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Background: Three-dimensional, ECG-gated, time-resolved, three-directional, velocity-encoded phase-contrast MRI (4D flow MRI) has been applied extensively to measure blood velocity in great vessels but has been much less used in diseased carotid arteries. Carotid artery webs (CaW) are non-inflammatory intraluminal shelf-like projections into the internal carotid artery (ICA) bulb that are associated with complex flow and cryptogenic stroke., Purpose: Optimize 4D flow MRI for measuring the velocity field of complex flow in the carotid artery bifurcation model that contains a CaW., Methods: A 3D printed phantom model created from computed tomography angiography (CTA) of a subject with CaW was placed in a pulsatile flow loop within the MRI scanner. 4D Flow MRI images of the phantom were acquired with five different spatial resolutions (0.50-2.00  mm 3 ) and four different temporal resolutions (23-96 ms) and compared to a computational fluid dynamics (CFD) solution of the flow field as a reference. We examined four planes perpendicular to the vessel centerline, one in the common carotid artery (CCA) and three in the internal carotid artery (ICA) where complex flow was expected. At these four planes pixel-by-pixel velocity values, flow, and time average wall shear stress (TAWSS) were compared between 4D flow MRI and CFD., Hypothesis: An optimized 4D flow MRI protocol will provide a good correlation with CFD velocity and TAWSS values in areas of complex flow within a clinically feasible scan time (~ 10 min)., Results: Spatial resolution affected the velocity values, time average flow, and TAWSS measurements. Qualitatively, a spatial resolution of 0.50  mm 3 resulted in higher noise, while a lower spatial resolution of 1.50-2.00  mm 3 did not adequately resolve the velocity profile. Isotropic spatial resolutions of 0.50-1.00  mm 3 showed no significant difference in total flow compared to CFD. Pixel-by-pixel velocity correlation coefficients between 4D flow MRI and CFD were > 0.75 for 0.50-1.00  mm 3 but were < 0.5 for 1.50 and 2.00  mm 3 . Regional TAWSS values determined from 4D flow MRI were generally lower than CFD and decreased at lower spatial resolutions (larger pixel sizes). TAWSS differences between 4D flow and CFD were not statistically significant at spatial resolutions of 0.50-1.00  mm 3 but were different at 1.50 and 2.00 mm 3 . Differences in temporal resolution only affected the flow values when temporal resolution was > 48.4 ms; temporal resolution did not affect TAWSS values., Conclusion: A spatial resolution of 0.74-1.00  mm 3 and a temporal resolution of 23-48 ms (1-2 k-space segments) provides a 4D flow MRI protocol capable of imaging velocity and TAWSS in regions of complex flow within the carotid bifurcation at a clinically acceptable scan time., (© 2023. The Author(s) under exclusive licence to Biomedical Engineering Society.)

Purpose: To evaluate the agreement of 4D flow cMRI-derived bulk flow features and fluid (blood) velocities in the carotid bifurcation using prospective and retrospective gating techniques., Methods: Prospective and retrospective ECG-gated three-dimensional (3D) cine phase-contrast cardiac MRI with three-direction velocity encoding (i.e., 4D flow cMRI) data were acquired in ten carotid bifurcations from men (n = 3) and women (n = 2) that were cardiovascular disease-free. MRI sequence parameters were held constant across all scans except temporal resolution values differed. Velocity data were extracted from the fluid domain and evaluated across the entire volume or at defined anatomic planes (common, internal, external carotid arteries). Qualitative agreement between gating techniques was performed by visualizing flow streamlines and topographical images, and statistical comparisons between gating techniques were performed across the fluid volume and defined anatomic regions., Results: Agreement in the kinematic data (e.g., bulk flow features and velocity data) were observed in the prospectively and retrospectively gated acquisitions. Voxel differences in time-averaged, peak systolic, and diastolic-averaged velocity magnitudes between gating techniques across all volunteers were 2.7%, 1.2%, and 6.4%, respectively. No significant differences in velocity magnitudes or components ([Formula: see text], [Formula: see text], [Formula: see text]) were observed. Importantly, retrospective acquisitions captured increased retrograde flow in the internal carotid artery (i.e., carotid sinus) compared to prospective acquisitions (10.4 ± 6.3% vs. 4.6 ± 5.3%; [Formula: see text] < 0.05)., Conclusion: Prospective and retrospective ECG-gated 4D flow cMRI acquisitions provide comparable evaluations of fluid velocities, including velocity vector components, in the carotid bifurcation. However, the increased temporal coverage of retrospective acquisitions depicts increased retrograde flow patterns (i.e., disturbed flow) not captured by the prospective gating technique., (© 2022. The Author(s) under exclusive licence to Biomedical Engineering Society.)

Understanding of the hemodynamics of Type B aortic dissection may improve outcomes by informing upon patient selection, device design, and deployment strategies. This project characterized changes to aortic hemodynamics as the result of dissection. We hypothesized that dissection would lead to elevated flow reversal and disrupted pulsatile flow patterns in the aorta that can be detected and quantified by non-invasive magnetic resonance imaging. Flexible, anatomic models of both normal aorta and dissected aorta, with a mobile intimal flap containing entry and exit tears, were perfused with a physiologic pulsatile waveform. Four-dimensional phase contrast magnetic resonance (4D PCMR) imaging was used to measure the hemodynamics. These images were processed to quantify pulsatile fluid velocities, flow rate, and flow reversal. Four-dimensional flow imaging in the dissected aorta revealed pockets of reverse flow and vortices primarily in the false lumen. The dissected aorta exhibited significantly greater flow reversal in the proximal-to-mid dissection as compared to normal (21.1 ± 3.8 vs. 1.98 ± 0.4%, p < 0.001). Pulsatility induced unsteady vortices and a pumping motion of the distal intimal flap corresponding to flow reversal. Summed true and false lumen flow rates in dissected models (4.0 ± 2.0 L/min) equaled normal flow rates (3.8 ± 0.1 L/min, p > 0.05), validated against external flow measurement. Pulsatile aortic hemodynamics in the presence of an anatomic, elastic dissection differed significantly from those of both steady flow through a dissection and pulsatile flow through a normal aorta. New hemodynamic features including flow reversal, large exit tear vortices, and pumping action of the mobile intimal flap, were observed. False lumen flow reversal would possess a time-averaged velocity close to stagnation, which may induce future thrombosis. Focal vortices may identify the location of tears that could be covered with a stent-graft. Future correlation of hemodynamics with outcomes may indicate which patients require earlier intervention.

Cardiac allograft vasculopathy (CAV) is one of the leading causes of morbidity and morality in orthotopic heart transplant (HTx) patients. While disturbed flow patterns have been linked to the spatial localization of atherosclerosis, the role of hemodynamics in CAV development has not been examined. HTx patients (n = 5) requiring percutaneous coronary intervention (PCI) for a focal, epicardial lesion were studied. Angiographic images were retrospectively obtained from baseline (i.e., in the presence of no observed disease) and follow-up catheterizations (i.e., at the time of PCI; 12.4 ± 2.6 years post-HTx). Patient-specific computational models were created from baseline images. Computational fluid dynamic techniques were employed to quantify the hemodynamic environment, which was expressed as normalized time-averaged WSS (TAWSSnorm; measure of temporal WSS magnitude) and normalized WSS angle deviation (WSSADnorm; measure of instantaneous WSS vector oscillation) values. Baseline hemodynamic and follow-up angiographic data were co-registered to investigate the association between WSS and subsequent occlusive CAV lesion location. Results indicate a high degree of co-localization between baseline low WSS data and follow-up occlusive CAV lesion. Local minima in TAWSSnorm were located 2.5 ± 0.6 mm from the site of PCI. Furthermore, local maxima in WSSADnorm were located 3.9 ± 0.7 mm from the site of PCI. In 3 patients, the occlusive lesion formed in a region that was subjected to both low and oscillatory WSS at baseline. There was discernable spatial co-localization between baseline disturbed flow patterns and follow-up CAV lesions requiring PCI. These results suggest a role of fluid mechanics in the development of focal, flow-limiting CAV lesions.

Purpose: We investigated whether local hemodynamics were associated with sites of plaque erosion and hypothesized that patients with plaque erosion have locally elevated WSS magnitude in regions where erosion has occurred., Methods: We generated 3D, patient-specific models of coronary arteries from biplane angiographic images in 3 human patients with plaque erosion diagnosed by optical coherence tomography (OCT). Using computational fluid dynamics, we simulated pulsatile blood flow and calculated both wall shear stress (WSS) and oscillatory shear index (OSI). We also investigated anatomic features of plaque erosion sites by examining branching and local curvature in x-ray angiograms of barium-perfused autopsy hearts., Results: Neither high nor low magnitudes of mean WSS were associated with sites of plaque erosion. OSI and local curvature were also not associated with erosion. Anatomically, 8 of 13 hearts had a nearby bifurcation upstream of the site of plaque erosion., Conclusions: This study provides preliminary evidence that neither hemodynamics nor anatomy are predictors of plaque erosion, based upon a very unique dataset. Our sample sizes are small, but this dataset suggests that high magnitudes of wall shear stress, one potential mechanism for inducing plaque erosion, are not necessary for erosion to occur.