#9 How Much Off-Focus Radiation Comes Out of the
Tube? 5 /1/13
Today's
blog is about off-focus (stem) radiation which until a couple of weeks ago I
had completely forgotten about. As a
reminder to all of you who haven't cracked open your physics books in a while;
off-focus radiation are x-rays produced by stray
electrons that interact at positions on the anode at points other than the
focal spot and are emitted at angles not
in the primary beam. This turns out to be a truly amazing subject
when it comes to patient safety and shielding.
So much so that I have also put it in my website as the newest Current
Research so you could see the photos, images and doses.
Last month my student Ian asked me if one
should shield a patient having a PA chest in the front or the back. I told him we had that same discussion in my
class 37 years ago and that I still had no idea what was the correct
answer. We had figured that shielding in
the back really didn't do much as most of the scatter occurred once the
radiation hit the body or happened inside the patient. Shielding in the front seemed like it might
be more beneficial because maybe there was a certain amount of the exit beam
that hit the Bucky and bounced off back at the patient.
I decided to test this out using an 8x10
CR cassette that I would run at 1200 speed, making it extremely sensitive to
any radiation. In an upcoming Current
Research I will show you exactly what we discovered when we put this age old
question of front or back shielding to the test. One thing that was extremely obvious right
off the bat was how much radiation hit the cassette when placed on the back
side (facing the tube). I had hung the
cassette lengthwise with paper clips taped on and every clip showed up on the
image. What made this so incredible was
that none of the cassette was in the
primary beam.
So where was all this radiation coming
from? We thought it might be radiation
that had scattered from the molecules in the air, but that seemed
unbelievable. After speaking with my
colleague Quinn Carroll (who recently wrote the physics book "Radiography
in the Digital Age") he told that this kind of scatter would be physically
impossible. We then thought it might be
leakage radiation coming through the collimators. After showing my research to an amazing group
of physicists I am in contact with, they determined that it is off-focus radiation.
I now changed the experiment to have as
few variables in it as possible. I
wanted to use my dosimeter so that I could have exact readings as to how much
dose was being emitted that was not in the primary beam and I already had ideas
on writing this up for a peer reviewed article for the ASRT Journal. I used
a large conference room and our AMX portable machine so that I could hang my ion
chamber with nothing remotely near it to cause back or side scatter. I then made close to 400 exposures (for a
peer reviewed experiment all "projections" need to be done 5 times
and then the average is taken from that).
I collimated the beam to 14x17 at a
72"SID with the bottom of the light field just above the ion chamber. I used 2 average chest x-ray techniques: 85
kV @ 3.2 mAs and 115 kV @ 4 mAs. Then I
moved the tube 1" higher and made those 10 exposures again and continued
an inch at a time. With the 85 kV
technique I needed to go 14" higher before the readouts were not accurate
anymore. With the 115 kV technique I was
still getting precise readouts at 28" above the ion chamber but the tube
could not go any higher. I also did the
experiment at a 40" SID using 85 kV @ 16 mAs to show how this would all
pan out for an average abdomen technique.
What has
been proven so far is there is definite radiation below the collimated light
field, more so on bigger techniques.
Although the dose is in the MicroR's (1/1000ths of a milliroentgen) one needs
to be aware that there is a noticeable dose hitting the body outside of the
primary beam. My hospital has a 100%
shielding policy, but if yours doesn't then you should definitely be aware of
how much extra radiation is getting to your patient.
Please go to the Current Research to see
what all this looks like.
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