Mean-Squared Error Beamforming for Coherent Plane-Wave Compounding

The goal of this paper is to implement a new minimum mean-squared error (MMSE) beamformer for coherent plane-wave compounding. In ultrasound imaging, an beamformer, in the MMSE approach, is usually derived as a multiplication of the minimum variance distortionless response (MVDR) output and a scalar, which is approximated by the coherent factor (CF). Beside the spatial smoothing adopted for estimating the data covariance matrix in MVDR beamforming, this approximation could be another factor that potentially reduces performance of the MMSE. In this paper, we extend the spatial coherent approach that we developed previously for MVDR beamforming, and show how this scalar can be estimated through an array of MVDR outputs. Advances of the proposed MMSE are evaluated on datasets available on PICMUS website. The imaging results show it offers improvements, in terms of spatial and contrast resolution, over the corresponding MVDR and a combination of MVDR beamforming and CF in the literature.Department of Engineerin

Deep Coherence Learning: An Unsupervised Deep Beamformer for High Quality Single Plane Wave Imaging in Medical Ultrasound

Plane wave imaging (PWI) in medical ultrasound is becoming an important reconstruction method with high frame rates and new clinical applications. Recently, single PWI based on deep learning (DL) has been studied to overcome lowered frame rates of traditional PWI with multiple PW transmissions. However, due to the lack of appropriate ground truth images, DL-based PWI still remains challenging for performance improvements. To address this issue, in this paper, we propose a new unsupervised learning approach, i.e., deep coherence learning (DCL)-based DL beamformer (DL-DCL), for high-quality single PWI. In DL-DCL, the DL network is trained to predict highly correlated signals with a unique loss function from a set of PW data, and the trained DL model encourages high-quality PWI from low-quality single PW data. In addition, the DL-DCL framework based on complex baseband signals enables a universal beamformer. To assess the performance of DL-DCL, simulation, phantom and in vivo studies were conducted with public datasets, and it was compared with traditional beamformers (i.e., DAS with 75-PWs and DMAS with 1-PW) and other DL-based methods (i.e., supervised learning approach with 1-PW and generative adversarial network (GAN) with 1-PW). From the experiments, the proposed DL-DCL showed comparable results with DMAS with 1-PW and DAS with 75-PWs in spatial resolution, and it outperformed all comparison methods in contrast resolution. These results demonstrated that the proposed unsupervised learning approach can address the inherent limitations of traditional PWIs based on DL, and it also showed great potential in clinical settings with minimal artifacts

Coherent Multi-Transducer Ultrasound Imaging

An extended aperture has the potential to greatly improve ultrasound imaging performance. This work extends the effective aperture size by coherently compounding the received radio frequency data from multiple transducers. A framework is developed in which an ultrasound imaging system consisting of $N$ synchronized matrix arrays, each with partly shared field of view, take turns to transmit plane waves. Only one individual transducer transmits at each time while all $N$ transducers simultaneously receive. The subwavelength localization accuracy required to combine information from multiple transducers is achieved without the use of any external tracking device. The method developed in this study is based on the study of the backscattered echoes received by the same transducer and resulting from a targeted scatterer point in the medium insonated by the multiple ultrasound probes of the system. The current transducer locations along with the speed of sound in the medium are deduced by optimizing the cross-correlation between these echoes. The method is demonstrated experimentally in 2-D using ultrasound point and anechoic lesion phantoms and a first demonstration of a free-hand experiment is also shown. Results demonstrate that the coherent multi-transducer imaging has the potential to improve ultrasound image quality, improving resolution and target detectability. Lateral resolution, contrast and contrast-to-noise ratio improved from 0.67 mm, -6.708 dB and 0.702, respectively, when using a single probe, to 0.18 mm, -7.251 dB and 0.721 in the coherent multi-transducer imaging case

Ultrafast Ultrasound Imaging

Among medical imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI), ultrasound imaging stands out due to its temporal resolution. Owing to the nature of medical ultrasound imaging, it has been used for not only observation of the morphology of living organs but also functional imaging, such as blood flow imaging and evaluation of the cardiac function. Ultrafast ultrasound imaging, which has recently become widely available, significantly increases the opportunities for medical functional imaging. Ultrafast ultrasound imaging typically enables imaging frame-rates of up to ten thousand frames per second (fps). Due to the extremely high temporal resolution, this enables visualization of rapid dynamic responses of biological tissues, which cannot be observed and analyzed by conventional ultrasound imaging. This Special Issue includes various studies of improvements to the performance of ultrafast ultrasoun

心臓内血流動態可視化のための血球エコーの高フレームレート超音波イメージング

Tohoku University金井浩課

Multi-Frame Rate Plane Wave Contrast-Enhance Ultrasound Imaging for Tumour Vasculature Imaging and Perfusion Quantification

A multi-frame rate plane wave imaging strategy is developed to simultaneously image tumor vasculature and quantify tumor perfusion. Customised imaging sequences interleaving a short but high frame rate (HFR) plane wave imaging sequence with a long but low frame rate imaging (LFR) sequence were implemented using a programmable ultrasound research platform. The results from a spatio-temporal coherence processing technique of ours demonstrated a significant improvement in the SNR and vasculature contrast when compared with the existing ultrafast Power Doppler (PD) using the same data. Initial perfusion quantification using LFR imaging was also demonstrated. Mean time intensity curve and some parametric measures were generated. Combining both structural and functional perfusion imaging using the multiframe rate sequences, a better evaluation of the tumour angiogenesis can be assessed

Feasibility study of the reverberant shear wave elastography technique for diabetic foot characterization

The diabetic foot is a complication that stems from diabetes when it is not properly managed. The underlying biomechanical changes that occur inside the plantar tissue as a result of glycation when a person begins to develop this complication suggest that there is a change in its elasticity and hardness. Consequently, these changes could be estimated quantitatively with elastography, which comprises many modalities that can be implemented with a magnetic resonator, an ultrasound equipment, among other devices. One of these modalities relies on the generation of a reverberant shear wave field inside the tissue or medium of interest, called reverberant shear wave elastography (RSWE). In that sense, the literature is scarce pertaining diabetic foot research with ultrasound elastography, while the RSWE modality is still in its early stages. The main objective of this work focuses on assessing the clinical application feasibility of this novel elastography technique by characterizing the elasticity of regions of the foot plant.Trabajo de investigació

Wavefront shaping approaches for spectral domain optical coherence tomography

Optical coherence tomography (OCT) enables sub-surface three dimensional imaging with micrometer resolution. The technique is based on the time-of-flight gated detection of light which is backscattered from a sample and has applications in non-destructive testing, metrology and contact-less and non-invasive medical diagnostics. With scattering media such as the human skin, the penetration depth is limited to just a few millimetres, on the other hand, and OCT imaging hence allows to investigate superficial sample layers only. Scattering of light is a deterministic process. As a consequence, manipulation of the beam incident to a turbid sample yields control over the scattered field. Following this approach, a number of groups demonstrated iterative wavefront optimization algorithms to be able to focus light transmitted through or backscattered from opaque media. First applications to optical coherence tomography were shown to extend the penetration depth as well as to improve the signal-to-noise ratio when imaging biological tissue. This work explores practical approaches to combine wavefront shaping techniques with OCT imaging. To this end, a compact spectral domain (SD-) OCT design is developed which enables single-pass and independent wavefront control at the reference and at sample beam. Iterative optimization of the phase pattern applied to the sample beam is shown to selectively enhance the amplitude of the OCT signal received from scattering media. In a more sophisticated approach, the acquisition of the time-resolved reflection matrix, which yields the linear dependence of the OCT signal on the field at the sample beam, is demonstrated. Subsequent wavefront optimization based on a phase conjugation algorithm is shown to enhance the OCT signal but not image artefacts, even though no attempt is made to actively suppress these artefacts. The approach is comparable to iterative wavefront optimization but yields a substantially improved acquisition speed. First imaging applications demonstrate the algorithm to enhance the signal-to-noise ratio and the penetration depth with scattering media, such as biological tissue, and to reduce the observed speckle contrast, similar to compounding algorithms. Furthermore, the acquisition of the reflection matrix and subsequent signal enhancement based on binary amplitude-only (on/off) beam shaping is presented for the first time. The technique can be implemented with digital micromirror devices which enable high-speed implementations. The presented techniques constitute substantial improvements compared to previous works and yield promising results in the context of depth-enhanced OCT imaging with scattering biological tissue. Approaches to further enhance the performance and the acquisition speed for real-time in-vivo imaging applications are discussed.Niedersächsisches Ministerium für Wissenschaft und Kultur (MWK)/Tailored Light/78904-63-6/16/E

Minimum variance beamformers for coherent plane-wave compounding

In this paper we present and analyse a technique for applying minimum variance distortionless response (MVDR) beamforming to a coherent plane-wave compounding (CPWC) acquisition system. In the past, this has been done using a spatial smoothing approach that reduces the effective size of the receive aperture and degrades the image resolution. In this paper, we apply the MVDR algorithms in a novel way to the acquired data from the individual transducer elements, before any summation or other compounding. This enables us to propose a new approach for estimation of the covariance matrix that decorrelates the coherence among the components at all the different acquisition angles. This results in a new approach to receive beamforming for CPWC acquisition. The new beamformer is demonstrated on imaging data acquired with a research scanner. We find the new beamformer offers substantial improvements over the DAS method. It also significantly outperforms the previously published MVDR/CPWC beamformer on phantom studies where the signal from the main target is dominated by noise and interference. These improvements motivate further study in this new approach for enhancing image quality

A biomechanical analysis of shear wave elastography in pediatric heart models

Early detection of cardiac disease in children is essential to optimize treatment and follow-up, but also to reduce its associated mortality and morbidity. Various cardiac imaging modalities are available for the cardiologist, mainly providing information on tissue morphology and structure with high temporal and/or spatial resolution. However, none of these imaging methods is able to directly measure stresses or intrinsic mechanical properties of the heart, which are potential key diagnostic markers to distinguish between normal and abnormal physiology. This thesis investigates the potential of a relatively new ultrasound-based technique, called shear wave elastography (SWE), to non-invasively measure myocardial stiffness. The technique generates an internal perturbation inside the tissue of interest, and consequently measures the propagation of the acoustically excited shear wave, of which the propagation speed is directly related to tissue stiffness. This allows SWE to identify regions with higher stiffness, which is associated with pathology. SWE has shown to be successful in detecting tumors in breast tissue and fibrosis in liver tissue, however application of SWE to the heart is more challenging due to the complex mechanical and structural properties of the heart. This thesis provides insights into the acoustically excited shear wave physics in the myocardium by using computer simulations in combination with experiments. Furthermore, these models also allow to assess the performance of currently used SWE-based material characterization algorithms