Resolution Ultrasonic Imaging Research Articles

Ultrasonic waveguide based super resolution imaging using structured channel metamaterial lenses

The extension of metamaterial concepts to the ultrasonic domain is challenging because of the shorter wavelength, which necessitates the use of spatially narrow band receiving techniques to capture wavefields past fine features of the metamaterial. Currently, the Laser Doppler Vibrometer is the only option with several drawbacks hampering its widespread practical implementation, including cost and sensitivity to external disturbances. This paper proposes a novel waveguide based reception technique to capture the amplified evanescent fields transmitted through the subwavelength features of the metamaterials. Numerical simulations and experiments are carried out on a structured channel metamaterial and a thin stainless steel waveguide attached to a commercial transducer. A practical super resolution ultrasonic imaging down to a third of the operating wavelength is successfully demonstrated in comparison with a commercial laser receiver. The physics of the imaging and dispersion characteristics of the waveguide enabling the process are discussed. The promising results showcase broadband, low-cost, portable alternatives with important implications for high-resolution ultrasonic imaging in industrial and biomedical applications.

Multikernel positional embedding convolutional neural network for photoacoustic reconstruction with sparse data.

Photoacoustic imaging (PAI) is an emerging noninvasive imaging modality that merges the high contrast of optical imaging with the high resolution of ultrasonic imaging. Low-quality photoacoustic reconstruction with sparse data due to sparse spatial sampling and limited view detection is a major obstacle to the popularization of PAI for medical applications. Deep learning has been considered as the best solution to this problem in the past decade. In this paper, we propose what we believe to be a novel architecture, named DPM-UNet, which consists of the U-Net backbone with additional position embedding block and two multi-kernel-size convolution blocks, a dilated dense block and dilated multi-kernel-size convolution block. Our method was experimentally validated with both simulated data and in vivo data, achieving a SSIM of 0.9824 and a PSNR of 33.2744dB. Furthermore, the reconstructed images of our proposed method were compared with those obtained by other advanced methods. The results have shown that our proposed DPM-UNet has a great advantage in PAI over other methods with respect to the imaging effect and memory consumption.

The Acoustic Field Distribution Inside the Ultrasonic Ring Array

This paper presents and analyses the results of a simulation of the acoustic field distribution in sectors of a 1024-element ring array, intended for the diagnosis of female breast tissue with the use of ultrasonic tomography. The array was tested for the possibility to equip an ultrasonic tomograph with an additional modality - conventional ultrasonic imaging with the use of individual fragments (sections) of the ring array. To determine the acoustic field for sectors of the ring array with a varying number of activated ultrasonic transducers, a combined sum of all acoustic fields created by each elementary transducer was calculated. By the use of MATLAB software, a unique algorithm was developed, for a numerical determination of the distribution of pressure of an ultrasonic wave on any surface or area of the medium generated by the concave curvilinear structure of rectangular ultrasound transducers with a geometric focus of the beam. The analysis of the obtained results of the acoustic field distribution inside the ultrasonic ring array used in tomography allows to conclude that the optimal number of transducers in a sector enabling to obtain ultrasound images using linear echographic scanning is 32 ≤ $n$ ≤ 128, taking into account that due to an increased temporal resolution of ultrasonic imaging, this number should be as low as possible.

Grating Lobe Reduction in Plane-Wave Imaging With Angular Compounding Using Subtraction of Coherent Signals.

Plane-wave imaging (PWI) with angular compounding has gained in popularity over recent years, because it provides high frame rates and good image properties. However, most linear arrays used in clinical practice have a pitch that is equal to than the wavelength of ultrasound. Hence, the presence of grating lobes is a concern for PWI using multiple transmit angles. The presence of grating lobes produces clutter in images and reduces the ability to observe tissue contrast. Techniques to reduce or eliminate the presence of grating lobes for PWI using multiple angles will result in improved image quality. Null subtraction imaging (NSI) is a nonlinear beamforming technique that has been explored for improving the lateral resolution of ultrasonic imaging. However, the apodization scheme used in NSI also eliminates or greatly reduces the presence of grating lobes. Imaging tasks using NSI were evaluated in simulations and physical experiments involving tissue-mimicking phantoms and rat tumors in vivo. Images created with NSI were compared with images created using traditional delay and sum (DAS) with Hann apodization and images created using a generalized coherence factor (GCF). NSI was observed to greatly reduce the presence of grating lobes in ultrasonic images, compared to DAS with Hann and GCF, while maintaining spatial resolution and contrast in the images. Therefore, NSI can provide a novel means of creating images using PWI with multiple steering angles on clinically available linear arrays while reducing the adverse effects associated with grating lobes.

RESEARCH ON MAGNETICALLY MEDIATED THERMOACOUSTIC IMAGING BASED ON B-SCAN

Magnetically Mediated Thermoacoustic Imaging (MMTAI) is a new imaging technology that uses the thermoacoustic effect of electromagnetic fields. It is capable of a high resolution in ultrasonic imaging and high contrast in electrical impedance imaging. It has considerable potential applications in the early diagnosis of diseases. First, this paper describes the theoretical analysis and numerical simulation of MMTAI. For B-scan thermoacoustic imaging, the thermoacoustic image of a two-dimensional model is simulated and analyzed. The numerical simulation results provide theoretical guidance for the design of the experiment. Second, the B-scan experimental system of MMTAI is designed and built. The imaging experiment was carried out using an imitation gel that contains sodium chloride as the target, and the imaging results were basically consistent with the conductivity distribution of the target. Numerical simulation and physical experiments verify the feasibility of the MMTAI method for low conductivity media. This preliminary study has shown the feasibility of the technique to detect conductivity boundaries, making it relevant for the future application of this method in the field of biomedical imaging.

Far-field ultrasonic imaging using hyperlenses

Hyperlenses for ultrasonic imaging in nondestructive evaluation and non-invasive diagnostics have not been widely discussed, likely due to the lack of understanding on their performance, as well as challenges with reception of the elastic wavefield past fine features. This paper discusses the development and application of a cylindrical hyperlens that can magnify subwavelength features and achieve super-resolution in the far-field. A radially symmetric structure composed of alternating metal and water layers is used to demonstrate the hyperlens. Numerical simulations are used to study the performance of cylindrical hyperlenses with regard to their geometrical parameters in imaging defects separated by a subwavelength distance, gaining insight into their construction for the ultrasonic domain. An elegant extension of the concept of cylindrical hyperlens to flat face hyperlens is also discussed, paving the way for a wider practical implementation of the technique. The paper also presents a novel waveguide-based reception technique that uses a conventional ultrasonic transducer as receiver to capture waves exiting from each fin of the hyperlens discretely. A metallic hyperlens is then custom-fabricated, and used to demonstrate for the first time, a super-resolved image with 5X magnification in the ultrasonic domain. The proposed hyperlens and the reception technique are among the first demonstrations in the ultrasonic domain, and well-suited for practical inspections. The results have important implications for higher resolution ultrasonic imaging in industrial and biomedical applications.

3D reconstruction and characterization of reef limestone pores based on optical and acoustic microscopic images

The pore network in reef limestone provides an important resource for fluid storage and flow. Three-dimensional (3D) reconstruction and characterization of the spatial structure of reef limestone pores offers a valuable way of identifying its physical and mechanical properties. This paper reports on a study where optical and acoustic microscope images were used to combine the advantages of the high resolution of optical testing with the high penetration capabilities of acoustic testing. This overcomes existing problems with multi-scale structural imaging of the pores in reef limestone caused by the small penetration depth of traditional optical imaging and the low resolution of ultrasonic imaging. First of all, a correlation function between the shallow and deep pore structure of reef limestone is established. This draws on high-resolution data from shallow optical microscopic images and deep acoustic microscopic images with a multi-layered cross-section. The contours of the shallow and deep pore structure are then digitized. After this, horizontal section control points and vertical transition contours are constructed that enable a deep integration of information about the multi-dimensional geometric morphology of the pore structure to be realized, providing in turn for a fine-grained 3D reconstruction of the pore structure at different scales. Finally, factors governing the characterization of contour similarity and center deviation are proposed that make it possible to generate spatial representations of the pore structure across multiple dimensions. A real-world case analysis is then used to verify the feasibility and reliability of the proposed method. The results show that the method can provide abundant data for the establishment of a reef limestone pore network and physical model, making it possible to improve the quality of 3D reconstructions of the pore structure and improve the 3D imaging of reef limestone pore networks. The proposed method can underpin new approaches to analysis of the pore structure of reef limestone and support a wide range of potential applications in the field of pore visualization and characterization.

Visual geometry Group-UNet: Deep learning ultrasonic image reconstruction for curved parts.

Detecting small defects in curved parts through classical monostatic pulse-echo ultrasonic imaging is known to be a challenge. Hence, a robot-assisted ultrasonic testing system with the track-scan imaging method is studied to improve the detecting coverage and contrast of ultrasonic images. To further improve the image resolution, we propose a visual geometry group-UNet (VGG-UNet) deep learning network to optimize the ultrasonic images reconstructed by the track-scan imaging method. The VGG-UNet uses VGG to extract advanced information from ultrasonic images and takes advantage of UNet for small dataset segmentation. A comparison of the reconstructed images on the simulation dataset with ground truth reveals that the peak signal-to-noise ratio (PSNR) and structural similarity index measure (SSIM) can reach 39 dB and 0.99, respectively. Meanwhile, the trained network is also robust against the noise and environmental factors according to experimental results. The experiments indicate that the PSNR and SSIM can reach 32 dB and 0.99, respectively. The resolution of ultrasonic images reconstructed by track-scan imaging method is increased approximately 10 times. All the results verify that the proposed method can improve the resolution of reconstructed ultrasonic images with high computation efficiency.

Implementation method for magneto-acoustic concentration tomography with magnetic induction (MACT-MI) based on the method of moments

Magneto-acoustic concentration tomography of magnetic nanoparticles with magnetic induction (MACT-MI) is a new imaging technology that combines the advantages of the high sensitivity of magnetic particle imaging and the high resolution of ultrasonic imaging. This technique has broad application prospects in the biomedical and molecular imaging fields. In this study, a reconstruction algorithm based on the method of moments (MoM) is proposed for the MACT-MI inverse problem. Image reconstructions of the acoustic source and superparamagnetic nanoparticle (SPN) concentration were performed using different shape models, and the reconstructed images were analyzed. In addition, the effect of the radius of the tissue region loaded with SPNs on the quality of the reconstructed images was evaluated. The results demonstrated that the new method could reconstruct the SPN concentration distribution well, and a negative correlation existed between the radius of the imaging model and reconstructed image quality. The finding of this research can potentially contribute to the development of MACT-MI in medicine.

Ultrasonic elastography to assess biomechanical properties of the optic nerve head and peripapillary sclera of the eye

PurposeTo quantitatively investigate both optic nerve head (ONH) and peripapillary sclera (PPS) biomechanical properties of porcine eyes through an ultrasonic elastography imaging system in response to both increasing and decreasing intraocular pressure (IOP). MethodsThe Young’s modulus of the ONH and PPS were assessed using our high resolution ultrasonic imaging system which utilized a mechanical shaker to induce shear waves and an off-axis aligned 40 MHz needle transducer to track micron-level displacement along the direction of wave propagation. In this study, imaging on a total of 8 ex vivo porcine eyes preloaded with IOPs from 6 mmHg to 30 mmHg was performed. To have a better understanding of the effect of varying IOP on biomechanics, both increasing and decreasing IOPs were investigated. ResultsThe increase of the Young’s modulus of ONH (92.4 ± 13.9 kPa at 6 mmHg to 224.7 ± 71.1 kPa at 30 mmHg) and PPS (176.8 ± 14.3 kPa at 6 mmHg to 573.5 ± 64.4 kPa at 30 mmHg) following IOP elevation could be observed in the reconstructed Young’s modulus of the shear wave elasticity (SWE) imaging while the B-mode structural images remained almost unchanged. In addition, for the same IOP level, both ONH and PPS have a tendency to be stiffer with decreasing IOP as compared to increasing IOP. ConclusionsOur results demonstrate the feasibility of using our ultrasonic elastography system to investigate the stiffness mapping of posterior eye with high resolution in both increasing and decreasing IOPs, making this technique potentially useful for glaucoma.

Joint Inversion of Electromagnetic and Acoustic Data With Edge-Preserving Regularization for Breast Imaging

Joint inversion of microwave and ultrasonic data for breast imaging is investigated with deterministic edge-preserving regularization by introducing auxiliary variables indicating whether a pixel is on an edge or not. These edge markers are shared by dielectric and acoustic parameters and are the link to fusion between modalities. They can be jointly optimized from the last parameter profiles of microwave and ultrasonic cases and guide the next optimization as coefficients of the regularization term. Alternate minimization is used to update acoustic contrast, edge markers and dielectric contrast. Comprehensive numerical experiments are carried out on breast phantoms, a simple synthetic one and three extracted from a database. The results show, with comparisons to more classical approaches involving total variation or cross-gradient regularization developed in parallel, that the joint inversion algorithm can gain from the high resolution of ultrasonic imaging and the high contrast of microwave imaging. The quality of microwave imaging is enhanced in clear fashion and small tumors detected.

Novel multi-layer-composites design for ultrasonic transducer applications

High quality ultrasonic transducers require high sensitivity and broad bandwidth to achieve high contrast and high spatial resolution ultrasonic imaging. Multi-layer-composites transducers were proposed, which facilitated high sensitivity and broad bandwidth. Multi-layer-composites were designed by W.A. Smith theory, simulated using finite element analysis based PZFlex software and fabricated using dice and fill method. 5 MHz transducers were fabricated and both electrical and acoustic properties of the transducers were characterized systematically. The multi-layer-composites transducers have a little larger center frequency (5.03 MHz), similar −6 dB bandwidth (40.6%) and a much higher peak-to-peak voltage (166.18 mV) compared to large pitch 1–3 composites transducers (4.605 MHz, 38.9%, and 98.74 mV, respectively) and small pitch 1–3 composites transducers (4.695 MHz, 46.6%, and 88.2 mV, respectively). These results suggest that multi-layer-composites have great potential in ultrasonic transducer applications. A theory was proposed to approximate the sensitivity, which has potential in guiding the design of multi-layer-composites transducers.

A stimulated liquid-gas phase transition nanoprobe dedicated to enhance the microwave thermoacoustic imaging contrast of breast tumors.

Microwave-induced thermoacoustic imaging (MTAI), combining the advantages of the high contrast of microwave imaging and the high resolution of ultrasonic imaging, is a potential candidate for breast tumor detection. MTAI probes have been used to extend thermoacoustic imaging to molecular imaging. However, due to the high content of water molecules in tissues, the thermoelastic expansion-based probes used in conventional MTAI are not capable of adequate enhancement. Herein, an MTAI nanoprobe for amplification of thermoacoustic (TA) signals by the stimulated liquid-gas phase transition mechanism has been developed, providing significantly higher signal amplitude than that from the conventional mechanism of thermoelastic expansion. The nanoprobe consists of liquid perfluorohexane (PFH) and tungsten disulfide (WS2) nanoparticles rich in defect electric dipoles. When irradiated with pulsed microwaves, the defect electric dipoles in WS2 were repeatedly polarized by gigahertz. This results in localized transient heating and an acoustic shockwave, which destroys the van der Waals forces between PFH molecules. Ultimately, liquid PFH droplets undergo a liquid-gas phase transition, generating dramatically enhanced TA signals. The practical feasibility was tested in vitro and in a breast tumor animal model. The results show that the proposed nanoprobe can greatly improve the contrast of tumor imaging. It will be a new generation probe for MTAI.

High resolution ultrasonic imaging based on frequency sweep in both of transducer element domain and imaging line domain

We developed the super resolution FM-chirp correlation method (SCM) and its high frame rate version called synthetic aperture-SCM (SA-SCM) based on frequency sweep in order to improve the range resolution in ultrasound imaging. However, in these methods, discontinuities in the lateral direction tend to occur, because the SCM processing is performed line by line in the image. In addition, in array transducers, grating lobes are more likely to occur when the spatial resolution is increased using high-frequencies. Therefore, another version called SCM-weighted SA was proposed. In the SCM-weighted SA, in order to avoid the lateral discontinuities, first the SCM processing is performed on each echo received by each transducer element, and then the obtained SCM result is used as a weight for SA processing. The SCM-weighted SA can generate multiple B-mode images each of which corresponds to each carrier frequency, and the appropriate low frequency images among them have no grating lobes. While this plurality of B-mode images can be used as multi-frequency images, it can also be integrated as one high SNR image. In the present study, we propose a method to generate the integrated one B-mode image by applying SCM again in the line direction of the obtained multi-frequency images, which is called SCM-weighted SA-SCM. This method can further improve the range resolution.

Improvement of Performance Degradation in Synthetic Aperture Extension of Enhanced Axial Resolution Ultrasound Imaging Based on Frequency Sweep.

We are studying a method based on the carrier frequency sweep for axial high resolution ultrasonic imaging to provide the range resolution that corresponds to the carrier wavelength. The first proposal for this type of method was based on the focused pulse transmission. Then, to improve the frame rate, the method was extended to a synthetic aperture-type method that transmits divergent pulses. While the method is effective in terms of the frame rate, degradation of the enhanced axial resolution performance is a concern. Therefore, using finite element method simulations and simple experiments, the performance of the synthetic aperture method with high axial resolution is evaluated via comparison with the original method using focused pulses. The evaluation confirmed that the performance degradation of the synthetic aperture method is caused by weakness in the transmitted wave intensity and deterioration of the phase coherence in the reception beamforming. Based on this result, we propose a method that is less affected by the latter cause and show its effectiveness.

High Resolution Ultrasonic Imaging in the Evaluation of the Posterior Segment Disorders

High Resolution Ultrasonic Imaging in the Evaluation of the Posterior Segment Disorders

High Resolution Ultrasonic Imaging in the Evaluation of the Posterior Segment Disorders

Purpose: to evaluate the role of high resolution 20 MHz ultra sound in assessment of posterior segment disorders. Patients and Methods: a prospective, case series study was conducted on 40 patients who attended the ophthalmology outpatient clinic of Al-Azhar university hospitals (2014-2018). All should have a posterior segment disorder either retinal, vascular, vitreo-macular, optic nerve head or choroidal disorder. They should be with clear media, therefor we can reach a diagnosis based on clinical examination and augmented by optical coherence tomography (OCT) and fundus fluorescein angiography (FFA) whenever indicated, the patients were evaluated using both high resolution 20 MHz ultrasound(US) and conventional 12.5 MHz US and compare their results to the gold standard clinical diagnosis to assess their sensitivity to reach the diagnosis. Results & Conclusion: we have two main characters for well diagnosis by any US tool, resolution and sensitivity, and neither the 20 MHz nor 12.5 MHz could combine both of them. Therefore, we recommended tocombine the examination by both of them as they are complementary to each other.

A Locally Adaptive Phase Aberration Correction (LAPAC) Method for Synthetic Aperture Sequences.

Phase aberration is a phenomenon caused by heterogeneity of the speed of sound in tissue, in which the actual speed of sound of the tissue is different than the assumed speed of sound used for beamforming. It reduces the quality and resolution of ultrasonic images and impairs clinical diagnostic capabilities. Although phase aberration correction (PAC) methods can reduce these detrimental effects, most practical implementations of PAC methods are based on the near field phase screen model, which have limited ability to represent the true aberration induced by inhomogeneous tissue. Accordingly, we propose a locally adaptive phase aberration correction (LAPAC) method that is applied through the use of synthetic aperture. The method is tested using full-wave simulations of models of human abdominal wall, experiments with tissue aberrators, and in vivo carotid images. LAPAC is compared with conventional phase aberration correction (cPAC) where aberration profiles are computed at a preselected depth and applied to the beamformer's time delays. For all experiments, LAPAC shows an average of 1 to 2 dB higher contrast than cPAC, and enhancements of 3 to 7 dB with respect to the uncorrected cases. We conclude that LAPAC may be a viable option to enhance ultrasound image quality images even in the presence of clinically relevant aberrating conditions.

超音波画像の高分解能化のための位相コヒーレンス因子の効果の増強

超音波画像の高分解能化のための位相コヒーレンス因子の効果の増強

Improvement of range spatial resolution of medical ultrasound imaging by element-domain signal processing

The range spatial resolution is an important factor determining the image quality in ultrasonic imaging. The range spatial resolution in ultrasonic imaging depends on the ultrasonic pulse length, which is determined by the mechanical response of the piezoelectric element in an ultrasonic probe. To improve the range spatial resolution without replacing the transducer element, in the present study, methods based on maximum likelihood (ML) estimation and multiple signal classification (MUSIC) were proposed. The proposed methods were applied to echo signals received by individual transducer elements in an ultrasonic probe. The basic experimental results showed that the axial half maximum of the echo from a string phantom was improved from 0.21 mm (conventional method) to 0.086 mm (ML) and 0.094 mm (MUSIC).