Dosimetry as a Function of Anatomy and Exposure Source

Transcription

1 Dosimetry as a Function of Anatomy and Exposure Source Andreas Christ, Marie-Christine Gosselin, Marcel Zefferer, Stefan Benkler, Eugenia Cabot, Sven Kühn and Niels Kuster Foundation for Research on Information Technologies in Society, Swiss Federal Institute of Technology, Zürich

2 Objectives review fundamental dosimetric quantities discuss characteristics of sources of RF and ELF electromagnetic fields and their numerical representation discuss impact of anatomical properties and incident field conditions on the absorption in the body present the latest developments in human body modeling

3 Dosimetry vs. Incident Exposure Human exposure assessment is the discipline of dosimetry. Incident field exposure assessment is only a proxy of human exposure since its correlation with induced field values is poor in the best case. However, incident field exposure is sufficient to demonstrate compliance because it is easy to measure and the following can be derived: X V/m ---> <Y W/kg anywhere in the body X A/m ---> <Z W/kg anywhere in the body In general, compliance is only achieved if both values, i.e., the electric and mangetic fields, are below the derived limits. Therefore, limits based on power density should not be used in the standards since it is only valid in the special case of plane-wave exposure. -> in summary: incident exposure quantities have little correlation with dosimetry but are suitable for compliance testing if E- and H-fields are considered.

4 Meaningful Quantities whole-body absorption (to protect from unwanted thermal loads) locally induced temperature increases (to protect from locally induced unwanted thermal bioresponses) locally induced field values (to protect from possible athermal and non-thermal field effects) SAR -> in summary: standards are rather unclear about the rationale of the selected quantities.

5 SAR Averaging Volume The averaging of the peak spatial SAR (pssar) in a cubical volume is a historical selection and has NO physical meaning. It is greatly direction dependent and therefore difficult/time consuming to determine (to our knowledge nobody has ever evaluated the maximum cube values). contiguous tissue is a large improvements over the cube (unique definition) but is more conservative and may overestimate the thermal correlations. -> in summary: the concept contiguous tissue is superior in terms of non-ambiguous definition of the assessment but has not been enforced yet.

6 Characteristics of EM Field Sources far-field like RF sources near-field RF sources ELF sources (quasistatic approximation)

7 Far-Field Like RF Sources electric field magnetic field measured E- and H-fields in front of a base station antenna at 1840MHz Normalized Field Strength in db geometry or distance to body > approx. λ/2 directed power flux (wave vector k is real), E and H correlated by Z (depending on dimensions of the source) incident field can be very inhomogeneous unless distance -> infinity negligible impact of loading (back scattering) of the exposed subject on the source

8 Near-Field RF Sources Normalized Electric Field in db geometry and distance < λ/2 little or no directional power flux (reactive fields, wave vector is complex or imaginary) complex field distribution, correlation of E and H not obvious generally strong interaction between source and exposed subject exposure of the user of a cell phone and a bystander

9 Quasistatic Approximation of ELF Fields Normalized Current Density in db geometry and distance << λ no power flux no correlation of E and H (generally only either component present in the source) new electro- and magnetoquasi-static solvers (FEbased) for dosimetric applications have been developed current density in a child model in front of generic induction hob

10 Summary: Modelling of Exposure Sources Far-field exposure is not necessarily plane wave exposure. The numerical evaluation of far-field and far-field-like exposure should be based on validated models or consider a worst-case set of incident fields. For the simulation of near-field exposure, the source model must correctly reproduce the effect of loading the reactive field with the exposed subject. A novel solver for dosimetric ELF simulations has been developed. It can handle anatomical models of more than 10 million cells.

11 Dependence of EM Absorption on Anatomy The whole body SAR is a rather robust quantity. Comparatively simple anatomical models yield consistent results depending on the ratio of the body cross section and the mass. For the correct rendering of the absorption in the skin, the resolution should be better than 2mm.

12 Whole Body Exposure from a Base Station Antenna 0.5m 1.0m 2.0m 4.0m Radom Patches Feedpoint Ground Normalized E-field in db 0 field distribution in the direction of the main beam in front of a base station antenna (1.9m height, 900MHz, 17dBi directivity) strong concentration of the fields in the center of the antenna even at 4m

13 Permissible Antenna Power for Human Models distance = 0.5m 450 MHz 900 MHz 1800 MHz 2140 MHz 250 Power (W) Cross- section on mass ratio (m 2 /kg) Different anatomical body models (male, female, 55kg - 101kg, 1.60m m) have been complemented by scaled models. The maximum permissible power decreases monotonically with the ratio of the exposed cross section to the mass of the body (CSM). x 10-3

14 Dependence of EM Absorption on Anatomy The whole body SAR is a rather robust quantity. Comparatively simple anatomical models yield consistent results depending on the ratio of the body cross section and the mass. For the correct rendering of the absorption in the skin, the resolution should not exceed 2mm. For local and far-field like exposure of larger sections of the body, the maximum pssar depends on partial body resonances and the tissue layering. They vary significantly among individuals and depend on the posture of the body.

15 Typical Locations of the Peak Spatial Average SAR hands, wrists calves nose penis Normalized Spatial Average SAR db JF JM VH JF Far-field like exposure from a base station antenna at 900MHz

16 Local SAR at 900MHz: Impact of Penis Length original Ratio 10g SAR in penis to wb SAR original length length=65mm length=125mm Penis length (mm) increase of the 10g SAR in the penis by approx. 1.8dB due to partial body resonance in the penis (900MHz)

17 SAR in the Abdominal Region at 1800MHz skin 50mm generic phone fat muscle generic phone with integrated antenna at 50mm distance in front of abdomen tissues: skin (2mm - 3.5mm), fat (7mm - 15mm), muscle

18 SAR Distribution in the Abdomen for the Generic Phone Normalized SAR [ db] anatomical homogeneous -25 SAR distribution at 1800 MHz normalized to output power av. SAR ratio (anat. / hom.) of 1.2dB (1g) and 0.6dB (10g) skin thickness: 3.3mm, fat thickness: 9mm (approximate values)

19 Dependence of EM Absorption on Anatomy The whole body SAR is a rather robust quantity. Comparatively simple anatomical models yield consistent results depending on the ratio of the body cross section and the mass. For the correct rendering of the absorption in the skin, the resolution should not exceed 2mm. For local and far-field like exposure of larger sections of the body, the maximum pssar depends on partial body resonances and the tissue layering. They vary significantly among individuals and depend on the posture of the body. The impact of age dependent changes of tissue properties has been a long term issue of discussion. Recent data show that the effects are small. Age dependent changes of the anatomy can have a large impact on the exposure of particular organs.

20 Assessment of Age Dependencies on SAR 3 anatomical head models: Visible Human (VH), 3 and 7 year old children (3YC, 7YC) generic dual band phone (900MHz and 1800MHz) with internal antenna touch and tilted positioning [Kainz et al., 2005, PMB, 50(14): ] assessment of Peak Spatial Av. SAR for the head without pinna tissue and for brain tissue comparison of 10g av. SAR without pinna to SAM phantom (IEEE 1528)

21 10g Average SAR at 900MHz in the Touch Position 140% SAR Ratio Anatomical vs. SAM 120% 100% 80% 60% 40% 20% Cole-Cole 10kg 50kg 250kg ADD COLORBAR Normalized SAR in db 0% Visible Human 3 year old child 7 year old child

22 10g Average SAR at 1800MHz in the Touch Position 140% SAR Ratio Anatomical vs. SAM 120% 100% 80% 60% 40% 20% 10kg 50kg 250kg ADD COLORBAR Normalized SAR in db 0% Visible Human 3 year old child 7 year old child

23 Exposure of the Brain at 1800MHz 0 ADD COLORBAR Normalized SAR in db Visible Human 3 Year Old Child SAR distribution at 1800MHz in brain tissue cube location at maximum 1g Peak Spatial Average SAR

24 Peak Spatial Average SAR in the Brain at 1800MHz Av. SAR Ratio Children vs. VH 500% 450% 400% 350% 300% 250% 200% 150% 100% 50% 3 year old child 7 year old child SAR maximum located in cerebellum of children current density maximum of the phone at the antenna strong increase of SAR in children because SAR maximum is directly located at current maximum no impact on compliance testing but significant for epidemiological studies 0% Touch Tilted Touch Tilted 1g 10g

25 Conclusions: Dependence of EM Absorption on Anatomy The whole body average SAR is a robust quantity which can be correlated with the cross section to mass ratio. The grid resolution should be better than 2mm for convergent results. Partial body resonances and standing wave effects in layered tissues depend on individual properties of the body model. They can lead to variations in local SAR of more than 3dB. Generally, a large set of different models and postures is necessary to completely assess such effects. This should be complemented by generic body models. The effect of age dependent changes of dielectric tissue parameters on the pssar is not larger than other intersubject variations (less than ±30%). Additional differences can occur due to different pinna thicknesses. Anatomical changes due to growth of the skull can lead to significant differences of the exposure of certain brain regions (above 6dB, depending on phone design).

26 Tasks for Conservative Human Exposure Assessment assessment of the operational conditions resulting in the maximum radiated power determination of conservative position/max incident field distribution determination if the scatterer (human body) may affect the source resulting in higher exposure determination of conservative posture determination of conservative tissue composition determination of conservative tissue parameters

27 Latest Developments in Human Body Modeling development of anatomical models of adults and children using novel techniques for segmentation and CAD modeling development of a pregnant woman model identification of functional brain regions (Talairach Transformation) poser software to change the posture of the models limbs

28 The Virtual Family adult male: 34 years, 1.74m, 70kg adult female: 26 years, 1.60m, 58kg girl: 11 years, 1.48m, 34kg boy: 6 years, 1.07m, 17kg 4 more children under development

29 Construction of CAD Objects reconstructed surface smoothed reduced complexity using the AMIRA 4.0 software surface reconstruction (marching cube), surface smoothing (spring model), reduction of complexity (triangle collapse) 84 different tissue types (CAD objects) adjacent CAD objects share common surfaces export of organs and tissues as watertight CAD parts in SAT format

30 Development of a Pregnant Woman Model MR scans of the torso of a pregnant woman (7 th gestational month) resolution 0.94 x 0.94 x 4.4mm 3, coronal and axial scans avaliable

31 Integration of the Torso into Virtual Family Model

32 Exposure Evaluation of Functional Brain Regions A S S R AC PC L R AC/PC L A AC PC P I I P axial orientation coronal orientation saggital orientation The Talairach Coordinate System defines a labeling hierarchy and coordinates of functional regions of the brain. The coordinate system is defined by the anterior and posterior commissurae and the bounding box of the brain. The outline of the brains of the anatomical models are normalized (rotated, scaled, warped) to match the Talairach coordinates.

33 Talairach Transformation in SEMCAD X 1. Hemisphere Left Cerebrum Right Cerebrum Left Cerebellum Right Cerebellum Left Brainstem Right Brainstem Inter-Hemispheric * 2. Lobe Anterior Lobe Frontal Lobe Frontal-Temp. Space Limbic Lobe Medulla Midbrain Occipital Lobe Parietal Lobe Pons Posterior Lobe Sub-lobar Temporal Lobe * 3. Gyrus Middle Occ. Gyrus Middle Temp. Gyrus Nodule Orbital Gyrus Precentral Gyrus Precuneus Pyramis Pyramis of Vermis Sup. Frontal Gyrus Sup. Occipital Gyrus Sup. Parietal Lobule Sup. Temporal Gyrus Thalamus 4. Tissue Type CSF Gray Matter White Matter * 5. Cell Type Brodmann area 37 Brodmann area 38 Caudate Body Caudate Head Caudate Tail Corpus Callosum Dentate Hippocampus Hypothalamus Lat.Dorsal Nucleus Lat. Geniculum Body Lat. Globus Pallidus Lat. Post. Nucleus The Talairach Coordinate System are identified in all anatomical models. A dialog in the post processor of SEMCAD X will allow the selection of brain regions on the hierarchical levels defined in by Talairach. All SAR extraction functions of SEMCAD X will be avaliable for the selected regions (statistics, averaging, visualization, etc.)

34 Outlook: Poser Software for Anatomical Models objective: anatomically correct repositioning of limbs and joints bending of skeleton, recalculation of soft tissues under approximate conservation of organ masses applicable to all virtual family models and future anatomical CAD models

35 Summary and Conclusions Human exposure assessment is a discipline of dosimetry; the correlation with incident exposure assessments is not straightforward. The evaluation of different exposure situations has shown that the induced SAR can vary by more than 3dB db depending on individual anatomical features. For the correlation of the incident fields with human exposure, the coupling mechanism of the particular exposure situations must be understood and the induced fields must be quantified considering the anatomical variability of the exposed group of the population. Significant progress has been made with respect to the accuracy of the modeling of the human body. Ongoing developments include the modeling of a pregnant women model, an obese adult model and a novel software to change the posture of the CAD models.