Answer to Question #3538 Submitted to "Ask the Experts"

I work at an airport on passenger hand-luggage screening using a modern x-ray screening machine. Our system of screening requires a member of staff to hand load luggage on to the x-ray machine belt for screening and another member of staff to take off the bag from the belt after the bag has passed through the x-ray machine at the other end. Due to time and passenger pressures, the airport security officer has set the machine belt to run continuously while screening instead of it stopping while making each image. I have noticed that the lead curtain is sometimes raised by a bag that is entering the machine while the x-ray warning light is lit as it screens a bag that is already in the machine. As we stand at the mouth of the x-ray machine, loading and removing bags during these occurrences, do you think there could be a risk from scattered radiation?

The bottom-line answer to your question is that, while there may be some scatter of the x rays from the luggage being scanned, it would be small and not a significant risk for you. That said, I still have a couple of other thoughts, one of which has nothing to do with radiation but is more of the security issue—are we seeing what we need to see if bags are going through that quickly and not stopped while being x rayed? That is not something I can address but it is an issue if we're using radiation, and possibly exposing a person, for something that is of little value.

My other initial thought was that I'm not sure what you mean by a "modern x-ray screening machine." There are a couple different types of x-ray screening devices used. One uses a standard x-ray tube (fluoroscopy—which is "live time" x ray) technology and one that uses computerized tomography (CT) technology.

Let me relate those technologies to your question about scatter. While the luggage gets more radiation exposure from a machine that uses CT technology, the person standing by the portals loading or unloading would get very little, if any, scattered radiation exposure. CT technology uses a very thin beam of x rays. That, combined with the fact that the beam is right in the middle of the big machine, makes it that much more difficult for scatter to exit the unit. The InVision Technologies machine, with which I am more familiar, has a loading port and an unloading port that are roughly six to seven feet from where the luggage is actually x rayed. There is very little chance for scatter to exit the machine.

The other type of scanner, referred to as a parcel scanner, is the type most of us are familiar with when we take our carry-ons through security. They actually are built to run continuously but with the parcels separated so only one is entering at a time. One of the manufacturers quotes the exposure to a piece of luggage at 0.1 mR (milliroentgen—a unit of radiation exposure). That's about one-tenth of the amount of daily background radiation exposure we receive from naturally occurring radioactive materials in our living environment. If that is the case, it is unlikely that any amount of scatter radiation would be measurable.

Bottom line—You're probably not receiving any radiation dose from being next to these units but they are intended to have the x-ray beam on only when the lead drape or curtain is closing the portal.

Kelly Classic
Certified Medical Physicist