ULTRA-SR Challenge: Assessment of Ultrasound Localization and TRacking Algorithms for Super-Resolution Imaging.

With the widespread interest and uptake of super-resolution ultrasound (SRUS) through localization and tracking of microbubbles, also known as ultrasound localization microscopy (ULM), many localization and tracking algorithms have been developed. ULM can image many centimeters into tissue in-vivo and track microvascular flow non-invasively with sub-diffraction resolution. In a significant community effort, we organized a challenge, Ultrasound Localization and TRacking Algorithms for Super-Resolution (ULTRA-SR). The aims of this paper are threefold: to describe the challenge organization, data generation, and winning algorithms; to present the metrics and methods for evaluating challenge entrants; and to report results and findings of the evaluation. Realistic ultrasound datasets containing microvascular flow for different clinical ultrasound frequencies were simulated, using vascular flow physics, acoustic field simulation and nonlinear bubble dynamics simulation. Based on these datasets, 38 submissions from 24 research groups were evaluated against ground truth using an evaluation framework with six metrics, three for localization and three for tracking. In-vivo mouse brain and human lymph node data were also provided, and performance assessed by an expert panel. Winning algorithms are described and discussed. The publicly available data with ground truth and the defined metrics for both localization and tracking present a valuable resource for researchers to benchmark algorithms and software, identify optimized methods/software for their data, and provide insight into the current limits of the field. In conclusion, Ultra-SR challenge has provided benchmarking data and tools as well as direct comparison and insights for a number of the state-of-the art localization and tracking algorithms.

Investigating the importance of left atrial compliance on fluid dynamics in a novel mock circulatory loop.

The left atrium (LA) hemodynamic indices hold prognostic value in various cardiac diseases and disorders. To understand the mechanisms of these conditions and to assess the performance of cardiac devices and interventions, in vitro models can be used to replicate the complex physiological interplay between the pulmonary veins, LA, and left ventricle. In this study, a comprehensive and adaptable in vitro model was created. The model includes a flexible LA made from silicone and allows distinct control over the systolic and diastolic functions of both the LA and left ventricle. The LA was mechanically matched with porcine LAs through expansion tests. Fluid dynamic measures were validated against the literature and pulmonary venous flows recorded on five healthy individuals using magnetic resonance flow imaging. Furthermore, the fluid dynamic measures were also used to construct LA pressure-volume loops. The in vitro pressure and flow recordings expressed a high resemblance to physiological waveforms. By decreasing the compliance of the LA, the model behaved realistically, elevating the a- and v-wave peaks of the LA pressure from 12 to 19 mmHg and 22 to 26 mmHg, respectively, while reducing the S/D ratio of the pulmonary venous flowrate from 1.5 to 0.3. This model provides a realistic platform and framework for developing and evaluating left heart procedures and interventions.

Diverging Polymer Acoustic Lens Design for High-Resolution Row-Column Array Ultrasound Transducers.

Spherical diverging acoustic lenses mounted on flat 2-D row-column-addressed (RCA) ultrasound transducers have shown the potential to extend the field of view (FOV) from a rectilinear to a curvilinear volume region and, thereby, enable 3-D imaging of large organs. Such lenses are usually designed for small aperture low-frequency transducers, which have limited resolution. Moreover, they are made of off-the-shelf pieces of materials, which leaves no room for optimization. We hypothesize that acoustic lenses can be designed to fit high-resolution transducers, and they can be fabricated in a fast, cost-effective, and flexible manner using a combination of 3-D printing and casting or computer numerical control (CNC) machining techniques. These lenses should increase the FOV of the array while preserving the image quality. In this work, such lenses are made in concave, convex, and compound spherical shapes and from thermoplastics and thermosetting polymers. Polymethylpentene (TPX), polystyrene (PS), polypropylene (PP), polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), and room-temperature-vulcanizing (RTV) silicone diverging lenses have been fabricated and mounted on a 128 + 128 6-MHz RCA transducer. The performances of the lenses have been assessed and compared in terms of FOV, signal-to-noise ratio (SNR), bandwidth, and potential artifacts. The largest FOV (24.0.) is obtained with a 42.64-mm radius PMMA-RTV compound lens, which maintains a decent fractional bandwidth (53%) and SNR at 6 MHz (.9.1-dB amplitude drop compared with the unlensed transducer). The simple PMMA TPX, PS, PP, PDMS, and RTV lenses provide an FOV of 12.2°, 6.3°, 8.1°, 11.7°, 0.6°, and 10.4°; a fractional bandwidth of 97%, 46%, 103%, 46%, 97%, 53%, and 49%; and an amplitude drop of -5.2, -4.4, -2.8, -15.4, -6.0, and -1.8 dB, respectively. This work demonstrates that thermoplastics are suitable materials for fabricating low-attenuation convex diverging lenses for large-aperture, high-frequency 2-D transducers. This is highly desired to achieve high-resolution volumetric imaging of large organs.

Super-Resolution Ultrasound Imaging of Renal Vascular Alterations in Zucker Diabetic Fatty Rats during the Development of Diabetic Kidney Disease.

Individuals with diabetes at risk of developing diabetic kidney disease (DKD) are challenging to identify using currently available clinical methods. Prognostic accuracy and initiation of treatment could be improved by a quantification of the renal microvascular rarefaction and the increased vascular tortuosity during the development of DKD. Super-resolution ultrasound (SRUS) imaging is an in vivo technique capable of visualizing blood vessels at sizes below 75 µm. This preclinical study aimed to investigate the alterations in renal blood vessels' density and tortuosity in a type 2 diabetes rat model, Zucker diabetic fatty (ZDF) rats, as a prediction of DKD. Lean age-matched Zucker rats were used as controls. A total of 36 rats were studied, subdivided into ages of 12, 22, and 40 weeks. Measured albuminuria indicated the early stage of DKD, and the SRUS was compared with the ex vivo micro-computed tomography (µCT) of the same kidneys. Assessed using the SRUS imaging, a significantly decreased cortical vascular density was detected in the ZDF rats from 22 weeks of age compared to the healthy controls, concomitant with a significantly increased albuminuria. Already by week 12, a trend towards a decreased cortical vascular density was found prior to the increased albuminuria. The quantified vascular density in µCT corresponded with the in vivo SRUS imaging, presenting a consistently lower vascular density in the ZDF rats. Regarding vessel tortuosity, an overall trend towards an increased tortuosity was present in the ZDF rats. SRUS shows promise for becoming an additional tool for monitoring and prognosing DKD. In the future, large-scale animal studies and human trials are needed for confirmation.

Intra-Cardiac Flow from Geometry Prescribed Computational Fluid Dynamics: Comparison with Ultrasound Vector Flow Imaging.

PURPOSE: This paper investigates the accuracy of blood flow velocities simulated from a geometry prescribed computational fluid dynamics (CFD) pipeline by applying it to a dynamic heart phantom. The CFD flow patterns are compared to a direct flow measurement by ultrasound vector flow imaging (VFI). The hypothesis is that the simulated velocity magnitudes are within one standard deviation of the measured velocities. METHODS: The CFD pipeline uses computed tomography angiography (CTA) images with 20 volumes per cardiac cycle as geometry input. Fluid domain movement is prescribed from volumetric image registration using the CTA image data. Inlet and outlet conditions are defined by the experimental setup. VFI is systematically measured in parallel planes, and compared to the corresponding planes in the simulated time dependent three dimensional fluid velocity field. RESULTS: The measured VFI and simulated CFD have similar flow patterns when compared qualitatively. A quantitative comparison of the velocity magnitude is also performed at specific regions of interest. These are evaluated at 11 non-overlapping time bins and compared by linear regression giving R2 = 0.809, SD = 0.060 m/s, intercept = - 0.039 m/s, and slope = 1.09. Excluding an outlier at the inlet, the correspondence between CFD and VFI improves to: R2 = 0.823, SD = 0.048 m/s, intercept = -0.030 m/s, and slope = 1.01. CONCLUSION: The direct comparison of flow patterns shows that the proposed CFD pipeline provide realistic flow patterns in a well-controlled experimental setup. The demanded accuracy is obtained close to the inlet and outlet, but not in locations far from these.

Universal Synthetic Aperture Sequence for Anatomic, Functional, and Super Resolution Imaging.

Synthetic aperture (SA) can be used for both anatomic and functional imaging, where tissue motion and blood velocity are revealed. Often, sequences optimized for anatomic B-mode imaging are different from functional sequences, as the best distribution and number of emissions are different. B-mode sequences demand many emissions for a high contrast, whereas flow sequences demand short sequences for high correlations yielding accurate velocity estimates. This article hypothesizes that a single, universal sequence can be developed for linear array SA imaging. This sequence yields high-quality linear and nonlinear B-mode images as well as accurate motion and flow estimates for high and low blood velocities and super-resolution images. Interleaved sequences with positive and negative pulse emissions for the same spherical virtual source were used to enable flow estimation for high velocities and make continuous long acquisitions for low-velocity estimation. An optimized pulse inversion (PI) sequence with 2 ×12 virtual sources was implemented for four different linear array probes connected to either a Verasonics Vantage 256 scanner or the SARUS experimental scanner. The virtual sources were evenly distributed over the whole aperture and permuted in emission order for making flow estimation possible using 4, 8, or 12 virtual sources. The frame rate was 208 Hz for fully independent images for a pulse repetition frequency of 5 kHz, and recursive imaging yielded 5000 images per second. Data were acquired from a phantom mimicking the carotid artery with pulsating flow and the kidney of a Sprague-Dawley rat. Examples include anatomic high contrast B-mode, non-linear B-mode, tissue motion, power Doppler, color flow mapping (CFM), vector velocity imaging, and super-resolution imaging (SRI) derived from the same dataset and demonstrate that all imaging modes can be shown retrospectively and quantitative data derived from it.

Precise Estimation of Intravascular Pressure Gradients.

This study presents a method for noninvasive pressure gradient estimation, which allows the detection of small pressure differences with higher precision compared to invasive catheters. It combines a new method for estimating the temporal acceleration of the flowing blood with the Navier-Stokes equation. The acceleration estimation is based on a double cross-correlation approach, which is hypothesized to minimize the influence of noise. Data are acquired using a 256-element, 6.5-MHz GE L3-12-D linear array transducer connected to a Verasonics research scanner. A synthetic aperture (SA) interleaved sequence with 2 ×12 virtual sources evenly distributed over the aperture and permuted in emission order is used in combination with recursive imaging. This enables a temporal resolution between correlation frames equal to the pulse repetition time at a frame rate of half the pulse repetition frequency. The accuracy of the method is evaluated against a computational fluid dynamic simulation. Here, the estimated total pressure difference complies with the CFD reference pressure difference, which yields an R -square of 0.985 and an RMSE of 3.03 Pa. The precision of the method is tested on experimental data, measured on a carotid phantom of the common carotid artery. The volume profile used during measurement was set to mimic flow in the carotid artery with a peak flow rate of 12.9 mL/s. The experimental setup showed that the measured pressure difference changes from -59.4 to 31 Pa throughout a single pulse cycle. This was estimated with a precision of 5.44% (3.22 Pa) across ten pulse cycles. The method was also compared to invasive catheter measurements in a phantom with a 60% cross-sectional area reduction. The ultrasound method detected a maximum pressure difference of 72.3 Pa with a precision of 3.3% (2.22 Pa). The catheters measured a maximum pressure difference of 105 Pa with a precision of 11.2% (11.4 Pa). This was measured over the same constriction and with a peak flow rate of 12.9 mL/s. The double cross-correlation approach revealed no improvement compared to a normal differential operator. The method's strength, thus, lies primarily in the ultrasound sequence, which allows precise and accurate velocity estimations, at which acceleration and pressure differences can be acquired.

Row-Column Beamformer for Fast Volumetric Imaging.

This work presents a beamforming procedure that significantly reduces the number of operations when performing volumetric synthetic aperture imaging with row-column addressed arrays (RCAs). The proposed beamformer uses that the image values along the elevation direction of the low-resolution volume (LRV) are approximately constant. It is thus hypothesized that the entire LRV could be reconstructed from a single 2-D cross section of the LRV. The presented method contains two stages. The first stage beamforms, for each emission, a cross section using the conventional RCA beamformer. The second stage extrapolates the rest of the image points in the volume from the 2-D cross sections. Assuming the image volume is covered by 3-D grid coordinates with a size of Nw×Nw ×Nz , i.e., Nw samples along the x - and y -axis and Nz samples along the z -axis, the proposed beamformer reduces the number of mathematical operations by a factor of approximately NNw/(NS+Nw) . Here, S is the ratio between the first- and second-stage axial sampling rates, and N is the receiving aperture's number of channels. Beamforming a 128×128×1024 volume from data acquired with N = 128 receiving channel can thus be achieved with 25.6 times fewer operations, when S = 4. A 9.23 times increase in the beamforming rate for a 100×100×200 volume was demonstrated on complex data from a 128 + 128 Vermon RCA probe. Real-time volumetric beamformation can, with this increase, be performed with a pulse repetition frequency of up to 1804.80 Hz. The proposed and conventional beamformer's output was visually indistinguishable, and the full width at half maximum (FWHM) and full width at tenth maximum (FWTM) were at most 1.19% larger with the proposed approach. The proposed beamformer can thus perform volumetric imaging significantly faster than the current approach, with a negligible difference in image quality.

Transverse oscillation tensor velocity imaging using a row-column addressed array: Experimental validation.

Tensor velocity imaging (TVI) performance with a row-column probe was assessed for constant flow in a straight vessel phantom and pulsatile flow in a carotid artery phantom. TVI, i.e., estimating the 3-D velocity vector as a function of time and spatial position, was performed using the transverse oscillation cross-correlation estimator, and the flow was acquired with a Vermon 128+128 row-column array probe connected to a Verasonics 256 research scanner. The emission sequence used 16 emissions per image, and a TVI volume rate of 234 Hz was obtained for a pulse repetition frequency (fprf) of 15 kHz. The TVI was validated by comparing estimates of the flow rate through several cross-sections with the flow rate set by the pump. For the constant 8 mL/s flow in the straight vessel phantom with relative estimator bias (RB) and standards deviation (RSD) was found in the range of -2.18% to 0.55% and 4.58% to 2.48% in measurements performed with an fprf of 15, 10, 8, and 5 kHz. The pulsatile flow in the carotid artery phantom the was set to an average flow rate of 2.44 mL/s, and the flow was acquired with an fprf of 15, 10, and 8 kHz. The pulsatile flow was estimated from two measurement sites: one at a straight section of the artery and one at the bifurcation. In the straight section, the estimator predicted the average flow rate with an RB value ranging from -7.99% to 0.10% and an RSD value ranging from 10.76% to 6.97%. At the bifurcation, RB and RSD values were between -7.47% to 2.02% and 14.46% to 8.89%. This demonstrates that an RCA with 128 receive elements can accurately capture the flow rate through any cross-section at a high sampling rate.

Contrast-enhanced ultrasound imaging using capacitive micromachined ultrasonic transducers.

Capacitive micromachined ultrasonic transducers (CMUTs) have a nonlinear relationship between the applied voltage and the emitted signal, which is detrimental to conventional contrast enhanced ultrasound (CEUS) techniques. Instead, a three-pulse amplitude modulation (AM) sequence has been proposed, which is not adversely affected by the nonlinearly emitted harmonics. In this paper, this is shown theoretically, and the performance of the sequence is verified using a 4.8 MHz linear capacitive micromachined ultrasonic transducer (CMUT) array, and a comparable lead zirconate titanate (PZT) array, across 6-60 V applied alternating current (AC) voltage. CEUS images of the contrast agent SonoVue flowing through a 3D printed hydrogel phantom showed an average enhancement in contrast-to-tissue ratio (CTR) between B-mode and CEUS images of 49.9 and 37.4 dB for the PZT array and CMUT, respectively. Furthermore, hydrophone recordings of the emitted signals showed that the nonlinear emissions from the CMUT did not significantly degrade the cancellation in the compounded AM signal, leaving an average of 2% of the emitted power between 26 and 60 V of AC. Thus, it is demonstrated that CMUTs are capable of CEUS imaging independent of the applied excitation voltage when using a three-pulse AM sequence.

Anatomic and Functional Imaging Using Row-Column Arrays.

Row-column (RC) arrays have the potential to yield full 3-D ultrasound imaging with a greatly reduced number of elements compared to fully populated arrays. They, however, have several challenges due to their special geometry. This review article summarizes the current literature for RC imaging and demonstrates that full anatomic and functional imaging can attain a high quality using synthetic aperture (SA) sequences and modified delay-and-sum beamforming. Resolution can approach the diffraction limit with an isotropic resolution of half a wavelength with low sidelobe levels, and the field of view can be expanded by using convex or lensed RC probes. GPU beamforming allows for three orthogonal planes to be beamformed at 30 Hz, providing near real-time imaging ideal for positioning the probe and improving the operator's workflow. Functional imaging is also attainable using transverse oscillation and dedicated SA sequence for tensor velocity imaging for revealing the full 3-D velocity vector as a function of spatial position and time for both blood velocity and tissue motion estimation. Using RC arrays with commercial contrast agents can reveal super-resolution imaging (SRI) with isotropic resolution below [Formula: see text]. RC arrays can, thus, yield full 3-D imaging at high resolution, contrast, and volumetric rates for both anatomic and functional imaging with the same number of receive channels as current commercial 1-D arrays.

Super-Resolution Ultrasound Imaging Provides Quantification of the Renal Cortical and Medullary Vasculature in Obese Zucker Rats: A Pilot Study.

Obesity is a risk factor of chronic kidney disease (CKD), leading to alterations in the renal vascular structure. This study tested if renal vascular density and tortuosity was quantifiable in vivo in obese rats using microbubble-based super-resolution ultrasound imaging. The kidneys of two 11-week-old and two 20-week-old male obese Zucker rats were compared with age-matched male lean Zucker rats. The super-resolution ultrasound images were manually divided into inner medulla, outer medulla, and cortex, and each area was subdivided into arteries and veins. We quantified vascular density and tortuosity, number of detected microbubbles, and generated tracks. For comparison, we assessed glomerular filtration rate, albumin/creatinine ratio, and renal histology to evaluate CKD. The number of detected microbubbles and generated tracks varied between animals and significantly affected quantification of vessel density. In areas with a comparable number of tracks, density increased in the obese animals, concomitant with a decrease in glomerular filtration rate and an increase in albumin/creatinine ratio, but without any pathology in the histological staining. The results indicate that super-resolution ultrasound imaging can be used to quantify structural alterations in the renal vasculature. Techniques to generate more comparable number of microbubble tracks and confirmation of the findings in larger-scale studies are needed.

Super-Resolution Ultrasound Imaging Can Quantify Alterations in Microbubble Velocities in the Renal Vasculature of Rats.

Super-resolution ultrasound imaging, based on the localization and tracking of single intravascular microbubbles, makes it possible to map vessels below 100 µm. Microbubble velocities can be estimated as a surrogate for blood velocity, but their clinical potential is unclear. We investigated if a decrease in microbubble velocity in the arterial and venous beds of the renal cortex, outer medulla, and inner medulla was detectable after intravenous administration of the α1-adrenoceptor antagonist prazosin. The left kidneys of seven rats were scanned with super-resolution ultrasound for 10 min before, during, and after prazosin administration using a bk5000 ultrasound scanner and hockey-stick probe. The super-resolution images were manually segmented, separating cortex, outer medulla, and inner medulla. Microbubble tracks from arteries/arterioles were separated from vein/venule tracks using the arterial blood flow direction. The mean microbubble velocities from each scan were compared. This showed a significant prazosin-induced velocity decrease only in the cortical arteries/arterioles (from 1.59 ± 0.38 to 1.14 ± 0.31 to 1.18 ± 0.33 mm/s, p = 0.013) and outer medulla descending vasa recta (from 0.70 ± 0.05 to 0.66 ± 0.04 to 0.69 ± 0.06 mm/s, p = 0.026). Conclusively, super-resolution ultrasound imaging makes it possible to detect and differentiate microbubble velocity responses to prazosin simultaneously in the renal cortical and medullary vascular beds.

Performance Assessment of Row-Column Transverse Oscillation Tensor Velocity Imaging Using Computational Fluid Dynamics Simulation of Carotid Bifurcation Flow.

In this work, the accuracy of row-column tensor velocity imaging (TVI), i.e., 3-D vector flow imaging (VFI) in 3-D space over time, is quantified on a complex, clinically relevant flow. The quantification is achieved by transferring the flow simulated using computational fluid dynamics (CFD) to a Field II simulation environment, and this allows for a direct comparison between the actual and estimated velocities. The carotid bifurcation flow simulations were performed with a peak inlet velocity of 80 cm/s, nonrigid vessel walls, and a flow cycle duration of 1.2 s. The flow was simulated from two observation angles, and it was acquired using a 3-MHz 62+62 row-column addressed array (RCA) at a pulse repetition frequency ( fprf ) of 10 and 20 kHz. The tensor velocities were obtained at a frame rate of 208.3 Hz, at fprf = 10 kHz , and the results from two velocity estimators were compared. The two estimators were the directional transverse oscillation (TO) cross correlation estimator and the proposed autocorrelation estimator. Linear regression between the actual and estimated velocity components yielded, for the cross correlation estimator, an R 2 value in the range of 0.89-0.91, 0.46-0.77, and 0.91-0.97 for the x -, y -, and z -components, and 0.87-0.89, 0.40-0.83, and 0.91-0.96 when using the autocorrelation estimator. The results demonstrate that an RCA can, with just 62 receive channels, measure complex 3-D flow fields at a high volume rate.

Ultrasound super-resolution imaging with a hierarchical Kalman tracker.

Microbubble (MB) tracking plays an important role in ultrasound super-resolution imaging (SRI) by enabling velocity estimation and improving image quality. This work presents a new hierarchical Kalman (HK) tracker to achieve better performance at scenarios with high concentrations of MBs and high localization uncertainty. The method attempts to follow MBs with different velocity ranges using different Kalman filters. An extended simulation framework for evaluating trackers is also presented and used for comparison of the proposed HK tracker with the nearest-neighbor (NN) and Kalman (K) trackers. The HK tracks were most similar to the ground truth with the highest Jaccard similarity coefficient in 79% of the scenarios and the lowest root-mean-square error in 72% of the scenarios. The HK tracker reconstructed vessels with a more accurate diameter. In a scenario with an uncertainty of 51.2µm in MB localization, a vessel diameter of 250µm was estimated as 257µm by HK tracker, compared with 329µm and 389µm for the K and NN trackers. In the same scenario, the HK tracker estimated MB velocities with a relative bias down to 1.7% and a relative standard deviation down to 8.3%. Finally, the different tracking techniques were applied to in vivo data from rat kidneys, and trends similar to the simulations were observed. Conclusively, the results showed an improvement in tracking performance, when the HK tracker was employed in comparison with the NN and K trackers.

Pressure Difference Estimation in Non-stenotic Carotid Bifurcation Phantoms Using Vector Flow Imaging.

Local pressure differences estimated using vector flow imaging (VFI) and direct catheterization in seven carotid bifurcation phantoms were compared with simulated pressure fields. VFI correlated strongly with simulated peak pressure differences (r = 0.99, p < 0.00001), with an average overestimation of 12.3 Pa (28.6%). The range between the lowest and highest pressure difference of VFI underestimated simulations by 4.6 Pa (8.06%; r = 0.99, p < 0.0001). The catheter method exhibited no correlation (r = -0.09, p = 0.85). Ten repeated measurements on one phantom revealed a small standard deviation (SD) for VFI (SD = 8.4%, mean estimated SD = 11.5%), but not for the catheter method (SD = 785.6%). An in vivo peak systolic pressure difference of 97.9 Pa (estimated SD = 30%) was measured using VFI in one healthy individual. This study indicates that VFI pressure difference estimation is feasible in phantoms and in vivo and realistic estimates of the SD can be attained from the data.

Evaluation of 2D super-resolution ultrasound imaging of the rat renal vasculature using ex vivo micro-computed tomography.

Super-resolution ultrasound imaging (SRUS) enables in vivo microvascular imaging of deeper-lying tissues and organs, such as the kidneys or liver. The technique allows new insights into microvascular anatomy and physiology and the development of disease-related microvascular abnormalities. However, the microvascular anatomy is intricate and challenging to depict with the currently available imaging techniques, and validation of the microvascular structures of deeper-lying organs obtained with SRUS remains difficult. Our study aimed to directly compare the vascular anatomy in two in vivo 2D SRUS images of a Sprague-Dawley rat kidney with ex vivo µCT of the same kidney. Co-registering the SRUS images to the µCT volume revealed visually very similar vascular features of vessels ranging from ~ 100 to 1300 µm in diameter and illustrated a high level of vessel branching complexity captured in the 2D SRUS images. Additionally, it was shown that it is difficult to use µCT data of a whole rat kidney specimen to validate the super-resolution capability of our ultrasound scans, i.e., validating the actual microvasculature of the rat kidney. Lastly, by comparing the two imaging modalities, fundamental challenges for 2D SRUS were demonstrated, including the complexity of projecting a 3D vessel network into 2D. These challenges should be considered when interpreting clinical or preclinical SRUS data in future studies.

Transthoracic Vector Flow Imaging in Pediatric Patients with Valvular Stenosis - A Proof of Concept Study.

Purpose Continuous wave Doppler ultrasound is routinely used to detect cardiac valve stenoses. Vector flow imaging (VFI) is an angle-independent real-time ultrasound method that can quantify flow complexity. We aimed to evaluate if quantification of flow complexity could reliably assess valvular stenosis in pediatric patients. Materials and Methods Nine pediatric patients with echocardiographically confirmed valvular stenosis were included in the study. VFI and Doppler measurements were compared with transvalvular peak-to-peak pressure differences derived from invasive endovascular catheterization. Results Vector concentration correlated with the catheter measurements before intervention after exclusion of one outlier (r=-0.83, p=0.01), whereas the Doppler method did not (r=0.49, p=0.22). The change in vector concentration after intervention correlated strongly with the change in the measured catheter pressure difference (r=-0.86, p=0.003), while Doppler showed a tendency for a moderate correlation (r=0.63, p=0.07). Conclusion Transthoracic flow complexity quantification calculated from VFI data is feasible and may be useful for assessing valvular stenosis severity in pediatric patients.

Common Carotid Artery Volume Flow: A Comparison Study between Ultrasound Vector Flow Imaging and Phase Contrast Magnetic Resonance Imaging.

Volume flow estimation in the common carotid artery (CCA) can assess the absolute hemodynamic effect of a carotid stenosis. The aim of this study was to compare a commercial vector flow imaging (VFI) setup against the reference method magnetic resonance phase contrast angiography (MRA) for volume flow estimation in the CCA. Ten healthy volunteers were scanned with VFI and MRA over the CCA. VFI had an improved precision of 19.2% compared to MRA of 31.9% (p = 0.061). VFI estimated significantly lower volume flow than MRA (mean difference: 63.2 mL/min, p = 0.017), whilst the correlation between VFI and MRA was strong (R2 = 0.81, p < 0.0001). A Bland-Altman plot indicated a systematic bias. After bias correction, the percentage error was reduced from 41.0% to 25.2%. This study indicated that a VFI setup for volume flow estimation is precise and strongly correlated to MRA volume flow estimation, and after correcting for the systematic bias, VFI and MRA become interchangeable.