On consideration of radiated power in RF field simulations for MRI (original) (raw)
Related papers
On the radio-frequency power requirements of human MRI
PIERS Online, 2007
In high and ultrahigh field magnetic resonance imaging (MRI) research, computational electromagnetic techniques are now taking an important role in the design and evaluation of MRI radiofrequency (RF) coils. This paper focuses on the RF power requirements and specific absorption rate (SAR) associated with the MRI operation at different field strength. This paper also presents new techniques for achieving high-quality transmit field homogeneity simultaneously with lower total RF power deposition. The studies are done utilizing the finite difference time domain (FDTD) method and the validation of the methods is performed using ultra high field MRI volume 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.
Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects
Magnetic Resonance in Medicine, 2011
Radiofrequency magnetic fields are critical to nuclear excitation and signal reception in Magnetic Resonance Imaging (MRI). The interactions between these fields and human tissues in anatomical geometries results in a variety of effects regarding image integrity and safety of the human subject. In recent decades numerical methods of calculation have been used increasingly to understand the effects of these interactions and aid in engineering better, faster, and safer equipment and methods. As MRI techniques and technology have evolved through the years, so too have the requirements for meaningful interpretation of calculation results. Here we review the basic physics of RF electromagnetics in MRI and discuss a variety of ways RF field calculations are used in MRI in engineering and safety assurance from simple systems and sequences through advanced methods of development for the future.
1996
Finite Element Method (FEM) using %-node isoparametric finite elements was applied for modeling saddleshaped head coils used in Magnetic Resonance Imaging (MM) generating linearly polarized radiofrequency (RF) pulses at 64 MHz. The human head was modeled from MR sciplls of a volunteer and additional information were taken from Atlas of Sectional Human Anatomy. The physical dimensions of the head coil and the head permit a calculation of the outside magnetic field by a quasistatic approach. Of course, a full-wave approach was applied within the head. Values of specific energy-specific absorption (SA)-% well as of specific power-specific absorption rate (SARI-were calculated by the method, simulating the real exposure conditions during MRI. Although the results of the used numerical method were compared previously to the results of the analytical solution with
A Numerical Analysis of Radio-Frequency Power Requirements in Magnetic Resonance Imaging Experiment
IEEE Transactions on Microwave Theory and Techniques, 2004
In this paper, a numerical analysis of the radio-frequency (RF) power requirements in magnetic resonance imaging (MRI) is presented at frequencies that span 200-362 MHz. This was performed utilizing an anatomically detailed human head model and a high-frequency RF coil utilized in high-field MRI systems. For axial slices through the brain region, it is demonstrated that the power required in order to obtain an average flip angle across the slice increases with frequency plateauing at a certain value, and then dropping as the frequency increases. The results demonstrate the significance of the electromagnetic interactions between the load and coil and their effects on the so important issue of power-frequency dependence in MRI.
2014
Magnetic Resonance Imaging (MRI) has been contraindicated in patients with pacemakers or implantable cardioverter-defibrillators (ICDs) due to safety concerns, such as the heating of adjacent bodily tissue due to radio frequency (RF) induced current. The ISO/IEC 10974 Joint Working Group (JWG) has developed a tiered approach in establishing the worst case RF heating conditions that active implantable devices may experience during MRI utilizing computer simulations. According to the ISO/IEC JWG tier 2 approach, we evaluated the electric fields induced in the implant regions of pacemakers and ICDs in five human body models during 1.5 T MRI scans. The maximum electrical field (Emax) can be used as a conservative estimation to test MRI induced heating. The SEMCAD software package was used to calculate the electric field distribution due to RF fields from high pass and low pass MRI birdcage coils. The variables studied in the simulations also included circularly polarized field rotations...
Improving RF safety in MRI by modifying the electric field distribution
2011 XXXth URSI General Assembly and Scientific Symposium, 2011
In this work we demonstrate that the radiofrequency (RF) electric field in magnetic resonance imaging (MRI) can be modified in order to enhance patient safety. The heating of metallic devices in MRI is directly related to electric field distribution. On the other hand the MR image homogeneity is related to forward polarized component of the magnetic field (transmit sensitivity). In order to prevent heating, electric field-free zones should be generated in the body without significantly altering the transmit sensitivity. For this purpose the linearly polarized birdcage coil is proposed as a metallic device friendly MRI coil. The zero electric field plane of the linear birdcage coil is coincided with the location of the metallic device and the heating is reduced as shown by simulations and experiments. One disadvantage of this approach is, the linear coils generate twice as much whole body average SAR when compared to quadrature birdcage coils. In order to solve this problem simulations are performed to find electromagnetic field solutions with reduced average SAR and uniform transmit sensitivity.
NMR in biomedicine, 2015
The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B1 (B1 (+) ) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32-rung birdcage body coil (driven either in a fixed quadrature mode or a two-channel transmit mode), two single-ring stripline arrays (with either 8 or 16 elements), and two multi-ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B1 (+) homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multich...
… , 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.
Increased rf power absorption in MR imaging due to rf coupling between body coil and surface coil
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 1989
Fears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rf field leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive-only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst-case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst-case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coup...