Numerical analysis of human sample effect on RF penetration and liver mr imaging at ultrahigh field

Magnetic resonance imaging (MRI) can provide clinically-valuable images for hepatic diseases and has become one of the most promising noninvasive methods in evaluating liver lesions. To facilitate the ultrahigh field human liver MRI, in this work, the RF penetration behavior in the conductive and high dielectric human body at the ultrahigh field of 7 Tesla (7T) is investigated and evaluated using the finite-difference time-domain numerical analysis. The study shows that in brain imaging at the ultrahigh field of 7T, the "dielectric resonance" effect dominates among other factors, resulting in improved B₁ penetration; while in liver imaging, due to its irregular geometry of the liver, the "dielectric resonance" effect is not readily to be established, leading to a reduced B₁ penetration or limited image coverage comparing to that in the brain. Therefore, it is necessary to build a large size coil to have deeper penetration to image human liver although the coil design may become more challenging due to the required high frequency. Based on this study, a bisected microstrip coil operating at 300 MHz range is designed and constructed. Three-dimensional in vivo liver images in axial, sagittal and coronal orientations are then acquired from healthy volunteers using this dedicated RF coil on a 7T whole body MR scanner.