# Oscillating Magnetic Field Heating up blood

1. Aug 24, 2014

### VVS

Hi,

I want to measure the effect of oscillating magnetic fields on oxidative stress in erythrocytes.
I will use frequencies ranging from 0Hz (i.e DC) up to 1MHz the field strength will be 10uT.
But I am not sure if I can do this. Wouldn't the oscillating magnetic field heat up the sample?
Or does that only happen with waves in the Microwave region?
How can I calculate the rise in temperature?

thanks
VVS

2. Aug 24, 2014

### B0b-A

At 10uT [ten microTesla], (which is less than Earth's magnetic field), I doubt it will have any measurable heating effect ...

http://www.ncbi.nlm.nih.gov/pubmed/2255234

Heat in the electromagnets themselves (due to the current passing through them) could be transferred to the blood though.

The electric fields necessary to effect RBCs have to be large too ... http://www.ncbi.nlm.nih.gov/pubmed/13222
http://www.ncbi.nlm.nih.gov/pubmed/8294130

Last edited: Aug 24, 2014
3. Aug 24, 2014

### Staff: Mentor

There will definitely be heating, although at the low frequencies you are using it should not be too much. However, I would recommend that you control the temperature of your samples so that they all have identical temperature. Otherwise you will be measuring thermal stress in combination with oxidative stress. If you control the temperature then calculating it will be irrelevant.

4. Aug 24, 2014

### Staff: Mentor

10 uT is within the range of field strength generated by modern RF coils in MRI. They definitely produce measurable heating. The heating effect is more related to the frequency than the field strength. Specifically, the heating is roughly proportional to the square of the frequency, so at these low frequencies it should be low.

5. Aug 24, 2014

### VVS

Thanks for everybody's answers. I already calculated the power dissipated by the resistance of the coil and the corresponding heat created. This is negligible. However after your answers I will have to calculate the heat created by the magnetic and the induced electric field. For that I guess I will have to use the Poynting Vector. Are oscillating magnetic fields and electric fields in inductors electromagnetic waves?
Thanks again

6. Aug 24, 2014

### davenn

no that's VERY tiny compared to a MRI unit :)

from wiki and other places ....
cheers
Dave

7. Aug 24, 2014

### Staff: Mentor

The main magnetic field strength is on the order of 1 T. The RF field strength is on the order of 10 uT. They are different fields.

8. Aug 24, 2014

### davenn

yes but we are not talking about RF field strength

9. Aug 25, 2014

### Staff: Mentor

Yes, we are talking about the RF field strength. The RF field in a MRI system is an oscillating magnetic field (see the title of the thread). The OP is not discussing the main magnetic field strength which does not oscillate. The main field is completely irrelevant to the discussion here.

Even though it is large, the main magnetic field does not cause any significant heating because it does not oscillate. Even though it is small, the RF magnetic field does cause significant heating precisely because it oscillates. The SAR (the measure of heating) is a significant safety concern for the RF system, not the main field. The danger of the main field is due to projectiles and other ferromagnetic forces, not heating.

In a MRI system a typical RF pulse causes a 180° precession in 1 ms. The gyromagnetic ratio for hydrogen is 42.58 MHz/T. That works out to 11.7 uT for that typical RF pulse. If you look at the FDA guidelines and the vendor safety sheets you can see that all of the SAR constraints are related to that ~10 uT RF magnetic field, not the ~3 T main magnetic field.

With all due respect, davenn, I do research and design of new MRI systems and techniques for a living.

Last edited: Aug 25, 2014