Objective To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be... Show moreObjective To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.Methods Electromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.Results Simulations of RF coil characteristics and B-1(+) transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180 degrees refocusing pulses of duration similar to 10 ms are used for TSE sequences does SAR need to be carefully considered.Conclusion This work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T-1 rho, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed. Show less
This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil... Show moreThis article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple "dielectric pads" for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed. Show less
Objective: The purpose of this work is to investigate the use of ceramic materials (based on BaTiO3 with ZrO2 and CeO2-additives) with very high relative permittivity (epsilon(r) similar to 4500)... Show moreObjective: The purpose of this work is to investigate the use of ceramic materials (based on BaTiO3 with ZrO2 and CeO2-additives) with very high relative permittivity (epsilon(r) similar to 4500) to increase the local transmit field and signal-to-noise ratio (SNR) for commercial extremity coils on a clinical 1.5 T MRI system.Methods: Electromagnetic simulations of transmit efficiency and specific absorption rate (SAR) were performed using four ferroelectric ceramic blocks placed around a cylindrical phantom, as well as placing these ceramics around the wrist of a human body model. Results were compared with experimental scans using the transmit body coil of the 1.5 T MRI system and an eight-element extremity receive array designed for the wrist. SNR measurements were also performed for both phantom and in vivo scans.Results: Electromagnetic simulations and phantom/in vivo experiments showed an increased in the local transmit efficiency from the body coil of similar to 20-30%, resulting in an similar to 50% lower transmit power level and a significant reduction in local and global SAR throughout the body. For in vivo wrist experiments, the SNR of a commercial eight-channel receive array, integrated over the entire volume, was improved by similar to 45% with the ceramic.Conclusion: The local transmit efficiency as well as the SNR can be increased for 1.5 T extremity MRI with commercial array coils by using materials with very high permittivity. (C) 2018 The Authors. Published by Elsevier Inc. Show less
Shchelokova, A.V.; Slobozhanyuk, A.P.; Bruin, P. de; Zivkovic, I.; Kallos, E.; Belov, P.A.; Webb, A. 2018
In this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high... Show moreIn this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high permittivity low loss medium. Placement of such a structure inside the scanner results in strong coupling of the radiofrequency field produced by the body coil with the lowest frequency electromagnetic eigenmode of the metasurface. This leads to spatial redistribution of the near fields with enhancement of the local magnetic field and an increase in the transmit efficiency per square root maximum specific absorption rate in the region-of-interest. We have investigated this structure in vivo and achieved a factor of 3.3 enhancement in the local radiofrequency transmit efficiency. (C) 2017 The Authors. Published by Elsevier Inc. Show less
