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14:00   Vascular III Chair: Jolanda Wentzel 14:00 15 mins Ultrafast Doppler diverging wave imaging of coronary flow under rapid tissue motion – Phantom experiments Yizhou Huang, Emilia Badescu, Ruud van den Sloun, Massimo Mischi Abstract: The advancement of ultrafast Doppler imaging enables more accurate flow measurements compared to conventional Doppler. Most studies have focused on ultrafast coronary imaging combined with singular value decomposition (SVD) for clutter filtering with favorable acquisition settings (open-chest, linear high-frequency probe, and plane waves). Here, we aim at verifying the feasibility of using phased-array diverging wave imaging to measure coronary flow at increased depth, for non-negligible cardiac tissue motion, mimicking a condition similar to clinical transthoracic echocardiography. This imposes an extra challenge on SVD for separating blood signals from clutter. To obtain the optimal Doppler performance, a set of acquisition and processing parameters, including the number of transmitting angles, the ensemble length, and different compounding strategies, along with different SVD thresholding methods, are examined in this study. To mimic coronary flow measurements, a 15% gelatin phantom with a 3-mm diameter channel (20 degrees, 9-12 cm depth) is connected to a syringe pump generating flow at 10 cm/s. A Philips S5-1 phased array probe operating at 3.125-MHz is mounted on a sinusoidal axial motion generator with a speed of ±6 cm/s. A Verasonics 256 system is programmed to generate alternate 1-, 2-, 4-, 8, and 14-angle (between ±7°) acquisitions with a pulse repetition frequency of 4400 Hz. The performance of different compounding strategies, including coherent compounding (CC), CC with a sliding window of 2 angles (CC-S), and linear interpolation of CC frames, is compared. Six different ensemble lengths from 0.025 to 1.2 s are analyzed. SVD with three literature thresholding methods (Baranger et al., TMI, 2018; Maresca et al., TUFFC, 2018) are investigated. A novel method to determine the clutter-blood threshold is proposed that relies on the minimum distance of the singular value curve from the origin, and an extensive threshold search method is also implemented as a reference. Contrast-to-Noise ratio (CNR) is employed for quantitative comparison. Using an alternate 2-angle acquisition, CC-S, an ensemble length of 0.1 s, with the proposed SVD thresholding method, returns the highest CNR. These promising results prove the feasibility of measuring coronary flow with ultrafast diverging wave imaging. 14:15 15 mins A mechanical implant to control flow of blood through an arteriovenous conduit Nicholas White, Koen van der Bogt, Joris Rotmans, Tim Horeman Abstract: End-stage renal disease (ESRD) is a long-term health problem in which the structure and function of the kidney is affected. Patients with ESRD are largely dependent on dialysis for survival, with 85% receiving haemodialysis. For chronic haemodialysis, an adequately functioning high-flow vascular access is required, usually achieved by surgical creation of an arteriovenous fistula (AVF) between a vein and artery in the arm. However, the durability of AVFs in providing high-flow vascular access is far from optimal, with ~40% requiring surgical intervention within 1 year. The primary limiting factor affecting durability of AVFs is occlusion induced by the constantly elevated flow. On the contrary, ~80% of patients with a well-functioning AVF eventually develop cardiac issues due to the constantly elevated cardiac output. The high flow caused by the AVF is always present, but only necessary during dialysis sessions, rarely exceeding 12 hrs/week. By enabling opening and closing of the AVF, the necessary high flow for dialysis will only present during the 12 hours of dialysis sessions per week. Circulation can return to normal outside these sessions, removing the core of the issue in vascular access: continuous high flow. Patient outcomes and quality of life should greatly improve, as well as greatly reducing costs of dialysis related issues. A novel implant to allow opening and closing of this AVF is being developed. Actuation occurs non-invasively through a magnetic drive placed subcutaneously in the upper arm. A set of magnets outside the skin can then be coupled to allow actuation of the shunt non-invasively and transcutaneously. The drive is connected through a flexible Bowden cable to a valve mechanism that manipulates the AVF. A functional prototype has been developed and recently implanted into a goat model for a duration of 13 weeks in a pilot study (n=2). Transcutaneous actuation of the drive remained possible throughout the study. However, excessive fibrosis formation appeared to be the limiting factor in enabling fully opening and closing the AVF. An upcoming second pilot with an improved design expected to solve this issue could provide first insights into the systemic effects of opening and closing an AVF. 14:30 15 mins 3D ultrasound-based mechanical and geometrical analysis of abdominal aortic aneurysms relating to growth Esther Maas, Arjet Nievergeld, Judith Fonken, Mirunalini Thirugnanasambandam, Marc van Sambeek, Richard Lopata Abstract: An abdominal aortic aneurysm (AAA), a local dilatation of the aorta, is a progressive disease. Between patients, there is a variety in growth rates and diameter at which the AAA ruptures [1-2]. In this study, these differences between patients were studied in a longitudinal study by analyzing 3D geometrical and mechanical parameters of AAAs using time-resolved 3D ultrasound (3D+t US). The mutual correlations between parameters, and their relationships with diameter and growth were studied. 3D+t US AAA images were acquired for 178 patients, who all had >3 follow-up diameter measurements with 2D US (d2D) from which the growth of their AAA was obtained. An automated analysis tool was developed to create a segmentation of the AAA wall for all time frames. From this, the maximal diameter (d3D) perpendicular to the centerline, volume (V), compliance (C) and distensibility (D) were determined. First, the geometry parameters were verified with computed tomography (CT) for a subset of patients (N=17), showing good correspondence with a median similarity index of 0.88. Correspondence between d3D and CT-based diameters is high (R2 = 0.90), although the d3D values are smaller (difference IQR [0.23 - 0.66] cm), a range similar to the differences for clinically used d2D values. Next, relationships between parameters were explored for all patients with Spearman correlations, and the parameters’ predictive value for growth were determined. A decrease in D with increasing blood pressure (p<0.0001) and diameter (p<0.05) was observed. Growth is related to d2D, d3D, V and C (p <0.005). Stepwise linear regression showed that C is a better predictor for growth than diameter (RMSE 1.70 and 1.78, respectively), despite diameter being a more established growth predictor [1,3]. In conclusion, an automated tool to determine geometrical and mechanical parameters of AAAs from 3D+t US has been developed, validated, and applied to a large group of patients. The AAA growth was found to be correlated to d2D, d3D, V and C, with C having the best predictive value for growth in a linear model. These findings contribute to the understanding of differences in AAA progression between patients, which is a step towards more patient-specific treatment. 14:45 15 mins Ultrasound shear wave elastography in the Carotid arteries: applications, results and future perspectives Judith Pruijssen, Stein Fekkes, Chris de Korte, Rik Hansen Abstract: Ultrasound is the most used imaging modality to assess vascular health. Carotid intima-media thickness (IMT) and stenosis degree are validated measures of atherosclerotic disease associated with cerebrovascular events at population level. However, they are not sufficiently specific for personalized treatment selection. IMT and stenosis degree are both geometrical measures. They do not provide information on wall composition and mechanics which are considered key parameters of vascular health. Alterations in vascular wall stiffness may proceed vascular wall thickening, and plaque composition correlates better with stroke risk than stenosis degree. New ultrasound techniques, including shear wave elastography (SWE), allow for tissue stiffness assessment. Carotid wall SWE has shown to correlate with cardiovascular risk factors[1]. Furthermore, SWE values seem to differ between symptomatic and asymptomatic and between stable and unstable plaques[2]. This study, therefore, aims to evaluate the applicability of SWE for vascular health assessment. Currently, longitudinal SWE imaging is commercially available. We evaluated this method in twenty-six patients ≥5 years after unilateral irradiation for head and neck neoplasms. We assessed radiation-induced changes in vascular thickness (IMT) and stiffness (SWE) in irradiated compared to unirradiated carotids. Measurements were performed in four segments (proximal/mid/distal common, and internal carotid artery). As patients received unilateral irradiation, the non-irradiated side served as internal control. We found higher IMTs, and a trend towards higher SWVs in some segments in irradiated compared to non-irradiated carotids. However, when applied to carotid plaques, not all plaques can be imaged with longitudinal SWE. Therefore, we developed a cross-sectional SWE-method with steered ultrasound beams to enable shear wave tracking along the entire arterial circumference. In this way, possibly all plaques can be visualized. To evaluate this method, we performed cross-sectional SWE measurements in vessel-mimicking phantoms with a stiff wall and soft plaque. Plaques could be detected, a great part of the wall circumference (~80%) could be visualized, and measurements were highly accurate. Concluding, SWE shows promising results in vascular health assessment. Longitudinal SWE could enable radiation-induced vasculopathy assessment. Furthermore, we developed a method for cross-sectional SWE enabling visualization of plaques located along the entire arterial circumference. We will further evaluate this in vivo. [1] Marais et al. Ultrasound Med Biol, 45(3): 758-772, 2019. [2] Pruijssen, et al., Ultrasound Med Biol, 46(9): 2145-2163, 2020. 15:00 15 mins Multi-perspective bistatic 2-D and 3-D ultrasound acquisitions and strain imaging of abdominal aortas Hein de Hoop, Vera van Hal, Marieke Vermeulen, Hans-Martin Schwab, Richard Lopata Abstract: Ultrasound imaging is a safe and accessible modality to visualize the abdomen. It is frequently used to assess the geometry of the abdominal aorta, which can provide clinicians with patient-specific information of aortic aneurysms. The anisotropy in resolution and contrast, caused by the limited aperture size and refraction, degrades image quality and restricts the estimation and precision of local wall deformation and mechanical properties. Expanding the imaging aperture with an additional transducer can improve the lateral resolution and extent the angular coverage of the vessel wall. This study demonstrates a dual-transducer system for aortic strain imaging that combines ultrasound transmits from two perspectives with a simultaneous receive (bistatic imaging). The acquisition sequence consists of fast interleaved transmits of diverging waves that can be received by both transducers. For 3-D imaging this was realized with the use of sparse random apertures on two matrix arrays. After registration, bistatic ultrasound images were fused and 2-D axial displacement fields were compounded, discarding all lateral tracking data. Strain estimates were compared for ex vivo porcine aortas in a mock loop of the abdomen and systemic circulation. In 3-D, aortic wall contrast was measured and spatial resolution was quantified in a phantom containing point sources, normal background scattering, and different contrast lesions. Compounding of multi-perspective axial displacements reduced motion tracking errors with a factor 10 compared to conventional tracking of focused scanline images. Consequently, strain precision and resolution increased, leading to more homogeneous circumferential strain patterns and enabling measurements of local radial strain at high resolution wall segments which can be further extended with the inclusion of trans-probe signals. In 3-D, coherent fusion of bistatic signals reduced the volumetric speckle size by 72% at a depth of 6 cm and wall-lumen contrast of the aorta increased by 16%. Future work includes bistatic strain estimations in 3-D, in vivo measurements, and mechanical characterization of patient-specific local wall properties of the abdominal aorta. 15:15 15 mins In-vitro investigation of the impact of monodisperse microbubble size on contrast-enhanced ultrasound super-localization imaging Peiran Chen, Andreas Pollet, Simona Turco, Miguel de Vargas, Lisa te Winkel, Wim van Hoeve, Jaap den Toonder, Hessel Wijkstra, Massimo Mischi Abstract: Introduction Contrast-enhanced ultrasound (CEUS) imaging provides real-time analysis of the vasculature with the help of injected ultrasound contrast agent (UCA) microbubbles. Recently, CEUS super-localization imaging has shown promise for the assessment of fine microvascular networks by localizing and tracking microbubbles, achieving resolutions beyond the diffraction limit. Compared to commercial UCAs, monodisperse microbubbles have a narrow size distribution. Therefore, proper frequency and pressure tuning have the potential to improve the signal to noise ratio and resolution of CEUS acquisitions, which can be expected to increase the performance of CEUS super-localization imaging. Moreover, both bubble size and insonating settings can be jointly optimized. In this work, the impact of monodisperse microbubble size on CEUS imaging quality and the efficacy of super-localization imaging was investigated by jointly tuning different frequencies and pressures for different monodisperse microbubble size. Methods A sugar-printed dual-bifurcation microvasculature phantom was perfused with UCA microbubbles. The channel diameter varies from 330 to 650 µm through the subsequent bifurcations. SP1 monodisperse microbubbles (Solstice) having dominant size of 3.7, 3.3, 2.9, 2.7 and 2.6 µm, corresponding respectively to resonance frequencies of 3.43, 4.09, 4.58, 5.08 and 5.42 MHz, were infused into the phantom at a flow rate of 0.08 mL/min. CEUS imaging was performed in contrast-specific mode using a Vantage 256 system (Verasonics Inc., USA) equipped with a L12-3 probe. For each microbubble size, seven transmit frequencies varying from 3.0 to 6.0 MHz with 0.5-MHz steps and four pressure values corresponding to mechanical index (MI) of 0.1, 0.13, 0.2 and 0.3 were used. The obtained CEUS acquisitions were then post-processed to generate a super-localization output using the Gaussian-centroid localization approach. Results/Discussion Metrics including generalized contrast-to-noise ratio of the CEUS acquisitions, full-width half-maximum of the pixel intensity profile extracted across a flow channel and number of localization events, were employed to quantify the CEUS imaging quality and super-localization performance. In general, jointly optimizing the transmit frequency and pressure for monodisperse microbubbles with smaller size lead to improved CEUS imaging and better super-localization performance. Yet, the weaker backscatter of smaller microbubbles must also be considered.

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