Placing an AC magnetic field inside a static DC field

  • Thread starter MrNewton
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Hello,

I am looking for some information on how 2 different types of magnetic fields interfere with eachother. And i dont mean, 2 magnets, but let me be specifically:
Lets say that you have a very strong static magnetic field, from a huge magnet. (for instance, the strength of the magnet of an MRI scanner. >3T)

Now im going to apply a second AC magnetic field, inside the strong magnetic field of a MRI scanner (for instance, a wireless charger). What would theoraticly happen to the magnetic fieldlines? How would the strong DC field interfere with the second AC field? And how would the second AC field interfere the strong DC field?

The reason im asking is because of this: A friend of mine was telling me about a project he is doing for work. He has to design a inductive coupling system for a hospital in his town. They have an MRI scanner, and normally, if a person is placed in the scanner, he will get some sensors attached to his body. (im not sure if they are called sensors, but OK). Those sensors will be connected to a ADC converter, which is placed next to the patient, inside the MRI scanner. Those ADC's need power supply, which means that a couple of cables will be routed from the power supply (which is in the other room) to the ADC's inside the MRI scanner. Now the hospital wants the powersupply of the ADC to be wireless (i.e. inductive coupling). The concept was (i think) that they will place coils at the outside of the bore (the tunnel in which the person in placed) and place the ADC's inside the bore. No more powersupply cables, but wireless powersupply via the coils on the outside of the bore. I thought this would be impossible, because those fields will interfere with eachother, but i cannot find any info on how those 2 fields will interfere.
 
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  • #2
Vanadium 50
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In principle, the fields add. In practice for that to happen the generator of the weak field needs to be unaffected by the strong field.
 
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  • #3
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Thanks for your reply. The reason im asking is because of this:
A friend of mine was telling me about a project he is doing for work. He has to design a inductive coupling system for a hospital in his town. They have an MRI scanner, and normally, if a person is placed in the scanner, he will get some sensors attached to his body. (im not sure if they are called sensors, but OK). Those sensors will be connected to a ADC converter, which is placed next to the patient, inside the MRI scanner. Those ADC's need power supply, which means that a couple of cables will be routed from the power supply (which is in the other room) to the ADC's inside the MRI scanner. Now the hospital wants the powersupply of the ADC to be wireless (i.e. inductive coupling). The concept was (i think) that they will place coils at the outside of the bore (the tunnel in which the person in placed) and place the ADC's inside the bore. No more powersupply cables, but wireless powersupply via the coils on the outside of the bore.

I thought this would be impossible, because those fields will interfere with eachother, but i cannot find any info on how those 2 fields will interfere.
 
  • #4
Dale
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Now im going to apply a second AC magnetic field, inside the strong magnetic field of a MRI scanner (for instance, a wireless charger).
Or for instance the oscillating magnetic field from the RF coil actually used for MRI.

What would theoraticly happen to the magnetic fieldlines? How would the strong DC field interfere with the second AC field? And how would the second AC field interfere the strong DC field?
The total field is just the sum of the two. That is pretty central to how MRI works
 
  • #5
Dale
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The concept was (i think) that they will place coils at the outside of the bore (the tunnel in which the person in placed) and place the ADC's inside the bore. No more powersupply cables, but wireless powersupply via the coils on the outside of the bore.

I thought this would be impossible, because those fields will interfere with eachother, but i cannot find any info on how those 2 fields will interfere.
The concept is indeed possible. Here is the relevant dissertation of a colleague whose research was exactly on this topic.

https://etd.ohiolink.edu/!etd.send_file?accession=case1377183452&disposition=inline
 
  • #6
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Or for instance the oscillating magnetic field from the RF coil actually used for MRI.

The total field is just the sum of the two. That is pretty central to how MRI works

Just like every other combinations of waves on the planet right? But the MRI is DC, so doesnt have waves. How will the small AC magnetic field interfere with the MRI? Will the images of the MRI be affected?
I read that if you bring a coil in which a AC current is flowing, will start to mechanicly vibrate, like the coil of a speaker. I also understand that the used coils cannot have a core, because in those strong magnetic fields the coils will go into saturation.

Thanks for the link Dale! I will read it carefully, but on the first glance im not sure if its going to help me.
 
  • #7
Dale
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But the MRI is DC, so doesnt have waves.
That doesn’t matter. The linearity of Maxwell’s equations is not frequency dependent. All solutions of Maxwell’s equations are linear, regardless of the frequency or even whether or not they are waves.

Will the images of the MRI be affected?
It is possible. If the the AC source is at the resonant frequency then you will have severe artifacts. At other frequencies you could get saturation transfer or thermal proton resonance shift effects.

I read that if you bring a coil in which a AC current is flowing, will start to mechanicly vibrate, like the coil of a speaker.
Yes. That is the physical principle behind the acoustic noise generated by the MRI gradient coils. If the AC frequency is in the audio range it will be audible.

Thanks for the link Dale! I will read it carefully, but on the first glance im not sure if its going to help me.
You are welcome. You can also go down the list of references and possibly find shorter papers that may be more focused on your exact needs.
 
  • #8
davenn
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They have an MRI scanner, and normally, if a person is placed in the scanner, he will get some sensors attached to his body. (im not sure if they are called sensors, but OK).

that's interesting
I have had 8-10 MRI's and several CT scans
Have never had sensors placed on my body, in fact quite the opposite, The MRI etc technicians go to
great lengths to make sure there are no other bits of anything else on the body ( other than hospital gown), particularly metal
 
  • #9
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Has your friend considered a battery powered data logger? Arduino comes to mind for a prototype.
 

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