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May 13 - Group 34

Stephen March

Spencer McAtee

Mike Senter

Kris Spoth

Daniel Stiner

Client Dr. David Jiles

Advisor Dr. Ravi Hadimani

Transcranial Magnetic Stimulation Coil for Mice

learn invent impact

May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Transcranial Magnetic Stimulation (TMS)

  • Uses stimulator, coils, to produce magnetic field, induced electric field in brain
  • Treat neurological disorders[1]
  • Noninvasive, painless
  • Existing coil designs
    • Stimulation depth, focality
  • Exact physiological function not well understood

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2

Photo by Jason Grow for PROTO [2]

Halo coil design from ISU [3]

May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Problem Statement

Design coils for TMS in mice

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Scope and Goals

Goal:

  • Deliver a coil system suitable for testing deep-brain stimulation in mice

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Scope:

  • Design, simulation, and characterize a coil system for deep-brain TMS in mice
  • Coils will be fabricated by Magstim Company and compatible with Magstim stimulator equipment
  • Helmet system was deemed necessary to relate simulation data to physical data, and give consistent reproducible results.

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Functional Decomposition

Input

  • Signal parameters
  • Coils and host

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Output

  • TMS

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Components

  • Magstim Stimulator
  • Time-varying signal
  • Impedance matching circuitry
  • Coils
  • E and B fields
  • Helmet

Induced E-Field

Neuron Firing --> TMS

Stimulator: Magstim Rapid2 - Biphasic

Magnetic Field

Coil System

Signal

5000A

10 Hz, 2.5 kHz pulse train

x E = -

∂B

∂t

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Functional Requirements

Coil System

  • Magnetic flux at the surface is 3T
  • Generate 150V/m E in brain[4]
  • Surface temperature may not exceed 41ºC (106ºF)[5]
  • System utilizes two coils sharing one power source
  • Both coils share one power source
  • System supports up to 5000A pulse train at 10 Hz
    • Each pulse in the train is 400μs in duration

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Helmet

  • Fits head dimensions of a C57BL/6J, a common lab mouse strain[6]
  • Does not constrict the mouse’s breathing
  • Maximum temperature at mouse skin below 41°C
  • Minimal interference with B-fields generated by coil systems

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Non-Functional Requirements

  • Compact packaging for ease of transport
  • Consists of helmet and a coil system that yield consistent results

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Coil System

  • Attaches to the helmet near the mouse's head
  • Connects to Magstim stimulators
  • Utilizes impedance matching system produced by Magstim to maximize output power

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Helmet

  • Conforms to different shapes and sizes of mouse heads
  • Does not constrict mouse breathing
  • Holds the mouse head firmly so as to generate more consistent results
  • Minimizes interference with B-fields generated by coils

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Process Overview

Research

  • Literature survey
  • Stimulation on mice
  • Halo coil field profiles

Early Design

  • Design sketches
  • Learning the software

Systematic Design Exploration

  • Different coil designs
  • Automation

Realistic Re-design

  • Spacing and insulation
  • Field and force simulations

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Process Overview

Helmet

  • 3D printed prototype design
  • High temp final design

Characterization

  • Heat
  • Forces
  • Field Profile

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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SEMCAD X Profiles

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 1

Dorsoventral Halo Coil

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 2

Anteroposterior Halo Coil

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 3

Perpendicular Halo Coil

Sagittal

Coronal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Submission for Fabrication

Submitted Version 3 Design to Magstim

  • Round 0.4mm radius wire
  • Six turns per coil
  • 0.2mm spacing between coil turns
  • Inner radii of coils:
    • Vertical: 24mm
    • Horizontal: 24mm

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Magstim's Feedback

  • Exciting design
  • Wire too thin
  • Rapid heat buildup
  • Low coil longevity
  • Use 2mm round wire
  • Fabrication queue up to four weeks

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 4

Rectangular Wire

Design Changes

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  • Larger rectangular wire (0.8x5mm)
    • Used in many Magstim coils

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  • Bent along narrow dimension of the wire cross section

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 4

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 5

Stacked Turns

Design Changes

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  • Horizontal coil turns bent along wider dimension of the wire cross section

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  • Stacking up away from brain instead of horizontally across it
    • This produced more focused field profiles

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 5

Stacked Turns

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Resubmission for Fabrication

Submitted Version 5 Design to Magstim

  • Rectangular 0.8x5mm wire
  • Ten turns per coil
  • 0.4mm spacing between coil turns
  • Inner radii of coils:
    • Vertical: 30mm
    • Horizontal: 32mm

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Magstim's Feedback

  • Thank you for the revised design.
  • Tight bend radius along 5mm edge of horizontal
  • Try a double decker

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 6

Double Decker Coil

Design Changes

​

  • Combination of version 4 and 5 based on Magstim feedback

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  • All turns bent along narrow dimension of the wire cross section

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  • Still stacks away from the brain instead of horizontally across it

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Version 6

Double Decker Coil

Coronal

Sagittal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Helmet System Design

  • Hold mouse and coils in place
  • Produce more consistent test results
  • Protect mouse from heat generated from coil

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Helmet System Design

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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shalf Analysis

Best suggested method for comparing coils

  • Empirically derived by other TMS research groups[7]
  • Quantified E-field focality and depth of penetration
  • Developed MATLAB script to analyze E-field data

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shalf ≡ focality and tangential field spread

vhalf ≡ half-value volume, brain volume ≥ 0.5Emax

dhalf ≡ half-value depth, penetration depth ≥ 0.5Emax

​

shalf =

vhalf

dhalf

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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shalf, dhalf, vhalf Geometrically

Emax

vhalf

dhalf

d1/2

Emax

vhalf

dhalf

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Best profile: focused spike

  • dhalf large
  • vhalf small
  • shalf small

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​

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vhalf large and dhalf small

==> Large shalf

vhalf small and dhalf large

==> Small shalf

shalf =

vhalf

dhalf

25

0.5Emax

May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Statistical Analysis: Design Comparison

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Best profile: small shalf

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Sample summary of simulation groupings

  • Group 1 - Ideal models and spacing
  • Group 2 - Realistic spacing (V4, V5, V6)�and geometry modifications
  • Group 3 - Current small animal coil

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Trends for smaller shalf

  • Larger horizontal coil
  • Small vertical coil
  • 10 horizontal and vertical turns

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​

​

shalf =

vhalf

dhalf

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Vertical

Horizontal

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Horizontal

Vertical

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Final Design

Specifications

General

Wire cross section: 0.8mm x 5mm

Spacing between turns: 0.4mm

Wire material: Copper

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Horizontal Coil

Inner to outer radius: 31.6mm - 37.2mm

Spacing between layers: 1.0mm

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Vertical Coil

Inner to outer radius: 19.6mm - 31.2mm

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Force Simulations

  • COMSOL Multiphysics
    • Simulated at 5000A DC
  • Intracoil forces
    • Internal stress
    • Coils must withstand
  • Intercoil forces
    • Expect coils to pull�together
    • Expect and rotate
    • Helmet system must withstand�magnetic forces

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​

z

y

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Force Simulations

Lorentz Force

  • Force/volume

f = J x B

  • Force in a given coil segment

Fx,segment = ∭ fx dVsegment

Fy,segment = ∭ fy dVsegment

Fz,segment = ∭ fz dVsegment

Rigid Body Force

  • Model Vsegment as entire coil

Segment Forces

  • Model Vsegment as quarter-turn�of coil

Z

X

Y

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Force Simulations

Rigid Body Results

  • Fmax pulling coils together �at 2.56N (0.56 lbf) in y component
  • Within helmet stress limit

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Segmented Results

  • Fmax of 19.51N (4.39 lbf) pulling coils together (y-component)
  • Slight rotation force
  • Overall small segment forces

Z

X

Z

Y

Y

X

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Thermal Simulations

Joule Heating

  • P(t)=I(t)2R
  • Simulate AC 5000A pulses
  • 20 pulses over 2s, then 60s pause

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Model Accuracy

  • COMSOL Multiphysics
  • Heat transfer
    • ρCp∂T/∂t+ρCpu∙∇T = ∇∙(k∇T)+Q
  • Convection
    • -n∙(-k∇T) = h∙(Text-T)
    • h = hair(L,pA,Text)

​

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Thermal Simulations

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May 13-34: TMS Coil for Mice

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seniord.ece.iastate.edu/may1334

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Prototype Characterization

Compare simulated vs. physical B-field

  • 3.0A DC
  • Hall probe: B-field and Temperature measurements
  • MATLAB

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Prototype Characterization

Simulated

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Technical Challenges

Tools

  • Software: SEMCAD X, COMSOL Multiphysics, Blender, Inventor
  • Data interpretation and output
  • Methods for testing focality

Size (Mouse Size vs Human Size)

  • Helmet Design
  • Coil System

Forces and Heat Dissipation of Coil Design

  • Design and machinable high temp materials

External Challenges

  • Open ended project with changing scope
  • Access to programs due to licensing
  • Magstim

​

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Future Work

  • Helmet Improvements
    • Glass ceramic mica version
    • Correct to final coil sizing
  • Final coil characterization
  • Mouse testing
  • Patent - ISURF

Credit: © pressmaster / Fotolia

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Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Questions?

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May 13-34: TMS Coil for Mice

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seniord.ece.iastate.edu/may1334

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References

[1] Wassermann, E.M., & Zimmerman, T. (2012). Transcranial magnetic brain stimulation: Therapeutic promises and scientific gaps. Pharmacology & Therapeutics 133, 98-107.

[2] Photo by Jason Grow for PROTO magazine. http://protomag.com/assets/out-of-despair.

[3] Crowther, L. J., Marketos, P., Williams, P. I., Melikhov, Y., Jiles, D. C., & Starzewski. (2011). Transcranial magnetic stimulation: Improved coil design for deep brain investigation. Journal of Applied Physics 109.

[4] Jarmo Ruohonen. Transcranial Magnetic Stimulation: Modelling and New Techniques. http://www.biomag.hus.fi/tms/Thesis/dt.html.

[5] General technical standard IEC 60601-1.

[6] Preliminary Skull Characterization and Comparison of C57BL/6J, C3H/HeSnJ, BALB/cByJ and DBA/2J Inbred Mice. http://craniofacial.jax.org/characteristics.html.

[7] Zhi-De Deng, Sarah H. Lisanby, Angel V. Peterchev, Electric field depth–focality tradeoff in transcranial magnetic stimulation: Simulation comparison of 50 coil designs, Brain Stimulation, Volume 6, Issue 1, January 2013, Pages 1–13 http://dx.doi.org/10.1016/j.brs.2012.02.005

[8] Crowther, J.L., Porzig, K., Hadimani, R. L., Brauer, H., and Jiles, D. C. (2012). Realistically modeled TMS coils for stress and Lorentz force calculations during MRI. Unpublished conference paper.

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Backup Slides

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seniord.ece.iastate.edu/may1334

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Task Responsibility

Dan: Team leader and webpage

  • SEMCAD simulations
  • COMSOL force and thermal analysis
  • Document creation
  • Physical coil characterization

Kris: Photographer

  • COMSOL force analysis
  • Document creation

Mike: Simulation

  • SEMCAD simulations
  • Helmet Design and Fabrication
  • Document creation

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Spencer: Design

  • Helmet design and fabrication
  • Patent
  • Document creation

Stephen: Co-leader

  • COMSOL force and thermal analysis
  • Shalf analysis in MATLAB
  • Document creation
  • Weekly reports
  • Physical coil characterization

​

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Task Responsibility

2nd semester

542 hours total

36.1 hours/week group average

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Stimulator Signal Model

99.6 ms

400 μs

5000A

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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All shalf data

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Lorentz Forces at maximum stress points

  • 2.8x109 Nm-3 observed for small animal coil in external B-field

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  • Values observed are 2 orders of magnitude lower

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Rotational Force Data

1

2

4

3

1

2

4

3

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334

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Rotational Force Data

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May 13-34: TMS Coil for Mice

Advisors: Dr. David C. Jiles and Dr. Ravi Hadimani

seniord.ece.iastate.edu/may1334