maia's phd summary for @hopperties, march 2013

Transcript

1. ROBOTS!!
   ! ! OR
 HOW I LEARNED TO GIVE UP AND FINISH MY PHD maia sauren
3. 1 10 100 1 10 100 1 10 100 1 10 100 1 10 100 Hz kHz MHz GHz THz Radiofrequency
 radiation Ionising radiation UV Infrared
 radiation visible light 50 Hz Power Lines AM radio FM radio Mobile Phones TV Satellite X rays Gamma rays Lasers
4. RF  SAFETY  STANDARDS • ICNIRP or IEEE recommendations • Different standards in different countries (humans aren’t so different)– no difference at 900 MHz
5. RF SAFETY STANDARDS • Basic restrictions vs. fundamental limits • ICNIRP basic restrictions: mandatory limits on direct modes of RF interaction with the body • Current density (mA/cm²) • Whole Body Averaged SAR (W/kg) • Localised SAR in the head and torso (W/kg) • Localised SAR in the limbs (W/kg) • Specific absorption per pulse (J/kg) • Power flux density (W/m²)
6. THIS WORD, ‘SAFE’... “4 W/kg of whole body exposure for ~30 minutes produces a temperature increase of 1oC, a level at which established biological effects begin to take place” (Health Phys, ’98) • safety factors •Based on conservative and crude assumptions •Too conservative = uneconomic
7. REAL- TIME EXPOSURE OF LIVING HUMANS • Can’t measure absorption, only exposure • Dead tissue? (Surowiec, Stuchley et al 1985) • Animals? (Forster 1979, Schwan 1989) • ...
8. Absorption is based on: a) size b) shape c) dielectric properties of tissues • How much do these vary in humans? • How much effect do these variations have on absorption? • Do some factors affect absorption more than others? • How can we test any of this?` PROBLEM STATEMENT
9. PHYSICAL MODELLING - MEET SAM specific anthoromorphic mannequin
10. COMPUTATIONAL MODELS • Canonical models: • Stuchley 2008, layered homogenous spheres & cubes • Watanabi 2006: cubes • slab models: Melbourne Uni • SAM and his homogeneous friends • CT or MRI-based models: Visible Human(s), Korean, Chinese, NORMAN • Child models: • simply scaled adults (Dimbylow 2002, 2005), • morphological scaling (Wang & Fujiwara 2006, 2008) (Wiart 2008)
11. NUMERICAL SOLVING TECHNIQUES •Solving Maxwell’s equations •Different methods • FDTD • MoM • FEM • Hybrids
12. PROBLEMS WITH COMPLIANCE • Assumptions in both kinds of models: • Size, thickness, relative position of tissues • Dielectric properties of tissue • Average human = Caucasian male • Women, children = scaled Caucasian males • Image-based models are one-offs • Contradictions, inconsistencies in the data • Testing assumptions is difficult
13. Relatively easy parametric adjustment of: • Tissue size • Tissue relative location • Dielectric properties • Morphology Skin - 2 mm thick Skull - 7 mm thick Brain – 80 mm radius Eyes – 15 mm radius Ears – 2 mm skin Nose – average head coated by 2 mm skin Fat around eyes – 7 mm Filler: average head Based on anatomic measurements taken from large sample of adult Caucasian males (Farkas, 1994) FEKO, finite element method/method of moments (FEM/MoM) Plane wave excitation (10 W/m), sagittal plane Dielectric properties as per Gabriel GEOMETRY HEAD WILL SAVE THE DAY
14. SAR  &  CRANIAL  THICKNESSES TRL Geometry Head VH Norman Phanto m 4mm 6mm 11mm Whole head average SAR (W/Kg) 0.043 0.036 0.036 0.035 0.034 0.043 10g Ave SAR (W/Kg) 0.27 0.12 0.13 0.13 0.15 0.19 Tissue ear head (near head (near ear) head (near top of brain back of neck Max peak SAR (W/Kg) 0.66 0.90 0.96 0.89 0.59 0.73 Position ear head (behind eye) skin (at ear) skin (at ear) back of neck ear 27 literature sources ! multiple measurements ! 3103 skulls • compromise  model   • point  of  diminishing  returns
15. SKIN THICKNESS
 Peak 10g SAR distribution 0.000000 0.000005 0.000010 0.000015 0.000020 0.000025 0.000030 0.000035 0.000040 0.000045 0 50 100 150 200 250 300 350 400 Penetration into head (mm) SAR (W/Kg) 1mm 2mm 5mm • 16 literature sources • multiple measurements • 1520 cadavers or live volunteers • Significant effect of skin thickness , but not at human anatomic ranges • Absorption is within safety limits • Skin in heterogeneous models is 1.5 mm or 2 mm thick • Predicted SAR results from Geometry Head model are comparable
16. DIELECTRIC  PROPERTIES • Original dielectric properties as per Gabriel (1996), varied to ±10, ± 20 and ± 30 % ! • Assumptions: • Approximately linear association with SAR • Variation for SAR of 30 % is an accepted uncertainty in SAR measurement standards • Skull tissue close to surface - a priori reason that it plays a significant role in energy absorption
17. DIELECTRIC PROPERTIES • SAR in skull increases as conductivity increases: a more lossy layer near surface • Increased shielding effect in the head – drop in whole head SAR as conductivity increases • Skin dielectric properties affect SAR – as expected – but not enough to exceed limits • As eye dielectric properties drop, SAR increases a little •All SAR variations seen are within safety limits
18. Head size scaling factor Testing: Effect of varying entire head size ± 30% ! Dimensions GHead: height = 230 mm width = 176 mm GHead 70%: height = 161 mm width= 123 mm GHead 130% height = 299 mm width = 229 mm VH: height= 292 mm width = 229 mm SAM: height = 228 mm width = 190 mm ! Previous work (scaled down adult): Dimbylow (1993) – higher SAR in eyes Dimbylow & Mann (1994) – lower SAR in smaller heads Ghandi (1996) – SAR increase in smaller heads Schönborn (1998 ) – some differences Guy (2002) – no characteristic differences Christ 2005 – no correlation between head size & SAR
19. HEAD  SIZE:  1-­‐GRAM  AND  10-­‐GRAM                    70%  -­‐  85%             90%  -­‐  100%              105  -­‐  110  %              115-­‐130  %
20. HEAD SIZE: RESONANCE Variables 70% 80% 85% 90% 95% 100% 105% 110% 115% 120% 130% Head diameter 123.20 140.80 149.60 158.40 167.20 176.00 184.80 193.60 202.40 211.20 228.80 (head dia)/λ 0.37 0.43 0.45 0.48 0.51 0.53 0.56 0.59 0.61 0.64 0.69 Skull diameter 120.40 137.60 146.20 154.80 163.40 172.00 180.60 189.20 197.80 206.40 223.60 (skull dia)/λ 0.36 0.42 0.44 0.47 0.50 0.52 0.55 0.57 0.60 0.63 0.68 Brain diameter 112.00 128.00 136.00 144.00 152.00 160.00 168.00 176.00 184.00 192.00 208.00 (brain dia)/λ 0.34 0.39 0.41 0.44 0.46 0.48 0.51 0.53 0.56 0.58 0.63 Head height 161.00 184.00 195.50 207.00 218.50 230.00 241.50 253.00 264.50 276.00 299.00 (head height)/λ 0.49 0.56 0.59 0.63 0.66 0.70 0.73 0.77 0.80 0.84 0.91 Skull height 158.20 180.80 192.10 203.40 214.70 226.00 237.30 248.60 259.90 271.20 293.80 (skull height)/λ 0.48 0.55 0.58 0.62 0.65 0.68 0.72 0.75 0.79 0.82 0.89 Brain cavity height 149.80 171.20 181.90 192.60 203.30 214.00 224.70 235.40 246.10 256.80 278.20 (brain cavity height)/λ 0.45 0.52 0.55 0.58 0.62 0.65 0.68 0.71 0.75 0.78 0.84 Freq = 900 MHz, λ = 330mm σ = 0.77 σ = 0.97
21. VALIDATION PROJECT
23. DISCUSSION • Confounding factors: Sources of uncertainty • Increased uncertainty in Geometry Head by simplifying • Simplification of the tissues into geometrical shapes • Computational methods: solver introduces uncertainty – tetrahedral meshes as estimate for tissue shapes • Dielectric properties – measured or guessed at • Decreased uncertainty by including fewer tissues • is variability in humans high enough to affect compliance with safety standards? • is this tool any good?
24. APPLICATIONS OF GEOMETRY HEAD • good tool for exploring the relationship between SAR and anatomical features • middle ground between homogeneous and highly heterogeneous models • Provides info on how limits should be set – uncertainty within the standard, vs. uncertainty with measurements • Can be used for looking at epidemiological population variations and how these affect standards – are all populations safe? Are kids safe?
25. HAVE I SOLVED THE PROBLEMS? Does the anatomic variability in humans place some people in danger of exposure above the safety limits? • How much do these factors vary in humans? • <tick> a fair bit, some factors more than others • How much effect do these variations have on absorption? • <tick> a fair bit, some factors more than others • Do some anatomic factors affect absorption more than others? • <tick> • How can we test any of this? • <tick> using my fabulous model!
26. NEXT (NOT ME) •What happens at extremes? •Remove sharp angle in the middle of the head? •Other tissue variations, more tissues? •Eventually – expand model • Mobile handset, not just dipole • Hand on mobile • Whole body • Other frequencies