Dielectric materials in magnetic resonance

Webb, Andrew

doi:10.1002/cmr.a.20219

Cited by 159 publications

(156 citation statements)

References 67 publications

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“…However, UHF MRI of deep organs is complicated due to the use of relatively high frequency of the radiofrequency (RF) signal as well as the effect of much shorter signal wavelength in biological tissue. These factors can lead to destructive interference within the sample, causing field inhomogeneity and lower signal-to-noise (SNR) at deep locations of the body [1]. In addition, the RF power deposition is increased due to the use of a higher frequency, and specific absorption rate (SAR) levels become an important concern from a radiation exposure safety point of view [2].…”

Section: Introductionmentioning

confidence: 99%

Radiative MRI Coil Design Using Parasitic Scatterers: MRI Yagi

Sánchez‐Heredia

Avendal²,

Bibic

et al. 2018

IEEE Trans. Antennas Propagat.

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Abstract-Conventionally, radiofrequency (RF) coils used for magnetic resonance imaging (MRI) are electrically small and designed for nearfield operation. Therefore, existing antenna design techniques are mostly irrelevant for RF coils. However, the use of higher frequencies in ultrahigh field (UHF) MRI allows for antenna design techniques to be adapted to RF coil designs. This study proposes the use of parasitic scatterers to improve the performance of an existing 7T MRI coil called the single-sided adapted dipole (SSAD) antenna. The results reveal that scatterers arranged in a Yagi fashion can be applied to reduce local specific absorption rate (SAR) maxima of a reference SSAD by 40% with only a 6% decrease in the propagated B1 + field at the tissue depth of 15 cm. The higher directivity of the proposed design also decreasing the coupling with additional elements, making this antenna suitable for use in high density arrays. These findings show the potential of parasitic scatterers as an effective method to improve the performance of existing radiative MRI coils.Index Terms-Magnetic resonance imaging, electromagnetic propagation in absorbing media, specific absorption rate, YagiUda antennas.

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Section: Introductionmentioning

confidence: 99%

Radiative MRI Coil Design Using Parasitic Scatterers: MRI Yagi

Sánchez‐Heredia

Avendal²,

Bibic

et al. 2018

IEEE Trans. Antennas Propagat.

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“…The optimum dielectric constant of the two conformal pads with a thickness of 5 mm placed left and right of the pelvis model was found to be about 400; meanwhile, the partial body SAR, maximum mother 10 g local SAR and maximum fetus 10 g local SAR were 1.75, 7.19 and 2.45 W/kg, respectively. The optimum dielectric constant of the two conformal pads with a thickness of 5 mm placed anterior and posterior of the The contribution to the B 1 field and SAR from the conductivity of HDM was much less than that from the relative dielectric constant due to the low ratio of conduction to displacement current [25], however, the conductivity does vary with different relative dielectric constant in different HDM. 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.…”

Section: Discussionmentioning

confidence: 96%

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Luo

Zhuang

et al. 2016

Biomedical Engineering / Biomedizinische Technik

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Abstract:The specific absorption rate (SAR) is an important issue to be considered in fetus MRI at 3 T due to the high radiofrequency energy deposited inside the body of pregnant woman. The high dielectric material (HDM) has shown its potential for enhancing B 1 field and reducing SAR in MRI. The aim of this study is to assess the feasibility of SAR reduction by adding an HDM to the fetus MRI. The feasibility of SAR reduction is numerically assessed in this study, using a birdcage coil in transmission loaded with an electromagnetic pregnant woman model in the SEMCAD-EM solver. The HDMs with different geometric arrangements and dielectric constants are manually optimized. TheB homogeneity is also considered while calculating the optimized fetus 10 g local SAR among different strategies in the application of HDM. The optimum maximum fetus 10 g local SAR was obtained as 2.25 W/kg, by using two conformal pads placed left and right with the dielectric constant to be 400, reduced by 24.75% compared to that without the HDM. It indicated that the SAR can be significantly reduced with strategic placement of the HDM and the use of HDM may provide a simple, effective and low-cost method for reducing the SAR for the fetus MRI at 3 T.

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“…dielectric materials, between a radiofrequency (RF) coil and the sample in magnetic resonance (MR) imaging applications [1,2]. In-vivo studies have demonstrated these benefits, for example, in cardiac imaging [3] and cerebral imaging [4].…”

Section: Introductionmentioning

confidence: 99%

“…Recently, dielectric materials with a high relative permittivity (εr = 1,200) have been proposed to improve coil performance for MR applications at 3 T [5]. Various studies have demonstrated the benefits of integrating dielectric materials with commercially manufactured coils [1,2]. In at least one case, a complete annular dielectric sleeve was simulated surrounding an entire head-sized birdcage coil [6].…”

Section: Introductionmentioning

confidence: 99%

Effect of an annular sleeve of high permittivity material on resonant modes and homogeneity of a birdcage coil

Vaidya

Chen

Zhang

et al. 2015

2015 IEEE 12th International Symposium on Biomedical Imaging (ISBI)

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Fig. 1 Simulation set-up. High pass head-sized birdcage with a dielectric (blue) sleeve (A, B) or brick (C) with dimensions 3.5 cm x 8.5 cm x 26 cm. ABSTRACTWe investigated how high permittivity materials affect the S-parameters and transmit efficiency of a head-sized birdcage coil. We also studied whether re-tuning and matching are necessary for improved performance. While adjusting the S-parameters is possible during coil development and for prototype coils, it is often unfeasible to re-tune and match commercially manufactured coils, and poses a challenge after adding dielectric material to such coils. Our work demonstrates the change in the S-parameters and the transmit efficiency after placing a dielectric sleeve within a head-sized birdcage coil, and reports that coil performance can be improved after re-tuning and matching the coil in the presence of the dielectric material.

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Dielectric materials in magnetic resonance

Cited by 159 publications

References 67 publications

Radiative MRI Coil Design Using Parasitic Scatterers: MRI Yagi

Radiative MRI Coil Design Using Parasitic Scatterers: MRI Yagi

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

Effect of an annular sleeve of high permittivity material on resonant modes and homogeneity of a birdcage coil

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