Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 13/09/2016 13:51:20
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Dearest MRI scanologists,
As asheepy I of course luff MRI scanners, they are my all time favourite magnetic things for looking inside bodies and stuff.
Below is a true bona fide non doctored piccy of me checking out the latest gadgets attached to my latest MRI toy !!
Why are MRI scanners so noisy !! ? it's well loud innit !?
Its like there a party inside and they are larging MRI style.
Can ewe help me understand the nature of noisy MRI scanners ?
ta
hugs and shmishes
mwah mwah
Sheepy
xxxxxx
MRI's they make some noise
Louder than the loudest toys
But why do they make such a hullabaloo ?
Please to answer cos I dont knooooooo !!....lol
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Most whole-body MRI scanners use an air-cored, liquid-cooled superconducting solenoid as the primary homogeneous field magnet. This is constructed from fairly lightweight materials to minimse thermal mass and maximise insulation (air and vacuum being good insulators).
The image is formed by adding radiofrequency electromagentic energy to voxels of the patient which are selectively brought to resonance by the further addition of "gradient" magnetic fields. The gradient field coils are excited by hefty current pulses, and thus behave like 10 kilowatt loudspeaker coils within the strong primary field. Whilst we can restrict their gross movement and warping with a degree of mechanical constraint. they also undergo inevitable magnetostriction, and what ewe hear is the residual rattliing and magnetostrictive acoustic pulse.
"Open" MRI machines generally use massive iron-cored magnets for the primary field, with water-cooled resistive coils, gas-cryocooled superconductors, or permanent magnets. Being inherently heavier and more rigid (one of mine weighed 200 tonnes, compared with about 3 tonnes for a conventional supercon tunnel) they allow us to bolt the gradient coils to a massive sound absorber, and the machine-gun rattle is reduced to a tolerable buzz.
Curiously, this presents problems with our "upright" MRI scanners. They are of course ideal for studying the bizarre complications of the human spine (sheep rarely suffer from slipped discs) but patients sometimes fall asleep sitting or even standing in the scanner. This is OK until they start dreaming, whereupon they twitch and jerk, so we have to wake them up and ask them to stay consciously still. So we fit each scanning room with a huge television and hi-fi sound system to make up for the lack of noise!
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Thank ewe very much Alan. two things I have learned here, the magneto words are really kewl and the fact that there are upright MRIs.
I realise that there must be all different types of MRI scan but how long does the average scan last ?
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There was a recent bit of interesting research looking at how dogs perceive the content of their master's voice - in terms of syllables and emotional tone.
They had to train a dog to sit absolutely still in an MRI scanner, while wearing earphones!
I am not sure how you would incentivize a sheep to be the subject in this experiment?
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Scan time depends on the volume being scanned, the required spatial resolution, the scan pulse sequence, and the tolerable signal/noise ratio. It can be as short as a few seconds, or up to an hour. We have some superb movies of an athlete's heart valves working as he runs on a treadmill (one of my upright scanners is enormous!) but whilst the effective temporal resolution is a few milliseconds, the actual acquisition took around 30 minutes of phase-locked sequencing gated to his cardiac waveform - a great academic study but hardly diagnostic for a sick patient. The NHS generally books patients at 20 - 30 minute intervals and we allow 40 minutes for multi-position musculoskeletal studies.
Regrettably I've not been challenged to image a sheep. I once offered to calibrate an x-ray machine for ovine use but the vet said that sheep were worth less than x-ray film so there was no point.
As for how dogs perceive the human voice, Sophie says "What a stupid question. As long as you can pronounce "dinner" and "walkies" clearly, nothing else matters. Sheep herding is a doddle: we've been doing it for millions of years but it sometimes takes ages to train the shepherd to whistle at the right moment. Guide dogs? Come on, would you step in front of a bus if you had eyes and ears? Again, it takes months to train the humans but eventually they learn to walk behind us. Now you must excuse me - rabbit at 2 o'clock, 300 yards, stationary. Tally-ho."
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CT (https://en.wikipedia.org/wiki/CT_scan) scanners have big moving-parts under the cover : like a giant washing-machine ...
https://www.youtube.com/watch?v=2CWpZKuy-NE (https://www.youtube.com/watch?v=2CWpZKuy-NE)
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But they are extremely quiet - at least the current generation of multislice machines. And my favorite https://en.wikipedia.org/wiki/Electron_beam_tomography
has no mechanical moving parts at all