Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 – 3×3×3 mm3, typical temporal resolution of 30–40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.

Background Ascending aortic dilation is important in bicuspid aortic valve disease (BAV), with increased risk of aortic dissection. We used cardiovascular magnetic resonance (CMR) to understand the pathophysiology better by examining the links between 3-dimensional flow abnormalities, aortic function and aortic dilation. Methods and Results 142 subjects underwent CMR (mean age 40 years; 95 with BAV, 47 healthy volunteers [HV]). BAV patients had predominantly abnormal right-handed helical flow in the ascending aorta, larger ascending aortas (18.3 ±3.3 vs. 15.2 ±2.2mm/m2, p<0.001), and higher rotational (helical) flow (31.7 ±15.8 vs. 2.9 ±3.9mm2/s, p<0.001), systolic flow angle (23.1 ±12.5 vs. 7.0 ±4.6°, p<0.001) and systolic wall shear stress (WSS) (0.85 ±0.28 vs. 0.59 ±0.17N/m2, p<0.001) compared to HV. BAV with right-handed flow and right-non coronary cusp fusion (n= 31) showed more severe flow abnormalities (rotational flow 38.5 ±16.5 vs. 27.8 ±12.4mm2/s, p<0.001; systolic flow angle 29.4 ±10.9 vs. 19.4 ±11.4°, p<0.001; in-plane WSS 0.64 ±0.23 vs. 0.47 ±0.22N/m2, p<0.001) and larger aortas (19.5 ±3.4 vs. 17.5 ±3.1mm/m2, p<0.05) than right-left cusp fusion (n=55). BAV patients with normal flow patterns had similar aortic dimensions and WSS to HV and younger BAV patients showed abnormal flow patterns but no aortic dilation, both further supporting the importance of flow pattern in the etiology of aortic dilation. Aortic function measures (distensibility, aortic strain and pulse wave velocity) were similar across all groups. Conclusions Flow abnormalities may be a major contributor to aortic dilation in BAV. Fusion type affects the severity of flow abnormalities, and may allow better risk prediction and selection of patients for earlier surgical intervention.