Phase-contrast Magnetic Resonance Imaging Data Research Articles

Resolution-based comparative analysis of 4D-phase-contrast magnetic resonance images and hemodynamic simulations of the aortic arch

Abstract Introduction: In this study, phase-contrast magnetic resonance imaging (PC-MRI) 4D flow data for patients with bicuspid aortic valve (BAV) were analyzed based on the spatial resolution of the images. BAV is a congenital heart defect characterized by the presence of two cusps in the aortic valve, leading to various complications. PC-MRI is a noninvasive imaging technique used to assess the hemodynamics in patients. However, interpretation of PC-MRI data can be challenging due to complex hemodynamics, which makes numerical simulations necessary to complement the results. Methods:Within this study PC-MRI 4D flow data in the aortic arch (AA) were compared with hemodynamic numerical simulations based on the MRI resolution for six patient-specific AAs with BAV. First, the segmentations were reconstructed and divided into three groups based on the resolution. Second, the numerical simulations were performed and resulting hemodynamics compared to PC-MRI results at three time points in the cardiac cycle using velocity and wall shear stress. Results: Results from group 1 (highest resolution) show an overestimation of wall shear stress (WSS) values during peaksystole in the three arteries and along the AA and descending aorta (DAo). Nevertheless, velocity mean values show an overall good agreement. Group 2 shows higher velocity magnitudes in the simulation than in PC-MRI. Within group 3, velocity values differ from the measurements. Conclusion: The study shows that with a low resolution, flow patterns can already be evaluated based on PC-MRI, but to get an insight into the flow quantitatively a higher resolution is necessary.

Structured analysis of the impact of fetal motion on phase-contrast MRI flow measurements with metric optimized gating

The impact of fetal motion on phase contrast magnetic resonance imaging (PC-MRI) with metric optimized gating (MOG) remains unknown, despite being a known limitation to prenatal MRI. This study aims to describe the effect of motion on fetal flow-measurements using PC-MRI with MOG and to generate a scoring-system that could be used to predict motion-corrupted datasets at the time of acquisition. Ten adult volunteers underwent PC-MRI with MOG using a motion-device to simulate reproducible in-plane motion encountered in fetuses. PC-MRI data were acquired on ten fetuses. All ungated images were rated on their quality from 0 (no motion) to 2 (severe motion). There was no significant difference in measured flows with in-plane motion during the first and last third of sequence acquisition. Movement in the middle section of acquisition produced a significant difference while all referring ungated images were rated with a score of 2. Intra-Class-Correlation (ICC) for flow-measurements in adult and fetal datasets was lower for datasets with scores of 2. For fetal applications, the use of a simple three-point scoring system reliably identifies motion-corrupted sequences from unprocessed data at the time of acquisition, with a high score corresponding to significant underestimation of flow values and increased interobserver variability.

A novel Fontan Y-graft for interrupted inferior vena cava and azygos continuation.

OBJECTIVESTo evaluate the hemodynamicdynamic advantage of a new Fontan surgical template that is intended for complex single-ventricle patients with interrupted inferior vena cava-azygos and hemi-azygos continuation. The new technique has emerged from a comprehensive pre-surgical simulation campaign conducted to facilitate a balanced hepatic flow and somatic Fontan pathway growth after Kawashima procedure.METHODSFor 9 patients, aged 2 to18 years, majority having poor preoperative oxygen saturation, a pre-surgical computational fluid dynamics customization is conducted. Both the traditional Fontan pathways and the proposed novel Y-graft templates are considered. Numerical model was validated against in vivo phase-contrast magnetic resonance imaging data and in vitro experiments.RESULTSThe proposed template is selected and executed for 6 out of the 9 patients based on its predicted superior hemodynamic performance. Pre-surgical simulations performed for this cohort indicated that flow from the hepatic veins (HEP) do not reach to the desired lung. The novel Y-graft template, customized via a right- or left-sided displacement of the total cavopulmonary connection anastomosis location resulted a drastic increase in HEP flow to the desired lung. Orientation of HEP to azygos direct shunt is found to be important as it can alter the flow pattern from 38% in the caudally located direct shunt to 3% in the cranial configuration with significantly reversed flow. The postoperative measurements prove that oxygen saturation increased significantly (P-value = 0.00009) to normal levels in 1 year follow-up.CONCLUSIONSThe new Y-graft template, if customized for the individual patient, is a viable alternative to the traditional surgical pathways. This template addresses the competing hemodynamic design factors of low physiological venous pressure, high postoperative oxygen saturation, low energy loss and balanced hepatic growth factor distribution possibly assuring adequate lung development.Date and number of IRB approval25 October 2019, 280011928-604.01.01.

Blood Flow Analysis of the Great Saphenous Vein in the Su-Pine Position in Clinical Manifestations of Varicose Veins of Different Severities: Application of Phase-Contrast Magnetic Resonance Imaging Data.

The progression of clinical manifestations of lower-limb varicose veins remains unclear. This study investigated changes in lower-limb venous blood flow using phase-contrast magnetic resonance angiography. Data were collected on veins from 141 legs. We compared legs with and without varicose veins and related symptoms and examined varying levels of varicose vein symptom severity. Legs without varicose veins exhibited a lower absolute stroke volume (ASV, p < 0.01) and mean flux (MF, p = 0.03) for the great saphenous vein (GSV) compared with legs with symptomatic varicose veins. Legs with asymptomatic varicose veins exhibited lower MF for the GSV (p = 0.02) compared with legs with symptomatic varicose veins. Among legs with varicose veins, asymptomatic legs exhibited lower ASV (p = 0.03) and MF (p = 0.046) for the GSV compared with legs that exhibited skin changes or ulcers; however, no significant differences were observed between legs presenting with discomfort or edema and legs with skin changes or ulcers, and between legs presenting with discomfort or edema and asymptomatic legs. In conclusion, in the supine position, increased blood flow rate and blood flow volume in the GSV were associated with symptomatic varicose veins and increased symptom severity.

Importance of through-plane heart motion correction for the assessment of aortic regurgitation severity using phase contrast magnetic resonance imaging

PurposeTo elucidate the influence of through-plane heart motion on the assessment of aortic regurgitation (AR) severity using phase contrast magnetic resonance imaging (PC-MRI). ApproachA patient cohort with chronic AR (n = 34) was examined with PC-MRI. The regurgitant volume (RVol) and fraction (RFrac) were extracted from the PC-MRI data before and after through-plane heart motion correction and was then used for assessment of AR severity. ResultsThe flow volume errors were strongly correlated to aortic diameter (R = 0.80, p < 0.001) with median (IQR 25%;75%): 16 (14; 17) ml for diameter>40mm, compared with 9 (7; 10) ml for normal aortic size (p < 0.001). RVol and RFrac were underestimated (uncorrected:64 ± 37 ml and 39 ± 17%; corrected:76 ± 37 ml and 44 ± 15%; p < 0.001) and ~ 20% of the patients received lower severity grade without correction. ConclusionThrough-plane heart motion introduces relevant flow volume errors, especially in patients with aortic dilatation that may result in underestimation of the severity grade in patients with chronic AR.

Parametric Sequential Method for MRI-Based Wall Shear Stress Quantification.

Wall shear stress (WSS) has been suggested as a potential biomarker in various cardiovascular diseases and it can be estimated from phase-contrast Magnetic Resonance Imaging (PC-MRI) velocity measurements. We present a parametric sequential method for MRI-based WSS quantification consisting of a geometry identification and a subsequent approximation of the velocity field. This work focuses on its validation, investigating well controlled high-resolution in vitro measurements of turbulent stationary flows and physiological pulsatile flows in phantoms. Initial tests for in vivo 2D PC-MRI data of the ascending aorta of three volunteers demonstrate basic applicability of the method to in vivo.

Fluid-Structure Interaction Simulation of an Intra-Atrial Fontan Connection.

Simple SummaryA fluid-structure interaction (FSI) simulation of an intra-atrial Fontan connection was performed. Power loss and pressure drop results fluctuated less during the FSI simulation than during the simulation run with rigid walls, but there were no observable differences in time-averaged pressure drop, connection power loss or hepatic flow distribution. These results suggested that employing a rigid wall is a reasonable assumption when evaluating time-averaged hemodynamic quantities of the Fontan connection under resting breath-held flow conditions.Total cavopulmonary connection (TCPC) hemodynamics has been hypothesized to be associated with long-term complications in single ventricle heart defect patients. Rigid wall assumption has been commonly used when evaluating TCPC hemodynamics using computational fluid dynamics (CFD) simulation. Previous study has evaluated impact of wall compliance on extra-cardiac TCPC hemodynamics using fluid-structure interaction (FSI) simulation. However, the impact of ignoring wall compliance on the presumably more compliant intra-atrial TCPC hemodynamics is not fully understood. To narrow this knowledge gap, this study aims to investigate impact of wall compliance on an intra-atrial TCPC hemodynamics. A patient-specific model of an intra-atrial TCPC is simulated with an FSI model. Patient-specific 3D TCPC anatomies were reconstructed from transverse cardiovascular magnetic resonance images. Patient-specific vessel flow rate from phase-contrast magnetic resonance imaging (MRI) at the Fontan pathway and the superior vena cava under resting condition were prescribed at the inlets. From the FSI simulation, the degree of wall deformation was compared with in vivo wall deformation from phase-contrast MRI data as validation of the FSI model. Then, TCPC flow structure, power loss and hepatic flow distribution (HFD) were compared between rigid wall and FSI simulation. There were differences in instantaneous pressure drop, power loss and HFD between rigid wall and FSI simulations, but no difference in the time-averaged quantities. The findings of this study support the use of a rigid wall assumption on evaluation of time-averaged intra-atrial TCPC hemodynamic metric under resting breath-held condition.

Enhanced in vitro model of the CSF dynamics

BackgroundFluid dynamics of the craniospinal system are complex and still not completely understood. In vivo flow and pressure measurements of the cerebrospinal fluid (CSF) are limited. Whereas in silico modeling can be an adequate pathway for parameter studies, in vitro modeling of the craniospinal system is essential for testing and evaluation of therapeutic measures associated with innovative implants relating to, for example, normal pressure hydrocephalus and other fluid disorders. Previously-reported in vitro models focused on the investigation of only one hypothesis of the fluid dynamics rather than developing a modular set-up to allow changes in focus of the investigation. The aim of this study is to present an enhanced and validated in vitro model of the CSF system which enables the future embedding of implants, the validation of in silico models or phase-contrast magnetic resonance imaging (PC-MRI) measurements and a variety of sensitivity analyses regarding pathological behavior, such as reduced CSF compliances, higher resistances or altered blood dynamics.MethodsThe in vitro model consists of a ventricular system which is connected via the aqueduct to the cranial and spinal subarachnoid spaces. Two compliance chambers are integrated to cushion the arteriovenous blood flow generated by a cam plate unit enabling the modeling of patient specific flow dynamics. The CSF dynamics are monitored using three cranial pressure sensors and a spinal ultrasound flow meter. Measurements of the in vitro spinal flow were compared to cervical flow data recorded with PC-MRI from nine healthy young volunteers, and pressure measurements were compared to the literature values reported for intracranial pressure (ICP) to validate the newly developed in vitro model.ResultsThe maximum spinal CSF flow recorded in the in vitro simulation was 133.60 ml/min in the caudal direction and 68.01 ml/min in the cranial direction, whereas the PC-MRI flow data of the subjects showed 122.82 ml/min in the caudal and 77.86 ml/min in the cranial direction. In addition, the mean ICP (in vitro) was 12.68 mmHg and the pressure wave amplitude, 4.86 mmHg, which is in the physiological range.ConclusionsThe in vitro pressure values were in the physiological range. The amplitudes of the flow results were in good agreement with PC-MRI data of young and healthy volunteers. However, the maximum cranial flow in the in vitro model occurred earlier than in the PC-MRI data, which might be due to a lack of an in vitro dynamic compliance. Implementing dynamic compliances and related sensitivity analyses are major aspects of our ongoing research.

Comparison of Aortic Flow Patterns in Patients with and without Aortic Valve Disease: Hemodynamic Simulation Based on PC-MRI and CTA Data

Recent studies have revealed that aortic valve diseases are associated with the increased incidence of the aortopathy development. However, the influence of aortic valve diseases on aortic hemodynamics remains unclear. The purpose of this study was therefore to investigate the hemodynamic differenecs in patients with and without aortic valve disease through patient-specific simulations performed on two aorta models (BAV with severe stenosis vs. normal tricuspid aortic valve (TAV)). Realistic geometries and boundary conditions were obtained from computed tomography angiography (CTA) and phase-contrast magnetic resonance imaging (PC-MRI) measurements, respectively. In addition, 4D-MRI were performed to validate the the numerical methods used to simulate transient flow characteristics. Obtained results shown that the 3D streamlines in the patient with normal TAV were relatively symmetric and evenly distributed. For the patient with BAV, concentrated and high-speed inflow jets were found to impinge on the ascending aorta accompanied by strong vortices. These results indicate that the aortic valve phenotype plays a crucial role in featuring the disturbed flows primarily in ascending aorta, which may relate to the development of aortic diseases.

Quantifying Hemodynamic Changes in Moyamoya Disease Based on Two-Dimensional Cine Phase-Contrast Magnetic Resonance Imaging and Computational Fluid Dynamics

To investigate hemodynamic changes in moyamoya disease (MMD) via two-dimensional cine phase-contrast magnetic resonance imaging and computational fluid dynamics. In 18 patients with MMD and 10 healthy control subjects, phase-contrast magnetic resonance imaging was performed to quantify flow rate of main supplying arteries, including internal carotid arteries (ICAs) and vertebral arteries. Mean flow rate in these vessels was adopted as the patient-specific boundary condition for computational fluid dynamics simulation of the circle of Willis in MMD and control groups. Pressure drop in both ICAs and their difference, wall shear stress and secondary flow in the carotid siphon of ICAs, and flow rate and size of posterior communicating arteries (PComAs) were compared between MMD and control groups. Four patients with MMD underwent follow-up scans for longitudinal comparison. Phase-contrast magnetic resonance imaging data revealed significantly different flow rate in the left ICAand right vertebral arteries between MMD and controlgroups. Computational fluid dynamics simulation demonstrated similar wall shear stress and similar secondary flow of both ICAs but significantly higher pressure drop in left ICA, higher pressure drop difference between left ICA and right ICA, and higher flow rate in PComAs in patients with MMD compared with control subjects. Significantly increased size of left PComA in patients with MMD was also found. Follow-up results confirmed that the combination of pressure drop difference, flow rate, and size of PComAs can potentially assist long-term prognosis after surgery. Pressure drop difference, flow rate, and size of PComAs can be used to evaluate impairments in cerebrovascular reserve and indicate long-term prognosis in MMD.

Blood Flow after Endovascular Repair in the Aortic Arch: A Computational Analysis.

Background The benefits of thoracic endovascular aortic repair (TEVAR) have encouraged stent graft deployment more proximally in the aortic arch. This study quantifies the hemodynamic impact of TEVAR in proximal landing zone 2 on the thoracic aorta and the proximal supra-aortic branches. Methods Patients treated with TEVAR in proximal landing zone 2 having available preoperative and 30-day postoperative computer tomography angiography and phase-contrast magnetic resonance imaging data were retrospectively selected. Blood flow was studied using patient-specific computational fluid dynamics simulations. Results Four patients were included. Following TEVAR in proximal landing zone 2, the mean flow in the left common carotid artery (LCCA) increased almost threefold, from 0.21 (0.12–0.41) L/min to 0.61 (0.24–1.08) L/min (+294%). The surface area of the LCCA had not yet increased commensurately and therefore maximum flow velocity in the LCCA increased from 44.9 (27.0–89.3) cm/s to 72.6 (40.8–135.0) cm/s (+62%). One of the patients presented with Type Ib endoleak at 1-year follow-up. The displacement force in this patient measured 32.1 N and was directed dorsocranial, perpendicular to the distal sealing zone. There was a linear correlation between the surface area of the stent graft and the resulting displacement force ( p = 0.04). Conclusion TEVAR in proximal landing zone 2 alters blood flow in the supra-aortic branches, resulting in increased flow with high flow velocities in the LCCA. High displacement forces were calculated and related to stent graft migration and Type I endoleak during 1-year follow-up.

Modern cerebrospinal fluid flow research and Heinrich Quincke's seminal 1872 article on the distribution of cinnabar in freely moving animals.

The discovery of the ‘glymphatic’ pathway has shed new light on the cerebrospinal (CSF)–brain interstitial fluid (ISF) exchange process (Iliff et al., 2012). In new anatomically defined terms, the glymphatic pathway now represents a brain-wide pathway where CSF flows through the brain and spinal cord parenchyma and exchanges with ISF from periarterial to perivenous spaces in a route interconnected by aquaporin 4 water channels present on glial cells (Iliff et al., 2012). The new concept of the glymphatic pathway has evolved from the older concept of bulk flow of ISF from brain parenchyma to the ventricular CSF referred to as the “ependymal pathway” (Brightman, 1965a,b). The glymphatic pathway is important for detoxification of the brain (and spinal cord), a cleaning process that appears to be driven by adrenergic tone (Xie et al., 2013), arterial pulsatility (Iliff et al., 2013b), level of arousal (Xie et al., 2013), and aging (Kress et al., 2014). For decades, brain pulsations have been observed directly during neurosurgical procedures. Pulsation of the CSF was first qualitatively described in vivo using fluoroscopy (Du Boulay, 1966) and then validated using non-invasively using magnetic resonance imaging (MRI) (Sherman and Citrin, 1986; Sherman et al., 1986). A comprehensive review of this topic by Wagshul and coworkers was recently published (Wagshul et al., 2011). Phase-contrast MRI was later used to quantify CSF pulsation in selected parts of the ventricular CSF spaces, followed by quantitative flow visualization in the entire subarachnoid and ventricular spaces through computational flow reconstruction from phase-contrast MRI data (Gupta et al., 2010; Cheng et al., 2012; Siyahhan et al., 2014). Major aspects of CSF flow dynamics and outflow pathways are now more fully characterized (Wagshul et al., 2006; Oreskovic and Klarica, 2010; Bulat and Klarica, 2011). All of these experimental and mathematical modeling studies have led the conclusion that for example ‘pressure pulsatility’ in the brain functions as an internal biosensor of intracranial compliance (Wagshul et al., 2011) which is heightened in chronic hydrocephalus (Greitz, 2004). CSF reabsorption via the arachnoid villi into the dural sinuses to the blood or through the nasal lymphatics have been previously documented in seminal papers (Boulton et al., 1996; Abbott, 2004; Johnston et al., 2004). Nevertheless, important questions pertaining to CSF drainage remain unanswered. For example, the exact anatomy of the subarachnoid spaces along cranial and spinal nerves, the functional (quantitative and directional) relationship between influx of CSF into the brain and lymphatic drainage of CSF-ISF, and the potentially fluctuating outflow volumes at the various exit points along the complex outflow pathways, which under normal conditions may be dependent on posture, activity and state of consciousness. The influence of body posture and respiration on CSF outflow and transport has been characterized; and continues to be investigated using MRI techniques with improved temporal resolution (Bradbury et al., 1981; Bradbury and Westrop, 1983; Alperin et al., 2005; Yamada et al., 2013). However, another exceedingly important area that is only partially understood is how physiological movement and various motor activities affects CSF movements and outflow patterns. This lack of information is paradoxical because the influence of motor activity is clearly playing a prominent role on CSF-ISF exchange and clearance (Bechter, 2011). For example, local muscle tone change associated with yawning changes CSF dynamics (Walusinski, 2014). To our knowledge, one of the best studies on CSF flow and outflow pathways have been performed by Heinrich Quincke and described in a paper he published in “Archiv fur Anatomie, Physiologie und wissenschaftliche Medicin” in 1872 (Quincke, 1872). In his studies, Quincke experimented with freely moving animals, injecting the particulate dye cinnabar (Mercury(II) sulfide, HgS) into the intrathecal spaces of dogs, cats, and rabbits, all animals with high motor activity, and analyzed the dye’s distribution throughout the body after days, weeks or months postmortem.

Systolic flow displacement correlates with future ascending aortic growth in patients with bicuspid aortic valves undergoing magnetic resonance surveillance.

Altered systolic blood flow in the ascending aorta has been correlated with increased aortic growth in patients with bicuspid aortic valves (BAVs). We used conventional, 2-dimensional (2D) phase contrast magnetic resonance imaging (PC-MRI) to assess the relationship between altered flow and future growth in patients with BAV. Aortic MRI data were reviewed for 17 adult patients with BAV with right-left leaflet fusion undergoing surveillance imaging who had 2D PC-MRI through their ascending aortas on an initial study, follow-up studies more than 1 year later, and an initial maximum aortic diameter of less than 4.5 cm. Diameters were measured at standard levels by 2 blinded reviewers. Normalized systolic flow displacement was calculated at peak systole from the PC-MRI data, and correlation with the interval aortic growth was performed, with adjustment for clinical/demographic factors. The average follow-up interval was 2.9 ± 1.3 years. Systolic flow displacement at the initial study strongly correlated with ascending aortic growth rate (r = 0.71, P < 0.005) with moderate, non-significant correlation between initial diameter and growth (r = 0.45, P = 0.214). Aortic growth was 4 times faster in patients with initial flow displacement of 0.2 or greater (n = 9) compared with those (n = 8) with initial flow displacement less than 0.2 (0.8 ± 0.4 vs 0.2 ± 0.3 mm/y; P = 0.002). Systolic flow displacement calculated from conventional 2D PC-MRI in the ascending aorta correlates with future aortic growth in patients undergoing routine surveillance imaging for BAV. With a cutoff valve of 0.2, flow displacement may be used to identify a subset of patients likely to have elevated growth rates and may better risk-stratify patients with BAV for aortic disease progression than vessel diameter alone.

Volumetric arterial wall shear stress calculation based on cine phase contrast MRI

To assess the accuracy and precision of a volumetric wall shear stress (WSS) calculation method applied to cine phase contrast magnetic resonance imaging (PC-MRI) data. Volumetric WSS vectors were calculated in software phantoms. WSS algorithm parameters were optimized and the influence of spatial resolution and segmentation was evaluated. Subsequently, 2D cine PC-MRI data in the carotid and the aorta at varying spatial resolutions were obtained (n = 2) and compared with the simulations. Finally, volumetric WSS was calculated in 3D cine PC-MRI data of the carotid bifurcation and the aorta (n = 6). We found that at least 8 voxels across the diameter are required to obtain a WSS accuracy of 5% and a precision of 20% in software phantoms. Systematic WSS quantification errors up to 40% were found in the case of segmentation errors. The in vivo measurements using 2D cine PC-MRI exhibited WSS increase at increasing spatial resolutions, similar to the results in software phantoms. Volumetric WSS vectors were successfully calculated in three healthy carotid bifurcations and aortas. The effects of resolution and segmentation on the accuracy and precision of the WSS algorithm were quantified. We were able to calculate volumetric WSS in the carotid bifurcation and the aorta.

Analysis of an automated background correction method for cardiovascular MR phase contrast imaging in children and young adults

Phase contrast magnetic resonance imaging (MRI) is a powerful tool for evaluating vessel blood flow. Inherent errors in acquisition, such as phase offset, eddy currents and gradient field effects, can cause significant inaccuracies in flow parameters. These errors can be rectified with the use of background correction software. To evaluate the performance of an automated phase contrast MRI background phase correction method in children and young adults undergoing cardiac MR imaging. We conducted a retrospective review of patients undergoing routine clinical cardiac MRI including phase contrast MRI for flow quantification in the aorta (Ao) and main pulmonary artery (MPA). When phase contrast MRI of the right and left pulmonary arteries was also performed, these data were included. We excluded patients with known shunts and metallic implants causing visible MRI artifact and those with more than mild to moderate aortic or pulmonary stenosis. Phase contrast MRI of the Ao, mid MPA, proximal right pulmonary artery (RPA) and left pulmonary artery (LPA) using 2-D gradient echo Fast Low Angle SHot (FLASH) imaging was acquired during normal respiration with retrospective cardiac gating. Standard phase image reconstruction and the automatic spatially dependent background-phase-corrected reconstruction were performed on each phase contrast MRI dataset. Non-background-corrected and background-phase-corrected net flow, forward flow, regurgitant volume, regurgitant fraction, and vessel cardiac output were recorded for each vessel. We compared standard non-background-corrected and background-phase-corrected mean flow values for the Ao and MPA. The ratio of pulmonary to systemic blood flow (Qp:Qs) was calculated for the standard non-background and background-phase-corrected data and these values were compared to each other and for proximity to 1. In a subset of patients who also underwent phase contrast MRI of the MPA, RPA, and LPA a comparison was made between standard non-background-corrected and background-phase-corrected mean combined flow in the branch pulmonary arteries and MPA flow. All comparisons were performed using the Wilcoxon sign rank test (α = 0.05). Eighty-five children and young adults (mean age 14 years; range 10 days to 32 years) met the criteria for inclusion. Background-phase-corrected mean flow values for the Ao and MPA were significantly lower than those for non-background-corrected standard Ao (P = 0.0004) and MPA flow values (P < 0.0001), respectively. However, no significant difference was seen between the standard non-background (P = 0.295) or background-phase-corrected (P = 0.0653) mean Ao and MPA flow values. Neither the mean standard non-background-corrected (P = 0.408) nor the background-phase-corrected (P = 0.0684) Qp:Qs was significantly different from 1. However in the 27 patients with standard non-background-corrected data, the difference between the Ao and MPA flow values was greater than 10%. There were 19 patients with background-phase-corrected data in which the difference between the Ao and MPA flow values was greater than 10%. In the subset of 43 patients who underwent MPA and branch pulmonary artery phase contrast MRI, the sum of the standard non-background-corrected mean RPA and LPA flow values was significantly different from the standard non-background-corrected mean MPA flow (P = 0.0337). The sum of the background-phase-corrected mean RPA and LPA flow values was not significantly different from the background-phase-corrected mean MPA flow value (P = 0.1328), suggesting improvement in pulmonary artery flow calculations using background-phase-correction. Our data suggest that background phase correction of phase contrast MRI data does not significantly change Qp:Qs quantification, and there are residual errors in expected Qp:Qs quantification despite background phase correction. However the use of background phase correction does improve quantification of MPA flow relative to combined RPA and LPA flow. Further work is needed to validate these findings in other patient populations, using other MRI units, and across vendors.

Neoaortic Root Aneurysm After Arterial Switch Operation With Lecompte Maneuver

Fig 2. Ateries had undergone an arterial switch operation with the Lecompte maneuver and mechanical valve replacement for neoaortic valve insufficiency at 1 and 8 years of age, respectively. Recently, a neoaortic aneurysm was diagnosed by magnetic resonance imaging. Right (blue) and left (red) heart structures are shown in Fig 1A, and isolated left heart structures can be seen in Fig 1B. Three-dimensional phase-contrast magnetic resonance imaging data was used to visualize flow patterns. Vortical flow in the aneurysm was fast (red path lines) during systole and persisted as slow (blue) flow into late diastole. Blood flow was accelerated in the narrowed distal main pulmonary artery (Video 1). Because the aneurysm showed progressive growth, reaching a diameter of 7 cm, replacement of the ascending aorta was indicated. The main pulmonary artery (Fig 2, MPA) was transected and the aneurysm was incised (Fig 2, asterisk). Implantation of a mechanical valve conduit (Fig 2, MVC) became necessary because of pannus formation at the valve prosthesis (Fig 2, AC 1⁄4 aortic cannula; LVOT 1⁄4 left ventricular outflow tract; VC 1⁄4 venous cannula). Based on intraoperative observations, we assume that the aneurysm evolved because of

Wall shear stress estimated with phase contrast MRI in an in vitro and in vivo intracranial aneurysm

To evaluate wall shear stress (WSS) estimations in an in vitro and in vivo intracranial aneurysm, WSS was estimated from phase contrast magnetic resonance imaging (PC-MRI) and compared with computational fluid dynamics (CFD). First, WSS was estimated using a high-resolution in vitro PC-MRI measurement under steady and pulsatile flow conditions and compared with CFD simulations. Second, WSS was estimated in steady PC-MRI data acquired at different spatial resolutions. Third, WSS estimations in pulsatile in vivo data were compared with CFD. The direction and magnitude of WSS vectors were computed and compared. Quantitative agreement between PC-MRI and CFD-based WSS estimations was moderate for the phantom (Spearman ρ = 0.69). The WSS magnitude derived from PC-MRI data was lower than CFD for both the in vitro and in vivo case. However, there was qualitative agreement between PC-MRI and CFD, i.e. WSS vector direction was similar for both modalities. Circular WSS patterns were found both in vitro and in vivo for PC-MRI and CFD. Increasing PC-MRI resolution increased mean WSS magnitude and uncovered complex WSS patterns. WSS patterns can be estimated based on PC-MRI data in in vitro and in vivo aneurysm geometries. Similar WSS directions as CFD can be discerned.

In Vivo Validation of Numerical Prediction for Turbulence Intensity in an Aortic Coarctation

This paper compares numerical predictions of turbulence intensity with in vivo measurement. Magnetic resonance imaging (MRI) was carried out on a 60-year-old female with a restenosed aortic coarctation. Time-resolved three-directional phase-contrast (PC) MRI data was acquired to enable turbulence intensity estimation. A contrast-enhanced MR angiography (MRA) and a time-resolved 2D PCMRI measurement were also performed to acquire data needed to perform subsequent image-based computational fluid dynamics (CFD) modeling. A 3D model of the aortic coarctation and surrounding vasculature was constructed from the MRA data, and physiologic boundary conditions were modeled to match 2D PCMRI and pressure pulse measurements. Blood flow velocity data was subsequently obtained by numerical simulation. Turbulent kinetic energy (TKE) was computed from the resulting CFD data. Results indicate relative agreement (error ≈10%) between the in vivo measurements and the CFD predictions of TKE. The discrepancies in modeled vs. measured TKE values were within expectations due to modeling and measurement errors.

In Vitro Validation of Finite-Element Model of AAA Hemodynamics Incorporating Realistic Outlet Boundary Conditions

The purpose of this study is to validate numerical simulations of flow and pressure in an abdominal aortic aneurysm (AAA) using phase-contrast magnetic resonance imaging (PCMRI) and an in vitro phantom under physiological flow and pressure conditions. We constructed a two-outlet physical flow phantom based on patient imaging data of an AAA and developed a physical Windkessel model to use as outlet boundary conditions. We then acquired PCMRI data in the phantom while it operated under conditions mimicking a resting and a light exercise physiological state. Next, we performed in silico numerical simulations and compared experimentally measured velocities, flows, and pressures in the in vitro phantom to those computed in the in silico simulations. There was a high degree of agreement in all of the pressure and flow waveform shapes and magnitudes between the experimental measurements and simulated results. The average pressures and flow split difference between experiment and simulation were all within 2%. Velocity patterns showed good agreement between experimental measurements and simulated results, especially in the case of whole-cycle averaged comparisons. We demonstrated methods to perform in vitro phantom experiments with physiological flows and pressures, showing good agreement between numerically simulated and experimentally measured velocity fields and pressure waveforms in a complex patient-specific AAA geometry.

Analysis of human myocardial dynamics using virtual markers based on magnetic resonance imaging

Myocardial dynamics are three-dimensional (3D) and time-varying. Cineradiography of surgically implanted makers in animals or patients is accurate for assessing these events, but this invasive method potentially alters myocardial motion. The aim of the study was to quantify myocardial motion using magnetic resonance imaging (MRI) and hence to provide a non-invasive approach to characterize 3D myocardial dynamics. Myocardial motion was quantified in ten normal volunteers by tracking the Lagrangian motion of individual points (i.e. virtual markers), based on time-resolved 3D phase-contrast MRI data and Fourier tracking. Nine points in the myocardium were tracked over the entire cardiac cycle, allowing a wire frame model to be generated and systolic and diastolic events identified. Radius of curvature of the left ventricular (LV) wall was calculated from the virtual markers; the ratio between the anterior-posterior (AP) and septal-lateral (SL) walls in the LV shows an oval shape of the apical short axis plane at end systole (ES) and more circular at end diastole (ED). The AP/SL ratio for the basal plane shows an oval shape at ES and ED. We found that the rotation of the basal plane in ES was less compared to the apical plane [-2.0 +/- 2.2 versus 4.1 +/- 2.6 degrees (P<0.005)]. The apical plane rotated counter clock wise as viewed from the apex. This new non-invasive tool, despite current limitations in temporal and spatial resolution, may provide a comprehensive set of virtual myocardial markers throughout the entire LV without the confounding effects introduced by surgical implantation.