This experiment is part 3 in my series on lead aprons and will show that the average lead apron we use to protect ourselves and our patient’s are supposed to protect against scatter radiation only. You will quickly see why these aprons should never be put in a primary beam.
As you can see in image 1, I put our Canon tethered detector on the floor so I would be able to use 40” and 72” SID’s. In image 2 I’ve opened up an 8”x8”collimated lightfield centered to the detector. I covered half the field with the .5mm apron. In the primary beam I placed my left marker and under the apron I put my right marker. Then I kept adding .5mm aprons until I couldn’t see the right marker anymore. You will be able to see how many aprons are stacked by the little arrows above the left marker.
I used three different techniques: 85 kV @ 16 mAs (AP hip), 113 kV @ 4 mAs (gridded chest) and 85 kV @ 3.2 mAs (non gridded chest). When I use this experiment in my PowerPoint presentations I have 14 slides total. As that would be overwhelming here, I will cut it to 2 for each technique.
Image 3 was taken at 40” SID, 85 kV @ 16 mAs and 1 apron. As you can see the left marker in the primary beam is almost burned out and the right marker, which is under the 1 apron, is seen perfectly. The next image (4) is the same SID and technique, but now has 4 of the .5mm lead aprons stacked on top of each other. The right marker is now barely seen and in my presentations the stack of 5 aprons would show that the right marker is not seen at all. This means to completely block primary beam at this technique one would need to use 5 lead aprons on top of each other.
Now we will move to a 72” SID and chest techniques. These next two were 113 kV @ 4 mAs. Image 5 has one apron and image 6 has 4 aprons and the right marker is just barely visible. This again means to completely block primary beam at this technique one would need to use 5 lead aprons on top of each other.
The final two images used 85 kV @ 3.2 mAs. Image 7 has one apron and image 8 has 3 aprons and the right marker is just barely visible. Because this technique uses noticeably less kV and mAs, 1 less apron was needed here. To completely block primary beam at this technique one would need to use 4 lead aprons on top of each other.
In summary, lead aprons are not designed to be used in a primary beam. Two exams where one sees a shield used in the primary beam are frogleg hips and pediatric chests. If a shield does end up in the beam most likely it will only soften the beam a bit but as you saw, most of it will still penetrate through the shield and radiate the patient.