So you've worked out which way to move but now how far?
The traditional way of centring your image in theatre radiography is to look at the image and then make a small movement in the direction indicated by the anatomy. (If you don't know which way to move without reference to anatomy: See Here )
This is called ‘Intuitive’ centring and is little more than a guess. To improve on this I looked into the geometry of the X-ray beam, it turns out that frequently it is possible to accurately estimate distances across the image. In this way the Radiographer (Radiologic Technologist) can know how far to move the cross arm forward or back; or how much to adjust height up or down on the lateral view, to centre in one move. Not maybe; definitely.
For cases where both AP and horizontal beam views are taken the distance to any anatomy can be simply and accurately checked on screen giving a fixed magnification point for both views. This technique can be accurate enough to confirm or dispute measurements of screws and implants but that has more complications and is the subject of an upcoming feature.
The diagram above shows how far your chosen anatomy is from the centre of the monitor. For example if you are centring a wrist on screen which shows at the bottom edge of the image, then it is six centimetres or three degrees from the centre. For cross arm movements forwards or back it is simple to move six centimetres in the right direction. For movements left to right you can use either the panoramic motion or track sideways to move the required distance. Some C-arms are equipped to measure panoramic swing (The Siemens Siremobil for example) others offer no facility for measuring this motion (Philips Endura or Libra for example). In the case of the Siemens machine examples I have seen have the pan scale marked in five degree increments which is not accurate enough. Five degrees of panoramic movement gives eleven centimetres of sideways arc motion of the intensifier which is most of the way across the screen. In practice lateral pan or ‘arc’ movements will usually be between one and three degrees.
Why not just use lasers for centring?
This is in fact a great question; in practice very few C-arms are equipped with lasers bright enough to be visible under the theatre main operating lights. There certainly are ways to use lasers for this but even when equipped in this way most trunk and femur anatomy is not externally apparent therefore lasers may not be very helpful in locating them.
How do I use this in practice?
Now you know which way to move to centre, the above diagram shows the distance to move in each direction. It is applicable whenever a sixteen centimetre intensifier (most mobile C-arms) is less than forty centimetres from the subject; which if you are working properly will be most of the time. The physical size of the intensifier is usually 30cm across with the image on screen a similar size. If your intensifier fits these approximate values then it is almost certainly a 16cm model.
What if we have a larger intensifier?
Some centres use larger intensifiers or even a flat plate detector. In order to find the correct values as in the diagram above take a horizontal lateral view of a test object of known length. Centre this on screen then rotate back to an AP view and check with a ruler how much larger the object appears on screen. This gives the system magnification at the working height. Now measure across the widest part of your image displayed on screen, divide by the working magnification factor and you have the working width.
Show me an example
In the image below the wrist is a little more than one half of the distance to the edge of the screen. In this case you can see that the centre of the image needs to be moved four centimetres forwards to centre the wrist on screen. Move the cross arm forwards four centimetres to achieve this. If you move in the wrong direction you need to go back to basics!
Measure your C-arm movements as you Pan left or right
This is a printable overlay scale for Siemens Siremobil C-arms which shows one degree increments. It's in Powerpoint format so that the majority of users will be able to print it. There is no need to print it in colour if that is not readily available. Cover with Helicopter tape and stick over existing scale with selotape double sided tape. Some care is required!