John R. de Bruyn, Leesa Fleury, Corey Rae McRae, Will Handler, Blaine Chronik Department of Physics and Astronomy Western University

Transcription

1 John R. de Bruyn, Leesa Fleury, Corey Rae McRae, Will Handler, Blaine Chronik Department of Physics and Astronomy Western University

2 the MR scanner the MR image

3 High dc magnetic field Rapidly varying magnetic fields due to Switching of gradient coils ( 1 khz) RF coils ( 100 MHz) High fields and field gradients lead to strong forces and torques on materials that are even slightly magnetic Changing fields induce eddy currents in conductors, leading to heating

4 Medical devices such as pacemakers or metallic implants will either cease to function in the MR environment heat up due to eddy current heating, potentially injuring the patient Patients with implanted medical devices are normally denied access to MRI Medical devices must be tested and certified as MRsafe Testing requires the use of materials that mimic the properties of human tissue in the MR environment phantoms

5 An ideal phantom would duplicate the electrical response electrical conductivty, dielectric constant (at the appropriate frequencies) magnetic response magnetic permeability thermal response thermal diffusivity, heat capacity mechanical response elastic and viscous moduli, yield stress of tissue. This is hard.

6 Currently used phantom materials: Salt water Adjust salt content to get the conductivity right. Local heating causes convection, so thermal response is wrong. Gelled saline Poly(acrylic acid)+water+nacl Hydroxyethylcellulose+water+NaCl

7 ASTM F a: Measurement of RF heating on or near passive implants during MRI Specifes properties of phantom: Size and shape Conductivity = 0.47 S/m ± 10% Dielectric constant = 80 ± 20 Thermal properties similar to that of the body Thermal diffusivity about m 2 /s Heat capacity 4150 J/kgK (close to water) Viscosity: The viscosity shall be great enough so that the phantom material does not allow bulk transport or convection currents.

8 ASTM a recipe: uses polyacrylic acid partial sodium salt, Aldrich product number No molecular weight provided H replaced by Na on an unknown fraction of carboxyl groups It is essential to strictly follow the mixing protocol. The following protocol needs to be followed precisely. blend the suspension into a slurry. A kitchen grade immersion blender with a blade has been found to be satisfactory. The blender is turned on intermittently for at least 20 min.

9 Electrical properties

10 The phantom In the scanner Measured temperature change

11 Prepared PAA materials according to the standard protocol. Measured viscosity in steady shear and viscous and elastic moduli under SAOS Studied effect of mixing time and aging

12 Effect of mixing time

13 Effect of mixing time

14 Effect of mixing time

15 Effect of age

16 Rheological properties and conductivity depend on age and on mixing time The gelled saline specified in the ASTM standard does not have a yield stress Material will flow when subjected to lateral T gradients This will affect heat transport and T changes measured in MR tests

17 Does this matter? What is required is a reproducible, conservative test of heating in MR environment The physical properties of the phantom material should allow for a conservative test Preparation method should be sufficiently clear and detailed that everyone will get the same result Material properties should not be sensitive to variations in preparation protocol Material properties should be stable over time These requirements are not met by the current standard/material

18 The plan: Further characterization of currently used phantom materials. Development of new & better materials Longer term: Model perfusion? Different phantoms for different kinds of tissue?

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21 What is MR used for? (circa 2007)

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23 the attractive force issue this is what makes MR so loud this is what causes heat deposition Magnet 1.5, 3, 7 T constant Gradients (3x) ~10 mt ~ 1 khz Transmit RF ~10 ut ~ 100 MHz

24 Superconducting magnet Gradient coils Radiofrequency (RF) coils

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26 Test procedure: Mount device to be tested in phantom container Add phantom material Put in MR scanner for specified time/mr pulse sequence Measure T increase at key points in phantom

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