Four-dimensional Flow MRI: Principles and Cardiovascular Applications

SA-CME LEARNING OBJECTIVES

After completing this journal-based SA-CME activity, participants will be able to:

• ■ Explain the technical principles of 4D flow MRI.

• ■ Describe the current and advanced tools provided by 4D flow MRI in cardiovascular disease.

• ■ Discuss the established and potential indications of 4D flow MRI in cardiovascular disease.

Introduction

Standard two-dimensional (2D) phase-contrast (PC) MRI was introduced in the late 1980s to enable through-plane assessment of blood flow fields and velocities, particularly in the cardiovascular system (1–3). Since then, the development of in-plane PC sequences has allowed acquisition of a time-resolved cine sequence amenable to three-dimensional (3D) velocity encoding, a technique known as four-dimensional (4D) (3D plus time) flow MRI. Although 4D flow MRI was introduced in the 1990s (4), it entered the field of clinical practice only recently, after multiple improvements produced faster sequences, lighter datasets, big-data analysis software that was more user-friendly and accurate, more powerful graphics processing units (GPUs), and cloud-based postprocessing. In addition to providing the basic PC findings such as peak velocity, flow volume, and flow direction, 4D flow MRI delivers unprecedented capabilities for comprehensive blood flow assessment, offering various modalities of blood flow pathway visualization and thereby helping one understand blood flow changes and retrospectively perform accurate flow measurements (5,6).

In this article, we review the main technical issues of 4D flow MRI and the parameters provided by it and describe the main applications in cardiovascular diseases, including congenital heart disease (CHD), cardiac valvular disease, aortic disease, and pulmonary hypertension.

From 2D to 4D Flow Imaging

The development of in-plane PC sequences has resulted in acquisition of a volumetric time-resolved cine sequence enabling 3D velocity encoding, a technique known as 4D (3D plus time) flow MRI.

Two-dimensional PC MR images are acquired with single-direction (through-plane) velocity encoding during a breath hold. When flow must be measured at several sites, the acquisition must be repeated at each site, taking care to position the acquisition plane perpendicular to the long axis of the vessel of interest, a technically challenging and time-consuming process that may also be difficult for patients, particularly those with complex CHD. Whereas the standard 2D PC sequence provides an image with a single velocity and magnitude, 4D flow MRI provides sets of 3D volumes over time (4D). Each 4D volume contains one magnitude volume and three velocity volumes encoded in the three dimensions of space (Fig 1).

Figure 1.

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Technical Considerations

Before starting 4D flow MRI acquisition, several technical points must be considered to ensure that signal-to-noise ratio and velocity-to-noise ratio are optimal and to produce sufficient spatial and temporal resolution. However, the parameters should be adapted and optimized for each case. A technical checklist can be useful (Table).

Table 1 Technical Checklist for 4D Flow MRI

Note.—CNR = contrast-to-noise ratio, ECG = electrocardiography, SNR = signal-to-noise ratio, VENC = velocity encoding, VNR = velocity-to-noise ratio.

Preparation

The patient should be informed that the acquisition time for 4D flow MRI is relatively long. To reduce artifacts and ensure optimal and homogeneous data acquisition, patients should be instructed to breathe naturally and regularly for 7–10 minutes, depending on the data acquisition time. In most cases, patients should also be prepared for an intravenous injection of gadolinium-based contrast agent via a power injector.

Electrocardiographic Gating

For all anatomic regions, 4D flow MRI is synchronized to the heartbeat. Synchronization must cover the entire cardiac cycle with consistency across R-R intervals (7). Thus, reliable retrospective electrocardiographic (ECG) gating is crucial for 4D flow MRI. Using retrospective rather than prospective gating enables full coverage of the cardiac cycle. New self-gating techniques are being evaluated by some manufacturers (8).

Respiratory Gating

Because respiratory motion may affect 4D flow MRI data, thoracic and abdominal images are typically acquired using adaptive diaphragm navigator gating. When a fixed acceptance gating window is used, acquisition efficiency can vary widely. Owing to interindividual variability in breathing patterns, total acquisition time can range from 6 to 15 minutes, depending on acquired volume or spatial resolution.

Many authors have described free-breathing 4D flow MRI (8) without respiratory gating, which results in shorter acquisition times. Several motion-suppression techniques are effective in minimizing errors associated with respiratory motion. Whether respiratory gating should be used for 4D flow MRI remains debated (9).

Pulse Sequence

Four-dimensional flow MRI is based on short echo time (TE) and repetition time (TR) radiofrequency-spoiled gradient-echo sequences (TR = 5–7 msec, TE = 2–4 msec).

Contrast Medium

Four-dimensional flow MRI can be performed without contrast agent. Use of contrast agent significantly improves signal-to-noise ratio in magnitude data and noise reduction in velocity data compared to measurements without contrast agent (10). The concentration and relaxivity of the contrast agent, the acquisition technique, and the injection protocol may influence image quality (11).

We recommend using a macromolecular contrast agent for safety reasons. Several injection protocols are available. We use a triphasic protocol, starting with a bolus of contrast agent (flow rate, 3–4 mL/sec) followed by a slow infusion (0.1–0.2 mL/sec) and finally by a small saline flush injection.

Acquisition Time

For thoracic 4D flow MRI, the acquisition time is 5–8 minutes without respiratory gating and up to 10–15 minutes with respiratory gating.

Velocity Encoding Sensitivity

As with 2D PC sequences, blood velocities exceeding the velocity encoding (VENC) result in velocity aliasing, precluding flow measurements. Hence, VENC is usually set at 10% above the expected maximum velocity (7). However, high VENC increases noise and decreases velocity-to-noise ratio, especially in regions of low velocity; therefore, the best compromise should be sought.

If velocity aliasing occurs, phase unwrapping algorithms may help, although the available applications use different strategies and do not always allow optimal measurements. Different investigators and manufacturers advocate new multi- or dual-VENC strategies during acquisition to reliably incorporate both low- and high-velocity fields with optimal velocity-to-noise ratio (12).

Temporal Resolution

Temporal resolution is ideally set below 40 msec. To reduce the acquisition time, decreasing temporal resolution up to 60 msec has been advocated in specific situations, despite the negative effect on flow quantification and visualization accuracy (7).

Spatial Resolution

The setting for spatial resolution depends on the region of interest and field of view. Ideally, isotropic spatial resolution of about 1.5–3.0 mm³ is recommended for the great vessels and thoracoabdominal area in adults (7) (Table). Coronal or sagittal acquisitions provide more extensive coverage. Axial acquisition is suitable for smaller coverage (less risk of wrapping artifact).

Pre- and Postprocessing of Raw Data

Four-dimensional flow MRI generates a huge number of datasets, from which a wide variety of flow-specific images and information can be derived. Retrospective navigation allows highly accurate analysis of the datasets at any moment after acquisition (Fig 1).

Big-Data Management

Local Software.—Various types of software are available for direct local transfer and processing of big data. The advantage of this approach is that pre- and postprocessing are performed locally. The main issues are the time-consuming nature of the processing and the need for powerful computers with high-performance GPUs (graphics processing units) and considerable random-access memory.

Online Software.—The dataset is uploaded to a dedicated Web-based software application (ie, Arterys; San Francisco, Calif), considering local regulations for protection of personal health information (PHI). A secure PHI system must be used. As dataset size ranges from 2 to 4 gigaoctets, a good Internet connection is crucial. Once the dataset is uploaded, the software ensures rapid online pre- and postprocessing and fluent retrospective analysis. This method improves workflow and can be used from any device anywhere in the world.

Dataset Preprocessing

As with 2D cine PC imaging, various changes in gradient fields result in varying spatial and temporal phase offsets. Data preprocessing should include intrinsic automatic or semiautomatic phase-offset corrections for Maxwell terms, eddy currents, and phase wraps resulting in velocity aliasing (7). Optimal corrections for background phase offsets may vary across MRI systems and postprocessing applications (Fig 1).

Dataset Postprocessing and Analysis

Four-dimensional flow MRI provides unprecedented capabilities for comprehensive blood flow assessment via blood flow visualization using color-coded 3D multiplanar reformations, streamlines, and velocity vectors.

Visual Analysis

Multiple options provide new capabilities for a comprehensive and interactive approach to flow analysis via application of various visualization modalities to the volumetric, time-resolved, velocity-encoded dataset (Fig 2).

Figure 2a.

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Figure 2b.

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Figure 2c.

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3D PC MR Angiography

Three-dimensional PC MR angiography provides anatomic orientation for flow visualization and regional flow quantification. Three-dimensional maximal intensity projection (MIP), isosurface rendering, or other options can be used.

Color Coding

As with echo Doppler US, adding color coding with an adjustable scale to 3D PC MR angiography enables visualization of low and high velocities within the volume at a glance. In general, red is used for high velocities and blue for low velocities.

Streamlines

Streamlines show the path a particle would take if released into the velocity field, with the field held constant (Fig 2b) (Movie 1). Streamlines were introduced for nongated carotid artery imaging in the 1990s (2) and were subsequently improved and time resolved by Buonocore (13). Streamlines allow characterization of the blood flow pattern as follows:

Laminar Flow.—Normal flow is laminar and central (Fig 2b).

Helical Flow.—The particles rotate around an axis of flow while also exhibiting net forward motion parallel to the axis, producing helical streamlines (Fig 3a). Initial jet flow may be eccentric, with high-velocity flows near the vessel wall (Fig 3b).

Figure 3a.

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Figure 3b.

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Figure 3c.

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Vortical Flow.—Rotating or swirling motion occurs in the flow field, with streamlines or path lines tending to curl back on themselves, as in a whirlpool. Hence, the streamlines are concentric circles (Fig 3c).

Velocity Vectors

Velocity vectors indicate the speed and direction of blood velocity (Fig 2c). An arrow can be used to represent both the magnitude and the direction of velocity at a given point, calculated from the magnitudes of the x, y, and z components of velocity.

Conventional Quantitative Analysis

Four-dimensional flow MRI allows retrospective navigation, thereby providing optimal measurement of any blood flow at any level of a given vessel within the acquired volume.

When performing quantitative analysis of 4D flow MRI data, several methodological issues must be considered.

Eddy currents may result in background phase errors that can seriously affect flow measurements. An appropriate strategy for correction of phase offset errors due to eddy currents should be applied before further processing. Using a stationary fluid-filled phantom is a complex and less used strategy to establish a baseline of zero velocity. The most commonly used approach is manual thresholding to identify static tissues, eventually associated with identification of different anatomic structures within the acquisition, or manually limiting the field of interest. Some postprocessing software programs propose already advanced algorithms enabling semiautomatic thresholding.

Vendors and postprocessing software programs are working on deep learning–based approaches already used for cardiac segmentation to improve the accuracy and speed of this correction process. However, quality control is essential for each 4D flow MRI study. Applying the conservation-of-mass principle is an excellent option for checking the reliability of flow volume quantification for each study, particularly when imaging the heart and great vessels. This option may be used as the first in-line quality control step (7). Furthermore, given its volumetric coverage, 4D flow MRI offers several opportunities for controlling internal data consistency. Use of routine phantom calibration to subtract potential background phase offsets when possible has been recommended (7).

Flow Measurement

Use of 3D or 4D PC MR angiography derived from 4D flow MRI data helps with anatomic orientation and identification of cross-sectional analysis planes for flow quantification. Retrospective navigation enables highly accurate positioning for blood flow measurement, usually perpendicular to the axis of the vessel. Streamline visualization improves positioning accuracy for flow measurements by avoiding areas with turbulent flow patterns such as vortical flows (Fig 3c). Flow measurements can be retrospectively calculated in regions not previously suspected to be relevant, and this is probably one of the most important advantages of 4D flow MRI compared with multiple 2D velocity sequences performed at different specific locations. New measurement sites can be assessed retrospectively and interactively, and new strategies can be developed to refine the diagnosis (6).

Once the boundaries of a given vessel are drawn, automatic segmentation is usually performed by the software for all cardiac time frames. This segmentation process can be expected to benefit from machine learning or deep learning. As always, automatically detected flow contours must be checked over the entire cardiac cycle to avoid measurement errors due to the involvement—for some phases—of other flows within the boundaries. This type of error is particularly likely to occur at the heart and great vessels. Forward flow, reverse flow (in milliliters per beat), regurgitation fraction (in percent), and peak velocity (in meters per second) are parameters currently provided by flow measurement (Fig 1).

Advanced Features for Research

Four-dimensional flow MRI provides new advanced parameters for use in research, such as wall shear stress (WSS), kinetic energy loss, and pressure difference fields.

Wall Shear Stress

WSS refers to the stress applied tangentially to the vessel wall, that is, the tangential viscous shear forces per unit area exerted by shear in the fluid layer immediately adjacent to the wall (fluid-wall shear stress). WSS reflects the effect of flow changes on endothelial-cell and extracellular-matrix function (14,15). Four-dimensional flow MRI–derived WSS helps determine the site of greatest shear stress on the vessel wall (Fig 4). A 2D or 3D WSS map may help identify areas at risk for rupture in aneurysms (16).

Figure 4.

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Pulse-Wave Velocity

Pulse-wave velocity is the velocity of pulse-wave propagation along a vessel, usually an artery. Pulse-wave velocity is normally several times faster than the blood flow velocities within the vessel (17).

Turbulence Kinetic Energy, Viscous Energy Loss, Pressure Difference Fields

These parameters have been described as new tools for research (17), although pitfalls in their determination persist and the best measurement strategies remain unclear (18). Pressure difference maps derived from 4D flow MRI may depict alterations in spatial pressure distribution (19), although further validation of this possibility is needed.

Clinical Applications

Although cardiac 4D flow MRI is chiefly used and studied in CHD, the spectrum of indications is expanding rapidly to other fields such as cardiac valvular disease, aortic aneurysm and stenosis, pulmonary hypertension, portal hypertension, and cerebral artery aneurysm.

In the following sections, we review the main indications for 4D flow MRI. This technique was readily adopted for evaluation of aortic abnormalities and CHD. However, the spectrum of 4D flow MRI indications is expanding steadily. For indications not discussed herein, readers are referred to the literature.

Congenital Heart Disease

Although transthoracic echocardiography is the first-line cardiovascular imaging modality for adults with CHD, 4D flow MRI is increasingly used as a means of obtaining accurate values for difficult flow measurements (eg, pulmonary regurgitation). Cardiac MRI is the current reference standard for measuring both right and left ventricular volumes (20,21). The more complex the CHD, the greater the usefulness of cardiac 4D flow MRI in understanding the disease and providing guidance for planning surgery or other treatments. In addition to diagnosis and comprehensive evaluation of CHD in infants and adults, areas that are benefiting from cardiac 4D flow MRI include follow-up of patients with complex surgically treated grown-up CHD.

Shunts and Anomalous Pulmonary Venous Return.—Echocardiography is the modality of choice for visualizing atrial septal defect (ASD), ventricular septal defect, and patent ductus arteriosus (PDA). Cardiac MRI allows clarification of the amount of shunting, assessment of biventricular size and function, and most important, detection of associated anomalies such as anomalous pulmonary venous return (APVR). These data are crucial for planning interventions (22,23). The volumetric coverage of cardiac 4D flow MRI improves visualization of sinus venosus, APVR, and other shunts that are difficult to assess with transthoracic echocardiography (Movie 2).

Systemic flow (Qs) in the ascending aorta and pulmonary flow (Qp) in the pulmonary artery can be estimated retrospectively to evaluate intracardiac left-to-right shunting (significant if Qp/Qs > 1.5). If PDA, septal defect, APVR, or other abnormalities are detected subsequently, flow measurements can be reliably assessed retrospectively in optimally positioned planes (Fig 5). Interestingly, in advanced ASD, temporary or permanent shunt inversion (right to left) due to pulmonary hypertension can be demonstrated by analyzing the velocity vectors (Fig 5g).

Figure 5a.

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Figure 5b.

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Figure 5c.

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Figure 5d.

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Figure 5e.

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Figure 5f.

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Figure 5g.

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Follow-up of Patients with Fontan Circulation.—Fontan operations are performed in children with a single effective ventricle (severe hypoplastic left or right heart syndrome). The systemic and pulmonary vascular beds are surgically connected in series downstream of the single effective ventricle. This cavopulmonary connection eliminates shunting at the cost of a major increase in systemic venous pressure, which maintains flow through the lungs (24).

Cardiac 4D flow MRI has been used to assess flow shunting in post–Fontan operation patients (25). Streamlines help illustrate the spatial distribution and dynamics of blood flow from the inferior vena cava (IVC) or superior vena cava (SVC) to each pulmonary artery (Movie 3). Nonuniform mixing of blood to the left and right pulmonary arteries has been reported, with substantial interindividual differences despite similar Fontan geometry (Fig 6). This uneven distribution of hepatic-rich venous return from the lower body to the left and right lungs (via the IVC) may influence development of serious complications such as pulmonary arteriovenous malformations and fistulas (25).

Figure 6a.

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Figure 6b.

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Figure 6c.

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Valverde et al (25) found good agreement between flow shunting measurements with cardiac 4D flow MRI and 2D PC MRI in patients with systemic-to-pulmonary collateral flow. In addition, cardiac 4D flow MRI allows quantification of intra- and extraventricular kinetic energy and comparison of energy loss in patients with Fontan circulation, which may be relevant for planning surgery or an interventional procedure (26).

Repaired Tetralogy of Fallot.—Major issues in many patients with repaired tetralogy of Fallot include evaluation of free or almost free pulmonary regurgitation, pulmonary artery patency, and right ventricular size and function. Cardiac MRI with 2D PC sequences provides most of the information needed during follow-up of patients after tetralogy of Fallot repair.

In this population, cardiac 4D flow MRI has documented variations in flow characteristics (27). Flow vortex formation in the pulmonary trunk and pulmonary arteries, as well as higher right/left pulmonary artery blood flow ratios, were observed (27). Vortical flows in an aneurysmal pulmonary artery may induce offsets in pulmonary regurgitation evaluation. Streamline visualization improves the accuracy of positioning for blood flow measurement by avoiding areas of vortical flow (Fig 3c).

Tetralogy of Fallot can occur in patients with Down syndrome or CHARGE syndrome, in whom the repeated breath-holds needed for cardiac MRI are difficult to achieve. Free-breathing cardiac 4D flow MRI overcomes this difficulty and improves measurement accuracy. After transcatheter pulmonary valve repair for pulmonary regurgitation, cardiac 4D flow MRI provides optimal flow measurements with minimal susceptibility artifacts (Fig 7).

Figure 7a.

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Figure 7b.

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Follow-up after Senning Atrial Switch Surgery.—Specialist knowledge is needed to assess grown-up patients with CHD who have undergone the Mustard or Senning operation for transposition of the great arteries. In the past decade, correction by atrial switch has gained popularity, as it makes the left ventricle the systemic ventricle. Nevertheless, many grown-up patients with CHD are still referred for follow-up after atrial switch surgery (24). Patency or stenosis of the reimplanted IVC and SVC and their course to the mitral valve are difficult to assess. Cardiac 4D flow MRI readily depicts these features and provides information on blood flow variations in the Senning circulation (Fig 8).

Figure 8.

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Cardiac Valvular Disease

Cardiac 4D flow MRI allows accurate quantification of net flow volumes through all four cardiac valves (28). The regurgitation fraction can be assessed with good intra- and interobserver agreement (28).

Pulmonary and Aortic Regurgitation.—Two-dimensional cine PC sequences are widely used for MRI assessment of pulmonary regurgitation (29). Cardiac 4D flow MRI has shown good reliability and reproducibility for this indication (30) (Fig 9) (Movie 4). Cardiac 4D flow MRI helps achieve optimal positioning for blood flow measurements, avoiding the areas of vortical flow that may occur in an aneurysmal pulmonary artery (Fig 3c).

Figure 9a.

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Figure 9b.

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Similarly, reported measurement errors with 2D PC MRI can be avoided when estimating the regurgitation fraction and volume in patients with moderate to severe aortic regurgitation and sinus of Valsalva dilatation (31). In patients with significant aortic regurgitation, major vortical diastolic reverse flow imposes an eccentric systolic course on the systolic outflow jet along the outer wall of the ascending aorta (Figs 10, 11) (Movie 5). Further large studies are needed to test the accuracy of cardiac 4D flow MRI in enabling discrimination between moderate and severe aortic regurgitation.

Figure 10a.

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Figure 10b.

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Figure 10c.

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Figure 10d.

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Figure 11a.

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Figure 11b.

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Figure 11c.

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Mitral and Tricuspid Regurgitation.—Regurgitation through the atrioventricular valves can be assessed using 4D velocity acquisition (Movie 6). Four-dimensional proximal isovelocity surface area measurement may contribute to quantitate mitral regurgitation without making any geometric assumptions (32). Echocardiography is the first-line imaging investigation for evaluating mitral regurgitation, but cardiac 4D flow MRI can help assess regurgitation severity in complex cases with eccentric mitral regurgitation (33) (Fig 12).

Figure 12.

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Aortic Stenosis.—Phantom and in vivo studies in patients with aortic stenosis showed promising results with cardiac 4D flow MRI, although further validation is required (34). Evaluation of turbulence kinetic energy using cardiac 4D flow MRI may provide complementary information to that of echocardiography for discriminating between moderate and severe aortic stenosis.

Prosthetic Valve.—Cardiac 4D flow MRI allows assessment of blood flow after bioprosthetic valve surgery or transcatheter aortic valve implantation (TAVI) with minimal susceptibility artifacts (Fig 13) (Movie 7). Flow patterns and turbulence intensity downstream from a prosthetic heart valve are dependent on the specific valve design (35). Improved understanding of the hemodynamic consequences of percutaneous or open interventions may contribute to optimize treatment strategies.

Figure 13a.

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Figure 13b.

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Dilated Cardiomyopathy.—Changes in the extent and size of rotating vortices, as well as other flow alterations such as a decrease in direct flows, have been reported (36) and deserve further investigation.

Abnormalities of the Great Vessels

Early in vivo studies of cardiac 4D flow MRI evaluated blood flow path lines in the thoracic aorta and 3D quantification of thoracic aorta hemodynamics (37,38). The comprehensive evaluation of great vessel blood flow features provided by cardiac 4D flow MRI was first used by researchers, then shown to provide clinically relevant findings (6,39).

Bicuspid Aortic Valve and Aortic Aneurysm.—Patients with bicuspid aortic valve (BAV) are at risk for ascending aortic aneurysm and dissection. Although vascular tissue changes have been described, BAV also induces hemodynamic flow turbulence that may contribute to aneurysm formation and growth (39). Recent data obtained using cardiac 4D flow MRI include high-velocity eccentric flow jets associated with changes in regional WSS distribution related to the aortic alterations seen in BAV. Flow eccentricity was the feature that was most sensitive to differences in BAV phenotype. WSS or jet impingement angle and jet flow eccentricity measured with cardiac 4D flow MRI may improve risk stratification in patients with ascending aortic aneurysms.

Vortices in the sinuses of Valsalva have been studied after different valve-sparing aortic root replacement procedures. The results contribute to the ongoing debate about the role of the sinuses and the importance of preserving them in valve-sparing surgical repair of aortic root ectasia (7,8,39). In patients with BAV, the WSS increase in the ascending aorta was most pronounced in the presence of aortic stenosis and in the absence of dilatation of the ascending aorta (40).

Aortic Coarctation.—Cardiac 4D flow MRI has been found reliable for assessing aortic coarctation and evaluating collateral blood flow in untreated patients and after repair surgery (40–42) (Fig 14) (Movie 8). Various methods of estimating pressure difference maps derived from cardiac 4D flow MRI have been described for depicting alterations in spatial pressure distribution in patients with repaired and unrepaired aortic coarctation (19,43). Further validation of these methods is needed.

Figure 14a.

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Figure 14b.

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Takayasu Disease.—Four-dimensional flow MRI may supply clinically relevant information in patients with inflammatory or noninflammatory aortic disease. Coronal acquisition and enhanced 4D flow MRI enable extensive coverage in an acquisition time of 8–10 minutes. Thus, the aorta can be covered from the proximal cerebral arteries to the aortic bifurcation (Fig 15). Blood flow data and regional WSS increases can be matched to anatomic changes such as wall thickening or aortic dilatation, ulceration, or stenosis (Fig 4).

Figure 15.

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Follow-up after Intervention.—Four-dimensional flow MRI at 1.5 T or 3 T can be used for follow-up after endovascular abdominal aortic aneurysm repair. Minimal artifacts are seen with these sequences, chiefly with the new nitinol endoprostheses. Four-dimensional flow MRI was more sensitive than CT angiography for detecting endoleaks (44). More interestingly, 4D flow MRI allows identification of concomitant multiple endoleaks, enables flow measurements, and shows via flow vector direction whether multiple vessels are involved in a type II endoleak (Fig 16).

Figure 16a.

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Figure 16b.

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Figure 16c.

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Thus, 4D flow MRI allows subclassification of type II endoleaks into type IIa (biphasic flow pattern from a branch vessel) and type IIb (monophasic flow pattern with inflow and outflow branches) (44) (Movie 9), thereby improving interventional mapping for further embolization. Similarly, 4D flow MRI can provide useful information for follow-up of type B dissection after endovascular thoracic aorta repair (Fig 17) (Movie 10).

Figure 17a.

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Pulmonary Hypertension.—Alterations in pulmonary artery flow hemodynamics have been abundantly documented in patients with pulmonary hypertension. Diastolic vorticity has been described as an indicator of mild pulmonary hypertension, whereas vorticity that becomes systolic indicates severe pulmonary hypertension (45). Helicity has also been found to have strong diagnostic potential, suggesting that characterizing flow hemodynamics may become an important component of the initial and follow-up evaluations in patients with pulmonary hypertension (45).

Increasing Number of New Applications

The field of application is expanding steadily. Thus, 4D flow MRI has been studied in portal hypertension (46), cerebral aneurysm risk stratification (47), and complex arteriovenous malformations (48) (Fig 18). The development of new sequences and technological advances can be expected to shorten acquisition times (49) and make postprocessing easier and faster, thus facilitating use of 4D flow MRI in clinical practice and allowing broader investigations in larger patient cohorts to further validate the technique in various applications.

Figure 18.

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When to Choose between 2D PC and 4D Flow MRI?

Through-plane 2D PC sequences are used in clinical routine, during breath holding, mostly for quantifying aortic or pulmonary artery blood flow. When the anatomic morphology of the patient is not complex and the patient can achieve breath holds, 2D PC MRI is a suitable and fast method to assess valvular regurgitation or left-to-right shunting using the pulmonary-to-systemic blood flow ratio (Q_P/Q_S).

When placement of the acquisition plane is challenging, the number of measurements is elevated, anatomic features of the disease are complex (as in CHD), and breath holding is difficult, 4D flow MRI will be more accurate. Turbulent aortic flows due to a bicuspid aortic valve or aneurysmal ascending aorta will also be more accurately assessed with 4D flow sequences, with retrospective placement of the measurement plane avoiding the helical or vortical flow areas. Differences between 2D PC and 4D flow measurements were found—especially for the widely used ascending aorta flow—by Bollache et al (50), indicating that reference values should be established for each technique.

Conclusion

Recent advances in 4D flow MRI have expanded the potential for assessing cardiovascular disease, thereby providing unprecedented capabilities for comprehensive quantitative evaluation of cardiovascular blood flows. Technological advances are now taking these capabilities into the realm of clinical practice. Early studies suggest that 4D flow MRI can play an important complementary role in cardiovascular assessments. New areas of application are emerging at a brisk pace. Multicenter studies in larger patient cohorts are needed for further validation. Four-dimensional flow MRI may soon become the reference standard for blood flow assessment in many indications.