High-frame-rate volume imaging using sparse-random-aperture compounding

Row Transmission for High Volume-Rate Ultrasound Imaging With a Matrix Array

The widely used Vermon 1024-element matrix array for 3-D ultrasound imaging has three blank rows in the elevational direction, which breaks the elevation periodicity, thus degrading volumetric image quality. To bypass the blank rows in elevation while maintaining the steering capability in azimuth, we proposed a row-transmission (RT) scheme to improve 3-D spatial resolution. Specifically, we divided the full array into four apertures, each with multiple rows along the elevation. Each multirow aperture (MRA) was further divided into subapertures to transmit diverging waves (DWs) sequentially. Coherent DW compounding (CDWC) was realized in azimuth, while the elevation was multielement synthetic aperture (M-SA) imaging by regarding each row as an array of dashed line elements. An in-house spatiotemporal coding strategy, cascaded synthetic aperture (CaSA), was incorporated into the RT scheme as RT-CaSA to increase the signal-to-noise ratio (SNR). We compared the proposed RT with conventional bank-by-bank transmission-reception (Bank) and sparse-random-aperture compounding (SRAC) in a wire phantom and the in vivo human abdominal aorta (AA) to assess the performance of anatomical imaging and aortic wall motion estimation. Phantom results demonstrated superior lateral resolution achieved by our RT scheme (+19.52% and +16.88% versus Bank, +15.32% and +19.72% versus SRAC, in the azimuth-depth and elevation-depth planes, respectively). Our RT-CaSA showed excellent contrast ratios (CRs) (+8.19 and +8.08 dB versus Bank, +6.81 and +5.85 dB versus SRAC, +0.99 and +0.90 dB versus RT) and the highest in vivo aortic wall motion estimation accuracy. The RT scheme was demonstrated to have potential for various matrix array-based 3-D imaging research.

Increasing abdominal aortic aneurysm curvature visibility using 3D dual probe bistatic ultrasound imaging combined with probe translation

High frame rate ultrasound (US) imaging techniques in 3D are promising tools for capturing abdominal aortic aneurysms (AAAs) over time, however, with the limited number of channel-to-element connections current footprints are small, which limits the field of view. Moreover, the maximal steering angle of the ultrasound beams in transmit and the maximal receptance angle in receive are insufficient for capturing the curvy shape of the AAA. Therefore, an approach is needed towards large arrays. In this study, high frame rate bistatic 3D US data (17 Hz) were acquired with two synchronized matrix arrays positioned at different locations (multi-aperture imaging) using a translation stage to simulate what a larger array with limited channel-to-element connections can potentially achieve. Acquisitions were performed along an AAA shaped phantom with different probe tilting angles (0 up to ± 30°). The performance of different multi-aperture configurations was quantified using the generalized contrast-to-noise ratio of the wall and lumen (gCNR). Furthermore, a parametric model of the multi-aperture system was used to estimate in which AAA wall regions the contrast is expected to be high. This was evaluated for AAAs with increasing diameters and curvature. With an eight-aperture 0° probe angle configuration a 69 % increase in field of view was measured in the longitudinal direction compared to the field of view of a single aperture configuration. When increasing the number of apertures from two to eight, the gCNR improved for the upper wall and lower wall by 35 % and 13 % (monostatic) and by 36 % and 13 % (bistatic). Contrast improvements up to 22 % (upper wall) and 12 % (lower wall) are achieved with tilted probe configurations compared to non-tilted configurations. Moreover, with bistatic imaging with tilted probe configurations gCNR improvements up to 4 % (upper wall) and 7 % (lower wall) are achieved compared to monostatic imaging. Furthermore, imaging with a larger inter-probe distance improved the gCNR for a ± 15° probe angle configuration. The gCNR has an expected pattern over time, where the contrast is lower when there is more wall motion (systole) and higher when motion is reduced (diastole). Furthermore, a higher frame rate (45 Hz) yields a lower gCNR, because fewer compound angles are used. The results of the parametric model suggest that a flat array is suitable for imaging AAA shapes with limited curvature, but that it is not suitable for imaging larger AAA shapes with more curvature. According to the model, tilted multi-aperture configurations combined with bistatic imaging can achieve a larger region with high contrast compared to non-tilted configurations. The findings of the model are in agreement with experimental findings. To conclude, this study demonstrates the vast improvements in field of view and AAA wall visibility that a large, sparsely populated 3D array can potentially achieve when imaging AAAs compared to single or dual aperture imaging. In the future, larger arrays, less thermal noise, more steering, and more channel-to-element connections combined with carefully chosen orientations of (sub-) apertures will likely advance 3D imaging of AAAs.

Sub-aperture ultrafast volumetric ultrasound imaging for fully sampled dual-mode matrix array

Fully sampled dual-mode matrix array ultrasound transducer is capable of performing imaging and therapeutic ultrasound in three dimensions (3D). It is a promising tool for many clinical applications because of its precise multi-focus therapy with imaging guidance by itself. Our team previously designed a 256-element fully sampled dual-mode matrix array transducer, while its imaging quality needs to be further improved. In this work, we propose a high-contrast sub-aperture volumetric imaging strategy to improve the imaging quality of the dual-mode matrix array. We first analyzed the effect of various parameters of sub-aperture imaging on the imaging quality by Field II. Based on the optimized parameters, we compared the resolution and signal to noise ratio (SNR) of sub-aperture imaging with those of full aperture imaging on phantoms and rabbit brain. The experimental results showed the proposed sub-aperture imaging method could obtain a comparable resolution to full aperture imaging. Moreover, the average intensity of noise signal near the wire phantom decreased by about 5 dB and the SNR of tissue phantom image increased by 8 %. The proposed sub-aperture imaging method also enabled clearer and more accurate imaging of the rabbit brain. The obtained results indicate the proposed sub-aperture imaging is a promising method for practical use of a fully sampled dual-mode matrix array for volumetric ultrasound imaging.

Coherent Bistatic 3-D Ultrasound Imaging Using Two Sparse Matrix Arrays

In the last decade, many advances have been made in high frame rate 3-D ultrasound imaging, including more flexible acquisition systems, transmit (TX) sequences, and transducer arrays. Compounding multiangle transmits of diverging waves has shown to be fast and effective for 2-D matrix arrays, where heterogeneity between transmits is key in optimizing the image quality. However, the anisotropy in contrast and resolution remains a drawback that cannot be overcome with a single transducer. In this study, a bistatic imaging aperture is demonstrated that consists of two synchronized matrix ( 32×32 ) arrays, allowing for fast interleaved transmits with a simultaneous receive (RX). First, for a single array, the aperture efficiency for high volume rate imaging was evaluated between sparse random arrays and fully multiplexed arrays. Second, the performance of the bistatic acquisition scheme was analyzed for various positions on a wire phantom and was showcased in a dynamic setup mimicking the human abdomen and aorta. Sparse array volume images were equal in resolution and lower in contrast compared to fully multiplexed arrays but can efficiently minimize decorrelation during motion for multiaperture imaging. The dual-array imaging aperture improved the spatial resolution in the direction of the second transducer, reducing the average volumetric speckle size with 72% and the axial-lateral eccentricity with 8%. In the aorta phantom, the angular coverage increased by a factor of 3 in the axial-lateral plane, raising the wall-lumen contrast with 16% compared to single-array images, despite accumulation of thermal noise in the lumen.

3-D H-scan ultrasound imaging of relative scatterer size using a matrix array transducer and sparse random aperture compounding

H-scan ultrasound (US) is a high-resolution imaging technique for soft tissue characterization. By acquiring data in volume space, H-scan US can provide insight into subtle tissue changes or heterogenous patterns that might be missed using traditional cross-sectional US imaging approaches. In this study, we introduce a 3-dimensional (3-D) H-scan US imaging technology for voxel-level tissue characterization in simulation and experimentation. Using a matrix array transducer, H-scan US imaging was developed to evaluate the relative size of US scattering aggregates in volume space. Experimental data was acquired using a programmable US system (Vantage 256, Verasonics Inc, Kirkland, WA) equipped with a 1024-element (32 × 32) matrix array transducer (Vermon Inc, Tours, France). Imaging was performed using the full array in transmission. Radiofrequency (RF) data sequences were collected using a sparse random aperture compounding technique with 6 different data compounding approaches. Plane wave imaging at five angles was performed at a center frequency of 8 MHz. Scan conversion and attenuation correction were applied. To generate the 3-D H-scan US images, a convolution filter bank (N = 256) was then used to process the RF data sequences and measure the spectral content of the backscattered US signals before volume reconstruction. Preliminary experimental studies were conducted using homogeneous phantom materials embedded with spherical US scatterers of varying diameter, i.e., 27 to 45, 63 to 75, or 106-126 μm. Both simulated and experimental results revealed that 3-D H-scan US images have a low spatial variance when tested with homogeneous phantom materials. Furthermore, H-scan US is considerably more sensitive than traditional B-mode US imaging for differentiating US scatterers of varying size (p = 0.001 and p = 0.93, respectively). Overall, this study demonstrates the feasibility of 3-D H-scan US imaging using a matrix array transducer for tissue characterization in volume space.

3D transcranial ultrasound localization microscopy for discrimination between ischemic and hemorrhagic stroke in early phase

Early diagnosis is a critical part of the emergency care of cerebral hemorrhages and ischemia. A rapid and accurate diagnosis of strokes reduces the delays to appropriate treatments and a better functional recovery. Currently, CTscan and MRI are the gold standards with constraints of accessibility, availability, and possibly some contraindications. The development of Ultrasound Localization Microscopy (ULM) has enabled new perspectives to conventional transcranial ultrasound imaging with increased sensitivity, penetration depth, and resolution. The possibility of volumetric imaging has increased the field-of-view and provided a more precise description of the microvascularisation. In this study, rats (n = 9) were subjected to thromboembolic ischemic stroke or intracerebral hemorrhages prior to volumetric ULM at the early phases after onsets. Although the volumetric ULM performed in the early phase of ischemic stroke revealed a large hypoperfused area in the cortical area of the occluded artery, it showed a more diffused hypoperfusion in the hemorrhagic model. Respective computations of a Microvascular Diffusion Index highlighted different patterns of perfusion loss during the first 24 h of these two strokes’ subtypes. Our study provides the first proof that this methodology should allow early discrimination between ischemic and hemorrhagic stroke with a potential toward diagnosis and monitoring in clinic.

Experimental Study of Aperiodic Plane Wave Imaging for Ultrafast 3-D Ultrasound Imaging

OBJECTIVE

Although plane wave imaging (PWI) with multiple plane waves (PWs) steered at different angles enables ultrafast three-dimensional (3-D) ultrasonic imaging, there is still a challenging tradeoff between image quality and frame rate. To address this challenge, we recently proposed the aperiodic PWI (APWI) with mathematical analysis and simulation study. In this paper, we demonstrate the feasibility of APWI and evaluate the performance with phantom and in vivo experiments.

METHODS

APWI with a concentric ring angle pattern (APWI-C) and APWI with a sunflower pattern (APWI-S) are evaluated. For experimental verification of the methods, the experimental results are compared with simulation results in terms of the mainlobe-to-sidelobe ratio. In addition, the performance of APWI is compared with that of conventional PWI by using a commercial phantom. To examine the potential for clinical use of APWI, a gallstone-mimicking phantom study and an in vivo carotid artery experiment are also conducted.

RESULTS

In the phantom study, the APWI methods provide a contrast ratio approximately 23 dB higher than that of PWI. In a gallstone mimicking experiment, the proposed methods yield 3-D rendered stone images more similar to the real stones than PWI. In the in vivo carotid artery images, APWI reduces the clutter artifacts inside the artery.

CONCLUSION

Phantom and in vivo studies show that the APWI enhances the contrast without compromising the spatial resolution and frame rate.

SIGNIFICANCE

This study experimentally demonstrates the feasibility and advantage of APWI for ultrafast 3-D ultrasonic imaging.

Ultrafast 3-D Ultrasound Imaging Using Row-Column Array-Specific Frame-Multiply-and-Sum Beamforming

Row-column arrays have been shown to be able to generate 3-D ultrafast ultrasound images with an order of magnitude less independent electronic channels than traditional 2-D matrix arrays. Unfortunately, row-column array images suffer from major imaging artifacts due to high sidelobes, particularly when operating at high frame rates. This article proposes a row-column-specific beamforming technique, for orthogonal plane-wave transmissions, row-column-specific frame multiply and sum (RC-FMAS), that exploits the incoherent nature of certain row-column array artifacts. A series of volumetric images is produced using row or column transmissions of 3-D plane waves. The voxelwise geometric mean of the beamformed volumetric images from each row and column pair is taken prior to compounding, which drastically reduces the incoherent imaging artifacts in the resulting image compared to traditional coherent compounding. The effectiveness of this technique was demonstrated in silico and in vitro, and the results show a significant reduction in sidelobe level with over 16-dB improvement in sidelobe to main-lobe energy ratio. Significantly improved contrast was demonstrated with contrast ratio increased by ~10 dB and generalized contrast-to-noise ratio increased by 158% when using the proposed new method compared to the existing delay and sum during in vitro studies. The new technique allowed for higher quality 3-D imaging while maintaining high frame rate potential.

3D Transcranial Ultrasound Localization Microscopy in the Rat Brain with a Multiplexed Matrix Probe

OBJECTIVE

Ultrasound Localization Microscopy (ULM) provides images of the microcirculation in-depth in living tissue. However, its implementation in two-dimension is limited by the elevation projection and tedious plane-by-plane acquisition. Volumetric ULM alleviates these issues and can map the vasculature of entire organs in one acquisition with isotropic resolution. However, its optimal implementation requires many independent acquisition channels, leading to complex custom hardware.

METHODS

In this article, we implemented volumetric ultrasound imaging with a multiplexed 32 x 32 probe driven by a single commercial ultrasound scanner. We propose and compare three different sub-aperture multiplexing combinations for localization microscopy in silico and in vitro with a flow of microbubbles in a canal. Finally, we evaluate the approach for micro-angiography of the rat brain.The "light" combination allows a higher maximal volume rate than the "full" combination while maintaining the field of view and resolution.

RESULTS

In the rat brain, 100,000 volumes were acquired within 7 min with a dedicated ultrasound sequence and revealed vessels down to 31 m in diameter with flows from 4.3 mm/s to 28.4 mm/s.

CONCLUSION

This work demonstrates the ability to perform a complete angiography with unprecedented resolution in the living rats brain with a simple and light setup through the intact skull.

SIGNIFICANCE

We foresee that it might contribute to democratize 3D ULM for both preclinical and clinical studies.

XDoppler: Cross-Correlation of Orthogonal Apertures for 3D Blood Flow Imaging

Row column addressing (RCA) transducers have the potential to provide volumetric imaging at ultrafast frame rate using a low channel count over a large field of view. In previous works we have shown that vascular imaging of large arteries as well as functional neuroimaging of the rat brain were feasible using RCA orthogonal plane wave imaging (OPW), but these applications required to transmit many plane waves, significantly reducing the frame rate. In this study, we introduce XDoppler imaging, a novel method to increase the performances of RCA flow imaging by taking advantage of the blood spatial decorrelation statistics combined with the limited spatial overlap of the point spread functions (PSF) of the two orthogonal apertures of the RCA transducer. We provide at first a theoretical basis to understand how the correlation operation reduces the sidelobe level. Then, we demonstrate both in vitro and in vivo in the human carotid artery and in the rat brain that XDoppler provides a significant gain in contrast-to-noise ratio (CNR) (between 3 and 6 dB depending on the application) compared to OPW. This improvement also leads to a sensitivity increase in the rat brain as more blood vessels are detected by XDoppler imaging.

High Frame Rate Volumetric Imaging of Microbubbles Using a Sparse Array and Spatial Coherence Beamforming

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.