Finite-difference time-domain simulations of radio frequency electromagnetic fields and signal inhomogeneities in ultrahigh-field magnetic resonance imaging systems

摘要

Severe inhomogeneities of radio frequency electromagnetic fields arise in ultrahigh-field magnetic resonance imaging (MRI) systems. The purpose of this study is to estimate the distribution of electromagnetic fields, magnetic resonance signals, and the specific absorption ratio (SAR) in the human head. Numerical simulations were carried out using the finite-difference time-domain method. An increase in frequency resulted in severe inhomogeneities in the magnetic resonance signals and the SAR. A reduction in the duty ratio is required to meet safety standards for the SAR. The results indicate the importance of developing coils and image-processing methods to compensate for the signal inhomogeneities in proton images. On the other hand, ultrahigh-field MRI systems are particularly useful for quantitative measurements of nonproton nuclei.