07 MRI device compliance - support.ansys.com › staticassets › ANSYS › Conference ›...

© 2011 ANSYS, Inc. October 24, 20111

MRI Device Compliance

Martin Vogel, PhDKimberley PolingApplication Engineering Team Eastern USA


Overview

High simulation efficiency for MRI

Method that enables a non‐EE to analyze the thermal effects of an MRI scan on an implant


Main tools

HFSS – 3D Full‐wave Electromagnetic simulator

Designer– Circuit simulator that interacts with HFSS

ANSYS Professional NLT– Thermal simulator that uses electromagnetic losses from HFSS as heat loads


1. Simulation efficiency

Minimize RAM and simulation time


Generic birdcage coil

Ring with 24 ports, phases 0 – 345 deg

Ring with 24 ports,phases -180 – 165 deg

Around the coil is acylindrical shield withopen ends.


Saving computer resources

A birdcage coil can be created with many, e.g. 48, sources that impose the required rotating field.

A realistic birdcage coil has few sources and many capacitors.

Such a coil can be simulated with less resources.

With a couple of ANSYS tools, we have created such realistic coils, one for 1.5 T and one for 3 T.


Realistic coil feed network

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Cap

C4

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Feed points are driven with phases 0, 90, 180 and 270 degrees.

Feed points simplified and optimized with ports and capacitors


S11 coils with capacitors

63.8 MHz

127.6 MHz


Fine‐tune capacitors for MRI image quality

1.5 T124.5 pF

3 T26.5 pF

B+/B‐ ratio is at least 20


Rotating H with capacitors

Feed points

Capacitors have been fine-tuned in HFSS for best rotating H after optimization in Designer.


Apply this design technique to birdcage coil with human‐body model

RAM 12.5 GB

Elapsed time 58 minutes


Rotating B for body in birdcage with 48 sources Compare with results of other simulations.


Rotating B for homogeneous body in birdcage with 4 sources and 44 capacitors is quite similar.

RAM 5.5 GB



Is it possible to simulate just the blue volume and IMPOSE fields from a separate coil simulation? YES, you can do that!


With imposed fields, the rotating B again looks very similar!

RAM 1.6 GB

Elapsed time 7.3 minutes


Now that we have such an efficient method, use it with a complicated heterogeneous human‐body model and a pacemaker.

RAM 4.3 GB



Local SAR in cut plane (3T coil)


R13 Schematic


Thermal resultMaximum occurs on neck, not at implant


Contours of 38.1 0C and higher

Tissue around pacemaker


Pacemaker, lead and heart


2. Thermal effects of MRI scan on implant

Minimize the implant designer’s exposure to electromagnetics


Generic birdcage coil

Around the coil is acylindrical shield withopen ends.


Where should implants be placed?

Implants should have worst‐case location and orientation.

Next few slides illustrate how this is achieved.


Phantom placement

|B| is almost uniform within coil (left). However, |E| is not (right).Make sure implants be located in a region with relative high E-field.

|B|

|E|


Worst‐case SAR: implant near phantom wall; does not stick out of RF coil


Worst case: implant parallel with electric‐field vectors


Workflow for implant testing

Have reusable coil+phantom model in HFSS

Have phantom+parameterized implant(s) in separate geometry system

This separate geometry will be exposed to the fields from the coils.

Thermal simulator will determine temperatures of phantom+implant(s) due to RF losses.

Next slides show this in more detail.


Workflow in R14 Schematic

CAD

No need to open HFSS.


Create in other CAD tool import with parameters into prepared HFSS design


Transfer geometry to thermal simulatorNo more need for File/Export and File/Import

Note the parameters from The CAD tool are available!


Library of thermal material properties is available


Have prepared a subset with the same material names as in HFSS.


Objects come in with correct material assignment!


Drag and Drop HFSS Solution to Thermal Setup

Drop and Drag

Resulting Schematic with extra Link


Assign Losses from HFSS to Objects in Thermal


Effect implants on local SAR


Temperatures on implants after 900 s


Add final maximum temperature as an output parameter


Input parameter:Implant length

Output parameter:Max. temperature


Attach DX to investigate design variations efficiently

Vary length and inspect final temperatures


Maximum temperature as a function of implant length (at reduced power, not to be compared with previous result)

40 60 80 100 120 140 160 180 Length implant (mm)


Observations

Temperatures tend to decrease when implant is buried deeper.This makes sense, as electromagnetic fields decay with depth.

For small implants, temperatures increase with implant length.This makes sense, because a longer implant can function as a more effective receiving antenna (as long as implant ≤ λ/2 at this frequency and in this environment).

For large implants, field inhomogeneity makes predictions harder to make.

Results are sensitive to the gel’s electrical and thermal material properties.


Summary

We have demonstrated

(1) A way to achieve very efficient simulations;(2) A simulation flow that enables non‐EM experts to determine MRI‐induced heating for different implant designs.

07 MRI device compliance - support.ansys.com › staticassets › ANSYS › Conference ›...

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