Magnetic susceptibility and R2* of myocardial reperfusion injury at 3T and 7T

Purpose: Magnetic susceptibility (Δχ) alterations have shown association with myocardial infarction (MI) iron deposition, yet there remains limited understanding of the relationship between relaxation rates and susceptibility or the effect of magnetic field strength. Hence, Δχ and $R_2^{\ast }$ in MI were compared at 3T and 7T.

Methods: Subacute MI was induced by coronary artery ligation in male Yorkshire swine. 3D multiecho gradient echo imaging was performed at 1-week postinfarction at 3T and 7T. Quantitative susceptibility mapping images were reconstructed using a morphology-enabled dipole inversion. $R_2^{\ast }$ maps and quantitative susceptibility mapping were generated to assess the relationship between $R_2^{\ast }$ , Δχ, and field strength. Infarct histopathology was investigated.

Results: Magnetic susceptibility was not significantly different across field strengths (7T: 126.8 ± 41.7 ppb; 3T: 110.2 ± 21.0 ppb, P = NS), unlike $R_2^{\ast }$ (7T: 247.0 ± 14.8 Hz; 3T: 106.1 ± 6.5 Hz, P < .001). Additionally, infarct Δχ and $R_2^{\ast }$ were significantly higher than remote myocardium. Magnetic susceptibility at 7T versus 3T had a significant association (β = 1.02, R² = 0.82, P < .001), as did $R_2^{\ast }$ (β = 2.35, R² = 0.98, P < .001). Infarct pathophysiology and iron deposition were detected through histology and compared with imaging findings.

Conclusion: $R_2^{\ast }$ showed dependence and Δχ showed independence of field strength. Histology validated the presence of iron and supported imaging findings.