The New Exposure Index (EI) Terminology

There are three new definitions all of the manufacturers are now using that will soon become standard terminology in our profession. The first and most important is that finally(!!) there will be only one name for the number that comes up after your exposure to show the measure ofthe radiation in the Region of Interest (ROI). It will now be called the Exposure Index (EI) number. This means the S, EXI, LgM, DEI and ReX numbers will be vanquished forever. Yayyyyy!!!!

The next term is the Target Exposure number (EIT) and this will be the EI number when an image is optimally exposed. Or in other words, the perfect goal EI number you are shooting for. Since every facility will have different ideas on what is a perfectly diagnostic image, these target numbers will be determined by each facility depending on body part, view, procedure, image receptor and radiologists.

The third term is the Deviation Index (DI) number and 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.

Now we get to the big problem though. A few years ago the American Association of Physicist’s in Medicine (AAPM) came up with an EI system that all the manufacturers were going to use with their CR and DR equipment. This was the Task Group (AAPM TG) 116 report issued in 2009. Unfortunately their system was not really useable in the real world and none of the vendors implemented their plan (or at least the whole plan).

The picture above shows what the AAPM plan looked like. The image has 4 distinct lines of information. At the bottom in white I have put in different mAs’ so you can easily see the difference between each DI number. I could have picked any mAs as the starting place so I chose 30. The colored circles show the plus and minus DI numbers. The system was set up so that a +1 DI number would be one average step up in mAs, +2 would be another step in mAs and +3 would now be one more step in mAs (or doubled a DI of 0). You can see this in the black numbers just under the black line. At the top is the place where the AAPM went off course. Here is where they actually quantified how many DI numbers would be over or under exposed and what would be a repeat or excessive.

I personally love that they want to set a standard but the ranges they gave were just impossible to uphold with the current equipment we use. From all of the experiments I have performed on corrupting an EI number with incorrect positioning or collimation, I have seen up to 75% number changes, even though the same technique was used. So to say that a +3 DI number would be excessively overexposed can really be just slightly overexposed but had really bad collimation and or positioning.

So all the manufacturers realized that this system was too unforgiving. The DI part of it was great, but how many plus or minuses make a repeat or is excessive will have to be decided in each facility.

In my next blog on September 1st I will discuss my version of the perfect Exposure Index system.

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