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 48″) 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 “exposure-distance conversion” chart below (and located in the All Charts section of my website).