Gradient coil inductance has been remarkably reduced by the minimum-inductance design technique, which minimizes the magnetic energy stored by the gradient coil. The planar gradient coil designed by this technique, however, often has poor magnetic field linearity. Scaling the spatial frequencies of the current density function derived by this method, the magnetic field linearity of the planar gradient coil can be greatly improved with a small sacrifice of gradient coil inductance. A figure of merit of the planar gradient coil has been found to be improved by scaling the spatial frequencies.
This study describes the MRI-related radio frequency (RF) safety evaluation of breast tissue expander devices to establish safety criteria. Numerical simulations and experimental measurements were performed at 64 MHz with a gel phantom containing a breast expander. Additionally, computational modeling was performed (64 and 128 MHz) with an adult female model, containing a virtually implanted breast tissue expander device for four imaging landmark positions. The presence of the breast tissue expander device led to significant alterations in specific absorption rate (SAR) and|B1+|distributions. The main source of SAR alterations with the use of the breast expander device was the saline-filled pouch of the expander. Conversely, the variation of RF magnetic field (B1+) was mainly caused by the metallic port. The measured values of electric field magnitude did not increase significantly due to the introduction of the expander device. The maximum 1g- or 10g-averaged SAR values in tissues near the implant were lower than those expected in other regions of the patient body with normalization of both|B1+|equal to 2 μT at the coil isocenter and whole body averaged SAR equal to 4W/kg.
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.
This study investigates the use of pads with high dielectric constant (HDC) materials to alter electromagnetic field distributions in patients during magnetic resonance imaging (MRI). The study was performed with numerical simulations and phantom measurements. An initial proof-of-concept and validation was performed using a phantom at 64 MHz, showing increases of up to 10% in electromagnetic field when using distilled water as the high dielectric material. Additionally, numerical simulations with computational models of human anatomy were performed at 128 MHz. Results of these simulations using barium titanate (BaTiO3) beads showed a 61% increase of
∣bold-italicB1+∣ with a quadrature driven RF coil and a 64% increase with a dual-transmit array. The presence of the HDC material also allowed for a decrease of SAR up to twofold (e.g., peak 10 g-averaged SAR from 54 to 22 W/kg with a quadrature driven RF coil and from 27 to 22 W/kg with a dual-transmit array using CaTiO3 powder at 128 MHz). The results of this study show that the use of HDC pads at 128 MHz for MRI spine applications could result in improved magnetic fields within the region of interest, while decreasing SAR outside the region.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.