The rotating inside of a medical CT scanner.

[coppice]

When I went in for my MRI the machine was a G.E. and it was perfectly quiet except for a light tapping sound. Very good bearings indeed on these machines.

What makes me wonder though are the bearings non magnetic, I suppose they would have to be?

MRI machines do not rotate.  They are purely magnetic & their magnetic fields are controlled via software.

They are also so noisy the patient has ear mufflers to dampen the noise. An MRI machine is quite uncomfortable. It feels like they could do a lot more to make is pleasant for the patient, like at least keep the temperature better controlled with better air flow.

[coppice]

I wonder how many people would opt out of the scan, if it had a glass cover. Or, at least, be very frightened. I think it looks cool.

You do see a strip of the rotating mechanics when you are inside a CT machine. The narrow ring where the x-rays go is exposed. You can't see the sheer bulk of what is spinning around, but you can see its speed, and its clearly not something small and light.

[SiliconWizard]

Good thing they don't make the patient spin instead.

[tooki]

They are also so noisy the patient has ear mufflers to dampen the noise. An MRI machine is quite uncomfortable. It feels like they could do a lot more to make is pleasant for the patient, like at least keep the temperature better controlled with better air flow.

That was never an issue in the MRIs I’ve had done. That sounds more like a design flaw in a particular model and/or some deficiency in the machine’s setup. (Like how the ambient air is handled.)

The machines I was in had earphones (using air hoses to carry the sound, like really old airplanes used to do). I don’t remember whether there were over-ear hearing protection over them or not. (Probably??  )

[tom66]

I had a sinus CT a few months ago, and whilst the machine wasn't silent, I wouldn't call it particularly loud.  It was about as loud as being in a conversation with a few people.

That said it was probably an older generation of machine so perhaps wasn't rotating as fast.

[Halcyon]

MRI's are fascinating machines. Watching the magnet being "quenched" is just as fascinating.

The helium expands at such a rate that if allowed to escape into a relatively well sealed room, it can blow out windows/doors and even the joint where the walls meet the ceiling, which it why it's vented outside the building.

Pushing that red emergency button results in a very expensive repair.

[BrianHG]

I had a sinus CT a few months ago, and whilst the machine wasn't silent, I wouldn't call it particularly loud.  It was about as loud as being in a conversation with a few people.

That said it was probably an older generation of machine so perhaps wasn't rotating as fast.

CTs are quiet compared to MRIs.
The field RF coils in MRIs are being modulated with RF inside a slower ramp signal, something like a slow 3 dimensional CRT monitor's yoke.  This heavy-duty ramp signal, when it deflects from one axis to the next creates those huge different buzzing noises as that scan rate is slow, down in the audible range.

[WatchfulEye]

I had a sinus CT a few months ago, and whilst the machine wasn't silent, I wouldn't call it particularly loud.  It was about as loud as being in a conversation with a few people.

That said it was probably an older generation of machine so perhaps wasn't rotating as fast.

CTs are quiet compared to MRIs.
The field RF coils in MRIs are being modulated with RF inside a slower ramp signal, something like a slow 3 dimensional CRT monitor's yoke.  This heavy-duty ramp signal, when it deflects from one axis to the next creates those huge different buzzing noises as that scan rate is slow, down in the audible range.

This is the same mistake as in the video above.

It is spatial modulation of the static magnetic field (e.g. A nominal 1.5 T static field may be modulated to 1.6T at the head and 1.4 T at the feet in order to extract location information in that axis), and not the RF.

The sound produced can be substantial. With the exception of some ultra low field portable MRI systems, all current medical MRI can operate at well above 99dB(A) although they may not necessarily do so for all examinations.

Higher gradient slew rates facilitate faster scanning and higher spatial resolution. Examinations of small body parts like hands, ankles, knees and inner ear as well as cine images, tend to be louder than most other scans. 120dB(A) can be reached by some scanners running these protocols.

[TimFox]

Yes:  an MRI scanner uses three different coil sets:  from the outside in.
1.  The superconducting solenoid establishes the DC magnetic field, which is uniform in the imaging volume, in the z-direction.
2.  The gradient coils, which are pulsed, vary the magnetic field in a programmed fashion, as a function of x, y, and z.  This coil set is also used to trim the uniformity of the DC magnetic field with appropriate DC bias currents.
3.  The RF coils (or resonator) excites the nuclear resonance of the protons in the patient, where the resonant frequency is proportional to the local magnetic field value.  They are also used to receive the radio-frequency signal (not waves) from the rotating net magnetization of the proton spins.  The complicated pulse sequence rotates the net magnetization away from the z-axis, and that magnetization vector then rotates in the x-y plane at the resonant frequency.
The system is very flexible and designing the appropriate pulse sequence (for gradients and RF) is an important exercise.

[IDEngineer]

I wonder what method they use to impart the spin motion. The first thought would probably be "electric motors" but they have their own magnetic fields which would have to be shielded from the diagnostic ones. Otherwise the magnetic crosstalk could affect both.

[tom66]

I wonder what method they use to impart the spin motion. The first thought would probably be "electric motors" but they have their own magnetic fields which would have to be shielded from the diagnostic ones. Otherwise the magnetic crosstalk could affect both.

Huh?  The CT scanner can use electric motors since it is sensing X-rays from the generator.

An MRI does not rotate the patient or scanning head, it detects everything via the RF coils.  There are probably cooling fans and the like inside the machine that need some care if positioned near sensitive components, and the bed itself and moved in/out using a motor, but I imagine the fields produced by those are quite small.

[BrianHG]

I wonder what method they use to impart the spin motion. The first thought would probably be "electric motors" but they have their own magnetic fields which would have to be shielded from the diagnostic ones. Otherwise the magnetic crosstalk could affect both.

You do realize that a CT scanner is a 3D X-ray machine.  It does not see any magnetic fields, just X-rays.
And an MRI is a 3D magnetic resonance imaging machine.  No moving parts.
Right?
The 2 are not the same thing.

[TimFox]

Yes, like any other expensive device, careful engineering is required to minimize unwanted noise in the signals.
For x-ray computed axial tomography (CAT), normal motors are required to rotate the gantry around the patient, and an oil pump with cooling fans on the rotating platform to run a heat-exchange loop on the x-ray tube's anode.
A motor built into the x-ray tube rotates a large anode disc to allow high-power anode dissipation for short times, since typical scans are done while the patient holds his breath;  if the tube's anode is out of balance, this will degrade the image.
Modern CAT scanners have a high-voltage supply on the rotating platform for the x-ray tube, and it must be shielded and filtered appropriately.
Most CAT scanners have the detector array (now two-dimensional) rotating on the same platform with the tube, with fancy data communication from the detector electronics to the stationary platform.
As posted above, MRI machines do not require motion (except for the patient couch), but the pulsed gradient currents could interfere with electronic signals;  the basic signals from the patient go through a resonant "receive" coil, tuned to a much higher frequency.
Sometimes, motors are required to operate vacuum-variable capacitors to tune the high-power RF coils:  these motors are an engineering problem to avoid loss of homogeneity in the DC field due to ferrous components near the main magnet.
Again, as mentioned above, the clearly-audible noise results from interactions between the gradient fields, the DC field, and the coil structure.  Also audible is a mechanical refrigerator that often accompanies the magnet to minimize loss of liquid helium.

[coppice]

An MRI does not rotate the patient or scanning head, it detects everything via the RF coils.  There are probably cooling fans and the like inside the machine that need some care if positioned near sensitive components, and the bed itself and moved in/out using a motor, but I imagine the fields produced by those are quite small.

The only motors in the heart of the machine are ultrasonic motors used to move the patient. Other types of motor are useless in such a powerful magnetic field. For cooling I believe they duct all the air, and the motors are well away from the magnetic field. Early MRI machines, before ultrasonic motors were commercially available, used electromagnetic motors to move the patient, but had to site them remotely, and used non-ferrous gears and long rods.

[IDEngineer]

The only motors in the heart of the machine are ultrasonic motors used to move the patient. Other types of motor are useless in such a powerful magnetic field. For cooling I believe they duct all the air, and the motors are well away from the magnetic field. Early MRI machines, before ultrasonic motors were commercially available, used electromagnetic motors to move the patient, but had to site them remotely, and used non-ferrous gears and long rods.

Interesting. I thought perhaps they used hydraulic or pneumatic motors/cylinders, with the pump remotely located for sound/cleanliness. Those can be made quite powerful yet with minimal crosstalk to magnetic fields. You can hold a 10HP hydraulic motor in the palm of your hand.

[coppice]

The only motors in the heart of the machine are ultrasonic motors used to move the patient. Other types of motor are useless in such a powerful magnetic field. For cooling I believe they duct all the air, and the motors are well away from the magnetic field. Early MRI machines, before ultrasonic motors were commercially available, used electromagnetic motors to move the patient, but had to site them remotely, and used non-ferrous gears and long rods.

Interesting. I thought perhaps they used hydraulic or pneumatic motors/cylinders, with the pump remotely located for sound/cleanliness. Those can be made quite powerful yet with minimal crosstalk to magnetic fields. You can hold a 10HP hydraulic motor in the palm of your hand.

Can you make good high lifetime hydraulic motors without using ferrous metals?

[WatchfulEye]

The only motors in the heart of the machine are ultrasonic motors used to move the patient. Other types of motor are useless in such a powerful magnetic field. For cooling I believe they duct all the air, and the motors are well away from the magnetic field. Early MRI machines, before ultrasonic motors were commercially available, used electromagnetic motors to move the patient, but had to site them remotely, and used non-ferrous gears and long rods.

When I happened to see an MRI system with the covers off, I was so surprised to see the fans bolted to the magnet, that I had to snap a picture!

[TimFox]

The warning about the magnetic part is extremely important!
I once set down a tweezers on the patient couch during a repair and it went flying point-first into the bore.
Field engineers were always finding paper clips stuck to the magnet (not good for homogeneity) despite the availability of plastic clips at office-supply stores.

[IDEngineer]

Can you make good high lifetime hydraulic motors without using ferrous metals?

The seals aren't metal so no problem there. Pump bodies are already available in aluminum. The big question is the shafts and (if a gear motor) the meshing gears. If it's a vane pump design the vanes are not metal and I suspect the shafts and carriers wouldn't need to be either.

I'm not a pump expert nor a metallurgist, but we had Danfoss (huge hydraulics manufacturer) quote a pump that would be submerged and operated in seawater indefinitely and they had no problem with it. I'm pretty certain it didn't have any ferrous metal content since that doesn't mix well with seawater (well, actually, it mixes a bit too well  ).

[tom66]

For MRIs there is a whole industry of 'MRI-safe' equipment.

The interesting ones are devices like headphones and televisions.  For headphones the answer is usually simple - typically the sound unit is placed far away from the MRI and air hoses conduct the sound.  It's not particularly good but it does work.  For analysing the brain's response to visual stimuli, a monitor is needed. This usually relies on mirrors from an external monitor, though there's a headset available which does this in VR. With sufficient shielding and minimising ferrous components the device can be made sufficiently safe for MR environments.