bio-medical rf simulations with cst microwave studio

www.cst.com | May-10

Bio-Medical RF Simulations

with CST Microwave Studio®

Biological Models

Specific Absorption Rate (SAR)

Bio-Medical Examples


Biological Models

The right choice of the biological model is essential for the reliability

of a SAR or EMI simulation.

Visible Human

voxel data

SAM Phantom,

homogeneous

models

other voxel data

CST Voxel Family


• New .obj import allows import of biological models, e.g. from

Poser® 8 (http://my.smithmicro.com/win/poser/index.html)

• For most high frequency applications fully sufficient

• Simulate much faster then voxel models

Homogeneous Hand/Body Models


SAM - Standard Anthropomorphic Model

www.sam-phantom.com

Frequency dependent material

properties (according to

standard) can be modelled by

dispersive materials via

tabulated input.

Only one simulation run for

broadband results!!

tissuesimulantliquid

(TSL)

plastic shell• Originally created for measurements

• Shape specified in IEEE/CENELEC/IEC

standards

• Filled with homogeneous glycol-containing

tissue-simulant liquid, only two materials

for simulation

• Virtual prototyping through simulation


Hand monoblock

Hand fold

Hand narrowdata

Hand PDA

CTIA Hand Models


CST Voxel Family

KATJA

(pregnant)

LAURA GUSTAV

BABY

CHILD

EMMADONNA


CST Voxel Family

Macros -> Solver -> Calculate

Human Material Properties


Available in

different

resolutions

Materials of

interest can be

chosen

Visible Human Project

produced by the National Library of

Medicine (NLM), Maryland

http://www.vr-laboratory.com/

HUGO


Cole-Cole-Materials


SAR: Overview and Background

SAR – Specific Absorbtion Rate

Unit of SAR: W/kg

P: Power loss density

E: Electric field strength

J: Current density

s: Conductivity

r: DensityTypically averaged over pre-defined mass

A measure for electromagnetic energy absorbed by biological

tissue mass when exposed to radiating device (e.g. mobile phone)

22

22 JEPSAR


Averaging Procedure

1. Point of avg. SAR calculation

2. Search for 10 g cube (iteratively)

3. Integrate losses in cube

At boundary treatment depends on

chosen averaging standard:

IEEE C95.3, IEEE 1528.1, CST C95.3

CST legacy

The „constant volume“ assumption uses an averaged cube size:

- Faster (no iterative search for cube with correct mass)

- Only approximative (not according to official SAR standard)


• Several guidelines and standards specify SAR safety limits (i.e.

ICNIRP).

• Standards like IEEE 1528 regulate measurement methods for

practical assessment of compliance.

• A simulation standard IEEE 1528.X is in development

• 1528.1 requirements for hexahedral time domain codes (end

of 2010)

• 1528.2 application to cars with passenger/bystander (~2011)

• 1528.3 application to mobile phones near head (~2011)

• 1528.4 requirements for tetrahedral frequency domain codes

• CST participates in standards committee.

• IEEE C95.3 Annex E specifies SAR averaging scheme for

simulation.

• CST MICROWAVE STUDIO® has already been approved by the FCC

(USA) to comply with hex td standard drafts.

SAR Standards under Development


Visualization of SAR

2D or 3D plot including information about position of the maximum


Visualization of Max. SAR Cube


Re(er) Im(er)

0.9 GHz 41.5 17.98 (= 0.9 S/m)

1.8 GHz 40.0 13.98 (= 1.4 S/m)

Dispersive Broadband Simulation

Typical requirement for dual band phones:

Frequency dependent

material definition:

Second order dispersive

fit for tabulated values,

only one simulation run

required


0.9 GHz, 1g 1.35 1.31 1.74

0.9 GHz, 10g 0.96 0.93 1.13

1.8 GHz, 1g 0.69 1.32 1.32

1.8 GHz, 10g 0.99 0.83 0.83

Dispersive Broadband Simulation

Compared material settings:

Constant settings for 0.9 GHz

sim. time 45 min.

Constant settings for 1.8 GHz

sim. time 45 min.

Dispersive broadband fit

total sim. time: 57 min.

Dispersive fit agrees very well for S-Parameter and SAR

values in both bands for only 25% extra simulation time

S-Parameter comparison:

SAR value comparison:


Measured vs. Computed SAR Distribution

0 dB = 2.8 W/kg

Example: 7T MRI endorectal coil

MeasurementSimulation

Overall:

SAR computed

SAR measured 1.08 – 1.15

Courtesy of Erwin L. Hahn Institute Essen,Germany


Magnetic Resonance Imaging (MRI)

Three EM-fields needed for imaging

• STRONG magnetostatic field (human: 1 – 9.4

Tesla, up to 21 T for animals)

Mostly superconducting magnets, aligning the

spinning protons

-> M-Statik Solver

• Gradient field for positioning (in kHz range)

-> Magneto-Quasistatik Solver, LT-Solver

• HF field to excite spinning protons and

receive relaxation signal (60 – 500 MHz)

Rotating B-Field most interesting (B1+)

-> Both T- and F-Solvers are of intererst!

Most interesting for MRI R&D


Design Challenge: Increase SNR of image

For 7T MRI -> fres = 297 MHz -> lbody ~ 13 cm

-> It is difficult to obtain homogeneous field

distribution inside body, specialized coils

required

Safety issue: SAR ~ fres2

-> SAR critical for higher fres

-> Alternative: queck directly body

temperature increase, bioheat solver!

SNR ~ static biasing field ~ spin resonance frequency fres

Advantages of CST: Complete Technology, Static, LF, T, F and

bioheat solvers in one frontend, Voxel Family, fast SAR, etc…


Courtesy of Erwin L. Hahn Institute Essen,Germany

8 Channel Head Coil


8 Channel Head Coil

arg(B1+)

Vs/m²

|B1+| SAR voxel

SAR 10g

[°]


8 Channel Head Coil

SAR10g

location of max. SAR10g in

left shoulder for off-centre

position of head

max. perm. power = 23 W

(CW)

location of max. SAR on

left side of the head

most critical aspectSAR10g SAR10g SARhead

max. perm. power25 W 27 W 33 W


• loops overlapped and shifted

• 70 cm cable length

• box with TR-switches + pre-amps

Spine Loop Array

z

x

43 cm

20 cm


Comparison to Measurement

Measurement

max B1+ = 15.9 µT

Simulation

max B1+ = 13.5 µT


SAR Compliance

critical aspect: localized SAR (10g averaged)


Microwave Breast Cancer detection

Dr. Maciej Klemm, Electromagnetics Group, Centre for Communications

Research (CCR), University of Bristol, United Kingdom

e-mail: [email protected]


Model setup and clinical results

• dipole antennas

• dispersive tissues

• inhomogeneous breast !

• model 30-40M cells

• full imaging (30 simulations)

takes about 10h (hardware

accelerated; 4 GPU cards)


Pace Maker Simulation

T-Solver

F-SolverT-Solver

Complete Technology:


Results at 400 MHz

Inside biological tissue phantom

Averaging Cube for

max SAR

E-Field

SAR


Pacemaker inside Human Body Model


Cardiac Pace Maker

Courtesy of Lehrstuhl für Theoretische Elektrotechnik, Bergische Universität Wuppertal, Germany

Frequency dependent field coupling into a Cardiac Pace Maker (CPM)


BABY besides Baby-Phone

Stimulated power: 500 mW at 865 MHz

Max. SAR value (averaged over 10g): 0.02 W/kg

(well below accepted maximum of 2 W/kg for public exposure)

Courtesy of Lehrstuhl für Theoretische Elektrotechnik, Bergische Universität Wuppertal, Germany


New CST Examples!

Can only be opened by customers who have

-Voxel Import

- BioModel License

-> offer for evaluation license!!


CST STUDIO SUITE offers a wide range of tools for

bio-medical simulations (MRI, cancer treatment,

diathermy, implants, etc.)

Both flexible homogeneous and detailed voxel models

are available

„Complete Technology“ allows combined simulations

from static to GHz including circuit simulation

SAR and Bio-Thermal simulations help to improve

performance and safety of medical devices

Summary


Appendix

bio-medical rf simulations with cst microwave studio

Documents

hahn institute

bergische

constant settings

complete technology

ieee c95

time 45 min

germany www

field