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