How Much Off-Focus Radiation Comes Out of the Tube?

How Much Off-Focus Radiation Comes Out of the Tube?

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 8×10 CR cassette that I would run at 1200 speed, making it extremely sensitive to any radiation.  In an upcoming Important Experiments 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 14×17 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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