mri induced heating of a...

© 2011 ANSYS, Inc. November 28, 2014 1

MRI Induced Heating of a Pacemaker

Peter Krenz, Application Engineer


Electric fields generated during MRI exposure are dissipated in tissue of the human body resulting in a temperature rise

Multi-physics simulation can predict this temperature increase

ANSYS Workbench workflow allows the user to pre-define simulation setups that enable non-experts to successfully execute this multi-physics simulation

Problem Statement


Electromagnetic power dissipated in the human body positioned inside an MRI coil: • ANSYS HFSS

Temperature rise in the human body due to dissipation of electromagnetic power: • ANSYS Mechanical

System level tool that enables HFSS and Mechanical to interact and exchange simulation results: • ANSYS Workbench

What Tools are Required?


What is HFSS?

Premier 3D Electromagnetic design tool

Solves • Any arbitrary 3D structure

Uses • Full Wave Finite Element Method

(FEM) • Transient Finite Element Solver • Integral Equation Solver • Physical Optics Solver


Comprehensive product solution for structural analysis

Analysis types available in ANSYS Mechanical:

Structural (static and transient): • Linear and nonlinear structural analyses

Dynamics: • Modal, harmonic, random vibration, flexible and rigid

dynamics

Heat Transfer (steady state and transient): • Solve for temperature field and heat flux • Temperature-dependent conductivity, convection, radiation

and materials allowed

What is ANSYS Mechanical?


ANSYS Workbench is a project-management tool that is the top-level interface linking all ANSYS software • Handles the passing of data between ANSYS Geometry / Mesh / Solver /

Post-processing tools • Manages project and individual files on disk (geometry, mesh etc.) and

provides graphical project details making it easy for the user to understand how a project has been built

Because Workbench manages the individual applications AND passes data between them, it is easy to automatically perform design studies (parametric analyses) for design optimization

What is ANSYS Workbench


Connections between cells indicate sharing of information: • Geometry • Simulation results from one simulation serve as a setup

(input) condition for a second simulation

ANSYS Workbench


The ‘Archive’ tool can be used to bundle the many files and directories of the Workbench project to send the simulation to colleagues for further setup/analysis

This enables leveraging of people in the organization with different strengths and disciplines

ANSYS Workbench


MRI Induced Implant Heating Simulation using ANSYS Workbench

HFSS Simulation Transient

Thermal Simulation

+ =

Temperature Profile


Workflow


Workflow Overview


The human body model and pacemaker are defined in a HFSS cell

The built-in 3D modeler was used for geometry creation and manipulation (note this is not a simulation)

External CAD programs can be used for object creation and directly linked into workflow

Geometry Definition


The human body model contains three levels of de-featuring to accommodate varying degrees of accuracy

Material properties that are included: • Frequency dependent values for electro-magnetic

simulations • Constant values for thermal simulations A user can include as much detail of the body model as required in an MRI induced heating simulation

Human Body Model


A pacemaker was created and positioned within the human body model using the HFSS 3D modeler

Pacemaker


MR coil and simulation setup is pre-defined: • Geometry • Boundary conditions • Excitation • Simulation setup

No user input is required

MRI coils operating at 64 and 128 MHz are available

Due to pre-defined HFSS simulation setup, non-expert HFSS users are able to successfully execute this simulation

HFSS Simulation


Geometry of human body model with pacemaker are inserted into pre-defined HFSS simulation containing MRI coil

Ready to simulate

HFSS Simulation

+ =


HFSS creates and refines mesh automatically

No user-input required

Meshing

Geometry Initial Mesh Final Mesh


HFSS simulation results can be visualized: • Electric field distribution • SAR distribution

HFSS Simulation Results


Geometry of human body and pacemaker are passed from HFSS to ANSYS Mechanical

Electric fields dissipated in human tissue or on surfaces of metal conductors are the source of the temperature rise calculated by ANSYS Mechanical

Thermal Simulation


Only keep what is required: • Do not include MRI coil in

thermal simulation, since it does not impact temperature distribution in human body

• Do not include organs and bones located away from pacemaker, since they will not influence the temperature distribution near the implant

Thermal Simulation


Define analysis setup to replicate measurement conditions • Specify duration of how long MRI is turned on and off

Analysis Setup

MR Coil: On Off


Meshing in ANSYS Mechanical works best on smaller volumes

Human body was split into several sub-sections to facilitate meshing

Meshing


Different ways to visualize the temperature rise in the entire human body: • Temperature rise on surface of

human body after 15 minutes of exposure to MRI is shown on left

• Maximum temperature rise in entire human body as a function of time is shown below

Temperature Profile


Temperature profile on individual components can be visualized

Pacemaker is shown here

Temperature Profile


MRI heating of human body with pacemaker was simulated using ANSYS Workbench linking HFSS and Mechanical

Simulation replicates physical structure of MRI coil and human body to ensure accurate results

Workflow is a simulation template: • Simulation setups are pre-defined to enable non-experts to

execute simulation successfully • Workflow allows sharing and collaborating on simulation –

leverage strengths of people on your team

Final Thoughts


Questions?

Thank you!

mri induced heating of a...

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