Quantitative magnetic resonance imaging of the atrioventricular conduction axis based on the rotating-frame...

Noninvasive visualization and characterization of the atrioventricular conduction axis (AVCA) can help avoid iatrogenic injury and improve the accuracy of heart rhythm therapy guidance.

This study aimed to identify the AVCA structure using noninvasive rotating-frame relaxation-time mappings (relaxation along a fictitious field in the second rotating-frame relaxation time [T_RAFF2] and longitudinal rotating-frame relaxation time [T_1ρ]) in ex vivo swine hearts.

Ex vivo swine heart tissue blocks were scanned on a 3T clinical magnetic resonance imaging system. The contrast among AVCA, central fibrous body, and remote myocardium was evaluated using T_RAFF2 and T_1ρ compared with T₁ and T₂ relaxation-time mappings and magnetization transfer imaging. Histologic sections at 4 levels were stained with Masson's trichrome. The blue channel of histology images was used for analysis. Regions of interest in the myocardium, AVCA, and central fibrous body were selected slice-wise and sample-wise using multi-Otsu thresholding. Relative relaxation-time difference and magnetization transfer contrast were calculated between AVCA and remote myocardial areas.

In the AVCA region, all quantitative magnetic resonance imaging (qMRI) parameters were significantly higher than in remote myocardium. T_RAFF2, T_1ρ, and T₂ showed the highest contrast between AVCA and myocardium. All qMRI parameters correlated with the blue channel of Masson's trichrome-stained histology images. Three-dimensional reconstruction of 2-dimensional T_RAFF2 maps revealed the AVCA structure with increased relaxation time compared with the surrounding myocardium.

This research demonstrates the potential of T_RAFF2, T_1ρ, and T₂ for visualizing the AVCA quantitatively in the swine heart without contrast agents. The contrast provided by the qMRI parameters is based on the fibrous content of the tissue.