Displacement current distribution on a high dielectric constant helmet and its effect on RF field at 10.5 T (447 MHz)

Abstract

Purpose Investigating the designs and effects of high dielectric constant (HDC) materials in the shape of a conformal helmet on the enhancement of RF field and reduction of specific absorption rate at 10.5 T for human brain studies. Methods A continuous and a segmented four‐piece HDC helmet fit to a human head inside an eight‐channel fractionated‐dipole array were constructed and studied with a phantom and a human head model using computer electromagnetic simulations. The simulated transmit efficiency and receive sensitivity were experimentally validated using a phantom with identical electric properties and helmet‐coil configurations of the computer model. The temporal and spatial distributions of displacement currents on the HDC helmets were analyzed. Results Using the continuous HDC helmet, simulation results in the human head model demonstrated an average transmit efficiency enhancement of 66%. A propagating displacement current was induced on the continuous helmet, leading to an inhomogeneous RF field enhancement in the brain. Using the segmented four‐piece helmet design to reduce this effect, an average 55% and 57% enhancement in the transmit efficiency and SNR was achieved in human head, respectively, along with 8% and 28% reductions in average and maximum local specific absorption rate. Conclusion The HDC helmets enhanced the transmit efficiency and SNR of the dipole array coil in the human head at 10.5 T. The segmentation of the helmet to disrupt the continuity of circumscribing displacement currents in the helmet produced a more uniform distribution of the transmit field and lower specific absorption rate in the human head compared with the continuous helmet design.

Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

“…They obtained images which showed similar signal to noise ratio (SNR) maps using a 20-channel receive coil with the HPM helmet to those obtained using a 64-channel coil without the HPM insert, as shown in Fig. 5 (this helmet approach has subsequently been used for neuroimaging at 7 T and 10.5 T with lower permittivity materials [ 35 , 36 ]). One of the questions which often arises is how can the SNR be increased when typically body loss dominates over coil loss at clinical field strengths?…”

A Preliminary Study for Reference RF Coil at 11.7 T MRI: Based on Electromagnetic Field Simulation of Hybrid-BC RF Coil According to Diameter and Length at 3.0, 7.0 and 11.7 T

“…In the 11.7 T result, the inhomogeneity in the B 1 + -field becomes severe due to the RF wavelength being 1.67 times shorter than the 7.0 T [70][71][72][73][74]. For this reason, it was difficult to find regularity for RF transmission efficiency changes due to the increase in the diameter and length (D/L-ratio fixed 0.7) of the hybrid-BC RF coil.…”