CMR 4D flow underestimates peak systolic flow measurements in patients with severe aortic stenosis

Abstract Funding Acknowledgements Type of funding sources: Public hospital(s). Main funding source(s): Helsinki University Hospital, Finland Background Aortic stenosis (AS) is the most prevalent valvular disease in the developed countries. 4D flow is an emerging cardiac magnetic resonance (CMR) imaging technique, which has been suggested to improve the evaluation of AS severity. The accuracy of peak flow measurement by 4D flow CMR in patients with severe AS has, however, remained unvalidated. Purpose We investigated the reliability of the novel 4D flow CMR technique in measuring transvalvular peak systolic flow in patients with severe aortic valve stenosis. Methods The study included 63 patients clinically evaluated for valve replacement due to severe symptomatic AS. All the patients underwent echocardiography, 2D phase-contrast and 4D flow CMR. CMR was performed on consecutive patients according to international guidelines. Mean age of the patients was 73.8 ± 11.5 years, mean aortic valve area 0.7 ± 0.2 cm², and 40 of the valves were tricuspid and 23 bicuspid. QFlow and QFlow 4D software were used for flow analyses. Bland-Altman analyses and Wilcoxon signed rank sum tests were performed using SPSS software. Results CMR 4D flow analyses underestimated peak flow values when compared with echocardiography (bias -1.1 m/s, limits of agreement ± 1.5 m/s) and with 2D flow analyses (bias -1.2 m/s, limits of agreement ± 1.7 m/s). The difference between values obtained by 4D flow (median 3.1 m/s, range 1.5 – 4.9 m/s) and echocardiography (median 4.3 m/s, range 2.1 – 6.1 m/s) as well as by 2D flow (median 4.3 m/s, range 2.0 – 8.4 m/s) were statistically significant (p < 0,001). The difference between 2D flow analyses and echocardiography remained statistically insignificant (bias 0.05 m/s, limits of agreement ± 1.6 m/s). Conclusions We found that 4D flow analysis significantly underestimates systolic peak flow values in patients with severe AS. This may be due to intra-voxel averaging of the narrow jets. In contrast to previous assumptions, traditional 2D flow technique may therefore outperform 4D flow in measuring valvular peak flow by CMR in patients with severe AS. This should be taken into consideration when assessing disease severity by CMR. Abstract Figure. Peak systolic flow in AS patients (n = 63)