Evaluating Perfusion Quantification for Phase-Resolved Functional Lung MRI Using Gaussian Filtering

Phase-resolved functional lung (PREFUL) MRI enables contrast-free lung perfusion quantification (Q_Quant). Current quantification uses a single median value per slice of maximal blood exchange fraction (ΔS), which limits regional assessment, whereas regional ΔS causes artifacts. This retrospective study aims to optimize Q_Quant by applying Gaussian filtering to ΔS to improve intrascan/interscan repeatability in healthy volunteers, while evaluating whether the optimized Q_Quant retains agreement with dynamic contrast-enhanced (DCE) MRI in chronic obstructive pulmonary disease (COPD) and chronic thromboembolic pulmonary hypertension (CTEPH) patients and exhibits a physiologically plausible gravity-dependent perfusion gradient. Analysis was performed on 29 healthy subjects (11 female, age: 19-33 years), 20 COPD patients (three female, age: 46-76 years) and 15 CTEPH patients (six female, age: 44-82 years), scanned at 1.5 T. Healthy subjects underwent four PREFUL scans in two sessions. Patients just underwent one PREFUL and DCE scan. Q_Quant maps were calculated while applying different Gaussian filter options to ΔS. In patients, PREFUL-derived Q_Quant maps were coregistered to DCE-derived pulmonary blood flow (PBF) maps for global and voxel-wise correlations. In healthy subjects, the gravity-dependent perfusion gradient was quantified by linear regression of slice-wise Q_Quant across five coronal slices. Gaussian filtering ΔS with σ = 1.5 improved intrascan and interscan repeatability in healthy subjects (mean bias: ≤ 2.94 mL/100 mL; Cohen's d: 0.2) compared with the current slice median setting (mean bias: ≥ 3.75 mL/100 mL; Cohen's d: 0.68). Correlations with PBF remained stable after filtering (σ = 1.5) in COPD (filtered vs. default ΔS: global r = 0.57 vs. r = 0.59, voxel-wise r = 0.32 vs. r = 0.32) and CTEPH (global r = 0.32 vs. r = 0.28, voxel-wise r = 0.40 vs. r = 0.41), all significant except global CTEPH. The filtered Q_Quant maps showed a significant gravity-dependent perfusion gradient (ΔS filtered σ = 1.5) of 1.47 mL/min/100 mL/cm. Gaussian filtering ΔS with σ = 1.5 improved Q_Quant repeatability while preserving DCE-MRI agreement and is therefore recommended. The observed gravity-dependent perfusion gradient confirms physiological plausibility, supporting potential in clinical use.