer-monomer electronic interactions), delocalization persists. Here we show that guanine (G) runs in double-stranded DNA support delocalization over 4-5 guanine bases. The weak interaction between delocalized G blocks on opposite DNA strands is known to support partially coherent long-range charge transport. The molecular-resolution model developed here finds that the coherence among these G blocks follows an even-odd orbital-symmetry rule and predicts that weakening the interaction between G blocks exaggerates the resistance oscillations. These findings indicate how sequence can be exploited to change the balance between coherent and incoherent transport. The predictions are tested and confirmed using break-junction experiments. Thus, tailored orbital symmetry and structural fluctuations may be used to produce coherent transport with a length scale of multiple nanometers in soft-matter assemblies, a length scale comparable to that of small proteins.
We extend our charge transport studies from linear molecules to branched molecules.
Self-assembling circuitry on the molecular scale demands building blocks with three or more terminals, the sine qua non for circuit elements like current splitters or combiners\cite{Molen2013,RN2,Tao2006}. A promising material for such building blocks is DNA, wherein multiple strands can self-assemble into multi-ended junctions and nucleobase stacks can transport charge over long distances\cite{Genereux2010,Cohen2005,RN6,RN7,RN8,RN9}. However, nucleobase stacking is often disrupted at the junction point, hindering electric charge transport between different terminals of the junction\cite{RN10,RN11}. Thus, the challenge of designing a multi-ended DNA circuit element remains open. Here, we address the challenge by using a guanine-quadruplex (G4) motif as the connector element of a multi-ended DNA junction, and designing the terminal groups of the motif to ensure efficient current splitting in the DNA junction with minimal carrier transport attenuation. We describe the design, assembly, and charge transport measurement\cite{RN46} of a 3-way G4 junction structure, in which charge can enter the structure from one terminal at one end of the G4, and exit from one of two terminals at the other end of the G4. We find that the charge transport characteristics are the same along the two pathways, and are also similar to those of the corresponding linear DNA duplexes. Thus, the G4-based junction indeed enables effective three-way transport, which is a necessary step towards building of DNA-based electrical networks. We optimize G4-based junction structures and interpret the charge-transport measurements with molecular dynamics and quantum chemistry simulations.
Energy transfer with an associated spin change of the donor and acceptor, Dexter energy transfer, is critically important in solar energy harvesting assemblies, damage protection schemes of photobiology, and organometallic opto-electronic materials. Dexter transfer between chemically linked donors and acceptors is bridge mediated, presenting an enticing analogy with bridge-mediated electron and hole transfer. However, Dexter coupling pathways must convey both an electron and a hole from donor to acceptor, and this adds considerable richness to the mediation process. We dissect the bridge-mediated Dexter coupling mechanisms and formulate a theory for triplet energy transfer coupling pathways. Virtual donor-acceptor charge-transfer exciton intermediates dominate at shorter distances or higher tunneling energy gaps, whereas virtual intermediates with an electron and a hole both on the bridge (virtual bridge excitons) dominate for longer distances or lower energy gaps. The effects of virtual bridge excitons were neglected in earlier treatments. The two-particle pathway framework developed here shows how Dexter energy-transfer rates depend on donor, bridge, and acceptor energetics, as well as on orbital symmetry and quantum interference among pathways.
Item Open Access Coherence in Dynamic Metasurface Aperture Microwave Imaging Systems(2020) Diebold, Aaron VincentMicrowave imaging systems often utilize electrically large arrays for remote characterization of spatial and spectral content. Image reconstruction involves computational processing, the success of which depends on adequate spatial and temporal sampling at the array as dictated by the nature of the radiation and sensing strategy. Effective design of an imaging system, consisting of its hardware and algorithmic components, thus requires detailed understanding of the nature of the involved fields and their impact on the processing capabilities. One can sufficiently characterize many of the properties of such fields and systems via coherence, which quantifies their interferometric capacity in terms of statistical correlations and point spread functions. Recent work on dynamic metasurface apertures (DMAs) for microwave imaging has demonstrated the utility of these structures in active, coherent systems, supplanting traditional array architectures with lower-cost designs capable of powerful wavefront shaping. In contrast to arrays of distinct antennas, DMAs are composed of electrically large arrays of dynamically-tunable, radiating metamaterial elements to realize diverse gain patterns that can function as an encoding mechanism for coded aperture image reconstruction. With the appropriate formulation, well-established concepts from the realm of Fourier optics can be transposed from conventional array systems to DMA architectures. This dissertation furthers that task by modeling DMA imaging systems involving partial coherence and incoherence, and demonstrating new reconstruction algorithms in these contexts. Such an undertaking provides convenient opportunities for examining the origins of coherence in computational and holographic imaging systems. This insight is necessary for the development of modern approaches that seek to smoothly integrate hardware and computational elements for powerful, efficient, and innovative imaging tasks.
Accommodating different degrees of coherence in a microwave imaging system can substantially relax demands on hardware components including phase stability and synchronization, or on algorithmic procedures such as calibration. In addition, incoherent operation can yield improved images free of coherent diffraction artifacts and speckle. Finally, an understanding of coherence can unlock fundamentally distinct applications, such as passive imaging and imaging with ambient illumination, that can benefit from the flexibility of a DMA system but have yet to be demonstrated under such an architecture. To this end, I formulate a unified framework for analyzing and processing array imaging systems in the Fourier domain, and demonstrate a method for transforming a DMA-based system to an equivalent array representation under active, coherent operation. I then investigate the role of spatial coherence in a two-dimensional holographic imaging system, and experimentally demonstrate some results using a collection of DMAs. I conduct a similar investigation in the context of single-pixel ghost imaging, which allows coherent and incoherent imaging directly from intensity measurements, thereby relaxing hardware phase requirements. I then formulate a model for partially coherent fields in a DMA imaging system, and provide several reconstruction strategies and example simulations. I finally restrict this general case to passive, spectral imaging of spatially and temporally incoherent sources, and experimentally demonstrate a compressive imaging strategy in this context.
Item Open Access Coherent flow power Doppler imaging(2017) Li, YouUltrasonic flow detection is a widely used technique to detect vessel, measure blood flow velocities, and monitor perfusion. Conventional techniques include color Doppler imaging and power Doppler (PD) imaging. These methods depend on either the measurement of phase change or the detection of the power of backscattered echoes from blood. Both techniques are susceptible to noise. Common noise sources include thermal noise and clutter. The noise significantly deteriorates the performance of color Doppler imaging, because color Doppler imaging estimates the axial blood velocity from temporal changes in the echo phase, and phase change measurement is sensitive to noise. Power Doppler imaging measures the power of the temporal differences in backscattered echoes, and can provide higher sensitivity with small vessel and slow flow detection than color Doppler imaging at the expense of direction and velocity information. However, it requires a large ensemble length, limiting the frame rate to a few frames per second. The limitations of color Doppler imaging and power Doppler imaging are more severe in deep body vessel imaging due to depth dependent attenuation of the ultrasound waves. Therefore, for deep body vessel imaging, including liver vessel imaging and placental spiral artery imaging, better vessel detection techniques are desirable.
Coherent flow power Doppler (CFPD) imaging was proposed as a sensitive flow detection and imaging technique for slow flow and small vessels. In this work, we present the study on CFPD from principles to clinical evaluation.
The CFPD imaging technique detects blood flow from the spatial coherence of the blood signal. The short-lag spatial coherence (SLSC) beamformer is used for the measurement of spatial coherence. Because blood signals and common noise sources, including thermal noise reverberation clutter, have different spatial coherence properties, CFPD can suppress the noise.
The performance of CFPD in flow detection was evaluated with simulations and flow phantom experiments under various imaging conditions, and compared with the performance of PD. It is found that CFPD provides an improvement of Doppler signal-to-noise ratio (SNR) of 7.5-12.5 dB over PD in slow flow and small vessel imaging. The improvement in SNR translates to higher Doppler image contrast, faster frame rate, or lower limit-of-detection (LOD). In similar imaging conditions of slow flow, CFPD may detect up to 50% slower flow than PD.
The CFPD imaging technique was also implemented with novel pulse sequences, including plane-wave synthetic transmit aperture imaging, and diverging-wave synthetic transmit aperture imaging. For plane-wave synthetic transmit aperture imaging, the angular coherence theory was proposed to describe the coherence of backscattered waves corresponding to plane wave transmits at different steering angles. In addition, we also propose the coherent Kasai and Loupas estimators, which utilizes the coherence information of flow signals to provide velocity estimates with reduced uncertainty.
To demonstrate the clinical relevance of CFPD, we built a real-time CFPD imaging system and conducted a pilot clinical study with it. In the system, the CFPD technique was implemented on a Verasonics Vantage 256 research scanner. The software beamformer and CFPD processing were implemented on the graphics processing unit (GPU). The Doppler frame rate of the system is 10 frames per second for a field-of-view (FOV) of 10 cm axially and 4 cm laterally.
In the pilot clinical study, the liver vasculatures of 15 healthy human volunteers were imaged by a trained sonographer using the real-time CFPD system. The raw data corresponding to a 132 Doppler videos were captured and processed offline. The SNR of the vessels in the CFPD and PD images were measured and analyzed. In all of the 132 data sets, CFPD provides higher SNR than PD. The average improvement in SNR is 8.6 dB. From the visual analysis of the images, it can be seen that the improvement in SNR leads to more sensitive detection of small vessels in deeper parts of the liver.
Item Open Access Covariance Decomposition of Ultrasonic Backscatter: Application to Estimation-based Image Formation(2020) Morgan, Matthew RobertMedical ultrasound imaging is portable, real-time, and inexpensive, with countless applications across a range of pathologies and imaging targets. Despite these advantages, many patients suffer from suboptimal image quality, hampered by acoustic clutter which can reduce contrast and obscure targets of interest. Obesity, in particular, has been linked to increased rates of inadequate visualization and reduced diagnostic efficacy of ultrasound imaging. Rising obesity rates support the need for improved image quality in challenging imaging environments.
Advanced beamforming methods may offer an opportunity to mitigate sources of acoustic clutter and improve image quality. Many methods have been proposed in the literature, which have been shown to improve aspects of image quality over conventional delay-and-sum beamforming. However, these methods often exchange enhanced contrast for coarse speckle texture, distort the native echogenicty of the imaging target, and/or employ ad hoc approaches to image formation that lack a sound basis in physical principles.
This dissertation presents a new paradigm for image formation: an estimation-based approach to image the statistical properties of tissue. The foundation for this approach is the fundamental observation that targets in medical ultrasound consist of inherently unresolvable, diffuse scatterers. Backscattered echoes from diffuse targets can be characterized by their statistical properties, which are classically described by the van Cittert-Zernike (VCZ) theorem under a statistically stationary, spatially incoherent scattering model.
This work applies the VCZ theorem to a piecewise-stationary scattering model. This application yields a key insight: the spatial covariance of the received echo data is the linear superposition of covariances from distinct spatial regions in the imaging target. This relationship is derived from first principles and validated through simulation studies demonstrating superposition and scaling.
Under the framework of spatial covariance decomposition, a novel method to image the statistical properties of stochastic targets is derived. Multi-covariate Imaging of Sub-resolution Targets, or MIST, employs an estimation-based method to image the on-axis contributions to the echo data covariance matrix. MIST covariance models are defined based on a spatial decomposition of the theoretical transmit intensity distribution into contributions received on- and off-axis. The mathematical foundations of the MIST estimator are analytically derived, and imaging performance is evaluated in simulation, phantom, and in vivo studies, which demonstrate consistent improvements in contrast-to-noise ratio (CNR) and speckle signal-to-noise ratio (SNR) across imaging targets, while preserving target echogenicity and lateral resolution.
In a pilot clinical study, MIST image quality was evaluated in fifteen patients at the Duke Fetal Diagnostic Center, using data collected with the Verasonics Vantage 256 research scanner from a variety of fetal structures in first- and second-trimester pregnancies. Patient body habitus varied from underweight to obese (body mass indices of 17.5--58.3). Across 152 images from all patients, MIST demonstrated improved contrast (93.2% of images), CNR (99.1%) and speckle SNR (99.5%) over matched B-Mode images. Image quality improvements were consistent across patient body habitus and between fundamental and harmonic imaging modes, showing promising indications for MIST in fetal applications.
To characterize the intrinsic tradeoffs associated with MIST, the effects of varying two key parameters on image quality were explored: (1) the spatial cutoff delineating the on- from off-axis covariance models and (2) the degree of spatial averaging of the measured echo data covariance matrix. The results demonstrated a fundamental tradeoff between resolution and speckle texture. This fundamental tradeoff was compared to similar tradeoffs in spatial and frequency compounding. MIST was shown to provide greater improvements in speckle texture at a comparable resolution to each method. Across these tunable parameters, MIST also demonstrated stable performance in noise and fidelity to native contrast. These results present a framework for parameter selection in MIST to maximize speckle SNR without an appreciable loss in resolution.
Like many coherence-based imaging methods, MIST suffers from reduced image quality outside the depth of field for focused ultrasound transmissions. To extend the depth of field, synthetic aperture focusing was applied to MIST under focused, plane wave and diverging wave transmit geometries. Synthetic aperture MIST demonstrated consistent improvements in image quality over conventional dynamic receive MIST, with approximately equivalent results between transmit geometries. In an in vivo liver example, synthetic aperture MIST images demonstrated 16.8 dB and 16.6% improvements in contrast and CNR, respectively, over dynamic receive MIST images, as well as 17.4 dB and 32.3% improvements over synthetic aperture B-Mode. Simulation and experimental results indicate wide applicability of MIST to synthetic aperture focusing methods.
Lastly, MIST imaging performance in multi-dimensional arrays was evaluated through a preliminary simulation study. MIST images were formed using 1-D, 1.75-D, and 2-D transducer geometries on a number of targets with a range of native contrast values. MIST image quality was demonstrated to be stable in the presence of noise across array geometries. Preliminary results showed substantial improvements in contrast, speckle SNR, and lesion detectability metrics with only a modest increase in system complexity.
In summary, Multi-covariate Imaging of Sub-resolution Targets is a novel approach to image the statistical properties of diffuse scattering targets, based on a spatial decomposition of aperture domain covariance into on- and off-axis contributions. Simulated and experimental results indicate significant improvements of image quality over conventional methods, promising preliminary clinical data, and feasibility under modern focusing schemes and advanced hardware. This work suggests MIST may greatly benefit image quality patients in patients for whom conventional methods fail.
Item Open Access Efficient Spatial Coherence Estimation for Improved Endocardial Border Visualization in Real-Time(2017) Hyun, DongwoonCoronary heart disease contributed to approximately one in four deaths in the United States in 2014, and is caused by a restriction of blood flow to myocardial tissue. Stress echocardiography is a clinical technique used to assess myocardial ischemia by observing changes (or lack thereof) in ventricular wall motion in response to cardiac stress. The American Society of Echocardiography (ASE) recommends that left ventricle functionality be quantified using a 16 or 17 segment model of the left ventricle (LV). To properly assess the function of the ventricle, clear endocardial border delineation is necessary.
However, an increasing prevalence of obesity has been linked to a rise in the number of unreadable ultrasound scans. Image degradation is attributed to tissue inhomogeneities and subcutaneous fat layers, giving rise to phase aberration errors and acoustical clutter from near-field reverberation. In the event that two or more segments are inadequately visualized, the ASE recommends the use of contrast agents. Though contrast agents are effective, they are invasive and increase the procedure time and costs.
Recent work has shown that clutter can be suppressed using a novel image reconstruction technique based on the second order statistics of ultrasound echoes called short-lag spatial coherence (SLSC). Unlike conventional B-mode imaging, which forms images of the echo magnitude, SLSC forms images of the spatial coherence of the echo. By suppressing clutter, a sufficient improvement in the visualization of the endocardial border could minimize the need for contrast agents and potentially reduce the level of expertise necessary to interpret images. Though promising in preliminary studies, SLSC has a high computational demand that limited previous studies to offline image reconstruction. The goal of this research was to implement spatial coherence imaging in real-time, and to assess its performance in echocardiography.
First, the existing spatial coherence estimation methodology was investigated, and three computationally efficient modifications were proposed: a reduced kernel, a downsampled receive aperture, and the use of an ensemble correlation coefficient. The proposed methods were implemented in simulation and in vivo studies. Reducing the kernel to a single sample improved computational throughput and improved axial resolution. Downsampling the receive aperture was found to have negligible effect on estimator variance, and improved computational throughput by an order of magnitude for a downsample factor of 4. The ensemble correlation estimator was found to have lower variance than the currently used average correlation estimator. Combining the three methods, the throughput was improved 105-fold in simulation with a downsample factor of 4 and 20-fold in vivo with a downsample factor of 2.
Spatial coherence estimation techniques were also expanded to 2D matrix array transducers. SLSC images generated with a 2D array yielded superior contrast-to-noise ratio (CNR) and texture signal-to-noise ratio (SNR) measurements over SLSC images made on a corresponding 1D array and over B-mode imaging. SLSC images generated with square subapertures were found to be superior to SLSC images generated with subapertures of equal surface area that spanned the whole array in one dimension. Subaperture beamforming was found to have little effect on SLSC imaging performance for subapertures up to 8x8 elements in size on a 64x64 element transducer. Additionally, the use of 8x8, 4x4, and 2x2 element subapertures provided an 8, 4, and 2 times improvement in channel SNR along with a 2640-, 328-, and 25-fold reduction in computation time, respectively.
The improved spatial coherence estimation methodology was implemented using a GPU-based software beamformer to develop a real-time SLSC imaging system suitable for echocardiography. The system went through several iterations, with the final form consisting of a stand-alone CUDA C++ library for GPU-based beamforming, and a second CUDA C++ library to interface a research ultrasound scanner with the first. The resulting system was capable of live spatial coherence imaging at more than 30 frames per second, a rate sufficient for echocardiography.
The system was then used in a clinical study to image 15 stress echocardiography patients with poor image quality. A fundamental and harmonic imaging study was conducted. The latter study, which had greater clinical significance, was an assessment of the visibility of 17 LV segments using conventional tissue harmonic imaging (THI) and harmonic spatial coherence imaging (HSCI). A cardiologist rated the visibility of each of 17 LV segments as 0=invisible, 1=poorly visualized, or 2=well visualized, where scores of 0 and 1 indicated a need for contrast agent. There was a clear superiority of HSCI over THI in a comparison of overall segment scores (p < 0.0001 by symmetry test unadjusted for clustering). When comparing the number of segments with clinically acceptable image quality per patient, HSCI again showed superiority over THI (p < 0.0001 by McNemar test adjusted for clustering). In one patient, HSCI improved visualization sufficiently to eliminate the need for contrast agents altogther. These results indicate that spatial coherence imaging may provide sufficient improvements in LV wall visualization in certain patients to proceed without contrast agents.
The research in spatial coherence estimation techniques also proved fruitful in other areas of ultrasound imaging, such as ultrasound molecular imaging (USMI). USMI is accomplished by detecting microbubble (MB) contrast agents that have bound to specific biomarkers, and can be used for the early detection of cancer. However, USMI in humans is challenging because of the signal degradation caused by the presence of heterogenous subcutaneous tissue. In a phantom and in vivo study, USMI performance was assessed using conventional contrast-enhanced ultrasound (CEUS) imaging and SLSC-CEUS. In a USMI-mimicking phantom, SLSC-CEUS was found to be more robust than DAS to additive thermal noise, with a 9 dB and 15 dB SNR improvement without and with -6 dB thermal noise, respectively. USMI performance was also measured in vivo using VEGFR2-targeted MBs in mice with subcutaneous human hepatocellular carcinoma tumors. SLSC-CEUS improved the SNR in each of 10 tumors by an average of 65%, corresponding to 4.3 dB SNR. These results indicate that the SLSC beamformer is well-suited for USMI applications because of its high sensitivity and robust properties.
These studies are a demonstration of the feasibility of real-time spatial coherence imaging using current technology, and an exposition of its utility in medical ultrasound imaging.