My Version of the Perfect Exposure Index (EI) System

The chart below is a system that Agfa has implemented with all of their new equipment. It’s a three color system with green being “within range’, yellow being “slightly over or under exposed” and red being “significantly over or under exposed”. I like that they took a stab at it, but think that they went the opposite of the AAPM in that their ranges are too wide open (see blog # 14 for the AAPM’s ranges). The green (within range) goes from 15 to 60 mAs
and 60 is quadruple 15. I like that the yellow zone on the overexposed side is only from 60-90 mAs but don’t really like that one needs to be 4 times their perfect 30 mAs or 8 times their bottom of the green 15 mAs to get to a place that is deemed significantly overexposed.

The chart above is the one I created to fix the problems I saw with both the AAPM and Agfa models. I’ve used a 7 section/color system as I do not think it’s a great idea to have the same color for over and under exposures (for those radiographers who are just looking at the colors and not the numbers). I’m not sure how difficult it would be to implement 7 colored light bulbs or filters in the control panel, but it would be the best set up and easiest to use and understand.

The reasons for the 7 sections instead of Agfa’s 5 is the radiographer can really fine tune their technique skills and get a much better understanding about how an EI number that is noticeably off can still look perfect. Keeping a nice tight range as to what is Very Good versus Within Range versus Significant and finally Excessive is the only way to truly combat Dose Creep.

I’ve put the green/Very good from a perfect EIT to +60%, or 2 normal steps up in mAs. From 61% to 100% (double the mAs) is the blue/Within Range. From 101% to 199% (double to triple) we’re now in the yellow/Significant region and from 200% up we’re in the red/Excessive zone.

If you remember from my last posting, I wrote about the Deviation Index (DI) number and that it quantifies how much the EI varies from the TEI. So unless the EI number comes out perfect (identical to the EIT) there will be a minus or plus DI number. It’s A great idea to have this EIT number, but what help is it if you now know that your exposure was a +4 but it doesn’t tell you how to fix the technique?

So my colleague Carter Doupnick and I created the “How to Fix an Incorrect mAs’ chart you see above. To use it you start in the middle column with the mAs that was just used (either by manual technique or with the AEC). Then you use the plus or minus rows depending on what the DI number was. Let’s say it was a +4 and the mAs used was 63. When you match them up you see that 25 mAs should have been used.

I am hoping that every vendor starts using this “How to Fix an Incorrect mAs’ chart as it really is the only way that a DI number will actually be useful to a radiographer. With it, every repeat that needs to have the technique changed will be incredibly easy to accomplish. It won’t matter one bit if you’re good or bad with math as all the calculations have already been done. Plus all techs and students will be able to use this chart to see what would/could have been the perfect technique even though the image taken was perfectly diagnostic, but still over or under exposed.

If you’re facility is using any of the newest equipment that displays the DI number and you’re wondering what to do with it, please contact me so I can set your department up with this chart and system.

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