Comparison of 2D and 4D Flow MRI in Neonates Without General Anesthesia

Highly accelerated 4D flow MRI with respiratory compensation and cardiac view sharing: a cross-sectional study of flow in the great vessels of pediatric congenital heart disease

BACKGROUND

Conventional four-dimensional (4D) flow magnetic resonance imaging (MRI) is limited by long scan times, particularly in pediatric congenital heart disease (CHD) patients.

OBJECTIVE

This study evaluates accelerated 4D flow MRI incorporating respiratory compensation and cardiac view sharing in healthy adults and pediatric CHD patients.

MATERIALS AND METHODS

Subjects underwent 5-min free-breathing protocol with a three-dimensional (3D) radial trajectory and compressed sensing reconstruction. The 4D flow MRI reconstruction pipeline was improved by respiratory soft-gating and cardiac view sharing. Flow in major thoracic vessels was compared with two-dimensional (2D) phase contrast MRI, the reference standard.

RESULTS

Fourteen pediatric CHD patients (median age: 13 years (interquartile range (IQR): 5)) and four healthy adult volunteers (median age: 26 years (IQR: 3)) were recruited. Soft-gating improved diaphragm sharpness and reduced respiratory-induced blur (image quality scores: healthy: 46.1 soft-gated vs. 47.2 non-gated; CHD: 47.8 soft-gated vs. 48.2 non-gated). View sharing reduced undersampling artifacts and enhanced the signal-to-noise ratio (SNR, healthy: +9.9%; CHD: +3.8%). In healthy adults, correlations with 2D phase contrast MRI were strong for mean flow (R2=0.94, slope=0.94±0.12, root mean square error (RMSE)=6.4 ml/s; bias=1.1±6.4 ml/s, P=0.45) and peak flow (R2=0.9, slope=0.86±0.13, RMSE=40.9 ml/s; bias=21.3±44.7 ml/s, P=0.04). Similarly, CHD patients showed a strong correlation for mean flow (R2=0.88, slope=0.93±0.09, RMSE=8.3 ml/s) and peak flow (R2=0.97, slope=0.98±0.03, RMSE=25.9 ml/s). Internal consistency for 4D flow MRI in CHD cases showed mean percent differences of 6.1% Main pulmonary artery=Left pulmonary artery+Right pulmonary artery and 6.5% Ascending aorta=Descending aorta+Superior vena cava.

CONCLUSION

The accelerated 4D flow MRI method provides robust flow quantification and visualization in pediatric CHD patients, strongly correlating with 2D phase contrast MRI and completing scans in 5 min for clinical use.

Three-dimensional aortic arch geometry and blood flow in neonates after surgical repair for aortic coarctation

Background

Recurrent coarctation of the aorta (re-CoA) is a well-known although not fully understood complication after surgical repair, typically occurring in 10%-20% of cases within months after discharge.

Objectives

To (1) characterize geometry of the aortic arch and blood flow from pre-discharge magnetic resonance imaging (MRI) in neonates after CoA repair; and (2) compare these measures between patients that developed re-CoA within 12 months after repair and patients who did not.

Methods

Neonates needing CoA repair, without associated major congenital heart defects, were included. Transthoracic echocardiography (echo) and 4D phase-contrast MRI were performed prior to discharge after CoA repair to assess 3D arch geometry, flow velocity and flow pattern in the distal aortic arch corresponding to the area at risk for re-CoA. Arch geometry was assessed by measuring angles of the aortic arch and its branches using 3D patient-specific geometries segmented from MRI. Continuous data are presented as median and interquartile range.

Results

The median age at CoA surgery was 9 days. Four out of the included 28 patients (14%) developed re-CoA within the first 12 months after surgery. Re-CoA was associated with repair technique (lateral thoracotomy 100% vs. 33%, p = 0.02), higher postoperative isthmic flow velocity by echocardiography (1.9 [0. 9] m/s vs. 1.25 [0.5] m/s, p = 0.04) and postoperative crenel aortic arch (100% vs. 21%, p = 0.007) with a larger distance between the first and last branching points (12.6 [3.1] mm vs. 7.3 [7.0] mm; p = 0.01). A smaller angle between the ascending aorta and the brachiocephalic artery (89 [58]° vs. 122 [37]°, p = 0.05) and between the proximal aortic arch and the left carotid artery (75° vs. 97 [37]°, p = 0.04), with a more pronounced caliber change between the ascending aorta and the proximal (1.85 vs. 0.86 [0.76]; p = 0.03) and distal aortic arch (2.19 [2.42] vs. 1.01 [0.94]; p = 0.03) were observed in re-CoA patients. Patients who developed re-CoA had more left-handed helical flow in systole (p = 0.045), more right-handed helical flow in diastole (p = 0.02), and less vortical flow (p = 0.05).

Conclusion

Subtle changes in aortic arch geometry and flow pattern early after neonatal CoA repair may contribute to the risk of re-CoA.

Going with the flow: Implementing a 4D flow MRI program at a children's hospital

Four-dimensional phase contrast MRI (4D flow) has emerged as a versatile imaging technique for comprehensive visualization and both qualitative and quantitative assessment of cardiovascular blood flow. 4D flow is a three-dimensional, time-resolved acquisition that is gated to the cardiac cycle. 4D flow provides cardiovascular velocity and flow assessment across the volume of acquisition and yields a multitude of advanced hemodynamic parameters that help to assess the impact of cardiovascular disease on flow and vice versa, guiding the clinical and surgical management of patients with congenital and acquired heart disease. In the past, lengthy scan acquisition and complex post-processing workflows hindered 4D flow adoption into routine clinical practice. Decreasing image acquisition times and improvements in post-processing techniques have made 4D flow a clinically useful tool. The purpose of this communication is to facilitate more widespread adoption of 4D flow by describing its clinical utility, technical acquisition, optimization, and post-processing in pediatric cardiovascular imaging at our center.

Multi-Modal in Vitro Experiments Mimicking the Flow Through a Mitral Heart Valve Phantom

PURPOSE

Fluid-structure interaction (FSI) models are more commonly applied in medical research as computational power is increasing. However, understanding the accuracy of FSI models is crucial, especially in the context of heart valve disease in patient-specific models. Therefore, this study aimed to create a multi-modal benchmarking data set for cardiac-inspired FSI models, based on clinically important parameters, such as the pressure, velocity, and valve opening, with an in vitro phantom setup.

METHOD

An in vitro setup was developed with a 3D-printed phantom mimicking the left heart, including a deforming mitral valve. A range of pulsatile flows were created with a computer-controlled motor-and-pump setup. Catheter pressure measurements, magnetic resonance imaging (MRI), and echocardiography (Echo) imaging were used to measure pressure and velocity in the domain. Furthermore, the valve opening was quantified based on cine MRI and Echo images.

RESULT

The experimental setup, with 0.5% cycle-to-cycle variation, was successfully built and six different flow cases were investigated. Higher velocity through the mitral valve was observed for increased cardiac output. The pressure difference across the valve also followed this trend. The flow in the phantom was qualitatively assessed by the velocity profile in the ventricle and by streamlines obtained from 4D phase-contrast MRI.

CONCLUSION

A multi-modal set of data for validation of FSI models has been created, based on parameters relevant for diagnosis of heart valve disease. All data is publicly available for future development of computational heart valve models.

Image reconstruction impacts haemodynamic parameters derived from 4D flow magnetic resonance imaging with compressed sensing

Aims

4D blood flow measurements by cardiac magnetic resonance imaging (CMR) can be used to simplify blood flow assessment. Compressed sensing (CS) can provide better flow measurements than conventional parallel imaging (PI), but clinical validation is needed. This study aimed to validate stroke volume (SV) measurements by 4D-CS in healthy volunteers and patients while also investigating the influence of the CS image reconstruction parameter λ on haemodynamic parameters.

Methods and results

Healthy participants (n = 9; 20-62 years) underwent CMR with 2D, 4D-CS, and 4D-PI flow. Patients (n = 30, 17 with congenital heart defect; 2-75 years) had 4D-CS added to their clinical examination. Impact of λ was assessed by reconstructing 4D-CS data for six different λ values. In healthy volunteers, 4D-CS and 4D-PI SV differed by 0.4 ± 6.5 mL [0.6 ± 9.1%; intraclass correlation coefficient (ICC) 0.98], and 4D-CS and 2D flow by 0.9 ± 7.0 mL (0.9 ± 10.6%; ICC 0.98). In patients, 4D-CS and 2D flow differed by -1.3 ± 6.0 mL (-7.2 ± 20%; ICC 0.97). SV was not dependent on λ in patients (P = 0.75) but an increase in λ by 0.001 led to increased differences between 4D-CS and 4D-PI of -0.4% (P = 0.0021) in healthy participants. There were significant differences for ventricular kinetic energy (systole: P < 0.0001; diastole: P < 0.0001) and haemodynamic forces (systole: P < 0.0001; diastole: P < 0.0001), where error increased with increasing λ values in both healthy participants and patients.

Conclusion

4D flow CMR with CS can be used clinically to assess SV in paediatric and adult patients. Ventricular kinetic energy and haemodynamic forces are however sensitive to the change in reconstruction parameter λ, and it is therefore important to validate advanced blood flow measurements before comparing data between scanners and centres.

Assessment of 4D flow MRI for quantification of left-to-right shunt in pediatric patients with ventricular septal defect: comparison with right heart catheterization

Objectives

The percentage of shunt fraction significantly impacts the management of patients with congenital shunts, influencing strategic choices such as surgical or interventional procedures. This study compared the estimated shunt fraction (the ratio of pulmonary-to-systemic flow, Qp/Qs) for quantifying the left-to-right shunt in children with ventricular septal defect (VSD) using heart catheterization, four-dimensional (4D) flow, and two-dimensional (2D) flow magnetic resonance imaging (MRI). The goal was to establish a non-invasive and reliable measurement ratio between pulmonary and systemic blood flow in these patients.

Methods

Between July 2022 and June 2023, patients scheduled to undergo invasive right heart catheterization were included in this study. MRI was performed one hour before the catheterization procedure. The correlation of shunt fraction was assessed between all methods after calculating the Qp/Qs ratio from 2D and 4D flow MRI and catheterization.

Results

A total of 24 patients (aged 3-15 years, eight females) were ultimately included in the study. The Qp/Qs ratios obtained from 4D flow had a robust correlation (correlation coefficient r = 0.962) compared to those obtained during catheterization. Cardiac catheterization recorded the mean shunt fraction at 1.499 ± 0.396, while 4D flow measured it at 1.403 ± 0.344, with no significant difference between the two techniques. Moreover, there was a reasonable correlation (r = 0.894) between 2D flow measurements of Qp/Qs and the results obtained from catheterization, with a mean shunt fraction of 1.326 ± 0.283.

Conclusion

4D flow MRI has the potential to be a non-invasive method for accurately measuring the left-to-right shunt in children with VSD.

Using Gaussian process for velocity reconstruction after coronary stenosis applicable in positron emission particle tracking: An in-silico study

Accurate velocity reconstruction is essential for assessing coronary artery disease. We propose a Gaussian process method to reconstruct the velocity profile using the sparse data of the positron emission particle tracking (PEPT) in a biological environment, which allows the measurement of tracer particle velocity to infer fluid velocity fields. We investigated the influence of tracer particle quantity and detection time interval on flow reconstruction accuracy. Three models were used to represent different levels of stenosis and anatomical complexity: a narrowed straight tube, an idealized coronary bifurcation with stenosis, and patient-specific coronary arteries with a stenotic left circumflex artery. Computational fluid dynamics (CFD), particle tracking, and the Gaussian process of kriging were employed to simulate and reconstruct the pulsatile flow field. The study examined the error and uncertainty in velocity profile reconstruction after stenosis by comparing particle-derived flow velocity with the CFD solution. Using 600 particles (15 batches of 40 particles) released in the main coronary artery, the time-averaged error in velocity reconstruction ranged from 13.4% (no occlusion) to 161% (70% occlusion) in patient-specific anatomy. The error in maximum cross-sectional velocity at peak flow was consistently below 10% in all cases. PEPT and kriging tended to overestimate area-averaged velocity in higher occlusion cases but accurately predicted maximum cross-sectional velocity, particularly at peak flow. Kriging was shown to be useful to estimate the maximum velocity after the stenosis in the absence of negative near-wall velocity.

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.