#14    The New Exposure Index (EI) Terminology  7/15/13

     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 of the 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 (EI_T) 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 EI_T) 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 1^st I will discuss my version of the perfect Exposure Index system.

 Link to DigitalRadiography Solutions Website

mAs = Dose

#13                                 mAs =  Dose                            7/1/13

     Today’s post will center on how the mAs you use is directly proportional to the dose you are giving your patients.  I could have just written “the dose the patient is getting” but I really want to make it clear that you the radiographer, or radiographer student, are the one setting the technique and making the exposure.  This is true whether you are actually setting a technique (both mAs and kV) or using an AEC and just setting the kV.

     By directly proportional I mean that the dose increases or decreases to the exact degree that the mAs increases or decreases.  For example, if the mAs is increased from 10 to 20 (doubled) then the dose is also exactly doubled.  If the mAs is decreased by 50% (cut in half) from say 30 to 15 then the dose is also exactly cut in half.

     To prove this point to you, I am going to present an experiment I conducted a while back with my DUKE phantom (see image 1). 

I did the experiment to prove that both the EI number and the dose will directly change with the mAs, so you will see Agfa’s LgM numbers also annotated on the images.  For this article I will only be concerned with the Entrance Dose (ESE) that was annotated on the magnified images that go with each exposure.

On image 2,we begin with a technique of 4 mAs@117 kV and an ESE of 20.81.  

 

As we go to image 3, the mAs is doubled to 8 mAs@117 kV and the ESE is now 41.40.  If it had been 41.62 that would have been exact, but even so it is only off by 1.06%.

   

                               

Looking at image 4, the mAs is now doubled again to 16 mAs@117 kV and the ESE is now 83.0.  If it had been 82.80 that would have been exact, but it is only off by -0.48%.

   

On the final image 5, the mAs is now doubled again to 32 mAs@117 kV and     the ESE is now 166.0.  This ESE is exactly double.

     

I hope this proves to you that changes to the mAs are equal to the dose.  Because of this I have been telling radiographers and students that a great way to keep yourself from needlessly increasing the mAs is to call it dose.  By this I mean you would say "I am going to use 50 or 100% more dose" instead of saying more mAs.  The word dose has a lot more impact and energy to it while the word mAs is no big deal.

Link to Digital Radiography Solutions Website