Aortic diseases, particularly aneurysms and dissections, present clinical challenges owing to their potential for catastrophic complications. Accurate assessment and timely intervention are critical for managing these conditions [1]. Central to management is strict blood pressure control, with surgical referral for patients at a high risk of life-threatening complications. Thoracic aortic surgery is complex and carries devastating risks, including cerebral infarction and permanent spinal cord ischemia. The mortality rate of thoracic aortic rupture is extremely high, with only 41% of patients with ruptured thoracic aneurysms surviving to hospital arrival [2]. Thus, deciding the timing and necessity of surgery requires balancing surgical risks with the risk of aortic rupture.

Generally, the timing of prophylactic intervention for thoracic aortic disease is based on the aortic size criteria [3], which are derived from natural history studies of thoracic aortic disease [4,5]. However, under certain conditions, prophylactic intervention may be beneficial before the aorta reaches a specific threshold. For example, dissection can occur in aortas with diameters smaller than the recommended threshold for surgery, a phenomenon known as the aortic size paradox [6]. Patients with acute aortic dissection and a nondilated aorta may experience rapid expansion within a few weeks [7]. Consequently, there is an ongoing debate on the feasibility of predicting individual patient risk and improving outcomes through timely endovascular or surgical interventions [8,9].

Traditionally, imaging modalities such as computed tomography angiography (CTA) and echocardiography have been used for anatomical evaluation. Recent studies have highlighted that high-risk features on CTA, such as aortic diameter, false lumen size, and the number of intimal tears, are associated with long-term adverse events in thoracic aortic dissection [10,11]. However, understanding how these anatomical factors influence aortic blood flow and contribute to disease progression remains challenging. Although a recent study using two-dimensional (2D) velocity-encoded phase-contrast magnetic resonance imaging (MRI) identified high systolic antegrade flow in the false lumen with significant diastolic retrograde flow as a high-risk factor for complications, it did not establish a clear link between these flow patterns and specific anatomical factors [12]. Moreover, there is a limitation in that the blood flow observed in a single cross-section of the aorta may not represent the overall disease state.

Recent advancements in time-resolved three-dimensional (3D) phase-contrast (4D flow) MRI have provided a powerful tool for noninvasive and comprehensive analysis of aortic hemodynamics, enabling better understanding and management of aortic diseases [13]. 4D flow MRI has shown promise in identifying abnormal flow patterns associated with disease progression, thereby facilitating patient-specific risk assessments and improving clinical decision-making. While it utilizes conventional MRI machines, 4D flow MRI requires specialized software for image acquisition and post-processing, such as 4D-flow tool of CVI42 (Circle Cardiovascular Imaging Inc.) or iT flow^TM (Cardio Flow Design Inc.). Although some advanced centers have integrated 4D flow MRI into routine practice for complex cardiovascular assessments, it is not yet widely available in emergency settings, where CTA remains the preferred method for rapid aortic assessment owing to its availability and speed. Expanding the use of 4D flow MRI in acute settings will require advancements in acquisition speed and wider access to postprocessing techniques (Table 1). Moreover, to use 4D flow MRI, it is essential to understand the parameters that can be determined using this imaging technique. These various parameters will be discussed in depth [14].

Table 1 . Comparison of 4D flow MRI, traditional aorta MRI, and aorta CTA.

	4D flow MRI	Traditional aorta MRI	Aorta CTA
Information provided	Comprehensive - detailed hemodynamic information such as flow patterns, wall shear stress, and vortex	Provides anatomical images without hemodynamic data	Excellent anatomical detail, limited functional data
Radiation exposure	None - MRI uses magnetic fields	None - MRI uses magnetic fields	Yes
Need for contrast agents	Optional - can be performed with or without contrast, although contrast may improve signal-to-noise ratio	Optional - can be performed with or without contrast, although contrast may improve signal-to-noise ratio	Required - iodinated contrast agents are standard
Temporal resolution	High - allows for time-resolved imaging capturing flow dynamics over the cardiac cycle	Variable - depends on the protocol, generally lower than 4D flow MRI	High - particularly in gated protocols, but less detailed functional data
Spatial resolution	High - provides detailed spatial resolution, particularly in larger vessels	High - excellent for structural imaging	Very high - superior for detailed anatomical structures
Functional information	Extensive - direct measurement of flow velocity, wall shear stress, and energy loss	Limited - primarily structural, some functional information with specific sequences (e.g., VENC)	Limited - primarily structural, functional information mainly through indirect means
Availability	Limited - available in specialized centers with advanced MRI capabilities	Somewhat available - standard in most tertiary care centers	Widely available - standard in most hospitals
Artifacts	Potential Issues - susceptible to artifacts from metallic implants (e.g., stents)	Moderate - can be affected by patient motion, metallic implants	Low - with advancements in CT technology, artifact issues are reduced
Utility in emergency settings	Low - limited by the absence of specialized technician and long scan time	Moderate - can be used in emergency settings with appropriate protocols	High - quick imaging makes it suitable for emergency situations
Patient comfort	Moderate - long scan times can be uncomfortable for some patients	High - shorter scan times and generally better tolerated	High - quick scan, generally well tolerated
Need for skilled technician and radiologist	High - requires specialized training for analysis of hemodynamic parameters	Moderate - standard MRI interpretation skills needed	Moderate - standard CTA interpretation skills needed
Examination time	Long - typically 20-30 min, can be reduced with advanced techniques	Moderate - typically 15-30 min	Short - typically 5-10 min
Cost	High - due to advanced technology and longer scan times	Moderate - standard MRI costs	Moderate - typically lower than 4D flow MRI but higher than standard X-rays

4D, four-dimensional; MRI, magnetic resonance imaging; CTA, computed tomography angiography; VENC, velocity encoding..

This review briefly introduces the parameters addressed in several studies and their relevance to aortic valve disease, aneurysms, and dissections.

The 4D flow MRI protocol begins by obtaining preliminary anatomical information regarding the aorta (Fig. 1). This is achieved by performing T2-weighted imaging or using a modified Dixon (mDIXON) sequence to capture a general overview of the aortic anatomy. Alternatively, a SURVEY sequence (echo time [TE] 1.3 ms; repetition time [TR] 2.5 ms; field of view [FOV] 340 mm; flip angle 50°, matrix size 256×256) for the thorax and abdomen can be used to acquire these preliminary data.

Figure 1. This showed the aorta 4D flow MRI protocol and post-processing. (A) An example of a protocol that involves confirming the anatomy of the aorta, obtaining contrast-enhanced information using time-resolved MR angiography, and then acquiring VENC and 4D flow data. The protocol can be adjusted based on the specific circumstances of each hospital and the information to be obtained, and it may be performed without contrast enhancement. (B) Post-processing for 2D VENC and 4D flow analysis. 4D, four-dimensional; MRI, magnetic resonance imaging; MRA, magnetic resonance angiography; T2 WI, T2 weighted image; mDIXON, modified Dixon; VENC, velocity encoding.

The next step involved time-resolved MR angiography (MRA) with the following parameters: TE 1.6 ms; TR 3.9 ms; FOV 500 mm; flip angle 25°; and matrix size 512×512. The images acquired from time-resolved MRA are postprocessed into maximal intensity projection images with a slice thickness of approximately 5 mm, providing a brief overview of the dynamic enhancement throughout the aorta. These images can then be reconstructed into sagittal and posteroanterior views and uploaded to the picture archiving and communication system. A 2D velocity encoding (VENC) sequence can be acquired to obtain velocity information and compare its accuracy with that of 4D flow data by selecting two or three representative sites within the lesion area. The 2D VENC sequence is performed with the following settings: TE 2.7-3.1 ms, TR 4.2-4.8 ms, FOV 240 mm, flip angle 15°, matrix size 320×320.

Finally, the 4D flow MRI sequence is executed with the following parameters: TE 2.5 ms, TR 4.4 ms, FOV 320-480 mm, flip angle 10°, and matrix size 128×128 or 192×192. The FOV and matrix size may vary depending on the imaging range. Since signal loss may occur in the peripheral portions of the acquired images due to limited k-space information, it is recommended to separate the thorax and abdomen and acquire two sets of 4D flow data when the lesion is extensive and cannot be captured within a single range.

Postprocessing begins by analyzing the acquired 2D VENC data using dedicated software to generate flow curves and extract information on velocity and flow direction. The region of interest (ROI) is drawn on the phase image using the magnitude image obtained from the VENC data. After ensuring that the ROI is accurately depicted across multiple-phase images, the results are extracted.

To analyze the 4D flow data, the aortic margins can be delineated on the 3D mDIXON sequence to help define the volume of interest for analysis. Alternatively, in certain software packages, the aortic margins can be directly drawn on the 3D reconstruction of 4D flow MRI data. Various 4D flow MRI parameters are calculated using specialized software. When specific areas of interest are identified, the values for these regions are extracted for further evaluation. This comprehensive post-processing approach ensures accurate assessment of hemodynamic parameters within the aorta.

1) Flow velocity and flow rate

Flow velocity and flow rate are fundamental parameters measured by 4D flow MRI, representing the speed, direction, and volume of blood flow through the aorta (Table 2). Alterations in these parameters can indicate pathological conditions such as aneurysms and dissections. Physiological blood flow in the aorta includes laminar flow, with peak velocities during systole. Conversely, abnormal flow patterns, such as turbulence and vortex formation, often occur in diseased states.

Table 2 . Hemodynamic parameters in 4D flow MRI.

4D, four-dimensional; MRI, magnetic resonance imaging; OSI, oscillatory shear index; RRT, relative residence time; TKE, turbulent kinetic energy; VEL, viscous energy loss; PWV, pulse wave velocity; SBP, systolic blood pressure; DBP, diastolic blood pressure; BAV, bicuspid aortic valve; MFS, Marfan syndrome; TAWSS, time-averaged wall shear stress; TAVR, transcatheter aortic valve replacement; WSS, wall shear stress..

A study investigating patients with bicuspid aortic valves (BAV) found that abnormal helical flow patterns were prevalent in the ascending aorta, particularly during peak systole [15]. Abnormal flow dynamics in patients with BAV can contribute to the development of ascending aortic aneurysms by inducing asymmetric wall stress and vessel wall degeneration [16].

Another important parameter associated with the flow velocity is the Reynolds number, which helps predict fluid flow patterns under different conditions by determining whether the flow is laminar or turbulent. A high Reynolds number typically indicates turbulent flow, often observed in pathological conditions such as stenosis or aneurysms. Because 4D flow MRI provides comprehensive velocity data within the volume of interest, it is particularly useful for quantifying flow rate distributions in complex vascular structures and estimating the peak velocity within specific regions.

2) Flow displacement

Flow displacement measures the deviation of the flow from the aortic centerline, which often occurs in aneurysmal and dissected aortas. Flow displacement is used to quantify outflow asymmetry, indicating deviation from a symmetric flow profile (Fig. 2). Sigovan et al. [17] demonstrated that flow displacement is one of the most reliable quantitative parameters for assessing eccentric systolic flow in the ascending aorta, particularly when compared to other measures. Other studies have shown that flow displacement is sensitive in detecting altered systolic outflow patterns in patients with various types and severities of aortic valve disease [18,19].

Figure 2. This illustrated an example of flow displacement at the ascending aorta in patients with severe aortic stenosis. (A) Peak systolic streamlines and flow velocity. (B) A cross-section perpendicular to the centerline of the ascending aorta shows eccentric flow. The asymmetric flow profile results in an off-center location of the areas of high velocities at peak systolic phase.

Flow displacement is a simplified parameter that can be used in 2D or 3D imaging environments as an alternative to the wall shear stress (WSS), which is more complex to calculate. It effectively captures the asymmetry in the flow associated with abnormal hemodynamic stresses, which are known to contribute to conditions such as aneurysm growth or dissection expansion [20].

3) Wall shear stress (WSS)

WSS is the tangential force per unit area exerted by blood flow on the vessel wall [14]. This force results from the frictional drag of blood moving along the endothelium and is caused by the tangential component of the force vector associated with the blood flow. The magnitude of WSS is smaller than that of the perpendicular force component linked to blood pressure.

The standard unit for measuring WSS is Pascal (Pa), equivalent to 10 dynes/cm². Abnormal WSS has been linked to various vascular pathologies, including aortic aneurysms and dissections. Elevated WSS levels play a role in modulating the behavior of endothelial cells lining the aortic wall, which can contribute to vascular pathologies. Recent studies have indicated that low and/or oscillatory WSS is a key factor in the initiation of atherosclerosis, as plaque formation tends to occur in regions of low shear stress where atherosclerosis preferentially develops [21,22].

Additionally, regions with a low WSS are prone to thrombus formation, further complicating the pathogenesis of vascular diseases [23]. In patients with aortic aneurysms, 4D flow MRI studies have shown alterations in WSS that correlate with regions of vascular remodeling and disease progression [16].

4) Oscillatory shear index (OSI)

The OSI reflects directional changes in the WSS over the cardiac cycle, indicating regions of disturbed flow. OSI values ranged from 0, indicating consistent unidirectional flow with no oscillations in the WSS vector, to 0.5, signifying frequent directional changes and significant oscillatory patterns. High OSI has been implicated in the development of atherosclerotic plaques and aneurysm formation [24]. The OSI is calculated by integrating temporal changes in the WSS vectors. Regions with high OSI experience fluctuations in shear forces, contributing to vascular inflammation and degeneration [25].

5) Relative residence time (RRT)

The RRT measures the duration that blood elements spend in a given region, serving as an important indicator of areas prone to thrombosis and plaque formation. Low WSS, high OSI, and elevated RRT have been correlated with increased uptake of inflammatory markers in the endothelial wall, as prolonged blood residence time near the vessel wall promotes the uptake of inflammatory cells and biomarkers. This prolonged residence time can contribute to aortic wall degeneration, leading to aneurysm growth and rupture.

In the context of abdominal aortic aneurysms, regions with high RRT are more prone to thrombus formation due to blood flow stasis and recirculation zones within the aneurysm [26]. In hypertensive rats, increased RRT was observed on the inner wall of the aortic arch, aligned with areas of significant elastin breakdown within the tunica media near this region [27].

6) Vortex

A vortex is defined as a fluid region in which streamlines tend to curl back, indicating rotational motion. Vorticity is a vector that quantifies the tendency of a fluid particle to rotate at a specific point. Vortical flow patterns are often observed in aneurysms and contribute to abnormal hemodynamics. Both physiological and pathological flows in the aorta are associated with rotational flow patterns. Vortex formation can be a sign of a disturbed flow associated with potential aneurysm growth [28]. Although vortices can be observed in healthy aortas, particularly in regions such as the aortic root, their presence in the ascending aorta is often indicative of a pathological condition.

7) Energy loss

The mean kinetic energy is a measure of the average energy possessed by particles in a fluid owing to their motion, averaged over a specific period. This is calculated using the phase-averaged velocity of the fluid flow. Turbulent kinetic energy (TKE) is defined as the kinetic energy per unit mass associated with eddies in a turbulence, representing energy loss due to turbulence. Elevated TKE is a marker of pathological flow conditions, such as those found in aortic aneurysms and dissections [29]. Healthy aortas exhibit higher mean kinetic energy and lower TKE, whereas aneurysmal aortas display elevated TKE and pressure loss. High TKE levels indicate an increased energy loss and inefficient blood flow, which can exacerbate vascular wall stress and contribute to disease progression [30].

8) Pulse wave velocity (PWV)

PWV is a measure of arterial stiffness and is computed from 4D flow MRI data by analyzing the transit time of blood flow pulses along the aorta. It is determined by the speed at which pressure waves move through the aorta and is influenced by the elasticity and structural integrity of the vessel.

In a previous study that explored the biomechanical properties of the ascending and descending aortas, with a focus on aortic distensibility and PWV in different patient groups, BAV, Marfan syndrome (MFS), and tricuspid aortic valve with aortic dilation were evaluated [31]. This study demonstrated that patients with nondilated BAV have aortic stiffness similar to that of healthy controls, whereas patients with MFS exhibit stiffer aortas. These findings highlight the importance of PWV as a robust marker of aortic stiffness and suggests its potential use in monitoring aortic aneurysm progression. The different biomechanical properties of patients with BAV and those with MFS suggest that tailored clinical management approaches are necessary.

9) Aortic distensibility

Aortic distensibility measures the ability of the aorta to expand and contract with pulsatile blood flow and is calculated as the relative change in the cross-sectional diameter (or area) for a given pressure step at the target vessel. Reduced distensibility is associated with aortic stiffness and disease progression and can increase the risk of adverse events in patients with aortic aneurysms. One study suggested that decreased aortic distensibility is an early marker of aortic disease in patients with MFS, preceding visible aortic dilation [32]. Patients with MFS had significantly lower aortic distensibility and higher PWV across different aortic segments, indicative of stiffer aortas.

10) Viscous energy loss (VEL)

VEL quantifies the inefficiency of blood flow, representing the amount of energy absorbed by the aorta. It reflects the overall energy dissipated owing to fluid motion and other factors, indicating regions of abnormal hemodynamic stress. VEL increases in pathological conditions such as aortic aneurysms.

Studies have shown that VEL is elevated in the dilated ascending aorta and in patients with aortic stenosis compared with healthy controls [33]. In cases with a BAV, VEL is higher than in age-matched healthy controls [34]. Peak systolic VEL in the ascending aorta is associated with the energy loss index and discrimination of aortic stenosis-related events [35]. These findings indicate that VEL can serve as a marker for evaluating the severity and prognosis of aortic aneurysms.

11) Stasis

Stasis indicates the regions of slow or stagnant blood flow. It is quantified as the percentage of cardiac time frames during which the flow velocity falls below a certain threshold (typically 0.1 m/s), indicating regions where blood flow is minimal or stagnant. Patients with BAV demonstrated significantly lower stasis (P<0.01) and higher reverse flow (P<0.01) throughout the aorta than healthy controls [36]. Abnormal hemodynamics, including reduced stasis, are associated with aortic dilation and the need for surgical intervention in patients with BAV.

A study analyzing the flow dynamics in the aortic sinus using particle image velocimetry after transcatheter aortic valve replacement (TAVR) utilized stasis as the key parameter [37]. However, accurate measurement of flow near the valve with 4D flow MRI is hampered by metallic artifacts from the valve prosthesis and limitations in spatial resolution. Despite these challenges, 4D flow MRI has been employed to observe changes in ascending aortic flow before and after TAVR [38].

1) Valve-related aortic disease

Valve-related aortic diseases, particularly aortic stenosis and BAV, have been extensively studied using 4D flow MRI. Traditional flow assessment parameters, such as the transvalvular gradient, valve area, and peak velocity, provide valuable insights, but have limitations in accuracy due to factors such as pressure recovery and low ejection fractions [39,40].

4D flow MRI provides a complementary approach to these traditional measurements by enabling direct assessment of energy loss via viscous dissipation and TKE [33]. VEL increases with the abnormal flow characteristics observed in aortic valve disease, whereas TKE, which accounts for energy dissipation into heat due to turbulence, provides a direct measurement of the irreversible pressure loss [41]. Patients with aortic stenosis exhibited markedly higher TKE levels than healthy individuals, with a robust correlation between the peak total TKE and the pressure loss index (r²=0.91, P<0.001). These findings suggest that the direct measurement of TKE using 4D flow MRI may be a noninvasive method for estimating irreversible pressure loss in patients with aortic stenosis.

BAV-related research using 4D flow MRI has shown that abnormal hemodynamics contribute to asymmetric aortic dilation, highlighting on hemodynamic markers predictive of disease progression [42]. Elevated WSS at the aortic convexity, linked to increased near-wall velocity gradients, has been implicated in the asymmetric dilation observed with BAV [43,44]. Flow displacement, which measures the deviation of peak systolic flow from the vessel centerline, is another promising marker. It demonstrates a clear differentiation between BAV phenotypes and correlates with aortic growth [17].

2) Aortic aneurysm

4D flow MRI has been utilized to study aortic aneurysms, revealing abnormal flow patterns, elevated WSS, and increased TKE. These hemodynamic abnormalities correlate with aneurysm size and growth rate, providing valuable information for risk stratification and management. Hemodynamic parameters, such as flow patterns, are crucial for understanding aneurysm development and evaluating rupture risk (Fig. 3).

Figure 3. This showed the four-dimensional (4D) flow magnetic resonance imaging of a 69-year-old male with a saccular infrarenal abdominal aorta. (A) The image shows the streamline of flow velocity and wall shear stress at peak systole, indicating decreased flow velocity and wall shear stress in the infrarenal abdominal aorta (arrows). (B) In the cross-sectional magnitude image of the 2D velocity encoding (VENC) at the mid portion of the aneurysm, a partial mural thrombus (asterisk) is visible. (C) Flow analysis of the 2D VENC reveals low-velocity blood flow with minimal regurgitation fraction (1.39%). The aneurysm in this patient has remained in a stable state, with a size change of approximately 1.5 mm over the past 2 years.

4D flow MRI allows a detailed analysis of these parameters, revealing that patients with ascending aortic dilatation exhibit altered flow patterns, including increased helical and vortex flows and decreased WSS, which are associated with higher peak velocities [24]. Dilation of the ascending aorta distorts normal flow patterns, resulting in retrograde and helical flow [45].

Furthermore, saccular aneurysms with high sac depth/neck width ratios exhibit low WSS, which may explain their poor outcomes [46]. Peak WSS decreased as the sac depth/neck width ratio increased, especially in saccular aneurysms with a sac depth/neck width ratio >0.8, indicating different flow dynamics compared to fusiform aneurysms. Low WSS within aneurysms contributes to the weakening of vessel walls and promotion of growth and may create a vicious cycle that increases the risk of rupture in saccular aneurysms.

3) Aortic dissection

In aortic dissections, 4D flow MRI allows a detailed visualization of flow dynamics within the true and false lumens, aiding in the identification of tear locations and thrombosis within the false lumen [47]. This capability is particularly beneficial in the presurgical planning and postoperative follow-up of patients undergoing interventions for aortic dissection. If malperfusion is caused by aortic dissection, endovascular treatment can achieve high technical success and contribute to improved mid-term outcomes [48]. The flow patterns in aortic dissections differ according to the extent of disease, vessel dilation, and post-therapeutic anatomy [30]. A slow helical flow, following a rapid entry jet into the false lumen of chronic aortic dissection, is associated with disease progression [49] (Fig. 4). The false lumen ejection fraction, defined as the ratio of the retrograde flow rate at the dominant entry tear during diastole to the antegrade systolic flow rate, is an independent predictor of aortic growth [50].

Figure 4. A 58-year-old male presented with a chronic type III aortic dissection during follow-up. (A) A computed tomography (CT) scan from 2017 shows a dissection in the descending thoracic aorta (DTA) with a proximal DTA diameter of 40.5 mm. (B) A CT scan from 2023 reveals a general increase in the DTA diameter, which is 48 mm at the proximal DTA. The axial image of the proximal DTA (box) clearly reveals the entry tear. (C) Four-dimensional flow MRI of the aorta performed around the same time in 2023 depicts streamlines of flow velocity during the end-systolic phase, indicating flow entering the false lumen through the entry tear (arrow). (D) During the same phase, wall shear stress is increased on the superior wall of the proximal DTA.

In a study involving 19 patients with type B aortic dissection, 4D flow MRI was used to detect and evaluate fenestration in the aortic dissection flap [51]. 4D flow MRI identified more fenestrations than standard MRI/MRA and had detection rates similar to those of CTA. Importantly, 4D flow MRI provided additional hemodynamic information, demonstrating that most fenestrations exhibited biphasic flow, with blood entering and exiting the false lumen during systole and diastole, respectively. This detailed flow analysis is crucial for understanding the dynamics of dissection and planning endovascular repair.

Despite its numerous advantages, 4D flow MRI has several limitations that hinder its widespread clinical application. One major drawback is the lengthy image acquisition time, which can extend beyond 10 minutes when electrocardiography and respiratory gating are used to attain optimal image quality. Although recent advancements in acceleration techniques have reduced the scan time to a few minutes, achieving high temporal and spatial resolutions still requires careful optimization of the scan parameters.

Another limitation is the potential need for gadolinium-based contrast agents to enhance the signal-to-noise ratio and improve the contrast between blood flow and surrounding tissues. This requirement poses a risk to patients with renal impairment, who may be susceptible to nephrogenic systemic fibrosis. Non-contrast 4D flow MRI techniques have been developed by leveraging the inherent T1-weighted contrast of blood; however, these methods require precise adjustment of imaging parameters, such as the flip angle, to maintain image quality.

Furthermore, the accurate measurement of parameters such as WSS can be problematic because of aortic pulsation during the cardiac cycle. Many studies have only considered peak WSS with time-averaged or time-maximum intensity projections, which might not accurately reflect diastolic phase WSS or OSI across the cardiac cycle. Improved postprocessing software for dynamic vessel segmentation throughout the cardiac cycle is necessary to enhance the reliability of these measurements.

In cases where a stent graft has been placed for an aneurysm, internal flow cannot be observed owing to metallic artifacts. Moreover, particularly when the site of leaks, such as endotension around the graft, is unclear, the assistance of 4D flow MRI has not yet been determined [52].

The processing of 4D flow MRI data remains labor-intensive and time-consuming, often requiring manual segmentation and analysis, which introduces user variability and affects workflow efficiency. Automation and advanced machine learning algorithms are being explored to streamline these processes; however, widespread implementation remains a future goal.

Finally, while 4D flow MRI offers the potential to reveal unique blood flow dynamics that cannot be detected by CTA, its added value over CTA has not yet been fully demonstrated. The high cost of MRI along with the need for specialized equipment, software, and trained radiologists has limited its widespread clinical use. To better determine its role, studies focusing on large prospective cohorts and comparing outcomes using CTA alone versus CTA combined with 4D flow MRI are particularly valuable.

In clinical practice, the integration of 4D flow MRI offers a promising avenue for the assessment and management of aortic diseases. By providing comprehensive hemodynamic information, 4D flow MRI enhances the understanding of disease mechanisms, facilitates risk stratification, and provides information regarding therapeutic strategies. Future studies should focus on validating the predictive value of 4D flow MRI parameters in large prospective cohorts to establish their role in routine clinical care.

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (No. RS-2023-00218630 and No. 2022R1A5A1022977).

The author has nothing to disclose.

Concept and design: HJK, HH. Analysis and interpretation: HJK, HH, GHL. Data collection: HJK, JEL, SP, KP, JWK, DHY. Writing the article: HJK. Critical revision of the article: all authors. Final approval of the article: all authors. Statistical analysis: none. Obtained funding: HJK, DHY. Overall responsibility: HJK.