# Placing an AC magnetic field inside a static DC field

• MrNewton
In summary, two different types of magnetic fields can interfere with each other, such as a strong static magnetic field and a second AC magnetic field. The total field is the sum of the two, but for this to happen the generator of the weak field must be unaffected by the strong field. This concept is relevant in designing an inductive coupling system for a hospital's MRI scanner, where wireless power supply via coils is being considered. This concept is possible, as shown in a relevant dissertation on the topic.
MrNewton
Hello,

I am looking for some information on how 2 different types of magnetic fields interfere with each other. And i don't 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 I am 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 I am 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 each other, but i cannot find any info on how those 2 fields will interfere.

Last edited:
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.

Nugatory and vanhees71
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 each other, but i cannot find any info on how those 2 fields will interfere.

MrNewton said:
Now I am 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.

MrNewton said:
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

vanhees71
MrNewton said:
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 each other, 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.

Dale said:
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 doesn't 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 I am not sure if its going to help me.

MrNewton said:
But the MRI is DC, so doesn't 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.

MrNewton said:
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.

MrNewton said:
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.

MrNewton said:
Thanks for the link Dale! I will read it carefully, but on the first glance I am 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.

vanhees71
MrNewton said:
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

Motore
Has your friend considered a battery powered data logger? Arduino comes to mind for a prototype.

## 1. How does placing an AC magnetic field inside a static DC field affect the overall magnetic field?

Placing an AC magnetic field inside a static DC field results in the superposition of the two fields, creating a new, more complex magnetic field. The resulting field will have components of both the AC and DC fields, and the overall strength and direction of the field will depend on the specific parameters of the two fields.

## 2. What is the purpose of placing an AC magnetic field inside a static DC field?

Placing an AC magnetic field inside a static DC field is often used in applications where a more complex and variable magnetic field is needed, such as in magnetic resonance imaging (MRI) or particle accelerators. It allows for greater control and manipulation of the magnetic field, which can be useful in various scientific and technological processes.

## 3. How does the frequency of the AC magnetic field affect the resulting field when placed inside a static DC field?

The frequency of the AC magnetic field can have a significant impact on the resulting field when placed inside a static DC field. At certain frequencies, the two fields can interact constructively or destructively, resulting in a stronger or weaker overall field. This phenomenon is known as resonance and is often used in applications such as wireless charging or wireless power transfer.

## 4. Are there any potential risks or dangers associated with placing an AC magnetic field inside a static DC field?

In general, there are no significant risks or dangers associated with placing an AC magnetic field inside a static DC field. However, it is important to carefully consider the specific parameters and strength of the fields being used, as well as any potential interactions or interference with nearby electronic devices or equipment.

## 5. Can the strength and direction of the resulting magnetic field be controlled when placing an AC magnetic field inside a static DC field?

Yes, the strength and direction of the resulting magnetic field can be controlled by adjusting the parameters of the AC and DC fields, such as their amplitudes, frequencies, and orientations. This allows for precise manipulation of the resulting field, making it a useful tool in various scientific and technological applications.

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