Doses – Entrance, Midline and Exit

Doses – Entrance, Midline and Exit

Today’s blog will be a shortened version of the experiment I posted in the Significant Experiments section last week which has the full write up.  This will be an overview only as I would love to have you go to the Sig. Exp. and see all of the photos (figures 2-5) along with the doses.   I know it will be worth your while!

I’ve always wondered how much dose occurs in the middle of the body (which is called the Midline dose).  My FLUKE dosimeter and 150cc ion chamber can read out the entrance and exit doses but I couldn’t figure out a way to get the Midline dose, which is where I figured the most internal damage would possibly happen.

As you can see in figure 1, 1
I created my own phantom using four 1” thick sheets of polyethylene and 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 attenuates and scatters radiation.  The four layers measure 12” or just about 30 cm.  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.

As I go through the next set of exposures, moving the ion chamber ¼, ½, ¾ and getting the exit dose, the dose drops dramatically (to see all of these you’ll need to go to the Significant Experiments).  Then we really get to what I could barely believe as I was doing this experiment.  The dose readout with the ion chamber in the Bucky (below the grid), registered only .4% or less than ½ of 1%.

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 and does not made it out the back side of the patient and through the grid to the Image Receptor (IR).  This hopefully will be a great reminder to use the least amount of radiation possible (How Low Can You Go?).  Remember that this doesn’t matter whether you’re using film, CR or DR.

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