Calibrating xray software

 Today I had an interest observation in the student clinics. It so happened that all the students I was put with were doing Root canal treatments at the same time. I noticed that when they were assessing their working lengths, they were all using the measure function on the dental xray program. This is fine for a preoperative assessment of the tooth and perhaps to estimate how short of working length you are but is by no means an exact science. Firstly, the measurement too l needs to be properly calibrated as the objects in the image can be elongated, foreshortened or distorted so a reference point is needed that is measured in the mouth eg the width of a tooth. This can then be used by the program to work out the ratio of enlargement of the xray compared to the actual objects imaged in the film. 

However, it turns out they were taught to measure the width of the film (41mm) and input that measurement into the software as a calibration. I may be wrong but this doesn’t make any sense to me as the width of the film is independent of the objects imaged in the film. I posed a question to explain this concept to them which I thought was quite a good analogy: if you’re looking out a window at two trees next to each other and you want to know the length of one of the trees, is it more useful to know the size of the window or the height of the other tree? In this way we can consider than an xray film is just a window that we can see through; the bigger the xray the larger field we can see. The calibration measurement needs to be in the same plane as the object we are trying to measure. Imagine two images of the same object, one image foreshortened and the other elongated. For both of these images, the width of the film does not change and so to use this as a calibration measurement is useless. The only useful situation is if the image is taken perfectly parallel with the object.

The other major issue I noticed was the importance that the students were putting on this digital measurement. When I asked what their working length measurement was they would refer me to the digital measurement e.g 23.58 rather than taking the file out and measuring it. It is important yet again to realise that this digital measurement is only an estimate and is limited by the accuracy of your calibration measurement, distortion of the film etc. the measurement that we should trust is the one with the least complexity and room for error which is just actually measuring the file.

I think education about certain aspects of dentistry are easily misconstrued and care must be taken to properly explain things to anyone at any level. An educator can't just assume that a student will have the knowledge to carry out a procedure the way they are expected to. Of course we would like to give people the benefit of the doubt but  it is important to test and question our students and ourselves. Of course there are many thing that I have to learn from the students as each of them will carry a unique take on the theory they have learnt and they will have access to up to date knowledge that I may not have come across. On my end I can share what experience I have and my interpretation of the first principles that underlie the daily dentistry we face.