Dielectric properties of biological tissues at frequencies below 1 MHz. Azadeh Peyman

Transcription

1 Dielectric properties of biological tissues at frequencies below 1 MHz Azadeh Peyman

2 Introduction Dielectric properties of tissues: One of the main inputs required in the dosimetry studies involving electromagnetic fields (EMF) and human body To calculate the internal electric field strength and current density, and the energy deposition when tissues are exposed to EMF Strongly frequency dependant 1.E+08 1.E+06 1.E+04 1.E+02 Dielectric properties of Pig Liver Permittivity n ˆ n 1 ( j ) n (1 n) i j 0 1.E+00 Conductivity (S/m) 1.E-02 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09 1.E+11 Frequency(Hz) 2 Low Frequency Dielectric Properties of Tissues

3 State of knowledge Comprehensive literature review 1996 database- LFand RF Experimental measurements over ten frequency decade using three different techniques Development of a parametric model to describe the variation of dielectric properties of tissues as a function of frequency Main source of tissue dielectric data, extensively used by scientific community specially in dosimetric studies- Gabriel et al 1996 (3 papers) Shortcomings: Data mainly collected under in-vitro condition Only provides a best estimate of dielectric data at frequencies below 1MHz based on available knowledge Lack of comprehensive uncertainty assessment Only provides a best estimate of dielectric data at frequencies below 1MHz based on available knowledge 3 Low Frequency Dielectric Properties of Tissues

4 State of knowledge MTHR ( )- RF Literature review of all relevant papers published post 1996 In-vivo data collection Data as a function of animal age Comprehensive uncertainty analysis Acquisition of dielectric data for tissues missing in 1996 database (e.g. glands and body fluids) Main outcomes: Results were comparable to the 1996 data base for most of the tissues ( brain and abdominal tissues) Exceptionally higher values on both skull and long bone of different pigs compared to those reported in the Gabriel et al 1996 data base. Could be due to differences across species and age of animals- bone tissues difficult to sample Differences between data collected under in-vivo and in-vitro conditions are not systematic at microwave frequencies 4 Low Frequency Dielectric Properties of Tissues

5 Low Frequency Study The topic was reviewed in 2004 through a study funded by EPRI, carried out by MCL (now MCL-T) and the UK s Defence Science and Technology Laboratory (DSTL). Critical review of data for the conductivity of tissues at <1 MHz with emphasis on the data published post 1996 data base to highlight their usefulness and limitations. Development of a probe comprising a rectilinear array of four pin electrodes A coherent set of capacitance and conductance values for water and low concentration saline solutions, down to 1 Hz was produced Measurements were also carried out on a selection of porcine tissues under invivo condition The effect of electrode polarisation at low frequencies, and some other highfrequency effects were identified as measurement artefacts that rendered the determination of the capacitive part of the permittivity. 5 Low Frequency Dielectric Properties of Tissues

6 The paper was received with great interest within the scientific community. Across all IOP journals 3% of articles were accessed over 500 times on that year (editor of IOP) It has become a featured article; available free of charge on the internet. To date, it has been downloaded more than 6000 times since publication. 6 Low Frequency Dielectric Properties of Tissues

7 Conductance (S) Capacitance (F) Conclusion made by Gabriel et al Further work is needed to correct the problems identified in the study. On the positive side, the regularity and reproducibility of artefacts augurs well for the possibility of their avoidance or correction and hence for the opportunity of making error-free permittivity and conductivity measurement in future. 1,E-03 Water M M 0.001M 1,E-05 1,E-06 Water M M 0.001M 1,E-07 1,E-04 1,E-08 1,E-09 1,E-05 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06 Frequency (Hz) 1,E-10 1,E-11 1,E+00 1,E+02 1,E+04 Frequency (Hz) 1,E+06 7 Low Frequency Dielectric Properties of Tissues

8 Why do we need high quality dielectric data < 1MHz Medical applications Imaging (cancer imaging -Electrical Impedance Tomography EIT) There is often a significant contrast between normal and pathological tissue Therapeutic e.g. Electroporation Improvements in dosimetric tools has been highlighted in recent recommendations (ICNIRP 2010 and WHO 2007). To provide a better understanding of childhood exposure to ELF fields and advancing computational models of pregnant women and the foetus. There have also been call for refined micro-dosimetric models, to take into account the sub organ systems identified as being more sensitive to induced electric field effects. 8 Low Frequency Dielectric Properties of Tissues

9 Proposal Part 1: Development of an improved measurement technique Part 2: Provision of a data base of low frequency dielectric properties of tissues 9 Low Frequency Dielectric Properties of Tissues

10 Part 1- Development of an improved measurement technique Investigation of the artefacts observed by Gabriel et al 2009 through further measurement and computational modelling Identifying the design factors affecting the frequency dependence of the impedance of the probe. Development and construction of a new probe design in which the various artefacts are shifted outside the frequency range of interest. Testing the probe on a selection of well-known standard liquids such as saline solutions (by means of both measurements and modelling) 10 Low Frequency Dielectric Properties of Tissues

11 Part 2- Provision of a data base of low frequency dielectric properties of tissues Measurements of dielectric properties of selection of animal tissues using the newly developed probe, focusing on bone marrow, brain and muscle. Childhood leukaemia studies Extended measurements of low frequency dielectric properties of tissues including that of ageing tissues. Investigation of the anisotropy nature of tissues such as muscle Characterisation of dielectric properties of skin at low frequencies (the effect of layering on dielectric behaviour of skin) Emphasis by ICNIRP 2010 on the requirement to improve dosimetry for PNS. Suggestion that skin is the conservative substitute for peripheral nerve exposure. 11 Low Frequency Dielectric Properties of Tissues

12 References Gabriel, C., Gabriel, S., Corthout, E., 1996a. The dielectric properties of biological tissues: I. Literature survey. Phys. Med. Biol. 41, Gabriel, S., Lau, R.W., Gabriel, C., 1996b. The dielectric properties of biological tissues: II. Measurements in the frequency range of 10 Hz to 20 GHz. Phys. Med. Biol. 41, Gabriel, S., Lau, R.W., Gabriel, C., 1996c. The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues. Phys. Med. Biol. 41, Gabriel, C., Peyman, A., Grant, E.H., Electrical conductivity of tissue at frequencies below 1 MHz. Phys. Med. Biol. 54, ICNIRP 2010, Guidelines for Limiting Exposure to Time-Varying Electric and Magnetic Fields (1 Hz khz). Health Physics 99(6): ; Peyman, A., Holden, S., Gabriel, C., Dielectric Properties of Tissues at Microwave Frequencies, RUM3 MTHR Final Technical Report. Peyman, A., Holden, S.J., Watts, S., Perrott, R., Gabriel, C., Dielectric properties of porcine cerebrospinal tissues at microwave frequencies: in vivo, in vitro and systematic variation with age. Phys. Med. Biol. 52, Peyman, A., Gabriel, C., Cole-Cole parameters for the dielectric properties of porcine tissues as a function of age at microwave frequencies. Phys. Med. Biol. 55, N413-N419. Peyman, A., Gabriel, C., 2012 Dielectric properties of porcine glands, gonads and body fluids Phys. Med. Biol. 57 (2012) N339 N344 WHO 2007, Environmental Health Criteria 238, EXTREMELY LOW FREQUENCY FIELDS 12 Low Frequency Dielectric Properties of Tissues