Answer to Question #9456 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

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

Q
I work in a busy emergency department. There have been recent questions regarding exposure of staff (we wear dosimeters) despite what we have been taught. Is 1.8 meters the proper distance from the source for us as well as for patients and families in adjacent rooms, knowing there are no lead barriers except for large trauma bays? I have read everything I can find related to this on your Web site and the Centers for Disease Control and Prevention Web site and still have these questions: (1) Is the radiation dose with proper distancing accurately measured using a dosimeter? (2) Knowing and understanding exposure values for certain radiographic studies conducted in the emergency department, is there a significant exposure or risk to those in an adjacent room who are not protected by a lead barrier in the wall (two-layer gypsum wallboard and no insulation)? Most common studies are: one-view chest x ray, one-view pelvis x ray, extremity three-to four-view films, and use of a c-arm fluoroscopic unit for fracture reduction. Thank you for your wonderful Web site and for considering this question.
A
The usual "rule of thumb" for personnel who are within 1.8 meters of the primary x-ray beam, is that they should wear leaded aprons when a portable x ray is taken and step back from the patient, if possible, as the exposure occurs. Radiation levels at distances of 1.8 meters or more are usually very low and do not require additional shielding. 

This applies to off-axis locations where the personnel are located outside of the primary x-ray beam. For radiographic exposures like chest x rays, pelvis or extremity views, you can see where the primary beam will be directed by observing the light field that the x-ray technologist will use to aim the x-ray tube just before taking an exposure. For the c-arm fluoroscope, the primary beam is located between the x-ray tube port and the image receptor on the other side of the c-arm. Before taking an exposure, the x-ray technologist should be sure that no one is located in the primary beam, except of course, the patient.

Permanent x-ray facilities are required to be shielded because they are used multiple times daily for x-ray purposes and the workload is large enough that shielding in the rooms is usually required by state codes.  The workloads for emergency rooms where only "portable" or "mobile" x rays are taken, are usually small enough so that wall shielding is not required.

For example, the skin entrance dose to a patient from a portable chest x ray is about 0.15 millisieverts and the exit dose entering the image receptor is about 0.02 millisieverts per exposure. At 0.9 meters, off of the primary x-ray beam axis, the dose to a person would be about 8E-7 millisieverts per exposure. The off-axis dose for an extremity (hand, foot, ankle, etc.) x ray would be similarly small. Even for a pelvic x ray, where the skin entrance dose to the patient may be as high as 10 millisieverts, the off-axis dose at 0.9 meters would be approximately 4E-5 millisieverts. With a mini c-arm fluoroscope, commonly used in the emergency room to perform extremity fracture reductions, the off-axis dose rate at 0.9 meters would be approximately 2E-6 millisieverts per minute of fluoroscopy time. Due to the inverse square law, at 1.8 meters the dose would be one-fourth of the dose at 0.9 meters.

To put this into perspective, natural background sources, such as cosmic, terrestrial, and natural internal sources of radiation cause a dose of approximately 0.01 millisieverts per day. So the off-axis dose, from an occasional procedure, is a small fraction of the daily dose received naturally by everyone. Your dosimeter is certified through the National institute of Standards and Technology (NIST), National Voluntary Laboratory Accreditation Program (NAVLAP) and sums up all of the doses that you receive occupationally, as long as you wear and return it properly. Ask your radiation safety officer (RSO) about your occupational exposure records. I would be surprised, if they were more than one-third (1 millisievert), of your annual background exposure.

Mike Bohan
Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 17 December 2010. 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.