What are the Limitations of Mumetal for Magnetic Shielding in 3T MRI Systems?

[csm09]

Hey all,

I need to come up with a solution to shield a control box from the magnetic field from a 3T MRI system. With approval (that I will be very persistent to get) I am hoping to be allowed to place the box at around or even outside the 150G fringe surface.

Mumetal seems to be the best possible solution. Does anyone know the limitations of this material? I can't seem to find much information on the web regarding it. At what field strength will it saturate? I took a sample into the shield room with an active 1.5T MRI system and it was obviously strongly attracted to the machine. How will it behave over a long period of time near such a strong field?

This may be a bit of a long shot, but any experience you have with magnetic shielding and huge superconducting electromagnets will be interesting and helpful!


PS. I read through the recent thread(s) on a very similar situation, but I felt that this would be a discussion focussed on actually shielding the control box at a reasonable field strength, as opposed to the 5 tesla that was previously being considered. Anything closer to the bore in this case is all fibre.

 

[berkeman]

Hey all,

I need to come up with a solution to shield a control box from the magnetic field from a 3T MRI system. With approval (that I will be very persistent to get) I am hoping to be allowed to place the box at around or even outside the 150G fringe surface.

Mumetal seems to be the best possible solution. Does anyone know the limitations of this material? I can't seem to find much information on the web regarding it. At what field strength will it saturate? I took a sample into the shield room with an active 1.5T MRI system and it was obviously strongly attracted to the machine. How will it behave over a long period of time near such a strong field?

This may be a bit of a long shot, but any experience you have with magnetic shielding and huge superconducting electromagnets will be interesting and helpful!


PS. I read through the recent thread(s) on a very similar situation, but I felt that this would be a discussion focussed on actually shielding the control box at a reasonable field strength, as opposed to the 5 tesla that was previously being considered. Anything closer to the bore in this case is all fibre.

We've used Magnetic Shield Corporation for several B-shielding applications, and have been happy with their products and support (I sound like a commercial...).

http://www.magnetic-shield.com/

Take a look at their product datasheets and application notes -- those should be of help. Also, remember that with mu metal, if it is bent to form it into a shape, it needs to be annealed at high temperature to get its high-mu characteristics back. You can't bend up a box yourself to see how well it works. Magnetic Shield Corp can do the bending/forming and annealing for you, either on one-up prototypes or in full production.

 

[mgb_phys]

You are usually lucky to get an attenuation of more than 25x with mu-metal and I would have thought a 1.5T field would saturate it anyway.
Do a search here for a previous thread, but be careful introducing big lumps of metal to the vicinity of MRI machines, especially if there is someone inside them!
People get killed regularly because of flying oxygen tanks or stretchers/tolleys when someone forgets.