Time-domain Method In Conjunction Research Articles

Design of miniaturized double-negative material for specific absorption rate reduction in human head.

In this study, a double-negative triangular metamaterial (TMM) structure, which exhibits a resounding electric response at microwave frequency, was developed by etching two concentric triangular rings of conducting materials. A finite-difference time-domain method in conjunction with the lossy-Drude model was used in this study. Simulations were performed using the CST Microwave Studio. The specific absorption rate (SAR) reduction technique is discussed, and the effects of the position of attachment, the distance, and the size of the metamaterials on the SAR reduction are explored. The performance of the double-negative TMMs in cellular phones was also measured in the cheek and the tilted positions using the COMOSAR system. The TMMs achieved a 52.28% reduction for the 10 g SAR. These results provide a guideline to determine the triangular design of metamaterials with the maximum SAR reducing effect for a mobile phone.

A comparative numerical study of rotating and stationary RF coils in terms of flip angle and specific absorption rate for 7 T MRI

While high-field magnetic resonance imaging promises improved image quality and faster scan time, it is affected by non-uniform flip angle distributions and unsafe specific absorption rate levels within the patient, as a result of the complicated radiofrequency (RF) field – tissue interactions. This numerical study explored the possibility of using a single mechanically rotating RF coil for RF shimming and specific absorption rate management applications at 7T. In particular, this new approach (with three different RF coil element arrangements) was compared against both an 8-channel parallel coil array and a birdcage volume coil, with and without RF current optimisation. The evaluation was conducted using an in-house developed and validated finite-difference time-domain method in conjunction with a tissue-equivalent human head model. It was found that, without current optimisation, the rotating RF coil method produced a more uniform flip angle distribution and a lower maximum global and local specific absorption rate compared to the 8-channel parallel coil array and birdcage resonator. In addition, due to the large number of degrees of freedom in the form of rotated sensitivity profiles, the rotating RF coil approach exhibited good RF shimming and specific absorption rate management performance. This suggests that the proposed method can be useful in the development of techniques that address contemporary RF issues associated with high-field magnetic resonance imaging.

NOVEL TRIANGULAR METAMATERIAL DESIGN FOR ELECTROMAGNETIC ABSORPTION REDUCTION IN HUMAN HEAD

In this paper, a novel triangular metamaterial (TMM) structure, which exhibits a resounding electric response at microwave frequency, is developed by etching two concentric triangular rings of conducting materials. A flnite-difierence time-domain method in conjunction with the lossy-Drude model was used in this study. Simulations were performed using the CST Microwave Studio R ∞ . The technique of speciflc absorption rate (SAR) reduction is discussed, and the efiects of the position of attachment, the distance, and the size of the metamaterials on the SAR reduction are explored. The performance of the novel TMMs in cellular phones was also measured in the cheek and the tilted positions using the COMOSAR system. The TMMs achieved a 50.82% reduction for 1gm SAR. These results provide a guideline to determine the triangular design of metamaterials with the maximum SAR reducing efiect for a cellular phone.

Design of photonic crystal power beam splitters via corrugated and gratinglike surfaces

In this work, we use the finite-difference time-domain method in conjunction with a genetic algorithm to design photonic crystal power beam splitters based upon a typical planar photonic crystal waveguide with corrugated surfaces or gratinglike surfaces covered behind the termination. Considering a power detector placed at different locations in the output field, we have obtained several beam splitters designs with different splitting angles. These beam splitters have high splitting efficiency and power intensity in the propagation direction.

Design of photonic crystal power couplers via gratinglike surfaces

This work deals with the inverse design in the field of photonic crystal based devices. We use the finite-difference time-domain method in conjunction with a genetic algorithm to design a power coupler between two photonic crystal waveguides. By optimizing the structure of gratinglike surfaces added near the photonic crystal waveguides, we have obtained several simple power couplers considering different coupling lengths. These couplers have high coupling efficiency as well as high power intensity in the propagation direction.

Genetic optimization of photonic crystal waveguide termination for both on-axis and off-axis highly efficient directional emission

In this work, we use the finite-difference time-domain method in conjunction with a genetic algorithm (GA) to design a photonic crystal waveguide with gratinglike surface added for highly-efficient directional emission. By placing a detector at different locations in the output field, we have obtained both on-axis and off-axis highly-efficient directional emission designs with the help of GA. The interference of light emitted from the outlet of waveguide and surfaces modes in gratinglike surface is believed to account for the directional beaming effect in our designs.

Development and Dosimetry Analysis of a 2-GHz Whole-Body Exposure Setup for Unrestrained Pregnant and Newborn Rats

For investigation of possible bio-effects of the 2-GHz wideband code division multiple access cellular system on pregnant and newborn rats, we have developed an unrestrained whole-body exposure setup, which employs two dipole antennas to induce a circularly polarized field at the location of the rats. The dosimetric results, by using the finite-difference time-domain method in conjunction with anatomical rat models, have confirmed the realization of a circular polarization in the exposure space, and have also revealed that the exposure setup can maintain a relative variation of the whole-body average specific absorption rate (SAR) within plusmn60% of the designed average level for various positions of rats. These findings show that the exposure setup is reasonable and acceptable in view of the actual SAR variation in a human body due to RF exposure from base stations.

Dosimetry Evaluation for Pregnant and Fetus Rats in a Near-Field Exposure System of 1.95-GHz Cellular Phones

In this letter, the dosimetry for pregnant rats and their fetuses in an in vivo exposure setup is conducted by using the finite-difference time-domain method in conjunction with anatomically based pregnant rat models. The exposure setup has been developed for testing possible indirect biological effects on fetuses when the pregnant rats are exposed to 1.95-GHz digital cellular phones. As a result, it is found that the whole-body average specific absorption rate (SAR) for the fetuses varies between 0.02-0.40 W/kg when the brain-average SAR for the mother rats varies between 1.4-2.6 W/kg.

Detection of buried dielectric cavities using the finite-difference time-domain method in conjunction with signal processing techniques

We address the problem of detecting low-dielectric contrast cavities buried deep in a lossy ground by using the finite-difference time-domain (FDTD) method in conjunction with signal processing techniques for extrapolation and object identification. It is well known that very low frequency probing is needed for deep penetration into the lossy ground, owing to a rapid decay of electromagnetic (EM) waves at higher frequencies. It is also recognized that numerical modeling using the FDTD method becomes very difficult, if not impossible, when the operating frequency becomes as low as 1 Hz. To circumvent this difficulty, we propose a hybrid approach in this paper that combines the FDTD method with signal processing techniques, e.g., rational function approximation and neural networks (NNs). Apart from the forward problem of modeling buried cavities, we also study the inverse scattering problem-that of estimating the depth of a buried object from the measured field values at the surface of the Earth or above. Numerical results for a buried prism are given to illustrate the application of the proposed technique.

A model for predicting electromagnetic interference of implanted cardiac pacemakers by mobile telephones

A prediction of the electromagnetic interference (EMI) of pacemakers due to mobile phones is significant in improving the immunity of pacemakers. The Pacemaker Committee of Japan recently conducted immunity tests of pacemakers for mobile phones, and consequently concluded that the connector between the pacemaker housing and the lead wire of the electrode plays a major role for the EMI due to mobile phones. Based on this finding, a computer model for predicting the EMI level has been presented, in which the internal impedance seen from the connector was considered as a load, and the metal portions consisting of the pacemaker housing and the lead wire of the electrode were considered as two elements of a receiving antenna. Interference voltages induced through the connector were analyzed by using the finite-difference time-domain method in conjunction with a torso and mobile phone model. The modeling was validated by comparison with previously reported experimental results.