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Answer to Question #8440 Submitted to "Ask the Experts"

Category: Security Screening — Airport Screening

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

Q

Please explain exactly how the new body scanners (x ray) work.

An article in our paper stated that low-level x rays "penetrate clothing but not skin" and use the "backscatter" to record the data. It just does not sound right or I have forgotten all my x-ray physics.

Don't most low-level x rays get absorbed by the body or clothing? I would imagine that you would have to have quite a bit of low-level energy to get enough backscatter to record anything.

I'm used to thinking of higher-level energies and the issues with the lower levels used in mammography. A good explanation of how and what intensity/quality x rays are used in these scanners and how the image is recorded would be helpful.

I work in a catheterization laboratory as a registered technologist in radiography so I am sure my annual work exposure would make walking through one of these scanners inconsequential. Still, I would rather not add to my total.

A

Thank you for your question. You are right, there is some radiation exposure absorbed by the body during these scans—about 0.05 µSv (microsievert, µSv, is a unit of effective radiation dose; daily background radiation is about 0.01 millisievert).

The idea behind the backscatter units is the energy spectrum of the scattered radiation as it is detected when the x rays scatter off of various substances including your body. Things containing organics (carbon, hydrogen), like drugs and explosives, scatter the x rays substantially while heavier elements will absorb the x rays more. Much like you see on the x rays you take—bone (more dense) absorbs more radiation while lung tissue (mostly air) scatters or allows the x rays to pass through.

The idea is pretty interesting, but the idea that these x rays "penetrate clothing and not skin" is misleading. While there isn't much absorbed radiation in the body, there is some. The airplane flight, though, will expose you to more radiation than this backscatter system.

Kelly Classic
Certified Medical Health Physicist

Answer posted on 13 August 2009. 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.
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