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

Abstract

Materials with high dielectric constant (HDC) have been used in high field MRI to decrease specific absorption rate (SAR), increase magnetic field intensity, and increase signal-to-noise ratio. In previous studies, the HDC materials were placed inside the RF coil decreasing the space available. This study describes an alternative approach that considers an HDC-based sleeve placed outside the RF coil. The effects of an HDC on the electromagnetic (EM) field were studied using numerical simulations with a coil unloaded and loaded with a human head model. In addition, experimental EM measurements at 128 MHz were performed inside a custom-made head coil, fitted with a distilled water sleeve. The numerical simulations showed up to 40% decrease in maximum 10 g-avg. SAR on the surface of the head model with an HDC material of barium titanate. Experimental measurements also showed up to 20% decrease of maximum electric field using an HDC material of distilled water. The proposed method can be incorporated in the design of high field transmit RF coils.

Improvement of Electromagnetic Field Distributions Using High Dielectric Constant (HDC) Materials for CTL-Spine MRI: Numerical Simulations and Experiments

“…Previous studies evaluating the effect of HDC materials had focused on specific imaging applications for brain [7]–[9], [17], [18], [22] and [23], knee [23], abdomen [12], [14], and [22] heart [13] or C-spine (rather than CTL spine) [6]. Additionally, our method, which uses the pad outside of the receive-only coil, does not constrain the spacing between the receive-only RF coil and the subject.…”

“…This was done due to the limitation of the field mapping robot allowing measurements of only the magnitude of the magnetic field [9]. Conversely, the numerical results with the human model and the transmit coil only were normalized to $\mid {\mathrm{bold-italicB}}_1^{+}\mid =2\mu \mathrm{T}$ at the center of the spine, to match a 90° flip angle of RF rectangular pulse, with a 3 ms duration.…”

“…Whereas, ${\mathrm{bold-italicB}}_1^{+}$ is the rotating RF magnetic field in transmit mode defined as ( B x + jB y ) / 2 [9], [24]. Therefore, an analysis of the effect of the HDC material on electromagnetic field needs to include ${\mathrm{bold-italicB}}_1^{+}$ and SAR, which characterize the transmitted electromagnetic fields, as well as ${\mathrm{bold-italicB}}_1^{-}$ for the receive field.…”

Valid Exposure Protocols Needed in Magnetic Resonance Imaging Genotoxic Research

“…For more detailed information, see, for instance, Kim and Kim [2017], Frankel et al [2018], andMcRobbie et al [2017]. There are also electric fields present in the MRI environment [Kangarlu et al, 2007;Park et al, 2015;Saniour et al, 2018]. These are not well characterized today, but definitely need more attention.…”

SAR and thermal response effects of a two-arm Archimedean spiral coil in a magnetic induction sensor on a human head

“…The SAR quantification can also be simulated by the commercial electromagnetic simulation software which employs these techniques [8]. A significant amount of research has been conducted on SAR and the thermal effect of mobile phone, antenna, and MRI on the human brain [9][10][11]. However, due to the facts that magnetic induction measurement is an emerging technique and its excitation frequency is relatively low, the SAR and thermal response are ignored in many of these studies.…”