Depth-of-field enhancement in Filtered-Delay Multiply and Sum beamformed images using Synthetic Aperture Focusing

A Deep Learning Approach for Beamforming and Contrast Enhancement of Ultrasound Images in Monostatic Synthetic Aperture Imaging: A Proof-of-Concept

Goal: In this study, we demonstrate that a deep neural network (DNN) can be trained to reconstruct high-contrast images, resembling those produced by the multistatic Synthetic Aperture (SA) method using a 128-element array, leveraging pre-beamforming radiofrequency (RF) signals acquired through the monostatic SA approach. Methods: A U-net was trained using 27200 pairs of RF signals, simulated considering a monostatic SA architecture, with their corresponding delay-and-sum beamformed target images in a multistatic 128-element SA configuration. The contrast was assessed on 500 simulated test images of anechoic/hyperechoic targets. The DNN's performance in reconstructing experimental images of a phantom and different in vivo scenarios was tested too. Results: The DNN, compared to the simple monostatic SA approach used to acquire pre-beamforming signals, generated better-quality images with higher contrast and reduced noise/artifacts. Conclusions: The obtained results suggest the potential for the development of a single-channel setup, simultaneously providing good-quality images and reducing hardware complexity.

F-number optimization for synthetic aperture delay-multiply-and-sum reconstruction

The choices of transmit and receive f-numbers impact both ultrasound image contrast and spatial resolution. Although previous studies have evaluated the impact of receive f-number in delay-and-sum (DAS) plane wave imaging, there has not been a systematic study of f-numbers in DAS or delay-multiply-and-sum (DMAS) synthetic aperture (SA) imaging. In this study, we measured the impact on main lobe to side lobe energy ratio (MSER), generalized contrast-to-noise ratio (gCNR), and spatial resolution when varying receive and transmit f-numbers from 1 to 5 in 0.2 increments in DAS and DMAS reconstructed SA images. A wire target in a water tank and a standard imaging phantom were used to measure these metrics. From the water tank wire target images, higher MSER values were achieved with middle-range transmit f-numbers (2-4) and high receive f-numbers (>4) for both DAS and DMAS. From the phantom contrast target images, DAS produced images with high gCNR when using high transmit f-numbers (>4) and high receive f-numbers (>4). This came at the cost of reduced spatial resolution. DMAS produced images with high gCNR when using low transmit f-numbers (<3) and high receive f-numbers (>4). DMAS was not found to have as severe of a tradeoff in spatial resolution when seeking maximum gCNR. However, gCNR was typically lower for DMAS than DAS. For both DAS and DMAS, point target images had high spatial resolution when using low receive f-numbers (<2). Spatial resolution was typically higher for DMAS than DAS. Hanning apodization was found to produce similar trends as those found with rectangular apodization. These findings give insight on the behaviors of DAS and DMAS SA reconstruction algorithms and could be used to guide f-number selection.

Enhancement of Ultrasound B-Mode Image Quality Using Nonlinear Filtered-Multiply-and-Sum Compounding for Improved Carotid Artery Segmentation

In ultrasound B-mode imaging, the axial resolution (AR) is commonly determined by the duration or bandwidth of an excitation signal. A shorter-duration pulse will produce better resolution compared to a longer one but with compromised penetration depth. Instead of relying on the pulse duration or bandwidth to improve the AR, an alternative method termed filtered multiply and sum (FMAS) has been introduced in our previous work. For spatial-compounding, FMAS uses the autocorrelation technique as used in filtered-delay multiply and sum (FDMAS), instead of conventional averaging. FMAS enables a higher frame rate and less computational complexity than conventional plane-wave compound imaging beamformed with delay and sum (DAS) and FDMAS. Moreover, it can provide an improved contrast ratio and AR. In previous work, no explanation was given on how FMAS was able to improve the AR. Thus, in this work, we discuss in detail the theory behind the proposed FMAS algorithm and how it is able to improve the spatial resolution mainly in the axial direction. Simulations, experimental phantom measurements and in vivo studies were conducted to benchmark the performance of the proposed method. We also demonstrate how the suggested new algorithm may be used in a practical biomedical imaging application. The balloon snake active contour segmentation technique was applied to the ultrasound B-mode image of a common carotid artery produced with FMAS. The suggested method is capable of reducing the number of iterations for the snake to settle on the region-of-interest contour, accelerating the segmentation process.

Pixel-based approach to delay multiply and sum beamforming in combination with Wiener filter for improving ultrasound image quality

Unified pixel-based (PB) beamforming has been implemented for ultrasound imaging, offering significant enhancements in lateral resolution compared to the conventional dynamic focusing. However, it still suffers from clutter and off-axis artifacts, limiting the contrast resolution. This paper proposes an efficient method to improve image quality by integrating filtered delay multiply and sum (F-DMAS) into the framework. This hybrid strategy incorporates the spatial coherence of the received data into the beamforming process to improve contrast resolution and clutter rejection in the generated image. We also integrate a Wiener filter to suppress the spatiotemporal spreading using signals echoed from a single scatterer at the transmit focus as a kernel for the deconvolution. The Wiener filter is applied to the received waveforms before performing the hybrid strategy. The Wiener filter is shown to reduce interference due to the interaction between the excitation pulse and the transfer functions of the transducer elements, thus benefiting the axial resolution of the generated images. We validate the proposed method and compare it with other beamforming strategies through a series of experiments, including simulation, phantom, and in vivo studies. The results show that our approach can substantially improve both spatial resolution and contrast over the unified PB algorithm, while still maintaining the good features of this beamformer. The simplicity and good performance of our method show its potential for use in clinical applications.

An optimized total focusing method based on delay-multiply-and-sum for nondestructive testing

Total focusing method (TFM) attracts much interest because of high image resolution and large inspection coverage. However, the synthetic focusing approach based on delay-and-sum beamforming employs only the defect information contained in the dataset while ignoring the spatial information of the array signals, leading to limited imaging performance mixed with artifacts and noise. In addition, the signal-to-noise ratio (SNR) suffers due to single-element emission of full matrix capture. This work combines a modified delay-multiply-and-sum (DMAS) beamforming approach with conventional synthetic focusing in the TFM algorithm, to achieve optimization of TFM imaging performance. DMAS-based TFM is able to take full advantage of the defect and spatial information in the array dataset, and to generate new frequency components for better image reconstruction. As demonstrated on a series of comparative simulation and experimental results, the imaging results of the optimized TFM provide a considerable improvement in SNR. Better lateral spatial resolution is also achieved due to the increased number of equivalent transducer elements and second harmonic component. Therefore, this work provides a quite promising alternative solution for the post-processing of ultrasonic phased array with improved imaging performance.

Novel spatio-temporal non-linear beamformers for sparse synthetic aperture ultrasound imaging

The development of two modified non-linear beamformers, Spatio-Temporal Delay Multiply and Sum (ST-DMAS) and Spatio-Temporal Delay Euclidian-Weighted Multiply and Sum (ST-DewMAS) is reported in this paper. A sparse-transmit scheme (with only 8 transmits) on Synthetic Transmit Aperture technique (sparse STA) was chosen to evaluate the beamformers ability to generate the high-resolution Ultrasound image. These methods allow for obtaining superior-quality imaging at enhanced frame rates. The different beamformers of ST-DewMAS, ST-DMAS, Filtered Delay Multiply and Sum (F-DMAS), and Delay and Sum (DAS), were compared in terms of the Axial and Lateral Resolutions, AR and LR, respectively, Contrast-to-Noise Ratio (CNR), Contrast Ratio (CR), and Generalized CNR (GCNR). Experimental results demonstrate that the developed ST-DMAS and ST-DewMAS reconstruction on sparse STA technique resulted in better quality images compared to those obtained using DAS and F-DMAS. Specifically, the metrics of AR, LR CR, CNR, and GCNR showed improvements of more than 25% (for ST-DMAS) and 40 % (for ST-DewMAS) over those from DAS and F-DMAS beamformed images, respectively. Thus, the results demonstrate that the frame rate and image quality of an US system can both be enhanced by ST-DewMAS compared to the beamformers of F-DMAS and DAS.

Optimized Simultaneous Axial Multifocal Imaging via Frequency Multiplexed Focusing

Simultaneous axial multifocal imaging (SAMI) using a single acoustical transmission was developed to enhance the depth of field. This technique transmits a superposition of axial multifoci waveforms in a single transmission, thus increasing the frame rate. However, since all the waveforms are transmitted at a constant center frequency, there is a tradeoff between attenuation and lateral resolution when choosing a constant frequency for all the axial depths. In this work, we developed an optimized SAMI method by adding frequency dependence to each axial multifocus. By gradually increasing the frequency as a function of the focal depth, this method makes it possible to compensate for the gradually increasing F-number in order to achieve constant lateral resolution across the entire field of view. Alternatively, by gradually decreasing the axial multifoci frequencies as a function of depth, enhanced penetration depth and contrast are obtained. This method, termed frequency multiplexed SAMI (FM-SAMI), is described analytically and validated by resolution and contrast experiments performed on resolution targets, tissue-mimicking phantoms, and ex vivo biological samples. This is the first real-time implementation of a frequency multiplexing approach for axial multifoci imaging that facilitates high-quality imaging at an increased frame rate.

Spatial Coherence in Medical Ultrasound: A Review

Traditional pulse-echo ultrasound imaging heavily relies on the discernment of signals based on their relative magnitudes but is limited in its ability to mitigate sources of image degradation, the most prevalent of which is acoustic clutter. Advances in computing power and data storage have made it possible for echo data to be alternatively analyzed through the lens of spatial coherence, a measure of the similarity of these signals received across an array. Spatial coherence is not currently explicitly calculated on diagnostic ultrasound scanners but a large number of studies indicate that it can be employed to describe image quality, to adaptively select system parameters and to improve imaging and target detection. With the additional insights provided by spatial coherence, it is poised to play a significant role in the future of medical ultrasound. This review details the theory of spatial coherence in pulse-echo ultrasound and key advances made over the last few decades since its introduction in the 1980s.

Spatial Coherence Beamforming With Multi-Line Transmission to Enhance the Contrast of Coherent Structures in Ultrasound Images Degraded by Acoustic Clutter

This work demonstrates that the combination of multi-line transmission (MLT) and short-lag spatial coherence (SLSC) imaging improves the contrast of highly coherent structures within soft tissues when compared to both traditional SLSC imaging and conventional delay and sum (DAS) beamforming. Experimental tests with small (i.e., [Formula: see text]-3 mm) targets embedded in homogeneous and heterogeneous backgrounds were conducted. DAS or SLSC images were reconstructed when implementing MLT with varying numbers of simultaneously transmitted beams. In images degraded by acoustic clutter, MLT SLSC achieved up to 34.1 dB better target contrast and up to 16 times higher frame rates when compared to the more conventional single-line transmission SLSC images, with lateral resolution improvements as large as 38.2%. MLT SLSC thus represents a promising technique for clinical applications in which ultrasound visualization of highly coherent targets is required (e.g., breast microcalcifications, kidney stones, and percutaneous biopsy needle tracking) and would otherwise be challenging due to the strong presence of acoustic clutter.

Optimally-weighted non-linear beamformer for conventional focused beam ultrasound imaging systems

A novel non-linear beamforming method, namely, filtered delay optimally-weighted multiply and sum (F-DowMAS) beamforming is reported for conventional focused beamforming (CFB) technique. The performance of F-DowMAS was compared against delay and sum (DAS), filtered delay multiply and sum (F-DMAS), filtered delay weight multiply and sum (F-DwMAS) and filter delay Euclidian weighted multiply and sum (F-DewMAS) methods. Notably, in the proposed method the optimal adaptive weights are computed for each imaging point to compensate for the effects due to spatial variations in beam pattern in CFB technique. F-DowMAS, F-DMAS, and DAS were compared in terms of the resulting image quality metrics, Lateral resolution (LR), axial resolution (AR), contrast ratio (CR) and contrast-to-noise ratio (CNR), estimated from experiments on a commercially available tissue-mimicking phantom. The results demonstrate that F-DowMAS improved the AR by 57.04% and 46.95%, LR by 58.21% and 53.40%, CR by 67.35% and 39.25%, and CNR by 44.04% and 30.57% compared to those obtained using DAS and F-DMAS, respectively. Thus, it can be concluded that the newly proposed F-DowMAS outperforms DAS and F-DMAS. As an aside, we also show that the optimal weighting strategy can be extended to benefit DAS.

Euclidian-Weighted Non-linear Beamformer for Conventional Focused Beam Ultrasound Imaging Systems

In this paper, the recently developed method of, Filtered Delay Euclidian-Weighted Multiply and Sum (F-DewMAS), is newly investigated for Conventional Focused Beamforming (CFB) technique. The performance of F-DewMAS method was compared with the established Delay and Sum (DAS) method and the popular non-linear beamforming method of F-DMAS. The different methods of F-DewMAS, F-DMAS, and DAS were compared in terms of the resulting image quality metrics, Lateral Resolution (LR), Axial Resolution (AR), Contrast Ratio (CR) and contrast-to-noise ratio (CNR), in experiments on Nylon point scatterer and CIRS Triple modality Abdominal phantoms. Experimental results show that F-DewMAS resulted in improvements of AR by 35.56% and 25.33%, LR by 42.97 % and 31.05 % and CR by 119.94% and 61.46% compared to those obtained using DAS and F-DMAS, respectively. The CNR of F-DewMAS is 46.33 % more compared to F-DMAS. Hence, it can be concluded that the image quality is improved appreciably by F-DewMAS compared to DAS and F-DMAS.Clinical Relevance-The developed method can improve the resolution and contrast of the image, which results in better visualization of finer details and thus may aid in better diagnosis.

Computationally efficient minimum-variance baseband delay-multiply-and-sum beamforming for adjustable enhancement of ultrasound image resolution

Baseband Delay-Multiply-and-Sum (BB-DMAS) beamforming takes advantage of the baseband spatial coherence of receiving aperture to improve image resolution and contrast. Meanwhile, the side-lobe clutter and noise level can also be effectively suppressed in BB-DMAS beamforming due to their low coherence when being detected by channels in different spatial locations. BB-DMAS scales the magnitude of channel signal by p-th root and restores the output dimensionality by p-th power after channel summation. Higher p value introduces more spatial coherence into DMAS beamforming and provides higher image resolution at the cost of background speckle quality. In this study, a computationally efficient integration of BB-DMAS with minimum-variance (MV) beamforming is developed so that the image resolution can be drastically improved with low p value (e.g. p < 2) while maintaining the speckle quality. For each image pixel, the proposed MV-DMAS only requires single MV estimation to optimize the aperture apodization for DMAS beamforming. Our simulation results show that, with p = 1.5, the -6-dB lateral width of wire reflector noticeably improves from 0.22 mm to 0.13 mm by adopting MV estimation in BB-DMAS beamforming. In MV-DMAS, the suppression of uncorrelated random noises also remains effective. Experimental results not only confirm the superior resolution in MV-DMAS beamforming but also demonstrates comparable image contrast and speckle quality to BB-DMAS counterpart. In conclusion, MV-DMAS beamforming can provide improvement in image resolution while maintaining the other image quality metrics using an efficient combination of moderate spatial coherence and MV estimation of receiving aperture apodization in ultrasonic imaging.

Weighted non-linear beamformers for low cost 2-element receive ultrasound imaging system

In this paper, the development of modified beamforming methods, Filtered Delay Weight Multiply and Sum (F-DwMAS) and Filtered Delay Euclidian-Weighted Multiply and Sum (F-DewMAS), are reported. These methods were investigated on a minimum-redundancy synthetic aperture technique, called as 2 element Receive Synthetic Aperture Focusing Technique (2R-SAFT), which uses one element on transmit and two consecutive elements on receive, for reducing hardware complexity without compromising much on the image quality. The performance of the developed F-DwMAS and F-DewMAS methods were compared with Delay and Sum (DAS) and recently introduced F-DMAS beamforming methods. Notably, in the proposed methods, an additional aperture window function is designed and incorporated to the F-DMAS method. The different methods of F-DwMAS, F-DewMAS, F-DMAS and DAS were compared in terms of the resulting image quality metrics, Lateral Resolution (LR), Axial Resolution (AR), Contrast Ratio (CR) and contrast-to-noise ratio (CNR), in simulation and experiments on tissue-mimicking phantoms. Experimental results show that (F-DwMAS) and {F-DewMAS} resulted in improvements of AR by (46.32% and 23.51%), {43.56% and 17.78%}, LR by (47.81% and 30.27%), {44.26% and 26.14%} and CR by (45.68% and 17.15%), {42.16% and 9.87%} compared to those obtained using DAS and F-DMAS, respectively. However, CNR of F-DwMAS and F-DewMAS was found to be 31.19% and 21.16% less compared to DAS, but 4.89% and 18.64% more than F-DMAS, respectively. Hence, it can be concluded that the image quality improved by both F-DwMAS and F-DewMAS compared to DAS and F-DMAS. Also, between F-DwMAS and F-DewMAS, the later has the advantage of ready applicability to different acquisition schemes and settings compared to the former also having an additional advantage of better CNR compared to both F-DMAS and F-DewMAS.

Synthetic Aperture Focusing for Multi-Covariate Imaging of Sub-Resolution Targets

Coherence-based imaging methods suffer from reduced image quality outside the depth of field for focused ultrasound transmissions. Synthetic aperture methods can extend the depth of field by coherently compounding time-delayed echo data from multiple transmit events. Recently, our group has presented the Multi-covariate Imaging of Sub-resolution Targets (MIST), an estimation-based method to image the statistical properties of diffuse targets. MIST has demonstrated improved image quality over conventional delay-and-sum, but like many coherence-based imaging methods, suffers from limited depth of field artifacts. This article applies synthetic aperture focusing to MIST, which is evaluated using focused, plane-wave, and diverging-wave transmit geometries. Synthetic aperture MIST is evaluated in simulation, phantom, and in vivo applications, demonstrating consistent improvements in contrast-to-noise ratio (CNR) over conventional dynamic receive MIST outside the transmit depth of field, with approximately equivalent results between synthetic transmit geometries. In vivo 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. MIST performance is characterized in the space of plane-wave imaging, where the total plane-wave count is reduced through coarse angular sampling or total angular span. Simulation and experimental results indicate wide applicability of MIST to synthetic aperture imaging methods.

Ultrasound Baseband Delay-Multiply-and-Sum (BB-DMAS) nonlinear beamforming

Compared to conventional Delay-and-Sum (DAS) beamforming, Delay-Multiply-and-Sum (DMAS) imaging uses multiplicative coupling of channel pairs for spatial coherence of receiving aperture to improve image resolution and contrast. However, present DMAS imaging is based on the radio-frequency (RF) channel signals (RF-DMAS) and thus requires large oversampling to avoid aliasing and switching of band-pass filtering to isolate the corresponding spectral components for imaging. Baseband DMAS (BB-DMAS) beamforming in this study is based on the demodulated channel signals to provide similar results but with simplified signal processing. The BB-DMAS beamforming scales the magnitude of time-delayed channel signal by p-th root while maintaining the phase. After channel sum, the output dimensionality is restored by p-th power. The multiplicative coupling in BB-DMAS always renders baseband signal and thus the need for oversampling is eliminated. Besides, the BB-DMAS can use any rational p values to provide flexible image quality and an explicit relation between BB-DMAS beamforming and channel-domain phase coherence exists. Our results show that the image characteristics between BB-DMAS and RF-DMAS are similar. The suppression of lateral side lobe level, grating lobe level and uncorrelated random noises gradually increases with the rational p value in BB-DMAS beamforming. The image contrast improves from -24.8 dB in DAS to -34.3 dB, -43.0 dB and -51.4 dB in BB-DMAS, respectively with p value of 1.5, 2.0 and 2.5. In conclusion, BB-DMAS beamforming provides flexible manipulation of image quality by introducing baseband spatial coherence in the ultrasonic imaging.

Multi-line acquisition with delay multiply and sum beamforming in phased array ultrasound imaging, validation of simulation and in vitro

Increasing the frame rate of medical ultrasound imaging is very important, especially in applications such as cardiac diagnostic imaging, where such an imaging should be able to facilitate the examination of the temporal behaviour of the short cardiac cycle. Frame rate can be increased by the multi-line acquisition (MLA) method, also called parallel receive beamforming (PRB), where several beams are received from a single transmit (Tx) beam. The shortage is that imaging performance would be sacrificed. Filtered-delay multiply and sum (F-DMAS) is a non-linear beamforming technique proven to be able to improve the contrast and resolution of the image compared to traditional delay and sum (DAS) beamforming. In this paper, we proposed to combine MLA and the lower complexity F-DMAS algorithm, and use synthetic transmit beams (STB) to reduce the artifacts of MLA. The simulations of point targets and cyst phantoms were all carried out in Matlab using Field II. The results show that 2 line acquisition with delay multiply and sum (DMAS 2MLA) beamforming presents an equivalent imaging performance to that of traditional DMAS beamforming, and obtains a 7.69% higher resolution and 2 times higher contrast ratio in comparison to DAS beamforming. A real RF data experiment was applied to support the feasibility and validity of our method. The low complexity of F-DMAS (O(N)) would make it easy to implement 2 parallel beamformers. Thus, by combining 2MLA and F-DMAS, the frame rate can be improved to 2-fold higher with a better image quality compared to that of DAS beamforming.

Simultaneous Axial Multifocal Imaging Using a Single Acoustical Transmission: A Practical Implementation

Standard ultrasound imaging techniques rely on sweeping a focused beam across a field of view; however, outside the transmission focal depth, image resolution and contrast are degraded. High-quality deep tissue in vivo imaging requires focusing the emitted field at multiple depths, yielding high-resolution and high-contrast ultrasound images but at the expense of a loss in frame rate. Recent developments in ultrasound technologies have led to user-programmable systems, which enable real-time dynamic control over the phase and apodization of each individual element in the imaging array. In this paper, we present a practical implementation of a method to achieve simultaneous axial multifoci using a single acoustical transmission. Our practical approach relies on the superposition of axial multifoci waveforms in a single transmission. The delay in transmission between different elements is set such that pulses constructively interfere at multiple focal depths. The proposed method achieves lateral resolution similar to successive focusing, but with an enhanced frame rate. The proposed method uses standard dynamic receive beamforming, identical to two-way focusing, and does not require additional postprocessing. Thus, the method can be implemented in real time on programmable ultrasound systems that allow different excitation signals for each element. The proposed method is described analytically and validated by laboratory experiments in phantoms and ex vivo biological samples.

Sparse Convolutional Beamforming for Ultrasound Imaging

The standard technique used by commercial medical ultrasound systems to form B-mode images is delay and sum (DAS) beamforming. However, DAS often results in limited image resolution and contrast that are governed by the center frequency and the aperture size of the ultrasound transducer. A large number of elements lead to improved resolution but at the same time increase the data size and the system cost due to the receive electronics required for each element. Therefore, reducing the number of receiving channels while producing high-quality images is of great importance. In this paper, we introduce a nonlinear beamformer called COnvolutional Beamforming Algorithm (COBA), which achieves significant improvement of lateral resolution and contrast. In addition, it can be implemented efficiently using the fast Fourier transform. Based on the COBA concept, we next present two sparse beamformers with closed-form expressions for the sensor locations, which result in the same beam pattern as DAS and COBA while using far fewer array elements. Optimization of the number of elements shows that they require a minimal number of elements that are on the order of the square root of the number used by DAS. The performance of the proposed methods is tested and validated using simulated data, phantom scans, and in vivo cardiac data. The results demonstrate that COBA outperforms DAS in terms of resolution and contrast and that the suggested beamformers offer a sizable element reduction while generating images with an equivalent or improved quality in comparison with DAS.

Signal Coherence and Image Amplitude With the Filtered Delay Multiply and Sum Beamformer

The filtered delay multiply and sum (F-DMAS) beamformer has recently been presented in the context of medical ultrasound image formation. This nonlinear beamformer produces images with improved contrast resolution and noise rejection when compared with the delay and sum (DAS) beamformer. In an attempt to better understand the origin of the improved image quality, this paper shows a theoretical study of the image amplitude statistics backed up by numerical simulations. The results show that the difference in image amplitude using the DAS or F-DMAS beamformers can be partly explained by the way signal coherence influences both beamformers. When using the F-DMAS compared with the DAS beamformer, the image amplitude is shown to be more dependent on the signal coherence. Experimental ultrasound images of a phantom confirm our findings.

Experimental evaluation of ultrasound higher-order harmonic imaging with Filtered-Delay Multiply And Sum (F-DMAS) non-linear beamforming

Tissue Harmonic Imaging (THI) mode is currently one of the preferred choices by the clinicians for its ability to provide enhanced ultrasound images, thanks to the use of the second harmonic component of backscattered echoes. This paper aims at investigating whether the combination of THI with Filtered-Delay Multiply And Sum (F-DMAS) beamforming can provide further improvements in image quality. F-DMAS is a new non-linear beamformer, which, similarly to THI, is based on the use of the second harmonics of beamformed signals and is known to increase image contrast resolution and noise rejection. Thus, we have first compared the images obtained by using F-DMAS and the standard Delay And Sum (DAS) beamformers when only the second harmonics of the received signals was selected. Moreover, possible improvements brought about by other harmonic components generated by the combined use of the fundamental plus second harmonics and F-DMAS beamforming have been explored. Experimental results demonstrate that, as compared to standard harmonic imaging with DAS, THI and F-DMAS can be joined to improve the -20 dB lateral resolution up to 1 mm, the contrast ratio up to 12 dB on a cyst-phantom and up to 9 dB on in vivo images.

Inverse Problem of Ultrasound Beamforming With Sparsity Constraints and Regularization

Ultrasound (US) beamforming is the process of reconstructing an image from acquired echo traces on several transducer elements. Typical beamforming approaches, such as delay-and-sum, perform simple projection operations, while techniques using statistical information also exist, e.g., adaptive, phase coherence, delay-multiply-and-sum, and sparse coding approaches. Inspired by the feasibility and success of inverse problem (IP) formulations in several image reconstruction problems, such as computed tomography, we herein devise an IP approach for US beamforming. We define a linear forward model for the synthesis of the beamformed image, and solve its IP thanks to several intuitive and physics-based constraints and regularization terms proposed. These reflect the prior knowledge about the spectra of carrier signal and spatial coherence of modulated signal. These constraints admit effective formulation through sparse representations. Our proposed method was evaluated for plane-wave imaging (PWI) that transmits unfocused waves, enabling high frame rates with large field of view at the expense of much lower image quality with conventional beamforming techniques. Results are evaluated in numerical simulations, as well as tissue-mimicking phantoms and in vivo data provided by PWI challenge in medical US. The best results achieved by our proposed techniques are 0.39-mm full-width at half-maximum for spatial resolution and 16.3-dB contrast-to-noise ratio, using a single plane-wave transmit.

Linear-array photoacoustic imaging using minimum variance-based delay multiply and sum adaptive beamforming algorithm

In photoacoustic imaging, delay-and-sum (DAS) beamformer is a common beamforming algorithm having a simple implementation. However, it results in a poor resolution and high sidelobes. To address these challenges, a new algorithm namely delay-multiply-and-sum (DMAS) was introduced having lower sidelobes compared to DAS. To improve the resolution of DMAS, a beamformer is introduced using minimum variance (MV) adaptive beamforming combined with DMAS, so-called minimum variance-based DMAS (MVB-DMAS). It is shown that expanding the DMAS equation results in multiple terms representing a DAS algebra. It is proposed to use the MV adaptive beamformer instead of the existing DAS. MVB-DMAS is evaluated numerically and experimentally. In particular, at the depth of 45 mm MVB-DMAS results in about 31, 18, and 8 dB sidelobes reduction compared to DAS, MV, and DMAS, respectively. The quantitative results of the simulations show that MVB-DMAS leads to improvement in full-width-half-maximum about 96%, 94%, and 45% and signal-to-noise ratio about 89%, 15%, and 35% compared to DAS, DMAS, MV, respectively. In particular, at the depth of 33 mm of the experimental images, MVB-DMAS results in about 20 dB sidelobes reduction in comparison with other beamformers.

An Experimental Protocol for Assessing the Performance of New Ultrasound Probes Based on CMUT Technology in Application to Brain Imaging

The possibility to perform an early and repeatable assessment of imaging performance is fundamental in the design and development process of new ultrasound (US) probes. Particularly, a more realistic analysis with application-specific imaging targets can be extremely valuable to assess the expected performance of US probes in their potential clinical field of application. The experimental protocol presented in this work was purposely designed to provide an application-specific assessment procedure for newly-developed US probe prototypes based on Capacitive Micromachined Ultrasonic Transducer (CMUT) technology in relation to brain imaging. The protocol combines the use of a bovine brain fixed in formalin as the imaging target, which ensures both realism and repeatability of the described procedures, and of neuronavigation techniques borrowed from neurosurgery. The US probe is in fact connected to a motion tracking system which acquires position data and enables the superposition of US images to reference Magnetic Resonance (MR) images of the brain. This provides a means for human experts to perform a visual qualitative assessment of the US probe imaging performance and to compare acquisitions made with different probes. Moreover, the protocol relies on the use of a complete and open research and development system for US image acquisition, i.e. the Ultrasound Advanced Open Platform (ULA-OP) scanner. The manuscript describes in detail the instruments and procedures involved in the protocol, in particular for the calibration, image acquisition and registration of US and MR images. The obtained results prove the effectiveness of the overall protocol presented, which is entirely open (within the limits of the instrumentation involved), repeatable, and covers the entire set of acquisition and processing activities for US images.