The science behind an MRI
Today I had an MRI, don’t worry it was for research purposes. It wasn’t a short MRI either, I was in the bore for almost two hours. Yep, stuck in a tube with nothing but my thoughts, some music, and the sounds of the MRI machine doing its thing. I figure we can discuss what an MRI is, why I had one, and why I don’t mind that it took so long. I may even show off some of the scans, but not today I don’t have them yet.
An MRI machine is an amazing tool. There are several different “strengths” of MRI and that is designated by 1.5 T, 3 T, or even 7 T. The T in 3 T is tesla and it measures the magnet strength. For comparison the earth’s magnetic field at the equator, 0.0000305 T. The how it works is complex, but we can go through a rough rundown for the layperson.
Basically water has magnetic poles, you have 2 hydrogen and 1 oxygen one side is weakly positive (the hydrogens) the other weakly negative (the oxygen). This is good because we’re made up of water! About 60% of the adult body is water and almost 75% of the brain is water! So when we put the body in strong magnetic field we cause the water molecules to align with the field. That’s step one, the MRI is always on so you always have this field going. To get the image we need to perturb the protons (the positively charged bits in the water), this causes them to flip! The flip creates a radio signal and we use a coil (well several) to measure this change. There are a few properties we can use to measure, one of the most common is how long it takes for them to flip back. Different tissues take different amounts of time for the protons to flip back, which is how we create the image.
How to we cause them to flip? We use radio waves… which is the noise you hear if you’ve ever been in an MRI. This brings up why I was in the scanner for so long! We can use different sequences of radio waves to create different types of images and highlight different tissues. They had to use several different sequences to find which ones highlighted the things they were interested in best. This can take some time because it’s research so they don’t know beforehand which sequence will give the result they want, instead we cycled through multiple sequences. All the data collected is useful, but the last sequence they used gave them some of the clearest shots.
My MRI was a cervical MRI, since the research I do focuses on the spinal cord, this data will help our lab and other labs develop better models for things like non-invasive spinal stimulation or other simulations that can teach us about how the spine works. The images are created using software so they are generated in real-time without any work. Once I was done I even got to see my scans. You could literally see the nerve rootlets on the spinal cord, which I think was the coolest part of the whole thing. The rootlets of the spinal cord are delicate and tiny structures, but we were still able to resolve them in the MRI. The image below shows what the rootlets look like, but don’t give you a real scale of how tiny they are. For reference your spinal cord is about as thick as your pinky! Yeah, the rootlets are tiny!!!
Now the higher the field strength the better resolution you can get because more protons end up aligning with the field. I was in a 3T today, but we have a 7T and I could’ve been in that, but we didn’t have the coils for it! There are several different types, but the coils I’m talking about are what capture the information to produce the image. Since we only have a head coil, we wouldn’t have been able to resolve the cervical spinal cord. 7T are really rare and we’re lucky to have access to one, but because they are so rare getting the equipment for it takes time. We’re in the process of getting the right coils for the job so there’s a good chance I’m going back in the MRI for another set of scans eventually. Then again, the scans we have now came out so good, that might not be needed!
So that’s the quick and dirty rundown of how an MRI works! Now, there’s a lot more to it than I went into and I probably oversimplified quite a bit of it. However, this gives you a good starting point in understanding and is probably far more than you’d ever need to know. If you’re interested in learning more there are a ton of websites that go in depth (read more) (read more) (read more).