# How do MRI scans work?

1. Feb 5, 2013

### leaamy

I'm a little confused regarding MRI scans.

So you add a strong magnetic field and the protons spin. However, some websites are saying the protons allign with or against the magnetic field. Other websites are saying the protons precess (not even sure what that is) and something about Larmor frequency?

2. Feb 5, 2013

### ZapperZ

Staff Emeritus
At a finite, non-zero temperature, there is a statistical distribution of the population of protons with their spin parallel and antiparallel to the external field. So a percentage of the protons will have spins in the same direction as the field, and the remainder will be in the opposite direction.

The NET result is the macroscopic magnetization, i.e. the net sum of ALL the spin, to produce something similar to a "net force". It is this net magnetization that one often say precesses about the external field.

Zz.

3. Feb 5, 2013

### Staff: Mentor

Hi leaamy, welcome to PF!

Yes, all of those things happen. When protons are placed inside a magnetic field they will align themselves parallel or anti-parallel to the magnetic field. There is a small energy differentc between the two states so there is a slight excess (~6 parts per million) of spins in one direction. This leads to a net magnetization along the direction of the field.

If this net magnetization is preturbed away from the direction of the field then it will precess about the direction of the external field in the same way that a gyroscope precesses about the vertical direction in a gravitational field. This precession is at a characteristic rate called the Larmor frequency. It is about 64 MHz at 1.5 T and about 128 MHz at 3.0 T. That is in the radio frequency range.

So basically, you put protons in the magnet, that turns them into little radio receivers and transmitters, you transmit RF signals in, and you listen for the RF signals that come out, and you build your image from that.

4. Feb 5, 2013

### leaamy

Thank you both. This is the first time I've studied MRI scans in depth and I have to prepare a presentation for tomorrow to present to my class. We're ~18 year olds so we don't know much about magnetic fields yet. If I was asked what precessing was, would I just say a rotation?

I was wondering if you could possibly check this paragraph and ensure it is correct, thank you so much.
MRI scans rely on the concept of NMR (Nuclear Magnetic Resonance).
Hydrogen atoms naturally spin about their axis. The spin produces a small magnetic field. When placed in a strong magnetic field, the protons align either with or against the magnetic field. The resulting magnetic force experiences a torque and precesses with a frequency called the Larmor frequency. The Larmor frequency matches the frequency of radio waves so resonance occurs, during which a large amount of energy is absorbed by the proton causing it to align against the field. As the proton relaxes, it emits energy in the form of radio waves. These radiowaves are then picked up by coils which are amplified and processed by a computer.

5. Feb 5, 2013

### leaamy

Could someone please tell me if this paragraph is correct:
Hydrogen atoms naturally spin about their axis. The spin produces a small magnetic field. When placed in a strong magnetic field, the protons align either with the magnetic field (low energy state) or against the magnetic field (high energy state). The resulting magnetic force experiences a torque and precesses with a frequency called the Larmor frequency. The Larmor frequency matches the frequency of the radio waves resulting in resonance, during which a large amount of energy is absorbed by the proton causing it to align against the field. As the proton relaxes, it emits energy in the form of radio waves. These radio waves are then picked up by coils which are amplified and processed by a computer.

I urgently need to know, thank you.

6. Feb 5, 2013

### Staff: Mentor

I would make a few small changes

Hydrogen nuclei naturally spin. The spin produces a small magnetic field. When placed in a strong magnetic field, the protons align either with the magnetic field (low energy state) or against the magnetic field (high energy state). The resulting net magnetization experiences a torque and precesses with a frequency called the Larmor frequency. The Larmor frequency matches the frequency of the radio waves resulting in resonance, during which a relatively large amount of energy is absorbed by the proton causing it to align against the field. As the proton relaxes, it emits energy in the form of radio waves. These radio waves are then picked up by coils which are amplified and processed by a computer.

7. Feb 6, 2013

### sophiecentaur

An important aspect of the MRI Scanner is that they use a magnetic field with a gradient so that only the atoms that actually lie along one particular line are stimulated. As the field is varied in level, the position of this line moves over your body and they build a picture, a slice at a time. The stronger the Field, the steeper the gradient of the field and the greater the resolution. Using this with three axes, you can get a 3 dimensional image by combining the scans. This tends to be treated as the 'poor relative' of the device technology in any discussions about MRI but the image processing is really sexy!!!

I understand that there is also a time variation of the RF signal (a ping) obtained and this gives a time signature for certain of the chemical compounds present on the scan.