The rotating inside of a medical CT scanner.

[BrianHG]

  How on earth did they balance that? 

[BradC]

I’d wager very carefully. Love to know what the rotating mass is.

[tom66]

I'm amazed by how relatively quiet it is at full whack. The bearings on that must be something else, too.

[BrianHG]

This is what I expected to see:

[Gyro]

I'm just wondering what happens to the rotation speed of all those cooling fans (depending on their direction of rotation).

Balancing must be a real work of art - maybe it's dynamically adjusted.

[Kleinstein]

The speed is not that high, more like slower than the washing machine.  Essentially all the ciruit is fixed and balancing the mass only needs to be done once (unless they use leaking capacitors).
With the still moderate speed they may get away with a static balance (1 weight to adjust) and may not need an individual dynamic balance (2 weights to adjust).
It is not just about noise and external vibrations, but vibrations in the system could also limit the resolution.


The fans should see not problem. At least the ones visible in the video have the axis more or less parallel to the axis of rotation and thus no extra forces. A fan at a different angle could see extra forces and may not be such a good idea and complicate the balancing.

[vidarr]

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.

edit: That washing machine video is hilarious.

[fourtytwo42]

OMG I have been in one of those things several times, if I had seen this first I might have had second thoughts, bit like an inside out meat mincer if things went wrong 

[themadhippy]

Be more fun if the head was stationary and the patient spun

[tom66]

Be more fun if the head was stationary and the patient spun

What's the x-ray transmissibility of vomit?

[TimFox]

Be more fun if the head was stationary and the patient spun

Many industrial computed axial tomography scanners have a stationary x-ray system and rotate the part on a turntable.
If the human patient were mounted on a rotisserie (horizontal axis) inside a stationary system, the internal organs would flop around and blur the image.
There was limited use of a patient sitting on a bar-stool-like seat with a vertical axis in medical CAT with radiation sources that could not rotate.

[themadhippy]

What's the x-ray transmissibility of vomit?

No idea,but if you like to fund my research,a £100k should get the ball rolling

If the human patient were mounted on a rotisserie (horizontal axis) inside a stationary system, the internal organs would flop around and blur the image.

so spin em faster ,then the centrifugal force should keep everything in place, scream if ya wanna go faster.

[Gyro]

OMG I have been in one of those things several times, if I had seen this first I might have had second thoughts, bit like an inside out meat mincer if things went wrong 

Yes, I've had quite a few brain ones and it puts you in an ideal position to see it spinning in the slot. The overriding impression is that looks like a lot of expensive stuff spinning quite fast!

[daqq]

Must have quite the brushes, both for data and power.

[rsjsouza]

And on a similar vein...

[TimFox]

Must have quite the brushes, both for data and power.

Before about 1980, the rotating platform did not rotate continuously (before brushes) but went maybe 1-1/2 turns before stopping and rotating back.
Flexible cables were used that had a cable-handling system next to the wheel.
As I remember it, Varian had the first continuously-rotating CT system, using a small diameter oil-filled slipring at a closed end of the assembly.
I worked on the first open-end continuous-rotation CT scanner (Toshiba TCT900) using large-diameter sliprings, with the high-voltage rings enclosed in SF₆ insulating gas.
Modern scanners have the high-voltage supplies on the rotating platform, fed through low-voltage sliprings, with different high-speed data communication paths (optical or electrical).

To simplify the discussion of computed axial tomography, one needs to look through every point in the object from every angle to get the data required to "reconstruct" what was there at each point to form the image.

[tooki]

Can you elaborate more on the data communication?

[TimFox]

I've been out of that field for a while, so I can't comment on current practice.
On the continuously-rotating TCT-900, the detector ring did not rotate (that configuration is now essentially obsolete), so the data communication was straightforward.
On other scanners, with rotating detector rings, I am aware of complex optical configurations (mirrors without smoke) and slip-ring-like structures with capacitive coupling between rotating and non-rotating platforms.
A German company, Schleifring (with a branch in Elgin, Illinois) supplied the latter.
I believe they have been acquired( since my time) by GE Medical.

[james_s]

I doubt the balancing is much different than balancing something small, it's just scaled up. In fact I would expect it's easier than balancing something that has to spin at tens of thousands of RPM. In this case you have a handful of components that have to go in specific locations, then you can see where you are and start putting the other components where their mass is most needed. Once you have that as well optimized as possible add ballast weight where needed to get it dialed in. Much of the process could probably be calculated.

[BrianHG]

Once you have that as well optimized as possible add ballast weight where needed to get it dialed in. Much of the process could probably be calculated.

That's just it.  Calculating a balanced design is one thing.  But after that, when it comes to all those wire harnesses, slight different weights of copper and plastic, hand tying and mounting.  To balance, where do you know right position to add some weight?

It's not like a car wheel where you have a specific machine to measure the wobble and tell you where to place some correction weight.

[james_s]

That's just it.  Calculating a balanced design is one thing.  But after that, when it comes to all those wire harnesses, slight different weights of copper and plastic, hand tying and mounting.  To balance, where do you know right position to add some weight?

It's not like a car wheel where you have a specific machine to measure the wobble and tell you where to place some correction weight.

Motor armatures are balanced by drilling away material. Flywheels of various types are sometimes balanced by the same method. It's not outside the realm of possibility that they have a special machine to balance the rotating assemblies, or perhaps they attach transducers to the frame of the machine and balance each one in place. The transducers might even be integral in order to detect if it ever goes out of balance. Likely only someone that has seen the manufacturing process knows exactly how it's done in practice. I don't know that small variations in wiring harnesses and such would be enough to throw off the balance. The rotating assembly weighs hundreds of pounds, especially the older ones that had iron core transformers for the xray generator.

[TimFox]

Even back in the early 1980s, Hewlett-Packard made dynamic analyzers that could find the oscillation modes of the structure and look for harmonic vibrations from the rotation.
The rotating masses are dominated by the lead-packaged x-ray tube, the detector ring with electronics, the lead beam stop behind the detectors, the oil-to-air heat exchanger, and the power supplies.
Like anything else at this scale, this must be done carefully, which is why mechanical engineering is still important.

[SiliconWizard]

And on a similar vein...

Yeah, that's why patients must absolutely say whether they have anything implanted before this kind of exam. It can end up pretty badly.

[TimFox]

I once set a sharp-pointed tweezers on the patient couch of an MRI scanner:  luckily it didn't stab anyone on its way into the bore.
The field engineers knew that they had to expect paper clips clinging to the magnet, disrupting the high field homogeneity required for good performance.
An anecdote I heard from the very early days of MRI involved an installation where a technician was pinned to the machine by a steel oxygen tank.
Unfortunately, the first responders were police officers nearby, who carried sidearms.
Another anecdote involved "aneurysm clips", a surgical appliance, that were pulled from their location.
(These were from the early days, before the dangers were widely appreciated.)
I attended an MRI conference back then, where I heard the desk clerks at the conference hotel wondering why so many American Express cards presented at check-in had "wiped" magnetic stripes.

[amyk]

I don't know that small variations in wiring harnesses and such would be enough to throw off the balance. The rotating assembly weighs hundreds of pounds, especially the older ones that had iron core transformers for the xray generator.

Probably even a few pounds out of balance wouldn't be a problem, as long as the base is massive enough, and the machine is firmly secured to the floor; look at the various machining videos on YouTube of very unbalanced parts being spun in a lathe for comparison.