Dielectric resonances and B(1) field inhomogeneity in UHFMRI: computational analysis and experimental findings.

The RF B(1) distribution was studied, theoretically and experimentally, in phantoms and in the head of volunteers using a 3 T MRI system equipped with a birdcage coil. Agreement between numerical simulation and experiment demonstrates that B(1) distortion at high field can be explained with 3D full-Maxwell calculations. It was found that the B(1) distribution in the transverse plane is strongly dependent on the dielectric properties of the sample. We show that this is a consequence of RF penetration effects combined with RF standing wave effects. In contrast, along the birdcage coil z-axis the B(1) distribution is determined mainly by the coil geometry. In the transverse plane, the region of B(1) uniformity (within 10% of the maximum) was 15 cm with oil, 6 cm with distilled water, 11 cm with saline, and 10 cm in the head. Along z the B(1) uniformity was 9 cm with phantoms and 7 cm in the head.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.1201

Radio frequency magnetic field mapping of a 3 Tesla birdcage coil: experimental and theoretical dependence on sample properties.

Numerical modelling of the interaction between electromagnetic fields (EMFs) and the dielectrically inhomogeneous human body provides a unique way of assessing the resulting spatial distributions of internal electric fields, currents and rate of energy deposition Knowledge of these parameters is of importance in understanding such interactions and is a prerequisite when assessing EMF exposure or when assessing or optimizing therapeutic or diagnostic medical applications that employ EMFs In this review, computational methods that provide this information through full time-dependent solutions of Maxwell's equations are summarized briefly This is followed by an overview of safety- and medical-related applications where modelling has contributed significantly to development and understanding of the techniques involved In particular, applications in the areas of mobile communications, magnetic resonance imaging, hyperthermal therapy and microwave radiometry are highlighted Finally, examples of modelling the potentially new medical applications of recent technologies such as ultra-wideband microwaves are discussed

https://iopscience.iop.org/article/10.1088/0031-9155/53/16/R01/pdf

Modelling the interaction of electromagnetic fields (10 MHz–10 GHz) with the human body: methods and applications

Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations.

Tissue heating during magnetic resonance measurements is a potential hazard at high-field MRI, and particularly, in the framework of parallel radiofrequency transmission. The heating is directly related to the radiofrequency energy absorbed during an magnetic resonance examination, that is, the specific absorption rate (SAR). SAR is a pivotal parameter in MRI safety regulations, requiring reliable estimation methods. Currently used methods are usually based on models which are neither patient-specific nor taken into account patient position and posture, which typically leads to the need for large safety margins. In this work, a novel approach is presented, which measures local SAR in a patient-specific manner. Using a specific formulation of Maxwell's equations, the local SAR is estimated via postprocessing of the complex transmit sensitivity of the radiofrequency antenna involved. The approximations involved in the proposed method are investigated. The presented approach yields a sufficiently accurate and patient-specific local SAR measurement of the brain within a scan time of less than 5 min.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.23322

Patient‐individual local SAR determination: In vivo measurements and numerical validation

The finite-difference time-domain (FDTD) method is combined with the method of moments (MoM) to compute the electromagnetic fields of shielded radio-frequency (RF) coils loaded with an anatomically accurate model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The combined method can predict both the specific energy absorption rate (SAR) and the magnetic field (known as the B/sub 1/ field) excited by any RF coils. Results for SAR and B/sub 1/ field distribution, excited by shielded and end-capped birdcage coils, are calculated at 64, 128, 171, and 256 MHz. The results show that the value of SAR increases when the frequency of the B/sub 1/ field increases and the B/sub 1/ field exhibits a strong inhomogeneity at high frequencies.

Numerical simulation of SAR and B/sub 1/-field inhomogeneity of shielded RF coils loaded with the human head

http://ieeexplore.ieee.org/iel4/10/15067/00686805.pdf

Zhaomei Feng

Papers

Numerical simulation of SAR and B/sub 1/-field inhomogeneity of shielded RF coils loaded with the human head

Correction to "Numerical Simulation Of SAR And B/sub 1/-field Inhomogeneity Of Shielded RF Coils Loaded With The Human Head"