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

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 72 inches 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 website and the Centers for Disease Control and Prevention website 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 website and for considering this question.

A

The usual "rule of thumb" for personnel who are within six feet (72 inches) 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 six feet 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 15 millirem (mrem) and the exit dose entering the image receptor is about 2 mrem per exposure. At three feet, off of the primary x-ray beam axis, the dose to a person would be about 0.00008 mrem 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 1,000 mrem, the off-axis dose at three feet would be approximately 0.004 mrem. With a mini c-arm fluoroscope, commonly used in the emergency room to perform extremity fracture reductions, the off-axis dose rate at three feet would be approximately 0.0002 mrem per minute of fluoroscopy time. Due to the inverse square law, at six feet the dose would be one-fourth of the dose at three feet.

To put this into perspective, natural background sources, such as cosmic, terrestrial, and natural internal sources of radiation cause a dose of approximately 1 mrem 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 (100 mrem), of your annual background exposure.

Mike Bohan