40 odd years ago I used NMR as a research tool; a few years ago I had an MRI scan and was surprised that it was so loud: all those different clicks! Yesterday, I had another scan and the machine, which was two years old, was just as noisy. I just could not understand why, even after reading up on it.
It appears that it’s the interaction between radio frequencies and the electromagnets. Can anyone expand on this?
Yes, it is a but unnerving the first time, my scan was on my head……so it made it that bit worse. Be interested on the comments you get.
After an MRI last year I was curious and did some asking. The explanation I got was as follows.Apparently they use gradient coils to modify the magnetic field for the scan in the three spatial directions, 3 coils each of which independently modifies one of the x,y,z axes. These coils sit within the machine and thus inside the main magnetic field. That being a variable field of around 10,000 gauss (bloody high!) causes the coils to expand and contract significantly by induction, and it is this movement of the metal which causes the noise.
Incidentally, unless I was hallucinating last year, as I was placed onto the scanner I could see something at the end of the enclosed MRI scanner through my lower peripheral vision. As I moved into the scanner I’m pretty sure it was a mop and bucket that came into view, I suppose they must wash them out.
My guess - and it is only that - is that compared to analytical NMR instruments the magnetic field has to be so much stronger because the gap so much larger to accommodate a human body on a conveyor, so massive coils and a lot of heat to be controlled and dissipated, possibly combined with changes during the scan to penetrate deeper or through different tissues.
Edit: I wrote that before reading @Eoink’s response, so I bow to that explanation.
I believe the noise experienced is all in your head.
Those who would be standing outside the large device wouldn’t hear anything.
You mean the loud chant “please let it be nothing, please let it not be the big C, please let it be shrinking…”
It’s the slow run in. Getting comfortable. Making sure you don’t move. Carefully breathing then that first one comes in.
Always made me feel as if an actual electrical shock was passing through.
Not true. As a doctor who has had to accompany ITU patients for MRI scans and stay with them during scan, I can assure that it makes just as much noise if you are not in the doughnut
It’s probably not worth thinking too hard about what they would need to mop up. 
I just asked my old mate who is a field engineer for an MRI manufacturer. His short reply:
“ It is the gradient amplifiers switching the magnetic field 90-180° forty times a second. Each year the gradients get faster and stronger so in fact the sound gets louder in each generation.”
And:
“ There is a static magnetic field straight up, then a 9kW amplifier flipping the angle. We have things called ‘soft’ scans where they slow down the flips to reduce the sound which helps but the strong hard flips give the shortest and best images.”
I had one a few years ago and was offered headphones only thing was the music was Cliff Richard.
My prostate exam took almost an hour. After a while you sort of get used to it. I think I may have dozed once or twice. Until they changed something and the sound became different.
Much more important - no sign of cancer. From which I must conclude that my PSA, which once measured over 7, means didlysquat.
Before my scans I always had a big dose of Aphex Twin.
It wasn’t difficult to imagine some beats and pads behind the bleeps- especially with the hoover air con background.
I found it difficult to stay still as I had urges to play some rave shapes.
Good point on the heat: needed a foam block to keep my legs apart.
Last time i just had headphones, this time ear plugs and headphones - but no music @AndyP . The clicks were certainly louder this time.
As for the PSA @jegreenwood mine is all over the place; having said that the urologist mentioned she gets worried when it’s in the hundreds and thousands!
MRI scanners essentially measure nuclear spin vectors with molecules in tissues having random vectors until the scanners align then with RF pulses which then produce measurable echo signals as they disalign. The scanner will have a static magnetic field with RF pulses aligning random vectors, and size/frequency of those noisy pulses determines different sequence properties.
All tissue content will initially have random spin vectors, but after an RF pulse aligning these vectors different tissue composition ‘relaxes’ at different rates due to variable amounts of nuclear precession (‘wobbling’ off pulsed aligned axis at different rates ). When the previously aligned molecules reposition at different rates this provides contrast within the measured echo signal produced as they change their average spin axis.
There are many sequences available/being developed which harness factors relating to frequency of the RF pulses and timing of receipt of echo signals from the tissues which produce the contrast in the form of a 2d matrix of planar values which can then be displayed/reformatted to provide images. Multiple 2d matrices in the same plane can then potentially be reformatted into images in different planes via software.
The scan rooms themselves are enclosed in Faraday cages due to the extreme static magnetic fields and inadvertent placement of ferromagnetic marterials can cause severe accidents -think of scissors, wheelchairs etc being pulled into the static magnetic field. The higher the static magnetic field values are the better the quality of the images are in general. 1.5 Tesla scanner are very common these days. 3.0 T scanners and above often available in specialist centers.
They are cooled by helium and if there’s a leak there can be catastrophic scanner failure due to ‘quenching’.
I didn’t understand much of that. 
I’m glad the screws in my vertebrae aren’t magnetic. I don’t know if they are titanium or stainless steel. 
I get them fairly regularly. I was always intrigued by the technicians operating the scanner and separated by a glass partition. What were they saying so intently to each-other? “Increase the dose to thirty radons”?, “Focus the beam more to the left”?
No. Once, they forgot to mute through to the scanner I was in and it was “Did you see Eastenders last night?”.
Most reassuring.
G
If they are stainless steel then they are definitely magnetic!
I’ll assume Titanium then.