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Technology / Re: Why Are MRI Scanners So Noisy ?
« on: 13/09/2016 14:43:04 »
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!
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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