Answer to Question #14746 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment and Shielding — Shielding

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

Q

I work in a dental practice where there is a cone-beam computed tomography (CT) scanner in a small room. The operator stands just 1 meter or so from this scanner in a separate space outside of this room, separated by a door (normal door without lead) and a wall. The exposure switch is fixed to this wall.

Here are my concerns:

  1. I am concerned that the distance between the cone-beam CT scanner and the operator is too short.  
  2. I am unclear whether this door/wall blocks any of the radiation.  
  3. Is there any radiation remaining in the room after the exposure—i.e, is it safe for the operator to walk into the room with the scanner immediately after CT scan is taken? 

I would be grateful for your answers.

A

It is common practice to evaluate all fixed x-ray units for shielding prior to installation and it is likely required for your facility. A number of items are taken into account when preparing a shielding plan, including location of the unit, use and occupancy of adjacent areas and distance to these areas, x-ray workload of the unit, annual dose limits, and others.

If a shielding plan was prepared for this room and the room was constructed to meet the recommendations of the shielding plan, then the annual radiation dose to persons in adjacent areas should be below required levels. Note that I did not say zero. However, levels averaged over an entire year would be acceptable in accordance with regulations.

Here are the answers to your questions:

  1. If the operator to unit distance is short, then the wall design would have relatively more shielding than if the distance were larger. A lower workload would also be sufficient in some cases. 
  2. If this room was constructed as designed, the materials present in the wall (drywall, leaded drywall, leaded glass, etc.) would provide adequate protection to the operator. 
  3. Once an x-ray exposure is over (when the button is released), there is no more radiation present until another exposure is made. Think of it as turning a light switch on and off.

In addition to the above, your x-ray facility is likely regulated and inspected by a local government authority (in the United States the individual states regulate use of x ray), and your facility should have utilized a qualified radiation expert to prepare the shielding plan and possibly a post-installation survey. The results of a survey and/or radiation dosimeters that may be worn by the operators would demonstrate that radiation doses are below regulatory limits.

Kennith "Duke" Lovins, CHP

Answer posted on 30 November 2022. 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.