ELI5…how does an MRI work?


I’ve had a few MRIs so far in my life and have always wondered (in simple terms) how does the MRI work? How does it get such detailed images?

In: Technology

Hydrogen in water have one proton that spins around on its axis like a spinning top. An MRI scanner makes the protons in hydrogen align in one direction it then uses another magnetic field to tip over the protond like when a spinning top starts to loose momentum and no longer points upwards. The tipped over proton spin at a certain frequency and can then induce a current in an Arial (those things they put over your body part being scanned) and be mathematically turned into a picture.

The frequency they spin at can be manipulated to highlight or remove certain things on the picture like water or fat to help the diagnosis. The sounds you hear is the magnetic fields working away which causes themselves to vibrate causing the noise.

Basically it can picture water and most the body is water, its not good at picturing the lungs as there is not much water. The detail of the image is determined on how much time you are willing to wait as it has to wait for the protons to be tipped over and recover again and again hence why they take so long.


Apologies in advance for the length of this post, but you’ve asked a complicated question.
Bear with me…

MRI – magnetic resonance imaging – uses the principle of nuclear magnetic resonance (NMR), specifically the NMR behavior of hydrogen atoms. H has just one proton in its nucleus. Hydrogen atoms are part of all different kinds of molecules in the body – water molecules, different kinds of fat molecules, collagen, etc. The nuclei (protons) of these various hydrogen atoms in different molecules exist in very different environments, influenced by the density of the electron clouds around them, which depends on what kind of molecule it is. MRI exploits these differences, so that the protons in a water molecule, for example, can give either a brighter or a darker signal than those in fat, depending on the settings on the MRI, which you’ll see from what’s written below means how the area being scanned is pulsed with radio waves. We call this a pulse sequence, and they have gotten quite ingenious with various pulse sequences.

Protons have available to them two separate quantum states, with respect to a phenomenon called “spin.” When the proton is put into a magnetic field, these two states separate into a higher and a lower energy state, and most of the protons will settle into the lower energy state. If the proton is hit (“pulsed”) with a radio wave pulse having the same amount of energy (which really means frequency) as the energy between those two spin states, protons in the lower energy spin state will be boosted into the higher energy spin state. The lower energy spin state is aligned with the magnetic field (magnetic fields have a directionality to them), but the higher energy spin state is not, and now with more protons in the higher energy spin state, their spin, all summed together as a group, produces a radio wave signal that can be detected by a coil placed on the body part. The coil serves as both the delivery system for the pulse and the receiving system for the signal. So the coil would be called a transducer. It delivers a radio wave pulse and collects a radio wave signal. Exactly how the pulses are delivered is what the pulse sequence is all about.

Over a period of milliseconds to seconds, the excited (which means they’re in the higher energy state) protons will relax back to the lower energy state, and these pulsating radio wave signals will disappear. How fast they disappear varies with the type of molecule the proton is part of, and this property is used to distinguish between tissue types. Pulsing the protons over and over again with the radio wave stimulus repeats this process (pulse – collect – pulse – collect, etc), and the radio wave signal that is collected each time can be summed together, so that electronic noise is reduced, and the true signal emerges. The signal emitted by the spin of the excited protons and detected by the coil has a frequency that is precisely the same as the difference between the energies of the low spin state and the high spin state. So like a radio that can receive a radio wave consisting of all different frequencies in a song, the coil in an MRI can pick up, and the electronics can distinguish all these frequencies as well. Now you have a bunch of different radio signals, all having different frequencies and intensities.

The trick is knowing exactly where each of the collected radio wave signals came from in the body part being scanned. This was the genius of MRI. The difference in energy between the low energy and high energy spin states of the proton is proportional to the strength of the magnetic field. Stronger magnetic field = bigger difference in the two energy states. So in an MRI machine, you have a high-intensity magnetic field in the form of a big permanent magnet, and then a smaller magnetic field that varies in a precise fashion across the length of the coil placed over the body part being scanned (a “gradient” field). So since you now know the total strength of the magnetic field at every spot on the body part, you can assign each frequency obtained in your radio signal to a specific spot on the body part under the coil, and you know the intensity of the signal, which is translated into brightness. This gives you 2D resolution – think of the xy axis. The third dimension is obtained by changing the phase of the signal along the third axis – think of the z axis. In the end, you have intensity of signal information for each volume element (voxel, the 3D equivalent of a 2D pixel) in the area scanned, and this is converted by the computer (big computer) into a 3D model. Then the computer creates 2D slices in different directions through the 3D model, which is what we end up looking at in the pictures.

To finish this all up with a little opinion, this invention was brilliant and deserved the Nobel Prize in Medicine (2003).