EI Calibration

      Justin Shek     0842536

 

      An exposure index, as known as EI, gives us an idea whether our image has got enough signals or not.  It is mostly used in film screen and computated radiography (CR) since the cassettes have to be processed. Just by looking at the EI, we can tell if we have gotten a reasonable image.  Each manufacturer of x-ray machines has a different EI acceptable range. For instance, the acceptable range of EI is from 1400 to 1700.  Any EI falls in between the range is considered reasonable meaning the image has captured enough photons and is diagnostic. Therefore, an accurate EI is crucial to information of an image. The purpose of this lab is to carry out three trials to see if CR is properly calibrated and would it give us a consistent EI within an acceptable range in the three trials.

       The reason why I chose putting EI calibration into my e-portfolio is because it was not just fun and easy to perform, but it also required a little bit of creativity of doing it in the first time.  As the procedures requested a 172cm SID between the x-ray tube and the sensor of a dosimeter, we actually had to think how to get the SID because the procedure could only be performed vertically.  However, the largest distance between the x-ray tube and detector was not greater than 140cm, so we had to figure out a solution to this problem. After a second, we recalled a trick from previous years, which was, we set a hand protocol on the control panel so the detector did not have to be used. Then we could put the dosimeter or the cassette on the floor with the required distance. 

 This is the bottom of the dosimeter that we used

       

      However, we made a mistake in our trials because we collimated too tight, and the x-ray machine’s collimator was a bit old so it was not as precise as before. As a result, we got such a small EI which was around 800 and the images were too dark and not acceptable.  However, once we opened up our coning, the exposure index of the three trials are 1600, 1690 and 1700 respectively and were reasonable and acceptable meaning there were enough photons captured per image. If the EI was not calibrated correctly, it would affect the image quality which would increase the rate of repeats and patient dose. However, since the technology is getting more advanced, we can always change the contrast and brightness afterwards if the EI is not too far off.