Modeling tissue heating from exposure to RF energy & relevance of tissue heating to exposure limits

Abstract

Modeling tissue heating from exposure to RF energy & relevance of tissue heating to exposure limits Foster, KR, Ziskin MC, Balzano Q; Bit-Babik G. Modeling Tissue Heating From Exposure to Radiofrequency Energy and Relevance of Tissue Heating to Exposure Limits: Heating Factor. Health Physics: 115(2):295-307. Aug 2018 Abstract This review/commentary addresses recent thermal and electromagnetic modeling studies that use image-based anthropomorphic human models to establish the local absorption of radiofrequency energy and the resulting increase in temperature in the body. The frequency range of present interest is from 100 MHz through the transition frequency (where the basic restrictions in exposure guidelines change from specific absorption rate to incident power density, which occurs at 3-10 GHz depending on the guideline). Several detailed thermal modeling studies are reviewed to compare a recently introduced dosimetric quantity, the heating factor, across different exposure conditions as related to the peak temperature rise in tissue that would be permitted by limits for local body exposure. The present review suggests that the heating factor is a robust quantity that is useful for normalizing exposures across different simulation models. Limitations include lack of information about the location in the body where peak absorption and peak temperature increases occur in each exposure scenario, which are needed for careful assessment of potential hazards. To the limited extent that comparisons are possible, the thermal model (which is based on Pennes' bioheat equation) agrees reasonably well with experimental data, notwithstanding the lack of theoretical rigor of the model and uncertainties in the model parameters. In particular, the blood flow parameter is both variable with physiological condition and largely determines the steady state temperature rise. We suggest an approach to define exposure limits above and below the transition frequency (the frequency at which the basic restriction changes from specific absorption rate to incident power density) to provide consistent levels of protection against thermal hazards. More research is needed to better validate the model and to improve thermal dosimetry in general. While modeling studies have considered the effects of variation in thickness of tissue layers, the effects of normal physiological variation in tissue blood flow have been relatively unexplored. ncbi.nlm.nih.gov Conclusion Advances in computational dosimetry and thermal modeling, reviewed in this paper, have filled in many details about the relation between psSAR and peak increase in temperature. These studies show that the heating factor is a robust measure of this relation. However, additional information is needed to assess potential thermal hazards of RF exposure, including the location and magnitude of the temperature increases in the body. This present and two previous reviews in this series (Foster et al. 2016, 2017) show that thermal models can be useful for revising and updating RF exposure limits. While the theory that underlies the models, Pennes' BHTE, is generally reliable, it is not exact. Thermal models for RF heating of tissue require additional experimental validation including effects of variability in tissue blood perfusion, possible thermoregulatory responses of the body to RF heating, and other factors. Finally, more experimental data are needed for thermal hazards of RF energy at frequencies above the threshold. Apart from thermal hazards, a comprehensive review of all reported biological effects of RF energy above the transition frequency in the standards is also needed. Q.B., K.R.F, and M.Z. were supported in this project by Mobile and Wireless Forum, which did not review and had no control over preparation of this manuscript. The authors thank A. Hirata, Nagoya Institute of Technology, Japan, for providing results used to prepare Fig. 3, and Dr. Vitas Anderson and Dr. C-K Chou for helpful discussion of earlier drafts of this paper. The views and opinions expressed herein are solely those of the authors and are not to be attributed to Motorola Solutions or any of its operating companies.

AI evidence extraction

Main findings

This review/commentary summarizes image-based anthropomorphic modeling studies (100 MHz to 3–10 GHz transition region) linking local RF absorption to resulting tissue temperature rise. It suggests the “heating factor” is a robust quantity for normalizing exposures across different simulation models and that thermal models based on Pennes’ bioheat equation agree reasonably well with experimental data to the limited extent comparisons are possible, but notes important uncertainties and missing information (e.g., locations of peak absorption/temperature rise, variability in blood flow) relevant to hazard assessment and exposure-limit setting.

Outcomes measured

• Local absorption of RF energy (dosimetry)
• Tissue temperature rise / tissue heating
• Heating factor (dosimetric quantity)
• Relevance to RF exposure limits (SAR vs incident power density transition frequency)

Limitations

• Lack of information about where in the body peak absorption and peak temperature increases occur for each exposure scenario
• Thermal model (Pennes’ bioheat equation) lacks theoretical rigor/is not exact and has uncertain parameters
• Blood flow (perfusion) parameter is variable with physiological condition and strongly affects steady-state temperature rise
• Need for additional experimental validation of thermal models, including variability in tissue blood perfusion and possible thermoregulatory responses
• Limited experimental data for thermal hazards at frequencies above the transition frequency
• Physiological variation in tissue blood flow relatively unexplored in modeling studies

Suggested hubs

• who-icnirp (0.62)
  Discusses revising/updating RF exposure limits and the SAR-to-power-density transition frequency used in guidelines.

{
    "study_type": "review",
    "exposure": {
        "band": "RF",
        "source": null,
        "frequency_mhz": null,
        "sar_wkg": null,
        "duration": null
    },
    "population": null,
    "sample_size": null,
    "outcomes": [
        "Local absorption of RF energy (dosimetry)",
        "Tissue temperature rise / tissue heating",
        "Heating factor (dosimetric quantity)",
        "Relevance to RF exposure limits (SAR vs incident power density transition frequency)"
    ],
    "main_findings": "This review/commentary summarizes image-based anthropomorphic modeling studies (100 MHz to 3–10 GHz transition region) linking local RF absorption to resulting tissue temperature rise. It suggests the “heating factor” is a robust quantity for normalizing exposures across different simulation models and that thermal models based on Pennes’ bioheat equation agree reasonably well with experimental data to the limited extent comparisons are possible, but notes important uncertainties and missing information (e.g., locations of peak absorption/temperature rise, variability in blood flow) relevant to hazard assessment and exposure-limit setting.",
    "effect_direction": "unclear",
    "limitations": [
        "Lack of information about where in the body peak absorption and peak temperature increases occur for each exposure scenario",
        "Thermal model (Pennes’ bioheat equation) lacks theoretical rigor/is not exact and has uncertain parameters",
        "Blood flow (perfusion) parameter is variable with physiological condition and strongly affects steady-state temperature rise",
        "Need for additional experimental validation of thermal models, including variability in tissue blood perfusion and possible thermoregulatory responses",
        "Limited experimental data for thermal hazards at frequencies above the transition frequency",
        "Physiological variation in tissue blood flow relatively unexplored in modeling studies"
    ],
    "evidence_strength": "insufficient",
    "confidence": 0.7399999999999999911182158029987476766109466552734375,
    "peer_reviewed_likely": "yes",
    "keywords": [
        "radiofrequency energy",
        "tissue heating",
        "thermal modeling",
        "computational dosimetry",
        "anthropomorphic human models",
        "heating factor",
        "Pennes bioheat equation",
        "specific absorption rate",
        "incident power density",
        "transition frequency",
        "exposure limits"
    ],
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            "reason": "Discusses revising/updating RF exposure limits and the SAR-to-power-density transition frequency used in guidelines."
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}

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