Structural Characterization of Cardiac Purkinje Fibers Using Inhomogeneous Magnetization Transfer (ihMT): A proof of Concept MRI-Histology Approach

BACKGROUND The cardiac Purkinje network plays a critical role in maintaining synchronized activation of ventricles but remains challenging to image due to its fine and unique structure. Conventional MRI techniques lack sufficient contrast to distinguish the underlying structural composition of Purkinje Fibers (PF). PURPOSE This study investigates the potential of inhomogeneous Magnetization Transfer (ihMT) as a novel contrast mechanism for visualizing and differentiating subregions of the PF.

Five fixed ex-vivo sheep hearts (n = 5) containing free running PF were scanned with a 9.4T MRI using a 2D ihMT RARE sequence. ASSESSMENT ihMTR maps were analyzed using manually defined regions-of-interest (ROIs) corresponding to free-running PF, insertion points, and myocardium. Histological analysis (light and polarized microscopy) was performed on matched sections to quantify collagen types I and III, adipocytes, Purkinje cells, and cardiomyocytes.

Three ihMT protocols, which produced high ihMTR values in free-running PF (9.25–10.83%) and strong absolute contrast relative to the myocardium (2.00–2.17%) and insertion points (2.99–3.40%) in one sample were selected and applied to all samples. Across all samples, mean ihMTR in free-running was consistently higher than in insertion points (11.5 ± 1.5% vs. 9.0 ± 2.9%). Histological analysis revealed a significantly greater collagen content in free-running regions compared with insertion points (72.4 ± 15.9% vs. 31.1 ± 13.1%; p = 0.001), along with higher adipocyte content at insertion points vs. free-running regions (12.3 ± 6.1% vs. 3.8 ± 2.7%, non-significant). Collagen type III was more prominent at insertion points but remained a minor component overall.

ihMT imaging can distinguish PF subregions based on microstructural differences, particularly collagen and adipocyte distribution. This study lays the groundwork for developing biophysical models to interpret ihMT signals in terms of tissue composition and microstructure, providing a foundation for future studies. Sponsor This study received financial support from the French Government by the National Research Agency (ANR; SYNATRA ANR-21-CE19-0014-01) and Région Nouvelle Aquitaine (convention N°AAPR2022-2021-16609210). Competing Interest Statement The authors have declared no competing interest. Footnotes ↵* New address: CRC Human Imaging, GIGA Department, University of Liege, Liege, Belgium Abbreviations - ihMT - inhomogeneous Magnetization Transfer - MT - Magnetization Transfer - PF - Purkinje fiber - CCS - Cardiac Conduction System - PL - Polarized Light - NEX - Number of excitations - SAR - Specific Absorption Rate