Answer to Question #11764 Submitted to "Ask the Experts"

I went to the computed tomography (CT) room of an emergency room with my relative last month because she was in a critical situation. The CT technician requested my help. There are two doors in CT rooms, one is for the patients to come in, and the other one is for the technician going into the CT room from the room with the operator's window. When my relative was having the CT, the technician asked me to stand outside the CT room and stay behind the wall on the same side as the operator's window, separated by the door. In other words, he sat behind the operator's window and the door for going into the CT room separated me and him. I am wondering whether the wall I was standing against has shielding for my protection? Also, he kept the door, which is used to get into the CT room and also to separate him from me, open during the operation. Was that safe?

From your description of the situation, it appears that you were protected from the CT scan radiation. Most, if not all, state agencies require that a shielding evaluation be performed around all rooms where x rays are generated, which would include CT rooms. Such evaluations include all doors, walls, and windows immediately around the CT room including the room where the operator is positioned during the CT scan. The amount of shielding in any given "barrier" (i.e., door, wall, window, etc.) is based upon a number of factors including who resides in that area and how often individuals are present in the area.

From your description of where you were standing during this CT scan, it sounds as though you were in the CT control room which would have been shielded to protect the operator. As such, you would have been protected as well. Such areas are typically shielded in such a way as to prevent an operator from receiving more than 5 millisieverts (mSv) in one year (NCRP 2005), based upon a conservative estimate of the number of CT scans performed in any one year. To put that number in perspective, the average member of the U.S. population receives slightly more than 3 mSv per year from "natural background" (e.g., radon, space radiation, radiation from soil and rocks, etc.) (NCRP 2009). Thus, standing in the control room during a single CT scan would subject you to some small fraction of a mSv, much less than you would receive from natural background in a year.

You mentioned the door to the CT control room was open during the CT scan. The only reason that could present a problem would be if someone were to stand in the doorway for a significant number of CT scans—hopefully the operator would not allow anyone to do that. Thus, it appears that the situation you described was safe.

Mack L. Richard, MS, CHP
Director of Health Physics/Radiation Safety Officer
IUPUI/Indiana University Medical Center

References
National Council on Radiation Protection and Measurements. Structural shielding design for medical x-ray imaging facilities. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 147; 2005.

National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 160; 2009.