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Some notes on x-ray backscatter imaging

Update 2012-03-11: I've also commented on Jonathon Corbett's demonstration of the ineffectiveness of these crude devices.


Hilary sometimes complains that the overall look of this site is that of a scientist's, rather than a poet's. It's a fair cop. I'm a scientist. Here's some science.

There's a lot of stuff in the news lately about the deployment of x-ray backscatter imagers in American airports. To be honest, being seen naked doesn't bother me, and I find the whole "nudity taboo" thing is one of the weirder aspects of human behaviour. But unlike the organs of the security-industrial complex, I respect people's right to their neuroses, particularly as I bloody well expect them to respect mine.

That said, how safe are these machines? And how reasonable are the standards that are being applied to them?

This open letter from a group of scientists raising concerns regarding the safety of these devices and questioning the appropriateness of the testing that has been done on them to date is entirely to the point, in my view as a radiation transport physicist who is intimately familiar with the processes by which x-rays deposit energy in tissue.

Ask any radiation oncologist what the big problems in treating cancer with radiation are, and at some point they will say the words, "skin dose". The reasons why skin dose is a problem for the kind of high-energy x-rays used in cancer treatment are a little different from the problem of skin dose with low energy x-rays used in back-scatter imagers, but the problem remains.

They say a picture is worth a thousand words, and I usually leave the art to Hilary for what are about to become obvious reasons, but in this case I've given it a shot:


In the picture we've got x-rays incident on the skin from the left. The skin is the thin layer between the two black verticals. To the right of skin is the rest of the body, labelled "meat", because that's what we're made out of. Superimposed on this cartoon of the body is a graph showing x-ray energy deposited as a function of depth, which is roughly exponential. As the blue curve was hand-drawn, perhaps that should be "very roughly".

The blue horizontal lines give the average dose over the whole curve, and the average dose to the thin layer of skin that is up-close-and-personal with the incoming x-ray beam.

The difference between the average dose--which is what has been measured--and the skin dose--which is what people without a pecuniary interest in the matter are concerned with--I've labelled with "SC", for "Skin Cancer".

I said above that I didn't much care about being seen naked, but I appreciate other people's right to differ on that question.

What I do not appreciate is being exposed to an unquantified risk of skin cancer from a potentially very high skin dose that has not been properly measured or accounted for in the standards-based radiation safety testing of these machines, which is not well-suited for systems that are designed primarily to give high surface doses.

It isn't rocket science to do the job of testing these things properly. I have a good deal of expertise in the niceties of radiation transport in thin-layered systems, and there are some nuances, but given the millions someone is making on these things there ought to be plenty of money available to ensure their basic safety.

As it stands, these machines are a pure act of state-sponsored violence against anyone who is misled into one. I would certainly not allow myself to be scanned by one of these machines, nor allow one of my minor children to be scanned by one.

Millimeter wave imagers have the same issues with privacy and nudity and prudity that x-ray backscatter imagers do, but almost certainly don't pose any major safety issues, although massive public exposure is a hell of a way to perform the experiment.

Based on a back of the envelope calculation with a 50 KVp Bremsstrahlung spectrum and the standard mass absorption coefficients for soft tissue, X-ray backscatter machines will, if widely deployed, almost certainly kill far more people than the terrorists they are supposed to protect us from.

So that's a little bit of science. Now back to poetry.

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