flickering ligthts in MRI rooms

[mshields]

Some vendors of luminaires made for MRI rooms indicate that the wiring method must be aluminum conduit. In your experience, is MC Cable provided its in accordance with the NEC; i.e. fine for normal and ok for 6 foot stretches to the fixture itself for Critical branch lighting. Is Alum MC cable a reliable wiring method; providing an equal level of shielding to Alum EMT?

Thanks,

Mike

 

[hbiss]

I don't know how what you are saying has anything to do with flickering lights. Aluminum is specified because of it's non-magnetic properties, not shielding. I would think that as long as you use MC light- (aluminum armor) for whips you should be OK but I don't know off hand whether it's allowed for critical branch lighting circuits.

-Hal

 

[roger]

I agree with Hal.


Most MRI's I have done use DC lighting to remedy lighting problems however, a few years back a Phillips unit allowed 130 V heavy duty incandescent.

Roger

 

[mshields]

your input

your input

they do all use DC lights with drivers outside the room. The DC protects against the lights interfering with image. The concern I've run into with DC lights is the magnet interfering with the lights such that you get a very annoying flicker.

I think you're right that it shouldn't matter so long as their is a conducting shield around the conductors be it MC cable or EMT (and you're right that the driving factor behind the aluminum is not shielding but that it is non-ferrous.

Anyway - thanks for the input. It is driven by paranoia. We've had multiple projects where light flicker has been a problem. Though certainly none where MC vs EMT proved to be the culprit.

 

[roger]

they do all use DC lights with drivers outside the room.

Not all, as I said, (I see I didn't make it clear it was AC) I did a Phillips unit a few years ago who's own drawings called for 130V AC incandescent lamps for lighting.

Roger

 

[hbiss]

We've had multiple projects where light flicker has been a problem.

I think there is something going on there other than what you believe. I think it's just poor fixture design. Aluminum will not shield the wiring from magnetic fields. Have you tried tightly twisting the conductors before you pulled them? And you know for certain that the voltage to the lighting is not varying with the magnet current?

-Hal

 

[gar]

190326-2134 EDT

This is an interesting discussion. I have virtually no knowledge about MRIs. But I have had a CAT scan. I found it unpleasant because I had to be still for 30 minutes with my arms above my head, and this caused nerve pinching and residual numbness in my fingers. I certainly did not feel confined in the space.

My understanding on MRI is that it somewhat the same kind of space, but a very high magnetic field intensity, and a very repetitive loud noise. Seems to be described as a clicking. A scan may take an hour. Because I have a pacemaker I could never experience an MRI.

The discussions on MRI are not very clear about the function of the machine. Is the magnetic field a pulsed field, and/or is it pulsed and reversed? The RF field is described as pulsed, and of variable frequency. The very loud clicking noise would seem to imply that the magnetic field is pulsed.

If we assume a pulsed magnetic field of high intensity, then an open loop of wire will have a voltage induced in it from the magnetic pulse. If we simply have two parallel wires (Romex structure), then we have an open loop, and a changing magnetic field can induce a voltage. If we assume the magnetic field is fairly uniform over space, then the net induced voltage can be minimized by twisting the wire pair.

Could the above be the cause of light flickering. If it is, then there would be a correlation between the clicking sound and the light flicker. DC power to the lights won't change the above effect.

One Internet reference seems to imply the MRI magnet is pulsed

At certain times during the scan, the scanner will make loud tapping noises. This is the electric current in the scanner coils being turned on and off.

See https://www.nhs.uk/conditions/mri-scan/ . Somewhat below the MRI picture.

On a side, but related subject, a sonogram uses a transducer with 256 elements. I suspect an array of possinly 16 x 16 or 8 x 32 or 4 x 64. The probe looked fairly square thus favoring the 16 x 16 array.. Even with this limited number of elements the moving picture resolution is quite good. A very great deal of quantitive information can be obtained from such a scan.

.

 

[winnie]

The big magnet in an MRI is a large superconducting solenoid. It is not pulsed; it has huge inductance and would require a very large power supply to pulse.

There may be ancillary magnets which get pulsed.

I believe that the big 'pulse' component is in the RF used to excite the protons. The available energy states of the protons (hydrogen nucleii) are set by the background DC magnetic field, and they absorb and release RF photons transitioning between high and low energy states.

The higher the field strength, the higher the frequency and energy of these photons.

-Jon

 

[gar]

190327-1105 EDT

Following are important discussions on MRI.

https://www.cis.rit.edu/htbooks/mri/chap-1/chap-1.htm
https://www.cis.rit.edu/htbooks/mri/chap-2/chap-2.htm not useful
https://www.cis.rit.edu/htbooks/mri/chap-3/chap-3.htm
https://www.cis.rit.edu/htbooks/mri/chap-4/chap-4.htm
https://www.cis.rit.edu/htbooks/mri/chap-5/chap-5.htm
https://www.cis.rit.edu/htbooks/mri/chap-6/chap-6.htm
https://www.cis.rit.edu/htbooks/mri/chap-7/chap-7.htm
https://www.cis.rit.edu/htbooks/mri/chap-8/chap-8.htm
https://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm this gets to describing a machine and thus relates to this thread.

Besides the main superconducting magnet there are modulating magnetic field coils, RF coils, and possibly separate RF receiving coils.

The superconducting magnet won't cause light flicker, it is constant throughout a very long time (a year), but either the modulating coils thru magnetic fields or the RF signals might cause light flicker. Possibly more likely is variation of the AC supply as a result of power variations from the power changes from pulsing the modulating coils, and/or the RF generator.

.

 

[steve66]

190326-2134 EDT

This is an interesting discussion.

You might also like doing a search for "MRI quench" videos. I also remember a couple videos where they let the MRI pull objects into the core. yikes.

The room needs pressure relief vents or doors that swing outward so people don't get trapped in the room by the increase in air pressure in the room if the unit quenches.

Most of the quench goes out the vent pipe, but I believe if its clogged or gets enough backpressure, some can be released into the room.

Here are a couple:

https://www.bing.com/videos/search?...8256533A816CB8ECE3F08256533A816CB8E&FORM=VIRE

https://www.bing.com/videos/search?...0033BE120D24F7D38CB90033BE120D24F7D&FORM=VIRE

 

[winnie]

The big magnet is a superconducting solenoid. As long as it is kept cold it is essentially a permanent magnet.

However there is _lots_ of energy stored in that magnetic field. The thing is a huge inductor with lots of current flowing in it.

If, for any reason at all, any part of the superconductor stops superconducting, then that stored energy starts to get dissipated in the ohmic portion of the conductor. The superconductor getting too warm is one of the reasons it might become ohmic. So if any small portion stops being a superconductor, energy will be dissipated warming up that portion (and everything around it) and you get a big cascade where essentially all of that stored energy goes to boiling off the liquid helium keeping the superconductor cold.

A too strong magnetic field will cause the material to not superconduct, as well as a few other things. Apparently when they build really big magnets for particle accelerators, one of the construction steps is to 'train' the magnet. They build the thing, cool it down, and then start applying current and building up the field. Eventually the magnetic forces on the current carrying conductors will make something move. You get frictional heating and the magnet quenches. Then they go and do it again. Each time they get to a higher and higher field, until they reach their design point.

I bet the lights flicker when that happens

-Jon