Histology-validated quantitative MRI characterization of collagen-rich Purkinje fiber-associated pathways in the ex vivo porcine heart.

OBJECTIVE: Collagen-rich Purkinje fiber-associated (PF-associated) pathways within the subendocardial layer are small, compositionally distinct structures that are challenging to characterize noninvasively. This study aimed to evaluate whether quantitative magnetic resonance imaging (qMRI) can characterize collagen-rich PF-associated pathways using histology as a structural ground truth. MATERIALS AND METHODS: Ex vivo porcine left ventricular specimens were imaged at 3 T using relaxation-based mapping techniques, including TRAFF, T1ρ, T2 and T1 mapping. Corresponding Masson's trichrome sections were processed into optical density maps to segment myocardium, collagen and PF-associated regions. Histology-defined regions of interest were used to identify collagen-enriched PF-associated pathways and remote myocardium. Relaxation time contrast was quantified using relative relaxation time difference (RRTD). Statistical comparisons were performed using paired t-tests with Benjamini-Hochberg correction. RESULTS: Histology confirmed collagen-enriched PF-associated pathways within subendocardial tissue and false tendons. Optical density-based segmentation reliably differentiated tissue components with low inter-section variability. Collagen-rich PF-associated regions showed significantly higher TRAFF, T1ρ, T2, and T1 values compared with remote myocardium. RRTD analysis indicated that TRAFF and T1ρ provided the strongest relative contrast among the evaluated parameters. CONCLUSION: Histology-validated qMRI enables characterization of collagen-rich PF-associated pathways in the ex vivo porcine heart. These findings demonstrate the sensitivity of relaxation-based MRI techniques to collagen-rich PF-associated pathways and support further investigation of cardiac conduction system imaging. KEY POINTS: Question Noninvasive MRI characterization of collagen-rich PF-associated pathways remains challenging because these small conduction structures are difficult to identify. Findings Histology-referenced MRI differentiated collagen-rich PF-associated pathways from those of the remote myocardium, with rotating-frame relaxation methods providing the strongest contrast. Relevance statement qMRI relaxation mapping enables histology-validated characterization of collagen-rich PF-associated pathways, supporting the development of noninvasive imaging approaches for studying cardiac conduction system microstructure.