MRI Electromagnet Interference in a Figure-8 cable

[Ruumahum]

The prevailing wisdom is that coiling excess signal cable under an MRI magnet for storage in a figure-8 configuration will reduce or eliminate induced noise onto the cable from the pulsing fields from gradient coils. I am starting to doubt this, however. A quick diagram of a figure-8 looped cable in a magnetic field doesn't really show me a way to buck the noise effectively. Does anyone have some data to support/disprove this?

 

[berkeman]

The prevailing wisdom is that coiling excess signal cable under an MRI magnet for storage in a figure-8 configuration will reduce or eliminate induced noise onto the cable from the pulsing fields from gradient coils. I am starting to doubt this, however. A quick diagram of a figure-8 looped cable in a magnetic field doesn't really show me a way to buck the noise effectively. Does anyone have some data to support/disprove this?

Welcome to the PF.

Do you understand how current gets induced in a loop when a changing magnetic field pierces the area enclosed by the loop? Are you familiar with the Right Hand Rule?

Look at how the direction of the Right Hand Rule vector flips for the two sides of the figure-8...

 

[BHS]

A figure 8 coil can be effective at reducing pick-up from time-dependent, but spatially uniform, magnetic fields. However, the time-dependent magnetic fields from an MRI machine are not uniform magnetic fields. An MRI machine uses a large constant field, which will not contribute directly to the noise, and many pulsed magnetic field gradient coils. The latter usually are designed to supply a linear gradient of one component of the magnetic field along a certain direction inside the magnet. It is thus quite likely that those fields are also non-uniform outside the magnet. That means that in some circumstances and orientations, a figure-8 coil might actually pick up this noise better than a circular coil.

If you have a figure 8 coil lying on the floor under your MRI magnet and you are getting lots of noise, try rotating the coil (about a vertical axis through the center of the coil). I would start with a 90 degree rotation. Even better, however, is to reduce the amount of cable near your time-dependent magnet fields.