Subtitles section Play video Print subtitles J. MICHAEL MCBRIDE: OK, welcome back. I hope you had a good break. Can you remember back to when we, before break, what we were talking about? The last thing we did was this gyroscope bicycle-wheel precession to show what would happen to a nucleus that was spinning in a magnetic field, or an electron for that matter. Just to rehearse it a little bit, remember the idea of a pulse, a 90 degree pulse, that if you have a big magnetic field-- the blue one there, really enormous-- and then the little magnet of the nucleus precesses at 100 MHz, for example, in a certain field. And that gives rise to a constant vertical field, but a rotating horizontal field from that one. So the question is whether that rotating horizontal field, which as you see it will be going back and forth and back and forth, will act as an antenna and give you a signal. You should be able to pick up a radio signal at 100 MHz. Indeed, you should be able to, except that there's not just one proton. There are lots of protons, and they're in different phases of precession. So although they all add vertically, and you have a substantial vertical magnetism from those, their horizontal components cancel, so you don't see anything. In fact, the energy is so small, of the interaction of each of these magnets with the field, that there are ones pointing the opposite direction, with higher energy, almost exactly the same population. Just a tiny, tiny difference. But we'll look just at the excess. Obviously, ones down will cancel ones up. But if we look just at the net ones up, even they cancel, because they're at different phases of rotation or precession about that axis. But you can do a trick. We could consider that we're rotating with them, so that they look like they're standing still. And then in our frame we'll put on a little bit of a magnetic field that's horizontal, a very weak field. And now, as far as we can see, we don't care about the big field anymore, because we've compensated for it by orbiting around this thing as we're looking at it. But what we see is that these will begin to process in our frame around that horizontal field, so they'll do a slow precession, all of them, in that direction. So they'll be going like this. They'll start here, they'll go there, then they'll go down, then they'll go back, and then back up again. But we're going to put on a pulse only long enough, of this field, so that they go down to here. That's called a 90 degree pulse, and they'll rotate down like that. And now forget the rotating frame. They'll look like they're just pointing out toward us in the rotating frame. But if we go back to the real frame, the laboratory frame, what we see is this whole bunch precessing around the field. And now as they precess around the field in the laboratory frame, there will be a net horizontal field that they're generating, a magnetic field. And that will be the antenna that broadcasts a signal that we can hear. And what will determine the frequency of that signal that's going to be coming out from going back and forth, and back and forth or around? What will its frequency be? How rapidly are they precessing? It says 100 MHz. And what determined the 100 MHz? The strength of this big magnetic field. Remember, the more you twist on something, the faster it precesses. So we could make it 100 MHz. If we had half as big a big field, it'd be 50 MHz. Or twice the field, it would be 200 MHz, and so on. So we get a signal that's 100 MHz radio frequency in the laboratory frame, that we could detect with an antenna. But in time it will relax. This is a non-equilibrium situation when we put the energy in to make it go down. And in time it'll come back to equilibrium. And that process is called relaxation. And there are various things that control how fast that happens. But it will reestablish equilibrium, and it will be very important later in the lecture about this relaxation, and you'll see why. So a 90 degree pulse makes the spinning nuclei, protons, or C-13s broadcast a frequency that tells what their local magnetic field is. The higher the field, the faster they precess, the higher the frequency. Now let's first look at this as it arises in magnetic resonance imaging, where the purpose is to locate protons within the body using a non-uniform magnetic field. Now, the idea of tomography is important here. I've taken this from a Colorado Physics 2000-page. So this is a slice through somebody's body, show their rib cage, spine, and so on, and they're wearing some sort of jacket that's opaque to X-rays. And we're interested in finding what it looks like inside, where these things are. So what we do is we take X-rays, and send an X-ray beam straight through and see how much gets through. And we scan the X-ray from top to bottom, and see how much gets through at every different x-coordinate. So we do a scan, and it starts at the top, but oops, there's something there. And there's even more there. And lots, then a little bit more. And for when we hit the spine there's going to be quite a bit, and so on. But that's just a one-dimensional picture of the density. Now what we're going to do is take that same picture and just smear it out to the right. So that's the profile, top to bottom, or stomach to back, of this particular slice through the body of bones or whatever it is. Now the neat trick is that you rotate that, rotate it by 15 degrees. And now do the same thing again, and superimpose the new one on the old one. And now rotate another 15 degrees and do the same trick, scan top to bottom and add it up. And do it again, and again, and again, and again, and again, again, again, and again. And see what you got now? When you superimpose all those, you get what it looked like, the two-dimensional slice through the thing. So that's called tomography. If you can get a one-dimensional projection, and do it in lots of different directions and add them together, you can get the two-dimensional, or in fact, a three-dimensional picture of what's going on inside. So that's the trick that's used, except you want to do it for protons, not for bone. So we want to find protons in the body. For example, let's find where there's fluid water in the body. So there's a body, and we put it inside this cylinder and wrap the cylinder with special wire, that if we cool it to liquid helium temperature is superconducting. So essentially, we've made a big solenoid magnet that goes along the body's axis. Now, what will happen? Well, suppose that field is 1.5 Tesla, which means 15,000 Gauss. A Gauss, you remember, is about the size of the earth's magnetic field more or less, so 15,000 times as strong as the earth's magnetic field. So what will happen to the protons in there? Well, they're going to precess. And in that field, at 15,000 Gauss or 1.5 Tesla, they'll precess at 63 MHz. So if we put an antenna in there and give a 90 degree pulse, we're going to hear a signal at 63 MHz. Our radio will pick that up. So we know there are protons in the body. Surprised? No. The question is, where are the protons in the body? Now here's an analogy to figure this out. Suppose we had a cricket in this room, and wondered where it was. But I'm blind. I can hear, but with only one ear, so I can't hear it. I don't have spatial resolution with my ear. How can I find out where the cricket is in this room?