Assessment of a PML Boundary Condition for Simulating an MRI Radio Frequency Coil (original) (raw)
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Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering
Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time-harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed met...
Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 2015
Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time-harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM-FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low-pass and high-pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils.
IEEE Transactions on Biomedical Engineering, 2016
This study aims at a systematic assessment of five computational models of a birdcage coil for magnetic resonance imaging (MRI) with respect to accuracy and computational cost. Methods: The models were implemented using the same geometrical model and numerical algorithm, but different driving methods (i.e., coil "defeaturing"). The defeatured models were labeled as: specific (S2), generic (G32, G16), and hybrid (H16, H16 fr-forced). The accuracy of the models was evaluated using the "Symmetric Mean Absolute Percentage Error" ("SMAPE"), by comparison with measurements in terms of frequency response, as well as electric () and magnetic () field magnitude. Results: All the models computed the within 35 % of the measurements, only the S2, G32, and H16 were able to accurately model the inside the phantom with a maximum SMAPE of 16 %. Outside the phantom, only the S2 showed a SMAPE lower than 11 %. Conclusions: Results showed that assessing the accuracy of based only on comparison along the central longitudinal line of the coil can be misleading. Generic or hybrid coilswhen properly modeling the currents along the rings/rungswere sufficient to accurately reproduce the fields inside a phantom while a specific model was needed to accurately model in the space between coil and phantom. Significance: Computational modeling of birdcage body coils is extensively used in the evaluation of RF-induced heating during MRI. Experimental validation of numerical models is needed to determine if a model is an accurate representation of a physical coil.
A fast and accurate simulator for the design of birdcage coils in MRI
Magma: Magnetic Resonance Materials in Physics, Biology, and Medicine, 2002
The birdcage coils are extensively used in MRI systems since they introduce a high signal to noise ratio and a high radiofrequency magnetic field homogeneity that guarantee a large field of view. The present article describes the implementation of a birdcage coil simulator, operating in high-pass and low-pass modes, using magnetostatic analysis of the coil. Respect to other simulators described in literature, our simulator allows to obtain in short time not only the dominant frequency mode, but also the complete resonant frequency spectrum and the relevant magnetic field pattern with high accuracy. Our simulator accounts for all the inductances including the mutual inductances between conductors. Moreover, the inductance calculation includes an accurately birdcage geometry description and the effect of a radiofrequency shield. The knowledge of all the resonance modes introduced by a birdcage coil is twofold useful during birdcage coil design: */higher order modes should be pushed far from the fundamental one, */for particular applications, it is necessary to localize other resonant modes (as the Helmholtz mode) jointly to the dominant mode. The knowledge of the magnetic field pattern allows to a priori verify the field homogeneity created inside the coil, when varying the coil dimension and mainly the number of the coil legs. The coil is analyzed using equivalent circuit method. Finally, the simulator is validated by implementing a low-pass birdcage coil and comparing our data with the literature. #
Progress In Electromagnetics Research M, 2013
Numerical methods based on solutions of Maxwell's equations are usually adopted for the electromagnetic characterization of Magnetic Resonance (MR) Radiofrequency (RF) coils. In this context, many different numerical methods can be employed, including time domain methods, e.g., the Finite-Difference Time-Domain (FDTD), and frequency domain methods, e.g., the Finite Element Methods (FEM) and the Method of Moments (MoM). We provide
IEEE Transactions on Electromagnetic Compatibility, 2000
Due to complex field/tissue interactions, high-field magnetic resonance (MR) images suffer significant image distortions that result in compromised diagnostic quality. A new method that attempts to remove these distortions is proposed in this paper and is based on the use of transceiver-phased arrays. The proposed system uses, in the examples presented herein, a shielded fourelement transceive-phased array head coil and involves performing two separate scans of the same slice with each scan using different excitations during transmission. By optimizing the amplitudes and phases for each scan, antipodal signal profiles can be obtained, and by combining both the images together, the image distortion can be reduced several fold. A combined hybrid method of mo-
… , IEEE Transactions on, 2005
In this paper, two TEM resonators were evaluated experimentally and numerically at 8 tesla (T) (340 MHz for 1 H imaging). The coils were constructed to be 21.2-cm long (standard) and 11-cm long (a proposed less claustrophobic design). The experimental evaluation was done on a single cadaver using an ultra high field, 8 T, whole-body magnet. The numerical modeling was performed using an in-house finite difference time domain packagethat treats the coil and the load (anatomically detailed human head model) as a single system. The coils were tested with quadrature excitation at different coil alignment positions with respect to human head. For head imaging at 8 T, the overall numerical and experimental results demonstrated that when compared to the longer coil, the shorter coil provides superior signal-to-noise ratio, coil sensitivity, and excite field in the biological regions that lie within both of the coils' structures. A study of the RF (excite/receive fields) homogeneity showed variations in the performance of both coils that are mostly dependant on the region of interest and the position of coil with respect to the head. As such, depending on the application, the shorter coil could be effectively utilized.
On consideration of radiated power in RF field simulations for MRI
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 2013
In numerical analyses of radiofrequency (RF) fields for MRI, RF power is often permitted to radiate out of the problem region. In reality, RF power will be confined by the magnet bore and RF screen enclosing the magnet room. We present numerical calculations at different frequencies for various surface and volume coils, with samples from simple spheres to the human body in environments from free space to a shielded RF room. Results for calculations within a limited problem region show radiated power increases with frequency. When the magnet room RF screen is included, nearly all the power is dissipated in the human subject. For limited problem regions, inclusion of a term for radiation loss results in an underestimation of transmit efficiency compared to results including the complete bore and RF screen. If the term for radiated power is not included, calculated coil efficiencies are slightly overestimated compared to the complete case.
Electromagnetic characterisation of MR RF coils using the transmission-line modelling method
Magma: Magnetic Resonance Materials in Physics, Biology, and Medicine, 2002
The Transmission-Line Modelling (TLM) method is applied to the electromagnetic characterisation of RF coils and samples for magnetic resonance imaging ()MRI. Theoretical verification was performed using a simple surtitce coil. Experimental verification was performed using Alderman-Grant and birdcage coils constructed for use on a 7 T micro-imaging system. The modelling method enabled electromagnetic characteristics of frequency response, electromagnetic field generation, energy stored and power loss to be determined. From these parameters, coil resonant modes, B~ field profiles, voltages, currents, quality factor (O), re/2 pulse length, and the equivalent lumped-element circuit components of resistance, inductance and capacitance were calculated. Equations are presented that enable a comprehensive electromagnetic characterisation of the RF coil and sample to be achieved based on the results of the TLM simulations. The use of the TLM method is extended to include the design of sate arbitrary multi-nuclear pulse sequences such that the specific absorption rate (SAR) of tissue, and RF coil component safety limits are not exceeded. ~3