Document Detail


Heating of tissues by microwaves: a model analysis.
MedLine Citation:
PMID:  9771585     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau1) pertains to heat convection by blood flow, and is of the order of 20-30 min for physiologically normal perfusion rates; the second (tau2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau1 and tau2. In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard.
Authors:
K R Foster; A Lozano-Nieto; P J Riu; T S Ely
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, Non-P.H.S.    
Journal Detail:
Title:  Bioelectromagnetics     Volume:  19     ISSN:  0197-8462     ISO Abbreviation:  Bioelectromagnetics     Publication Date:  1998  
Date Detail:
Created Date:  1998-12-15     Completed Date:  1998-12-15     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  8008281     Medline TA:  Bioelectromagnetics     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  420-8     Citation Subset:  IM; S    
Affiliation:
Department of Bioengineering, University of Pennsylvania, Philadelphia 19104-6392, USA. kfoster@eniac.seas.upenn.edu
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MeSH Terms
Descriptor/Qualifier:
Heating*
Humans
Microwaves*
Models, Biological
Pain
Time Factors

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine


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