In recent years, transcranial photoacoustic (TPA) imaging has become a popular modality for diagnosis of brain disorders. However, due to the presence of skull, TPA images are strongly degraded. Acoustically, this degradation is mainly categorized into the phase aberration, mode-converted shear waves, and multiple scattering. Previous studies numerically investigated the effects of mode-converted shear waves and multiple scattering on TPA images while the phase aberration caused by the skull was ignored and a conventional delay-and-sum method was employed for reconstructing TPA images. In this paper, we investigate these effects while a refraction-corrected image reconstruction approach is used to form TPA images. This approach enables separating the effects of phase aberration, mode-converted shear wave and multiple scattering. A realistic human temporal bone based on a MicroCT was used in the numerical model. In average for all the absorbers, the power of the artifacts caused by the mode-converted shear wave and multiple scattering are -13.7 dB and -20.1 dB when the refraction is corrected during image formation, respectively. These values were -7.9 and -18.8 if the conventional reconstruction is used. Accounting for phase aberration enables accurate quantification of the effects of the mode-converted shear waves and multiple scattering, which is necessary for evaluating the methods developed for degrading these effects.

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High frame rate three-dimensional (3D) ultrasound imaging would offer excellent possibilities for the accurate assessment of carotid artery diseases. This calls for a matrix transducer with a large aperture and a vast number of elements. Such a matrix transducer should be interfaced with an application-specific integrated circuit (ASIC) for channel reduction. However, the fabrication of such a transducer integrated with one very large ASIC is very challenging and expensive. In this study, we develop a prototype matrix transducer mounted on top of multiple identical ASICs in a tiled configuration. The matrix was designed to have 7680 piezoelectric elements with a pitch of 300 μm × 150 μm integrated with an array of 8 × 1 tiled ASICs. The performance of the prototype is characterized by a series of measurements. The transducer exhibits a uniform behavior with the majority of the elements working within the −6 dB sensitivity range. In transmit, the individual elements show a center frequency of 7.5 MHz, a −6 dB bandwidth of 45%, and a transmit efficiency of 30 Pa/V at 200 mm. In receive, the dynamic range is 81 dB, and the minimum detectable pressure is 60 Pa per element. To demonstrate the imaging capabilities, we acquired 3D images using a commercial wire phantom.

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In a recent study, we proposed a technique to correct aberration caused by the skull and reconstruct a transcranial B-mode image with a refraction-corrected synthetic aperture imaging (SAI) scheme. Given a sound speed map, the arrival times were calculated using a fast marching technique (FMT), which solves the Eikonal equation and, therefore, is computationally expensive for real-time imaging. In this article, we introduce a two-point ray tracing method, based on Fermat's principle, for fast calculation of the travel times in the presence of a layered aberrator in front of the ultrasound probe. The ray tracing method along with the reconstruction technique is implemented on a graphical processing unite (GPU). The point spread function (PSF) in a wire phantom image reconstructed with the FMT and the GPU implementation was studied with numerical synthetic data and experiments with a bone-mimicking plate and a sagittally cut human skull. The numerical analysis showed that the error on travel times is less than 10% of the ultrasound temporal period at 2.5 MHz. As a result, the lateral resolution was not significantly degraded compared with images reconstructed with FMT-calculated travel times. The results using the synthetic, bone-mimicking plate, and skull dataset showed that the GPU implementation causes a lateral/axial localization error of 0.10/0.20, 0.15/0.13, and 0.26/0.32 mm compared with a reference measurement (no aberrator in front of the ultrasound probe), respectively. For an imaging depth of 70 mm, the proposed GPU implementation allows reconstructing 19 frames/s with full synthetic aperture (96 transmission events) and 32 frames/s with multiangle plane wave imaging schemes (with 11 steering angles) for a pixel size of $200~\mu \text{m}$. Finally, refraction-corrected power Doppler imaging is demonstrated with a string phantom and a bone-mimicking plate placed between the probe and the moving string. The proposed approach achieves a suitable frame rate for clinical scanning while maintaining the image quality.

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While transcranial ultrasound imaging is a promising diagnostic modality, it is still hindered due to phase aberration and multiple scattering caused by the skull. In this paper, we compare near-field phase-screen modeling (PS) to a geometry-based phase aberration correction technique (GB) when an ultrafast imaging sequence (five plane waves tilted from −15 to +15 degrees in the cutaneous tissue layer) is used for data acquisition. With simulation data, the aberration profile (AP) of two aberrator models (flat and realistic temporal bone) was estimated in five isoplanatic patches, while the wave-speed of the brain tissue surrounding the point targets was either modeled homogeneously (ideal) or slightly heterogeneously to generate speckle (for mimicking a more realistic brain tissue). For the experiment, a phased array P4-1 transducer was used to image a wire phantom; a 4.2-mm-thick bone-mimicking plate was placed in front of the probe. The AP of the plate was estimated in three isoplanatic patches. The numerical results indicate that, while all the scatterers are detectable in the image reconstructed by the GB method, many scatterers are not detected with the PS method when the dataset used for AP estimation is generated with a realistic bone model and heterogeneous brain tissue. The experimental results show that the GB method increases the signal-to-clutter ratio (SCR) by 7.5 dB and 6.5 dB compared to the PS and conventional reconstruction methods, respectively. The GB method reduces the axial/lateral localization error by 1.97/0.66 mm and 2.08/0.7 mm compared to the PS method and conventional reconstruction, respectively. The lateral spatial resolution (full-width-half-maximum) is also improved by 0.1 mm and 1.06 mm compared to the PS method and conventional reconstruction, respectively. Our comparison study suggests that GB aberration correction outperforms the PS method when an ultrafast multi-angle plane wave sequence is used for transcranial imaging with a single transducer. @en

Transcranial ultrasound imaging (TUI) is a diagnostic modality with numerous applications, but unfortunately, it is hindered by phase aberration caused by the skull. In this article, we propose to reconstruct a transcranial B-mode image with a refraction-corrected synthetic aperture imaging (SAI) scheme. First, the compressional sound velocity of the aberrator (i.e., the skull) is estimated using the bidirectional headwave technique. The medium is described with four layers (i.e., lens, water, skull, and water), and a fast marching method calculates the travel times between individual array elements and image pixels. Finally, a delay-and-sum algorithm is used for image reconstruction with coherent compounding. The point spread function (PSF) in a wire phantom image and reconstructed with the conventional technique (using a constant sound speed throughout the medium), and the proposed method was quantified with numerical synthetic data and experiments with a bone-mimicking plate and a human skull, compared with the PSF achieved in a ground truth image of the medium without the aberrator (i.e., the bone plate or skull). A phased-array transducer (P4-1, ATL/Philips, 2.5 MHz, 96 elements, pitch $=$ 0.295 mm) was used for the experiments. The results with the synthetic signals, the bone-mimicking plate, and the skull indicated that the proposed method reconstructs the scatterers with an average lateral/axial localization error of 0.06/0.14 mm, 0.11/0.13 mm, and 1.0/0.32 mm, respectively. With the human skull, an average contrast ratio (CR) and full-width-half-maximum (FWHM) of 37.1 dB and 1.75 mm were obtained with the proposed approach, respectively. This corresponds to an improvement of CR and FWHM by 7.1 dB and 36% compared with the conventional method, respectively. These numbers were 12.7 dB and 41% with the bone-mimicking plate.

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In photoacoustic tomography (PAT) systems, the tangential resolution decreases due to the finite size of the transducer as the off-center distance increases. To address this problem, we propose a multi-angle detection approach in which the transducer used for data acquisition rotates around its center (with specific angles) as well as around the scanning center. The angles are calculated based on the central frequency and diameter of the transducer and the radius of the region-of-interest (ROI). Simulations with point-like absorbers (for point-spread-function evaluation) and a vasculature phantom (for quality assessment), and experiments with ten 0.5 mm-diameter pencil leads and a leaf skeleton phantom are used for evaluation of the proposed approach. The results show that a location-independent tangential resolution is achieved with 150 spatial sampling and central rotations with angles of ±8°/±16°. With further developments, the proposed detection strategy can replace the conventional detection (rotating a transducer around ROI) in PAT.

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Photoacoustic imaging (PAI) is an emerging functional and molecular imaging technology that has attracted much attention in the past decade. Recently, many researchers have used the vantage system from Verasonics for simultaneous ultrasound (US) and photoacoustic (PA) imaging. This was the motivation to write on the details of US/PA imaging system implementation and characterization using Verasonics platform. We have discussed the experimental considerations for linear array based PAI due to its popularity, simple setup, and high potential for clinical translatability. Specifically, we describe the strategies of US/PA imaging system setup, signal generation, amplification, data processing and study the system performance.

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In circular photoacoustic computed tomography, for data acquisition, a single-element transducer rotates around the region of interest (ROI). Due to the limited acceptance angle of the finite size transducer, the reconstructed image becomes blurred, and tangential resolution/contrast degrades in off-center locations in the ROI. In this paper, we propose a compensation method in which in addition to the circular scanning, the transducer rotates around its center (with specific angles) at each detection point. The superposition of these central rotations and non-rotated transducer mimics a virtual detector with a wide acceptance angle. The angles are calculated based on the central frequency and diameter of the transducer and the radius of the region-of-interest. Three types of numerical phantom (point-like, vasculature and Derenzo) were used to evaluate the performance of our method. Features of Olympus NDT, V326-SU transducer were used to assemble the numerical data. The results show that the proposed method provides better structural information by lowering the image blurring, improves the tangential resolution by 90% and increases peak signal-to-noise ratio by 14%.@en

Simultaneous visualization of the teeth and periodontium is of significant clinical interest for image-based monitoring of periodontal health. We recently reported the application of a dual-modality photoacoustic-ultrasound (PA-US) imaging system for resolving periodontal anatomy and periodontal pocket depths in humans. This work utilized a linear array transducer attached to a stepper motor to generate 3D images via maximum intensity projection. This prior work also used a medical head immobilizer to reduce artifacts during volume rendering caused by motion from the subject (e.g., breathing, minor head movements). However, this solution does not completely eliminate motion artifacts while also complicating the imaging procedure and causing patient discomfort. To address this issue, we report the implementation of an image registration technique to correctly align B-mode PA-US images and generate artifact-free 2D cross-sections. Application of the deshaking technique to PA phantoms revealed 80% similarity to the ground truth when shaking was intentionally applied during stepper motor scans. Images from handheld sweeps could also be deshaken using an LED PA-US scanner. In ex vivo porcine mandibles, pigmentation of the enamel was well-estimated within 0.1 mm error. The pocket depth measured in a healthy human subject was also in good agreement with our prior study. This report demonstrates that a modality-independent registration technique can be applied to clinically relevant PA-US scans of the periodontium to reduce operator burden of skill and subject discomfort while showing potential for handheld clinical periodontal imaging.

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Transcranial ultrasound imaging is a suitable technology for diagnosis of strokes as it is safe, portable, relatively inexpensive and available in emergency medicine services, however it currently offers poor image quality due to the phase aberration caused by the human skull. In this work, we evaluate an approach for two-dimensional transcranial ultrasound imaging through the temporal window of a sagittally-cut human skull using the commercial P4-1 phased-array probe, where the position and true geometry of the bone layer is estimated for accurate phase aberration correction. The medium is described with four layers (probe lens, soft tissue, skull, soft tissue). A synthetic aperture imaging scheme is used as the transmission of spherical wave-fronts facilitates the modeling of refraction. First, the bidirectional headwave method estimates the compressional wave-speed in the temporal bone. Next, a fast marching method calculates the travel times between individual array elements and image pixels to be used with delay-and-sum reconstruction algorithm. Sound speed maps are generated with adaptive beamforming including the successive segmentation of the near and far surfaces of the cortical bone layer. The proposed method reconstructs the scatterers with an average lateral and axial localization error of about 1.25 mm and 0.37 mm, compared to the ground truth, respectively. In average, it improves the contrast ratio and lateral resolution by 7 dB and 36%, compared to the conventional method, respectively.@en

Recently, we realized a prototype matrix transducer consisting of 48 rows of 80 elements on top of a tiled set of Application Specific Integrated Circuits (ASICs) implementing a row-level control connecting one transmit/receive channel to an arbitrary subset of elements per row. A fully sampled array data acquisition is implemented by a column-by-column (CBC) imaging scheme (80 transmit-receive shots) which achieves 250 volumes/second (V/s) at a pulse repetition frequency of 20 kHz. However, for several clinical applications such as carotid pulse wave imaging (CPWI), a volume rate of 1000 per second is needed. This allows only 20 transmit-receive shots per 3D image. In this study, we propose a shifting aperture scheme and investigate the effects of receive/transmit aperture size and aperture shifting step in the elevation direction. The row-level circuit is used to interconnect elements of a receive aperture in the elevation (row) direction. An angular weighting method is used to suppress the grating lobes caused by the enlargement of the effective elevation pitch of the array, as a result of element interconnection in the elevation direction. The effective aperture size, level of grating lobes, and resolution/sidelobes are used to select suitable reception/transmission parameters. Based on our assessment, the proposed imaging sequence is a full transmission (all 80 elements excited at the same time), a receive aperture size of 5 and an aperture shifting step of 3. Numerical results obtained at depths of 10, 15, and 20 mm show that, compared to the fully sampled array, the 1000 V/s is achieved at the expense of, on average, about two times wider point spread function and 4 dB higher clutter level. The resulting grating lobes were at -27 dB. The proposed imaging sequence can be used for carotid pulse wave imaging to generate an informative 3D arterial stiffness map, for cardiovascular disease assessment.

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Phase aberration in transcranial ultrasound imaging (TUI) caused by the human skull leads to an inaccurate image reconstruction. In this article, we present a novel method for estimating the speed of sound and an adaptive beamforming technique for phase aberration correction in a flat polyvinylchloride (PVC) slab as a model for the human skull. First, the speed of sound of the PVC slab is found by extracting the overlapping quasi-longitudinal wave velocities of symmetrical Lamb waves in the frequency-wavenumber domain. Then, the thickness of the plate is determined by the echoes from its front and back side. Next, an adaptive beamforming method is developed, utilizing the measured sound speed map of the imaging medium. Finally, to minimize reverberation artifacts caused by strong scatterers (i.e., needles), a dual probe setup is proposed. In this setup, we image the medium from two opposite directions, and the final image can be the minimum intensity projection of the inherently co-registered images of the opposed probes. Our results confirm that the Lamb wave method estimates the longitudinal speed of the slab with an error of 3.5% and is independent of its shear wave speed. Benefiting from the acquired sound speed map, our adaptive beamformer reduces (in real time) a mislocation error of 3.1, caused by an 8 mm slab, to 0.1 mm. Finally, the dual probe configuration shows 7 dB improvement in removing reverberation artifacts of the needle, at the cost of only 2.4-dB contrast loss. The proposed image formation method can be used, e.g., to monitor deep brain stimulation procedures and localization of the electrode(s) deep inside the brain from two temporal bones on the sides of the human skull.

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In the above article [1], we mentioned that the superposition of the different symmetric (S) modes in the frequencywavenumber (f-k) domain results in a high-intensity region where its slope corresponds to the longitudinal wave speed in the slab. However, we have recently understood that this highintensity region belongs to the propagation of a wave called lateral wave or head wave [2]-[5]. It is generated if the longitudinal sound speed of the aberrator (i.e., the PVC slab) is larger than that of water and if the incident wavefront is curved. When the incidence angle at the interface between water and PVC is near the critical angle, the refracted wave in PVC reradiates a small part of its energy into the fluid (i.e., the head wave). As discussed in [4], if the thickness of the waveguide is larger than the wavelength, the first arriving signal is the head wave. This is also the case in our study [1] where the ultrasound wavelength of a compressional wave in PVC was close to 1 mm, and a PVC slab with a thickness of 8 mm was used. In this Erratum, numerical simulations (with SimSonic solver [5]) and experimental measurements (with the same PVC slab used in [1]) are conducted to investigate the propagation of the Lamb waves and head wave in detail, for the specific configuration studied in [1]. The pitch and element width of the P4-1 probe were used to assemble the numerical signals [see Fig. 1(a)]. If all the data simulated for the P4-1 probe is used, there indeed is a region with a slope [see Fig. 1(b)], but this has a low intensity, meaning that the head wave has a relatively low amplitude compared to the specular reflections. Once the head wave is isolated, the sound speed can be estimated with a 0.3% error from the f-k domain plot [see Fig. 1(c)]. No significant difference is observed between Fig. 1(b) and (d), in which the head wave is muted. Our experimental results show that if only the head wave (the first arriving signal) is used [see Fig. 2(b)], the slope of the linear fitting in the f-k domain also yields the longitudinal sound speed of the PVC with a 0.3% error. Of note, the signal processing (i.e., linear fitting in the f-k domain) used in our study [1] still works for the head wave and is correct provided that the aberrator is parallel to the probe [6]. Also, in [1, p. 6], it is mentioned that “the curved structure of the skull might lead to other types of modes, such as the torsional modes.” Here, we acknowledge that this sentence is not correct, as torsional modes only exist in cylindrical waveguides or rectangular bars. We would like to mention that Guillaume Renaud is added as a coauthor to acknowledge his contribution to the findings reported in this Erratum.

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This paper presents the integration steps towards a prototype forward-looking intra-vascular ultrasound probe. An ASIC (Application Specific Integrated Circuit) is laser cut to create a 1.6mm circular shaped die that can be integrated at the tip of a small catheter. The ASIC is designed such that it can be used to transmit and receive on 64 piezo-electric transducers while using a single cable for the supply, communication, transmit signals and amplified receive signals. Piezo elements are directly integrated on top of the ASIC to create a miniature probe.Electrical tests show that the circuitry still operates correctly after laser cutting. The functionality of a prototype has been successfully demonstrated in a 3D imaging experiment.

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To accurately investigate the state of the carotid artery by the local haemodynamics and motion of the plaque using ultrasound, high-frame rate volumetric imaging is necessary. We have specifically designed a matrix array for this purpose. In this proceeding we will focus on imaging a volumetric flow profile using this matrix. For this purpose, we extend a fast frequency domain vector flow imaging method to 3D and perform measurements on a flow phantom. The results indicate that it is feasible to estimate 3D velocity vectors on a 3D grid using our matrix transducer and the proposed algorithm.

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Photoacoustic imaging (PAI) is an emerging label-free and non-invasive modality for imaging biological tissues. PAI has been implemented in different configurations, one of which is photoacoustic computed tomography (PACT) with a potential wide range of applications, including brain and breast imaging. Hemispherical Array PACT (HA-PACT) is a variation of PACT that has solved the limited detection-view problem. Here, we designed an HA-PACT system consisting of 50 single element transducers. For implementation, we initially performed a simulation study, with parameters close to those in practice, to determine the relationship between the number of transducers and the quality of the reconstructed image. We then used the greatest number of transducers possible on the hemisphere and imaged copper wire phantoms coated with a light absorbing material to evaluate the performance of the system. Several practical issues such as light illumination, arrangement of the transducers, and an image reconstruction algorithm have been comprehensively studied.

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Double-stage delay-multiply-and-sum (DS-DMAS) is one of the algorithms proposed for photoacoustic image reconstruction where a linear-array transducer is used to detect signals. This algorithm provides a higher contrast image in comparison with the conventional delay-multiply-and-sum (DMAS) and delay-and-sum (DAS), but it imposes a high computational complexity. In this paper, open accelerators (OpenACC) GPU computation parallel approach is used to lessen the computational time and address the high computational time of the DSDMAS for photoacoustic image reconstruction process. Compared with sequential execution of the DS-DMAS on CPU, a speed-up of approximately 74× is achieved (for an image having 1024 × 1024 pixels). The proposed approach provides possibility to have an accurate reconstructed photoacoustic image with a reasonable frame rate. In addition, the higher the number of the image pixels, the higher speed-up is achieved. Using the suggested GPU implementation, it is feasible to reconstruct photoacoustic images having a size of 128 × 128, and 256 × 256 with a frame rate of 3 and 2, respectively.

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Double-stage delay-multiply-and-sum (DS-DMAS) is an algorithm proposed for photoacoustic image reconstruction. The DS-DMAS algorithm offers a higher contrast than conventional delay-and-sum and delay-multiply and-sum but at the expense of higher computational complexity. Here, we utilized a compute unified device architecture (CUDA) graphics processing unit (GPU) parallel computation approach to address the high complexity of the DS-DMAS for photoacoustic image reconstruction generated from a commercial light-emitting diode (LED)–based photoacoustic scanner. In comparison with a single-threaded central processing unit (CPU), the GPU approach increased speeds by nearly 140-fold for 1024 × 1024 pixel image; there was no decrease in accuracy. The proposed implementation makes it possible to reconstruct photoacoustic images with frame rates of 250, 125, and 83.3 when the images are 64 × 64, 128 × 128, and 256 × 256, respectively. Thus, DS-DMAS can be efficiently used in clinical devices when coupled with CUDA GPU parallel computation.

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Phase aberration of focused ultrasound by tissue structure causes focus degradation and reduces the quality of B-mode images. Refraction at the boundary between subcutaneous fat and muscle is one of the dominant factors behind such degradation. To correct this, we propose a refraction compensation method in which ultrasound is transmitted and received twice. The boundary shape between different tissues is detected by the first ultrasound transmission. Next, ultrasound rays from probe elements to the target are calculated taking refraction into account. Corrected delay times are calculated from the length of the rays and the sound velocity of the medium. Finally, ultrasound is transmitted a second time using the corrected delay time and a B-mode image is created. We evaluate the correction effect of the proposed method by numerical simulation and experiments with non-compensated and refraction-compensated cases of intensity distribution of the focused ultrasound. Results show that focus degradation is effectively corrected by the proposed method.

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