• Question: hello i was wondering if you could explain how the MRI machine works?

    Asked by to Joe on 15 Mar 2010 in Categories: . This question was also asked by .
    • Photo: Joseph Devlin

      Joseph Devlin answered on 15 Mar 2010:

      Yes, I wonder if I can explain it too 🙂 Normally I use diagrams because it’s easier to do with pictures, but I haven’t figured out a way to embed pictures in these replies yet so I’ll try with just words.

      And MRI scanner is basically a BIG magnet. Typical ones are about 1.5 to 3 Teslas in strength — about 50,000 to 100,000 times as strong as the earth’s magnetic field and roughly 10,000 times stronger than a fridge magnet. These are considerably stronger than the magnets used to pick up cars and trucks at junk yards. It’s strong!

      Anyway, if you put a person in an MRI scanner, the magnetic field makes hydrogen nuclei essentially “line up” with the magnetic field lines. Because your body is made of a lot of water (H2O) there are lots of hydrogen atoms floating around. Within their nucleus is a single proton which has a positive charge. It is also spinning (it’s a quantum thing). Anyway, a spinning charge makes it act like a very tiny bar magnet and when you put a bar magnet in a stronger magnetic field, it lines up with it. That’s just what our protons are doing.

      If you then introduce energy at very specific frequencies (again, determined by quantum mechanics), you can make those bar magnets flip so they line up against the magnetic field. Gradually they will relax and flip back and the speed of this depends on the type of tissue you’re looking at. As they relax they give up radio waves and we can record these with special antenna in the MRI scanner. So different types of tissue such as grey matter vs. white matter in the brain, for instance, send out slightly different signals which we record and show differently in the images. Because bones have very little free water in them, they don’t give off much signal at all, so MRIs aren’t good at imaging bones but are very good at soft tissues like skin, organs, tumours, ligaments, etc.

      That’s it in a nutshell. If I can figure out how to add images, I can illustrate this more clearly. I’ll check it out.

Comments

  • luke commented on :

    ok thank you, ok thank you, i was also wondering what the difference between a MRI scan and a CAT scan.

    • Photo: Joe

      Joe commented on :

      Hi Luke,

      The basic difference is that CAT scans use X-rays while the MRI scanner uses strong magnetic fields. There are good and bad to both of these, though.

      CAT scans are pretty safe in that just about anyone can safely have one. But you can’t have lots of them because the effects of the radiation build up and that can be dangerous.

      MRIs are great if the person has no metal in their body (fillings in your teeth and some old surgeries are fine). But if there is metal and it could move when placed in the magnetic field, then it is dangerous to do an MRI on that person. So if a person arrives in casualty unconscious and you don’t know anything about them other than they’ve had a head injury, you can’t put in them in an MRI scanner without first checking they have no metal in their bodies. You can put them into a CAT scanner though, and that will also show any metal.

      Overall, the images from an MRI scanner are much clearer, particularly for soft tissues but they don’t show bones very well. X-rays are really good for seeing bones, so CAT scanner are much better for that.

      Hope that helps!