How to Digitally Measure Distance Between C-Arm X-Ray Tips With 2.5m Span and 2cm Play

Hi, help gratefully requested please. I am a surgeon and use a 'c' arm xray in surgery. The radiation passes between the tips of the C. The C can be moved to take vertical or horizontal images, or anywhere in between as required. I would like to measure the distance between the two points of the C as they possibly open and close by up to 2cms which can distort the images.
I would like to digitally measure the distance as I move the c arm.
Many thanks
Anthony

Never seen one of these machines before. Looking at photos, it appears that the 'C' is fixed structure, so I assume that the source or imaging head can be moved to change the distance. If what you describe is a common problem, I would have assumed that the means to physically move the heads is already built into the machine. If it is done by machine control, then there may be a way to extract the information from the machine itself.
If it has to be done manually, there are many non contact ways of measuring distance, time of flight (ultrasonic and light), doppler radar etc. which will all work well over the distance you wish to measure. None will work while the patient is between the arms obviously, a real time measurement system if needed would need to be attached to the heads with a physical offset. The choice comes down to the precision needed, ease of implementation (electronics need around the device) and how the device could be retro engineered into the system. There are development kits available for all of these techniques available from manufacturers which would get you started, they would all need re packaging if they need regular cleaning to be kept sterile.
A lot of questions here, so a lot more detail might allow me to comment further.
Cheers,
Richard

Hi
A solution may be to use the ultrasonic Parallax sensors. This is the link:

https://www.parallax.com/product/ping-ultrasonic-distance-sensor/

It is easy to use and it must used with a MCU.

Cheers
Giovanni

Parallax PING ultrasonic sensor provides precise non-contact distance measurements from approximately 2cm to 3 meters. It is a very simple method for measuring distance by sonar.
An ultrasonic pulse is transmitted from the unit and the distance to the target is determined by measuring the time required for the echo return. The output from the PING is a variable-width pulse that corresponds to the distance to the target.

thank you so much.
What is an MCU? I am sorry but I am a novice.

Hi Richard,
the C can rotate around to take images from different directions. The ends are supposed to be a fixed distance so that reproducible images can be taken. I want to investigate whether this is the case or wether there is movement in the arms. The type of imaging is subject to distortion and I am trying to find out why. So this is not to be don e in s sterile way- I plan to just take xrays of a grid of known dimensions in different positions ranging from vertical to horizontal ( over the top and from under neath).
kind regards

What is the nature of the distortion? Is it rectilinear? What sort of depth of field (to borrow from optics) do you have from this sort of system. Would it be possible to use grids of slightly different sizes or styles (so they may be differentiated in the resulting image)and stack them vertically with a suitable displacement over the depth you need to be able to work over and see what happens.
I would assume that the system is rigid enough that gravity is not going to displace the heads slightly depending on the C orientation.
Cheers,
Richard

hi Richard,
thank you for this.
Acutally i think that there is some movement between the ends of the arms as it is moved from vertical through to horizontal. I want to measure the movement and simultaneously the 3D position of the arms. I also need to measure background electromagentic field.
kind regards
Anthony

If there is movement, then it likely would have an angular component, a twisting moment may occur as it goes from the vertical to horizontal. Seems there may be quite a bit to measure. There are simple electronic levels in hardware shops that would give you your 'C' angle. They would not have the resolution to determine if the heads themselves were twisting on the end of the arm. It could also be that only one end might shift. In that case you could put a platform in the middle on the axis about which the 'C' arm turns (It would need to be rigid enough not to be affected by gravity itself) projecting out as far as the heads to support a target grid on either side of it, and place a camera on each of the heads pointing at the targets.
When you say you need to measure background radiation, is that just an aside, and you are equipped to do so, or are you needing advice on that as well. Not my area if so, but this makes and interesting read.
http://www-naweb.iaea.org/nahu/DMRP/documents/Chapter4.pdf
There are usually companies leasing out test gear for such purposes, the equipment should have a current test certificates issued by a legitimate testing source.
Cheers,
Richard

Parallax PING ultrasonic sensor provides precise non-contact distance measurements from approximately 2cm to 3 meters. It is a very simple method for measuring distance by sonar.