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Two-dimensional (2-D) arrays offer volumetric imaging capabilities without the need for probe translation or rotation. A sparse array with elements seeded in a tapering spiral pattern enables one-to-one connection to an ultrasound machine, thus allowing flexible transmission and reception strategies. To test the concept of sparse spiral array imaging, we have designed, realized, and characterized two prototype probes designed at 2.5-MHz low-frequency (LF) and 5-MHz high-frequency (HF) center frequencies. Both probes share the same electronic design, based on piezoelectric ceramics and rapid prototyping with printed circuit board substrates to wire the elements to external connectors. Different center frequencies were achieved by adjusting the piezoelectric layer thickness. The LF and HF prototype probes had 88% and 95% of working elements, producing peak pressures of 21 and 96 kPa/V when focused at 5 and 3 cm, respectively. The one-way -3-dB bandwidths were 26% and 32%. These results, together with experimental tests on tissue-mimicking phantoms, show that the probes are viable for volumetric imaging.

Spiral array transducers with a sparse 2-D aperture have demonstrated their potential in realizing 3-D ultrasound imaging with reduced data rates. Nevertheless, their feasibility in high-volume-rate imaging based on unfocused transmissions has yet to be established. From a metrology standpoint, it is essential to characterize the acoustic field of unfocused transmissions from spiral arrays not only to assess their safety but also to identify the root cause of imaging irregularities due to the array's sparse aperture. Here, we present a field profile analysis of unfocused transmissions from a density-tapered spiral array transducer (256 hexagonal elements, 220- [Formula: see text] element diameter, and 1-cm aperture diameter) through both simulations and hydrophone measurements. We investigated plane- and diverging-wave transmissions (five-cycle, 7.5-MHz pulses) from 0° to 10° steering for their beam intensity characteristics and wavefront arrival time profiles. Unfocused firings were also tested for B-mode imaging performance (ten compounded angles, -5° to 5° span). The array was found to produce unfocused transmissions with a peak negative pressure of 93.9 kPa at 2 cm depth. All transmissions steered up to 5° were free of secondary lobes within 12 dB of the main beam peak intensity. All wavefront arrival time profiles were found to closely match the expected profiles with maximum root-mean-squared errors of [Formula: see text] for plane wave (PW) and [Formula: see text] for diverging wave. The B-mode images showed good spatial resolution with a penetration depth of 22 mm in PW imaging. Overall, these results demonstrate that the density-tapered spiral array can facilitate unfocused transmissions below regulatory limits (mechanical index: 0.034; spatial-peak, pulse-average intensity: 0.298 W/cm2) and with suppressed secondary lobes while maintaining smooth wavefronts.

An ultrasound sparse array consists of a sparse distribution of elements over a 2-D aperture. Such an array is typically characterized by a limited number of elements, which in most cases is compatible with the channel number of the available scanners. Sparse arrays represent an attractive alternative to full 2-D arrays that may require the control of thousands of elements through expensive application-specific integrated circuits (ASICs). However, their massive use is hindered by two main drawbacks: the possible beam profile deterioration, which may worsen the image contrast, and the limited signal-to-noise ratio (SNR), which may result too low for some applications. This article reviews the work done for three decades on 2-D ultrasound sparse arrays for medical applications. First, random, optimized, and deterministic design methods are reviewed together with their main influencing factors. Then, experimental 2-D sparse array implementations based on piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) technologies are presented. Sample applications to 3-D (Doppler) imaging, super-resolution imaging, photo-acoustic imaging, and therapy are reported. The final sections discuss the main shortcomings associated with the use of sparse arrays, the related countermeasures, and the next steps envisaged in the development of innovative arrays.

Stress echocardiography helps to diagnose cardiac diseases that cannot easily be detected or do not even manifest at rest. In clinical practice, assessment of the stress test is usually performed visually and, therefore, in a qualitative and subjective way. Although speckle tracking echocardiography (STE) has been proposed for the quantification of function during stress, its time resolution is inadequate at high heart rates. Recently, high-frame-rate (HFR) imaging approaches have been proposed together with dedicated STE algorithms capable of handling small interframe displacements. The aim of this study was to determine if HFR STE is effective in assessing strain and strain rate parameters during echocardiographic stress testing. Specifically, stress echocardiography, at four different workload intensities, was performed in 25 healthy volunteers. At each stress level, HFR images from the apical four-chamber view were recorded using the ULA-OP 256 experimental scanner. Then, the myocardium was tracked with HFR STE, and strain and strain rate biomarkers were extracted to further analyze systolic and diastolic (early and late) peaks, as well as a short-lived isovolumic relaxation peak during stress testing. The global systolic strain response was monophasic, revealing a significant (p0.001) increase at low stress but then reaching a plateau. In contrast, all strain rate indices linearly increased (p0.001) with increasing stress level. These findings are in line with those reported using tissue Doppler imaging and, thus, indicate that HFR STE can be a useful tool in assessing cardiac function during stress echocardiography.

Since the 1970s, when portable transducer arrays were first introduced for medical ultrasound imaging, they have undergone substantial technological developments. The development of advanced arrays is often motivated by the need to achieve high diagnostic and therapeutic efficacy and to serve new fields of application. In the past few decades, medical ultrasound array design has been an active research area with challenging technical requirements that continually seek to reduce physical size, improve sensitivity, optimize the number of array elements, realize wide bandwidth, and achieve high output power. The need to devise arrays with increased performance has concurrently stimulated advances in transducer technologies, microelectronics, and array layout design. Nowadays, representative examples can be found for both 2-D and 3-D applications such as high-intensity-focused ultrasound arrays, very-high-frequency or dual-frequency probes, kerf-less arrays, 2-D sparse arrays, and probes with embedded application-specific integrated circuits. The emergence of these advanced arrays has, in turn, stimulated the development of novel, customized transmission and reception approaches, image reconstruction algorithms, and data recovery strategies to exploit or deal with the peculiarities of a specific array.

High-frame-rate vector Doppler methods are used to measure blood velocities over large 2-D regions, but their accuracy is often estimated over a short range of depths. This article thoroughly examines the dependence of velocity measurement accuracy on the target position. Simulations were carried out on flat and parabolic flow profiles, for different Doppler angles, and considering a 2-D vector flow imaging (2-D VFI) method based on plane wave transmission and speckle tracking. The results were also compared with those obtained by the reference spectral Doppler (SD) method. Although, as expected, the bias and standard deviation generally tend to worsen at increasing depths, the measurements also show the following. First, the errors are much lower for the flat profile (from ≈ -4 ± 3% at 20 mm to ≈ -17 ± 4% at 100 mm) than for the parabolic profile (from ≈ -4 ± 3% to ≈ -38 ±%). Second, only part of the relative estimation error is related to the inherent low resolution of the 2-D VFI method. For example, even for SD, the error bias increases (on average) from -0.7% (20 mm) to -17% (60 mm) up to -26% (100 mm). Third, conversely, the beam divergence associated with the linear array acoustic lens was found to have a great impact on the velocity measurements. By simply removing such lens, the average bias for 2-D VFI at 60 and 100 mm dropped down to -9.4% and -19.4%, respectively. In conclusion, the results indicate that the transmission beam broadening on the elevation plane, which is not limited by reception dynamic focusing, is the main cause of velocity underestimation in the presence of high spatial gradients.

Background As many cardiac diseases do not lead to symptoms under resting conditions, subjecting the patient to a stress (e.g. exercise) test is clinical routine to reveal disease. Quantification of cardiac function using speckle tracking echocardiography (STE) during such stress test remains a challenge not only as overall image quality typically drops but also as the heart rate increases which results in an unfavourable heart-rate-to-frame-rate ratio as required for robust tracking. Moreover, conventional STE runs at relatively low frame rate thereby under-resolving peak strain rate (SR) values – which is particularly true at high heart rates – while this biomarker has been shown to better disclose stress-induced disease than strain using Tissue Doppler Imaging (TDI)-based strain methodologies. Purpose The aim of this study was therefore to test whether HFR STE could reveal the strain / SR response described previously in the TDI literature. Methods Stress echocardiography was performed in 25 healthy volunteers at four different stress levels (baseline, 25%, 50% and 66% of maximum workload) using a supine bike. Apical 4-chamber views were recorded using the ULA-OP 256 experimental scanner running a HFR sequence based on the compounding of 6 diverging wave transmits to achieve a frame rate of 833Hz. A myocardial contour was manually drawn on the reconstructed images and tracked during the cardiac cycle by a custom-made 2D HFR STE algorithm (DOI: 10.1109/TUFFC.2020.2985451). Then, strain and strain rate curves were computed from which systolic (S) and diastolic (early (E) and late (A)) peak values, as well as the short-lived isovolumic relaxation peak (IVR) were extracted. Finally, these values were compared amongst the different stress levels using a single factor ANOVA. Results Some datasets had to be discarded as the contour was not properly tracked at a visual assessment of an expert cardiologist. Tracking was feasible in 92%, 98%, 80% and 64% of cases, for baseline, 25%, 50% and 66% of maximum workload, respectively. The decreasing feasibility with exercise level is in line with what is seen clinically, as volunteers' breathing is faster and heavier with increasing exercise level making the imaging more challenging. The extracted strain and SR curves showed a physiological pattern (Fig. 1a). As shown in Fig. 1b, the global systolic strain response was biphasic, showing a significant increase at low stress level but then reaching a plateau. In contrast, all SR indices linearly increased at each stress level. Conclusion Strain rate clinical markers extracted with HFR STE are concordant with what was reported in TDI literature. These findings show that HFR STE allows to assess cardiac function adequately during stress echocardiography. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): FWO - Research Foundation Flanders

Background Left ventricular (LV) strain rate (SR) during isovolumic relaxation (SRIVR) and early diastolic filling (SRe) has previously been shown to correlate with the invasive gold standard for LV diastolic function (i.e. the time constant of LV pressure decay tau). However, the translation of these biomarkers to the clinic has been hampered by technical limitations. Indeed, conventional speckle tracking (STE) is limited by its temporal resolution, whereas tissue Doppler imaging (TDI) is angle-dependent, labor-intensive and thus rarely used clinically nowadays. Purpose The aim of this study was to show that these limitations could be overcome by using a recently proposed STE algorithm operating on high frame rate (HFR) imaging data. Methods 37 subjects (age: 64±12, 81% male) were included in the study; 16 had cardiac amyloidosis, 12 were undergoing clinically indicated left and/or right heart cardiac catheterization and 9 were healthy volunteers. Since the sequence of left ventricular activation and thus the repolarization process (i.e. relaxation) starts at mid septum, we measured SRIVR and SRe in the mid septal segment in an apical 4 chamber view using a commercially available clinical system with: (1) TDI (frame rate (FR) ∼142 Hz); (2) STE (FR ∼65 Hz). Moreover, subjects were scanned with HD-PULSE, an experimental high frame ultrasound scanner (FR ∼915 Hz) and then a manually placed contour was tracked during the cardiac cycle by a custom-made 2D HFR STE algorithm, to compute and extract SRIVR and SRe from the mid septum. Since TDI is considered the reference method to assess SR, conventional as well as HFR STE values were correlated against the TDI SR values. Results In 3 subjects, SRIVR could not be reliably assessed with the clinical STE approach, which we attributed to the relatively low temporal resolution of the images; all other measurements could be made in all subjects. For both biomarkers, HFR STE values correlated better with the TDI reference measurements than the clinical STE estimates (Fig.1). The latter estimates showed a systematic underestimation (bias −0.19 1/s (p Conclusions These results show that HFR STE offers a reliable way to assess novel biomarkers of diastolic function in a user-friendly manner and can therefore facilitate their incorporation to the clinical practice. Funding Acknowledgement Type of funding sources: None.

Accurate assessment of diastolic function is essential not only for diagnostic purposes, but also for estimates on prognosis. Several studies have shown the correlation of novel diastolic biomarkers, i.e., Strain Rate (SR) during isovolumic relaxation (SR IVR ) and early filling (SR e ), with the invasive gold standard assessment of left ventricular (LV) diastolic function, thus indicating their additive value for an improved patient classification. The incorporation of these biomarkers in clinical practice, however, has proven problematic due to the technical limitations of the current methods used for deformation imaging: tissue Doppler imaging (TDI) and speckle tracking echocardiography (STE). This study aimed to overcome these limitations by using a STE algorithm on high frame rate (HFR) acquisitions. SR IVR and SRe values were estimated from 37 subjects with all three abovementioned techniques. HFR STE values correlated better with the TDI reference measurements, while the clinical STE values systematically underestimated them. These results show that HFR STE can assess novel biomarkers of diastolic function accurately, in a user-friendly manner.

The use of wide beams in transmission and parallel beamforming in reception allows increasing the frame rate of ultrasound images. Any misalignment between the transmitted beams and the received lines determines spatial discontinuities that appear as strip-like imaging artifacts. The use of synthetic transmit beams (STB) has been shown effective to reduce these artifacts, although with an increased computational burden that might compromise its real-time implementation. This paper shows how STB was implemented on the ULA-OP 256 research scanner in combination with both multiline and diverging wave transmissions. The system performance is demonstrated in cardiac tissue Doppler imaging applications, showing that strip-like artifacts can be attenuated and, at the same time, real-time B-mode images can be obtained at a rate of 688 frames/s, which is high enough to process tissue Doppler images up to 230 frame/s, depending on the ensemble length.

The transmission of plane waves and parallel beamforming have been successfully used to improve the performance of Colour Flow Mapping (CFM) systems, as they allow the exploitation of effective wall filters in continuous time and autocorrelations with very long ensemble lengths. However, although hundreds of colour maps could be produced in realtime, the computing power requirements still compromise either the frame-rate, the region of interest (ROI) size or the spatial resolution. In this paper, the combination of real-time PW CFM @ 250 frames/sec with the virtual real-time modality of ULA-OP 256, is shown ideal for the investigation of rapidly changing flow at improved spatial resolution and extended ROI size.

Ultrasound open platforms are ideal instruments to experiment innovative modalities in transmission and reception. So far, the high number of channels, involved in applications based on 2-D arrays, has been managed through complex combinations of scanners and personal computers, at the expense of ease of use, portability, and real-time performance. This work presents a new flexible architecture, based on the full (hardware and software) synchronization of multiple ULA-OP 256 scanners, in a Master-Slave architecture, and controlled by a single computer. For the hardware synchronization, a Master scanner provides two digital signals (clock and transmission start) to the Slave scanners, while the software synchronization is managed by two purposely developed MATLAB classes that allow the user to interface the scanners as a single system. To demonstrate the effectiveness of the architecture, a 512-channel prototype, composed of two ULA-OP 256 connected to a host PC and to a sparse array, was implemented and tested through applications that demonstrate the high real-time processing power.

Recently, high frame rate (HFR) imaging approaches have been proposed together with dedicated speckle tracking (STE) algorithms capable of handling small interframe displacements. However, the compromised quality of HFR images affects the feasibility of STE. Therefore, the aim of this study was to assess whether Singular Value Decomposition (SVD) filtering could reduce HFR imaging artifacts and, possibly, improve STE feasibility. Specifically, 2-step SVD filter approach was proposed and tested on in vivo recordings, which were characterized by different heart rates, artefacts and noise levels. For both steps, the filter parameters were optimized to reject artifacts/noise. Then, filtered and original B-mode clips were shown side-by-side to an expert cardiologist, to select the preferred image based on visual reading. Although the clutter was suppressed as expected, the expert cardiologist preferred filtered images in 25% of the cases only. Moreover, the extracted systolic peak values of the Global Longitudinal Strain were not significantly different (p = 0.07), even if there was a tendency for slightly lower values after filtering. In general, these preliminary findings suggest that SVD filtering might not significantly affect HFR STE in a clinical setting.

Coherent multi-transducer ultrasound (CoMTUS) creates an extended effective aperture through coherent combination of multiple arrays that results in significantly improved images, with enhanced resolution, extended field-of-view, and higher sensitivity. The approach has been previously validated in 2-D imaging using two linear arrays constrained into the same elevational plane. However, any misalignment between the arrays in the elevational plane could compromise the performance of the method. Extending CoMTUS to 3-D would generalise the framework and provide a mitigation for misalignment issues. In this study, CoMTUS is implemented and demonstrated for the first time in 3-D imaging using a pair of 2-D 256-element sparse arrays, which allows the channel count and the amount of data to be kept within feasible bounds. Simulated results show that, in comparison to a single array system using the same number of active elements, CoMTUS leads to significant improvements in resolution, contrast, and contrast-to-noise-ratio.

Non-standard scan modes, based on the transmission of plane and diverging waves, are the key to enable high frame rate and volumetric imaging modalities and are promising in the implementation of novel system architectures for ultra-integrated (high channel count) scanners. So far, capacitive micromachined ultrasonic transducer (CMUT) arrays have been benchmarked against standard piezoelectric transducers only in standard modalities. In this work, a 256-element CMUT probe and a state-of-the art single crystal composite piezoelectric probe, sharing the same array geometry and bandwidth, are compared in plane wave (PW) imaging mode. The CMUT probe provided, on average, improved signal power (+9 dB) and contrast (+3 dB) for all PW aperture angles. Also, compared to the piezoelectric probe, the CMUT one offered comparable axial resolution, but slightly worse (10%) lateral resolution.

The market of portable, mid-range ultrasound imaging systems for ambulatory care has steadily grown. Hence, high-resource-demanding modalities must be adapted to be compliant with these systems. A high frame rate (HFR) 2-D vector flow imaging (VFI) algorithm was recently shown suitable for real-time implementation in a high-end system over a limited-size region of interest (ROI). In this work, we propose a solution to lower the computational load of HFR VFI without reducing the size of the ROI. The VFI algorithm estimates the displacement between kernels in consecutive frames. The pulse repetition interval (PRI) impacts the aliasing limit, but the needed estimation rate depends on the flow temporal gradients and may correspond to several PRIs. The VFI algorithm can thus be limited to pairs of consecutive frames, having a time-lag longer than the PRI. The higher the time-lag, the fewer the required estimates per second. The novel algorithm was tested in simulation on steady parabolic flow conditions. For time lags between 0.4 and 4 ms, the accuracy was in the range [2.4%, 3.1%] while the standard deviation slightly increased from 7.4% to 10.9%. On the other hand, the computational load decreases up to 85% for a time-lag equal to 10 times the PRI. Preliminary tests on pulsatile flow confirm the efficacy of such an approach.

Several high frame-rate ultrasound imaging techniques have been recently proposed which allow to increase image acquisition rates by transmitting multiple focused beams simultaneously or defocused waves, and reconstructing multiple scan lines in parallel. These methods, which include for example multi-line transmission, plane/diverging wave imaging, and parallel beamforming, have proved to be successful in increasing the frame-rate, but on the other hand they have also showed some limitations in terms of the achievable image quality, partly degrading image contrast and resolution. Different solutions have been proposed to address this problem so far, such as the use of advanced receive beamforming algorithms able to improve the beam shape and the quality of the obtained image. Among these, beamformers based on backscattered signals spatial coherence, as e.g. Filtered Delay Multiply and Sum beamforming, Coherence-Factor-based methods, and Short Lag Spatial Coherence Imaging, have gained increasing attention for their ability to enhance image contrast and suppress clutter. In this presentation, an overview of some recent results obtained by jointly exploiting high frame-rate ultrasound imaging and coherence-based beamforming methods will be presented, showing through experimental tests the performance improvement achievable with these techniques.

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.

Volumetric ultrasound imaging of blood flow with microbubbles enables a more complete visualization of the microvasculature. Sparse arrays are ideal candidates to perform volumetric imaging at reduced manufacturing complexity and cable count. However, due to the small number of transducer elements, sparse arrays often come with high clutter levels, especially when wide beams are transmitted to increase the frame rate. In this study, we demonstrate with a prototype sparse array probe and a diverging wave transmission strategy, that a uniform transmission field can be achieved. With the implementation of a spatial coherence beamformer, the background clutter signal can be effectively suppressed, leading to a signal to background ratio improvement of 25 dB. With this approach, we demonstrate the volumetric visualization of single microbubbles in a tissue-mimicking phantom as well as vasculature mapping in a live chicken embryo chorioallantoic membrane.