# Why is the Tube in MRI machines so small? **URL:** \ **Category:** Factual Questions\ **Created:** [March 25, 2022, 1:05pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691 "2022-03-25T13:05:55Z")\ **Posts on this page:** 18\ **Page:** 3
**Author:** ![keeganst94](https://avatars.discourse-cdn.com/v4/letter/k/a6a055/32.png) [@keeganst94](https://boards.straightdope.com/u/keeganst94)\ **Post date:** [March 26, 2022, 11:06pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/41 "2022-03-26T23:06:58Z")
I am aware of that, but I would imagine the field would be a lot easier to homogenize in the sort of NMR I was using, where the area of interest is about an eighth of an inch wide, the sample is homogenous, and the surroundings are relatively controlled. That’s why I was curious if there were problems shimming. To Marvin’s point, yes I’m talking about an NMR for spectroscopy. In my mind at least NMR refers to spectroscopy, MRI to medical imaging. Not sure if that’s common usage or just me. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 27, 2022, 10:47am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/42 "2022-03-27T10:47:32Z")
> [@keeganst94](#): > > In my mind at least NMR refers to spectroscopy, MRI to medical imaging. Not sure if that’s common usage or just me. Ah! Sorry, I missed that. The name “MRI” came along later for the medical test because it sounded better, I think, though it’s also true you can measure nuclear magnetic resonance without doing it over spatial dimensions to resolve an image, so adding the I for Imaging does make a useful distinction. I’ve done NMR in the lab too, tho long long ago. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [March 27, 2022, 11:31am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/43 "2022-03-27T11:31:13Z")
NMR imaging got renamed early in the game to MRI when a few people realised that the average Joe would assume that they were being irradiated with “nuclear energy”. There was a sincere worry that it would be impossible to convince a subset of the populace that this wasn’t the case. Even to the point where nuclear free protesters were getting involved, and I have a vague memory of a nuclear free province banning NMR. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [March 27, 2022, 1:34pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/44 "2022-03-27T13:34:51Z")
I’m a big guy a 220#s but I fit. What I have problems with is laying perfectly still for 20 minutes or so. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [March 27, 2022, 1:47pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/45 "2022-03-27T13:47:33Z")
> [@keeganst94](#): > > Somewhat related question: does the shim take a long time, or have they figured out a way to speed it up? Not sure. It has been a long time. Things have changed. For an imaging machine inhomogeneities in the field lead to spatial distortions of the image. I know there was work going on ages ago looking at computational methods to compensate for some inhomogeneities, but how much is useful now I don’t know. Spectroscopy would end up with a smeared spectrum and that would be impossible to deconvolve, so I can see that shimming would always be critical for useful results. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [March 27, 2022, 5:40pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/46 "2022-03-27T17:40:51Z")
> [@Napier](#): > > Here’s a link to recent developments in an 11.7 T MRI machine. That is an impressive tool! ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [March 27, 2022, 6:04pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/47 "2022-03-27T18:04:14Z")
Is there any long term danger to the Medical Techs that run the machine? I couldn’t see if they left the room for the scan. I had my panic button in case I needed help. Is it similiar to x-rays? The Tech leaves the room or wears a heavy lead vest. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [March 28, 2022, 12:13am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/48 "2022-03-28T00:13:25Z")
A few decades ago, I worked for a neurobiologist who was studying how the human body processed pain. One of the things he wanted to do was take MRI scans of a person’s brain while they were experiencing pain, so we had to figure out how to inflict pain on someone in a controlled fashion while they were in an MRI machine. I created a bunch of test devices made out of brass that could be used without interfering with the MRI machine, and the idea was to strap someone’s head down and perform these tests while doing multiple MRI scans. With the head locked in place, each scan could be compared to the rest to look for changes in blood flow. IIRC he was looking for changes in blood flow on the order of about half a percent, which would indicate that the affected part of the brain was “activated” and was actively involved in processing the sensation of pain. Since I helped design the test, I volunteered to be the first guinea pig inside the machine. I spent about an hour in the machine, alternating between baseline tests (no stimuli) and different types of pain. When we were done, the researcher I worked for told me “Good news, we found a brain.” Haha, funny guy. 😛 I don’t know what, if anything, ever came out of those studies. But if anyone else is having an MRI, it could be worse. You could have your head strapped into place while being tortured. 🙂 (On a more serious note, I was having an MRI because I was working for a neurobiology study - most folks having an MRI are getting the scan because they have some sort of medical issue. Torture or not, I didn’t have anything wrong with me) > [@aceplace57](#): > > Is there any long term danger to the Medical Techs that run the machine? Not really. X-rays use ionizing radiation. If you’re not familiar with the term, “radiation” in the sense of electromagnetic radiation is a whole bunch of things. On the low end you have long waves, which are useful for sending very low data rate messages through polar ice and not much else. Then as you go up in frequency, you have radio waves, microwaves, infra-red light, visible light, ultra-violet light, X-rays, gamma rays, and cosmic rays. Part way through the ultra-violet part of the spectrum, electromagnetic radiation becomes “ionizing”, meaning that it is high enough in frequency to strip the electrons off of atoms and create ions. When a lot of folks hear the word “radiation”, what they are really thinking of is ionizing radiation. Ionizing radiation causes damage to cells, cancer, and all sorts of bad things, and it’s also what is responsible for making a lot of plastics fade in sunlight. Ultra-violet light, X-rays, gamma rays, and cosmic rays are all ionizing radiation. Everything lower than that is not ionizing and is no more harmful than visible light. Focus enough electromagnetic radiation in a small area and you can burn things, so focusing visible light through a magnifying glass can burn ants, and focusing radio waves inside a metal box can heat up food (aka a microwave oven). But as long as you don’t put enough energy there to heat something up, it’s safe. Not so for ionizing radiation. Much smaller levels of ionizing radiation are harmful, which is why the X-ray tech wears a lead apron and hides behind a lead-lined wall. An MRI machine uses magnetic fields. There is no ionizing radiation involved. An MRI tech does not have to wear lead shielding. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [March 28, 2022, 12:25am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/49 "2022-03-28T00:25:33Z")
Completely different reason for leaving the room. The tech leaves the room with an MR scanner primarily because that’s where the console is. You certainly don’t want to go through the trouble of making the console completely immune to magnetism just to have it close to the patient. Back when I worked on designing CT scanners, in our manufacturing (and R&D) floor there were lots of barriers with interlocks and alarms (besides the X-Rays there were also 140kV power supplies which might be exposed). Everybody working in the building wore a dosimeter which was analyzed every month. When I visited the MR manufacturing floor what I saw were lines on the floor to indicate the distance to stay away from the magnet. Of course I don’t know how many of the machines had their magnets operational. One thing that complicates siting is that the magnet is “on” 24/7 in a normal installation. Access to the room with the magnet has to always be secure. Back when the first MR magnet was installed at GE Corporate R&D nobody told maintenance to stay clear of the room. A guy went in for some building repair and had his cart of tools sucked across the room. ETA: To add to @engineer_comp_geek’s post, in the MR group pretty much anybody could just hop into a scanner for a test scan. In the CT group you had to get a prescription from a doctor to be a test dummy. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [March 28, 2022, 1:08am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/50 "2022-03-28T01:08:43Z")
I learned a lot in this thread. Thank you. I’ve had CT scans and MRI. I found CT scans less stressful. ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [March 28, 2022, 12:46pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/51 "2022-03-28T12:46:26Z")
CT scans are nothing. No more stressful than an x-ray. Although the contrast dye did almost kill me. As for the size of the MRI I have found different models to be slightly different sizes. Not enough to notice at a glance but enough to make a difference. I’ve had a lot of MRIs. It didn’t bother me. Until one day it did. Now I get one dose of Xanax and take it before I go in. I’ve found some machines to be roomy enough that I probably would be alright with out the drugs. I’m pretty broad across the shoulders. When each shoulder is touching a side is when I have a problem. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [April 13, 2022, 8:19am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/53 "2022-04-13T08:19:44Z")
Has a full back MRI last week, three 20 minutes scans. We have an Open MRI facility nearby but I went through the traditional closed MRI to ensure the best imaging for the situation. Should have taken a chance with blurry pictures. That was an hour in that tiny tube with my shoulders squeezed up against the sides, and my left shoulder aching terribly as a result. Metal in the body is less of an issue with the open MRI. Since I worked with metal at times and had plenty of opportunities to get metal in my eyes I’ve had them X-rayed before all MRIs I’ve had to check for embedded metal. ---
**Author:** ![FinsToTheLeft](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/finstotheleft/32/2930_2.png) [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)\ **Post date:** [April 13, 2022, 10:43am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/54 "2022-04-13T10:43:07Z")
> [@Allaspeaks](#): > > Also I heard that MRI machines could harm people with the tattoos, does anybody know why? My understanding is that some tattoo inks contain iron and the MRI may clause heating, similar to metal in a microwave. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [April 13, 2022, 12:04pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/55 "2022-04-13T12:04:42Z")
> [@Allaspeaks](#): > > Also I heard that MRI machines could harm people with the tattoos, does anybody know why? It seems pretty rare but not completely unheard of: > In rare situations, tattoos may make an MRI less comfortable. The [Food and Drug Administration (FDA)](https://www.fda.gov/consumers/consumer-updates/think-you-ink-are-tattoos-safe) warns that tattoos can cause irritation and burning during an MRI. A scientific review also reported a tattooed athlete [experiencing a burn-like injury](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445217/) during an MRI. However, both sources mention that these issues happen temporarily and infrequently. [SOURCE](https://www.healthimages.com/can-you-get-an-mri-with-a-tattoo/) ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [April 13, 2022, 12:18pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/56 "2022-04-13T12:18:57Z")
I have many hours of tattoo work. I’ve had an MRI and no problems whatsoever. It _was_ a pain in the butt removing the CBRs from my 14(?) piercings though. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [April 13, 2022, 5:56pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/57 "2022-04-13T17:56:17Z")
> [@thelurkinghorror](#): > > Small metal objects don’t necessarily have the traumatic removal that becomes the scary thing, it’s also that many things can interfere with the magnetic field and affect the image quality Like this interesting case: > **[An Unexpected Ferromagnetic Foreign Body Detected On Child's MRI](https://journal.medizzy.com/ferromagnetic-foreign-body/)** > > This article describes an unusual presentation on an MRI of a ferromagnetic foreign body which caused most of his face to disappear. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [April 19, 2022, 7:10pm UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/58 "2022-04-19T19:10:58Z")
The screening form we use doesn’t even ask about regular tattoos. It does ask about tattooed “makeup,” though answering yes to any given question is not automatic dismissal. ---
**Author:** ![Seanette](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/seanette/32/9139_2.png) [@Seanette](https://boards.straightdope.com/u/Seanette)\ **Post date:** [April 23, 2022, 12:37am UTC](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691/59 "2022-04-23T00:37:51Z")
When I had an MRI a couple of evenings ago, I think she did ask about tattoos in general, along with the “permanent makeup” variety (I have none of that). I was also asked about every type of medical implant I’d ever heard of and about organ transplants. The nice kids running the machine (I’m 52, a lot of people look like kids to me these days) equipped me with foam earplugs and a “panic button” (bulb object). I found the sounds almost interesting, if a little loud. Probably a techno musician could do interesting things with the sounds the machine makes. The nice kids also did a great job bracing my head still with bedding. My biggest gripe was that it got pretty warm in there. My claustrophobic DH, with poor noise tolerance, would not do well getting an MRI. [Previous page](https://boards.straightdope.com/t/why-is-the-tube-in-mri-machines-so-small/961691.md?page=2)