Answer to Question #7659 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

I just found out that the wall in the control room facing the CT (computerized tomography) scanner is not lead lined. Is this safe? I put a cassette outside the room and left it there for five hours and the film was exposed, so I'm assuming there is scatter reaching that wall. How can I find out if there is a safety issue in the department? Management says it's safe. What is considered safe in a CT department?

Regulations for x-ray shielding vary from state to state. Typically, the wall between a fixed x-ray unit and the control panel will need to be shielded, especially in CT. Shielding plans are performed by a qualified radiation expert (medical or health physicist) and take into account the workload of the x-ray unit, distance to the walls, and occupancy factors of surrounding areas. Due to the proximity to the scanner and because of the higher radiation output of a CT (compared to other diagnostic x-ray equipment), it is typical to have lead shielding required in the control-area wall.

Also, it is important to understand that even when a wall has shielding added (such as lead), there are still x rays that pass through that barrier. It is essentially impossible to stop every single x ray that strikes the wall. This is acceptable, however, because as mentioned above we take into account the amount of radiation that penetrates the barrier and perform a calculation based on workload and occupancy. So, it is possible to have enough x rays penetrate a shielded barrier to fog film or a digital x-ray plate, but still be acceptable to personnel outside that barrier (whether they are occupational radiation workers or members of the general public).

Without knowing how you determined that there is no lead in the CT control wall and not knowing what the shielding requirements are in your state, I can't determine what is considered "safe" for your application. Shielding is always designed to keep radiation dose to workers and the public below set limits. You can ask the medical physicist/health physicist at your facility about the shielding that may or may not be in the room you are concerned about. Also, I would watch the results of your dosimetry reports. If you do CT full time and your results are well below 10 percent of the annual dose limits, it is likely that your control wall is shielded.

Ken "Duke" Lovins, CHP

Answer posted on 3 July 2008. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.