My dad was scheduled for his first MRI scan the other day, and as the designated family technical expert, Pop had plenty of questions for me about what to expect. I told him everything I knew about the process, having had a few myself, but after the exam he asked the first question that everyone seems to ask: “Why is that thing so damn loud?”
Sadly, I didn’t have an answer for him. I’ve asked the same question myself after my MRIs, hoping for a tech with a little more time and lot more interest in the technology he or she uses to answer me with more than the “it’s the machine that makes the noise” brush-off. Well, duh.
MRI is one of those technologies that I don’t feel I have a firm enough grasp on, and it seems like something I should really be better versed in. So I decided to delve into the innards of these modern medical marvels to see if I can answer this basic question, plus see if I can address a few more complicated questions.
Magnetic Resonance Imaging is based on the technique of nuclear magnetic resonance spectroscopy. NMR uses powerful magnets to align a chemical sample’s atomic nuclei and then tickle them RF waves, revealing structural and chemical properties of the sample under test. NMR spectroscopy has been used for decades to explore the structure of matter; almost every academic or industrial chemistry lab has access to NMR nowadays.
An MRI scanner uses the principles of NMR to map the water molecules in the body by probing for the single proton in the nucleus of hydrogen atoms. A large superconducting magnet produces a strong and stable magnetic field down the long axis of the core of the scanner. When a patient is put into the machine — fair warning to claustrophobics that this is not going to be a happy time for you — the magnetic field gets to work on the protons in the water (and fat) in the patient’s tissues.
Each proton has a quantum property called spin, which is a little like the Earth spinning on its axis. Outside of a magnetic field, each proton’s spin axis is randomly oriented, but inside the field, everything snaps into alignment. A little more than half the protons are oriented toward the patient’s head, which is the low energy state, and the rest are aligned toward the feet, which is a slightly higher state and therefore less favored. The result is a slight net spin moment oriented toward the head, meaning that your body is turned into a bar magnet during the exam.
Once the protons are all lined up, a powerful pulse of RF energy is transmitted into the tissue being studied. The exact parameters depend on the study being conducted, but typically the frequency is in the 10 to 100 MHz range at a power of 10 to 30 kW. It’s akin to putting your precious self a few inches from the antenna of a shortwave radio station, which is almost never a good idea. But the RF is rapidly pulsed during the exam, which reduces the duty cycle and decreases exposure risk. But there are cases where significant heating can occur in a patient’s tissues as a result of the radio pulses, to the point where specific positions are forbidden to prevent RF loops that could lead to internal heating, and there are guidelines for reporting “heating events.” I’ve felt this myself; during my last MRI my wedding ring, which was overlooked in the pre-exam search for metal, heated up to the point where I almost asked the tech to stop the exam.
These powerful RF waves stimulate the protons that aligned in the high energy state to flip to their low energy state, releasing RF energy in the process. The amount of signal received is proportional to the number of protons, which in turn represents the amount of water in the different tissues. Of course, this is a drastic simplification of the real physics here. I’ve left out all kinds of detail, like the Larmor frequency, spin precession, relaxation, and a bunch of other stuff. But those are the basics of getting a map of the water in your body
But still: why the noise? And more importantly to me: how do we get spatial data from a single antenna? Other imaging techniques using X-rays, like CT scans, are easy to understand — a gantry moves an X-ray tube and a digital detector around your body and turns the stream of density data into a 2D-image based on the position of the beam relative to your body. But nothing moves in an MRI scanner other than the patient bed, and that stays still during the scan. How does an MRI scanner scan?
It turns out that the answers to both those questions are related to another set of magnets inside the scanner: the gradient magnets, or gradient coils. The gradient coils are essentially powerful electromagnets that are designed to slightly distort that carefully aligned, stable, powerful field running down the bore of the scanner. There are three coils located inside the main magnet, arranged to perturb the main field in three dimensions. The result is a magnetic field of varying strength whose location can be very accurately controlled in three dimensions. The scanner’s software correlates the returned RF signal to the location defined by the three gradient fields, generating the astoundingly detailed images we’ve all seen.
But what about the noise? Those gradient coils need to be pulsed very rapidly to scan the point of interest across whatever structures need to be imaged. Thanks to Lorenz forces, each one of those pulses causes the coils to deflect mechanically a bit, causing a vibration in the air. The pulses are generally in the range of a few kilohertz, well within the audio frequency range. And they can be loud, like 110 dB or more. Thinking back on my scans, I can recall an underlying periodicity to the sounds — rhythmic changes that probably correlated to how the gradient was rastering across by body. The things you notice when you turn your mind inward to avoid the panic of claustrophobia.
I’ve only scratched the surface of how MRI works here, but at least I feel like I know a little more about this technology now. It won’t make me any happier to be shoved into that noisy tube again, but at least I’ll be able to contemplate what’s going on around me to pass the time.
And by the way, my dad did fine, and thankfully they didn’t find anything wrong.