64-channel Radio Frequency Head Coil Simulations

INTRODUCTION Currently with 7 tesla (T) and other ultra-high-field magnetic resonance imaging (MRI), the B1 magnetic fields have a high degree of inhomogeneity with stock MRI configuration. B1 inhomogeneity can lead to the creation of artifacts in the medical image and result in misdiagnosis of medical ailments [1]. Along with artifacts, distortions of both the geometry and intensity of the resulting medical image can occur and further diminish the coverage of the MRI system and therefore reduces the quality of the medical examination [2]. To combat the B1 inhomogeneity experienced with many ultra-high-field MRIs, researcher have created radio frequency (RF) coils to homogenize the magnetic fields emitted from the machine. The basis of an RF coil is to match the same Larmor frequency as the incoming RF source. The coil is tuned and then set to resonate at that specific frequency through the use of capacitive and inductive components [2]. After implementing an RF coil, the resulting image quality is significantly improved and therefore many researchers have explored coils that include loops, plates, and birdcage designs [2]. Another large problem with ultra-high-field MRI is the energy emitted into the patient during the scan, which is measured as specific absorption rate (SAR). The energy distribution inside the subject is extremely subject dependent and non-uniform. Depending on the load size from the (magnetic resonance) MR system, the SAR can be higher or lower in localized regions of the brain [3]. Higher SAR values can lead to increased temperature in the affected region and potentially cause irreversible damage to the tissue of the patient. Scanning with the addition of an RF coil may focus the energy into a localized region and further increase the SAR measured in the tissue. Therefore, RF coil designs must be verified for SAR values using Finite-Difference Time Domain (FDTD) simulations. Additionally, by using FDTD simulations, field homogeneity can be calculated and therefore proves to be an excellent verification method prior to the manufacturing of the RF coil. Current MR technologies include a 16-channel TTT head coil that increased field homogeneity and overall coverage. The design of the 16-channel TTT coil is a single row with a copper shield every 90 degrees (forming a square) [4]. With the development of a 64-channel RF head coil, improvements will be made to the 16-channel TTT coil and the issues associated with field inhomogeneities, increased SAR, and low coverage will hopefully be solved. The proposed coil design consists of two rows with 32 channels each row, totaling in 64 channels. Figure 1 represents the proposed double-octagon coil design. Figure 1. Proposed 64-channel double-octagon head coil for 7T imaging.