Clinical Intra-Cardiac 4D Flow CMR: Acquisition, Analysis, and Clinical Applications
Four-dimensional flow cardiovascular magnetic resonance imaging (4D flow CMR) has shown great promise as a tool for assessing flow patterns. It can encode velocity in all 3 directions, representing a significant step up in flow assessment from the 1-directional non-invasive imaging modalities typically used in clinical practice.
While 4D flow CMR is increasingly used in clinical research to further understand the relationship between cardiovascular disease and flow, analysis of its data can be complex. At present, this represents an obstacle to widespread use in clinical practice.
Researchers from Amsterdam UMC and Leiden University in the Netherlands produced an article that summarized the acquisition and analysis processes of 4D flow CMR. The study also considered the future directions of the technique from a clinical perspective.
Increasing data in intra-cardiac 4D flow CMR applications
Reliable blood flow analysis is important for accurate interpretation and management of cardiovascular diseases, but many of the non-invasive modalities used in clinical practice are restricted to 1 direction. As flow is multi-dimensional and -directional, this prohibits a critical assessment of blood flow in the heart and its two main arteries.
The review highlighted how 4D flow CMR is quickly developing as a novel tool for flow visualization and quantification, with recent technical improvements enhancing dynamic cardiac chamber segmentation over the cardiac cycle. This has resulted in the influx of a large amount of data relating to intra-cardiac 4D flow CMR applications. New 4D flow data-derived hemodynamic parameters are providing insights into cardiac energetics, and there are now a variety of tools for 4D flow data analysis.
In light of the increase of data, the review discussed 4D flow acquisition and analysis processes, before covering clinical applications of 4D flow CMR regarding cardiac diseases. The review then considered the 4D flow imaging potential of other modalities and looked at how 4D flow CMR may develop over the coming years.
Acceleration of acquisition duration ‘subject to caution’
The review described 4D flow image acquisition, which can take up to 15 minutes, as “time-consuming”. It noted acceleration approaches such as parallel imaging, compressed sensing, and non-Cartesian trajectory techniques, but highlighted that accelerating image acquisition risks impacting essential parameters such as signal-to-noise ratio (SNR), spatial and temporal resolution, and demands caution.
High-strength magnetic resonance imaging (MRI) systems may be needed to compensate for SNR loss in accelerated acquisition techniques, and this may affect the quantification of parameters such as novel hemodynamic parameters. It was also noted that combining techniques may reduce the scanning time, but impact the efficiency of each other.
Sources of errors
The review discussed the various sources of error that can impact the quality and accuracy of 4D flow data and require correction prior to flow visualization and quantification.
Correction methods can vary according to the MRI system, with some vendors providing correction for concomitant gradient field effects during standard reconstruction, and additional correction sometimes being required.
Clinical relevance of 4D flow CMR in intra-cardiac diseases
Here is a selection of some of the clinical applications of 4D flow CMR discussed in the following cardiac diseases:
By enabling detailed evaluation of flow in the atria, 4D flow CMR was used to report vortical flow in the left atrium (LA) during systole and diastole in healthy individuals. The ability of 4D flow CMR to provide global quantification of flow metrics within the heart was also highlighted. In one study, patients with persistent atrial fibrillation (AF) demonstrated lower mean blood flow velocity within the LA compared to patients in sinus rhythm, while other research uncovered a reduction of blood flow velocities in patients in sinus rhythm with a history of AF.
Valvular heart diseases
Significantly, 4D flow CMR allows the accurate quantification of valvular regurgitation, which the review underlined as “of great importance for the clinical management”. This is done by modifying analysis planes according to the location and angulation of moving valvular structures.
Retrospective valve tracking – which is superior with 4D flow CMR than it is with conventional 2D 1-directional velocity mapping in patients with mitral and tricuspid valve regurgitations – allows reproducible and robust trans-valvular flow assessment at each of the four heart valves. It was noted that an automated valve tracking method significantly cuts the analysis time required for retrospective valve tracking, compared to manual placement and tracking.
Congenital heart diseases
Congenital heart diseases (CHD) often result in changes to cardiovascular flow dynamics. Patients in this group also require accurate quantification of flow parameters for the best possible clinical management. Faster scanning and retrospective placement of analysis planes at any arbitrary location within the acquisition volume were highlighted as advantages of 4D flow CMR over 2D flow CMR in CHD. It has also been shown that 4D flow may provide superior time-efficient flow quantification.
Clinical perspective and future directions
The review recommends that “optimal severity grading values for cardiac valvular diseases should still be established for 4D flow CMR” as “existing severity classifications are based on echocardiographic and 2D flow assessment”.
Important advantages of using 4D flow CMR were emphasized in congenital cases requiring follow-up (obtaining comprehensive accurate hemodynamic data without using radiation). The review described the 4D flow acquisition process as “time-consuming” but “relatively straightforward” in contrast to the “quite complex” process of analyzing data. “Further development and establishment of a standardized analysis process” is recommended, with machine learning cited as an area of potential.
The article concluded that “it is of the high likelihood that 4D flow CMR will be an essential clinical tool within the upcoming 5–10 years and massively enhance the understanding of flow within the heart”. The technique offers “clear advantages over other [4D] imaging modalities,” which are held back by a “lack of analysis tools that allow for standardized quantification of 4D flow values”.