Permittivity and performance of dielectric pads with sintered ceramic beads in MRI: early experiments and simulations at 3 T

Abstract

Passive dielectric materials have been used to improve aspects of MRI by affecting the distribution of radiofrequency electromagnetic fields. Recently, interest in such materials has increased with the number of high-field MRI sites. Here, we introduce a new material composed of sintered high-permittivity ceramic beads in deuterated water. This arrangement maintains the ability to create flexible pads for conforming to individual subjects. The properties of the material are measured and the performance of the material is compared to previously used materials in both simulation and experiment at 3 T. Results show that both permittivity of the beads and effect on signal-to-noise ratio and required transmit power in MRI are greater than those of materials consisting of ceramic powder in water. Importantly, use of beads results in both higher permittivity and lower conductivity than use of powder.

Numerical assessment of the reduction of specific absorption rate by adding high dielectric materials for fetus MRI at 3 T

“…To further explore the influence of the variances of conductivity of HDM on B 1 field and SAR, the simulation was implemented as follows: relative dielectric constant set as 515, conductivity varied from 0.1 to 0.6 S/m with a step of 0.1 S/m. The obtained B 1 field and SAR distribution were compared with those when the conductivity was set as 0.35 S/m [16]. The results indicated that the influence of the variances of conductivity was negligible (Figure 4).…”

“…The dielectric constant of the HDM used in the simulations was optimized manually with a step of 100. The conductivity of the HDM was assumed as 0.35 S/m (when the relative dielectric constant was set equal/ < 500), or 0.5 S/m (when the relative dielectric constant was set equal/above 600) [5,15,16]. The post-processing of the simulation results was performed in MATLAB (The Mathworks, Natick, MA, USA).…”

“…This is because of the enhancement of the B 1 field for a given input power in the regions that are surrounded by the HDM [16,25]. The B 1 field distribution of the coil is altered by the strong displacement currents caused by the existence of the HDM, resulting in stronger B 1 fields in the region of interest (ROI) [30,32].…”

A Novel Method to Decrease Electric Field and SAR Using an External High Dielectric Sleeve at 3T Head MRI: Numerical and Experimental Results

“…The magnetic field generated by an RF coil in the MRI can be explained using Ampere's law [3], [4], [7], [10] ( 2) where J c is the conduction current, J d is the displacement current, i.e., secondary field source supporting the propagation of RF electromagnetic field, μ is the magnetic permeability (H/m), , ω is the angular frequency (rad/s), and ε 0 = 8.854 × 10 −12 F/m is the electrical permittivity in free space. Additionally, the is defined as , where B x and B y are the complex amplitudes of x-and y-oriented RF magnetic fields, respectively [19].…”

“…The study showed an improvement in the SNR of about 24%, obtained by manipulating the circulation of electric field within the coil with the HDC material. Other experimental and numerical studies have been performed for high field MRI and MR spectroscopy [8] with several different HDC materials, including distilled water [4], D 2 O [6], calcium titanate (CaTiO 3 ) [10] and barium titanate (BaTiO 3 ) [2], [7]. The electrical permittivity of these materials ranges between 78 [6] and 515 [7], the latter being much higher compared to biological tissues.…”