One of the current challenges in ultrasound imaging is achieving higher frame rates, particularly in cardiac applications, where tracking the heart motion and other rapid events can provide potential valuable diagnostic information. The main drawback of ultrasound high-frame-rate strategies is that usually they partly sacrifice image quality in order to speed up the acquisition time. In particular, multi-line transmission (MLT), which consists in transmitting multiple ultrasound beams simultaneously in different directions, has been proven able to improve frame rates in echocardiography, unfortunately generating artifacts due to inter-beam crosstalk interferences. This work investigates the possibility to effectively suppress crosstalk artifacts in MLT while improving image quality by applying beamforming techniques based on backscattered signals spatial coherence. Several coherence-based algorithms (i.e., short-lag filtered-delay multiply and sum beamforming, coherence and generalized coherence factor, phase and sign coherence, and nonlinear beamforming with $p$ th root compression) are implemented and compared, and their performance trends are evaluated when varying their design parameters. Indeed, experimental results of phantom and in vivo cardiac acquisitions demonstrate that this class of algorithms can provide significant benefits compared with delay and sum, well-suppressing artifacts (up to 48.5-dB lower crosstalk), and increasing image resolution (by up to 46.3%) and contrast (by up to 30 dB in terms of contrast ratio and 12.6% for generalized contrast-to-noise ratio) at the same time.

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A Comparison of Coherence-Based Beamforming Techniques in High-Frame-Rate Ultrasound Imaging With Multi-Line Transmission

In conventional ultrasound (US) imaging, it is common to transmit several focused beams at multiple locations to generate a multi-focus image with constant lateral resolution throughout the image. However, this method comes at the expense of a loss in temporal resolution, which is important in applications requiring both high-frame rate and constant lateral resolution. Moreover, relative motions of the target with respect to the probe often exist due to hand tremors or biological motions, causing blurring artifacts in the multi-focus image. This article introduces a novel approach for multi-focus US image recovery based on Generative Adversarial Network (GAN) without a reduction in the frame-rate. Herein, a mapping function between the single-focus US image and multi-focus version for having a constant lateral resolution everywhere is estimated through different GANs. We use adversarial loss functions in addition to Mean Square Error (MSE) to generate more realistic ultrasound images. Moreover, we use the boundary seeking method for improving the stability of training, which is currently the main challenge in using GANs. Experiments on simulated and real phantoms as well as on ex vivo data are performed. Results confirm that having both adversarial loss function and boundary seeking training provides better results in terms of the mean opinion score test. Furthermore, the proposed method enhances the resolution and contrast indexes without sacrificing the frame-rate. As for the comparison with other approaches which are not based on NNs, the proposed approach gives similar results while requiring neither channel data nor computationally expensive algorithms.

Fast Multi-Focus Ultrasound Image Recovery Using Generative Adversarial Networks

Plane-wave (PW) ultrasound imaging allows for ultrafast image acquisition rates, thus enabling new biomedical applications, such as ultrasound-based blood flow and tissue motion characterization. We propose two novel Fourier domain techniques for PW ultrasound image reconstruction that can be used as an alternative to conventional delay-and-sum beamforming. In particular, we show how to modify two classic algorithms used for geophysical data processing (namely, Stolt’s and slant-stack depth migration under zero-offset constant-velocity assumptions), so that their new versions can be used for PW ultrasound data processing. To demonstrate the merits and limitations of our approach, we provide qualitative and quantitative comparisons with other Fourier domain methods reported in the ultrasound literature. Our evaluation results are based on the image resolution, contrast, and similarity metrics obtained for several public-domain experimental benchmark data sets.

Fourier Domain Depth Migration for Plane-Wave Ultrasound Imaging

Diverging beam transmit through limited aperture: A method to reduce ultrasound system complexity and yet obtain better image quality at higher frame rates.

In elastography, conventional linear array (CLA)-based RF data acquisition provides more accurate displacement measurements in the direction of beam propagation (axial direction) when compared to the perpendicular direction (lateral). Obtaining good quality lateral displacement estimates in ultrasound (US) elastography will lead to several benefits such as obtaining accurate inverse solutions, improving shear strain elastogram quality, getting good quality poroelastograms, and obtaining reliable rotation elastograms. For accomplishing high-precision lateral displacement estimation (LDE), one of the popular methods is by interpolating additional A-lines in between neighboring RF A-lines. We describe a method wherein true RF A-lines (not interpolated) are acquired and augmented at subpitch locations using CLA transducer, instead of interpolating the data, and using this new frame data for further image formation and/or processing to yield better lateral resolution and LDE. We demonstrate the proposed method by translating the US beam of CLA transducer in subpitch range by the following two approaches: 1) actuator-assisted beam translation and 2) electronic translation of subaperture of a CLA by activating odd and even number of consecutive elements sequentially, referred to as electronic beam translation. The performances of the different methods were studied through simulations and experiments on phantoms. The results demonstrate that these methods yield better quality LDE compared to those obtained from interpolation of RF A-lines. These methods may provide affordable ways to obtain subpitch precision LDE using CLA.

Strategies to Obtain Subpitch Precision in Lateral Motion Estimation in Ultrasound Elastography

Rotation Elastogram Estimation Using Synthetic Transmit-aperture Technique: A Feasibility Study.

In this work, an adaptive weighting function based on the phase dispersion, namely, Sign Coherence Factor (SCF) is adapted to plane wave ultrasound imaging. SCF weighting is tested on both simulation and experimental data provided by the Plane Wave Imaging Challenge in Medical Ultrasound (PICMUS) organizers. The results demonstrate that the spatial resolution in plane wave imaging can be improved using SCF weighting without compromising the other image quality metrics. Furthermore, the results are compared with conventional delay and sum beamforming algorithm.

Spatial resolution improvement in Plane Wave Imaging using adaptive Sign Coherence Factor weighting

In this paper, a new method inspired by the synthetic aperture approach is proposed that aims at reducing the system's complexity (only 8 or 16 active elements) without compromising the image quality, and at frame rates comparable to or higher than conventional focused linear array technique. The novel method has been investigated in simulations using Field II software and experiments performed on a wire phantom using an ultrasound scanner. Results show that the proposed method provides better Lateral Resolution (LR) to that obtained when conventional focused linear array technique is used. The estimated LR at the focal point was 1.09 mm and 0.29 mm for conventional and the proposed method, respectively, in simulations. These were estimated to be 1.03 mm and 0.38 mm, respectively, in experiments. The image quality is shown to improve further when sign coherence factor weighting is incorporated during beamforming.

Design of a low cost ultrasound system using diverging beams and synthetic aperture approach: Preliminary study

Rotation Elastogram (RE) is a 2D spatial distribution map of the estimated local rigid-body rotation undergone by a target when subjected to an external compression, which is one of the recent variants in elastographic imaging A recent study has shown that inclusion-contrast in RE is independent of inclusion-background modulus contrast and thus may be helpful in distinguishing between barely-stiff benign and malignant lesions However, estimation of quality RE requires not only precise axial displacement estimates but also lateral displacement estimates The widely used conventional focused beamforming technique using linear array (CFB-LA) provides better lateral resolution only over the depth of focus, which still results in poorer quality lateral displacement estimates compared to the axial displacement estimates As an alternative to overcome this depth-dependent lateral resolution and obtain an improved lateral resolution, synthetic aperture-based approaches have been proposed in literature Recently, we developed a synthetic aperture-based method, diverging beam with synthetic aperture technique (DB-SAT) that was aimed to not only reduce the ultrasound system complexity, but also provide improved lateral resolution throughout the depth of imaging and at higher frame-rate than that is possible in CFB-LA In this paper, we report the preliminary experimental findings on the use of DB-SAT on RE and compare the resultant image quality against that obtained using often-employed CFB-LA and the synthetic transmit aperture (STA) technique The investigation was done on tissue-mimicking phantoms and using contrast-to-noise ratio (CNR) as the metric for performance evaluation The estimated CNR values from the REs obtained using CFB-LA, STA, and DB-SAT were 269  ±  081, 135  ±  022, and 1471  ±  983, respectively, for inclusion present at 55 mm depth The obtained results clearly demonstrated that the quality of RE can be improved significantly, especially at larger depth, using DB-SAT compared to that obtained using CFB-LA and STA technique

Lokesh B

Papers

Diverging beam transmit through limited aperture: A method to reduce ultrasound system complexity and yet obtain better image quality at higher frame rates.

Rotation Elastogram Estimation Using Synthetic Transmit-aperture Technique: A Feasibility Study.

Spatial resolution improvement in Plane Wave Imaging using adaptive Sign Coherence Factor weighting

Design of a low cost ultrasound system using diverging beams and synthetic aperture approach: Preliminary study

Diverging beam with synthetic aperture technique for rotation elastography: preliminary experimental results.