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

Abstract

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.

Radiofrequency safety of high permittivity pads in MRI—Impact of insulation material

“…5 As with the development of customized RF transmit coils, the application of passive materials to manipulate the RF field warrants dedicated RF safety assessment, typically involving numerical simulations and validation experiments such as B 1 + mapping and thermometry. 6 Despite the large body of literature reporting benign SAR effects of high permittivity materials, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] their "universal" safety remains difficult to establish as their design may vary considerably. When the materials enhance the B 1 + efficiency in the region of interest, reductions in SAR can be anticipated by virtue of the reduced RF power required by the system.…”

“…In practice, such insulation would always be present, both in the form of the plastic container that is used to seal the materials, as well as in the form of the patient gown that is used during scanning. The most realistic approach would be to account for this insulating layer explicitly in the simulation model, however this requires a highly refined spatial discretization grid at the interface between the body model and the dielectric pad, which can be computationally demanding 13,14,19 . Some groups have recommended implementing a deliberate separation of 10 mm to reduce these computational demands, however such countermeasures can also reduce the B 1 + enhancement and hamper the placement of pads in close‐fitting receive arrays 23 .…”

“…The most realistic approach would be to account for this insulating layer explicitly in the simulation model, however this requires a highly refined spatial discretization grid at the interface between the body model and the dielectric pad, which can be computationally demanding. 13,14,19 Some groups have recommended implementing a deliberate separation of 10 mm to reduce these computational demands, however such countermeasures can also reduce the B 1 + enhancement and hamper the placement of pads in close-fitting receive arrays. 23 Further insights into the relevance of this insulation layer could also provide a handle into what numerical modeling strategy should be considered for obtaining a realistic RF safety assessment of high permittivity pads in MRI.…”

Safety and imaging performance of two‐channel RF shimming for fetal MRI at 3T

“…11,17 However, pTx can significantly increase local SAR exposure. 18 Even though several studies have investigated safety at 1.5T and 3T for conventional birdcage volume coils, 17,[19][20][21][22][23][24][25][26][27][28][29] the safety of pTx for fetal MRI is understudied. RF shimming is a simplified form of pTx, where the complex weights of different transmit channels are optimized to improve image quality.…”

A simulation study on the effect of optimized high permittivity materials on fetal imaging at 3T

“…Several studies performed in adults at 3T have reported a higher B 1 + efficiency and/or homogeneity, leading to a reduced input power requirement and reduced SAR wb and local SAR . The potential benefits of using high permittivity materials in fetal MRI have been indicated previously, but these studies did not address the full parametric design of the materials and did not take into account the different gestational stages.…”