10:30   Vascular I Chair: Jan R. Buitenweg Strain Imaging in Abdominal Aortic Aneurysms Using Bistatic Dual-Transducer Ultrasound Vera van Hal, Lisanne Passier, Marc van Sambeek, Hans-Martin Schwab, Richard Lopata Abstract: Abdominal aortic aneurysms (AAA) are large dilatations of the abdominal aorta, that are typically asymptomatic until a life-threatening rupture occurs. Ultrasound (US) imaging is widely used in the clinic to monitor the diameter of AAAs. However, knowledge of full AAA geometry and local, mechanical wall parameters can contribute to a better assessment of the mechanical state and prediction of rupture risk. On top of that, the wall strength and wall stress are affected by the intraluminal thrombus (ILT), which is present in 75% of the patients. US has a high spatial and temporal resolution, which allows to study tissue function by estimating the wall deformation and strain. However, such an assessment is currently limited by the lateral lumen-wall contrast and resolution of conventional US. The recent introduction of ultrafast dual-aperture imaging has boosted image quality, using two transducers that alternately transmit and both probes receive simultaneously upon each transmit event (``bistatic" US). In this first-in-man study, the potential of dual-aperture, bistatic US to improve image quality and the estimation of local strains in the wall and the ILT is demonstrated in 40 AAA patients. Strain imaging was performed, leveraging the availability of axial displacement estimates derived from the four individual US signals. Results were compared to conventional, single-probe (SP) ultrafast imaging. The wall-lumen generalized contrast-to-noise ratio (gCNR) was significantly increased by 0.13 (+27%) on average, compared to SP US, yielding more accurate strain estimates inside, and for a larger part of, the vessel wall (+6dB). The resulting strains display physiological patterns with a certain amount of heterogeneity, which can be expected based on variation in local tissue properties. In 13 out of 25 patients with ILT, successful motion tracking was performed not only in the vessel wall, but also inside the thrombus. Vast improvements in strain accuracy were found, with a 78% reduction in the median tracking error, yielding significantly improved SNRecirc by 9.2dB (+124%) using bistatic dual-aperture US. In the end, strain imaging inside the vessel wall and thrombus can give important information on the mechanical state of the AAA in an effort to improve rupture risk assessment. Ultrasound-based exploration of the arterial dynamic behaviour during a non-invasive blood pressure measurement Agata Barbagini, Laura Bogatu, Simona Turco, Frederique De Raat, Esmée de Boer, Arthur Bouwman, Massimo Mischi, Lars Schmitt, Jens Muehlsteff Abstract: Background, Motivation and Objective Arterial blood pressure (ABP) is a key physiological parameter in hemodynamic monitoring. Previous research has shown how the current non-invasive method for ABP measurement (NIBP) is inaccurate and that the oscillometric device is not used to its full potential [1]. Characterization of the arterial adaption to cuff-based perturbation could provide relevant understandings for improving NIBP. Models have been developed to characterize cuff-induced mechanisms, based on physiological insights [2,3]. These models do not consider flow, but only variation in ABP, resistance and pulse transit time (PTT) induced by the cuff. In this study, we use ultrasound to investigate arterial behaviour and blood flow during deflation of a BP cuff. In addition, invasive ABP and photoplethysmography are acquired, using intensive care unit (ICU) hospital equipment, permitting an accurate measurement of PTT vs. cuff pressure (𝑃𝑐𝑢𝑓𝑓) upstream and downstream the cuff. Methods Ultrasound data is acquired with Philips Affiniti70 in ICU patients wearing a BP cuff connected to a pump with sphygmomanometer. After a 30 s occlusion, B-mode and pulsed Doppler acquisitions are performed proximally and distally from the cuff over eight decreasing 𝑃𝑐𝑢𝑓𝑓 plateaus. Arterial diameter, blood velocity and flow are measured at each 𝑃𝑐𝑢𝑓𝑓. Results and discussion Arterial area upstream the cuff increases with 𝑃𝑐𝑢𝑓𝑓, while this is not the case for downstream area. The presence of backflow varies with 𝑃𝑐𝑢𝑓𝑓, without following a monotonous trend, both upstream and downstream. This shows that the arterial behaviour is not fully understood yet and suggests how NIBP models should also consider flow. Ultrasound is essential to improve the understanding of the arterial response to cuff-based perturbation. This preliminary study helps identify unexpected effects requiring further study. In vivo Evaluation of the Hemodynamic Consequences of Carotid Endarterectomy by using high-frame-rate Velocity Vector Imaging Janna Ruisch, Joosje de Bakker, Suzanne Holewijn, Michel Reijnen, Chris de Korte, Anne Saris Abstract: Approximately 20% of strokes originate from an atherosclerotic plaque rupture in the carotid artery. To reduce the risk of stroke recurrence, symptomatic patients can undergo surgical plaque removal, i.e. carotid endarterectomy (CEA). However, restenosis after CEA was reported to range between 5-22%. Clinical associations are unknown, but CEA changes the carotid geometry and might induce an abnormal, atheroprone hemodynamic condition. The advent of ultrafast ultrasound makes continuous tracking of the blood flow profile in the imaging plane feasible. In this project, we aim to evaluate the hemodynamic consequences of CEA by using high-frame-rate velocity vector imaging (VVI). VVI of the carotid bifurcation was performed in patients that recently underwent CEA with patch angioplasty (n=16, 15 males, median age 69.0 [62.3-73.3] years) and age-matched healthy subjects without cardiovascular diseases (n=9, 5 males, median age 72.0 [69.0-73.0] years). The hemodynamic consequences of CEA were evaluated by comparing different blood flow features between both groups. These parameters aim to provide insight in potentially abnormal flow conditions, such as the presence of vortices including vortex duration as percentage of the cardiac cycle, vector complexity (VC) at peak systole, and time-averaged wall shear stress (TA-WSS). Data was reported as median [interquartile range]. One or multiple vortices were detected in 94% of patients, whereas vortices were only present in 56% of healthy subjects. Moreover, longer vortex durations were observed in patients (45 [36-61] %) compared to healthy subjects (13 [6-19] %), despite normal peak systolic velocities. The vortices mainly evolved during the systolic phase and disappeared during diastolic phase. Higher VC values were observed at peak systole in patients (0.31 [0.21-0.47]) compared to healthy subjects (0.24 [0.12-0.29]). Overall, lower TA-WSS values were observed in patients (0.44 [0.39-0.55] Pa versus 0.69 [0.58-0.79] Pa). In conclusion, this preliminary study shows the potential of VVI in identifying complex and recirculating blood flow patterns. The observed flow patterns after CEA might indicate an abnormal, potentially atheroprone, condition and might be related to the high restenosis rate after CEA. Expanding the dataset is needed to confirm these findings and gain more insight in the hemodynamic consequences of surgical carotid plaque removal. Optical Coherence Tomography based versus Computed Tomography Angiography based Computational Fluid Dynamics simulations in the stented femoropopliteal tract Lisa Rutten, Lennart van de Velde, Michel Versluis, Michel Reijnen, Kartik Jain Abstract: Introduction: Computational fluid dynamics (CFD) models offer a valuable approach for enhancing our understanding and potentially prediction of in-stent restenosis (ISR) in the femoropopliteal tract. While computed tomography angiography (CTA) is commonly employed to capture vessel geometry, its limited resolution (~300 µm) and calcium blooming artifacts introduce uncertainties that can impact CFD outcomes. Intravascular optical coherence tomography (OCT) can be used to improve the geometry estimation as it has a better spatial resolution (15 – 20 µm) and minimal artifacts. However, OCT is invasive and typically has a limited field of view of 10 mm in diameter. In this study, CTA and OCT-based segmentations fed into a Lattice-Boltzmann CFD simulation of the stented femoropopliteal tract are compared to identify differences in geometrical and flow features. We also investigate a patient case with ISR. Methods CTA and OCT images of three stented femoropopliteal arteries were segmented. The radial difference between segmentations along the center lumen line was calculated (mean ± standard deviation). For one case that developed a proximal ISR 13 months after treatment, CFD simulations were performed using the Lattice-Boltzmann Method (LBM). The center lumen line velocity was calculated, and the time-averaged wall shear stress (TAWSS) reported [median (interquartile range)]. Results The CTA radius tended to be smaller than the OCT radius for the stented segment (-0.26 ± 0.27 mm), but not for the native vessel (0.04 ± 0.28 mm). The stent structure and apposition of the stent against the vessel wall were clearly visualized with OCT, but could not be resolved with CTA. The TAWSS in the complete geometry was lower in the OCT-based CFD simulation (1.37 [0.54 – 2.21] Pa vs. 1.81 [0.56 – 3.06] Pa). Regions of low TAWSS (<0.04 Pa) were found at the site of ISR (proximal stent) for the OCT-based, but not for the CTA-based CFD simulation. Conclusion These preliminary results show that the imaging modality affects global (radius) and local (stent structure and apposition) geometrical features that can impact CFD outcomes. Furthermore, first results suggest a correlation between OCT-CFD results and the occurrence of ISR. Data analysis of additional CTA and OCT-based CFD simulations is ongoing. Ultrasound based framework for CFD modeling of peripheral arteries using an optical tracking approach Milan Gillissen, Frans van de Vosse, Marc van Sambeek, Richard Lopata Abstract: Cardiovascular diseases (CVDs) are a leading cause of death, representing an estimated 32% of all global deaths in 2019. It is a group of diseases that involve the heart and the circulatory system, including peripheral artery disease (PAD) which affects over 230 million people worldwide. With atherosclerosis the most common underlying mechanism, lower extremity peripheral arterial occlusive disease (PAOD) is an increasingly common condition in all countries. Currently, the length of the occlusion is used as criteria for endovascular therapy or surgical bypass. However, main challenges of endovascular therapy include the durability of stents in the femoro-popliteal region where the artery is very mobile and the more typical restenosis, which can occur within six to twelve months. Several studies have shown that finite element analysis (FEA) can predict restenosis regions in different arteries. Despite promising results, these patient-specific models are not used in the clinical setting yet. The current computed tomography (CT) or magnetic resonance (MR) approaches suffer from several drawbacks, e.g. ionizing radiation and high cost. To address these issues and capture vessel motion during the cardiac cycle, we propose a framework based on time-resolved twodimensional ultrasound (US). This study introduces a novel screening method using an optical tracking system to locate US images in three-dimensional (3D) space with high accuracy. Multiple sweeps of the entire upper leg were performed to segment the peripheral artery of healthy volunteers. An in-house developed framework is demonstrated in this study, which automatically segments US images and meshes the obtained geometries for computational fluid dynamics analysis. The method was successfully applied in vivo, preliminary results show the segmented arteries as well as computational fluid dynamics (CFD) results of healthy volunteers. In the future, CFD models will be applied to patients with PAD and can be used for intervention planning as they will be expanded to enable the application of procedures to a digital twin. This includes virtually removing the stenosis and examining the circulatory system for any additional anomalies. Off-grid Ultrasound Imaging by Stochastic Optimization Vincent van de Schaft, Oisín Nolan, Ruud J. G. van Sloun Abstract: One of the key challenges in B-mode ultrasound imaging is increasing sample efficiency: obtaining a high-quality image from the smallest number of transmissions. Sample efficiency is required to achieve high framerates, and to conserve power in handheld applications. However, beamformers based on the Delay-and-Sum (DAS) principle improve image quality by compounding a large number of beamformed transmissions. As such, they are inherently not sample efficient. Additionally DAS beamforming does not produce echogenicity maps that are plausible under a measurement model. An alternative approach is to pose imaging as an inverse problem, in which we define a forward model that predicts the measured radio frequency (RF) data from a scatter map and then invert this model to find an image from very few transmissions. This method poses challenges because forward models demand substantial storage and compute and the exact values for many hardware and tissue properties are unknown. We present a matrix-free, and grid-free, stochastic algorithm to solve the inverse scattering problem in ultrasound imaging. Our functional definition and stochastic sampling allows us to jointly optimize on multiple transmissions without increasing the runtime and obviates the need for recomputing large model matrices with parameter changes. Optimizing for positions of the scatterers allows for a coarser grid and makes it possible to focus compute in regions with more tissue. We assess our method on a tissue mimicking phantom and on in-vivo acquisitions with a linear and a phased array transducer. Our method tends to produce images with significantly less clutter and better resolution in the far-field.

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