Summary of Oral Evidence presented to IEGMP by Dr R A Lerski, Medical Physics Department, Tayside University Hospitals, on Friday 12 November 1999

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The presentation by Dr Lerski was based mainly on a recent publication by Van Leeuwen et al. (Physics in Medicine and Biology, 44, 2367–79 (1999)) regarding modelling of temperature rises in the head during use of a mobile phone. Essentially, two approaches may be used to estimate temperatures in the head, measurement in a physical phantom, or mathematical modelling. The problem with the former is that it is impossible to model the effects of blood flow. Early mathematical models also failed to take account of blood flow and produced specific energy absorption (SAR) values of around 0.16–2.8 W/kg. However, Van Leeuwen et al. have used data from three-dimensional magnetic resonance imaging (MRI) at a resolution of 1 mm to produce a mathematical model with realistic anatomy and vasculature. The model is segmented into different tissues, each of which is then assigned values for thermal and dielectric properties. By performing perfusion calculations based on Pennes bioheat transfer, it is possible to show that although localised SARs in very small volumes may be quite high, temperature rises are negligible and comply with basic restrictions; the greatest increases were found in surface tissues, particularly muscle. Dr Lerski is of the opinion that more work is needed to validate this model. In particular, he questions its robustness, segmentation into tissue types, and reliability of dielectric and thermal values for different tissues; the authors need to assess how inaccuracies in these variables might affect the result. He would also like to see models for a child's head, a head with reduced perfusion, and a head with reduced skin cooling. Experimental verification of the model would help, but is likely to be problematic. MRI could be used, but mobile phone exposure might not be possible in a scanner. Thermocouples would perturb the field and non-perturbing probes lack adequate resolution.

It was noted that the work was essentially a development of the modelling work previously presented to the Group by Dr Dimbylow; SAR values would be critically dependent on the proximity of the antenna on the phone to the head. Whilst models that incorporate blood flow were seen as a definite advance, it was felt that more information on the model was required. It was noted that the output from any mathematical model depends on the reliability of the input values.

There was some discussion on the magnitude of temperature changes that could be regarded as acceptable. Although temperature rises in excess of 1°C may occur during fever without obvious adverse consequences, small changes in other parameters such as blood pressure could be important even though they fall within the range of normal variation. There are animal data suggesting that temperature increases of up to 4°C do not cause significant damage, although increases as low as 1°C may cause damage in the eye. It was felt that it may be useful to examine data about heating during MRI and ultrasound investigations.

The robustness of the model could be tested using statistical techniques. It was recognised that a model based on data from a single individual would have limitations, and it was suggested that it may be possible to develop a more general model by combining data from several subjects. It was also suggested that segmentation of brain tissue may have been restricted to grey and white matter, and so could be improved.

First issued 5 April 2000