maia's phd summary for @hopperties, march 2013

Transcript

1. ROBOTS!!
   !
   !
   OR

   HOW I LEARNED TO GIVE UP AND FINISH MY PHD
   maia sauren
   

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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
   

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4. RF  SAFETY  STANDARDS
   • ICNIRP or IEEE recommendations
   • Different standards in different countries (humans aren’t so different)–
   no difference at 900 MHz
   

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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)
   

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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
   

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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)
   • ...
   

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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
   

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9. PHYSICAL MODELLING - MEET SAM
   specific anthoromorphic mannequin
   

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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)
    

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11. NUMERICAL SOLVING TECHNIQUES
    •Solving Maxwell’s equations
    
    •Different methods
    
    • FDTD
    
    • MoM
    
    • FEM
    
    • Hybrids
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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19. HEAD  SIZE:  1-­‐GRAM  AND  10-­‐GRAM  
                     70%  -­‐  85%             90%  -­‐  100%  
               105  -­‐  110  %              115-­‐130  %
    

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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
    

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21. VALIDATION PROJECT
    

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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?
    

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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?
    

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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?
    • a fair bit, some factors more than others
    • How much effect do these variations have on absorption?
    • a fair bit, some factors more than others
    • Do some anatomic factors affect absorption more than others?
    •
    • How can we test any of this?
    • using my fabulous model!
    

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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
    

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