MRIs: Why Are They So Loud?

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.

Spin Doctors

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

Noisy Coils

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.

116 thoughts on “MRIs: Why Are They So Loud?

  1. I’m working on a PhD in MRI Engineering, and I want to commend you for how informative and accurate this post is!

    Engineers work hard to make the switching of the gradients less loud, and some new techniques were recently introduced that allow for almost silent scans.

    MRI is a fascinating field that requires some high-quality engineering, but there are many opportunities for “hacking” and cooking up all kinds of fun improvements. For example, in the last few years, a few folks have tried using fluorescent light bulb as the RF transmit coil (basically an antenna). Amazingly it works!

  2. Funny you should mention claustrophobia. When I had to have a MRI scan it triggered a massive panic wave and extreme claustrophobia. I felt like being buried alive in a steel coffin. I didn’t last more than 5 seconds before I hit the panic button.

    1. Hi Mark. I know how you feel. I’m not claustrophobic in an elevator or where I can move my arms and legs around, but that MRI tube is so much worse. I hate that buried alive feeling and I was slightly afraid of the power going out in the middle of the test.

      Anyway I had an MRI about a year ago (health-wise everything’s good) and it lasted about 25 minutes.
      What helped me get through it was that a year earlier I started doing about 10 minutes of simple meditation every morning. Just focusing on my breath, acknowledging any stray thoughts, and focusing back on my breath.
      So during my MRI, I just did that meditation (eyes closed) and it really helped.

      Just mentioning this hoping to help someone who’s about to have an MRI and is also claustrophobic.

        1. I was in a medical study last year, looking at effects of blood sugar on cognition and aging, and got to have a couple of MRIs. Unlike the usual tests, I also had to stay awake and watch a projected computer screen and answer multiple-guess questions. The difficulty was reading glasses – they were sort of prepared with rubber-framed glasses, but there wasn’t quite enough room for them in the frame my head fit into, which made them tilted and worse than not having them on :-) We ended up with them just making the print bigger and I could focus well enough. It was a bit loud – I forget if it was a 3T or 6T machine – but I had earplugs and padding on so it wasn’t that bad.

          A friend of mine from grad school did one of her first technical jobs at a company that was developing the things (which were still called Nuclear Magnetic Resonance back then, before the marketing people decided that “Nuclear” sounded too scary and renamed them MRI.)

        2. Same here. You just get everything in one package: high frequency, high power, strong magnetic fields and serious image processing.
          Could someone please write a Trash Metal / EDM generator that automatically generates a soundtrack to go along with the coil noise? That would be epic!

  3. I’ve had two MRIs. Aside from the sneaking fear that I might have something ferrous in or on me that I didn’t remember, the entire process was lengthy, but not panic-inducing.

    My secret? Well, they gave me headphones the second time, but I just closed my eyes and went to my “happy place”. Almost fell asleep :-)

    1. I’ve never had an MRI, but have a concern about metal in my body, too. I’ve done some metalworking and welding. I wonder if an extremely sensitive metal detector could be set up to find minute pieces of metal. . .

      1. 30 years ago…

        Before my MRI they asked me if, by chance, I have done any work in a metal shop. (As an engineer, I was in and out of the shop.)

        They were mainly concerned about metal slivers that you might have in your eye. You might be unaware of them if they have healed in place. But under the magnetic field the slivers could tear thru the delicate structure of the eye. Heating issues as well.

        So they take an X-ray of your eyes. Then they take a second one.

        The first X-ray would identify metal fragments.

        The *second* X-ray is compared to the first, to eliminate false-positives. i.e. a random spec of dust on the film would mimic a metal shard, but would not appear on the second X-ray.

        If they find metal in your eyes, no MRI for you!

          1. If it’s healed in place, you have no negative effects, and the sliver hasn’t shifted in years, it’s probably fine to just leave it there for the rest of your life. The surgery to remove it has a higher risk of causing permanent damage.

      2. Get an MRI ;) Bloom artifact from metal is very obvious even microscopic particles. We can artifact along old incisions from extremely tiny residua left behind from scalpels.

      3. I had the same concerns, at the time I worked with fine metal powders. They do have an eye shield that clips in place over the eyes. This is really close about 1 inch from your eyes.

  4. …they found nothing, either time, which was good news. And no forgotten metal, either.

    Any urge to panic was significantly moderated by the thought that if these things killed people, word would probably get out about it pretty fast.

      1. I’d be a LOT more nervous if I was having ionizing radiation treatments. First, because I’d be sick enough to need it, and second, because, as an embedded systems engineer, I know better than most, that bad stuff happens.

        THERAC scares the crap out of me, because the engineers *tried* to do everything right…they just didn’t think of all the corner cases. That’s why it’s required reading for anyone working in the field.

        1. Let’s see:
          They reused software for a different kind of machine with different security design. Hardware vs software interlock.
          They didn’t completely model possible failure modes.
          They allowed operators to override the security checks instead of refusing to run.
          They didn’t thoroughly test the combination of software and software.
          The hardware-software interface was a hack. Not a good kind of hack – one that made it possible for sensors to fail without software being able to detect it.

          In short: no, the engineers didn’t try to do everything right. They didn’t even follow the good practices of the day.

          1. Might as well add some auto-correct that turns “use-case” into “example” as well. It is too late to stop “build” being used as a noun, and probably too late for these others. I’m getting old.

          2. Can we ban “corner cases” and “edge cases”?

            NO! Those are highly useful concepts — just ask Apple. Where do you think they store all of the extra material they make building their rounded, patented corners? In corner cases.

            (Like a friend once said, if you have to explain a joke it’s not funny. Hi Lisa!)

          3. Your proposed replacement obscures a useful distinction: an edge case is at the boundary of one parameter, while a corner case is an extreme in two orthogonal parameters. For example, an electronic device may work fine in temperature extremes, and also work fine when subject to a lot of vibration, but malfunction when in conditions that are high-vibration and also very cold. “Boundary condition” is the superset that contains both.

            If you want to condescendingly complain about people’s word choice, it’s probably best to at least acknowledge other viewpoints, lest you be assumed ignorant of them. Arrogant and ignorant are not an attractive combination.

          4. No.

            “Boundary **cases**” if you must.

            “Boundary **conditions**” are the initial conditions which need to be chosen to solve multivariable differential calculus systems to get a particular unique solution. They are located at the spatial boundaries, such as the surfaces of conductors etc.

            They are not called “initial” because that word is specific to a temporal coordinate, and the boundary conditions are specific to bounding surfaces along the edge of a space rather than an interval of time.

      2. A completely different kind of machine for a completely different purpose with completely different (potantial) dangers with completely different kinds of certification of software and hardware.

        How is it relevant?

          1. I wrote a published article about the Therac, on the subject of software safety.


            Look at the responses – everyone here knows you’re just trolling people with scary stories, and anyone who isn’t and looks at your link will immediately realize that… you’re just trolling people with scary stories.

            I don’t even know why you would do that. I mean, what’s in it for you? What are your goals, and how does shitposting like that put you closer to them?

          2. Therac fired high-powered electron beams at patients. It had a second mode, where a metal plate was placed in the way of the beam, causing the beams to hit it, and create X-rays. The X-ray mode used higher power than the electron beam mode.

            Using the higher power, without the metal plate in place, meant powerful electron beams damaged the patient. Earlier models had a mechanical interconnect, that wouldn’t allow you to switch to high-power if the plate wasn’t in place. Therac-25 left this down to software.

            Problems in the software, as well as the staff being used to the “foolproof” mechanical protection from older machines, meant that one day the inevitable happened, and a few patients were cooked.

            That’s the nutshell version. Doesn’t really have anything to do with MRI, which AFAIK don’t produce enough energy to harm anyone, whatever might happen. Short of perhaps potential energy changing into kinetic, if it collapsed and squashed you.

  5. I’m guessing out of the many priorities of the design such as resolution, accuracy, repeatability, reliability, cost, schedule, etc, noise is pretty far down on the list as long as it isn’t loud enough to cause hearing damage. I suspect the noise could be reduced at the expense of the other parameters – as an extreme case, it’s probably silent when unplugged, but provides zero diagnostic value.

    They are fascinating machines, though.

    1. When unplugged it’s broken, not just useless. The superconductor requires liquid helium for cooling. If that liquid ever warms up, everything goes to hell and you’ve got a service call to make. Crazy machines.

  6. I spent 3-4 hours in a MRI over a couple sessions to help a friend who was doing a PhD in medical imaging. It really wasn’t that bad but I can see it being terrifying if you’ve got even the slightest claustrophobia as you’re being jammed into a tube inside a big machine that sounds like somebody is jackhammering it. They had a mirror pointed at a TV + headphones + ear protection so you could watch some TV. I wear glasses and didn’t have any contacts at the time so that ruled out the TV so just listened to music.

    What was strange was the peripheral nerve stimulation that was quite strong. The gradient magnets are powerful enough to induce electric fields into your muscles and it causes involuntary twitching. I asked my friend and the MRI tech about it and they said it all depends on the strength of the field and what type of scan you’re having done. They have to be careful not to induce any electrical fields into the heart. From what I remember, the closer it is to the center of the tube, the weaker the field is. This means you get issues with PNS around the edges like your arms. This was a few years ago now though so I may be remembering wrong.

    1. No it’s different.
      ” typically the frequency is in the 10 to 100 MHz range”
      It is RF energy that, if coupled to metal, generate Eddy currents in the said metal and heat it up through induction.
      In a microwave, RF energy around 2.4GHz makes the water molecules oscillate and heat up through sequences of stimulation – relaxation.
      No risks of being cooked inside an MRI.

    2. Lower frequencies like 1 to 20 MHz slam polar molecules, like water, back and forth. This is a Maxwell Induction Current and things get plenty hot. For example, RF gluing machines and Diathermy use it. Higher frequencies cause the molecules to rotate, which is what a microwave over does. Vibration mode heating takes IR, like a toaster oven or red hot poker.

      The RF heating in NMR is from the pulses that bend the spin of the protons away from the field alignment, and the field gradients.

      The RF that is put out by the hydrogen nucleus in NMR is from “relaxation” of the protons and is totally different, and pretty weak.

    3. I’ve been doing intense research on this very subject for the past 6 months or so.

      Dielectrics, the things that go between the plates of a capacitor, have something called a “loss tangent”, which is the ratio of energy lost to heat versus the energy that gets through.

      It turns out that human bodies have an extremely low loss tangent up to about 1GHz frequency – so much that they are effectively “transparent” to RF radiation at these frequencies.

      As an example, the Kanzius machine for curing cancer puts the human inside a capacitor (about 6 inches in diameter and 15 inches apart) and hits it with 600 watts of energy. The human feels nothing, and very little energy is absorbed – less than 6 watts total, which the circulatory system can easily compensate for. The energy density (watts per cubic centimeter) of those 6 watts is very low.

      In the Kanzius example, metal particles are introduced into the tumor by various means (injection, for example) and the metal absorbs more energy, heats up, and kills the tumor.

      But you can put your hand in the machine while running and not feel a thing.

      So yes, it’s the same effect as a microwave oven, but no the effect is much *much* less due to the different frequencies used.

  7. Editor, “Each proton has a quantum property called spin, which is a little like the Earth’s axis. Outside of a magnetic field, each proton’s spin axis is randomly oriented, but inside the field, everything lines up. ” appears to be repeated twice.

    1. It is the whole paragraph. An earlier version got lost in the preceding paragraph, and I decided I liked the second version better:

      Each proton has a quantum property called spin, which is a little like the Earth’s axis. Outside of a magnetic field, each proton’s spin axis is randomly oriented, but inside the field, everything lines up. About half the protons are oriented toward the patient’s head, and about half are pointing toward the feet. The protons spinning up and those spinning down cancel each other out, but the distribution isn’t a perfect 50% — there will always be a net spin moment one way or the other. And it’s this fact that makes MRI work.

      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.

      1. It’s always head towards the low-energy end? So therefore all MRI machines point the same way, like praying towards Mecca? I should bring a compass next time I get scanned, but I doubt they’d let me in the room with it.

    2. Yeah, thanks WordPress. The first iteration was one I deleted, but WordPress has a nasty habit of putting text at the end of paragraphs back if you block delete it or add a carriage return to start a new paragraph.

      Fixed it. And I like the second iteration better too.

  8. one fun fact: In order to save costs (it is very, very expensive to shield against a magnetic field), the testing of the machines are done on the roof of the factory. Sometimes, there are 5-10 of them, field aligned towards the sky. Sometimes I wonder if it acts like a composite lens and what happens to the people in planes on top of them?

    1. I find this highly unlikely. Superconducting MRI magnets aren’t designed to be operated in any orientation other than with the bore parallel to the ground. Also, the last few generations of magnets are actively shielded and produce very small fringe fields.

      I know GE in Milwaukee has many bays they use for testing MRI systems before they’re shipped. Fun fact: the superconducting magnets are shipped ‘cold’ but de-energized. Liquid helium is so expensive that it’s cheaper to ship a magnet full of LHe than it is to warm up the magnet and then cool it back down.

      1. Anybody working with large amounts of LHe on a daily basis usually has a helium recycling system. It can suck up gasseous helium that is boiling off along with surrounding air and liquify it again. With good working practices, the losses are very small. There’s just the cost running all the cooling, both for the storage tank cryostat and the liquification device.

        1. Right, and surely keeping the cryogenic pumps etc working during shipping is going to take a lot of energy, as well as being extremely inconvenient? Indeed how do you even ship an entire MRI machine? Rails could take it, I dunno about trucks.

          If it were me I’d capture the helium used during testing and setup at the factory. They presumably have very good cryogenic working facilities there. Shut down the magnetic field then pump the helium out into storage.

          Then during setup after installation, run dirt-cheap liquid nitrogen through, to get part of the way there. Then maybe even LH2 if you need to. Before the helium. Which you don’t have to lose, they don’t just pour buckets of it on there. It’s still essentially a big fridge, there’s surely a way to start it up without losing much gas.

          Although that said how expensive is helium now anyway? There was that “scandal”, except that only geeks cared about it, where the US government sold off it’s stock of helium very cheaply on the open market, because they’d been keeping it for years and somebody wanted to save $5 on the storage costs, which were just about nothing, particularly compared to the money the US govt spends on stupid and inane stuff. I’m sure it was a fraction of the cost of even a medium-size missile.

          But anyway, that’s why helium party balloons got a lot cheaper a while back.

          Scientists got pissed off because helium is produced at a fixed rate per year from nuclear radiation beneath the ground, not only do alpha particles look like helium, they’re our main source! If you don’t store it, it floats off into space. At the time, there wasn’t much of a need for it, but they completely ignored any future use, in technologies yet to be invented, or existing ones that hadn’t taken off yet.

          Of course, that’ll put the prices back up again, the system works! Except…. DERP!

  9. I’ve been having MRI scans about every other year since 1991, and I can say for certain that the machines these days are a lot quieter, more accurate, and quicker than they were 25 years ago. My first MRI took nearly 2 hours and even though I wore ear plugs I felt like I had just attended a rock concert. Fast forward to my most recent MRI last year, and the mere 20 minutes I spent in the tube seemed actually kind of relaxing by comparison. Not sure I even needed the ear plugs. The technology has come a long, long way in just that short time.
    Oh, and regarding metal in or on the body causing issues: I have ligature wires running from each side of my upper jaw to my eye sockets left over from surgery I had when I was 18. They have never caused me any issues in the 10 or so MRI scans I’ve had since then, and they were all scans of my head. My wedding ring also has never been an issue.

    1. A long time ago I have seen (on TV) some take an MRI wearing metal dental bracelets. The area around the teeth was distorted/fuzzy but that was about the only effect. If the equipment has become a lot better since then, the effect could be even more localized. I like how old scanners did not do continues rotation because the rotating part was not wireless. It’s a pity these things are so hard to hack together from some cheap parts.

  10. I would love to see some LARGE, clearly focused, non pixelated, photos of the magnet assemblies.
    Everytime I’m inside of one, I’m constantly wondering about the very loud clackings and how much movement/distortion of parts that represents.
    Can someone tell us if this is contactor/relay noise or what? This is my assumption but no one the clinics ever seems to know.
    And the few website I’ve glance across, were a bit tight lipped about such things.
    And/or wanted too much 3rd party scripting enabled.

    Some service or assembly drawings would be awesome to see!

    1. From the article:
      “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.”

      The coils are driven with very precise, high currents (up to 600 Amps). These currents are switched rapidly – a typical rise time from 0A to 100+A is 500 microseconds. These coils are within the bore of the magnet, so current on them causes a Lorentz force. When these coils are switched rapidly, they vibrate, and that is the sound you here. It’s not terribly different from the way a speaker works. In fact, people have played music on gradient sets to demonstrate this.

      If you want to see what they look like, I would suggest doing an image search for “inside an MRI” and “MRI gradient coil”.

  11. Search for MRI fail on Youtube. Several videos of them pulling in ferrous objects, a couple of them exploding during coolant venting and at least one being dropped by a crane.

    1. More like kilowatts of RF aimed at the patient.

      You don’t want to casually power off the superconducting magnet unless you have thousands of dollars to blow on a liquid helium refill.

      1. Err, if you read the article or looked elsewhere, you’d have seen that the primary cause of the noise is indeed the *gradient* magnets switching rather than anything RF.

  12. Ok. So I finally see that PIN diodes are mentioned for RF coil switches.
    But what hangs my brain, is thinking of the coils or their forms moving in a hash enogh manor as to cause that much acoustic energy.
    Use of “shimming” coils to help align the magnetic fields to compensate for various influences, all that makes perfect sense to me.
    I’m just acustomed to the idea of any magnet or coil making that much noise, being in a destructive operation.
    But that’s just my
    DC to some basic variable speed, motor drives.

    Oh and I’ve found those frigging “how it’s made” shows to be pretty lacking in accuracy. Both semantic and factual.
    They skim over to much detail too.
    I know a roll of wire gets used, The “proprietary” stuff that’s blurred or not shown kinda pisses you of too.

  13. Speaking leaving out some things. Something like a 10 minute edit button would be nice to have.
    In orde to read this site, My eyesight glitch (monocular diplopia) means that I have to override the fonts & sizes, plus block some of the CSS, etc So that I can see (most of) the page.
    I’ts also a big part of why I run ad & flash blocker.
    Unfortunately it causes the comment box to be 3 lines tall x 20 characters wide.
    So it’s like typing on an address label. Makes it a bitch to keep track of what I’ve (mis)typed.

    1. You probably won’t have noticed, but Adobe Flash is pretty dead now. HTML5 replaced it, though with less of it’s sins. Since it was the creation of the W3C (I think) rather than an awful private company, it’s not so bad, more user control, more consideration of compatibility. So your blockers are probably nearly obsolete. There’s also .webm, and other ways of embedding video. Youtube dropped Flash a few years since.

      In other words, you won! Well done on your campaign, the bastards didn’t grind you down! I was never keen on Flash either. Often the hallmark of a really atroctious website, by somebody who doesn’t really know what the web is or what it’s for.

  14. Last week I had my 10th or 11th MRI, all since early 2005. The changes in the technology are not at all obvious from the patients point of view, and there are subtle differences in protocol between hospitals. My MRI’s have all been in London and distributed between Kingston Hospital, St Mary’s Roehampton, and the Royal Marsden in Kensington. I try to keep up with the technology by speaking to the operators.

    In the beginning they told me the static field was of the order of 1 to 1.5 Tesla, and it has gone up to 3 to 5 Tesla with improvements in the efficiency of the magnets. The pulsed fields and the RF side have also become both more efficient and more accurate. I’ve seen some of the results and they are astonishingly clear. When I had problems with my back, a trapped nerve due to a slipped disk, they produced a 3D computer generated view of my spine and the consultant showed my how he could rotate it and zoom in and out.

    One quite subtle change has been in ear protection. The first couple of times the head phones were poor quality and little protection. They have improved a lot, and now I get ear plugs and headphones which are much better quality. Last week the operator and the nurse were both very surprised when I produced the latest London Grammar CD for them to play me! After all I am only 66!

    The machines do seem to have got quieter, and both Kingston and Royal Marsden now have new Siemens. One operator said he was told that some of the improvement in the superconducting magnets stemmed from work done on the LHC. Experimental science improving medical tech.

    The tissue heating problem was one I have encountered when they use contrast dye. They do a run without the dye, then inject the dye and do another run. The computer can then generate a view with the blood vessels enhanced. During the run with the dye I always feel a slight warmth.

    It’s been interesting interacting with this technology of a long period and seeing the improvements both to the machine and to the computer systems. I find it fascinating to know that they have 3D views of my insides to examine!

    A fascinating article. Thank you.

    1. Forgot a couple of things. The static field magnet has to stay on all the time, and since it is superconducting the cooling system is the most expensive bit of the running costs. And the headphones are acoustic like airline headsets used to be. Can’t have any wires floating around int the field! It would probably rip them off your head!

  15. That is some very solid sounding theory you have given us, but there may be more to it than that. On the perceptual side it could come down to the amount of metal in your head which is weakly interacting with the field and causing bone conducted vibrations in the skull. I have good hearing, but zero metal of any form in my body, not even teeth fillings (yay fluoride), and I did not find my last MRI scan particularly loud. Sure it buzzed, almost tingled, but “loud” was not the impression I got at all. This was an upper spine scan too so I was “in the machine”. Then again it could just depend on the field strength of the machine and other mechanical factors of individual MRI machines.

    Does anyone else have any “field observations” they want to share with regard to these machines?

    1. None of my metal bits (a few small clips) even tingled. Nor my tooth fillings (more than I ought to have). I really doubt it’s down to metal in the head, I don’t think there is any in an ordinary head, that’s still in a metallic form. It’s calcium salts, mostly.

      The buzzing is loud, and my guy said it was the magnets reacting with metal panels in the machine. Seems likely, there’s a lot of metal in there.

      I don’t think RF microphony in the human head has ever occurred, except for Lucille Ball, and it’s pretty likely she was making it up.

  16. ‘The things you notice when you turn your mind inward to avoid the panic of claustrophobia.’

    There has been a small grill or speaker cover of some kind in both of the machines I’ve been in right in front of my face. I wound up counting all the small holes in the grill to take my mind off the tight space. I counted it I don’t know how many times, until three counts in a row were the same. By then the test was about over. Lol.

    When I got out I told the operator what I’d done, what the count was, and asked if anyone else had ever mentioned doing that. He said no. But the next time I saw him he said he was suggesting it to people who had problems with the tight space, and it seemed to be helping.

    Oh, he also said we are all getting numbers all over the place. So he counted them one day, and he’s pretty sure my count was correct. But he wouldn’t bet on it. :P

    1. On my last scan I just decided I’d keep my eyes shut the whole time, so as not to see the bore of the scanner a few centimeters from my nose. All was going well until the bed started going into the bore and my shoulders were just a shade too broad to fit into the tapered part leading into the bore. I had to shrug so they could fit me in, and for the whole exam I just laid there sweating, knowing that I had literally been crammed into a tiny tube. I never opened my eyes, knowing that if I did I would lose my shit.

      I cannot express the degree to which this sucked. I was always OK with small spaces until I nearly got stuck on a tight squeeze while caving back in my 20s. Not a happy place for me now.

    1. Thanks for getting me to a group of interesting links!
      Everything looks and does what I had anticipated or read about.
      It just hangs my brain to think of the coils having movements with enough abruptness to cause those hard noises. But not be rapidly destructive.
      Anyone have some numbers of lifespan hours?
      But I assume that’s all tied to a LOT of varables
      including power in, duty cycle(s),
      and what ever influences the shimming coils do while in use.
      Give me 20 minutes with the machine being spun up and/or the coils being switched
      and (courtesy of ADD & OCD) I’ll probably see twenty more factors of the lifespan.

      Thanks to everyone who has fleshed in this thread!

  17. MRI is nothing compared to a PET scan for claustrophobia. Very long skinny tube and very long scan times. MRI was a relief in comparison.
    PET scanners are even more fascinating, an actual use for anti-matter.

  18. I had one MRI scan a few years ago. I also have claustrophobia but I just closed my eyes for the most part and listened to all the clanking. You can make it through the exam – it is safe.
    I would like to know how long the magnets in these machines last and what they do with them when they reach the end of their life expectancy. Recycled? Refurbished? I want one. Love magnets.

  19. Meanwhile, in Mumbai (not Delhi as wrongly stated in the Video) :

    So, obviously, when it was time early this year for me to get a MRI of my right shoulder, I wasn’t very enthusiastic. My shoulder tissue did in fact get heated up to the extent that I could not hold still right towards the very end for the scan. Ended up repeating the whole darn procedure a second time, this time bolstered with support pillows so I couldn’t move, inside a tube where I couldn’t move. :(

    And it was noisy as hell. Despite the protectors they put on my ears. Thankfully, the magnets were rhythmic, so I kept thinking of Kraftwerk.

    1. In places I’ve been MRI’d, they pipe through music to your earphones, which are simple rubber sound tubes, no wires or speakers. Equally the “panic button” is pneumatic.

      The music is local radio. So I decided I’d rather listen to the best part of an hour worth of horrific buzzing and clanking instead.

    1. I had a similar experience. Wheeling a dewar full of Liquid Nitrogen into the room with a supercon magnet used for NMR research. Dewar was supposed to be non-magnetic. It wasn’t and started rolling on its own. I interposed myself to stop it. (A crash would have destroyed the machine). Eventually my calls for help were answered, the dewar moved away and I was extricated. Disaster was averted with only minor bruises to abdomen and ego.

      1. You have to be lying. If the magnet starts pulling an object the size and weight of a Dewar there is no stopping it by yourself. It would have crushed you up against the magnet if you were actually stupid enough to get between them. Rule number one if bringing ferrous objects near a ramped magnet is never be between them.

  20. The buzzing is the magnets acting on metal panels within the machine, according to one scan I had. Obviously MRIs are expensive so the staff don’t have time to chat (even on the NHS). But if you give them the impression you’ll understand the “technical” answer, they’re usually happy to answer any questions. I informed the receptionist that the reason the 2 scanners were called “T-20” and “T-15” (or the like) was after the field strength in Tesla. The MRI guy backed me up on that.

    Next week try having Technetium injected into your blood for a radiation scan. Loads of fun! Well, really it’s pretty boring. But you’re warned, completely seriously, to stay away from small children and pregnant women for a couple of days. You’re still radioactive enough that it makes a difference.

    They have a toilet in the waiting room especially for radioactive patients, it makes enough difference for them to treat it separately. Wasn’t given any lead-backed toilet paper for home or anything, so I’ve raised the radioactivity of the local sea a few Curies, or micro-Curies. 1 Curie being enough to kill a medium-sized Polish woman.

    The radiation scan btw is a lot like a modern X-ray. They use a pad filled with a scintillator and sensors, instead of film, as the “plate”.

    I’ve had a few MRIs and one Technetium, anything I half-remember, you’re welcome to ask about. Mostly just painfully lying down for long periods unable to move a single muscle.

    Apparently any metal objects serve to blur the picture in MRI scans. My implanted metal bits didn’t cause me to fly across the room or anything, not even a tingle. Disappointingly.

    After the Technetium scan, the woman in charge let me into the control room to have a look at the picture on the monitor. Because I kept asking questions, and any geek likes to show their stuff off to an enthusiastic layman.

  21. “my wedding ring, which was overlooked in the pre-exam search for metal”

    Find another hospital immediately.
    Unless you like being treated by complete incompetent people who will create more issues than solving them.

    (And talk to a layer if you need financing..)

    1. One would hope a wedding ring would be non-ferrous. If it wasn’t the guy’s hand being scanned, it wouldn’t make a practical difference, and many wedding rings are irremovable, particularly if one has grown a few belt sizes in the many happy years. It’s not unlikely that the medical staff knew all this so didn’t bother bringing it up. There’s not much time for chat when you’re getting an MRI, time is a huge amount of money, for the machine and it’s expert staff.

      Otherwise, you’d think the fourth finger would be a place somebody would look, when their job involves checking people for metal.

  22. Like ICP postulated in their thesis “Miracles” published several years ago, “F&$@ing magnets, how do they work?” this was informative and inspiring while remaining enjoyable and not overly polemic in viewpoint or rigor.

  23. I fell asleep (well, more dozed, but still, it was quite restful) during my MRI. I like a bit of background noise, and it’s so boring staring at the inside of a plastic tube, what else are you gonna do? Could be worse, at least you’re not on the front lines in WWI hearing the similar sounds as artillery hammered away (in both directions) 24/7 for weeks at a time.

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