How much scatter is created from an AP chest x-ray?

Recently I attended a lecture where an educator’s student had held a CR cassette in front of his chest (he was wearing a full lead apron) while he made an exposure during a PCXR. Although he was a good 10 feet from the patient, the bones in his hand were clearly visible when the cassette was processed. This is what that image looked like!!

hand

We decided to take that concept and turn it into a full blown researched project. Using our abdomen, thorax and head/neck phantoms, we were able to create a good facsimile of an upright patient in a gurney.

We then performed the experiment a couple of different methods. One way used our Fluke 150 cc ion chamber and dosimeter where we measured the scatter from 1 foot from the patient out until the reading showed zero.

The other experiment used our hand phantom and an Agfa cassette which was subsequently run at 1200 speed (the usual
being 400 speed).

For all of experiments we put either a plain CR cassette behind the thorax or a gridded CR cassette. For the plain cassette we always used 85kV @ 3.2 mAs and with the gridded cassette we used 117 kV @ 4 mAs.

We tested the scatter coming from the patient 2 different ways. We began with the ion chamber in 3 diverse positions, but at many different distances. The first position we called 0 degrees and it was directly in front of the patient (scattering straight back at the tube x-ray source).

Next we put the chamber at 45 degrees to the patient

and ended with it at 90 degrees to the patient.

We always started with the ion chamber 1 foot from the patient and worked our way back 1 foot at a time. We did this until the reading on the dosimeter gave a zero reading.

So we have those two sets of charts as we did the experiment with the two different techniques. All doses were measured in uR’s which are also called MicroR’s. As a quick reminder, a MilliR (mR) is 1/1000ths of a Rad (R) and a MicroR (or UR) is 1/1000ths of a MR.

As you can see from the charts there is obviously more scatter coming from a patient with the higher technique being used. Also if the radiographer is closer than 6 feet, the dose is quite considerable. Also you will notice that the worst scatter occurs at 0 degrees and gets less at 45 degrees and much less at 90 degrees.

You can see from the chart below that the average person gets 320 mrem (milliR’s) per year. Below the chart I did the math to figure out how much radiation that equals per day and it is 844 microR (UR).

So this brings us to the second part of this experiment, which was to see what the scatter that comes off of a patient can do on a CR cassette. Following are 12 images of the hand phantom. Each one was annotated at the top so you will know the following information:

1- Was it 85kV @ 3.2 mAs or 117 kV @ 4 mAs?

2- Was the hand phantom at 0, 45 or 90 degrees?

3- Was the tube at 6 or 12 feet?

So here are our results. As you can plainly see, even at 12 feet there is enough radiation scattering from your patient to penetrate your hand enough to show the bones.

We have set up this next section so that the photo shows whether we ere at 6 or 12 feet, and if the phantom is at 0, 45 or 90 degrees. The information is also annotated on each image.

 


 


 


 


 


 


What we didn’t understand at first was the ion chamber (and this is a $10,000.00 dosimeter) was far less sensitive than our 1200 speed CR cassette. This is why we were still getting images of the hand at distances where our dosimeter was reading zero. We spoke in depth to our physicist who said we would need an ion chamber the size of a 5 gallon jug to be sensitive enough to pick up 5-10 uR’s and that my 150cc ion chamber is incapable of such sensitivity. As an important aside, your badge monitor is also incapable of picking up such minute radiation, so be aware that you can be getting radiated daily with small enough amounts that it doesn’t show up on your monthly badge reports.

What we concluded from this research was that although the dose is small, it is not insignificant. If you go out and shoot 5 PCXR’s in a day, you could already double the dose you will naturally get in a given day. As a radiographer is potentially around radiation all day long, this means one should always be wearing a full lead apron with a thyroid shield, and standing behind the tower of the portable machine when doing mobile x-rays.

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2 Comments to “How much scatter is created from an AP chest x-ray?”

  1. Hi,
    Love the articles and experiments – I get them on linkedin too.
    I have a question. You say daily background is 844 microroengtens of dose
    per day. In another article you wrote 85. Also, when I use this converter http://www.convert-me.com/en/convert/radiation/ using 320 millirem as the annual background natural dose, I get 1008 microroengtens a day. That’s not too far off from 844 and I’m sure the ’85’ was a mistake.
    Thanks for clearing that up. I was shocked and dismayed by what I read in your ‘Off focus” experiment a year or two ago and by the ‘AP chest scatter radiation’ experiment. I’ve taken hundreds of portable AP CXRs, usually with no lead, and only 6-10 ft away. How wrong was that? I needed to be at least 14 ft away if not more. Or of course realize I must WEAR LEAD ALL THE TIME!
    Thank you and I wish I knew this 8 years ago!
    ~joe

    • Hi Joe,
      Sorry I didn’t reply sooner, but I just saw your note this morning. Weird. Anyway, In my talks I write that the annual background natural dose is 302 millirem, not 320. What article did I write 85? I’d love to go back and see if I could change that because for sure that’s a typo I didn’t catch.
      All the Best,
      Dennis

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