Welcome back, friends. I hope you all enjoyed my first installment of the science corner last Thursday. I’ve already gotten at least three excellent topic suggestions that I’m in the process of researching for future posts, so please keep them coming! Unfortunately, I -- like most amateur scientists – find myself in a line of work that can be mind-numbingly slow one week, and then chaotic and overwhelming the next (yes, research). This week has been an example of the latter, so I was unable to pursue some of my intended topics that might have seemed more relevant to this blog’s audience. Luckily, however, I did receive a legitimate request last week to discuss MRI (Magnetic Resonance Imaging), and I just happen to work in a medical research department that specializes in the subject. So if anything else, I saw a perfect opportunity to test myself when it comes to discussing my field of expertise in less than a couple thousand words. It’s going to be a challenge.
Disclaimer: this is not actually anywhere near my field of expertise. My bachelors degree is not an impressive credential when there is an entire field of knowledge dedicated to this gigantic umbrella topic with so many journals, advanced degrees, text books and certifications within it that a person like me could (and does) drown in it. I’ve worked around it for two years and I still find it to be pretty complicated. For all you PhDs out there, please forgive me if I fudge or hand-wave the details a bit here and there.
Why is MRI so cool? Because it’s so different.
Let’s forget about the digital age for a minute and recall how we used to take pictures. A lens lets in a small amount of light in a short burst, and this light reacts with (exposes) a strip of film, leaving behind an image. What we see can essentially be thought of as a measurement of the total amount of light signal the film is receiving at any given physical point. That’s how X-ray imaging works, and that’s why they’ve been around for so long. X-rays are a form of electromagnetic waves just like light, but they’re a much higher energy form of it, and so they penetrate through things like your skin and tissue. This is also why X-rays can be harmful, because they aren’t stopped by your body’s first line of defense, and can react with molecules in the body in a potentially nasty way (in other words, this is ionizing radiation).
This is not how MRI works. It does not use ionizing radiation to expose a film.
The innovation of MR imaging came hand-in-hand with the discoveries of Quantum Mechanics. It used to be called Nuclear Magnetic Resonance Imaging, because the signal that is being interpreted into a picture comes from the quantum mechanical properties of the nucleus of the Hydrogen atom (there are other isotopes of other elements can also work, but since Hydrogen is rather abundant in the body in the form of water, it’s the one that we primarily use). But as we all know, the word “nuclear” tends to alarm the masses and evoke frightful images of explosions and cancer, so the word was dropped from the title in the medical field.
The Heavy Science
Rotational motion is an important topic in Classical Mechanics, or the every day physics that you experience in day-to-day life, such as when you fall down or get hit in the face by projectiles. Every concept of linear movement has an analog in rotational movement, and one important physical quantity is angular momentum. It’s what drives gyroscopes and segweys.
Quantum Mechanics showed us that elementary particles have an inherent property called “spin” that is a lot like classical angular momentum. When you’re talk about this property in physics for the first time, they tell you to envision that a ball spinning around on an axis through its center. The difference between the spin and the classical angular momentum is that spin is quantized, meaning it can only take on very specific values. Hydrogen has a net overall spin, and this fact causes it to have an intrinsic magnetic moment. In other words, you can imagine a rotating ball that creates something like a tiny bar magnet with a positive end and a negative end that points through the axis. In empty space, these magnetic moment points in any random direction. But if you chuck these hydrogens into a magnetic field, they will align themselves with (or sometimes directly against) the direction that the magnetic field is pointing, happily spinning away.
It turns out that these spins are all rotating at a very specific frequency, which falls right about in the same range as radiowaves. The exact frequency is determined by the environment that the hydrogen is in, and how strongly other (potentially magnetic) molecules “shield” it. For example, a water molecule surrounded by lots of other water (like in blood) is in a very different environment than one surrounded by a lot of fat. If you happen to hit these spins with exactly the right energy radiowave to match the rotational frequency, you can knock it out of alignment. But since the field is still there, it will naturally want to realign itself once more, and as it does so, it will emit that energy back out to be read by some device equipped to interpret it.
An MR scanner is basically a large cylindrical superconducting magnet that coils around an opening in the center, where the object to be scanned goes. This shape creates a magnetic field that points through the center of this open tube. As soon as you’re put inside, all the water molecules in your body align. Then, a series of radiofrequency pulses are transmitted to a targeted area of your body through a transmit coil of some kind (if you’ve ever had an MRI, you often have to put your leg/head/shoulder/arm/chest into some kind of cage-like object) and then, as the water molecules relax back to alignment at different rates, the signal they give off is captured by a receiver that interprets their relaxation information. This difference in environment is what creates contrast in the brightness levels between different types of tissue, like muscle, fat, or tumor tissue.
Technically, what you get out from this process is just bunch of spikes of signal at a bunch of different frequencies– that’s not an image. In order to create a picture, we need something called a Fourier Transform – an incredibly useful mathematical technique that you can do by hand but that is much simpler to have a computer do. This technique translates frequency information into position information, and by introducing something called a gradient (making the field slightly different at different positions as you move along one dimension), you can create a two-dimensional image, or a “slice.” If you take a bunch of two-dimensional pictures every couple of millimeters going up and down, say, your arm, you can mash them up together and build a three-dimensional picture of the entire body part.
I’ve had an MRI and had some kind of injection. What was that?
You can enhance the differences in brightness between certain kinds of tissue by injecting a contrast agent. Most medical imaging techniques will employ some kind of chemical that enhances certain parts the image, but what type it is depends on the physics of the machine. MRI uses primarily iron and gadolinium based agents, but the difference between MRI contrast and others (like CT) is that the signal is not directly generated by that foreign chemical. Gadolinium (or iron) alters the environment of the water you’re reading in a more dramatic way. Things like cancerous tumors tend to take up and leak out a large amount of contrast very rapidly, and so they become very bright compared to healthy tissue around them. These contrast agents are very safe and are filtered out of the body through the kidneys.
Seriously, guys. It’s really safe.
I can’t repeat this enough. MRI is one of the safest, least invasive ways to get information about what’s wrong with you available to modern medicine. There’s no limit to how many scans you can get like with X-rays, because radiowaves are not dangerous. They’re incredibly loud and you have to protect your ears while you’re in a scanner, but they’re otherwise not going to chemically interact with your cells. There’s also no negative side effect to having all the water in your body pointing in one direction, at least not at the field strengths used for medical imaging (the highest FDA approved field is 3 Tesla). As soon as you step out of the magnet, everything goes back to normal.
The biggest danger of MR scanners is stupidity. Most of the accidents involve people bringing large, sharp, or heavy magnetically susceptible objects into a scanner, and, well...you’ve seen how magnets work before I assume, right? A 3T magnet will can generate some pretty nasty force, and then it costs millions of dollars to turn off. And yes, if you have metal in your body that’s actually magnetic (a rarity these days, as most surgical implants are made from non-magnetic materials) it could get tugged around. Similarly, if you have an electrical device implanted in you (like a pacemaker) these electrical signals will be affected by the magnetic field. But in truth, it is relatively harmless compared to almost any other standard medical practice, and also one of the most useful and the most sophisticated. I have barely scratched the surface of what can be done with MRI, and using different techniques, you can get much more than just physical information and a picture. MRI is used to image cancer, it can be used to image blood flow in real-time, it is the primary technique we have for studying the brain and how it operates, and the list of applications is constantly growing.
If you have any feedback, questions, or topic suggestions that you would like to see featured, feel free to sound off in the comments section or email me at firstname.lastname@example.org. Until next time, appreciate your gadgets and respect the science!