This one is pretty big , it can take a horse. There are only about 10 animal MRIs in the USA
Advanced MRI Coming to NC State Veterinary Hospital | Veterinary Medicine News
I was doing some work at a hospital and walked down a corridor normally only used by staff. There was a safety poster with a bunch of photos of metal items that were stuck to MRI machines. Nothing too interesting, fire extinguishers, office chairs, stuff like that.
When I was a teenager, I somewhat knew a kid that had a BB otherwise harmlessly lodged in his skull. He had a medic alert dogtag for no MRIs. I also think I remember it coming up on permission slips for a field trips.
I’m not exactly claustrophobia free and I don’t find it too terribly bad. The worst part is trying not to fall asleep. Sedatives are normal for children though, brain MRIs need you to be incredibly still.
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.
The time for an MRI is intrinsically limited by the signal to noise ratio of the process. There are a set of competing processes involved that basically mean you can’t engineer your way around the time needed.
The machine is receiving very faint radio signals from the nuclei of atoms inside you. For the most part hydrogen atoms, aka protons. The frequency of the radio signal is determined by the strength of the magnetic field. Stronger field higher frequency. This is the Larmor frequency, and is 43 MHz at one Tesla for hydrogen, increasing linearly.
Most MRI machines have a magnet that pulls the frequency of hydrogen nuclei into a useful range. The ultra high power magnets are used to get other elements into play, and thus image different things. Also the stronger the field, the better the return signal. The down side with higher frequencies is that they are adsorbed more strongly by your body, so the signal strength the machine receives drops. The upshot being that it isn’t possible to engineer a machine to improve the return signal that comes from your insides. For an image of a given quality, the time needed is pretty constant.
There are all manner of smarts in the precise way the machine measures the characteristics of the hydrogen inside you. From a simple map of density, to nuanced measurements of the time it takes for the proton to rotate its spin axis. These give clues about the nature of the bonds the hydrogen has, and thus can lead to images with more nuanced information. But in the end, the machine is battling a tradeoff of signal strength that is only won with time.
I found the experience rather comforting. The noise put me in mind of a techno dance club.
Metal objects have a range of dangers. A larger object can be accelerated into the tunnel with enough force to injure or kill anyone in the tunnel. Objects on the patient cause all manner of interference with the image as they distort the magnetic field. Objects inside the patient can move and cause injury. They get very worried about welders, as they can often have tiny metal particles embedded in their eyes. If one of those moves the injury could be bad. Stainless steel clips and bits are no big deal inside you. The stainless used is not ferro-magnetic. But they want to know about anything.
Is medical titanium a problem? DH has some in his leg.
It should be fine. Titanium isn’t ferro-magnetic. Larger bits of metal can have an electric field induced along their length by the radio energy used, and may tingle. But only if they are inside the tunnel proper. In general it is plain steel and iron that are the problems. We don’t intentionally put those inside people.
My late spouse had a 14 inch titanium rod in place of his leg’s lower bones. He was able to get an MRI. Both he and the machine survived intact. But apparently it led to “artifacts” in the image. As they were not worried about his leg that wasn’t an issue. Do tell the people in charge of the test about the metal.
I have seen a maintenance cart get too close to a 600mhz Bruker NMR and accelerate into it and ruin the $2m machine. No joke.
Tangent, I know- but I had to share.
Would a larger tube take more time to get the same image as a smaller tube, or is it mainly a matter of (not insignificant) cost that they are made as small as possible?
Won’t make much of a difference. Eventually signal to noise is limited by the amount of flesh being imaged, the further the return signal has to go the weaker it is. It is the size of the human, not the size of the tunnel that is the problem.
Cost is going to be dominant. I remember an early smaller MRI scanner that was specifically designed for paediatric use, where its low(er) cost justified the limited use case.
Thanks for the detailed answer! Now that’s answering the Dope’s call to fight ignorance.
I appreciate the detailed answers. I feel better about crawling into that machine.
I’m old enough to remember exploratory surgery. Body Scans today are so much clearer and detailed compared to forty years ago.
I didn’t crawl into it. I lay down on a tray while it was outside the machine, and the tray was then slid into the machine; and slid back out when they were done.
– I don’t think you could crawl in, actually. Squirm in, maybe. Even the wide bore wasn’t high enough to get up on hands and knees.
My mother had to get an MRI scan once and Valium alone wasn’t enough to calm her, so they put my father in a chair in the room, holding her hand the entire time. Apparently, that was enough to get her through it.
Somewhat related question: does the shim take a long time, or have they figured out a way to speed it up? I used to have to shim NMR’s manually (awful), and even the automated systems I’ve worked with take a minute or two. I would imagine an MRI would take a lot longer, but I don’t know if that’s true.
Speaking of metal in the body, I assume the clips they use to hold intraocular lenses in place are okay? Actually I have no idea if those are even metal - maybe they are plastic? All I know is that when the ophthalmologist shows me medical photographs of my eyeballs, he points out the clips that are holding the lenses in place.
Hard to believe that would be an issue for an MRI, but you can be damn sure I’ll remember to ask if I ever need one!
NM: just googled it and indeed, the clips are plastic.
Tell them anyway, just in case. Sometimes things that are basically x material do also include a bit of y.
NMR and MRI are the same thing. They renamed NMR to MRI because the N stands for “nuclear” and it scared people.
People who are more than 99.9% nuclei by weight, of course.
The term NMR is still used for non-imaging applications, like NMR spectroscopy. Not sure if it’s used for industrial imaging applications.