#2 Problems with Exposure Index (EI) Numbers 01-15-2013
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:
1- They are easily corrupted or skewed by poor collimation and/or centering (up to 75% I’ve found in my research).
2- 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 coolimation and centering are good.
The Problems of
“Creeping mAs/Dose” in America
As this is my first blog, I decided to jump right in and tackle what I believe is one of the biggest problems in radiology today; and that is the over radiation of the general public using digital x-ray (CR and DR). As of January 1st, 2013, there probably isn’t a city in the United States that doesn’t have facilities using digital x-ray equipment.
The first CR equipment came out in 1983, which means it has now been around for 30 years. The problem is most of us never had proper training when it came to the new techniques which implements using at least 15% more kV and cutting the mAs in half. By doing this the patient’s entrance dose is cut by 33%. Most vendors either had facilities utilize the same techniques that were used with film, a slightly higher kV with the same or even more mAs.
Most training consisted of being shown how to correctly set up and get an exposure and post process the image, and that was about it. The whole concept of a “light” image on your monitor no longer exists but if the patient is under radiated the image could have mottle/noise. To take no chances, many radiographers slowly started to increase the mAs little by little to guarantee a perfect “looking” image every time. The problem is the computer has the ability to automatically rescale the image and make it look perfect even though 5, 10, 50 times too much mas/dose/radiation is used. The Exposure Index (EI) numbers show that the patient has been over radiated but these numbers are not utilized nearly enough (see my next blog on the EI numbers later this month).
To make matters even worse, students don’t really learn to be as discriminating in determining the exact size of a patient so that they can use the perfect technique. Also there is so much latitude with how much mAs can be used in digital radiography, it’s impossible for any radiographer or student to be as good with manual techniques as we had to be with film. So each year the mAs slowly and insidiously creeps up a little bit at a time but over a 10-20 year period departments are now using double and triple the mAs that is needed
Finally, how do we combat this problem?
1. Everyone needs to be better educated about using higher kV and lower mAs.
2. Radiographers need to teach the students how to visually access the correct size of a patient (or use calipers).
3. Everyone needs to be better trained in using the EI numbers.