Comparison of free-running whole-heart 5D and 4D flow imaging to standard 2D phase contrast in patients with right-sided congenital heart disease
Authors: Rutz T , Tenisch E , Falcao M , Roy C , Faessler S , Rodrigues D , Ma L , Markl M , Stuber M , Piccini D , Schwitter J , Prsa M 
Published: June 2023

Abstract

Funding Acknowledgements
Type of funding sources: None.

Background
Flow measurements by cardiac magnetic resonance (CMR) are part of a routine assessment in patients with congenital heart disease (CHD). Standard 2D phase contrast (2DPC) sequences, however, require precise prescription of image planes leading to long scan times which could be overcome by 3D-acquisitons covering the entire cardiac anatomy in a single acquisition. Thus, recently introduced accelerated free-breathing 3D whole heart flow sequences promise to simplify CMR exams allowing an easier acquisition and retrospective flow measurements in virtually all vessels.

Purpose
Comparison of standard 2DPC to an accelerated 4D flow sequence with prospective respiratory navigation (1), and a free-running radial fully self-gated respiratory and cardiac motion-resolved 5D flow sequence (5D flow) (2,3).

Methods
Patients with right-sided CHD and no intra- or extracardiac shunt underwent a CMR on a 1.5T scanner. 2DPC flow sequences and a 4D flow data set were obtained (1). In addition, a whole-heart free-running 3D radial PC CMR sequence was acquired (2). Using a compressed-sensing-based image reconstruction framework (2,3), 5D flow images were obtained, and the end-expiratory phase was used for analysis. 3D vessel segmentation was based on retrospectively computed phase-contrast angiography images for each 4D and 5D flow datasets. Flow measured in the ascending, descending aorta (AA, DA), main, right and left pulmonary artery (MPA, RPA, LPA) and superior vena cava (SVC) as well as the internal consistency for determination of systemic and pulmonary output were compared between all three sequences by Bland-Altman analyses.

Results
Twenty CHD patients (age 31±11y, 7 women, tetralogy of Fallot N=7, Ross operation N=10; pulmonary regurgitation, atrial switch and Ebstein’s disease each N=1) were included. Mean peak velocity of MPA/ right ventricle to pulmonary artery conduit was 1.9 (range 0.9; 3.5) m/s.
Compared to 2DPC, both 4D and 5D flow moderately underestimated absolute flow with significant lower values for the RPA for 4D flow (Table 1).
The bias (i.e. the mean absolute difference) of MPA flow between 2DPC and 4D flow was related to the MPA peak velocity (r2=0.3, p = 0.03) with a similar trend for the comparison of 2DPC vs. 5D flow (r2=0.142, p = 0.141).
The comparison of the internal consistency for determination of systemic and pulmonary output showed similar results for 2DPC and 5D flow and a slightly lower agreement for the 4D flow sequence (Table 2).

Conclusions
In this preliminary study, 4D and 5D flow provide results comparable to 2DPC, however, both sequences showed a trend towards lower absolute flow values. Complex flow patterns due to the right-sided pathologies appear to have an impact on the difference observed between sequences as shown for the MPA.
The internal consistency of flow quantification was similar for the 2DPC and 5D flow sequences. The presented observations are currently being evaluated in a larger study.