Reliability of 4D Flow MRI-Derived Pulmonary Regurgitant Fraction in Repaired Tetralogy of Fallot: Impact of Measurement Location and Pulmonary Artery Geometry.

Background: Pulmonary regurgitant fraction (RF) in patients with repaired tetralogy of Fallot (rTOF) is an important indicator for therapeutic intervention. However, the impact of pulmonary artery geometry and turbulent flow on measurement accuracy is unclear.

Purpose: To evaluate the reliability of 4D flow MRI-derived and 2D phase contrast (2D-PC) RF assessments, identify clinically useful measurement locations, and investigate factors contributing to variability.

Study type: Retrospective.

Population/subjects: Thirty-four rTOF patients (21 female; age 45.2 ± 14.4 years).

Field strength/sequence: 3.0 T; balanced steady-state free precession (bSSFP) cine, time-resolved 2D-PC segmented gradient echo (2D-PC), and time-resolved velocity-sensitized 3D gradient echo with echo planar imaging readout (4D flow) sequences.

Assessment: MRI was performed before and 3 months after transcatheter pulmonary valve replacement (TPVI). Indexed right ventricular end-diastolic volume (RVEDVi) was assessed in pre/post bSSFP cine data to evaluate RV remodeling. MPA backward and forward flow volumes (BFV, FFV) and RF were measured using 2D-PC and 4D flow. Six equidistant 4D flow planes from the right ventricular outflow tract to the distal MPA were analyzed, with one additional plane matched to the 2D-PC location. MPA and vortex areas were also assessed for each 4D flow plane.

Statistical tests: Spearman's rho assessed correlations between 2D-PC and 4D flow parameters, and area under the receiver operating characteristic curve (AUC) assessed the association between RF and the pre/post TVPI percentage change in RVEDVi.

Significance: p < 0.05.

Results: 2D-PC and 4D flow measurements at the matched MPA plane correlated significantly (RF: ρ = 0.65, FFV: ρ = 0.76, BFV: ρ = 0.87). RF varied by plane, and the plane 10 mm distal to the valve best associated with RVEDVi reduction after TPVI (AUC = 0.84). Changes in FFV between planes significantly correlated with dilatation and stenosis (ρ = 0.432, -0.618), whereas BFV correlated with dilatation alone (ρ = 0.581); however, vortex area enlargement showed no significant association with flow volume changes.

Data conclusion: 2D-PC and 4D flow measurements show high correlations, but 4D flow accuracy depends on plane location and geometry.

Evidence level: 3.

Technical efficacy: Stage 2.