Last fall, I took a class called “Medical Imaging: Theory and Implementation”, one of the graduate level classes offered as a capstone for my specific concentration within my biomedical engineering major. The course, taught by a really cool professor named Dr. Parker (he’s done a lot of REALLY dope and novel research in several medical imaging fields), took us through a semester long journey unveiling the history of medical imaging. It started with x-ray, moved into computed tomography (CT) (which is sort of like 3D x-raying), then ultrasound (US), magnetic resonance imaging (MRI), and more advanced and developing techniques such as positron emission tomography (PET) and optical coherence tomography (OCT). It’s okay if none of these words mean anything to you, because a lot of them still don’t mean anything to me. It’s pretty crazy stuff, and the class was really hard. But, when we got to ultrasound (fun fact: ultrasound as a medical imaging technique was actually inspired by SONAR scanning during WWI), I was really relieved. This class was super hard, but because I had been studying ultrasound for almost an entire year and had just presented research at my first conference ever, I was like, “yes! please teach me something I already know, because I would like to get a good grade!” It was cool to finally learn about ultrasound in a classroom setting, because up until then, I had taught myself or learned through my PI.
Then, we learned about MRI, and it blew my socks right off. This imaging modality is just downright cool. While ultrasound works by transmitting an acoustic signal (a pressure wave) into a medium, and then measuring the reflected wave back from the signal (a lot like how bats use echolocation to see), MRI works by manipulating a magnetic field that surrounds a medium in such a way that it sort of almost “talks” to the atoms inside. The way my professor described it, instead of sending in a signal and measuring whatever gets reflected back out, MRI actually listens to the object “sing”. To the right is an image of an MRI machine from General Electric.
Let me explain how this actually works in a way that (hopefully) someone who doesn’t understand (sort of me) or enjoy (def not me) physics could appreciate. MRI deals a lot with quantum physics, of which I know very little (aka, I have a LOT of learning to do). But basically, if you remember from chemistry class, atoms are made of protons, neutrons, and electrons. A principle of chemistry that I really can’t explain (because I don’t quite get it myself) is that atoms with an odd number of protons and/or neutrons have this property called “spin angular momentum”, which causes the protons to spin and produces a magnetic dipole (basically, the proton becomes a tiny bit magnetic). For each element, the protons are all “spinning” at the same rate. For example, all hydrogen atoms spin at the same frequency, which is different from the frequency at which all sodium atoms spin.
Now, a principle of physics called Faraday’s Law of Induction implies that, when you run a current through a coil (think like a spring or a slinky), it will interact and align a magnetic field inside of the coil, as shown to the left. (Note the coordinate system. A scientists NEVER forgets to define their coordinate system!) So how does this apply to MRI? Well, an MRI system is basically just a bunch of big magnetic coils, and the patient gets put directly in the middle. So, when this current turns on, it induces a magnetic moment inside of the patient – AND THEN IT ALIGNS THE MAGNETIC MOMENTS OF ATOMS IN THE BODY. Because different types of atoms spin at certain frequencies, due to physical relationship between these spins and the magnetic field, if we create a magnetic field at a certain strength, we can target specific atoms in the body. Further, adding even more fields (just by using more coils) allows us to be more specific in the atoms we target. In MRI terms, we are targeting spin frequencies, which are called Lamour frequencies.
Another helpful analogy in this imaging modality is that of a spinning top. We can think of each of these atoms as an individual top. The first field we apply, called B0, aligns all of the atomic spins, and is basically what happens when the top first starts to spin. It’s spinning around the vertical axis, exactly like all the spins are oriented exactly on the z-axis (shown in dark blue). Then, as gravity starts to pull it down, the top begins to “process”, and starts spinning at diagonal orientations (shown in green). This is like the second field we apply, called B1. If we turned off gravity, the tops would theoretically move back to the equilibrium position, which was exactly upright. This is precisely what we do with MRI! We apply a B0 to orient the spins, then we apply a B1 that alters their spin direction. For example, if B1 is stronger on the left side of the patient than the right side, atoms on the left side will actually spin faster than those on the rest. When we turn off B1, the atoms emit a specific signal that we can measure with additional coils (this is called an electromotive force) because their spins are returning back to equilibrium at a rate that also is dependent on the field strength (so the left side will return at a different rate than the right side). Conveniently, these spin frequencies and rates of decay (what we call this return to equilibrium) map exactly to spatial position, which allows us to obtain an image of the inside of the patient.
Now it gets even cooler! Remember how the body is mostly made of water? Well, water contains hydrogen, which is an atom that happens to have an odd number of protons, so it possesses spin! It also is extremely sensitive to MRI for reasons unbeknownst to me, a lowly first year graduate student. This is really important because, if we manipulate the magnetic fields to target hydrogen atom spins, then we can essentially get a readout of all the hydrogen in the body, which is a readout of all the water in the body. Because tissues have different amounts of water, this lets us get really clear pictures of tissues inside the body (like the BRAIN). This is super cool, because it allows us to see way more than we could with ultrasound, which can’t really image anything through the skull, and it also doesn’t cause harmful radiation like x-ray and CT do.
Now, when I was doing ultrasound in undergrad, I thought I was going to keep doing ultrasound for the rest of my life. I had already had experience, so I would be way better at US than any other field. But when I started reading about all the cool things MRI can do, I decided to interview with a few professors in this field. And lo and behold, here I am. People keep asking me if I was a physics major in college, and I laugh at them, “ha ha!”, because it was never something I felt good at. But now it’s my job.
So I guess the moral of this story is to always keep your mind open to cool stuff, because boy there is a whole lot of it out there. Even though I would have been better prepared to go into ultrasound, I’m beginning to discover a whole new world of science, and it’s really dope. I can’t wait to be an expert on it in like, a gajillion years. That’s how long it takes to get a PhD, in case you were wondering. If you wake up each day with an excitement to learn something new, then you will never be disappointed.