That Neuroscience Guy - MRI

Episode Date: October 17, 2021

Magnetic Resonance Imaging, or MRI, is an essential clinical tool for diagnosing injuries of both the brain and body. As it turns out, it's also incredibly important for scientific research on the brain. In today's episode of That Neuroscience Guy, we discuss how MRI works, and why it's so well-suited for studying the brain. 

Hi, my name is Olive Kregolson, and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. For a lot of people, an MRI scan, magnetic resonance imaging, is something scary. It basically means that your doctor needs to see what's going on inside your body or your brain. So we try to avoid it at all costs. However, on the research side of things, magnetic resonance imaging and a specialized version of it, functional magnetic resonance imaging, plays a crucial role in modern neuroscience research and helping us understand

what's happening in the brain. On today's episode, we're going to talk about how MRI and fMRI work and why researchers use these tools. The holy grail, if you will, of neuroscience research is a problem of space and time. By space, I mean where in the brain something is happening. So where is their activity? Because different structures, as you've learned, are responsible for different things. So it's crucial for us to understand the where in the brain. The flip side of this is time. We want to know when that activity is happening and the time course of that activity. We want to know when that activity is happening and the time course of that activity. For instance, if you show someone a picture, there's going to be activity in the eyes and the optic nerve,

and then eventually at the back of the head in the primary visual cortex. But that activity processes from there, and it moves through the temporal cortex like we talked about last season, or up through the parietal cortex, and the time course of that activity is really important. We talked about EEG last season, electroencephalography, and that's the strength of EEG. It allows us to see the time course of brain activity, but it's really not that good at the space problem. EEG typically comes from a bunch of electrodes over the top of the head. Just because one electrode has more activity than another doesn't mean that that's where or the regions underneath the brain are where that activity is happening. MRI and fMRI are quite the opposite. They provide excellent spatial resolution. We can see through the entire brain structure, and we can see it to a resolution of one millimeter

by one millimeter by one millimeter little cubes, which are called voxels. Now, some scanners have higher resolution, but the one by one by one is pretty much an industry standard. So, how does it work? Imagine your doctor suspected that something was wrong inside the brain. They might send you for an MRI scan. And just so you know, this is equally applicable to if you're getting an MRI done of your knee or another part of your body. The logic's the same. Within your body, there's a whole bunch of hydrogen. And these hydrogen molecules have little electric fields and magnetic fields associated with them. Literally, like a bar magnet from elementary school, a North Pole and a South Pole.

And the next magnetization of your body is basically it all cancels out. And there's, of course, some influence from the Earth's gravitational fields. So what is an MRI scanner and how does it actually work? Well, like the name says, it's magnetic resonance imaging. So the simplest way to visualize an MRI scanner is it's a massive magnet. Now, it's actually an electromagnet. MRI scanners typically require massive amounts of electricity to function and create the magnetic fields that are needed to probe what's going on inside the brain or the body.

And basically, when the magnetic field is on, what happens is all of those hydrogen molecules become aligned. The north and south poles line up with the field, and you have a net magnetization. Now, this is completely harmless. It doesn't hurt you whatsoever. I've had MRI scans done to me multiple times for medical reasons and for research reasons. It doesn't hurt you whatsoever. I've had MRI scans done to me multiple times for medical reasons and for research reasons. It's completely safe. Now, think about what's happened. Your head is in the head coil of the magnet. And just so you know, it's incredibly loud in there. Typically, they give you music to listen to or something because you can hear the magnetic field and it's almost like a pounding of a hammer. And that's because as you're going to learn, the magnetic field is actually switched

on and off over and over again, very rapidly. Now you have all those hydrogen atoms aligned in your brain. And how does that get us a picture? Because if you think about what MRI is, you see pictures of the brain, or if you do fMRI, pictures of neural activity. Well, basically, a radio frequency pulse is applied, what we call RF energy. So what that does is it actually knocks the magnetic fields of these hydrogen atoms back out of alignment. All right, so you've got these things lined up due to the magnetic field. You quickly switch it off, you hit it with an RF energy pulse, and then these things get knocked out of alignment. Now this gets a little bit technical, and I don't want to get into the physics of all this, but essentially at the end of the day,

when these things return back to being aligned, because remember you're still in that magnet, RF energy is released, literally radio frequency energy. Now what's cool about that and how it works is the energy that's released differs for different types of material. What I mean by that literally is you can differentiate different types of material from the type of energy that's given off and it shows up as different colors. If you've seen an fMRI scan, you would have seen that different pieces of tissue have different colors. The bone typically comes out as white, especially in what we call a T1 scan. And brain tissue comes out a little bit more gray. The white matter itself is a little bit lighter again, and other regions are dark because they

don't give off RF energy. Now, typically typically for MRI we have two types of scans. I don't want to get too technical but we either do what's called a T1 scan where we wait a very short time before we take a look at what's happening with the RF energy released or a T2 scan where basically the computer running it just waits a bit longer. And the reason we do that is by waiting a little bit longer the energy that's given off changes a bit longer. And the reason we do that is by waiting a little bit longer, the energy that's given off changes a little bit. What happens is different types of tissue are easier or harder to see. So T1 scans are usually most useful for anatomical detail. They allow the structures of the brain to be very clearly seen. T2 scans are used more for

pathological changes. So for instance, if you want to see a change in a brain structure due to a neurological condition. Your doctor, of course, and the neurologist knows the type of scan that you use. And different researchers look at these scans that are studying different things. In my lab, we use T1 scans a lot because we actually want to see the anatomical structure of the brain and it'll make a little bit more sense when I talk about functional magnetic resonance imaging. So what is fMRI? Well, the basic idea is the same. You're gonna put people in a very large magnet and fMRI, typically we're interested in brain activity.

So there's actually an additional head coil that goes around your head. And this creates a different field. Now, what we're doing with fMRI, we're not actually looking at brain tissue, but with the additional head coil, we can actually see RF energy released and how it's influenced by oxygenation in the brain. Of course, when brain regions are active, there's increases in blood flow because you need blood to recharge the neurons so they can fire again. And this changes the oxygenation levels of the brain. Specifically, and we'll get a little bit technical here, we're looking at the proportion of deoxyhemoglobin to oxyhemoglobin,

and we can see those differences as changes in blood flow. And from that, we can infer more brain activity. Think of it this way. I'll give you an example. Let's say I asked you to make a hard decision. So I put you in an fMRI scanner and I say, look, do you want to have a 60% chance to win $100 or a 40% chance to win $200? Now, we've talked about expected value, so you could probably do the math and figure out which one you want to take. But that's going to cause neural activity. Neurons in the prefrontal cortex and other brain regions are going to activate. And because they're activated, there's going to be changes in blood flow. And with this fMRI signal, by scanning the brain and by looking at the images that we generate, we can see these changes.

It's actually called the hemodynamic response function. It's the change in blood flow in a particular part of the brain. And remember, it goes back to those little voxels I talked about at the start. We actually go through in little tiny cubes. So what's actually happening to pull it together a bit more is as you're lying there, these radio, the magnetic fields aligning things, the radio pulses are knocking them out. And we're taking a whole bunch of pictures, sometimes 26 every two seconds, sometimes it's 32. It all depends on the scanner. But by going through those pictures, we can reconstruct the brain,

if you will. We can build a nice 3D representation, and then we can hunt through those little voxels and look for changes in blood flow. If we see more blood flow, we believe there's more brain activity. If we see less blood flow, we believe there's more brain activity. If we see less blood flow, we believe there's less brain activity. Now, these come up literally as just different shades of gray. The regions, if there's an increased proportion of deoxyhemoglobin, tend to be a little bit darker. If there's a decreased proportion of deoxyhemoglobin, they tend to be a little bit brighter. But by looking at these differences in color, people have figured out how to infer those differences in blood flow. Let's use another example to try to bring it all together.

Imagine you were interested in studying learning, something that I do quite a bit of. Well, you could imagine giving someone something to learn in an fMRI scanner. All right, you teach them a new language, do you teach them some geography they don't know? So they're lying there, and as you give them feedback, you tell them, hey, you got that question right. Or no, you got that question wrong. You're going to get a series of images associated with those events.

You're going to get images that were taken while people were processing the feedback that they get. And by looking for those differences in color, we can say, hey, when people were processing positive feedback, saying they got it right, there was more brain activity or more blood flow in a place called the ventral striatum. You're right to remember it from season one. We talked about it during our episode on learning. And you might also say, hey, there's decreased activity following negative feedback in a different part of the brain. So that's literally what fMRI researchers do. They have people perform experiments in the fMRI scanner, and it's really tricky. You're lying on your back in the scanner, your head's in a head coil. You might be holding a little button box to make sure that you can respond yes or no or ABC for multiple choice.

And because it's a big magnet, you can't put metal in there. So all of this stuff has to be made of materials that can't be magnetized. And then the actual monitor that you're watching is typically in the back of the room and you use a little periscope. It's kind of a weird way to do research. is typically in the back of the room and you use a little periscope. It's kind of a weird way to do research. But by studying those images that come out of there, we can infer which brain regions were activated and which ones weren't. And that allows researchers to draw any number of conclusions about what's going on in the brain and how it all works. I will tell you one of the problems with fMRI research is that the hemodynamic response for

neural activity typically takes about eight seconds and a lot can go on in eight seconds so this is one of the things that fMRI researchers have to worry about and deal with is how do we design our experiments to take into account the fact that hey it might take a little bit longer than we hoped to see this difference in brain activity. Anyway, that's a little bit about how MRI and fMRI works. The summary is pretty straightforward. We use a magnet to align all of those hydrogen atoms that comprise the tissue in our brain. Then we knock them out of alignment with a radio pulse. And as the hydrogen atoms come back into alignment, they give off energy. And that energy shows up in the images that we take. Because at the end of the day, an fMRI scan or an MRI scan just generates a picture.

It actually generates a whole bunch of pictures because you're taking a whole bunch every couple of seconds. But you can put all those images back together and you can draw insight into what's going on in the brain. Well, I hope you enjoyed today's episode. I know it's a little bit technical, but I feel like if we're going to talk about all these tools that researchers use, we should explain a little bit about how they work. My name is Olof Kregolsen and I'm that neuroscience guy. Remember, you can follow me on twitter at that neuroscience guy or we have our youtube channel now that neuroscience guy or you can email the podcast if you've got some ideas for episodes that neuroscience guy at gmail.com and if you

haven't done so please check out my tedx talk it's all about some cool research i did in support of a nasa mars mission my name is Olaf Krigolsen, and I'm that neuroscience guy. Thanks for listening and see you on the podcast next week.