Rotating Frame Relaxation Time Mapping for the Visualization of the Sinoatrial Node Without Contrast Agent

Late gadolinium-enhanced cardiovascular magnetic resonance (LGE-CMR) has been used to visualize the sinoatrial node (SAN) structure. In this study, we aimed to investigate the rotating frame relaxation (RAFF2 and T_1ρ) time mapping to characterize the SAN structure in the heart ex vivo without a contrast agent. Ex vivo swine heart tissues were scanned on a 7 T vertical bore preclinical and a 3 T clinical MRI system. The contrast between the SAN and the surrounding atria was assessed using T_RAFF2, T_1ρ, T₁, and T₂ relaxation time mappings, as well as magnetization transfer (MT) imaging. Masson's trichrome stained histological sections were prepared to validate the location of the SAN. Relative relaxation time difference (RRTD) and MT contrast (MTC) were calculated for the SAN and remote myocardium areas. Significant differences between the SAN and myocardium were observed in all endogenous MRI contrasts. T_RAFF2 and T_1ρ showed the highest contrast between the SAN and myocardium, although the differences in contrast among T_RAFF2, T_1ρ and T₂ were not significant at 3 T. Furthermore, SAN sizes determined based on T_RAFF2 and T_1ρ maps were highly correlated with the corresponding fibrous structures in the histological sections. The 3D reconstruction of 2D T_RAFF2 maps revealed the SAN region with increased relaxation time compared to the surrounding myocardium, located lateral to the crista terminalis. This study demonstrates the potential of T_RAFF2 and T_1ρ for visualizing the SAN in the swine heart without the use of contrast agents. Detection of the SAN location using T_RAFF2 and T_1ρ relaxation time mapping could offer a non-invasive alternative to LGE-CMR.