What do the gradient magnets do on MRI scanners?

[kkid]

I am trying to understand MRI scanners.

I know that MRIs work by aligning the protons in the direction of the large magnetic field and the radio frequency sets the frequency of the oscillations to the lamour frequency - also raising its energy level. Then when the RF is switched off, the energy level drops by releasing radio waves instead which are detected by the same coil that emitted them in the first place.


I do not see where the gradient magnets come into this. I know the help with location of the atoms but how?



Thanks.

 

[mfb]

RF and the permanent magnetic field strength need some specific relation to get a signal. With a gradient field, this happens in a small region of the scanned volume only - therefore, you know where the signal comes from.

 

[marcusl]

The Larmor frequency of a spin is proportional to the magnetic field it is sitting in, call it $\mathbf{B}_z=\mathbf{\hat z} B_0$. All spins sit in the same field and share the same resonance. If you now add a field gradient onto the static B_0 field $$\mathbf{B_z}=\mathbf{\hat z}\left( B_0 + \frac{\partial B}{\partial a}\right),$$ where a = x, y or z, then you spread the frequencies along the chosen direction. That is, since you've made the z field depend linearly on position in one direction, spins at different positions in that direction resonate at different frequencies. You have taken the first step towards localizing (i.e. imaging) the spins in a 3D body. Similar games are played for to localize in the other two directions.