oa Artificial Ground Plane For Magnetic Resonance Coils To Reduce The Energy Specific Absorption Rate

Background & Objectives The anti-phase currents and the propagation of surface waves on the conventional metallic ground plane placed underneath a radio frequency (RF) coil for high field magnetic resonance imaging (MRI), represent the reasons for the reduction in RF magnetic flux density above this coil (inside the phantom). The objective of this paper is to overcome on the aforementioned problems by replacing this metallic reflector with a high impedance surface electromagnetic band gap (EBG) structure to improve the efficiency of a well-established meander dipole for 7Tesla MRI. A novel multilayer offset stacked polarization dependent EBG structure has been designed to work as an artificial ground plane (in particular as a soft surface) for 7Tesla MRI RF coils. The performance of a meander dipole element when it is backed by our proposed soft surface is compared in a fair manner to the performance of the design using the metallic ground plane by simulating the distribution of magnetic field, electric field, and the energy specific absorption rate (SAR) 1cm inside a homogeneous phantom. Materials and Methods A multilayer EBG structure is introduced, which consists of two arrays of metal patches diagonally offset from each other. The top layer consists of 4x3 patches each of 8% of λ300MHz in length and 3% of λ300MHz in width. These patches are connected to the metal backed dielectric substrate by vertical pins. The lower layer consists of solid patches and is floating. The HFSS full wave simulator (based on FEM) and the FDTD simulator EMPIRE XCcel were used to characterize and analyze the EBG structure. A homogeneous phantom is placed 2cm above the coil in order to emulate the human body at the MRI operating frequency of 300MHz. Results and Conclusions The FDTD results showed that the normalized total electric field for the meander dipole backed by a metallic reflector 1cm inside the phantom was 54 V/m/√W compared to 40 V/m/√W for the case when the RF coil backed by the proposed surface. Thereby, the peak localized specific absorption rate SAR values (hot spots), which is a dominant restriction for MRI of high field strengths, is also reduced. The 10g-SAR and 1g-SAR values for the conventional metallic and the proposed EBG ground planes are reduced from 2.612 W/kg to 1.478 W/kg and from 3.45 W/kg to 1.91 W/kg respectively, a significant reduction (by around 43%) in the local SAR is observed.