Answer to Question #14425 Submitted to "Ask the Experts"

I am a nurse working in a diagnostic department that performs computed tomography (CT) scans. Actually, the room that I am in all day is the controller's room. I noticed that the door of the CT room has a gap of 3 to 4 cm. Maybe this is a mistake from when the shielded door was put there. Is it possible for radiation to scatter through the gap? Also, today a paper dropped in front of the door and I bent down to take it, so my head was totally in front of the gap increasing my worries about radiation on me.

As you pointed out in your question, there is certainly some scattering associated with all types of x-ray devices, including CT scanners. This scattering and the amount of radiation exposure from x-ray scatter is dependent upon a number of factors. Those factors include the machine settings (commonly referred to as technique factors), the distance from the machine and patient to the point of interest (i.e., the distance to the bottom of the control room door), and the dimensions of the x-ray field. The last two parameters (distance and field size) really work to your advantage in terms of what the intensity of the scattered radiation might be under the control room door. Given those factors, the amount of radiation scattered under the door is likely to be quite small. Let's look at why that is likely to be the case.

Typically, control room doors are several feet away from the machine and patient, simply because there must be plenty of room for patients and staff to enter the room and navigate around the CT unit. As one moves away from the patient (the primary source of x-ray scatter), the intensity drops off by a factor of about 4 for every "doubling" of the distance—this is referred to as the "inverse square law." For example, if the radiation exposure at 1 meter from the isocenter of the CT scanner (where the CT beam enters the patient) is 50 microgray (uGy), at 2 meters away, the radiation exposure would be 50 divided by 4 or about 12.5 uGy. At 3 meters away, the radiation exposure would be 50 divided by 9 or about 5.6 uGy. Note that the 50 uGy value is simply for illustrative purposes. The actual radiation exposure will vary considerably, depending upon the other factors mentioned.

The x-ray field size from a CT scanner is actually quite small. CT scanners project a very narrow beam of radiation on the patient (only a few millimeters wide) which corresponds to a very small field size where that beam enters the patient. The amount of scattered radiation increases with field size, so when compared to other types of x-ray devices (e.g., radiographic and/or fluoroscopy units), the field size and scattered radiation is pretty small from CT units.

Another factor that would affect the scatter to the control room door would be the position of the CT scanner in the room. You will note that the CT scanner gantry is quite large and as such, the gantry itself provides considerable shielding from the scattered radiation from the isocenter. The only areas where any significant radiation scatter occurs would be at the "head" and "foot" end of the patient table. The gantry provides considerable shielding at 90 degrees in all directions from the table and increases as that angle changes.

You mentioned that your head was near the gap beneath the door as you picked up a dropped paper. As indicated above, the scattered radiation under the door is likely to be very small, so having your head in that position would not pose any significant hazard to you.

In summary, while the gap under the control room door might seem to allow scattered radiation from CT scans to enter the control room, for the reasons stated above, that amount of exposure is not likely to be significant. Typically, CT scanners are evaluated by a medical physicist or health physicist on an annual basis. You could ask that physicist to take some scatter measurements under the door, just to see what, if any exposures are measurable in that location.

Mack L. Richard, MS, CHP