EVALUATION OF RESONANCE PROPERTIES OF THE HUMAN BODY MODELS SITUATED IN THE RF FIELD

EVALUATION OF RESONANCE PROPERTIES OF THE HUMAN BODY MODELS SITUATED IN THE RF FIELD Elena COCHEROVA 1, Gabriel MALICKY 1, Peter KUPEC 1, Vladimir STOFANIK 1, Joze PUCIK 1 1 Institute o Eletronis and Photonis, Faulty o Eletrial Engineering and Inormation Tehnology, Slovak University o Tehnology in Bratislava, Ilkoviova 3, Bratislava, Slovak Republi elena.oherova@stuba.sk, g.maliky@gmail.om, peter.kupe@stuba.sk, vladimir.stoanik@savba.sk, joze.puik@stuba.sk Abstrat. The radio-requeny (RF) eletromagneti ield absorption in the human body model is investigated in this artile. Absorption o prolate model o the biologial body exhibits resonane properties in the ase o so alled E polarization. This ours in the arment when the longitudinal axis o the body is parallel with an eletrial omponent o the eletromagneti ield. Absorption properties o a homogeneous human model have been explored in the requeny rom MHz to 5 MHz in the CST Mirowave Studio programme environment. The simulation results o RF ield absorption in the model o the human body are presented or ellipsoidal, ylindrial and anthropomorphi Laura model shapes, and or various heights and widths o the model body. The values o the resonane requeny and also whole-body absorption values at these requenies are evaluated. Keywords Absorption, CST MW studio, human body model, resonane properties, RF ield, SAR.. Radio-Frequeny Dosimetry A speii absorption rate (SAR) is utilized as the basi measurement in RF ield dosimetry, [1], [], [3]. Beore SAR an be quantiied inside the exposed biologial body, the internal eletri ield E must be omputed. Consequently, SAR values an be evaluated rom internal eletri ield E values: σe SAR, (1) ρ where: is tissue ondutivity and is the mass density o the biologial tissue. Although SAR is a point variable, it is usually evaluated as an average value over 1 grams o tissue, or over the whole body as total SAR. Absorption in the human body depends on many parameters o the ield and many parameters within the irradiated body [], [4]. For ar-ield exposure onditions (plane wave), absorption diers signiiantly or the three basi E-, H- and P- polarizations [5], when the eletri, magneti or Poynting vetor is parallel to the longitudinal axis o the body. Here, an ellipsoidal representation o the human body is shown in Fig. 1. 1. Introdution There has been ontinuously inreasing industrial, domesti and other appliations o RF eletromagneti ields in reent deades. Consequently, the question o biologial and health eets o these ields on human beings requires examination. Sine the thermal eets o RF ields are o onsiderable importane, an assessment o the ield absorption inside the human body is the primary task [1], []. Even though, external RF ields are relatively easy to measure, the measurement o the internal ield inside the human body is not possible in the majority o ases. Numerial omputation [3] is onsidered most suitable or assessment o internal body ields. E-polarization H-polarization P-polarization Fig. 1: Plane wave polarization dependent on the orientation o the longitudinal axis (z-axis) in an ellipsoidal model. 1 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 345

Normalized SAR [W/kg per W/m ] Partial-body (head) Sub-resonane Resonane Whole-body Hot-spot Surae absorption 3 3 4 3 [MHz] Fig. : Normalized whole-body average SAR dependene on the requeny in E-polarization. With respet to the ield absorption in a human body or E-polarization, the RF an be subdivided into our regions [], whih are shown in Fig. : The sub-resonane has requenies less than 3 MHz. In this, the whole-body average absorption inreases rapidly with requeny. The resonane or whole-body absorption resonane is rom 3 MHz to approximately 3 MHz, and higher requenies or partial body resonanes, or example, in the head. In this, the absorption reahes its maximum at resonane requeny. The "hot-spot" extends rom approximately 4 MHz to about 3 GHz, and loalized energy absorption an our in this. The surae absorption here has requenies greater than approximately 3 GHz, so that energy absorption and temperature elevation is loalized at the body surae. Fig. 3: The Laura model o a woman s body. Models were exposed to the plane wave (representing ar-ield exposure onditions) with power density o 1 W m -. Sine prolate bodies exhibit pronouned resonane properties only or E-polarization, most o the simulations were perormed or E- polarization. I not speiied, E-polarization is applied. Laura model was irradiated laterally. Simulations were perormed at requenies rom MHz to 5 MHz in the CST Mirowave Studio programme environment [7]. 3. Model Parameters For assessment o energy absorption in the human body, we use several homogeneous model types: the simpliied models o ylindrial and ellipsoidal shape and the anthropomorphi model o the emale body (the Laura model in Fig. 3). In order to simulate new-born, younger and older hildren and adults bodies, the ylindrial and ellipsoidal model maintains a onstant height to width ratio o 3:1 while the body height is inreasing. For example, models with height h =,5 m,,6 m, 1, m and 1,8 m have widths w =,17 m,, m,,4 m and,6 m, respetively. The height o the Laura model is 1,8 m. These models onsist o musle tissue. Relative magneti permeability o μ r = 1, and a mass density o 13 kg m -3 were assumed or this tissue. The requeny dependent parameter values o permittivity and eletrial ondutivity were adopted rom reerene number [6]. The omputational spae around the model was illed by vauum. 4. Simulation Results An example o spatial distribution o SAR (1 g average) values or the ellipsoidal model with height h = 1,8 m at requenies o 6 MHz and 5 MHz or H-polarization is shown in Fig. 4. Spatial distributions o SAR values or ylindrial models with height h =,6 m and h = 1,8 m or E-polarization are shown in Fig. 5 to Fig. 8. Fig. 4: Distribution o SAR (1 g) in the ellipsoidal model o h = 1,8 m and ratio 3:1 (height: width), at 6 MHz and 5 MHz, H-polarization. 1 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 346

SAR [W/kg] SAR or the onstant height to width ratio o 3:1 or ellipsoidal models with heights h =,5 m,,6 m, 1, m and 1,8 m are shown in Fig. 9. It is obvious that smaller model heights have higher resonane requeny, and a higher total SAR at the resonane requeny. Thereore, the body resonane requeny o hildren is higher than that or adults, and the total SAR at the resonane requeny is also higher in hildren. Fig. 5: Distribution o SAR (1 g) in the ylindrial model o h =,6 m and ratio 3:1 (height: width) at 1 MHz. Fig. 6: Distribution o SAR (1 g) in the ylindrial model o h =,6 m and ratio 3:1 (height: width) at MHz. Fig. 9: Frequeny dependene o whole-body average (total) SAR in the ellipsoidal model o the human body with dierent height h (onstant height to width ratio o 3:1). 9 x 1-3 8 7 ylinder: h =.6 m w =. m, E-pol. elipsoid: h =.6 m w =. m, E-pol. ylinder: h = 1.8 m w =.6 m, E-pol. elipsoid: h = 1.8 m w =.6 m, E-pol. elipsoid: h = 1.8 m w =.6 m, H-pol. 6 5 4 3 1 1 3 4 5 6 [MHz] Fig. 7: Distribution o SAR (1 g) in the ylindrial model o h = 1,8 m and ratio 3:1 (height: width) at 6 MHz (near the resonane requeny). Fig. 8: Distribution o SAR (1 g) in the ylindrial model o h = 1,8 m and ratio 3:1 (height: width) at MHz. Frequeny dependenes o whole-body average Fig. 1: Frequeny dependene o whole-body average (total) SAR in the ellipsoidal and ylindrial model o the human body with dierent height h (onstant height to width ratio o 3:1). Frequeny dependenies o whole-body average SAR or the ylindrial and ellipsoidal models o heights h =,6 m and 1,8 m with onstant height to width ratio o 3:1 are shown in Fig. 1. There are depited dependenies or E-polarization and or omparison also one example or H-polarization. Contrary to E-polarization, the resulting wholebody average SAR is inreasing monotonially with requeny or H-polarization. As shown, the resonane requeny is somewhat smaller or the ylindrial model, while the total SAR at the resonane requeny is a little higher in the ylindrial model than in an ellipsoidal model with the same height. 1 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 347

SAR at r [W/kg] r [MHz] SAR [W/kg]..18.16.14.1.1.8.6.4. elipsoid: h = 1.8 m, w =. m elipsoid: h = 1.8 m, w =.3 m Laura: h = 1.8 m ylinder: h = 1.8 m, w =.3 m ylinder: h = 1.8 m, w =.6 m 4 6 8 1 1 14 16 18 [MHz] Fig. 11: Frequeny dependene o whole-body average SAR in the ellipsoidal, ylindrial and Laura model o the emale body with height h = 1,8 m and dierent model widths. 4 35 3 5 15 1 5 r1 (h) = / / h r (h) = / / h / 1. r3 (h) = / / h / 1.5 elipsoid, ratio (h/w) o 3:1 ylinder, ratio (h/w) o 3:1 Laura, h = 1.8 m.5 1 1.5 h [m] Fig. 1: Resonane requeny dependene on the height h o the ellipsoidal and ylindrial body model (with the onstant 3:1 ratio o height to width) and the Laura model..1.1.8.6.4. SAR r (h) =.8 / ( h -.3 ) SAR r3 (h) =.31 / ( h -.4 ) elipsoid, ratio (h/w) o 3:1 ylinder, ratio (h/w) o 3:1.4.6.8 1 1. 1.4 1.6 1.8. h [m] Fig. 13: Whole-body average SAR at the resonane requeny or the ellipsoidal body model and ylindrial model (with onstant height to width ratio o 3:1). models are quite similar (it is slightly smaller or the ylindrial models), sine it is mainly determined by equal model heights. Total SAR at the resonane requeny is higher or the Laura model than or an ellipsoidal model with,3 m width and lower than or the ellipsoidal model with, m width. We presume that this is a onsequene o lateral irradiation o the Laura model, whih is via the smaller side dimension. The values or resonane requenies or dierent models resulting rom the simulations are in Fig. 1. From [8], it is onsidered that the absorption reahes a maximum (resonane requeny r ) when the height o the model equals hal the wavelength (λ): where is the speed o light. h 1 h, () r The resonane requeny values or the ellipsoidal and ylindrial models (Fig. 1) are lower than estimated by the relationship in (). Thereore, we suggest that a more aurate approximation o resonane requeny or these models is satisied by the relationship: r h 1, h, (3) or ellipsoidal models o onstant height to width ratio o 3:1. Similarly, we an orret the relationship or ylinders with a onstant height to width ratio o 3:1 by: r3 h 1,5 h. (4) Furthermore, we suggest, that the approximate relationship o the whole-body average SAR at the resonane requeny shown in Fig. 13 or the ellipsoidal model with a onstant height to width ratio o 3:1 is: SAR r,8 h,3 h. (5) In addition, the whole-body average SAR at the resonane requeny or the ylindrial model with a onstant height to width ratio o 3:1 is in the orm: SAR r3,31 h,4 h. (6) The whole-body average SAR at the resonane requeny o the ylindrial model is very lose to that o the ellipsoidal model with a onstant height to width ratio o 3:1. Figure 11 depits requeny dependenies o the whole-body average SAR or several models o the same height: the ellipsoidal model with height h = 1,8 m (and width w =, m or w =,3 m), the ylindrial model with height h = 1,8 m (and width w =,3 m or w =,6 m) and or the anthropomorphi emale model o Laura with a height o h = 1,8 m. The resonane requenies or all 5. Conlusion The dependenies o resonane attributions involving the resonane requeny and the whole-body average SAR at the resonane requeny on parameters in a human model 1 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 348

were investigated in this artile. We observed that the values o resonane requeny are quite similar or the seleted ellipsoidal, ylindrial and anthropomorphi body model types when idential height was involved. The results o simulations show that although the body resonane requeny is mainly dependent on the body height, body shape parameters must also be inluded or more preise estimation. The whole-body average SAR at the resonane requeny or models o the same height depends markedly on additional model parameters, and this espeially reers to the width o the model. Resonane properties are present at requenies were wavelength is omparable with body height. For lower requenies, penetration depth is quite large (tens o entimeters), so only small part o transmitted eletromagneti wave in the body is absorbed. For muh higher requenies than resonane requeny, the penetration depth is only ew millimeters, so pratially all transmitted wave is absorbed. Thereore, the wholebody average SAR is not urther inreasing. Aknowledgements This work was supported by the Ministry o Eduation o Slovak Republi under grants VEGA No. 1/55/1 and KEGA No. 3/7411/9. Reerenes [1] GAJSEK, P., J. A. D'ANDREA, P. A. MASON, J. M. ZIRIA, T. J. WALTERS and W. D. HURT. Mathematial modeling using experimental and theoretial methods in evaluating speii absorption rates. Biologial Eets o Eletromagneti Radiation. Berlin: Springer, 3. ISBN 3-54-4989-1. [] BULMAN, A. Eletromagneti Fields (3 Hz to 3 GHz) Environmental Health Criteria: No: 137. Oupational and Environmental Mediine. 1994, vol. 51, iss. 1, pp. 7. ISSN 147-796. DOI: 1.1136/oem.51.1.7-a. [3] COCHEROVA, E. and V. STOFANIK. Numerial methods or solution o bioeletromagneti ields. Bratislava: Nakladatelstvo STU, 1. ISBN 978-8-7-37-7. [4] HIRATA, A., S. KODERA, J. WANG and O. OSAMU FUJIWARA. Dominant ators inluening whole-body average SAR due to ar-ield exposure in whole-body resonane requeny and GHz regions. Bioeletromagnetis. 7, vol. 8, iss. 6, pp. 484-487. ISSN 151-186X. DOI: 1.1/bem.335. [5] COCHEROVA, E., J. SURDA, O. ONDRACEK and V. STOFANIK. RF ield orientation inluene on the speii absorption rate in a biologial objet. In: Proeedings o the 14th Conerene on Mirowave Tehniques (COMITE 8). Prague: IEEE, 8, pp. 61-63. ISBN 978-1-444-137-4. DOI: 1.119/COMITE.8.4569938. [6] GABRIEL, C. and S. GABRIEL. Compilation o the dieletri properties o body tissues at RF and mirowave requenies. Home Page NirEm Ia [online]. 1997. Available at: http://nirem.ia.nr.it/dos/dielectric/home.html. [7] CST mirowave studio: HF design and analysis. Tutorials. Ben Gurions University: Department o Eletrial and Computer Engineering [online]. 6. Available at: http://www.ee.bgu.a.il/~mirowav/cst/tutorials.pd. [8] COCHEROVA, E., J. SURDA, J. PUCIK and V. STOFANIK. Dependene o the RF Field Absorption on the Human Body Dimensions. In: Proeedings o 19th International Conerene Radioelektronika 9. Bratislava: IEEE, 9, pp. 37-39. ISBN 978-1-444-3537-1. DOI: 1.119/RADIOELEK.9.515879. About Authors Elena COCHEROVA was born in Trenin in 197. She reeived her M.S. degree in radioeletronis in 1993 and Ph.D. degree in eletronis () rom the Faulty o Eletrial Engineering and Inormation Tehnology (FEI), the Slovak University o Tehnology (SUT) in Bratislava. Her researh interests inlude health aspets o non-ionizing radiation, biosignal proessing and simulation o eletrial ativities o exitable tissues. Gabriel MALICKY was born in Bojnie in 199. Currently, he is a student at the Institute o Eletronis and Photonis, Faulty o Eletrial Engineering and Inormation Tehnology o the Slovak University o Tehnology in Bratislava. His researh interests inlude modelling radio-requeny ield absorption in biologial objets. Peter KUPEC was born in 1983 in Slovakia. In 9, he obtained his M.S. degree in eletrial engineering rom the Slovak University o Tehnology in Bratislava. Currently, he is a Ph.D. student at the Institute o Eletronis and Photonis, Faulty o Eletrial Engineering and Inormation Tehnology o the Slovak University o Tehnology in Bratislava. His main ield o interest involves the modelling o radio-requeny ield inluene on biologial objets. Vladimir STOFANIK was born in Bratislava, Slovakia, in 1975. He obtained his M.S. degree in 1999 and the Ph.D. degree in 6 in eletrial engineering rom the Slovak University o Tehnology in Bratislava. Currently, he is a leturer at the Institute o Eletronis and Photonis, Faulty o Eletrial Engineering and Inormation Tehnology o the Slovak University o Tehnology in Bratislava. His main ield o interest overs design and simulation o eletroni equipment and radio ommuniations and navigation systems. Joze PUCIK was born in Bratislava, Slovakia, in 197. He obtained his M.S. degree in 1996 and the Ph.D. degree in in eletrial engineering rom the Slovak University o Tehnology in Bratislava. Currently, he is a leturer at the Institute o Eletronis and Photonis, Faulty o Eletrial Engineering and Inormation Tehnology o the Slovak University o Tehnology in Bratislava. His main ield o interest overs digital signal proessing, espeially biomedial signal proessing. 1 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 349