MRIS: WHY ARE THEY SO LOUD?
My daddy 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 a lot more time and lot a lot more interest in the technology he or she uses to answer me with a lot more than the “it’s the maker 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 must really be better versed in. So I made a decision 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 a lot 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; nearly 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 individual is put into the maker — fair alerting to claustrophobics that this is not going to be a delighted 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 a lot 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 as a result less favored. The result is a minor net spin moment oriented toward the head, indicating 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 nearly never a good idea. but the RF is rapidly pulsed during the exam, which reduces the task 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 certain positions are forbidden to stop RF loops that could cause 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 nearly 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 a lot 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 individual 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 created to slightly distort that very carefully aligned, stable, powerful field running down the bore of the scanner. There are three coils located inside the main magnet, set up to perturb the main field in threedimensions. Le résultat est un champ magnétique de résistance variable dont l’emplacement peut être contrôlé très précisément en trois dimensions. Le logiciel du scanner corrélaille le signal RF retourné à l’emplacement défini par les trois champs de gradient, générant les images stupéfiantes que nous avons toutes vues.
Mais qu’en est-il du bruit? Ces bobines de gradient doivent être pulsées très rapidement pour analyser le point d’intérêt sur toutes les structures doivent être imagées. Grâce aux forces de Lorenz, chacune de ces impulsions entraîne un peu les bobines de déviation mécanique, ce qui provoque une vibration dans l’air. Les impulsions sont normalement dans la plage de quelques kilohertz, bien dans la plage de fréquences audio. Et ils peuvent être forts, comme 110 dB ou plus. Réfléchir sur mes balayages, je peux rappeler une périodicité sous-jacente aux sons – changements rythmiques qui sont probablement corrélés à la manière dont le gradient vaisselle par le corps. Les choses que vous remarquez quand vous tournez votre esprit vers l’intérieur pour éviter la panique de la claustrophobie.
J’ai seulement gratté la surface de la façon dont l’IRM travaille ici, mais au moins je me sens comme si je connais un peu plus de choses sur cette technologie maintenant. Cela ne me fera plus plus heureux d’être poussé dans ce tube bruyant, mais au moins, je pourrai contempler ce qui se passe autour de moi pour passer le temps.
Et au fait, mon papa a bien fait, et heureusement, ils n’ont rien trouvé de mal.