The temperature rises in the human eye for plane wave exposures are investigated in the frequency range between 600 MHz and 6 GHz, which covers the hot spot frequency range. As a first step, the specific absorption rates (SARs) are calculated with the use of the finite-difference time-domain (FDTD) method and the mechanism of hot-spot formation is discussed. Then the temperature rises in the human eye are calculated by using Pennes' bioheat equation. In addition, the dependence of SARs and temperature rises on the electromagnetic (EM) wave polarization and the eye dimension is discussed. Furthermore, the temperature rises calculated are compared with the values found in the literature pertaining to microwave-induced cataract formation. Numerical results show that hot spots appear in a certain frequency range and that the location and number of hot spots depend on the frequencies of the incident wave. In particular, the averaged SARs and the temperature rise are found to depend obviously on the polarization of the EM wave. Additionally, the deviations in the SAR and the temperature rise caused by the eye size are found to be within 10%. Furthermore, the maximum temperature rise due to the incident EM power density of 5.0 mW/cm/sup 2/, which is the maximum permissible exposure limit for controlled environments, is found to be 0.30/spl deg/C at 6.0 GHz. This value is small but not negligible, as compared with the threshold temperature rise 3.0/spl deg/C for cataract formation.

Temperature rises in the human eye exposed to EM waves in the frequency range 0.6-6 GHz

Purpose: To present and discuss numerical calculations of the specific absorption rate (SAR) and temperature in comparison to regulatory limits. While it is possible to monitor whole-body or whole-head average SAR and/or core body temperature during MRI in practice, this is not generally true for local SAR values or local temperatures throughout the body. While methods of calculation for SAR and temperature are constantly being refined, methods for interpreting results of these calculations in light of regulatory limits also warrant discussion. Materials and Methods: Numerical calculations of SAR and temperature for the human head in a volume coil for MRI at several different frequencies are presented. Results: Just as the field pattern changes with the frequency, so do the temperature distribution and the ratio of maximum local SAR (in 1-g or 10-g regions) to whole-head average SAR. In all of the cases studied here this ratio is far greater than that in the regulatory limits, indicating that existing limits on local SAR will be exceeded before limits on whole-body or whole-head average SAR are reached. Conclusion: Calculations indicate that both SAR and temperature distributions vary greatly with B1 field frequency, that temperature distributions do not always correlate well with SAR distributions, and that regulatory limits on local temperature may not be exceeded as readily as those on local SAR.

/pdf/sar-and-temperature-simulations-and-comparison-to-regulatory-53enh3jg7o.pdf

SAR and temperature: simulations and comparison to regulatory limits for MRI.

This paper presents electromagnetic energy absorption in the homogeneous and layered human body models due to body-worn UWB antennas, at frequencies of 3, 6 and 8 GHz. Typical small planar UWB antennas are used in this study: printed UWB disc monopole and UWB slot antenna. Distances of 2, 5 and 10 mm (reactive near-field region) between antennas and human body were chosen, approximating realistic scenarios of operation in Wireless Body Area Networks. To approximate different parts of the human body, or body variations among different users, we compare results obtained for the planar homogeneous (muscle) model with those for three-layer body models (skin, fat and muscle), with different thicknesses of the skin (0.5- 2 mm) and fat (1-9 mm) tissue. For these body models we investigate the electromagnetic energy absorption mechanism by examining the peak 1-g SAR and peak SAR (without mass averaging). Based on our results we present and discuss new finding concerning the general electromagnetic energy absorption mechanism in human tissues under reactive near-fields exposure conditions.

/pdf/em-energy-absorption-in-the-human-body-tissues-due-to-uwb-4p149tzheu.pdf

Em energy absorption in the human body tissues due to uwb antennas

This paper attempts to correlate the maximum temperature increase in the head and brain with the peak specific absorption rate (SAR) value due to handset antennas. The rationale for this study is that physiological effects and damage to humans through electromagnetic-wave exposure are induced by temperature increases, while the safety standards are regulated in terms of the local peak SAR. For investigating these correlations thoroughly, the total of 660 situations is considered. The numerical results are analyzed on the basis of statistics. We find that the maximum temperature increases in the head and brain can be estimated in terms of peak SARs averaged over 1 and 10 g of tissue in these regions. These correlations are less affected by the positions, polarizations, and frequencies of a dipole antenna. Also, they are reasonably valid for different antennas and head models. Further, we discuss possible maximum temperature increases in the head and brain for the SAR values prescribed in the safety standards. They are found to be 0.31/spl deg/C and 0.13/spl deg/C for the Federal Communications Commission Standard (1.6 W/kg for 1 g of tissue), while 0.60/spl deg/C and 0.25/spl deg/C for the International Commission on Non-Ionizing Radiation Protection Standard (2.0 W/kg for 10 g of tissue).

Correlation of maximum temperature increase and peak SAR in the human head due to handset antennas

The specific absorption rate (SAR) is a limiting factor in high-field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single-channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so-called Q-matrices and further computational optimizations allow for a real-time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight-channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high-field MR.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.24138

A specific absorption rate prediction concept for parallel transmission MR.

The finite-difference time-domain (FDTD) is the most often used method for evaluation of electromagnetic fields in human tissue. This is of particular importance for compliance testing of portable telephones. The specific absorption rate (SAR) averaged over 1 or 10 g tissue mass is required by regulations not to exceed a prescribed value. A computer algorithm is developed for post-processing the FDTD data. This algorithm can be used for uniform as well as graded meshes, while the required cubical shape is maintained. Extensive investigations are shown of the resultant SAR due to the number of electric field components used in field averaging, and the volume of air within the cube. The latter is easily controlled in the algorithm, but the regulations on the permissible SAR do not specify it at the present time.

An algorithm for computations of the power deposition in human tissue

Many wireless communication devices are used with their antennas in close proximity to the human head. This results in changes in the antenna characteristics that affect the communications performance and in the deposition of microwave energy in the user's head. The amount of power deposited in the human body is limited to a given maximum value, according to current USA (FCC) and other regulation. We show here the feasibility and effectiveness of the FDTD method for complete evaluation of the human interaction with the performance of antennas for hand-held devices. A dual-frequency antenna and single-band monopole antennas are evaluated. Additionally, an accurate algorithm for computations of the power deposited in 1 g of tissue is introduced.

M. Okoniewski

Papers

An algorithm for computations of the power deposition in human tissue

Modeling of human interaction with antennas using the finite difference time domain technique