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Editorial for “Neopulmonary Outflow Tract Obstruction Assessment by 4D Flow Magnetic Resonance Imaging in Adults With Transposition of the Great Arteries After Arterial Switch Operation”

Cardiovascular magnetic resonance imaging (MRI) has become the standard in follow-up of adult congenital heart disease. Its advantages compared to transthoracic echocardiogram include easier quantitative assessment of regurgitation, more accurate assessment of cardiac ejection fraction, and no limitation in imaging windows, which is especially beneficial in assessment of the pulmonary tract, as this often has a limited window on echocardiography. Three-dimensional cine (time-resolved) phase contrast MRI with 3D velocity-encoding (4D flow MRI) has been studied with increasing interest. The major drawbacks of a long acquisition time and lengthy analysis protocol have previously hindered adoption into clinical practice. Since new accelerated 4D flow MRI sequences have become available, acquisition is now possible in as little as 5 minutes, making it feasible to incorporate into a clinical protocol. In addition, commercially available 4D flow MRI analysis software has opened the technique to a much wider clinical audience. The natural next steps towards clinical adoption are validation and assessment of clinical usefulness. To date, potential areas for clinical use are still being explored, with only a small number of longitudinal follow-up studies published. In the majority of valve disease and associated aortopathies, clinical progression is slow, hampering timely clinical outcome studies using 4D flow MRI. Therefore, much of the community’s effort has been focused on validation. This raises the question: Which modality should one compare 4D flow MRI to? The absolute gold standard is invasive hemodynamic measurements, which often require animal models for validation studies. More readily accessible in humans are echocardiogram and 2D cine phase contrast MRI with velocity encoding in a single direction (2D cine PC-MRI). While the consensus statement advises using 2D cine PC-MRI for validating net flow in 4D flow MRI, one could argue that the gold standard for peak velocity should be echocardiography, as 2D cine PC-MRI can underestimate peak velocities, something 4D flow MRI can potentially mitigate. While 4D flow MRI suffers from the same artifact problems as 2D cine PC-MRI, such as background phase offsets, the benefits of 4D flow MRI include easy acquisition, even by noncardiac radiographers, and the option of retrospective plane placement by expert reviewers offline. Plane placement can be altered to ensure that even complex jets are fully interrogated when quantifying peak velocity. Velocity measures in all spatial directions may further help to fully capture the peak velocity of a stenotic jet, although a lower temporal resolution (40–50 msec) may make it more likely to miss the true peak velocity timepoint. These technical considerations aside, as the article in this edition rightly explores, the important question is: Does 4D flow MRI add any additional clinically relevant information? A difference in peak velocity quantification of 0.2 m/s has quite rightly been deemed as not a clinically significant difference. Achieving velocity quantification in 100% of patients instead of just 50% of patients is of course highly clinically significant. But it does leave the reader wondering: Did 2D cine PC-MRI assessment also achieve velocity quantification in 100% of patients? Would 2D cine PC-MRI have been good enough? Did we really need 4D flow MRI to better image the pulmonary outflow tract and branch pulmonary arteries? This is especially relevant, as 2D cine PC-MRI acquisition was necessary in all patients to decide on the velocity for the 4D flow MRI scan; therefore, 4D flow MRI did not replace the 2D cine PC-MRI acquisitions. Nonetheless, what we do know is that 4D flow MRI can be as good as 2D cine PC-MRI and has the potential to provide additional information. So what we need to do as a community, in order to find 4D flow MRI’s unique selling point, is to use 4D flow MRI clinically, and use it routinely in all patients so its full potential can unfold over time in large-scale clinical studies.