EMC/EMI Issues in Biomedical Research Research Ji Chen Department - PDF document

3/9/2009 EMC/EMI Issues in Biomedical Research Research Ji Chen Department of Electrical and Computer Engineering University of Houston Houston, TX 77204 Email: jchen18@uh.edu UH: close to downtown of Houston 35,066 students ECE Department: 35 faculty members, 250 graduate students Electromagnetic Research at University of Houston: NSF Center For Electromagnetic Compatibility Research Areas: Faculty Members: Computational Electromagnetics Computational Electromagnetics 6 faculty members 6 faculty members Antennas IEEE Board of Directors High ‐ Speed Signal Propagation past president of AP society Bioelectromagnetics 4 IEEE Fellows Nano ‐ devices Wireless Propagation 1

3/9/2009 medical safety in MRI Design of periodic structures PEC patches PEC patches PEC patches W W W L L L y y y a a a x x x ε ε ε h h h silver substrate silver substrate silver substrate r r r aperture aperture Nano ‐ scale FSS modeling Outline � Introduction � Human subject models � Methodologies in modeling � Applications � Pregnant woman exposed to walk ‐ through metal detector � Pregnant woman under exposure to magnetic resonance imaging � Safety evaluation of metallic implants in magnetic resonance imaging � Interactions between medical implants and vehicular mounted antennas � Summary and future work 2

3/9/2009 Introduction 10 4 10 4 10 8 10 8 10 12 10 12 10 14 10 14 10 18 10 18 10 20 10 20 F F Frequency (Hz) Frequency (Hz) (H ) (H ) EM fields EM fields Magnetic stimulation in human head (low frequency) • severe depression • auditory hallucinations aud o y a uc a o s • migraine headaches • tinnitus Magnetic resonance imaging (radio frequency) visualize the inside of living organisms 6 3

3/9/2009 A head ‐ to ‐ toe uniform detection field Pinpoint Detection with DSP Chip • The problem of human exposure to high/low frequency electromagnetic fields has been the subject of many studies. • Electromagnetic and temperature analysis of high ‐ frequency exposure • SAR (energy deposition) • Temperature (thermal distribution) p ( ) Energy Tissue EM fields deposition heating • Calculate induced current density and induced electric field in human body due to extremely ‐ low ‐ frequency exposure • J (current density) & E (electric field) Induced EM fields current Anti theft device model 8 4

3/9/2009 Approach 1: Experimental measurement disadvantages: disadvantages: I. I. difficult to make models. difficult to make models. II. II. filling material is homogeneous. filling material is homogeneous. III. III. difficult to make measurement equipments for various EM exposure. difficult to make measurement equipments for various EM exposure. 9 Approach 2: Numerical simulation CAD model + external EM source Numerical method advantages: advantages: I. I. easy to make CAD models (difficult to make for experiments). easy to make CAD models ( II. II. able to analyze inhomogeneous models able to analyze inhomogeneous models III. III. easy to model various external EM fields. easy to model various external EM fields. 10 5

3/9/2009 Human Subject Models Models Month 1 Month 1 Month 3 Month 3 Month 3 Month 3 Month 4 Month 4 Month 5 Month 5 Month 6 Month 6 Month 7 Month 7 Month 8 Month 8 Virtual Family Models 6

3/9/2009 Tissue parameters 64 MHz 128 MHz Tissue Dielectric & thermal properties Dielectric & thermal properties ρ [kg/m 3 ] σ [S/m] ε r σ [S/m] ε r Body 1006 0.49 52.54 0.51 46.23 Placenta Placenta CAD Model (including different CAD Model (including different 1058 1058 0 95 0.95 86 50 86.50 1 00 1.00 73 19 73.19 Embryonic Fluid internal organs/tissues) 1055 1.50 69.13 1.51 69.06 Bladder 1055 0.29 24.59 0.30 21.86 Bone 1990 0.06 16.69 0.067 14.72 Fetus 987 0.39 42.68 0.412 37.60 Assign tissue parameters for each Uterus 1052 0.91 92.19 0.961 75.47 internal organs/tissues C K B 0 A 0 Tissue [J/kg/ o C] [W/m/ o C] [W/m 3 / o C] [W/m 3 ] B d Body 3270 0.43 2400 537 Final model (realistic human body) Placenta 3840 0.50 0 0 Embryonic Fluid 3840 0.50 0 0 Bladder 3300 0.43 9000 1600 Numerical simulation Bone 1260 0.40 3300 610 Fetus 3105 0.39 2250 461 Uterus 3430 0.51 6000 1075 13 Modeling Techniques • Low frequency bio ‐ electromagnetic modeling – Impedance method Impedance method Induced current & electric fields Induced current & electric fields • High frequency bio ‐ electromagnetic modeling – Finite difference time domain (FDTD) method Specific absorption rate • Thermal modeling in bio ‐ electromagnetic – Finite difference solution of bio ‐ heat equation T Temperature distribution t di t ib ti • Equivalent source Generate required magnetic fields for impedance method 14 7

3/9/2009 Method1: Impedance method • Impedance method Impedance method • Efficient for ELF calculation Efficient for ELF calculation • Easy to implement Easy to implement Equivalent circuit network for impedance method + i j , 1, k Z x , + 1, i j k Z z + , , i 1, , j k i j k Z Z y y , , i j k Z x , , i j k Z z Δ x = Δ Δ Z x σ + ωε ( ) y z j x x 15 Impedance method % + i j , 1, k I x % , + 1, i j k I z % % I + 1, , , , i j k i j k I y y , , , , i j k i j k I I z , , I i j k x % i j k , , I x % i j k , , I z , , i j k I y % + + % + − + % + − % = Kirchhoff voltage equations , , , , 1, , 1, , , 1, , 1, , , , , , , i j k i j k i j k i j k i j k i j k i j k i j k i j k Z I Z I Z I Z I emf x x y y x x y y z ∂ ∫∫ ∑ ∑ % ∂ + + ωμ ωμ ˆ = = ฀ ฀ IZ IZ j j H n H n V V = − ∂ ∫∫ 0 emf f B ds B d ฀ ฀ t 3 ∑ , , = ≤ ≤ % + + i j k ( , , ) ; 1 3 , , = , , + 1, , − , 1, − , , a I i j k emf m i j k i j k i j k i j k i j k I I I I I mn n m z x y x y = 1 n 8

3/9/2009 Numerical validation example radius=0.25m σ =0.1 B = 1 Tesla freq =60 Hz Method 2: FDTD Modeling of interaction of electromagnetic fields with human bodies at high frequency SAR (energy deposition) r ∂ r ρ ′ r 1 H = − ∇× − E H ∂ μ μ t Efficient numerical technique to solve electro ‐ magnetic wave problems r r r ∂ σ 1 E = ∇× − H E ∂ ε ε t � Finite Difference Time Domain Method � Direct solution method for Maxwell’s time dependent curl equations � Direct solution method for Maxwell s time dependent curl equations � Avoids solving simultaneous equations ‐‐ matrix inversion � Provides for complexities of structure shape and material composition � Very easy to implement compared to FEM/MOM method 18 9

3/9/2009 Method2: FDTD Yee’s FDTD Scheme 1 1 − + E − n n n 1 2 n 2 H E H Explicit update scheme •Easy to implement 1 1 + + H ( , i j , k ) x 2 2 •Able to be parallelized 1 1 ( + , , + ) H i j k 1 y 2 2 E i j k + ( , , ) z 2 1 1 1 + E i j ( , , ) k + + ( , , ) y H i j k 2 z 2 2 1 ( + , , ) E i j k x 2 19 Method2: FDTD Specific absorption rate (SAR) calculation 2 2 2 2 σ σ + + + + σ σ ( ( ) ) E E E E E E E E = = x y z SAR ρ ρ 2 2 12 ‐ field components approach + + + + + + E E E E E E E E + + + + = _ , , _ , 1, _ , , 1 _ , 1, 1 x i j k x i j k x i j k x i j k E _ _ , , x center i j k 4 20 10

3/9/2009 Method 2: FDTD Symbol Physical Property Value Units r r cylinder radius cylinder radius 0.05 0.05 m m plane wave incident power P density 1000 W/m 2 f plane wave frequency 2.45 GHz ε relative permittivity 47 ----- σ conductivity 2.21 S/m ρ mass density 1070 Kg/m 3 Δ x spatial resolution 0.5 mm 21 Method 3: Thermal modeling • Thermal modeling/bio Thermal modeling/bio ‐ heat equation heat equation Temperature Temperature ‐ rise computation rise computation When a human subject in a thermal equilibrium state is exposed to EM fields, the When a human subject in a thermal equilibrium state is exposed to EM fields, the resultant temperature rises may be obtained from thermal modeling (bio resultant temperature rises may be obtained from thermal modeling (bio ‐ ‐ heat heat equation), which takes into account such heat exchange mechanisms as heat equation), which takes into account such heat exchange mechanisms as heat i i ) ) hi h hi h k k i i h h h h h h h h i i h h conduction, blood flow, and EM heating . conduction, blood flow, and EM heating Bioheat transfer equation (BHTE): Bioheat transfer equation (BHTE): ∂ T ρ = ∇ + − − + 2 ( ) C K T A B T T Q ∂ 0 b EM t = ρ Q SAR from FDTD calculation from FDTD calculation EM Boundary condition: Boundary condition: ∂ T = − − ( ) K H T T ∂ a a n 22 11