#16    Why Should You Increase the SID for Abdomen’s?               8/15/13

     For the first 10 years of my career (late 70’s, early 80’s) the SID for abdomen x-rays was 40” and that was written in stone.  I still remember the first new GE room we got at Watsonville hospital that had a 44” detente.  It seemed blasphemous that a company would think, much less do, such an outlandish thing.  Was nothing sacred?

     Then about 6 or 7 years ago one of our students told me that he had just come from a facility that did all of their supine abdomen x-rays  with the tube as high and table as low as possible and their upright’s were done at 72”.  Needless to say this seemed unbelievable to me but here was an x-ray department in my own area that had obviously been doing it for quite a while.  He explained that they did it because with the greater SID there was less divergence, so more anatomy was seen.

     Now that I knew this was even possible (and ethical), I asked our radiologist’s what they thought about it.  It turned out they loved the idea.  Since their entire job is to make a diagnosis with the information given to them, the more information they have to work with the better they like it.

     So I decided to do a simple experiment to see exactly how much more anatomy we would see when we increased the SID.  I took my abdomen phantom and exposed it at 40”, 50”, 60” and 72”. 

     As you can see in Figure 1,

at 40” the top of L1 is at the very top of the image and there's about a marker’s width below the ischial tuberosity at the bottom.  There is also just over a markers width on the sides. 

     In Figure 2, at 50”

we can now see about one third of a vertebra of room at the top and a marker and a half at the bottom and on the sides.

     In Figure 3,at 60”

we can now see about half of a vertebra at the top and a marker and three fourths on the bottom and sides.

     Finally in Figure 4,

at 72” we can now see a solid three fourths of a vertebra of room at the top and two markers on the bottom and sides.

     Since a vertebra is approximately one and a half inches in height, this means we gained at least two and a half inches of extra anatomy top to bottom and just under that on the sides.

     Plus there is a second reason to use a greater SID and this is to save the patient entrance dose.  Or at least I believe this is so.  I am almost positive I have read an article in the ASRT Journal that stated that an increased SID 40" to 4*"") would save a patient about 8% entrance dose.  Unfortunately when I went to look up that article for this posting I couldn't locate it.  If any of you know about this dose savings article, please write or call me and I'll let everyone know about it in my next blog or somewhere on my website.                     

     Whether there really is a dose savings or not, getting close to 3 more inches of anatomy on an upright abdomen surely is worth talking to your radiologist about this.   Just remember that the more SID you have the higher you need to center so that all the new anatomy you are showing is the upper abdomen and not the symphysis or below.

     If you are taking these images portably and need to technique it, remember that the mAs will need to be increased as the distance is increased.  If you want to know exactly what these changes are, please use the “exp-dist conversion” chart below (and located in the All Charts section of my website). 

 

  Link to DigitalRadiography Solutions Website

#15       My Version of the Perfect Exposure Index (EI) System         8/1/13

     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 EI_T 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 EI_T) there will be a minus or plus DI number.  It’s A great idea to have this EI_T 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.

 Link to Digital Radiography Solutions Websitedigitalradiographysolutions.com