When Fuji first developed CR back in the early 80’s, they were also the first company to invent an Exposure Index (EI) number system. As most of us know, they went with S numbers and decided to have those numbers correspond with the speed of film –screen systems so that the higher the number the less the dose. Unfortunately they didn’t ask one radiographer anywhere in the world who would have told them that this is was not a good idea as it’s not intuitive and it’s backwards from the way we think. Pretty much every CR vendor after them followed suite as they decided to use Logarithms. Finally DR came into the picture and all the vendors were able to develop EI numbers that were more user friendly. Almost all of them came up with systems that had an EI number that would double when the mAs was doubled.
Pretty much all digital radiography equipment have some sort of EI number. There are S, LgM, DEI, EXI, REX, EI and probably others. These numbers were developed to tell us how much radiation made it through the patient and exposed the image receptor (IR). The problems with the numbers are twofold:
- They are easily corrupted or skewed by poor collimation and/or centering (up to 75% I’ve found in my research).
- Unless one is good with math the EI numbers are difficult to use to figure out how to fix a bad technique.
Because of these two reasons, many radiographers do not use the EI numbers when critiquing their images. Instead they use the same methods that were used with film, which are incorrect (see my next blog on Feb 1st that will cover how to properly critique a digital image). These EI numbers are quite reliable, even more so when the collimation and centering are good. This is why everyone should be trying their hardest to collimate as tightly as possible.
There is no doubt that the EI system is not a great system by any stretch of the imagination, but it is the ONLY system we have to help us critique our technique at this point in time. Hopefully in the near future the manufacturers will be able to create better and truer EI numbers, so all radiographers will truly know if they have used the correct technique.
I’m not a big government guy, but I think I would be OK with Big Daddy telling the vendors that they have to all play nice with each other and figure out 1 EI system that will be universal. It’s all going to be too much hassle (and costly) for them to do it, so unless they are forced into it, I don’t think it will ever happen. And that’s just a cryin’ shame.