Four-dimensional (4D) flow of the whole heart and great vessels using real-time respiratory self-gating

Four-dimensional flow MRI for quantitative assessment of cerebrospinal fluid dynamics: Status and opportunities

Neurological disorders can manifest with altered neurofluid dynamics in different compartments of the central nervous system. These include alterations in cerebral blood flow, cerebrospinal fluid (CSF) flow, and tissue biomechanics. Noninvasive quantitative assessment of neurofluid flow and tissue motion is feasible with phase contrast magnetic resonance imaging (PC MRI). While two-dimensional (2D) PC MRI is routinely utilized in research and clinical settings to assess flow dynamics through a single imaging slice, comprehensive neurofluid dynamic assessment can be limited or impractical. Recently, four-dimensional (4D) flow MRI (or time-resolved three-dimensional PC with three-directional velocity encoding) has emerged as a powerful extension of 2D PC, allowing for large volumetric coverage of fluid velocities at high spatiotemporal resolution within clinically reasonable scan times. Yet, most 4D flow studies have focused on blood flow imaging. Characterizing CSF flow dynamics with 4D flow (i.e., 4D CSF flow) is of high interest to understand normal brain and spine physiology, but also to study neurological disorders such as dysfunctional brain metabolite waste clearance, where CSF dynamics appear to play an important role. However, 4D CSF flow imaging is challenged by the long T1 time of CSF and slower velocities compared with blood flow, which can result in longer scan times from low flip angles and extended motion-sensitive gradients, hindering clinical adoption. In this work, we review the state of 4D CSF flow MRI including challenges, novel solutions from current research and ongoing needs, examples of clinical and research applications, and discuss an outlook on the future of 4D CSF flow.

Modeling cerebrospinal fluid dynamics across the entire intracranial space through integration of four-dimensional flow and intravoxel incoherent motion magnetic resonance imaging

BACKGROUND

Bidirectional reciprocal motion of cerebrospinal fluid (CSF) was quantified using four-dimensional (4D) flow magnetic resonance imaging (MRI) and intravoxel incoherent motion (IVIM) MRI. To estimate various CSF motions in the entire intracranial region, we attempted to integrate the flow parameters calculated using the two MRI sequences. To elucidate how CSF dynamics deteriorate in Hakim's disease, an age-dependent chronic hydrocephalus, flow parameters were estimated from the two MRI sequences to assess CSF motion in the entire intracranial region.

METHODS

This study included 127 healthy volunteers aged ≥ 20 years and 44 patients with Hakim's disease. On 4D flow MRI for measuring CSF motion, velocity encoding was set at 5 cm/s. For the IVIM MRI analysis, the diffusion-weighted sequence was set at six b-values (i.e., 0, 50, 100, 250, 500, and 1000 s/mm2), and the biexponential IVIM fitting method was adapted. The relationships between the fraction of incoherent perfusion (f) on IVIM MRI and 4D flow MRI parameters including velocity amplitude (VA), absolute maximum velocity, stroke volume, net flow volume, and reverse flow rate were comprehensively evaluated in seven locations in the ventricles and subarachnoid spaces. Furthermore, we developed a new parameter for fluid oscillation, the Fluid Oscillation Index (FOI), by integrating these two measurements. In addition, we investigated the relationship between the measurements and indices specific to Hakim's disease and the FOIs in the entire intracranial space.

RESULTS

The VA on 4D flow MRI was significantly associated with the mean f-values on IVIM MRI. Therefore, we estimated VA that could not be directly measured on 4D flow MRI from the mean f-values on IVIM MRI in the intracranial CSF space, using the following formula; e0.2(f-85) + 0.25. To quantify fluid oscillation using one integrated parameter with weighting, FOI was calculated as VA × 10 + f × 0.02. In addition, the FOIs at the left foramen of Luschka had the strongest correlations with the Evans index (Pearson's correlation coefficient: 0.78). The other indices related with Hakim's disease were significantly associated with the FOIs at the cerebral aqueduct and bilateral foramina of Luschka. FOI at the cerebral aqueduct was also elevated in healthy controls aged ≥ 60 years.

CONCLUSIONS

We estimated pulsatile CSF movements in the entire intracranial CSF space in healthy individuals and patients with Hakim's disease using FOI integrating VA from 4D flow MRI and f-values from IVIM MRI. FOI is useful for quantifying the CSF oscillation.

Deep learning based automated left ventricle segmentation and flow quantification in 4D flow cardiac MRI

BACKGROUND

4D flow MRI enables assessment of cardiac function and intra-cardiac blood flow dynamics from a single acquisition. However, due to the poor contrast between the chambers and surrounding tissue, quantitative analysis relies on the segmentation derived from a registered cine MRI acquisition. This requires an additional acquisition and is prone to imperfect spatial and temporal inter-scan alignment. Therefore, in this work we developed and evaluated deep learning-based methods to segment the left ventricle (LV) from 4D flow MRI directly.

METHODS

We compared five deep learning-based approaches with different network structures, data pre-processing and feature fusion methods. For the data pre-processing, the 4D flow MRI data was reformatted into a stack of short-axis view slices. Two feature fusion approaches were proposed to integrate the features from magnitude and velocity images. The networks were trained and evaluated on an in-house dataset of 101 subjects with 67,567 2D images and 3030 3D volumes. The performance was evaluated using various metrics including Dice, average surface distance (ASD), end-diastolic volume (EDV), end-systolic volume (ESV), LV ejection fraction (LVEF), LV blood flow kinetic energy (KE) and LV flow components. The Monte Carlo dropout method was used to assess the confidence and to describe the uncertainty area in the segmentation results.

RESULTS

Among the five models, the model combining 2D U-Net with late fusion method operating on short-axis reformatted 4D flow volumes achieved the best results with Dice of 84.52% and ASD of 3.14 mm. The best averaged absolute and relative error between manual and automated segmentation for EDV, ESV, LVEF and KE was 19.93 ml (10.39%), 17.38 ml (22.22%), 7.37% (13.93%) and 0.07 mJ (5.61%), respectively. Flow component results derived from automated segmentation showed high correlation and small average error compared to results derived from manual segmentation.

CONCLUSIONS

Deep learning-based methods can achieve accurate automated LV segmentation and subsequent quantification of volumetric and hemodynamic LV parameters from 4D flow MRI without requiring an additional cine MRI acquisition.

A Deep Learning Approach to Using Wearable Seismocardiography (SCG) for Diagnosing Aortic Valve Stenosis and Predicting Aortic Hemodynamics Obtained by 4D Flow MRI

In this paper, we explored the use of deep learning for the prediction of aortic flow metrics obtained using 4-dimensional (4D) flow magnetic resonance imaging (MRI) using wearable seismocardiography (SCG) devices. 4D flow MRI provides a comprehensive assessment of cardiovascular hemodynamics, but it is costly and time-consuming. We hypothesized that deep learning could be used to identify pathological changes in blood flow, such as elevated peak systolic velocity ([Formula: see text]) in patients with heart valve diseases, from SCG signals. We also investigated the ability of this deep learning technique to differentiate between patients diagnosed with aortic valve stenosis (AS), non-AS patients with a bicuspid aortic valve (BAV), non-AS patients with a mechanical aortic valve (MAV), and healthy subjects with a normal tricuspid aortic valve (TAV). In a study of 77 subjects who underwent same-day 4D flow MRI and SCG, we found that the [Formula: see text] values obtained using deep learning and SCGs were in good agreement with those obtained by 4D flow MRI. Additionally, subjects with non-AS TAV, non-AS BAV, non-AS MAV, and AS could be classified with ROC-AUC (area under the receiver operating characteristic curves) values of 92%, 95%, 81%, and 83%, respectively. This suggests that SCG obtained using low-cost wearable electronics may be used as a supplement to 4D flow MRI exams or as a screening tool for aortic valve disease.

Four-Dimensional Flow MR Imaging: Technique and Advances

4D Flow MRI is an advanced imaging technique for comprehensive non-invasive assessment of the cardiovascular system. The capture of the blood velocity vector field throughout the cardiac cycle enables measures of flow, pulse wave velocity, kinetic energy, wall shear stress, and more. Advances in hardware, MRI data acquisition and reconstruction methodology allow for clinically feasible scan times. The availability of 4D Flow analysis packages allows for more widespread use in research and the clinic and will facilitate much needed multi-center, multi-vendor studies in order to establish consistency across scanner platforms and to enable larger scale studies to demonstrate clinical value.

4D Flow cardiovascular magnetic resonance consensus statement: 2023 update

Hemodynamic assessment is an integral part of the diagnosis and management of cardiovascular disease. Four-dimensional cardiovascular magnetic resonance flow imaging (4D Flow CMR) allows comprehensive and accurate assessment of flow in a single acquisition. This consensus paper is an update from the 2015 '4D Flow CMR Consensus Statement'. We elaborate on 4D Flow CMR sequence options and imaging considerations. The document aims to assist centers starting out with 4D Flow CMR of the heart and great vessels with advice on acquisition parameters, post-processing workflows and integration into clinical practice. Furthermore, we define minimum quality assurance and validation standards for clinical centers. We also address the challenges faced in quality assurance and validation in the research setting. We also include a checklist for recommended publication standards, specifically for 4D Flow CMR. Finally, we discuss the current limitations and the future of 4D Flow CMR. This updated consensus paper will further facilitate widespread adoption of 4D Flow CMR in the clinical workflow across the globe and aid consistently high-quality publication standards.

Comparison of Improved Unidirectional Dual Velocity-Encoding MRI Methods

BACKGROUND

In phase-contrast (PC) MRI, several dual velocity encoding methods have been proposed to robustly increase velocity-to-noise ratio (VNR), including a standard dual-VENC (SDV), an optimal dual-VENC (ODV), and bi- and triconditional methods.

PURPOSE

To develop a correction method for the ODV approach and to perform a comparison between methods.

STUDY TYPE

Case-control study.

POPULATION

Twenty-six volunteers.

FIELD STRENGTH/SEQUENCE

1.5 T phase-contrast MRI with VENCs of 50, 75, and 150 cm/second.

ASSESSMENT

Since we acquired single-VENC protocols, we used the background phase from high-VENC (VENC_H ) to reconstruct the low-VENC (VENC_L ) phase. We implemented and compared the unwrapping methods for different noise levels and also developed a correction of the ODV method.

STATISTICAL TESTS

Shapiro-Wilk's normality test, two-way analysis of variance with homogeneity of variances was performed using Levene's test, and the significance level was adjusted by Tukey's multiple post hoc analysis with Bonferroni (P < 0.05).

RESULTS

Statistical analysis revealed no extreme outliers, normally distributed residuals, and homogeneous variances. We found statistically significant interaction between noise levels and the unwrapping methods. This implies that the number of non-unwrapped pixels increased with the noise level. We found that for β = VENC_L /VENC_H  = 1/2, unwrapping methods were more robust to noise. The post hoc test showed a significant difference between the ODV corrected and the other methods, offering the best results regarding the number of unwrapped pixels.

DATA CONCLUSIONS

All methods performed similarly without noise, but the ODV corrected method was more robust to noise at the price of a higher computational time.

LEVEL OF EVIDENCE

4 TECHNICAL EFFICACY STAGE: 1.

Impact of Respiratory Gating on Hemodynamic Parameters from 4D Flow MRI

The hemodynamic parameters from 4D flow datasets have shown promising diagnostic value in different cardiovascular pathologies. However, the behavior of these parameters can be affected when the 4D flow data are corrupted by respiratory motion. The purpose of this work was to perform a quantitative comparison between hemodynamic parameters computed from 4D flow cardiac MRI both with and without respiratory self-gating. We considered 4D flow MRI data from 15 healthy volunteers (10 men and 5 women, 30.40 ± 6.23 years of age) that were acquired at 3T. Using a semiautomatic segmentation process of the aorta, we obtained the hemodynamic parameters from the 4D flow MRI, with and without respiratory self-gating. A statistical analysis, using the Wilcoxon signed-rank test and Bland–Altman, was performed to compare the hemodynamic parameters from both acquisitions. We found that the calculations of the hemodynamic parameters from 4D flow data that were acquired without respiratory self-gating showed underestimated values in the aortic arch, and the descending and diaphragmatic aorta. We also found a significant variability of the hemodynamic parameters in the ascending aorta of healthy volunteers when comparing both methods. The 4D flow MRI requires respiratory compensation to provide reliable calculations of hemodynamic parameters.

Cardiac MR: From Theory to Practice

Cardiovascular disease (CVD) is the leading single cause of morbidity and mortality, causing over 17. 9 million deaths worldwide per year with associated costs of over $800 billion. Improving prevention, diagnosis, and treatment of CVD is therefore a global priority. Cardiovascular magnetic resonance (CMR) has emerged as a clinically important technique for the assessment of cardiovascular anatomy, function, perfusion, and viability. However, diversity and complexity of imaging, reconstruction and analysis methods pose some limitations to the widespread use of CMR. Especially in view of recent developments in the field of machine learning that provide novel solutions to address existing problems, it is necessary to bridge the gap between the clinical and scientific communities. This review covers five essential aspects of CMR to provide a comprehensive overview ranging from CVDs to CMR pulse sequence design, acquisition protocols, motion handling, image reconstruction and quantitative analysis of the obtained data. (1) The basic MR physics of CMR is introduced. Basic pulse sequence building blocks that are commonly used in CMR imaging are presented. Sequences containing these building blocks are formed for parametric mapping and functional imaging techniques. Commonly perceived artifacts and potential countermeasures are discussed for these methods. (2) CMR methods for identifying CVDs are illustrated. Basic anatomy and functional processes are described to understand the cardiac pathologies and how they can be captured by CMR imaging. (3) The planning and conduct of a complete CMR exam which is targeted for the respective pathology is shown. Building blocks are illustrated to create an efficient and patient-centered workflow. Further strategies to cope with challenging patients are discussed. (4) Imaging acceleration and reconstruction techniques are presented that enable acquisition of spatial, temporal, and parametric dynamics of the cardiac cycle. The handling of respiratory and cardiac motion strategies as well as their integration into the reconstruction processes is showcased. (5) Recent advances on deep learning-based reconstructions for this purpose are summarized. Furthermore, an overview of novel deep learning image segmentation and analysis methods is provided with a focus on automatic, fast and reliable extraction of biomarkers and parameters of clinical relevance.

Echo planar imaging-induced errors in intracardiac 4D flow MRI quantification

Purpose

To assess errors associated with EPI‐accelerated intracardiac 4D flow MRI (4DEPI) with EPI factor 5, compared with non‐EPI gradient echo (4DGRE).

Methods

Three 3T MRI experiments were performed comparing 4DEPI to 4DGRE: steady flow through straight tubes, pulsatile flow in a left‐ventricle phantom, and intracardiac flow in 10 healthy volunteers. For each experiment, 4DEPI was repeated with readout and blip phase‐encoding gradient in different orientations, parallel or perpendicular to the flow direction. In vitro flow rates were compared with timed volumetric collection. In the left‐ventricle phantom and in vivo, voxel‐based speed and spatio‐temporal median speed were compared between sequences, as well as mitral and aortic transvalvular net forward volume.

Results

In steady‐flow phantoms, the flow rate error was largest (12%) for high velocity (>2 m/s) with 4DEPI readout gradient parallel to the flow. Voxel‐based speed and median speed in the left‐ventricle phantom were ≤5.5% different between sequences. In vivo, mean net forward volume inconsistency was largest (6.4 ± 8.5%) for 4DEPI with nonblip phase‐encoding gradient parallel to the main flow. The difference in median speed for 4DEPI versus 4DGRE was largest (9%) when the 4DEPI readout gradient was parallel to the flow.

Conclusions

Velocity and flow rate are inaccurate for 4DEPI with EPI factor 5 when flow is parallel to the readout or blip phase‐encoding gradient. However, mean differences in flow rate, voxel‐based speed, and spatio‐temporal median speed were acceptable (≤10%) when comparing 4DEPI to 4DGRE for intracardiac flow in healthy volunteers.

Validation of 4D Flow based relative pressure maps in aortic flows

While the clinical gold standard for pressure difference measurements is invasive catheterization, 4D Flow MRI is a promising tool for enabling a non-invasive quantification, by linking highly spatially resolved velocity measurements with pressure differences via the incompressible Navier-Stokes equations. In this work we provide a validation and comparison with phantom and clinical patient data of pressure difference maps estimators. We compare the classical Pressure Poisson Estimator (PPE) and the new Stokes Estimator (STE) against catheter pressure measurements under a variety of stenosis severities and flow intensities. Specifically, we use several 4D Flow data sets of realistic aortic phantoms with different anatomic and hemodynamic severities and two patients with aortic coarctation. The phantom data sets are enriched by subsampling to lower resolutions, modification of the segmentation and addition of synthetic noise, in order to study the sensitivity of the pressure difference estimators to these factors. Overall, the STE method yields more accurate results than the PPE method compared to catheterization data. The superiority of the STE becomes more evident at increasing Reynolds numbers with a better capacity of capturing pressure gradients in strongly convective flow regimes. The results indicate an improved robustness of the STE method with respect to variation in lumen segmentation. However, with heuristic removal of the wall-voxels, the PPE can reach a comparable accuracy for lower Reynolds' numbers.

Analysis of right ventricular flow with 4-dimensional flow cardiovascular magnetic resonance imaging in patients with pulmonary arterial hypertension

Background

Cardiac flow closely interact with function, however, the correlation of right ventricular (RV) flow and function remains unknown, thus our objective is to observe right ventricular flow with four-dimensional phase-contrast cardiovascular magnetic resonance imaging (4D flow CMR) in patients with pulmonary arterial hypertension (PAH) and to analyze flow components with RV function and hemodynamics.

Methods

This study retrospectively enrolled 30 patients with PAH (mean age: 49±13 years, 16 females) and 14 age- and sex-matched healthy volunteers as controls (mean age: 44±12 years, 9 females). All patients who underwent CMR and right heart catheterization (RHC) within 1 week between January 2019 and July 2020 were included. Hemodynamics were measured with RHC. RV flow components, including the percentages of direct flow (RVPDF), retained inflow (RVPRI), delayed ejection flow (RVPDEF) and residual volume (RVPRVo) were quantified using 4D flow CMR. The associations between RV flow components and other CMR metrics, clinical data, and hemodynamics were analyzed by Spearman's correlation analysis.

Results

In patients with PAH, RVPDF was decreased and RVPRVo was increased compared with the normal control group. The sum of RVPDF and RVPDEF RV was significantly correlated with RV ejection fraction (RVEF) (r=0.802, P<0.001), and there was no notable difference between RVEF and the sum of RVPDF and RVPDEF (t=0.251, P=0.831). Both RVPDF and RVPRVo were correlated (in opposite directions) with the RV end-diastolic volume index, RV end-systolic volume index, RV global longitudinal strain, and RVEF. RVPDF was negatively correlated with pulmonary vascular resistance (PVR), and positively correlated with cardiac output and cardiac index. RVPRVo was positively correlated with PVR and negatively correlated with cardiac output and cardiac index.

Conclusions

RV blood flow components qualified with 4D flow CMR is a valuable noninvasive method for the assessment of RV function and hemodynamics in patients with PAH.

Impact of aortic arch curvature in flow haemodynamics in patients with transposition of the great arteries after arterial switch operation

AIMS 

In this study, we will describe a comprehensive haemodynamic analysis and its relationship to the dilation of the aorta in transposition of the great artery (TGA) patients post-arterial switch operation (ASO) and controls using 4D-flow magnetic resonance imaging (MRI) data.

METHODS AND RESULTS 

Using 4D-flow MRI data of 14 TGA young patients and 8 age-matched normal controls obtained with 1.5 T GE-MR scanner, we evaluate 3D maps of 15 different haemodynamics parameters in six regions; three of them in the aortic root and three of them in the ascending aorta (anterior-left, -right, and posterior for both cases) to find its relationship with the aortic arch curvature and root dilation. Differences between controls and patients were evaluated using Mann-Whitney U test, and the relationship with the curvature was accessed by unpaired t-test. For statistical significance, we consider a P-value of 0.05. The aortic arch curvature was significantly different between patients 46.238 ± 5.581 m-1 and controls 41.066 ± 5.323 m-1. Haemodynamic parameters as wall shear stress circumferential (WSS-C), and eccentricity (ECC), were significantly different between TGA patients and controls in both the root and ascending aorta regions. The distribution of forces along the ascending aorta is highly inhomogeneous in TGA patients. We found that the backward velocity (B-VEL), WSS-C, velocity angle (VEL-A), regurgitation fraction (RF), and ECC are highly correlated with the aortic arch curvature and root dilatation.

CONCLUSION 

We have identified six potential biomarkers (B-VEL, WSS-C, VEL-A, RF, and ECC), which may be helpful for follow-up evaluation and early prediction of aortic root dilatation in this patient population.

Monitoring and Synchronization of Cardiac and Respiratory Traces in Magnetic Resonance Imaging: A Review

Synchronization of human vital signs, namely the cardiac cycle and respiratory excursions, is necessary during magnetic resonance imaging of the cardiovascular system and the abdominal cavity to achieve optimal image quality with minimized artifacts. This review summarizes techniques currently available in clinical practice, as well as methods under development, outlines the benefits and disadvantages of each approach, and offers some unique solutions for consideration.

Velocity quantification in 44 healthy volunteers using accelerated multi-VENC 4D flow CMR

BACKGROUND

To evaluate the feasibility of a k-t accelerated multi-VENC 4D phase contrast flow MRI acquisition of the main heart-surrounding vessels, its benefits over a traditional single-VENC acquisition and to present reference flow and velocity values in a large cohort of volunteers.

METHODS

44 healthy volunteers were examined on a 3 T MRI scanner (Ingenia, Philips, Best, The Netherlands). 4D flow measurements were obtained with a FOV including the aorta and the pulmonary arteries. VENC values were set to 40, 100 and 200 cm/s and unfolded based on an MRI signal model. Unfolded multi-VENC data was compared to the single-VENC with VENC 200 cm/s. Flow and velocity quantification was performed in several regions of interest (ROI) placed in the ascending aorta and in the main pulmonary artery. Conservation of mass analysis was performed for single- and multi-VENC datasets. Values for mean and maximal flow velocity and stroke volume were calculated and compared to the literature.

RESULTS

Mean scan time was 13.8 ± 4 min. Differences between stroke volumes between the ascending aorta and the main pulmonary artery were significantly lower in multi-VENC datasets compared to single-VENC datasets (9.6 ± 7.8 mL vs. 25.4 ± 26.4 mL, p < 0.001). This was also true for differences in stroke volume between up- and downstream ROIs in the ascending aorta and pulmonary artery. Values for mean and maximal velocities and stroke volume were in-line with previous studies. To highlight potential clinical applications two exemplary 4D flow measurements in patients with different pathologies are shown and compared to single-VENC datasets.

CONCLUSIONS

k-t accelerated multi-VENC 4D phase contrast flow MRI acquisition of the great vessels is feasible in a clinically acceptable scan duration. It offers improvements over traditional single-VENC 4D flow, expectedly being valuable when vessels with different flow velocities or complex flow phenomena are evaluated.

Intraindividual validation of 4D flow measurement against 2D flow measurements in complex flow patterns in routine clinical cardiovascular magnetic resonance of bicuspid aortic valve

Background

Comparing four-dimensional flow against two-dimensional flow measurements in patients with complex flow pattern is still lacking. This study aimed to compare four-dimensional against the two-dimensional flow measurement in patients with bicuspid aortic valve and to test potentials of four-dimensional operator-dependent sources of error.

Results

The two- and four-dimensional flow data sets of sixteen patients with bicuspid aortic valve and eighteen healthy subjects were studied. Flow analyses were performed by two observers blindly. Patients with bicuspid aortic valve mean differences between the two- and four-dimensional measurements in both observers were − 8 and − 4 ml, respectively. Four-dimensional measurements resulted in systematically higher flow values than the two-dimensional flow in bicuspid aortic valve patients. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 12/− 28 ml and + 14/− 21 ml, respectively. In the healthy volunteers, mean differences between the two- and four-dimensional measurements in both observers were ± 0 and + 1 ml, respectively. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 21/− 18 ml and + 12/− 13 ml, respectively. Inter-observer variability in four-dimensional flow measurement was 4% mean net forward flow in bicuspid aortic valve patients and 8% in healthy volunteers.

Conclusion

Inter-observer variability in four-dimensional flow assessment is 8% or less which is acceptable for clinical cardiac MRI routine. There is close agreement of two- and four-dimensional flow tools in normal and complex flow pattern. In complex flow pattern, however, four-dimensional flow measurement picks up 4–9% higher flow values. It seems, therefore, that four-dimensional flow is closer to real flow values than two-dimensional flow, which is however to be proven by further studies.

Cardiac magnetic resonance using fused 3D cine and 4D flow sequences:Validation of ventricular and blood flow measurements

PURPOSE

Current cardiovascular magnetic resonance (CMR) examinations require expert planning, multiple breath holds, and 2D imaging. To address this, we sought to develop and validate a comprehensive free-breathing 3D cine function and flow CMR examination using a steady-state free precession (SSFP) sequence to depict anatomy fused with a spatially registered phase contrast (PC) sequence for blood flow analysis.

METHODS

In a prospective study, 25 patients underwent a CMR examination which included a 3D cine SSFP sequence and a 3D cine PC (also known as 4D flow) sequence acquired during free-breathing and after the administration of a gadolinium-based contrast agent. Both 3D sequences covered the heart and mediastinum, and used retrospective vectorcardiogram gating (20 phases/beat interpolated to 30 phases/beat) and prospective respiratory motion compensation confining data acquisition to end-expiration. Cardiovascular measurements derived from the 3D cine SSFP and PC images were then compared with those from standard 2D imaging.

RESULTS

All 3D cine SSFP and PC acquisitions were completed successfully. The mean time for the 3D cine sequences including prescription was shorter than that for the corresponding 2D sequences (21 min vs. 36 min, P-value <0.001). Left and right ventricular end-diastolic volumes and stroke volumes by 3D cine SSFP were slightly smaller than those from 2D cine SSFP (all biases ≤5%). The blood flow measurements from the 3D and 2D sequences had close agreement in the ascending aorta (bias -2.6%) but main pulmonary artery flow was lower with the 3D cine sequence (bias -11.2%).

CONCLUSION

Compared to the conventional 2D cine approach, a comprehensive 3D cine function and flow examination was faster and yielded slightly lower left and right end-diastolic volumes, stroke volumes, and main pulmonary artery blood flow. This free-breathing 3D cine approach allows flexible post-examination data analysis and has the potential to make examinations more comfortable for patients and easier to perform for the operator.

Validation of non-contrast multiple overlapping thin-slab 4D-flow cardiac magnetic resonance imaging

BACKGROUND

Cardiac magnetic resonance (CMR) flow quantification is typically performed using 2D phase-contrast (PC) imaging of a plane perpendicular to flow. 3D-PC imaging (4D-flow) allows offline quantification anywhere in a thick slab, but is often limited by suboptimal signal, potentially alleviated by contrast enhancement. We developed a non-contrast 4D-flow sequence, which acquires multiple overlapping thin slabs (MOTS) to minimize signal loss, and hypothesized that it could improve image quality, diagnostic accuracy, and aortic flow measurements compared to non-contrast single-slab approach.

METHODS

We prospectively studied 20 patients referred for transesophageal echocardiography (TEE), who underwent CMR (GE, 3 T). 2D-PC images of the aortic valve and three 4D-flow datasets covering the heart were acquired, including single-slab, pre- and post-contrast, and non-contrast MOTS. Each 4D-flow dataset was interpreted blindly for ≥moderate valve disease and compared to TEE. Flow visualization through each valve was scored (0 to 4), and aortic-valve flow measured on each 4D-flow dataset and compared to 2D-PC reference.

RESULTS

Diagnostic quality visualization was achieved with the pre- and post-contrast 4D-flow acquisitions in 25% and 100% valves, respectively (scores 0.9 ± 1.1 and 3.8 ± 0.5), and in 58% with the non-contrast MOTS (1.6 ± 1.1). Accuracy of detection of valve disease was 75%, 92% and 82%, respectively. Aortic flow measurements were possible in 53%, 95% and in 89% patients, respectively. The correlation between pre-contrast single-slab measurements and 2D-PC reference was weak (r = 0.21), but improved with both contrast enhancement (r = 0.71) and with MOTS (r = 0.67).

CONCLUSIONS

Although non-contrast MOTS 4D-flow improves valve function visualization and diagnostic accuracy, a significant proportion of valves cannot be accurately assessed. However, aortic flow measurements using non-contrast MOTS is feasible and reaches similar accuracy to that of contrast-enhanced 4D-flow.

Fully self-gated whole-heart 4D flow imaging from a 5-minute scan

PURPOSE

To develop and validate an acquisition and processing technique that enables fully self-gated 4D flow imaging with whole-heart coverage in a fixed 5-minute scan.

THEORY AND METHODS

The data are acquired continuously using Cartesian sampling and sorted into respiratory and cardiac bins using the self-gating signal. The reconstruction is performed using a recently proposed Bayesian method called ReVEAL4D. ReVEAL4D is validated using data from 8 healthy volunteers and 2 patients and compared with compressed sensing technique, L1-SENSE.

RESULTS

Healthy subjects-Compared with 2D phase-contrast MRI (2D-PC), flow quantification from ReVEAL4D shows no significant bias. In contrast, the peak velocity and peak flow rate for L1-SENSE are significantly underestimated. Compared with traditional parallel MRI-based 4D flow imaging, ReVEAL4D demonstrates small but significant biases in net flow and peak flow rate, with no significant bias in peak velocity. All 3 indices are significantly and more markedly underestimated by L1-SENSE. Patients-Flow quantification from ReVEAL4D agrees well with the 2D-PC reference. In contrast, L1-SENSE markedly underestimated peak velocity.

CONCLUSIONS

The combination of highly accelerated 5-minute Cartesian acquisition, self-gating, and ReVEAL4D enables whole-heart 4D flow imaging with accurate flow quantification.

4D flow imaging of the thoracic aorta: is there an added clinical value?

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment.

4D flow MRI applications in congenital heart disease

Advances in the diagnosis and management of congenital heart disease (CHD) have resulted in a growing population of patients surviving well into adulthood and requiring lifelong follow-up. Flow quantification is a central component in the assessment of patients with CHD. 4D flow magnetic resonance imaging (MRI) has emerged as a tool that enables comprehensive study of flow. It involves the acquisition of a three-dimensional time-resolved volume with velocity encoding in all three spatial directions along the cardiac cycle. This allows flow quantification and visualization of blood flow patterns as well as the study of advanced hemodynamic parameters as kinetic energy and wall shear stress. 4D flow MRI-based study of flow has given insight into the altered hemodynamics in CHD particularly in bicuspid aortic valve disease and Fontan circulation. The aim of this review is to discuss the expanding clinical and research applications of 4D flow MRI in CHD as well its limitations.Key Points• Three-dimensional velocity encoding allows not only flow quantification but also the visualization of multidirectional flow patterns and the study of advanced hemodynamic parameters.• 4D flow MRI has added insight into the abnormal hemodynamics involved in congenital heart disease in particular in bicuspid aortic valve and Fontan circulation.• The main limitation of 4D flow MRI in congenital heart disease is the relatively long scan duration required for the complete coverage of the heart and great vessels with adequate spatiotemporal resolution.

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

BACKGROUND

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.

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSION

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.

Hypertrophic Cardiomyopathy Is Associated with Altered Left Ventricular 3D Blood Flow Dynamics

Purpose

To employ four-dimensional (4D) flow MRI to investigate associations between hemodynamic parameters with systolic anterior motion (SAM), mitral regurgitation (MR), stroke volume, and cardiac mass in patients with hypertrophic cardiomyopathy (HCM).

Materials and Methods

A total of 13 patients with HCM (51 years ± 16 [standard deviation]; 10 men) and 11 age-matched healthy control subjects (54 years ± 15; eight men) underwent cardiac 4D flow MRI data analysis including calculation of peak systolic and diastolic control-averaged left ventricular (LV) velocity maps to quantify volumes of elevated velocity (EVV) in the left ventricle. Standard-of-care cine imaging was performed in short-axis, LV outflow tract (LVOT), and two-, three-, and four-chamber views on which the presence of SAM, presence of MR, total stroke volume, and cardiac mass were assessed.

Results

Systolic EVV in patients with HCM was 7 mL ± 5, which was significantly associated with elevated aortic peak velocity (R = 0.87; P < .001), decreased LVOT diameter (R = 0.68; P = .01), and increased cardiac mass (R = 0.62; P = .02). In addition, EVV differed significantly between patients with and those without SAM (10 mL ± 4.7 vs 3 mL ± 2.3; P = .03) and those with and those without MR (9.9 mL ± 4.8 vs 4.0 mL ± 3.2; P < .05). In the atrial systolic phase, peak diastolic velocity in the LV correlated with septal thickness (R = 0.66; P = .01).

Conclusion

Quantification and visualization of EVV in the LV is feasible and may provide further insight into the clinical manifestations of altered hemodynamics in HCM.© RSNA, 2020.

Enhancing Magnetic Resonance Imaging With Computational Fluid Dynamics

The analysis of the blood flow in the great thoracic arteries does provide valuable information about the cardiac function and can diagnose the potential development of vascular diseases. Flow-sensitive four-dimensional flow cardiovascular magnetic resonance imaging (4D flow CMR) is often used to characterize patients' blood flow in the clinical environment. Nevertheless, limited spatial and temporal resolution hinders a detailed assessment of the hemodynamics. Computational fluid dynamics (CFD) could expand this information and, integrated with experimental velocity field, enable to derive the pressure maps. However, the limited resolution of the 4D flow CMR and the simplifications of CFD modeling compromise the accuracy of the computed flow parameters. In this article, a novel approach is proposed, where 4D flow CMR and CFD velocity fields are integrated synergistically to obtain an enhanced MR imaging (EMRI). The approach was first tested on a two-dimensional (2D) portion of a pipe, to understand the behavior of the parameters of the model in this novel framework, and afterwards in vivo, to apply it to the analysis of blood flow in a patient-specific human aorta. The outcomes of EMRI are assessed by comparing the computed velocities with the experimental one. The results demonstrate that EMRI preserves flow structures while correcting for experimental noise. Therefore, it can provide better insights into the hemodynamics of cardiovascular problems, overcoming the limitations of MRI and CFD, even when considering a small region of interest. EMRI confirmed its potential to provide more accurate noninvasive estimation of major cardiovascular risk predictors (e.g., flow patterns, endothelial shear stress) and become a novel diagnostic tool.

Comparison of two accelerated 4D-flow sequences for aortic flow quantification

To compare two broadly used 4D-flow- with a 2D-flow-sequence in healthy volunteers, regarding absolute flow parameters, image quality (IQ), and eddy current correction (ECC). Forty volunteers (42 ± 11.8 years, 22 females) were examined with a 3T scanner. Thoracic aortic flow was assessed using a 3D-T2w-SPACE-STIR-sequence for morphology and two accelerated 4D-flow sequences for comparison, one with k-t undersampling and one with standard GRAPPA parallel-imaging. 2D-flow was used as reference standard. The custom-made software tool Bloodline enabled flow measurements for all analyses at the same location. Quantitative flow analyses were performed with and without ECC. One reader assessed pathline IQ (IQ-PATH) and occurrence of motion artefacts (IQ-ART) on a 3-point grading scale, the higher the better. k-t GRAPPA allowed a significant mean scan time reduction of 46% (17:56 ± 5:26 min vs. 10:40 ± 3:15 min) and provided significantly fewer motion artefacts than standard GRAPPA (IQ-ART 1.57 ± 0.55 vs. 0.84 ± 0.48; p < 0.001). Neither 4D-flow sequence significantly differed in flow volume nor peak velocity results with or without ECC. Nevertheless, the correlation between both 4D-flow sequences and 2D-flow was better with ECC; the k-t GRAPPA sequence performed best (R = 0.96 vs. 0.90). k-t GRAPPA 4D-flow was not inferior to a standard GRAPPA-sequence, showed fewer artefacts, comparable IQ and was almost two-fold faster.

Free-breathing cine imaging with motion-corrected reconstruction at 3T using SPiral Acquisition with Respiratory correction and Cardiac Self-gating (SPARCS)

PURPOSE

To develop a continuous-acquisition cardiac self-gated spiral pulse sequence and a respiratory motion-compensated reconstruction strategy for free-breathing cine imaging.

METHODS

Cine data were acquired continuously on a 3T scanner for 8 seconds per slice without ECG gating or breath-holding, using a golden-angle gradient echo spiral pulse sequence. Cardiac motion information was extracted by applying principal component analysis on the gridded 8 × 8 k-space center data. Respiratory motion was corrected by rigid registration on each heartbeat. Images were reconstructed using a low-rank and sparse (L+S) technique. This strategy was evaluated in 37 healthy subjects and 8 subjects undergoing clinical cardiac MR studies. Image quality was scored (1-5 scale) in a blinded fashion by 2 experienced cardiologists. In 13 subjects with whole-heart coverage, left ventricular ejection fraction (LVEF) from SPiral Acquisition with Respiratory correction and Cardiac Self-gating (SPARCS) was compared to that from a standard ECG-gated breath-hold balanced steady-state free precession (bSSFP) cine sequence.

RESULTS

The self-gated signal was successfully extracted in all cases and demonstrated close agreement with the acquired ECG signal (mean bias, -0.22 ms). The mean image score across all subjects was 4.0 for reconstruction using the L+S model. There was good agreement between the LVEF derived from SPARCS and the gold-standard bSSFP technique.

CONCLUSION

SPARCS successfully images cardiac function without the need for ECG gating or breath-holding. With an 8-second data acquisition per slice, whole-heart cine images with clinically acceptable spatial and temporal resolution and image quality can be acquired in <90 seconds of free-breathing acquisition.

Four-dimensional Flow MRI: Principles and Cardiovascular Applications

In-plane phase-contrast (PC) imaging is now a routine component of MRI of regional blood flow in the heart and great vessels. In-plane PC MRI provides a volumetric, isotropic, time-resolved cine sequence that enables three-directional velocity encoding, a technique known as four-dimensional (4D) flow MRI. Recent advances in 4D flow MRI have shortened imaging times, while progress in big-data processing has improved dataset pre- and postprocessing, thereby increasing the feasibility of 4D flow MRI in clinical practice. Important technical issues include selection of the optimal velocity-encoding sensitivity before acquisition and preprocessing of the raw data for phase-offset corrections. Four-dimensional flow MRI provides unprecedented capabilities for comprehensive analysis of complex blood flow patterns using new visualization tools such as streamlines and velocity vectors. Retrospective multiplanar navigation enables flexible retrospective flow quantification through any plane across the volume with good accuracy. Current flow parameters include forward flow, reverse flow, regurgitation fraction, and peak velocity. Four-dimensional flow MRI also supplies advanced flow parameters of use for research, such as wall shear stress. The vigorous burgeoning of new applications indicates that 4D flow MRI is becoming an important imaging modality for cardiovascular disorders. This article reviews the main technical issues of 4D flow MRI and the different parameters provided by it and describes the main applications in cardiovascular diseases, including congenital heart disease, cardiac valvular disease, aortic disease, and pulmonary hypertension. Online supplemental material is available for this article. ^©RSNA, 2019 See discussion on this article by Ordovas .

Validation of two accelerated 4D flow MRI sequences at 3 T: a phantom study

Background

Four-dimensional (4D) flow magnetic resonance imaging (MRI) sequences with advanced parallel imaging have the potential to reduce scan time with equivalent image quality and accuracy compared with standard two-dimensional (2D) flow MRI. We compared 4D flow to standard 2D flow sequences using a constant and pulsatile flow phantom at 3 T.

Methods

Two accelerated 4D flow sequences (GRAPPA2 and k-t-GRAPPA5) were evaluated regarding the concordance of flow volumes, flow velocities, and reproducibility as well as dependency on measuring plane and velocity encoding (V_enc). The calculated flow volumes and peak velocities of the phantom were used as reference standard. Flow analysis was performed using the custom-made software “Bloodline”.

Results

No significant differences in flow volume were found between the 2D, both 4D flow MRI sequences, and the pump reference (p = 0.994) or flow velocities (p = 0.998) in continuous and pulsatile flow. An excellent correlation (R = 0.99–1.0) with a reference standard and excellent reproducibility of measurements (R = 0.99) was achieved for all sequences. A V_enc overestimated by up to two times had no impact on flow measurements. However, misaligned measuring planes led to an increasing underestimation of flow volume and mean velocity in 2D flow accuracy, while both 4D flow measurements were not affected. Scan time was significantly shorter for k-t-GRAPPA5 (1:54 ± 0:01 min, mean ± standard deviation) compared to GRAPPA2 (3:56 ± 0:02 min) (p = 0.002).

Conclusions

Both 4D flow sequences demonstrated equal agreement with 2D flow measurements, without impact of V_enc overestimation and plane misalignment. The highly accelerated k-t-GRAPPA5 sequence yielded results similar to those of GRAPPA2.

Left ventricular blood flow patterns at rest and under dobutamine stress in healthy pigs

Intracardiac blood flow patterns are affected by the morphology of cardiac structures and are set up to support the heart's pump function. Exercise affects contractility and chamber size as well as pre- and afterload. The aim of this study was to test the feasibility of four-dimensional phase contrast cardiovascular MRI under pharmacological stress and to study left ventricular blood flow under stress. 4D flow data were successfully acquired and analysed in 12 animals. During dobutamine infusion, heart rate and ejection fraction increased (82 ± 5 bpm versus 124 ± 3 bpm/46 ± 9% versus 65 ± 7%; both p < 0.05). A decrease in left ventricular end-diastolic volume (72 ± 14 mL versus 55 ± 8 mL; p < 0.05) and end-systolic volume (40 ± 15 mL versus 19 ± 6 mL; p < 0.05) but no change in stroke volume were observed. Trans-mitral diastolic inflow velocity increased under dobutamine and the trajectory of inflowing blood was directed towards the anterior septum with increased inflow angle (26 ± 5°) when compared with controls (15 ± 2°). In 5/6 animals undergoing stress diastolic vortices developed later, and in 3/6 animals vortices collapsed earlier with significantly smaller cross-sectional area during diastole. The vorticity index was not affected. Under the stress condition direct flow (% ejection within the next heart beat) increased from 43 ± 6% to 53 ± 8%. 4D MRI blood flow acquisition and analysis are feasible in pig hearts under dobutamine-induced stress. Flow patterns characterized by high blood velocity and antero-septally oriented diastolic inflow as well as decreased ventricular volumes are unfavourable conditions for diastolic vortex development under pharmacological stress, and cardiac output is increased by a rise in heart rate and directly ejected left ventricular blood volume.

On the limitations of partial Fourier acquisition in phase-contrast MRI of turbulent kinetic energy

PURPOSE

To investigate limitations of partial Fourier acquisition in phase-contrast MRI of turbulent kinetic energy (TKE).

METHODS

To assess the validity of partial Fourier reconstruction of TKE and phase images, computational fluid dynamics data of mean and turbulent velocities in a stenotic U-bend phantom was used. Partial Fourier acquisition with 75% k-space coverage was simulated and TKE data were reconstructed using zero-filling, homodyne reconstruction, and the method of projections onto convex sets (POCS). Results were compared to data from fully sampled k-space and 75% symmetric sampling. In addition, compressed sensing (CS) reconstruction was compared for a standard variable density sampling pattern and a variable density sampling pattern combined with 75% partial Fourier. For illustration purposes, in vivo examples of velocity magnitude and TKE maps of aortic flow reconstructed with the different methods are provided.

RESULTS

In accordance with theory, partial Fourier reconstruction of TKE maps from phase-contrast data results in artifacts relative to fully sampled data. It is demonstrated that neither homodyne reconstruction nor POCS can improve reconstruction of TKE data with respect to zero-filling reconstruction when compared to ground-truth (RMS error: 4.70%, 4.34%, and 2.45% for homodyne, POCS, and zero-filling reconstruction of in vivo data, respectively). CS reconstruction from data acquired with partial Fourier did not recover the resolution loss incurred by partial Fourier sampling.

CONCLUSION

Partial Fourier reconstruction of TKE maps from phase-contrast data does not yield a benefit over zero-filling reconstruction. In consequence, symmetric sampling is preferred over partial Fourier acquisition for a given number of phase-encodes in phase-contrast MRI.

Comparison of respiratory motion suppression techniques for 4D flow MRI

PURPOSE

The purpose of this work was to assess the impact of respiratory motion and to compare methods for suppression of respiratory motion artifacts in 4D Flow MRI.

METHODS

A numerical 3D aorta phantom was designed based on an aorta velocity field obtained by computational fluid mechanics. Motion-distorted 4D Flow MRI measurements were simulated and several different motion-suppression techniques were evaluated: Gating with fixed acceptance window size, gating with different window sizes in inner and outer k-space, and k-space reordering. Additionally, different spatial resolutions were simulated.

RESULTS

Respiratory motion reduced the image quality. All motion-suppression techniques improved the data quality. Flow rate errors of up to 30% without gating could be reduced to less than 2.5% with the most successful motion suppression methods. Weighted gating and gating combined with k-space reordering were advantageous compared with conventional fixed-window gating. Spatial resolutions finer than the amount of accepted motion did not lead to improved results.

CONCLUSION

Respiratory motion affects 4D Flow MRI data. Several different motion suppression techniques exist that are capable of reducing the errors associated with respiratory motion. Spatial resolutions finer than the degree of accepted respiratory motion do not result in improved data quality. Magn Reson Med 78:1877-1882, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Modeling Left Ventricular Blood Flow Using Smoothed Particle Hydrodynamics

This study aims to investigate the capability of smoothed particle hydrodynamics (SPH), a fully Lagrangian mesh-free method, to simulate the bulk blood flow dynamics in two realistic left ventricular (LV) models. Three dimensional geometries and motion of the LV, proximal left atrium and aortic root are extracted from cardiac magnetic resonance imaging and multi-slice computed tomography imaging data. SPH simulation results are analyzed and compared with those obtained using a traditional finite volume-based numerical method, and to in vivo phase contrast magnetic resonance imaging and echocardiography data, in terms of the large-scale blood flow phenomena usually clinically measured. A quantitative comparison of the velocity fields and global flow parameters between the in silico models and the in vivo data shows a reasonable agreement, given the inherent uncertainties and limitations in the modeling and imaging techniques. The results indicate the capability of SPH as a promising tool for predicting clinically relevant large-scale LV flow information.

Comparison of radial 4D Flow-MRI with perivascular ultrasound to quantify blood flow in the abdomen and introduction of a porcine model of pre-hepatic portal hypertension

OBJECTIVES

Objectives of this study were to compare radial time-resolved phase contrast magnetic resonance imaging (4D Flow-MRI) with perivascular ultrasound (pvUS) and to explore a porcine model of acute pre-hepatic portal hypertension (PHTN).

METHODS

Abdominal 4D Flow-MRI and pvUS in portal and splenic vein, hepatic and both renal arteries were performed in 13 pigs of approximately 60 kg. In six pigs, measurements were repeated after partial portal vein (PV) ligature. Inter- and intra-reader comparisons and statistical analysis including Bland-Altman (BA) comparison, paired Student's t tests and linear regression were performed.

RESULTS

PvUS and 4D Flow-MRI measurements agreed well; flow before partial PV ligature was 322 ± 30 ml/min in pvUS and 297 ± 27 ml/min in MRI (p = 0.294), and average BA difference was 25 ml/min [-322; 372]. Inter- and intra-reader results differed very little, revealed excellent correlation (R ² = 0.98 and 0.99, respectively) and resulted in BA differences of -5 ml/min [-161; 150] and -2 ml/min [-28; 25], respectively. After PV ligature, PV flow decreased from 356 ± 50 to 298 ± 61 ml/min (p = 0.02), and hepatic arterial flow increased from 277 ± 36 to 331 ± 65 ml/min (p = n.s.).

CONCLUSION

The successful in vivo comparison of radial 4D Flow-MRI to perivascular ultrasound revealed good agreement of abdominal blood flow although with considerable spread of results. A model of pre-hepatic PHTN was successfully introduced and acute responses monitored.

KEY POINTS

• Radial 4D Flow-MRI in the abdomen was successfully compared to perivascular ultrasound. • Inter- and intra-reader testing demonstrated excellent reproducibility of upper abdominal 4D Flow-MRI. • A porcine model of acute pre-hepatic portal hypertension was successfully introduced. • 4D Flow-MRI successfully monitored acute changes in a model of portal hypertension.

Utility of cardiac magnetic resonance for evaluation of mitral regurgitation prior to mitral valve surgery

Mitral regurgitation (MR) is a common cause of morbidity worldwide and an accepted indication for interventional therapies which aim to reduce or resolve adverse clinical outcomes associated with MR. Cardiac magnetic resonance (CMR) provides highly accurate means of assessing MR, including a variety of approaches that can measure MR based on quantitative flow. Additionally, CMR is widely accepted as a reference standard for cardiac chamber quantification, enabling reliable detection of subtle changes in cardiac chamber size and function so as to guide decision-making regarding timing of mitral valve directed therapies. Beyond geometric imaging, CMR enables tissue characterization of ischemia and infarction in the left ventricular (LV) myocardium as well as within the mitral valve apparatus, thus enabling identification of structural substrates for MR. This review provides an overview of established and emerging CMR approaches to measure valvular regurgitation, including relative utility of different approaches for patients with primary or secondary MR. Clinical outcomes studies are discussed with focus on data demonstrating advantages of CMR for guiding diagnosis, risk stratification, and management of patients with known or suspected MR. Comparative data is reviewed with focus on diagnostic performance of CMR in comparison to conventional assessment via echocardiography (echo). Emerging literature is reviewed concerning potential new approaches that utilize CMR tissue characterization to guide clinical decision-making in order to improve therapeutic outcomes and clinical prognosis for patients with MR.

k-t accelerated aortic 4D flow MRI in under two minutes: Feasibility and impact of resolution, k-space sampling patterns, and respiratory navigator gating on hemodynamic measurements

PURPOSE

To assess the performance of highly accelerated free-breathing aortic four-dimensional (4D) flow MRI acquired in under 2 minutes compared to conventional respiratory gated 4D flow.

METHODS

Eight k-t accelerated nongated 4D flow MRI (parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisition kernels [PEAK GRAPPA], R = 5, TRes = 67.2 ms) using four k_y -k_z Cartesian sampling patterns (linear, center-out, out-center-out, random) and two spatial resolutions (SRes1 = 3.5 × 2.3 × 2.6 mm³ , SRes2 = 4.5 × 2.3 × 2.6 mm³ ) were compared in vitro (aortic coarctation flow phantom) and in 10 healthy volunteers, to conventional 4D flow (16 mm-navigator acceptance window; R = 2; TRes = 39.2 ms; SRes = 3.2 × 2.3 × 2.4 mm³ ). The best k-t accelerated approach was further assessed in 10 patients with aortic disease.

RESULTS

The k-t accelerated in vitro aortic peak flow (Qmax), net flow (Qnet), and peak velocity (Vmax) were lower than conventional 4D flow indices by ≤4.7%, ≤ 11%, and ≤22%, respectively. In vivo k-t accelerated acquisitions were significantly shorter but showed a trend to lower image quality compared to conventional 4D flow. Hemodynamic indices for linear and out-center-out k-space samplings were in agreement with conventional 4D flow (Qmax ≤ 13%, Qnet ≤ 13%, Vmax ≤ 17%, P > 0.05).

CONCLUSION

Aortic 4D flow MRI in under 2 minutes is feasible with moderate underestimation of flow indices. Differences in k-space sampling patterns suggest an opportunity to mitigate image artifacts by an optimal trade-off between scan time, acceleration, and k-space sampling. Magn Reson Med 79:195-207, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Unravelling cardiovascular disease using four dimensional flow cardiovascular magnetic resonance

Knowledge of normal and abnormal flow patterns in the human cardiovascular system increases our understanding of normal physiology and may help unravel the complex pathophysiological mechanisms leading to cardiovascular disease. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has emerged as a suitable technique that enables visualization of in vivo blood flow patterns and quantification of parameters that could potentially be of prognostic value in the disease process. In this review, current image processing tools that are used for comprehensive visualization and quantification of blood flow and energy distribution in the heart and great vessels will be discussed. Also, imaging biomarkers extracted from 4D flow CMR will be reviewed that have been shown to distinguish between normal and abnormal flow patterns. Furthermore, current applications of 4D flow CMR in the heart and great vessels will be discussed, showing its potential as an additional diagnostic modality which could aid in disease management and timing of surgical intervention.

Left Atrial 4-Dimensional Flow Magnetic Resonance Imaging: Stasis and Velocity Mapping in Patients With Atrial Fibrillation

OBJECTIVES

Left atrial (LA) 4-dimensional flow magnetic resonance imaging (MRI) was used to derive anatomic maps of LA stasis, peak velocity, and time-to-peak (TTP) velocity in patients with atrial fibrillation (AF) and to identify relationships between LA flow with LA volume and patient characteristics.

MATERIALS AND METHODS

Four-dimensional flow MRI for the in vivo assessment of time-resolved 3-dimensional LA blood flow velocities was performed in 111 subjects: 42 patients with a history of AF and in sinus rhythm (AF-sinus), 39 patients with persistent AF (AF-afib), 10 young healthy volunteers (HVs), and 20 age-appropriate controls (CTRL). Data analysis included the 3-dimensional segmentation of the LA and the calculation of LA stasis, peak velocity, and TTP maps. Regional LA flow dynamics were quantified by calculating mean stasis, peak velocity, and TTP in the LA center region and the region adjacent to the LA wall.

RESULTS

A sensitivity analysis identified thresholds for global LA stasis (<0.1 m/s) and peak velocity (top 5% LA velocities), which detected significant differences between AF patients and controls for global LA stasis (HV, 25% ± 5%; CTRL, 29% ± 10%; AF-sinus, 41% ± 13%; AF-afib, 52% ± 17%) and peak velocity (HV, 0.43 ± 0.02 m/s; CTRL, 0.37 ± 0.04 m/s; AF-sinus, 0.33 ± 0.05 m/s; AF-afib, 0.30 ± 0.05 m/s). Regional analysis revealed significantly increased stasis at both LA center and wall for AF patients compared with age-appropriate controls (29%-84% difference, P < 0.006) and for AF-afib versus AF-sinus patients (22%-30% difference, P < 0.004). In addition, stasis close to the LA wall was significantly elevated (P < 0.001) compared with the LA center for all subject groups. Multiple regressions revealed significant (RAdj = 0.45-0.50, P < 0.001) relationships between impaired global LA flow (reduced velocity and increased stasis) with age (|β| = 0.27-0.50, P < 0.002) and LA volume (|β| = 0.26-0.50, P < 0.003).

CONCLUSIONS

Atrial 4-dimensional flow MRI detected changes in global and regional LA flow dynamics associated with AF, age, and LA volume. Longitudinal studies are needed to test the diagnostic value of LA flow metrics as potential risk factors for thromboembolic events.

Evaluation of blood flow distribution asymmetry and vascular geometry in patients with Fontan circulation using 4-D flow MRI

BACKGROUND

Asymmetrical caval to pulmonary blood flow is suspected to cause complications in patients with Fontan circulation. The aim of this study was to test the feasibility of 4-D flow MRI for characterizing the relationship between 3-D blood flow distribution and vascular geometry.

OBJECTIVE

We hypothesized that both flow distribution and geometry can be calculated with low interobserver variability and will detect a direct relationship between flow distribution and Fontan geometry.

MATERIALS AND METHODS

Four-dimensional flow MRI was acquired in 10 Fontan patients (age: 16 ± 4 years [mean ± standard deviation], range: 9-21 years). The Fontan connection was isolated by 3-D segmentation to evaluate flow distribution from the inferior vena cava (IVC) and superior vena cava (SVC) to the left and right pulmonary arteries (LPA, RPA) and to characterize geometry (cross-sectional area, caval offset, vessel angle).

RESULTS

Flow distribution results indicated SVC flow tended toward the RPA while IVC flow was more evenly distributed (SVC to RPA: 78% ± 28 [9-100], IVC to LPA: 54% ± 28 [4-98]). There was a significant relationship between pulmonary artery cross-sectional area and flow distribution (IVC to RPA: R(2)=0.50, P=0.02; SVC to LPA: R(2)=0.81, P=0.0004). Good agreement was found between observers and for flow distribution when compared to net flow values.

CONCLUSION

Four-dimensional flow MRI was able to detect relationships between flow distribution and vessel geometry. Future studies are warranted to investigate the potential of patient specific hemodynamic analysis to improve diagnostic capability.

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.