3D Phase-contrast Magnetic Resonance Research Articles

Higher intracranial arterial pulsatility is associated with presumed imaging markers of the glymphatic system: An explorative study

BackgroundArterial pulsation has been suggested as a key driver of paravascular cerebrospinal fluid flow, which is the foundation of glymphatic clearance. However, whether intracranial arterial pulsatility is associated with glymphatic markers in humans has not yet been studied. MethodsSeventy-three community participants were enrolled in the study. 4D phase-contrast magnetic resonance imaging (MRI) was used to quantify the hemodynamic parameters including flow pulsatility index (PIflow) and area pulsatility index (PIarea) from 13 major intracerebral arterial segments. Three presumed neuroimaging markers of the glymphatic system were measured: including dilation of perivascular space (PVS), diffusivity along the perivascular space (ALPS), and volume fraction of free water (FW) in white matter. We explored the relationships between PIarea, PIflow, and the presumed glymphatic markers, controlling for related covariates. ResultsPIflow in the internal carotid artery (ICA) C2 segment (OR, 1.05; 95 % CI, 1.01–1.10, per 0.01 increase in PI) and C4 segment (OR, 1.05; 95 % CI, 1.01–1.09) was positively associated with the dilation of basal ganglia PVS, and PIflow in the ICA C4 segment (OR, 1.06, 95 % CI, 1.02–1.10) was correlated with the dilation of PVS in the white matter. ALPS was associated with PIflow in the basilar artery (β, -0.273, p, 0.046) and PIarea in the ICA C2 (β, -0.239, p, 0.041) and C7 segments (β, -0.238, p, 0.037). Conclusions: Intracranial arterial pulsatility was associated with presumed neuroimaging markers of the glymphatic system, but the results were not consistent across different markers. Further studies are warranted to confirm these findings.

Venous 3D Phase Contrast Magnetic Resonance Angiography Increases Diagnostic Certainty in Children with Ventriculoperitoneal Shunt and Suspected Shunt Failure

BackgroundClinical symptoms in children with suspected malfunction of ventriculoperitoneal shunt may not be specific and difficult to interpret. The presence or absence of ventricular enlargement on magnetic resonance imaging (MRI) does not reliably predict raised intracranial pressure (ICP) in these patients. Therefore, the aim was to investigate the diagnostic utility of 3D venous phase-contrast MR angiography (vPCA) in these patients.MaterialsThe MR studies of two groups of patients at two different examination dates were retrospectively analyzed; one group without clinical symptoms on both examinations and one with symptoms of shunt dysfunction on one examination receiving surgery. Both MRI examinations had to have been performed including axial T2 weighted (T2-w) images and 3D vPCA. Two (neuro)radiologists evaluated T2-w images alone and in combination with 3D vPCA in terms of suspected elevated ICP. Interrater reliability, sensitivity and specificity were assessed.ResultsCompression of venous sinuses was seen significantly more often in patients with shunt failure (p = 0.00003). Consequently, evaluation of 3D vPCA and T2-w images increases sensitivity to 0.92/1.0 compared to T2-w images alone with 0.69/0.77, the interrater agreement for the diagnosis of shunt failure rises from κ = 0.71 to κ = 0.837. Concerning imaging markers, three groups could be identified in children with shunt failure.ConclusionIn accordance with the literature, the results show that ventricular morphology alone is an unreliable marker for elevated ICP in children with shunt malfunction. The findings confirmed 3D vPCA as a valuable supplemental diagnostic tool improving diagnostic certainty for children with unchanged ventricular size in cases of shunt failure.

Segmentation of 4D Flow MRI: Comparison between 3D Deep Learning and Velocity-Based Level Sets.

A thoracic aortic aneurysm is an abnormal dilatation of the aorta that can progress and lead to rupture. The decision to conduct surgery is made by considering the maximum diameter, but it is now well known that this metric alone is not completely reliable. The advent of 4D flow magnetic resonance imaging has allowed for the calculation of new biomarkers for the study of aortic diseases, such as wall shear stress. However, the calculation of these biomarkers requires the precise segmentation of the aorta during all phases of the cardiac cycle. The objective of this work was to compare two different methods for automatically segmenting the thoracic aorta in the systolic phase using 4D flow MRI. The first method is based on a level set framework and uses the velocity field in addition to 3D phase contrast magnetic resonance imaging. The second method is a U-Net-like approach that is only applied to magnitude images from 4D flow MRI. The used dataset was composed of 36 exams from different patients, with ground truth data for the systolic phase of the cardiac cycle. The comparison was performed based on selected metrics, such as the Dice similarity coefficient (DSC) and Hausdorf distance (HD), for the whole aorta and also three aortic regions. Wall shear stress was also assessed and the maximum wall shear stress values were used for comparison. The U-Net-based approach provided statistically better results for the 3D segmentation of the aorta, with a DSC of 0.92 ± 0.02 vs. 0.86 ± 0.5 and an HD of 21.49 ± 24.8 mm vs. 35.79 ± 31.33 mm for the whole aorta. The absolute difference between the wall shear stress and ground truth slightly favored the level set method, but not significantly (0.754 ± 1.07 Pa vs. 0.737 ± 0.79 Pa). The results showed that the deep learning-based method should be considered for the segmentation of all time steps in order to evaluate biomarkers based on 4D flow MRI.

Data-driven generation of 4D velocity profiles in the aneurysmal ascending aorta

Background and Objective: Numerical simulations of blood flow are a valuable tool to investigate the pathophysiology of ascending thoratic aortic aneurysms (ATAA). To accurately reproduce in vivo hemodynamics, computational fluid dynamics (CFD) models must employ realistic inflow boundary conditions (BCs). However, the limited availability of in vivo velocity measurements, still makes researchers resort to idealized BCs. The aim of this study was to generate and thoroughly characterize a large dataset of synthetic 4D aortic velocity profiles sampled on a 2D cross-section along the ascending aorta with features similar to clinical cohorts of patients with ATAA.Methods: Time-resolved 3D phase contrast magnetic resonance (4D flow MRI) scans of 30 subjects with ATAA were processed through in-house code to extract anatomically consistent cross-sectional planes along the ascending aorta, ensuring spatial alignment among all planes and interpolating all velocity fields to a reference configuration. Velocity profiles of the clinical cohort were extensively characterized by computing flow morphology descriptors of both spatial and temporal features. By exploiting principal component analysis (PCA), a statistical shape model (SSM) of 4D aortic velocity profiles was built and a dataset of 437 synthetic cases with realistic properties was generated.Results: Comparison between clinical and synthetic datasets showed that the synthetic data presented similar characteristics as the clinical population in terms of key morphological parameters. The average velocity profile qualitatively resembled a parabolic-shaped profile, but was quantitatively characterized by more complex flow patterns which an idealized profile would not replicate. Statistically significant correlations were found between PCA principal modes of variation and flow descriptors.Conclusions: We built a data-driven generative model of 4D aortic inlet velocity profiles, suitable to be used in computational studies of blood flow. The proposed software system also allows to map any of the generated velocity profiles to the inlet plane of any virtual subject given its coordinate set.

Computational study of fenestration and parallel grafts used in TEVAR of aortic arch aneurysms.

To explore the differences between fenestration technique and parallel grafts technique of thoracic endovascular aortic repair, and evaluate the risk of complications after interventional treatment of aortic arch aneurysms. A three-dimensional aortic model was established from the follow-up imaging data of patient who reconstructed the superior arch vessel by the chimney technique, which was called the chimney model. Based on the chimney model, the geometric of the reconstructed vessel was modified by virtual surgery, and the normal model, fenestration model and periscope model were established. The blood flow waveforms measured by 2D phase contrast magnetic resonance imaging were processed as the boundary conditions of the ascending aorta inlet and the superior arch vessels outlets of the normal model. The pressure waveform of descending aorta was obtained using three-element Windkessel model, and specific pressure boundary conditions were imposed at reconstructed branches for the postoperative models. Through computational fluid dynamics simulations, the hemodynamic parameters of each model were obtained. The reconstructed vessel flow rate of the periscope model and the fenestration model are 33% and 50% of that of the normal model, respectively. The pressure difference between the inner and outer walls of the fenestration stent and periscope stent is 3.15 times and 7.56 times that of the chimney stent. The velocity in the fenestration stent and periscope stent is uneven. The high relative residence time is concentrated in the region around the branch stents, which is prone to thrombosis. The "gutter" part of the chimney model may become larger due to the effect of the stent-graft DF, increasing the risk of endoleak. For patients with incomplete circle of Willis, the periscope technique to reconstruct the supra-arch vessels may affect blood perfusion. It is recommended to use balloon-expandable stent for fenestration stent and periscope stent, and self-expanding stent for chimney stent. For patients with aortic arch aneurysms, the fenestration technique may be superior to the parallel grafts technique.

Hemodynamic Changes Before and After Endovascular Treatment of Type B Aortic Dissection by 4D Flow MRI.

Objective:The standard treatment for complicated Stanford type B aortic dissection (TBAD) is thoracic endovascular aortic repair (TEVAR). Functional parameters, specifically blood flow, are not measured in the clinical assessment of TEVAR, yet they are of outmost importance in patient outcome. Consequently, we investigated the impact of TEVAR on the flows in the aorta and its branches in TBAD using 4D Phase-Contrast Magnetic Resonance Imaging (4D Flow MRI).MethodsSeven patients with TBAD scheduled for TEVAR underwent pre and post-operative 4D Flow MRI. An experienced reader assessed the presence of helical flow in the false lumen (FL) using streamlines and measured net flow at specific locations. In addition, forward and reverse flows, stasis, helicity, and absolute helicity were computed automatically along the aorta centerline. Average values were then computed in the segmented vessels. Impact of TEVAR on these parameters was assessed with a Wilcoxon signed rank test. Impact of the metallic stent on the velocity quantification was assessed using intra-class correlation coefficient (ICC) between velocities measured intra-stent and in adjacent stent-free regions.ResultsFL helical flow was observed proximally in 6 cases and distally in 2 cases pre-operatively. Helical flow disappeared post-TEVAR proximally, but developed distally for 2 patients. Intra-stent measures were similar to stent-free with a median difference of 0.1 L/min and an ICC equal to 0.967 (p < 0.01). Forward flow increased from 59.9 to 81.6% in the TL and significantly decreased in the FL from 15.9 to 3.3%. Similarly, reverse flow increased in the TL from 4.36 to 10.8% and decreased in the FL from 10.3 to 4.6%. No significant changes were observed in net flow for aortic branches (p > 0.05). A significant increase in FL stasis was observed (p = 0.04).DiscussionTEVAR significantly increased forward flow in the TL and significantly decreased both forward and reverse flows in the FL. Interestingly, reverse flow in the TL increased post-TEVAR, which could be due to increased rigidity of the wall, due to the metallic stent. User independent helicity quantification enabled detection of elevated helicity at the level of secondary entry tears which had been missed by streamline visualization.

Hemodynamics in a three-dimensional printed aortic model: a comparison of four-dimensional phase-contrast magnetic resonance and image-based computational fluid dynamics.

This study aims to compare an electrocardiogram (ECG)-gated four-dimensional (4D) phase-contrast (PC) magnetic resonance imaging (MRI) technique and computational fluid dynamics (CFD) using variables controlled in a laboratory environment to minimize bias factors. Data from 4D PC-MRI were compared with computational fluid dynamics using steady and pulsatile flows at various inlet velocities. Anatomically realistic models for a normal aorta, a penetrating atherosclerotic ulcer, and an abdominal aortic aneurysm were constructed using a three-dimensional printer. For the normal aorta model, the errors in the peak and the average velocities were within 5%. The peak velocities of the penetrating atherosclerotic ulcer and the abdominal aortic aneurysm models displayed a more extensive range of differences because of the high-speed and vortical fluid flows generated by the shape of the blood vessel. However, the average velocities revealed only relatively minor differences. This study compared the characteristics of PC-MRI and CFD through a phantom study that only included controllable experimental parameters. Based on these results, 4D PC-MRI and CFD are powerful tools for analyzing blood flow patterns in vivo. However, there is room for future developments to improve velocity measurement accuracy.

Numerical simulation in the abdominal aorta and the visceral arteries with or without stenosis based on 2D PCMRI.

It is important to obtain accurate boundary conditions (BCs) in hemodynamic simulations. This article aimed to improve the accuracy of BCs in computational fluid dynamics (CFD) simulation and analyze the differences in hemodynamics between healthy volunteers and patients with visceral arterial stenosis (VAS). The geometric models of seven cases were reconstructed using the magnetic resonance angiogram (MRA) or computed tomography angiogram (CTA) imaging data. The physiological flow waveforms obtained from 2D Phase Contrast Magnetic Resonance Imaging (PCMRI) were imposed on the aortic inlet and the visceral arteries' outlets. The individualized RCR values of the three-element Windkessel model were imposed on the aortic outlet. CFD simulations were run in the open-source software: svSolver. Two specific time points were selected to compare the hemodynamics of healthy volunteers and patients with VAS. The results suggested that blood in the stenotic visceral arteries flowed at high speed throughout the cardiac cycle. The low pressure is distributed at stenotic lesions. The wall shear stress (WSS) reached 4Pa near stenotic locations. The low time average wall shear stress (TAWSS), high oscillatory shear index (OSI), and high relative residence time (RRT) concentrated in the abdominal aorta. Besides, the ratios of the areas with low TAWSS, high OSI, and high RRT to the computational domain were higher in patients with VAS than which in the healthy volunteers. The individualized BCs were used for hemodynamic simulations and results suggest that patients with stenosis have a higher risk of blood retention and atherosclerosis formation in the abdominal aorta.

Comprehensive mechanical &amp; metabolic imaging of abdominal aortic aneurysm with 4D flow/ FDG PET on an integrated PETMRI: a feasibility study

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): NIHR Biomedical Research Centre, University College London Hospitals. Background A number of non-invasive imaging derived parameters have been implicated in the development and progression of abdominal aortic aneurysm, although the mechanism, and relationships of many of these are yet to be precisely determined. Mechanical parameters can now be studied using 4D phase contrast magnetic resonance (PCMR), and inflammatory cellular activity can be detected with FDG PET. Purpose It may be postulated that inflammation of the aortic wall may be the intermediary at the tissue level linking mechanical wall shear stress (WSS) to aneurysm progression. It may be feasible to study 4D PCMR and FDG PET at the same patient visit on a PETMRI platform, with the potential to enhance temporal and spatial co-registration and improving the understanding of any relationship between these two parameters. Our study aims to assess feasibility of studying these on an integrated PETMRI system. Methods 7 patients with known aortic aneurysm were recruited in a vascular ultrasound screening follow up clinic. During a single visit following 6 hours fasting, all patients underwent FDG injection and 60 minutes uptake period. With quiet breathing, list mode PET acquisition and concurrent 4D PCMR was acquired using stacks of spiral acquisition, with ECG trace information for retrospective gating. Images from the 4D PCMR and FDG PET were assessed qualitatively for image quality and visual matching. Results All 7 patients completed the study. Overall image quality was adequate to good. There is qualitatively a good concordance with impression of positive correlation between wall shear stress and inflammatory signal (see attached image). Conclusion We have demonstrated feasibility of combined assessment of mechanical and metabolic imaging parameters using an integrated PETMRI system. Initial findings show there to be a broad concordance of wall shear stress and inflammatory signal in the abdominal aneurysm.

Blood Flow Velocity Pulsatility and Arterial Diameter Pulsatility Measurements of the Intracranial Arteries Using 4D PC-MRI

4D phase contrast magnetic resonance imaging (PC-MRI) allows for the visualization and quantification of the cerebral blood flow. A drawback of software that is used to quantify the cerebral blood flow is that it oftentimes assumes a static arterial luminal area over the cardiac cycle. Quantifying the lumen area pulsatility index (aPI), i.e. the change in lumen area due to an increase in distending pressure over the cardiac cycle, can provide insight in the stiffness of the arteries. Arterial stiffness has received increased attention as a predictor in the development of cerebrovascular disease. In this study, we introduce software that allows for measurement of the aPI as well as the blood flow velocity pulsatility index (vPI) from 4D PC-MRI. The internal carotid arteries of seven volunteers were imaged using 7 T MRI. The aPI and vPI measurements from 4D PC-MRI were validated against measurements from 2D PC-MRI at two levels of the internal carotid arteries (C3 and C7). The aPI and vPI computed from 4D PC-MRI were comparable to those measured from 2D PC-MRI (aPI: mean difference: 0.03 (limits of agreement: −0.14 – 0.23); vPI: 0.03 (−0.17–0.23)). The measured blood flow rate for the C3 and C7 segments was similar, indicating that our proposed software correctly captures the variation in arterial lumen area and blood flow velocity that exists along the distal end of the carotid artery. Our software may potentially aid in identifying changes in arterial stiffness of the intracranial arteries caused by pathological changes to the vessel wall.

Quantification and mapping of cerebral hemodynamics before and after carotid endarterectomy, using four-dimensional flow magnetic resonance imaging

BackgroundCarotid stenosis can profoundly affect cerebral hemodynamics, which cannot simply be inferred from the degree of stenosis. We quantified and mapped the distribution of the blood flow rate (BFR) in the cerebral arteries before and after carotid endarterectomy using four-dimensional (4D) phase-contrast (PC) magnetic resonance imaging (MRI). MethodsNineteen patients (age, 71 ± 6 years; 2 women) with symptomatic carotid stenosis (≥50%) undergoing carotid endarterectomy (CEA) were investigated using 4D PC-MRI before and after surgery. The BFR was measured in 17 cerebral arteries and the ophthalmic arteries. Collateral recruitment through the anterior and posterior communicating arteries, ophthalmic arteries, and leptomeningeal arteries was quantified. BFR laterality was significantly different between the paired contralateral and ipsilateral arteries. Subgroups were defined according to the presence of collateral recruitment. ResultsThe total cerebral blood flow had increased by 15% (P < .01) after CEA. Before CEA, laterality was seen in the internal carotid artery, anterior cerebral artery, and middle cerebral artery (MCA). On the ipsilateral side, an increased BFR was found after CEA in the internal carotid artery (246 ± 62 mL/min vs 135 ± 80 mL/min; P < .001), anterior cerebral artery (87 ± mL/min vs 38 ± 58 mL/min; P < .01), and MCA (149 ± 43 mL/min vs 119 ± 34 mL/min; P < .01), resulting in a postoperative BFR distribution without signs of laterality. In the nine patients with preoperatively recruited collaterals, BFR laterality was found in the MCA before, but not after, CEA (P < .01). This laterality was not found in the 10 patients without collateral recruitment (P = .2). The degree of stenosis did not differ between the groups with and without collateral recruitment (P = .85). ConclusionsUsing 4D PC-MRI, we have presented a comprehensive and noninvasive method to evaluate the cerebral hemodynamics due to carotid stenosis before and after CEA. MCA laterality, seen in the patients with collateral recruitment before CEA, pointed toward a hemodynamic disturbance in MCA territory for those patients. This methodologic advancement provides an insight into the pathophysiology of cerebral hemodynamics in patients with carotid stenosis.

Numerical simulation of time-resolved 3D phase-contrast magnetic resonance imaging.

A numerical approach is presented to efficiently simulate time-resolved 3D phase-contrast Magnetic resonance Imaging (or 4D Flow MRI) acquisitions under realistic flow conditions. The Navier-Stokes and Bloch equations are simultaneously solved with an Eulerian-Lagrangian formalism. A semi-analytic solution for the Bloch equations as well as a periodic particle seeding strategy are developed to reduce the computational cost. The velocity reconstruction pipeline is first validated by considering a Poiseuille flow configuration. The 4D Flow MRI simulation procedure is then applied to the flow within an in vitro flow phantom typical of the cardiovascular system. The simulated MR velocity images compare favorably to both the flow computed by solving the Navier-Stokes equations and experimental 4D Flow MRI measurements. A practical application is finally presented in which the MRI simulation framework is used to identify the origins of the MRI measurement errors.

Daikenchuto increases blood flow in the superior mesenteric artery in humans: A comparison study between four-dimensional phase-contrast vastly undersampled isotropic projection reconstruction magnetic resonance imaging and Doppler ultrasound

Respiratory-gated four-dimensional phase-contrast vastly undersampled isotropic projection reconstruction (4D PC-VIPR) is magnetic resonance (MR) imaging technique that enables analysis of vascular morphology and hemodynamics in a single examination using cardiac phase resolved 3D phase-contrast magnetic resonance imaging. The present study aimed to assess the usefulness of 4D PC-VIPR for the superior mesenteric artery (SMA) flowmetry before and after flow increase was induced by the herbal medicine Daikenchuto (TJ-100) by comparing it with Doppler ultrasound (DUS) as a current standard. Twenty healthy volunteers were enrolled in this prospective single-arm study. The peak cross-sectionally averaged velocity was measured by 4D PC-VIPR, peak velocity was measured by DUS, and flow volume (FV) of SMA and aorta were measured by 4D PC-VIPR and DUS 25 min before and after the peroral administration of TJ-100. The peak cross-sectionally averaged velocity, peak velocity, and FV of SMA measured by 4D PC-VIPR and DUS significantly increased after administration of TJ-100 (4D PC-VIPR: the peak cross-sectionally averaged velocity; p = 0.004, FV; p = 0.035, DUS: the peak velocity; p = 0.003, FV; p = 0.010). Furthermore, 4D PC-VIPR can analyze multiple blood vessels simultaneously. The ratio of the SMA FV to the aorta, before and after oral administration on the 4D PC-VIPR test also increased (p = 0.015). The rate of change assessed by 4D PC-VIPR and DUS were significantly correlated (the peak cross-sectionally averaged velocity and peak velocity: r = 0.650; p = 0.005, FV: r = 0.659; p = 0.004). Retrospective 4D PC-VIPR was a useful modality for morphological and hemodynamic analysis of SMA.

Non-contrast-enhanced magnetic resonance angiography of facial arteries for pre-operative evaluation of vascularized submental lymph node flaps

BackgroundThe aim of this study was to compare non-contrast-enhanced 3D phase contrast magnetic resonance angiography (3D PC-MRA) and conventional intravenous administration of contrast media, i.e., contrast-enhanced MRA (CE-MRA), to evaluate the courses of facial arteries for the preparation of vascularized submental lymph node flap (VSLN flap) transfer.MethodsThe head and neck regions of 20 patients with limb lymphedema were imaged using a 3 T MRI scanner. To improve the evaluation of facial artery courses, MRA was fused with anatomical structures generated by high-resolution T1-weighted imaging. The diagnostic and image qualities of facial arteries for VSLN flap planning were independently rated by two radiologists. Interobserver agreement was evaluated using Cohen’s kappa. Differences between 3D PC-MRA and CE-MRA in terms of the diagnostic quality of facial arteries were evaluated using McNemar’s test.ResultsCohen’s kappa indicated fair to good interobserver agreement for the diagnostic and image qualities of the bilateral facial arteries. No significant difference in terms of the diagnostic quality of the left and right facial arteries between 3D PC-MRA and CE-MRA, respectively, was identified.ConclusionsNon-contrast 3D PC-MRA is a reliable method for the evaluation of facial artery courses prior to VSLN flap transfer and could serve as an alternative to CE-MRA for patients with renal insufficiency or severe adverse reactions to contrast media.

Towards validating stent induced micro flow patterns in left main coronary artery bifurcations.

We investigated if blood flow changes induced through the presence of a stent could be detected using in vitro dynamically scaled 4D Phase-Contrast Magnetic Resonance Imaging (PC-MRI). Using idealized and patient-specific left main coronary artery bifurcations, we 3D-printed the dynamically large scaled geometries and incorporated them into a flow circuit for non-invasive acquisition with a higher effective spatial resolution. We tested the effects of using non-Newtonian and Newtonian fluids for the experiment. We also numerically simulated the same geometries in true scale for comparison using computational fluid dynamics (CFD). We found that the experimental setup increased the effective spatial resolution enough to reveal stent induced blood flow changes close to the vessel wall. Non-Newtonian fluid replicated all of the flow field well with a strong agreement with the computed flow field (R2 > 0.9). Fine flow structures were not as prominent for the Newtonian compared to non-Newtonian fluid consideration. In the patient-specific geometry, arterial non-planarity increased the difficulty to capture the near wall slow velocity changes. Findings demonstrate the potential to dynamically scale in vitro 4D MRI flow acquisition for micro blood flow considerations.

Comparison of turbulent flow measurement schemes for 4D flow MRI

Recently, time-resolved 3D phase contrast magnetic resonance imaging, which is also called 4D flow MRI or 4D PC-MRI, has been widely utilized to investigate spatial and temporal variations in the hemodynamic characteristics of blood flows. The conventional 4D flow MRI only provides the ensemble-averaged mean flow information but misses flow fluctuation because of turbulence. There have been some new manners for turbulence quantification of the blood flow using 4D flow MRI to acquire the fluctuation intensity of the turbulent flow. Although the previous studies showed that a minimum of six flow encoding is needed to perfectly describe six independent components of the 3 × 3 symmetric Reynolds stress tensor, the optimum flow encoding scheme for turbulence has not been clearly revealed. In this study, we compared and evaluated the uncertainty of mean velocity and Reynolds stress tensor in 4D flow MRI with various flow encoding schemes. The results showed that six- to fifteen-direction measurements could result 29–51% less noise in the velocity field and 34–57% less noise in the TKE field. In addition, higher number of flow encoding had less noise in Reynolds stress estimation.

Bulk Flow and Near Wall Hemodynamics of the Rabbit Aortic Arch and Descending Thoracic Aorta: A 4D PC-MRI Derived Computational Fluid Dynamics Study.

Animal models offer a flexible experimental environment for studying atherosclerosis. The mouse is the most commonly used animal, however, the underlying hemodynamics in larger animals such as the rabbit are far closer to that of humans. The aortic arch is a vessel with complex helical flow and highly heterogeneous shear stress patterns which may influence where atherosclerotic lesions form. A better understanding of intraspecies flow variation and the impact of geometry on flow may improve our understanding of where disease forms. In this work, we use magnetic resonance angiography (MRA) and 4D phase contrast magnetic resonance imaging (PC-MRI) to image and measure blood velocity in the rabbit aortic arch. Measured flow rates from the PC-MRI were used as boundary conditions in computational fluid dynamics (CFD) models of the arches. Helical flow, cross flow index (CFI), and time-averaged wall shear stress (TAWSS) were determined from the simulated flow field. Both traditional geometric metrics and shape modes derived from statistical shape analysis were analyzed with respect to flow helicity. High CFI and low TAWSS were found to colocalize in the ascending aorta and to a lesser extent on the inner curvature of the aortic arch. The Reynolds number was linearly associated with an increase in helical flow intensity (R = 0.85, p < 0.05). Both traditional and statistical shape analyses correlated with increased helical flow symmetry. However, a stronger correlation was obtained from the statistical shape analysis demonstrating its potential for discerning the role of shape in hemodynamic studies.

In-vitro validation of 4D flow MRI measurements with an experimental pulsatile flow model.

The purpose of this study was to assess the precision of four-dimensional (4D) phase-contrast magnetic resonance imaging (PCMRI) to measure mean flow and peak velocity (Vmax) in a pulsatile flow phantom and to test its sensitivity to spatial resolution and Venc. The pulsatile flow phantom consisted of a straight tube connected to the systemic circulation of an experimental mock circulatory system. Four-dimensional-PCMR images were acquired using different spatial resolutions (minimum pixel size: 1.5×1.5×1.5mm3) and velocity encoding sensitivities (up to three times Vmax). Mean flow and Vmax calculated from 4D-PCMRI were compared respectively to the reference phantom flow parameters and to Vmax obtained from two-dimensional (2D)-PCMRI. 4D-PCI measured mean flow with a precision of -0.04% to+5.46%, but slightly underestimated Vmax when compared to 2D-PCMRI (differences ranging from -1.71% to -3.85%). 4D PCMRI mean flow measurement was influenced by spatial resolution (P<0.001) with better results obtained with smaller voxel size. There was no effect of Venc on mean flow measurement. Regarding Vmax, neither spatial resolution nor Venc did influence the precision of the measurement. Using an experimental pulsatile flow model 4D-PCMRI is accurate to measure mean flow and Vmax with better results obtained with higher spatial resolution. We also show that Venc up to 3 times higher than Vmax may be used with no effect on these measurements.

Augmented Reality with Virtual Cerebral Aneurysms: A Feasibility Study

Advanced magnetic resonance imaging (MRI) acquisition methods such as 4D phase contrast magnetic resonance imaging (4D pcMRI) have found widespread applications in assessing hemodynamics in cerebral aneurysms. One of the goals is the identification of a hemodynamic scenario predictive of aneurysm rupture. Because of the technical complexity the 4D pcMRI technology, challenges exist in extracting and communicating the wealth of information contained in the MRI image data, in particular, to a multidisciplinary team of health professionals and basic scientists. Here we introduce a new resource in the form of a commercially available optical see-through head-mounted display, which allows augmenting the real world by computer-generated virtual objects. An image processing algorithm was presented for integrating 3D surface models derived from 3D rotational angiography data, 3D time-of-flight MRI data, and 4D pcMRI data into the augmented reality environment. This new approach was successfully evaluated on blood flow data acquired before and after flow diverter treatment in 6 patients harboring a cerebral aneurysm. Augmented reality may become an additional means for visualizing complex medical data, in particular complex flows as illustrated here.

Computational study of aortic hemodynamics for patients with an abnormal aortic valve: The importance of secondary flow at the ascending aorta inlet.

Blood flow in the aorta is helical, but most computational studies ignore the presence of secondary flow components at the ascending aorta (AAo) inlet. The aim of this study is to ascertain the importance of inlet boundary conditions (BCs) in computational analysis of flow patterns in the thoracic aorta based on patient-specific images, with a particular focus on patients with an abnormal aortic valve. Two cases were studied: one presenting a severe aortic valve stenosis and the other with a mechanical valve. For both aorta models, three inlet BCs were compared; these included the flat profile and 1D through-plane velocity and 3D phase-contrast magnetic resonance imaging derived velocity profiles, with the latter being used for benchmarking. Our results showed that peak and mean velocities at the proximal end of the ascending aorta were underestimated by up to 41% when the secondary flow components were neglected. The results for helical flow descriptors highlighted the strong influence of secondary velocities on the helical flow structure in the AAo. Differences in all wall shear stress (WSS)-derived indices were much more pronounced in the AAo and aortic arch (AA) than in the descending aorta (DAo). Overall, this study demonstrates that using 3D velocity profiles as inlet BC is essential for patient-specific analysis of hemodynamics and WSS in the AAo and AA in the presence of an abnormal aortic valve. However, predicted flow in the DAo is less sensitive to the secondary velocities imposed at the inlet; hence, the 1D through-plane profile could be a sufficient inlet BC for studies focusing on distal regions of the thoracic aorta.