4D Flow Magnetic Resonance Imaging Research Articles

In vitro experiments on ICOSA6 4D flow MRI measurement for the quantification of velocity and turbulence parameters

PurposeTo perform comprehensive in vitro experiments using six-directional icosahedral flow encoding (ICOSA6) 4D flow magnetic resonance imaging (MRI) under various scan conditions to analyze the robustness of velocity and turbulence quantification. Materials and methodsIn vitro flow phantoms with steady flow rates of 10 and 20 L/min were scanned using both conventional 4D flow MRI and ICOSA6. Experiments focused on comparisons between ICOSA6 and conventional four point (4P) methods, and the effects of contrast agents, velocity encoding range (Venc), and scan direction on velocity and turbulence quantification. ResultsThe results demonstrated that 1) ICOSA6 improves the velocity-to-noise ratio (VNR) of velocity estimation by 33% (on average) and results in similar turbulent kinetic energy (TKE) estimation as the 4P method. 2) Measurements with a contrast agent resulted in more than a 2.5 fold increase in average VNR. However, the improvement of total TKE quantification was not obvious. 3) TKE estimation was less affected by Venc and the scan direction, whereas turbulence production (TP) estimation was largely affected by these measurement conditions. The effects of Venc and scan direction accounted for less than 11.63% of TKE estimation, but up to 33.89% of TP estimation. ConclusionThe ICOSA6 scheme is compatible with conventional 4D flow MRI for velocity and TKE measurement. Contrast agents are effective at increasing VNR, but not signal-to-noise ratio for TKE quantification. The effects of Venc and scan direction influence total TP more than total TKE.

In Vitro Modeling of Human Brain Arteriovenous Malformation for Endovascular Simulation and Flow Analysis

Current invitro models for human brain arteriovenous malformation (AVM) analyzing the efficacy of embolic materials or flow conditions are limited by a lack of realistic anatomic features of complex AVM nidus. The purpose of this study was to evaluate a newly developed invitro AVM model for embolic material testing, preclinical training, and flow analysis. Three-dimensional (3D) images of the AVM nidus were extracted from 3D rotational angiography from a patient. Inner vascular mold was printed using a 3D printer, coated with polydimethylsiloxanes, and then was removed by acetone, leaving a hollow AVM model. Injections of liquid embolic material and 4-dimensional (4D) flow magnetic resonance imaging (MRI) were performed using the AVM models. Additionally, computational fluid dynamics analysis was performed to examine the flow volume rate as compared with 4D flow MRI. The manufacture of 3D invitro AVM models delivers a realistic representation of human nidus vasculature and complexity derived from patients. The injection of liquid embolic agents performed in the invitro model successfully replicated real-life treatment conditions. The model simulated the plug and push technique before penetration of the liquid embolic material into the AVM nidus. The 4D flow MRI results were comparable to computational fluid dynamics analysis. An invitro human brain AVM model with realistic geometric complexities of nidus was successfully created using 3D printing technology. This AVM model offers a useful tool for training of embolization techniques and analysis of hemodynamics analysis, and development of new devices and materials.

Four-dimensional Flow Magnetic Resonance Imaging Quantification of Blood Flow in Bicuspid Aortic Valve.

Four-dimensional (D) flow magnetic resonance imaging (MRI) is limited by time-consuming and nonstandardized data analysis. We aimed to test the efficiency and interobserver reproducibility of a dedicated 4D flow MRI analysis workflow. Thirty retrospectively identified patients with bicuspid aortic valve (BAV, age=47.8±11.8 y, 9 male) and 30 healthy controls (age=48.8±12.5 y, 21 male) underwent Aortic 4D flow MRI using 1.5 and 3 T MRI systems. Two independent readers performed 4D flow analysis on a dedicated workstation including preprocessing, aorta segmentation, and placement of four 2D planes throughout the aorta for quantification of net flow, peak velocity, and regurgitant fraction. 3D flow visualization using streamlines was used to grade aortic valve outflow jets and extent of helical flow. 4D flow analysis workflow time for both observers: 5.0±1.4 minutes per case (range=3 to 10 min). Valve outflow jets and flow derangement was visible in all 30 BAV patients (both observers). Net flow, peak velocity, and regurgitant fraction was significantly elevated in BAV patients compared with controls except for regurgitant fraction in plane 4 (91.1±29.7 vs. 62.6±19.6 mL/s, 37.1% difference; 121.7±49.7 vs. 90.9±26.4 cm/s, 28.9% difference; 9.3±10.1% vs. 2.0±3.4%, 128.0% difference, respectively; P<0.001). Excellent intraclass correlation coefficient agreement for net flow: 0.979, peak velocity: 0.931, and regurgitant fraction: 0.928. Our study demonstrates the potential of an efficient data analysis workflow to perform standardized 4D flow MRI processing in under 10 minutes and with good-to-excellent reproducibility for flow and velocity quantification in the thoracic aorta.

Measuring atrial stasis during sinus rhythm in patients with paroxysmal atrial fibrillation using 4 Dimensional flow imaging: 4D flow imaging of atrial stasis

BackgroundParoxysmal atrial fibrillation (PAF) is associated with cardioembolic risk, however events may occur during sinus rhythm (SR). 4D-flow cardiac magnetic resonance (CMR) imaging allows visualisation of left atrial blood flow, to determine the residence time distribution (RTD), an assessment of atrial transit time. ObjectiveTo determine if atrial transit time is prolonged in PAF patients during SR, consistent with underlying atrial stasis. Method91 participants with PAF and 18 healthy volunteers underwent 4D flow analysis in SR. Velocity fields were produced RTDs, calculated by seeding virtual ‘particles’ at the right upper pulmonary vein and counting them exiting the mitral valve. An exponential decay curve quantified residence time of particles in the left atrium, and atrial stasis was expressed as the derived constant (RTDTC) based on heartbeats. The RTDTC was evaluated within the PAF group, and compared to healthy volunteers. ResultsPatients with PAF (n = 91) had higher RTDTC compared with gender-matched controls (n = 18) consistent with greater atrial stasis (1.68 ± 0.46 beats vs 1.51 ± 0.20 beats; p = .005). PAF patients with greater thromboembolic risk had greater atrial stasis (median RTDTC of 1.72 beats in CHA₂DS₂-VASc≥2 vs 1.52 beats in CHA₂DS₂-VASc<2; p = .03), only female gender and left ventricular ejection fraction contributed significantly to the atrial RTDTC (p = .006 and p = .023 respectively). ConclusionsAtrial stasis quantified by 4D flow is greater in PAF, correlating with higher CHA₂DS₂-VASc scores. Female gender and systolic dysfunction are associated with atrial stasis. RTD offers an insight into atrial flow that may be developed to provide a personalised assessment of thromboembolic risk.

4DFlowNet: Super-Resolution 4D Flow MRI Using Deep Learning and Computational Fluid Dynamics

4D-flow magnetic resonance imaging (MRI) is an emerging imaging technique where spatiotemporal 3D blood velocity can be captured with full volumetric coverage in a single non-invasive examination. This enables qualitative and quantitative analysis of hemodynamic flow parameters of the heart and great vessels. An increase in the image resolution would provide more accuracy and allow better assessment of the blood flow, especially for patients with abnormal flows. However, this must be balanced with increasing imaging time. The recent success of deep learning in generating super resolution images shows promise for implementation in medical images. We utilized computational fluid dynamics simulations to generate fluid flow simulations and represent them as synthetic 4D flow MRI data. We built our training dataset to mimic actual 4D flow MRI data with its corresponding noise distribution. Our novel 4DFlowNet network was trained on this synthetic 4D flow data and was capable in producing noise-free super resolution 4D flow phase images with upsample factor of 2. We also tested the 4DFlowNet in actual 4D flow MR images of a phantom and normal volunteer data, and demonstrated comparable results with the actual flow rate measurements giving an absolute relative error of 0.6 to 5.8% and 1.1 to 3.8% in the phantom data and normal volunteer data, respectively.

Deep variational network for rapid 4D flow MRI reconstruction

Phase-contrast magnetic resonance imaging (MRI) provides time-resolved quantification of blood flow dynamics that can aid clinical diagnosis. Long in vivo scan times due to repeated three-dimensional (3D) volume sampling over cardiac phases and breathing cycles necessitate accelerated imaging techniques that leverage data correlations. Standard compressed sensing reconstruction methods require tuning of hyperparameters and are computationally expensive, which diminishes the potential reduction of examination times. We propose an efficient model-based deep neural reconstruction network and evaluate its performance on clinical aortic flow data. The network is shown to reconstruct undersampled 4D flow MRI data in under a minute on standard consumer hardware. Remarkably, the relatively low amounts of tunable parameters allowed the network to be trained on images from 11 reference scans while generalizing well to retrospective and prospective undersampled data for various acceleration factors and anatomies.

A Valveless Pulsatile Pump for Heart Failure with Preserved Ejection Fraction: Hemo- and Fluid Dynamic Feasibility

Treatment of heart failure with preserved ejection fraction (HFpEF) remains a major unmet medical need. An implantable valveless pulsatile pump with a single cannula—the CoPulse pump—may provide beneficial hemodynamic support for select HFpEF patients when connected to the failing ventricle. We aimed to demonstrate hemodynamic efficacy and hemocompatible design feasibility for this novel assist device. The hemodynamic effect of the pump was investigated with an in vitro circulatory mock loop and an ex vivo isolated porcine heart model. The hydraulic design was optimized using computational fluid dynamics (CFD), and validated by 4D-flow magnetic resonance imaging (MRI). The pump reduced left atrial pressure (> 27%) and increased cardiac output (> 14%) in vitro. Ex vivo experiments revealed elevated total stroke volume at increased end-systolic volume during pump support. Asymmetric cannula positioning indicated superior washout, decreased stagnation (8.06 mm2 vs. 31.42 mm2), and marginal blood trauma potential with moderate shear stresses (< 24 Pa) in silico. Good agreement in flow velocities was evident among CFD and 4D-flow MRI data (r > 0.76). The CoPulse pump proved hemodynamically effective. Hemocompatibility metrics were comparable to those of a previously reported, typical pulsatile pump with two cannulae. The encouraging in vitro, ex vivo, and hemocompatibility results substantiate further development of the CoPulse pump.

4D Flow MR Imaging to Improve Microwave Ablation Prediction Models: A Feasibility Study in an In Vivo Porcine Liver

PurposeTo characterize the effect of hepatic vessel flow using 4-dimensional (4D) flow magnetic resonance (MR) imaging and correlate their effect on microwave ablation volumes in an in vivo non-cirrhotic porcine liver model. Materials and MethodsMicrowave ablation antennas were placed under ultrasound guidance in each liver lobe of swine (n = 3 in each animal) for a total of 9 ablations. Pre- and post-ablation 4D flow MR imaging was acquired to quantify flow changes in the hepatic vasculature. Flow measurements, along with encompassed vessel size and vessel-antenna spacing, were then correlated with final ablation volume from segmented MR images. ResultsThe linear regression model demonstrated that the preablation measurement of encompassed hepatic vein size (β = –0.80 ± 0.25, 95% confidence interval [CI] –1.15 to –0.22; P = .02) was significantly correlated to final ablation zone volume. The addition of hepatic vein flow rate found via 4D flow MRI (β = –0.83 ± 0.65, 95% CI –2.50 to 0.84; P = .26), and distance from antenna to hepatic vein (β = 0.26 ± 0.26, 95% CI –0.40 to 0.92; P = .36) improved the model accuracy but not significantly so (multivariate adjusted R2 = 0.70 vs univariate (vessel size) adjusted R2 = 0.63, P = .24). ConclusionsHepatic vein size in an encompassed ablation zone was found to be significantly correlated with final ablation zone volume. Although the univariate 4D flow MR imaging-acquired measurements alone were not found to be statistically significant, its addition to hepatic vein size improved the accuracy of the ablation volume regression model. Pre-ablation 4D flow MR imaging of the liver may assist in prospectively optimizing thermal ablation treatment.

4D Flow with MRI.

Magnetic resonance imaging (MRI) has become an important tool for the clinical evaluation of patients with cardiac and vascular diseases. Since its introduction in the late 1980s, quantitative flow imaging with MRI has become a routine part of standard-of-care cardiothoracic and vascular MRI for the assessment of pathological changes in blood flow in patients with cardiovascular disease. More recently, time-resolved flow imaging with velocity encoding along all three flow directions and three-dimensional (3D) anatomic coverage (4D flow MRI) has been developed and applied to enable comprehensive 3D visualization and quantification of hemodynamics throughout the human circulatory system. This article provides an overview of the use of 4D flow applications in different cardiac and vascular regions in the human circulatory system, with a focus on using 4D flow MRI in cardiothoracic and cerebrovascular diseases.

The role of flow rotation in the adult right atrium: a 4D flow cardiovascular magnetic resonance study

Objective: In healthy adults, the right atrium (RA) serves as a reservoir for the systemic flow return from the superior vena cava (SVC) and inferior vena cava (IVC), preparing the two flows to be transferred to the right ventricle (RV) and pulmonary circulation. This study aims to quantify the haemodynamics of the RA and the associated SVC and IVC inflows, which have not been fully understood to date. Approach: Eighteen adults with structurally normal hearts underwent 4D flow magnetic resonance imaging. The cardiac cycle was resolved to 20 temporal phases with a spatial resolution of 3 × 3 × 3 mm3. Analysis included objective visualisation of the flow structures in the RA identified by three different vortex identification criteria, kinetic energy (KE), enstrophy and dissipation. KE and helicity flux were also assessed in both caval veins. Main results: Vortex identification methods confirmed that in the majority of participants the blood flow from the caval veins filling the RA during ventricular systole is not chaotic, but rather forms an organised pattern of a single coherent forward turning vortex structure. Thirteen participants displayed a single vortex flow structure, four showed multiple vortices and one had a helical flow pattern without a clear vortex structure. A strong positive correlation exists between the flow KE and enstrophy density. Significance: This suggests that flow energy in the RA is mainly rotational, part of which is convected by the highly helical SVC and IVC inflows. Multiple vortices tend to be associated with higher dissipation rates in the central RA region due to turbulence. The rotational nature of the flow in the RA maintains KE better than non-rotational flow. RA flow characteristics are highly related to the helicity content in the caval veins, as well as the KE flux intensity. Lower caval helicity or IVC KE flux dominance tends to favour single vortex formation while the opposite tends to lead to multiple vortices or the rare helical flow patterns. Atria lacking single vortex flow are inclined to have a larger energy input from atrial contraction.

4D Flow MR Imaging for Pulmonary Hypertension

Pulmonary hypertension is defined as a mean pulmonary artery pressure of 25 mmHg or higher as detected by right heart catheterization. It is broadly classified into 5 categories based on the etiology. Various imaging modalities are used to evaluate patients with or suspected of pulmonary hypertension. Transthoracic echocardiography is the most frequently used modality for detecting pulmonary hypertension. Magnetic resonance imaging (MRI) is indicated during examination, since it allows for comprehensive image analysis of morphology, cardiac function, and hemodynamics in the great arteries. The recently developed 4D Flow MRI can visualize three-dimensional pulmonary vascular flow and measure advanced hemodynamic parameters. The degree of vertical flow in the pulmonary trunk may be a key aspect that reflects pulmonary hypertension. 4D Flow MRI can highlight the value of MRI for the clinical diagnosis and management of patients with pulmonary hypertension.

Simulation of semilunar valve function: computer-aided design, 3D printing and flow assessment with MR

BackgroundThe structure of the valve leaflets and sinuses are crucial in supporting the proper function of the semilunar valve and ensuring leaflet durability. Therefore, an enhanced understanding of the structural characteristics of the semilunar valves is fundamental to the evaluation and staging of semilunar valve pathology, as well as the development of prosthetic or bioprosthetic valves. This paper illustrates the process of combining computer-aided design (CAD), 3D printing and flow assessment with 4-dimensional flow magnetic resonance imaging (MRI) to provide detailed assessment of the structural and hemodynamic characteristics of the normal semilunar valve.MethodsPreviously published geometric data on the aortic valve was used to model the ‘normal’ tricuspid aortic valve with a CAD software package and 3D printed. An MRI compatible flow pump with the capacity to mimic physiological flows was connected to the phantom. A peak flow rate of 100 mL/s and heart rate of 60 beats per minute were used. MRI measurements included cine imaging, 2D and 4D phase-contrast imaging to assess valve motion, flow velocity and complex flow patterns.ResultsCine MRI data showed normal valve function and competency throughout the cardiac cycle in the 3D-printed phantom. Quantitative analysis of 4D Flow data showed net flow through 2D planes proximal and distal to the valve were very consistent (26.03 mL/s and 26.09 mL/s, respectively). Measurements of net flow value agreed closely with the flow waveform provided to the pump (27.74 mL/s), confirming 4D flow acquisition in relation to the pump output. Peak flow values proximal and distal to the valve were 78.4 mL/s and 63.3 mL/s, respectively.Particle traces of flow from 4D-phase contrast MRI data demonstrated flow through the valve into the ascending aorta and vortices within the aortic sinuses, which are expected during ventricular diastole.ConclusionIn this proof of concept study, we have demonstrated the ability to generate physiological 3D-printed aortic valve phantoms and evaluate their function with cine- and 4D Flow MRI. This technology can work synergistically with promising tissue engineering research to develop optimal aortic valve replacements, which closely reproduces the complex function of the normal aortic valve.

Fluid-structure interaction simulations outperform computational fluid dynamics in the description of thoracic aorta haemodynamics and in the differentiation of progressive dilation in Marfan syndrome patients.

Abnormal fluid dynamics at the ascending aorta may be at the origin of aortic aneurysms. This study was aimed at comparing the performance of computational fluid dynamics (CFD) and fluid–structure interaction (FSI) simulations against four-dimensional (4D) flow magnetic resonance imaging (MRI) data; and to assess the capacity of advanced fluid dynamics markers to stratify aneurysm progression risk. Eight Marfan syndrome (MFS) patients, four with stable and four with dilating aneurysms of the proximal aorta, and four healthy controls were studied. FSI and CFD simulations were performed with MRI-derived geometry, inlet velocity field and Young's modulus. Flow displacement, jet angle and maximum velocity evaluated from FSI and CFD simulations were compared to 4D flow MRI data. A dimensionless parameter, the shear stress ratio (SSR), was evaluated from FSI and CFD simulations and assessed as potential correlate of aneurysm progression. FSI simulations successfully matched MRI data regarding descending to ascending aorta flow rates (R2 = 0.92) and pulse wave velocity (R2 = 0.99). Compared to CFD, FSI simulations showed significantly lower percentage errors in ascending and descending aorta in flow displacement (−46% ascending, −41% descending), jet angle (−28% ascending, −50% descending) and maximum velocity (−37% ascending, −34% descending) with respect to 4D flow MRI. FSI- but not CFD-derived SSR differentiated between stable and dilating MFS patients. Fluid dynamic simulations of the thoracic aorta require fluid–solid interaction to properly reproduce complex haemodynamics. FSI- but not CFD-derived SSR could help stratifying MFS patients.

Right Pulmonary Vein Atresia in a Mildly Symptomatic Boy: Comprehensive Analysis of Flow Dynamics Using Non-contrast-enhanced 4D Flow MR Imaging

We present multimodal imaging in the rare case of isolated unilateral pulmonary vein atresia in a 6 year-old boy, including analysis of hemodynamics by magnetic resonance acquisition technique of time-resolved three-dimensional phase contrast imaging (4D flow magnetic resonance imaging). This novel imaging method enables the quantification and especially comprehensive visualization of blood flow patterns, even in complex congenital anomalies which abducted detailed assessment so far, and therefore constitutes a promising alternative to conventional vascular imaging techniques.

4D flow magnetic resonance imaging in congenital heart diseases: Who can benefit?

Four-dimensional flow cardiac magnetic resonance (4D CMR) is a promising imaging modality, while its distribution and use remain limited suggesting practical or technical difficulties. We sought to report our preliminary experience in real life of 4D CMR in unselected children and adults with congenital heart disease (CHD). We focused on its feasibility and ability, compared with adults with CHD as control. We herein report 50 examinations over a one-year study period. This modality has been applied as a complementary imaging when conventional imaging modalities were unsatisfactory. Quality was classified according to qualitative and quantitative criteria by two blinded radiologists. This study included 22 children and 28 adults. In infant and children mean age was 10.6 ± 6.1 years old [0.3; 18], mean weight was 36.8 ± 24.5 kg [3.8; 105.0], mean height was 149.4 ± 42.1 cm [52; 192]. Clinical indications were 36% of tetralogy of Fallot, 18% of aorta disease, 27% of complex CHD, 9% of ventricular septal defect and 10% of valvulopathy. The feasibility of this examination was excellent, while 100% exam were performed with no need for general anesthesia whatever the indication or patient (infant, child or adult). Average duration of exams were 465 ± 90s [339–610]. The overall quality of exams was satisfactory; 63% of good quality and 23% of medium quality). The evolution of quality over time has shown a progressive improvement which seems to correspond to a 3-month long learning curve. The only predictive factor identified for quality was the experience (X 2 = 4.8; P = 0.03 in CHD). Based on our preliminary experience, 4D Flow has become a complementary imaging modality accessible in current practice and open to all patients. The quality of this examination was satisfactory and seems to require an estimated learning curve of 3 months according to our experience.

Four-Dimensional Flow Magnetic Resonance Imaging for Assessment of Velocity Magnitudes and Flow Patterns in The Human Carotid Artery Bifurcation: Comparison with Computational Fluid Dynamics.

Purpose: Knowledge of the hemodynamics in the vascular system is important to understand and treat vascular pathology. The present study aimed to evaluate the hemodynamics in the human carotid artery bifurcation measured by four-dimensional (4D) flow magnetic resonance imaging (MRI) as compared to computational fluid dynamics (CFD). Methods: This protocol used MRI data of 12 healthy volunteers for the 3D vascular models and 4D flow MRI measurements for the boundary conditions in CFD simulation. We compared the velocities measured at the carotid bifurcation and the 3D velocity streamlines of the carotid arteries obtained by these two methods. Results: There was a good agreement for both maximum and minimum velocity values between the 2 methods for velocity magnitude at the bifurcation plane. However, on the 3D blood flow visualization, secondary flows, and recirculation regions are of poorer quality when visualized through the 4D flow MRI. Conclusion: 4D flow MRI and CFD show reasonable agreement in demonstrated velocity magnitudes at the carotid artery bifurcation. However, the visualization of blood flow at the recirculation regions and the assessment of secondary flow characteristics should be enhanced for the use of 4D flow MRI in clinical situations.

False lumen ejection fraction predicts growth in type B aortic dissection: preliminary results.

Current risk assessment strategies in type B aortic dissection are focused on anatomic parameters, although haemodynamic abnormalities that result in false lumen (FL) pressurization are thought to play a significant role in aortic growth. The objective of this study was to evaluate blood flow of the FL using 4D flow magnetic resonance imaging (MRI) and identify haemodynamic and anatomic factors that independently predict the rate of aortic growth. Patients with dissection of the descending thoraco-abdominal aorta (n = 18) were enrolled in a prospective observational study and underwent 4D flow MRI for haemodynamic assessment of the entry tear and FL. Anatomic parameters were obtained by magnetic resonance angiography and baseline computed tomography. False lumen ejection fraction (FL EF) was defined the ratio of retrograde flow rate at the dominant entry tear during diastole over the antegrade systolic flow rate. The median aortic growth rate was 3.5 mm/year (interquartile range 0.5-8.1 mm/year). Entry tear peak velocity was lower in patients with enlarging aortic dimensions (95.5 ± 24.1 vs 128.1 ± 37.4 cm/s, P = 0.039). After adjusting for co-variates FL EF (β = 0.15, P = 0.004), baseline maximal aortic diameter (β = 0.37, P = 0.001) and the entry tear distance from the left subclavian artery (β = 0.07, P = 0.016) were significant predictors of aortic growth rate. Beyond standard anatomic risk factors, FL EF is an independent predictor of aortic growth rate and may represent an intuitive, non-invasive method to estimate FL pressurization and improve patient-specific risk assessment in patients with type B aortic dissection.

4D Flow MRI in Patients with Asymptomatic Abdominal Aortic Aneurysms: Reproducibility and Clinical Analysis

Introduction: Hemodynamics play an as of yet incompletely understood role in the progression (i.e. growth and rupture) of abdominal aortic aneurysms (AAAs), and are therefore relevant to study. Unfortunately, there are few methods to measure in vivo hemodynamics. One method is 4D flow magnetic resonance imaging (MRI), where in vivo blood flow is visualised for the estimation of wall shear stress (WSS). If robust, 4D flow MRI can be a valuable tool in the study of AAA hemodynamics. Therefore, this pilot study evaluated reproducibility of 4D flow MRI and performed a clinical analysis. Methods: Adult patients with an asymptomatic AAA of at least 30 millimetre in diameter were included in this study. 4D flow acquisitions were performed on a 3 Tesla MRI scanner (Philips, Best, Netherlands) in a sagittal oblique volume which ideally covered the abdominal aorta from the celiac artery to both common iliac arteries. Pulse sequence parameters were: spatial resolution = 2.5×2.5×2.5 mm; field of view = 315×275×77.5 mm; temporal resolution = 24 heart phases; velocity encoding = 0.52 metre per second (m/s) in all three velocity encoding directions. Participants were scanned twice with an interval of 1 week. Mimics (Materialise, Leuven, Belgium) was used to segment the aortic lumen between the celiac artery and the aortic bifurcation for the reproducibility analysis, while the aortic lumen inside the AAA (AAA lumen) was segmented for the clinical analysis. In-house MATLAB software was used to measure velocity and to estimate WSS in Pascal (Pa). Reproducibility was tested on a voxel-by-voxel basis by the calculating mean difference and limits of agreement in a Bland-Altman analysis, and with orthogonal regression analysis to calculate the slope, intercept and Pearson's r of the agreement between the two scans. The clinical analysis summarised mean and maximal velocity/WSS, and estimated Pearson's r between maximal AAA lumen diameter and mean velocity/WSS. A P value < .05 was considered significant. Results: Thirteen male participants with a mean maximal AAA diameter of 48 ± 10.4 millimetre (mm) and a mean maximal AAA lumen diameter of 35.92 ± 8 mm were included. Figure 1 shows velocity (A) and WSS (B) for six participants. As shown in table 1, Bland-Altman analysis revealed small differences in velocity and WSS (respectively 0.0004 m/s and 0.0124 Pa), while orthogonal regression showed moderate agreement (slope: 0.99077/0.9885, r = 0.8338/0.7131 for respectively velocity/WSS). Clinical analysis demonstrated a negative correlation between maximal AAA lumen diameter and WSS (r = -0.551, P = .051), but a positive correlation with velocity (r = 0.411, P = .163). Conclusion: 4D flow MRI is a robust method and can therefore be used to study AAA hemodynamics in larger longitudinal studies, with the aim of studying whether velocity and WSS may predict AAA rupture.Table 1Reproducibility and clinical analysisReproducibility in the entire segmentation volume (Celiac artery - aortic bifurcation)Clinical analysis in AAA lumenBland-Altmanorthogonal regression analysismean differencelimits of agreementslopeinterceptPearson´s raveraged mean over 13 participantsaveraged maximal over 13 participantsPearson´s rP valuevelocity (in metre per second)0.0004 ± 0.02890.1230 ± 0.03470.99077 ± 0.18060.0042 ± 0.02560.8338 ± 0.06430.1696 ± 0.03690.3191 ± 0.14010.411.163wall shear stress (in Pascal)0.0124 ± 0.05950.2621 ± 0.1210.9885 ± 0.21050.0487 ± 0.20070.7131 ± 0.16030.2386 ± 0.07900.4733 ± 0.1053-0.551.051 Open table in a new tab Disclosure: Nothing to disclose

4D Flow MRI Pressure Estimation Using Velocity Measurement-Error-Based Weighted Least-Squares.

This work introduces a 4D flow magnetic resonance imaging (MRI) pressure reconstruction method which employs weighted least-squares (WLS) for pressure integration. Pressure gradients are calculated from the velocity fields, and velocity errors are estimated from the velocity divergence for incompressible flow. Pressure gradient errors are estimated by propagating the velocity errors through Navier-Stokes momentum equation. A weight matrix is generated based on the pressure gradient errors, then employed for pressure reconstruction. The pressure reconstruction method was demonstrated and analyzed using synthetic velocity fields as well as Poiseuille flow measured using in vitro 4D flow MRI. Performance of the proposed WLS method was compared to the method of solving the pressure Poisson equation which has been the primary method used in the previous studies. Error analysis indicated that the proposed method is more robust to velocity measurement errors. Improvement on pressure results was found to be more significant for the cases with spatially-varying velocity error level, with reductions in error ranging from 50% to over 200%. Finally, the method was applied to flow in patient-specific cerebral aneurysms. Validation was performed with in vitro flow data collected using Particle Tracking Velocimetry (PTV) and in vivo flow measurement obtained using 4D flow MRI. Pressure calculated by WLS, as opposed to the Poisson equation, was more consistent with the flow structures and showed better agreement between the in vivo and in vitro data. These results suggest the utility of WLS method to obtain reliable pressure field from clinical flow measurement data.

An isolated beating pig heart platform for a comprehensive evaluation of intracardiac blood flow with 4D flow MRI: a feasibility study

BackgroundCardiac magnetic resonance imaging (MRI) in large animals is cumbersome for various reasons, including ethical considerations, costs of housing and maintenance, and need for anaesthesia. Our primary purpose was to show the feasibility of an isolated beating pig heart model for four-dimensional (4D) flow MRI for investigating intracardiac blood flow patterns and flow parameters using slaughterhouse side products. In addition, the feasibility of evaluating transcatheter aortic valve replacement (TAVR) in the model was investigated.MethodsSeven slaughterhouse pig hearts were installed in the MRI-compatible isolated beating pig heart platform. First, Langendorff perfusion mode was established; then, the system switched to working mode, in which blood was actively pumped by the left ventricle. A pacemaker ensured a stable HR during 3-T MRI scanning. All hearts were submitted to human physiological conditions of cardiac output and stayed vital for several hours. Aortic flow was measured from which stroke volume, cardiac output, and regurgitation fraction were calculated.Results4D flow MRI acquisitions were successfully conducted in all hearts. Stroke volume was 31 ± 6 mL (mean ± standard deviation), cardiac output 3.3 ± 0.9 L/min, and regurgitation fraction 16% ± 9%. With 4D flow, intracardiac and coronary flow patterns could be visualised in all hearts. In addition, we could study valve function and regurgitation in two hearts after TAVR.ConclusionsThe feasibility of 4D flow MRI in an isolated beating pig heart loaded to physiological conditions was demonstrated. The platform is promising for preclinical assessment of cardiac blood flow and function.