A while back I started to wonder about how much dose actually occurred in the middle of the body (the Midline Dose). My good old trusty FLUKE dosimeter and 150cc ion chamber could easily tell me the entrance and exit doses but I couldn’t figure out a way to get the dose that was occurring in the middle of the body which is where I figured the most internal damage possibly occurs.
I spoke to all kinds of medical physicist’s but it turned out that Midline doses aren’t really anything that they are that interested in as they are way more concerned about effective doses. Since I don’t have one of those amazing $25,000 phantoms which has compartments for the ion chamber at different depths, I created my own “poor man’s phantom”.
I’ve used four 1” thick sheets of polyethylene. Then I got a case of 500CC bags of saline and taped them as close together as I could. I figured that most medical phantoms are just pure plastic, so a phantom made of plastic and salt water should be even closer to how a human body actually attenuates and scatters radiation. The four layers together measure 12” or just about 30 cm.
I used both my ion chamber (round) and kV meter (rectangle). Afterwards I realized that the kV meter really didn’t give me any extra useful information but what I got from the ion chamber became more and more surprising, as you will see. The technique I used was 85 kV @ 14 mAs which would be the abdomen technique on a medium sized patient (weighing around 180 lbs). I had the collimation opened to 14”x17” and the SID was 45”. The entrance dose was 2.13 R.
I’ve moved the ion chamber ¼ of the way into the “body”. The dose now dropped almost exactly half as the dose was 1.05 R.
the chamber is now in the middle, so this is my Midline dose. It registered 0.469 R which is a decrease of 78% of the entrance dose. Another way of stating this would be that only 22% of the radiation that entered the body made it to the middle.
we are now the chamber 3/4ths of the way back and the dose has dropped to 0.195 R which is a decrease of almost 92% of the entrance dose.
now has the ion chamber below the “phantom” so it is now measuring the exit dose which was 0.051 R or a decrease of 97.6% of the entrance dose.
doing what a grid is supposed to do and absorbing scattered radiation. The dose readout now was 0.0075 R or a decrease of 99.6% of the original entrance dose. Or in other words, .4% (that’s less than ½ of 1%) of the original exposure actually made it out the back side of the patient and through the grid to the Image Receptor (IR).
What this means is that when using a grid, 99+% of the original x-ray dose that the patient is exposed to is not useful in creating your image as it is either absorbed or reflected. This should be a great reminder to use the least amount of radiation possible, as most of what is coming out of the tube never makes it to your IR and it doesn’t matter whether you’re using film, CR or DR, it’s all the same.