Interdependencies of aortic arch secondary flow patterns, geometry, and age analysed by 4-dimensional phase contrast magnetic resonance imaging at 3 Tesla

Enhancing the Velocity Data From 4D Flow MR Images by Reducing its Divergence

Velocity measurements from 4D flow MRI are prone to be affected by several imperfections of the MR system. Assuming that blood is incompressible, we propose a novel method for enhancing the velocity field by reducing its divergence. To enhance the velocity data, we added a corrector velocity to each voxel such that the divergence is minimized. The method was validated using an analytical Womersley flow model for different settings of resolution and noise levels. The performance of the proposed method was also assessed in volunteers and patients. Results demonstrated a significant reduction of the divergence depending on the size of the regularization term, obtaining a reduction close to 50% of the mean divergence with negligible modification of flow parameters. Remarkably, we found that the reduction of the divergence, in percentage, was independent of volunteers, resolution or noise.

Volumetric quantification of absolute local normalized helicity in patients with bicuspid aortic valve and aortic dilatation

PURPOSE

Absolute local normalized helicity (LNH) can differentiate flow alterations in the aorta between healthy controls and bicuspid aortic valve (BAV) patients.

METHODS

A total of 65 controls and 50 subjects with BAV underwent in vivo four-dimensional (4D) flow MRI. Data analysis included the three-dimensional (3D) segmentation of the thoracic aorta (ascending aorta, aortic arch, and descending aorta) and calculation of absolute LNH. The mean velocity in the entire aorta was used to identify peak systole, systolic deceleration, and middiastole. A sensitivity analysis was performed to identify the optimal absolute LNH threshold, comparing control and BAV groups. A reproducibility test was performed for 3D segmentation and absolute LNH.

RESULTS

Absolute LNH above 0.6 was significantly higher (P < 0.001) in BAV in comparison to controls for all aortic segments and cardiac time frames. Absolute LNH in the ascending aorta correlated with maximal aortic diameter (R = 0.83, P < 0.001, at peak systole; r = 0.84, P < 0.001, at systolic deceleration; R = 0.88, P < 0.001, at middiastole) and significantly increased (P < 0.001) with aortic stenosis severity. Intra- and interobserver errors were 5 ± 2% and 12 ± 6% for 3D segmentation and 7 ± 6% and 12 ± 7% for absolute LNH.

CONCLUSION

Absolute LNH can differentiate between controls and subjects with aortic dilatation, and was associated with maximal aortic diameter and aortic stenosis severity. Magn Reson Med 78:689-701, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Altered aortic shape in bicuspid aortic valve relatives influences blood flow patterns

AIMS

Bicuspid aortic valve (BAV) is known to exhibit familial inheritance and is associated with aortopathy and altered aortic haemodynamics. However, it remains unclear whether BAV-related aortopathy can be inherited independently of valve morphology.

METHODS AND RESULTS

Four-dimensional flow magnetic resonance imaging for the in vivo assessment of thoracic aortic 3D blood flow was performed in 24 BAV relatives with trileaflet aortic valves (age = 40 ± 14 years) and 15 healthy controls (age = 37 ± 10 years). Data analysis included aortic dimensions, shape (round/gothic/cubic), and 3D blood flow characteristics (semi-quantitative vortex/helix grading and peak velocities). Cubic and gothic aortic shapes were markedly more prevalent in BAV relatives compared with controls (38 vs. 7%). Ascending aorta (AAo) vortex flow in BAV relatives was significantly increased compared with controls (grading = 1.5 ± 1.0 vs. 0.6 ± 0.9, P = 0.015). Aortic haemodynamics were influenced by aortic shape: peak velocities were reduced for gothic aortas vs. round aortas (P = 0.003); vortex flow was increased for cubic aortas in the AAo (P < 0.001) and aortic arch (P = 0.004); vortex and helix flows were elevated for gothic aortas in the AAo and descending aorta (P = 0.003, P = 0.029). Logistic regression demonstrated significant associations of shape with severity of vortex flow in AAo (P < 0.001) and aortic arch (P = 0.016) in BAV relatives.

CONCLUSION

BAV relatives expressed altered aortic shape and increased vortex flow despite the absence of valvular disease or aortic dilatation. These data suggest a heritable component of BAV-related aortopathy affecting aortic shape and aberrant blood flow, independent of valve morphology.

Hemodynamic Measurement Using Four-Dimensional Phase-Contrast MRI: Quantification of Hemodynamic Parameters and Clinical Applications

Recent improvements have been made to the use of time-resolved, three-dimensional phase-contrast (PC) magnetic resonance imaging (MRI), which is also named four-dimensional (4D) PC-MRI or 4D flow MRI, in the investigation of spatial and temporal variations in hemodynamic features in cardiovascular blood flow. The present article reviews the principle and analytical procedures of 4D PC-MRI. Various fluid dynamic biomarkers for possible clinical usage are also described, including wall shear stress, turbulent kinetic energy, and relative pressure. Lastly, this article provides an overview of the clinical applications of 4D PC-MRI in various cardiovascular regions.

Analysis of Systolic Backflow and Secondary Helical Blood Flow in the Ascending Aorta Using Vector Flow Imaging

Secondary rotational flow and systolic backflow are seen in the ascending aorta and, in this study, were analyzed with the vector velocity method transverse oscillation. Twenty-five patients were scanned intra-operatively, and the vector velocities were related to estimates of transesophageal echocardiography and pulmonary artery catheter thermodilution, and associated with gender, age, aortic diameter, atherosclerotic plaques, left ventricular ejection fraction and previous myocardial infarctions. Secondary flow was present for all patients. The duration and rotational frequency (p < 0.001) and the duration and flow direction of the secondary flow (p < 0.002) were associated. Systolic backflow was present in 40% of the patients and associated with systolic velocities (p < 0.002) and the presence of atherosclerotic plaques (p < 0.001). No other significant associations were observed. The study indicates that backflow is injurious and that secondary flow is a normal flow phenomenon. The study also shows that transverse oscillation can provide new information on blood flow in the ascending aorta.

Differences in aortic vortex flow pattern between normal and patients with stroke: qualitative and quantitative assessment using transesophageal contrast echocardiography

The flow in the aorta forms a vortex, which is a critical determinant of the flow dynamics in the aorta. Arteriosclerosis can alter the blood flow pattern of the aorta and cause characteristic alterations of the vortex. However, this change in aortic vortex has not yet been studied. This study aimed to characterize aortic vortex flow pattern using transesophageal contrast echocardiography in normal and stroke patients. A total of 85 patients who diagnosed with ischemic stroke and 16 normal controls were recruited for this study. The 16 normal control subjects were designated as the control group, and the 85 ischemic stroke patients were designated as the stroke group. All subjects underwent contrast transesophageal echocardiography (TEE), and particle image velocimetry was used to assess aortic vortex flow. Qualitative and quantitative analyses of vortex flow morphology, location, phasic variation, and pulsatility were undertaken and compared between the groups. In the control group, multiple irregularly-shaped vortices were observed in a peripheral location in the descending thoracic aorta. In contrast, the stroke group had a single, round, merged, and more centrally located aortic vortex flow. In the quantitative analysis of vortex, vortex depth, which represents the location of the major vortex in the aorta, was significantly higher in the control group than in the stroke group (0.599 ± 0.159 vs. 0.522 ± 0.101, respectively, P = 0.013). Vortex relative strength, which is the pulsatility parameter of the vortex itself, was significantly higher in the stroke group than in the control group (0.367 ± 0.148 vs. 0.304 ± 0.087, respectively, P = 0.025). It was feasible to visualize and quantify the characteristic morphology and pulsatility of the aortic vortex flow using contrast TEE, and aortic vortex pattern significantly differed between normal and stroke patients.

Quantitative Analysis of Vortical Blood Flow in the Thoracic Aorta Using 4D Phase Contrast MRI

Introduction

Phase contrast MRI allows for the examination of complex hemodynamics in the heart and adjacent great vessels. Vortex flow patterns seem to play an important role in certain vascular pathologies. We propose two- and three-dimensional metrics for the objective quantification of aortic vortex blood flow in 4D phase contrast MRI.

Materials and Methods

For two-dimensional vorticity assessment, a standardized set of 6 regions-of-interest (ROIs) was defined throughout the course of the aorta. For each ROI, a heatmap of time-resolved vorticity values ω→=∇v→ was computed. Evolution of minimum, maximum, and average values as well as opposing rotational flow components were analyzed. For three-dimensional analysis, vortex core detection was implemented combining the predictor-corrector method with λ₂ correction. Strength, elongation, and radial expansion of the detected vortex core were recorded over time. All methods were applied to 4D flow MRI datasets of 9 healthy subjects, 2 patients with mildly dilated aorta, and 1 patient with aortic aneurysm.

Results

Vorticity quantification in the 6 standardized ROIs enabled the description of physiological vortex flow in the healthy aorta. Helical flow developed early in the ascending aorta (absolute vorticity = 166.4±86.4 s^-1 at 12% of cardiac cycle) followed by maximum values in mid-systole in the aortic arch (240.1±45.2 s^-1 at 16%). Strength, elongation, and radial expansion of 3D vortex cores escalated in early systole, reaching a peak in mid systole (strength = 241.2±30.7 s^-1 at 17%, elongation = 65.1±34.6 mm at 18%, expansion = 80.1±48.8 mm² at 20%), before all three parameters similarly decreased to overall low values in diastole. Flow patterns were considerably altered in patient data: Vortex flow developed late in mid/end-systole close to the aortic bulb and no physiological helix was found in the aortic arch.

Conclusions

We have introduced objective measures for quantification of vortical flow in 4D phase contrast MRI. Vortex blood flow in the thoracic aorta could be consistently described in all healthy volunteers. In patient data, pathologically altered vortex flow was observed.

4D flow cardiovascular magnetic resonance consensus statement

Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 - 3×3×3 mm(3), typical temporal resolution of 30-40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.

Automatic extraction of three-dimensional thoracic aorta geometric model from phase contrast MRI for morphometric and hemodynamic characterization

PURPOSE

To propose and assess a new method that automatically extracts a three-dimensional (3D) geometric model of the thoracic aorta (TA) from 3D cine phase contrast MRI (PCMRI) acquisitions.

METHODS

The proposed method is composed of two steps: segmentation of the TA and creation of the 3D geometric model. The segmentation algorithm, based on Level Set, was set and applied to healthy subjects acquired in three different modalities (with and without SENSE reduction factors). Accuracy was evaluated using standard quality indices. The 3D model is characterized by the vessel surface mesh and its centerline; the comparison of models obtained from the three different datasets was also carried out in terms of radius of curvature (RC) and average tortuosity (AT).

RESULTS

In all datasets, the segmentation quality indices confirmed very good agreement between manual and automatic contours (average symmetric distance < 1.44 mm, DICE Similarity Coefficient > 0.88). The 3D models extracted from the three datasets were found to be comparable, with differences of less than 10% for RC and 11% for AT.

CONCLUSION

Our method was found effective on PCMRI data to provide a 3D geometric model of the TA, to support morphometric and hemodynamic characterization of the aorta.

A rational approach to defining principal axes of multidirectional wall shear stress in realistic vascular geometries, with application to the study of the influence of helical flow on wall shear stress directionality in aorta

The distribution of arterial lesions is attributed by the prevalent mechanistic theory to the proatherogenic role played by low and oscillatory wall shear stress (WSS). However, discrepancies observed when comparing WSS distribution with location of regions with lesion prevalence challenge this theory and have recently stimulated the idea that a role in endothelial mechanosensing is played by WSS multidirectionality, which could contribute to explain the observed discrepancies. Here an approach is presented for analyzing the multidirectional nature of WSS in complex vascular geometries. Using an essential geometric attribute of the vessel (its centerline), the local WSS vector is projected along an "axial" direction (aligned with the tangent to the vessel׳s centerline), and "secondary" direction (orthogonal to centerline׳s tangent), which is related to secondary flow. The WSS projection scheme is applied: (1) to a realistic computational hemodynamic model of human aorta, with the aim to come up with a plausibility checking regarding its consistency; and (2) to investigate if an aortic hemodynamics characterized by different amount and topology of helical flow (HF) could influence WSS directionality. The projection scheme confirmed its consistency and plausibility in realistic arterial geometries and allowed to get insight into the relationship between aortic intravascular fluid structures and WSS directionality. The findings of this study clearly show the potential of the projection scheme as quantitative tool for an in depth investigation of the WSS multidirectional nature. The proposed approach enriches the arsenal of tools available to study and exploit the role played by local hemodynamics in vascular disease.

In vivo analysis of physiological 3D blood flow of cerebral veins

OBJECTIVES

To visualize and quantify physiological blood flow of intracranial veins in vivo using time-resolved, 3D phase-contrast MRI (4D flow MRI), and to test measurement accuracy.

METHODS

Fifteen healthy volunteers underwent repeated ECG-triggered 4D flow MRI (3 Tesla, 32-channel head coil). Intracranial venous blood flow was analysed using dedicated software allowing for blood flow visualization and quantification in analysis planes at the superior sagittal, straight, and transverse sinuses. MRI was evaluated for intra- and inter-observer agreement and scan-rescan reproducibility. Measurements of the transverse sinuses were compared with transcranial two-dimensional duplex ultrasound.

RESULTS

Visualization of 3D blood flow within cerebral sinuses was feasible in 100 % and within at least one deep cerebral vein in 87 % of the volunteers. Blood flow velocity/volume increased along the superior sagittal sinus and was lower in the left compared to the right transverse sinus. Intra- and inter-observer reliability and reproducibility of blood flow velocity (mean difference 0.01/0.02/0.02 m/s) and volume (mean difference 0.0002/-0.0003/0.00003 l/s) were good to excellent. High/low velocities were more pronounced (8 % overestimation/9 % underestimation) in MRI compared to ultrasound.

CONCLUSIONS

Four-dimensional flow MRI reliably visualizes and quantifies three-dimensional cerebral venous blood flow in vivo and is promising for studies in patients with sinus thrombosis and related diseases.

KEY POINTS

• 4D flow MRI can be used to visualize and quantify physiological cerebral venous haemodynamics • Flow quantification within cerebral sinuses reveals high reliability and accuracy of 4D flow MRI • Blood flow volume and velocity increase along the superior sagittal sinus • Limited spatial resolution currently precludes flow quantification in small cerebral veins.

Viscous energy loss in the presence of abnormal aortic flow

PURPOSE

To present a theoretical basis for noninvasively characterizing in vivo fluid-mechanical energy losses and to apply it in a pilot study of patients known to express abnormal aortic flow patterns.

METHODS

Four-dimensional flow MRI was used to characterize laminar viscous energy losses in the aorta of normal controls (n = 12, age = 37 ± 10 yr), patients with aortic dilation (n = 16, age = 52 ± 8 yr), and patients with aortic valve stenosis matched for age and aortic size (n = 14, age = 46 ± 15 yr), using a relationship between the three-dimensional velocity field and viscous energy dissipation.

RESULTS

Viscous energy loss was elevated significantly in the thoracic aorta in patients with dilated aorta (3.6 ± 1.3 mW, P = 0.024) and patients with aortic stenosis (14.3 ± 8.2 mW, P < 0.001) compared with healthy volunteers (2.3 ± 0.9 mW). The same pattern of significant differences was seen in the ascending aorta, where viscous energy losses in patients with dilated aortas (2.2 ± 1.1 mW, P = 0.021) and patients with aortic stenosis (10.9 ± 6.8 mW, P < 0.001) were elevated compared with healthy volunteers (1.2 ± 0.6 mW).

CONCLUSION

This technique provides a capability to quantify the contribution of abnormal laminar blood flow to increased ventricular afterload. In this pilot study, viscous energy loss in patient cohorts was significantly elevated and indicates that cardiac afterload is increased due to abnormal flow.

Accelerated analysis of three-dimensional blood flow of the thoracic aorta in stroke patients

To test if new software accelerates analysis of in vivo acquired 4D flow MRI data. Respiration-gated and ECG-synchronized 4D flow MRI of the aorta was performed in 20 stroke patients using a routine 3-Tesla MRI system (TIMTRIO, Siemens, Germany). 3D blood flow data was processed by one experienced observer using new (A = MEVISFlow) and widely-used software (B = EnSight + Velomap-/FlowTool). Evaluation included: inter-/intra-observer variability of software A and inter-software comparison regarding (1) blood flow quantification (total-/peak flow) and (2) flow visualisation, plus (3) measurement of the time required for visualization and quantification of data (software A&B). (1) Inter-/intra-observer agreement of software A (mean difference ≤5.2 and ≤0.9 %, respectively) and inter-software agreement (mean difference ≤ 2.2 %) was high with high correlation of peak and total blood flow (r ≥ 0.74; p < 0.001 and r ≥ 0.91; p < 0.001). (2) Comparison of blood flow visualization showed substantial agreement (κ ≥ 0.68). (3) Data-analysis was three times faster when using software A [18:10 (±1:29) vs. 58:30 (±5:28) min; p < 0.0001]. Acceleration of blood flow quantification and visualisation using new software strongly facilitates future applications of 4D flow MRI and thus enables its usage in larger patient cohorts in clinical research and routine.

Bovine arch and carotid artery atherosclerosis: are they related?

OBJECTIVES

The prevalence of the "bovine" arch in the population is known (8-25%). However, its prevalence in patients with significant carotid atherosclerosis has never been investigated. Altered flow patterns or turbulence that may occur in these patients may play a causative role in the development of atherosclerotic lesions. The primary purpose of this study was to retrospectively compare the prevalence of aortic arch variants in patients with and without significant carotid artery atherosclerosis, as we hypothesize that carotid atherosclerosis may be more prevalent in patients with a bovine arch due to hemodynamic alterations. A secondary objective was to review radiologist reporting of arch anatomy.

METHODS

Single-center, retrospective, case-control study in which 79 patients with hemodynamically significant carotid artery atherosclerosis who underwent computed tomography angiography, magnetic resonance angiography, or unenhanced computed tomography (CT) imaging including the aortic arch were identified. These patients were then compared with 95 randomly selected controls without carotid atherosclerosis that underwent similar imaging during the same time period. Images were independently reviewed by two blinded radiologists, who assessed arch anatomy as normal, bovine, or other variant. The original radiology reports were reviewed for reporting of arch anatomy.

RESULTS

In controls, 70% had normal arch anatomy, and 24% had a bovine arch. Among patients with significant carotid disease, these numbers were 70% and 20%, respectively. There was no statistically significant difference between incidence of arch variants in subjects with and without carotid artery atherosclerosis (P=.97). There was good interreader agreement. Among patients with aortic arch anomalies, 20% of the original radiology reports did not mention arch anatomy.

CONCLUSIONS

In our experience, percentage of bovine arch anomalies in patients with significant carotid atherosclerosis is not significantly different from those without disease. Clinicians should be aware of the high prevalence of arch anomalies, which can impact endovascular approach and management, and radiologists should be aware of the clinical importance of reporting such variants.

Quantification of thoracic blood flow using volumetric magnetic resonance imaging with radial velocity encoding: in vivo validation

OBJECTIVES

The objective of this study was to validate radially undersampled 5-point velocity-encoded time-resolved flow-sensitive magnetic resonance imaging (MRI) ("PC-VIPR", phase contrast vastly undersampled imaging with isotropic resolution projection reconstruction magnetic resonance) for the quantification of ascending aortic (AAO) and main pulmonary artery (MPA) flow in vivo.

MATERIALS AND METHODS

Data from 18 healthy volunteers (41.6 ± 16.2 years [range, 22-73 years]; body mass index, 26.0 ± 3.5 [19.1-31.4]) scanned at 3 T with a 32-channel coil were included. The left and right ventricular stroke volumes calculated from contiguous short-axis CINE-balanced steady state free precession (CINE-bSSFP) slices were used as the primary reference for cardiac output. Flow measured from 2-dimensional phase contrast MRI (2D-PC-MRI) in the AAO and the MPA served as the secondary reference. Time-resolved 4-dimensional flow-sensitive MRI (4D flow MRI) using PC-VIPR was performed with a velocity sensitivity of Venc = 150 cm/s reconstructed to 20 time frames at 1.4-mm isotropic spatial resolution. In 11 of 20 volunteers, phantom-corrected 4D flow MRI data were also assessed. Differences between methods of calculating the left ventricular and right ventricular cardiac output were assessed with the Bland-Altman analysis (BA, mean difference ±2SD). The QP/QS-ratio was calculated for each method.

RESULTS

Initially, PC-VIPR compared unfavorably with CINE-bSSFP (left ventricular stroke volume: 96.5 ± 14.4 mL; right ventricular stroke volume: 93.6 ± 14.0 mL vs 81.2 ± 24.3 mL [AAO] and 85.6 ± 25.4 mL [MPA]; P = 0.027 and 0.25) with BA differences of -14.6 ± 44.0 mL (AAO) and -9.0 ± 45.9 mL (MPA). Whereas phantom correction had minor effects on 2D-PC-MRI results and comparison with CINE-bSSFP, it improved PC-VIPR results: BA differences between CINE-bSSFP and PC-VIPR after correction were -1.4 ± 15.3 mL (AAO) and -4.1 ± 16.1 mL (MPA); BA comparison with 2D-PC-MRI improved to -12.0 ± 48.1 mL (AAO) and -2.2 ± 19.5 mL (MPA). QP/QS-ratio results for all techniques were within physiologic limits.

CONCLUSIONS

Accurate quantification of AAO and MPA flows with radially undersampled 4D flow MRI applying 5-point velocity encoding is achievable when phantom correction is used.

Finite element analysis of helical flows in human aortic arch: a novel index

This study investigates the helical secondary flows in the aortic arch using finite element analysis. The relationship between helical flow and the configuration of the aorta in patients of whose three-dimensional images constructed from computed tomography scans was examined. A finite element model of the pressurized root, arch, and supra-aortic vessels was developed to simulate the pattern of helical secondary flows. Calculations indicate that most of the helical secondary flow was formed in the ascending aorta. Angle α between the zero reference point and the aortic ostium (correlation coefficient (r) = -0.851, P = 0.001), the dispersion index of the cross section of the ascending (r = 0.683, P = 0.021) and descending aorta (r = 0.732, P = 0.010), all correlated closely with the presence of helical flow (P < 0.05). Stepwise multiple linear regression analysis confirmed angel α to be independently associated with the helical flow pattern in therein (standardized coefficients = -0.721, P = 0.023). The presence of helical fluid motion based on the atherosclerotic risks of patients, including those associated with diabetes, hypertension, hyperlipidemia, or renal insufficiency, was also evaluated. Numerical simulation of the flow patterns in aortas incorporating the atherosclerotic risks may better explain the mechanism of formation of helical flows and provide insight into causative factors that underlie them.

Flow-sensitive four-dimensional velocity-encoded magnetic resonance imaging reveals abnormal blood flow patterns in the aorta and pulmonary trunk of patients with transposition

BACKGROUND AND OBJECTIVES

Flow profiles are important determinants of fluid-vessel wall interactions. The aim of this study was to assess blood flow profiles in the aorta and pulmonary trunk in patients with transposition and different ventriculoarterial connection, and hence different mechanics of the coherent pump.

METHODS

In all, 29 patients with operated transposition--concordant atrioventricular and discordant ventriculoarterial connection, and no other cardiac malformation--and eight healthy volunteers were assessed with cardiac magnetic resonance imaging: n = 17 patients after atrial redirection, with a morphologic right ventricle acting as systemic pump and a morphologic left ventricle connected to the pulmonary trunk, and n = 12 patients after the arterial switch procedure, with physiologic ventriculoarterial connections. Flow-sensitive four-dimensional velocity-encoded magnetic resonance imaging was used to analyse systolic flow patterns in the aorta and pulmonary trunk, relating to helical flow and vortex formation.

RESULTS

In the aorta, overall helicity was present in healthy volunteers, but it was absent in all patients independent on the operation technique. Partial helices were observed in the ascending aorta of 58% of patients after arterial switch. In the pulmonary trunk, mostly parallel flow was seen in healthy volunteers and in patients after arterial switch, whereas vortex formation was present in 88% of patients after atrial redirection.

CONCLUSION

Blood flow patterns differ substantially between the groups. In addition to varying mechanics of the coherent pumping ventricles, the absent overall helicity in all patients might be explained by the missing looping of the aorta in transposition.

Evaluation of elevated mean pulmonary arterial pressure based on magnetic resonance 4D velocity mapping: comparison of visualization techniques

Purpose

Three-dimensional (3D) magnetic resonance phase contrast imaging (PC-MRI) allows non-invasive diagnosis of pulmonary hypertension (PH) and estimation of elevated mean pulmonary arterial pressure (mPAP) based on vortical motion of blood in the main pulmonary artery. The purpose of the present study was to compare the presence and duration of PH-associated vortices derived from different flow visualization techniques with special respect to their performance for non-invasive assessment of elevated mPAP and diagnosis of PH.

Methods

Fifty patients with suspected PH (23 patients with and 27 without PH) were investigated by right heart catheterization and time-resolved PC-MRI of the main pulmonary artery. PC-MRI data were visualized with dedicated prototype software, providing 3D vector, multi-planar reformatted (MPR) 2D vector, streamline, and particle trace representation of flow patterns. Persistence of PH-associated vortical blood flow (t_vortex) was evaluated with all visualization techniques. Dependencies of t_vortex on visualization techniques were analyzed by means of correlation and receiver operating characteristic (ROC) curve analysis.

Results

t_vortex values from 3D vector visualization correlated strongly with those from other visualization techniques (r = 0.98, 0.98 and 0.97 for MPR, streamline and particle trace visualization, respectively). Areas under ROC curves for diagnosis of PH based on t_vortex did not differ significantly and were 0.998 for 3D vector, MPR vector and particle trace visualization and 0.999 for streamline visualization. Correlations between elevated mPAP and t_vortex in patients with PH were r = 0.96, 0.93, 0.95 and 0.92 for 3D vector, MPR vector, streamline and particle trace visualization, respectively. Corresponding standard deviations from the linear regression lines ranged between 3 and 4 mmHg.

Conclusion

3D vector, MPR vector, streamline as well as particle trace visualization of time-resolved 3D PC-MRI data of the main pulmonary artery can be employed for accurate vortex-based diagnosis of PH and estimation of elevated mPAP.

The impact of MRI-based inflow for the hemodynamic evaluation of aortic coarctation

Aortic coarctation (CoA) accounting for 3-11% of congenital heart disease can be successfully treated. Long-term results, however, have revealed decreased life expectancy associated with abnormal hemodynamics. Accordingly, an assessment of hemodynamics is the key factor in treatment decisions and successful long-term results. In this study, 3D angiography whole heart (3DWH) and 4D phase-contrast magnetic resonance imaging (MRI) data were acquired. Geometries of the thoracic aorta with CoAs were reconstructed using ZIB-Amira software. X-ray angiograms were used to evaluate the post-treatment geometry. Computational fluid dynamics models in three patients were created to simulate pre- and post-treatment situations using the FLUENT program. The aim of the study was to investigate the impact of the inlet velocity profile (plug vs. MRI-based) with a focus on the peak systole pressure gradient and wall shear stress (WSS). Results show that helical flow at the aorta inlet can significantly affect the assessment of pressure drop and WSS. Simplified plug inlet velocity profiles significantly (p < 0.05) overestimate the pressure drop in pre- and post-treatment geometries and significantly (p < 0.05) underestimate surface-averaged WSS. We conclude that the use of the physiologically correct but time-expensive 4D MRI-based in vivo velocity profile in CFD studies may be an important step towards a patient-specific analysis of CoA hemodynamics.

Contemporary Role of Computational Analysis in Endovascular Treatment for Thoracic Aortic Disease

In the past decade, thoracic endovascular aortic repair (TEVAR) has become the primary treatment option in descending aneurysm and dissection. The clinical outcome of this minimally invasive technique is strictly related to an appropriate patient/stent graft selection, hemodynamic interactions, and operator skills. In this context, a quantitative assessment of the biomechanical stress induced in the aortic wall due to the stent graft may support the planning of the procedure. Different techniques of medical imaging, like computed tomography or magnetic resonance imaging, can be used to evaluate dynamics in the thoracic aorta. Such information can also be combined with dedicated patient-specific computer-based simulations, to provide a further insight into the biomechanical aspects. In clinical practice, computational analysis might show the development of aortic disease, such as the aortic wall segments which experience higher stress in places where rupture and dissection may occur. In aortic dissections, the intimal tear is usually located at the level of the sino-tubular junction and/or at the origin of the left subclavian artery. Besides, computational models may potentially be used preoperatively to predict stent graft behavior, virtually testing the optimal stent graft sizing, deployment, and conformability, in order to provide the best endovascular treatment. The present study reviews the current literature regarding the use of computational tools for TEVAR biomechanics, highlighting their potential clinical applications.

Aortic outflow cannula tip design and orientation impacts cerebral perfusion during pediatric cardiopulmonary bypass procedures

Poor perfusion of the aortic arch is a suspected cause for peri- and post-operative neurological complications associated with cardiopulmonary bypass (CPB). High-speed jets from 8 to 10FR pediatric/neonatal cannulae delivering ~1 L/min of blood can accrue sub-lethal hemolytic damage while also subjecting the aorta to non-physiologic flow conditions that compromise cerebral perfusion. Therefore, we emphasize the importance of cannulation strategy and hypothesize engineering better CPB perfusion through a redesigned aortic cannula tip. This study employs computational fluid dynamics to investigate novel diffuser-tipped aortic cannulae for shape sensitivity to cerebral perfusion, in an in silico cross-clamped aortic arch model modeled with fixed outflow resistances. 17 parametrically altered configurations of an 8FR end-hole and several diffuser cone angled tips in combination with jet incidence angles toward or away from the head-neck vessels were studied. Experimental pressure-flow characterizations were also conducted on these cannula tip designs. An 8FR end-hole aortic cannula delivering 1 L/min along the transverse aortic arch was found to give rise to backflow from the brachicephalic artery (BCA), irrespective of angular orientation, for the chosen ascending aortic insertion location. Parametric alteration of the cannula tip to include a diffuser cone angle (tested up to 7°) eliminated BCA backflow for any tested angle of jet incidence. Experiments revealed that a 1 cm long 10° diffuser cone tip demonstrated the best pressure-flow performance improvement in contrast with either an end-hole tip or diffuser cone angles greater than 10°. Performance further improved when the diffuser was preceded by an expanded four-lobe swirl inducer attachment-a novel component. In conclusion, aortic cannula orientation is crucial in determining net head-neck perfusion but precise angulations and insertion-depths are difficult to achieve practically. Altering the cannula tip to include a diffuser cone angle has been shown for the first time to have potential in ensuring a net positive outflow at the BCA. Cannula insertion distanced from the BCA inlet may also avoid backflow owing to the Venturi effect, but the diffuser tipped cannula design presents a promising solution to mitigate this issue irrespective of in vivo cannula tip orientation.

4D flow magnetic resonance imaging in bicuspid aortic valve disease demonstrates altered distribution of aortic blood flow helicity

PURPOSE

Changes in aortic geometry or presence of aortic valve (AoV) disease can result in substantially altered aortic hemodynamics. Dilatation of the ascending aorta or AoV abnormalities can result in an increase in helical flow.

METHODS

4D flow magnetic resonance imaging was used to test the feasibility of quantitative helicity analysis using equidistantly distributed 2D planes along the entire aorta. The evaluation of the method included three parts: (1) the quantification of helicity in 12 healthy subjects, (2) an evaluation of observer variability and test-retest reliability, and (3) the quantification of helical flow in 16 patients with congenitally altered bicuspid AoVs.

RESULTS

Helicity quantification in healthy subjects revealed consistent directions of flow rotation along the entire aorta with high clockwise helicity in the aortic arch and an opposite rotation sense in the ascending and descending aorta. The results demonstrated good scan-rescan and inter- and intraobserver agreement of the helicity parameters. Helicity quantification in patients revealed a significant increase in absolute peak relative helicity during systole and a considerably greater heterogeneous distribution of mean helicity in the aorta.

CONCLUSION

The method has the potential to serve as a reference distribution for comparisons of helical flow between healthy subjects and patients or between different patient groups.

Left atrial flow velocity distribution and flow coherence using four-dimensional FLOW MRI: a pilot study investigating the impact of age and Pre- and Postintervention atrial fibrillation on atrial hemodynamics

PURPOSE

To use four-dimensional (4D) flow MRI to characterize and quantify 3D blood flow in the left atria (LA) of patients with a history of atrial fibrillation (AF).

MATERIALS AND METHODS

The 4D flow MRI was acquired in 19 volunteers (n = 9<30 years, n = 10>50 years) and 10 patients with AF (62 ± 9.6 years; n = 4 in persistent AF, n = 6 postintervention). The LA in each dataset was segmented, and intra-atrial blood flow velocity was quantified. Flow coherence was measured as the consistency of the net blood flow vector.

RESULTS

Quantification of atrial flow revealed significant differences in atrial hemodynamics between age groups. Postintervention AF patients had a mean blood flow of 0.22 ± 0.04 m/s, which was not significantly different than age-matched volunteers (0.21 ± 0.03 m/s). Patients with persistent AF had a mean blood flow of 0.13 ± 0.01 m/s, lower than AF patients in sinus rhythm (0.22 ± 0.04 m/s, P = 0.005), or age-matched volunteers (0.21 ± 0.03 m/s, P < 0.001). Flow coherence was significantly impaired in patients in AF.

CONCLUSION

Flow-sensitive MRI shows that patients with a history of AF had global hemodynamics in the LA similar to those of age-matched volunteers. Additional studies with larger cohorts of AF patients and correlation with outcome are needed to further investigate the potential of atrial 4D flow MRI to flow patterns indicative of stroke risk in AF.

Mechanism of development of aortic transection: a possible new angle

Aortic transection injury is a frequently fatal injury secondary to sudden deceleration. To date magnitude of deceleration is the only factor known to influence the development of an aortic transection injury. We hypothesise that different 3D geometries of the aortic arch in healthy young adult patients as a possible predisposing factor for transection injuries when undergoing sudden deceleration. We extend this to hypothesise that the direction of deceleration may be important as well. In addition we hypothesise that the stage in the cardiac cycle, which determines central aortic blood pressure, when the deceleration occurs as an important factor. We utilise known engineering principles such as Newton's second Law of motion, moment of inertia, law of Laplace, and the theory of superposition to explain our hypothesis. We present limited data to demonstrate the wide variation in aortic arch 3D geometry to explain the possible an individual's variable susceptibility to transection injuries via the principle of moment of inertia. Engineering principles suggest that 3D aortic arch geometry, direction of deceleration and stage in the cardiac cycle, in addition to the magnitude of deceleration are potentially important factors in predisposing certain individuals in a given situation to aortic transaction injuries.