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X-Ray Backscatter Imaging Safety From Basic Principles

First things first: I'm not a physicist, nor do I play one on TV, nor am I a medical doctor. However, I do happen to have a PhD that was involved pretty heavily with radiation effects on electronics, specifically figuring out how to build circuits that can survive the high radiation environment of deep space. Therefore, the recent furore about the TSA's insistence on herding people through X-ray backscatter imagers worried me significantly -- though the TSA was claiming that these machines were safe, it just didn't ring true on a few levels, so I decided to do a bit of digging of my own. Since I don't want to confuse issues of privacy with radiation safety, I will talk here only about the latter (though, trust me, I have plenty of opinions on the former, too).

Firstly, I'll say one thing: there is no such thing as a truly safe radiation dose. At a fundamental level, the main effect we need to be concerned with is high energy particles smashing into a piece of DNA and doing just the right amount of damage to cause the cell concerned to start doing something it shouldn't. If we get lucky, the DNA molecule is damaged enough that it's no longer viable, making the cell concerned no longer viable. No big deal, we've all got plenty of cells. However, if the particle impact has the effect of changing the information stored in the DNA, things can get weird. The cell can start reproducing wildly, carring its radiation-affected DNA programming with it and thereby causing all its children to go similarly bananas. Hey presto, cancer, leukaemia.

So what is a radiation dose anyway?

In effect, a radiation dose is being exposed to a particular strength of a particular kind of radiation for a particular length of time. The strength of the radiation is basically a measure of how many particles per second are flying at you. The kind of radiation can vary widely, and consequentially the chances that a particle interaction will cause damage also varies. Duration of exposure is pretty self-explanatory. Putting this another way, it's like every single cell of your body all playing Russian roulette, all at once, many million times a second. Though each gun has only one bullet and a (very) large number of empty chambers, it's worth remembering that one hit that gives you cancer or leukaemia is enough to kill you. Just one. Even though most interactions won't kill you, it only takes one good one to finish you off. It is also worth remembering that the hit that kills you will not be apparent for weeks, months or years -- this is why it can be so difficult to prove a causal link between teratogenic effects and cancer or leukaemia.

The next thing that's worth talking about is that there are many kinds of radiation, and they all have wildly different properties, and thus wildly different chances of damaging you. One of the problems here is that the lay intuition here can trip you up. People, particularly in the media, talk about 'high energy' and 'low energy' radiation, with the implication that high energy radiation is more likely to kill you than low energy radiation. Makes sense, yes? Actually, this is hugely misleading. Firstly, remember that the real risks are based on how much, what kind, and for how long. Low and high energy refers only to the 'what kind' part of the equation. Worse, it is typically the case that low energy radiation is actually more dangerous -- potentially MUCH more dangerous -- than high energy radiation.

Like everyone who is wrong side of 40, I grew up during the cold war, when the association between terror and radiation was due to the threat of global thermonuclear war, rather than with X-ray scanners in airports. Though the consequences of a thermonuclear detonation is far worse (duh!) than a trip through a body scanner, there is a useful lesson here about X-rays. Here is a diagram of the classic Teller-Ulam hydrogen bomb design:


(from http://en.wikipedia.org/wiki/Teller%E2%80%93Ulam_design)

In a Teller-Ulam bomb, a fission bomb (the thing at the top of the diagram) detonates with enough energy in its own right to destroy a small city. Though a very rapid blast wave propagates outwards from the detonation, a huge pulse of soft (low energy) X-rays moving out at the speed of light hugely outruns the blast front, causing the fusion bomb (the thing at the bottom of the diagram) to implode, fuse, and thereby create a far larger secondary explosion. But, remember: soft X-rays. Low energy X-rays, yet they comprise arguably about 80% of the energy output of the fission device. Though usually omitted from public sources about H-bombs, including all of the sources I managed to find this morning with a bit of Googling, I remember hearing somewhere, years ago, that there is an extra component between the primary and the secondary that slows down hard X-rays to soft X-rays, because in the form of hard X-rays they would otherwise just go through the secondary without significantly interacting with it. Though, obviously, the radiation level inside a detonating H-bomb is many orders of magnitude greater than what we're dealing with in medical imaging or TSA-style backscatter imaging, the critical take-away point here is that higher energy particles tend to go straight through and out the other side, whereas lower energy particles have a greater tendency to be absorbed. Consequentially, it's just these lower energy particles that are more concerning from the point of view of radiation safety.

Let's start doing some numbers to see what's really going on here.

Rapiscan themselves are a little wooly about detail, so I'm pulling information from an article on the diagnosticimaging.com web site (http://www.diagnosticimaging.com/safety/content/article/113619/1521147). The Rapiscan is given as having an average beam energy of 30keV (bigger numbers mean higher energy), which equates to half the energy being deposited in the first 5cm. As with visible light, intensity within an absorbing material follows the Beer-Lambert law. Working backwards from these numbers gives us the following curve:


In the above graph, the curves represent X-ray intensity over depth within the human body. I'm arbitrarily deciding that we're all 20cm thick, here, just to make things simple. The upper curve represents the effects of the AS&E scanner, the bottom line represents the Rapiscan. The difference here is that the AS&E device uses higher energy X-rays, which penetrate further. Clearly, with the AS&E device, a bit more than half of the radiation goes all the way through the body and out the other side, whereas with the Rapiscan, nearly all of it is absorbed, but (more scarily) much of that is concentrated in the first few centimeters. Though the X-ray energies are lower than a medical X-ray machine, they aren't strictly speaking 'soft' X-ray sources -- actually they class as intermediate, being in the range used for mammography and CT scans.

Let's remind ourselves what the TSA had to say about this on their blog (http://blog.tsa.gov/2009/11/response-to-oops-backscatter-x-ray.html):

The Transportation Security Administration (TSA) has assessed multiple types of AIT systems including X-ray backscatter and millimeter wave. Both offer safe and effective whole body screening for weapons and explosives concealed on a person’s body. Backscatter X-ray technology uses X-rays that penetrate clothing, but not skin, to create an image.

This is clearly false, as demonstrated not only by the physics, but also by the TSA's own published images.

Here's one from a Rapiscan 1000:


Um... I'm sorry, but those things in the legs. You know, between the knees and the feet? I'm thinking that they look awfully like bones to me. In that part of the body, as with the face, the bones are very close to the surface, so they are showing up clearly. I'm pretty sure I'm not kidding myself that I can also see the prefrontal sinus, the brain, and on the other image, the spine. On these images, brightness represents reflected X-rays, so anything appearing dark is a good indication that X-rays have been absorbed. The brain looks nice and dark on there. Here's another one, this time from an AS&E scanner:


Bones, lots of bones there too. Even clearer this time, which is what the graph above would lead me to expect. And what looks suspiciously like lungs, an oesophagus, a stomach, and some other junk.

So what is the actual dose? And what is the risk?

Unfortunately, this is a lot harder to figure out from published sources. The Rapiscan 1000 specfication claims a scan time of 7 seconds, with a total claimed dose of 10 microrem. The same TSA blog post quoted above claims:

... the X - ray dose received from the backscatter system is equivalent to the radiation received in two minutes of airplane flight at altitude (0.003 millirem by backscatter (2 scans) compared to .0552 millirem for two minutes of flight). Newer technologies require less scanning time, reducing individual X - ray exposure to .002 millirem for the entire process.

The Rapiscan specification itself claims 10 microrems (0.010 millirems) per scan, though the TSA blog is claiming 0.003 millirems for two scans, so the Rapiscan is giving six times the dose that is being claimed. AS&E also claim a 10 microrem limit.

According to the NOAA web site (http://www.swpc.noaa.gov/info/RadHaz.html), the dose-equivalent rate is (picking a representative number) 6.78 microsieverts per hour, assuming 35 degrees North and an altitude of 40000 feet, at solar minimum (I'm being a bit generous to the TSA by picking one of the worse options). 6.78 microseiverts per hour equates to 0.678 millirems per hour, so a 5 hour flight (say) gives a total dose of 3.39 millirems, equivalent to being put through the scanner 339 times. In comparison, a chest X-ray (data courtesy http://www.xrayrisk.com/faq.php) gives us a dose of 10 millrem, equivalent to going through the scanner 1000 times. At the other end of the scale, it takes 200-1000 rems (200000 to 1000000 millirems) to cause acute radiation sickness, or a mindboggling number of trips through the machine.

So what is a (milli)rem anyway?

The unit 'rem' is actually an acronym: Roentgen Equivalent Man, and is the result of multiplying the radiation dose in rads (more on those later) with a weighting factor, WR, representing the effectiveness for the radiation in causing biological damage to mammalian cells. The underlying unit, the rad, is defined as the dose causing 100 ergs of energy to be absorbed by 1 gram of matter -- equivalently, it may also be defined as the dose causing 0.01 joule of energy to be absorbed by 1kg of matter. So, the figures should at least in principle be comparable. However, the kinds of radiation are extremely different, so reducing the comparison to a single number is a dangerous oversimplification. Where an X-ray dose is analogous to an extremely large number of relatively low energy particles all at once, a cosmic ray dose is equivalent to a relatively small number of extremely high energy particles far less often. Though there is a considerable amount of variability within the particle energy levels seen, the most extreme examples will commonly penetrate several metres of lead. The more commonly seen particles are less extreme than that, but practically speaking, adding a third line to the graph:


effectively gives you a nearly straight line. Therefore, the dose is effectively distributed evenly throughout the body, whereas the kinds of X-rays used in the scanners have a tendency to concentrate their dose toward the surface, or in the skin. It is also notable that, on attempting to research this, that whilst there is quite a lot of material available on medical X-ray imaging and its health consequences, there is little or nothing to be found that is related to more frequent lower dose exposures, or to the effects of these longer wavelengths over longer periods. Surprisingly, there is also relatively little to be found about cosmic ray exposure effects. Though the rem unit corrects for different kinds of radiation, this is only as accurate as the research on which the weighting factors themselves were based.

So, to sum up, it seems that the TSA don't really understand what they are using. Their X-ray transparent pants are on fire, frankly, with regard to their claims that backscatter X-ray technology does not penetrate skin. What is harder to establish is the health issues. Whilst this little from-basic-principles study doesn't show unequivocally that scanners are definitely a very significant radiation risk, it doesn't show that they are safe, either. Looking at the curves, it really wouldn't surprise me at all to find, several years hence, correlation between repeated exposure and elevated risks of skin and eye cancers, and possibly also brain cancers. I'm not alone in these worries, however -- http://www.whitehouse.gov/sites/default/files/microsites/ostp/ucsf-jph-letter.pdf is an open letter from a group of concerned scientists at UCSF, which is well worth reading, as is the US government's reply (http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/SecuritySystems/ucm231857.htm).

[Note: Feel free to link and/or repost as you see fit]

[ETA: A friend (who really does play a physicist on TV!) pointed out that whilst the radiation levels travelers see (which he points out are actually just 10 times background) are probably going to be fine, those machines have essentially no shielding for their operators, who will get many thousands of times the dose that travelers get just by standing next to the machines all day every day. Whilst I don't approve of the groping, I would not wish to see TSA people dying from cancer in 20 years either. I'm inclined to agree, though the jury is definitely still out.]


erm... i'll take the grope, please.
I wonder what would happen if I went for the grope in a kilt, with, er, what I hear a "True Scotsman" wears under his kilt. Tempting.
Thanks for making the very important does-penetrate-skin point. I already went and fixed my most recent LJ comment saying the XRays didn't penetrate skin - I had seen the same example images you did but had not understood them properly.
The dose-concentrated-in-the-skin point is the most worrying part of the post.

Betcha we hear a lot of reassuring propaganda based on a whole-body exposure limits.

If you take the whole body as a neutral zone for 95% of all ionising radiation, with the remaining 5% consisting of the actively-dividing cells that are involved in almost all malignant cancers - skin cells, the gut, lung tissue and the liver - concentrating the dose in the skin carries a x20 risk multiplier to any safety limit based on whole-body radiation dosage.

Who can put some hard numbers on thse guesstimates?
I'm a little bit in love with you.
Her brains are AMAZING, aren't they??
I had thought they were millimeter wave, which I would not expect to cause problems (being somewhere between IR and radio), but xray is quite another matter, as they penetrate a cm or so, the effective dose is likely to be what, 100x the whole body?
The TSA is very careful not to say what exactly they are using. But both millimeter-wave and backscatter low-energy X-ray systems are in use.
I Am A Radiochemist(TM), which makes me a fair but imperfect approximation of a health physicist.

I have been meaning to do a full analysis of the backscatter X-ray dose question but I haven't gotten around to it (and this isn't it).

But I want to point out that (A) the entire *point* of the rem is to be comparable between different kinds of radiation, which is why it has Wr, and (B) the questions of "is this amount of radiation safe" or "how does this amount of radiation compare to background" are utterly meaningless. The only salient question to my mind, is "does this scan represent a meaningful increase in the radiation dose you have volunteered for by choosing to fly". And while, as I said, I haven't done a full analysis, I would be astounded if the answer were not "no".

I am *fully* opposed to these scanners. But I am fully opposed to them on civil-liberties, privacy, safety-of-minorities, misuse-of-funds, abuse-of-discretion, procurement-unethics, and so forth grounds. The radiation bugaboo is just that and frankly it's a disservice to civilization given that the only way we can possibly hope to have any kind of industrial culture without destroying the planet in very short order is to get the fuck over our terror of all things nuclear.

That said, ifwhen I get around to doing (or find) a full, competent analysis and it shows the answer to that question to be "yes", I will wholeheartedly recant.

(UPDATE: this version corrects a logical transposition in the original.)
Indeed, I totally get it that the rem is intended to allow comparison, but what's not entirely clear is what the weighting factors were based on -- was it cells in a petri dish? Short duration exposure? Long duration exposure? Epidemiological studies? Since that particular spectrum hasn't been used like that on humans before in any scale, it's not possible to be certain that the comparison is valid, particularly since the TSA's main argument compares the backscatter scanner's dose with cosmic ray flux at airliner cruising altitude, which couldn't really be more different. Sure, the rem weighting facter should fix that, but does it? And if it doesn't, a lot of people will get cooked before it's proven.

On the plus side, it seems likely that, even if it's an order of magnitude off, it's probably not significant for occasional travelers, but anyone going through the scanner daily, and (much worse) anyone standing next to the things all day operating them, are going to be at significantly greater risk.

For myself, I'd probably choose the scanner over a grope-down, having done the numbers for myself. I wouldn't have said that this morning, however.
For another analysis, with significantly different conclusions, see this post.
Not that different, really -- most of the same points are made.
I work in medical diagnostic imaging on one of the biggest scariest beasts in the pack, the PET/CT scanner. The radiation sources I work with are putting out near 2GeV during maintenance but that gets brought down to 3MeV once the machines are put back together.

The 2GeV doesn't bother me. I change the scintillator tubes and load the isotopes wearing as little as I can get away with. When the thing is on, and running a test cycle, that's when I like to be behind as much lead as I can get. Gamma rays at 511keV scare the crap out of me.

So, taking as much radiation as I do on the job, I'll just let TSA grab my package. Embarrassing, inhumane, but I just can't take more low energy radiation.
Probably just as well they only use rubber gloves -- I'd hate to see the results of them pointing a Geiger counter at that package of yours, after all that!

(Yes, I know, I know, bad joke.)



Has anyone considered what happens if the beam stops, whilst emitting it's 30 keV at you? Has anyone seen any evidence of protection or interlocks to shut down the emitter if this happens?

Re: Safety?

Good question. The short answer is, no, not that I'm aware of, or at least nothing I've seen actually mentions this at all.

Realistically, the control systems should be implemented to safety critical standards, as used for aircraft flight control systems, e.g. ARINC 653 Part 1 running on something like Green Hills Integrity DO178B, or the VxWorks equivalent, with the software formally verified.

Yeah right, they totally did that, I'm sure.
This is relevant (and broadly in agreement? I don't have much of a physics-comprehending brain at the moment.)
It's referring to the same UCSD open letter that I linked above.
First, thank you for this. You're amazing.

Second, I'm curious what option you will take when you are presented with the scan vs. the pat down?
Thanks. :-)

As for which option, that's a tough question. Given the amount that I fly, I don't think the scanners are a significant risk for me, though I'd avoid them if I was flying every day, and as I've said elsewhere, I think the TSA operators standing next to them are probably the people who should really be concerned.

This post (deliberately) only addressed radiation safety concerns, so I'll be writing more about this later. I've written to the TSA asking for clarification on their policy regards transsexual and transgendered travelers, so I'll wait for a reply before saying much more.
What you need to see is the work of Prof Allison of Oxford University who studied the radiation doses vs death rates of victims from Chernobyl and Hiroshima. It is the most complete understanding of what radiation doses are and are not safe, and by extension how radiation kills people.

Off the top of my head, I think you need to be exposed to more than 200mSieverts of radiation (per month I think) for damage to start happening to your body. Damage follows an S shaped curve, in that increasing the radiation above that doesn't do much. Increasing it further suddenly has a huge effect and increasing it more is always fatal.

He also makes the point that the body has self repair mechanism that repairs radiation damage, which is why the body can take a certain level of radiation and be fine. Radiation is only harmful if the self-repair mechanism is overwhelmed. This is something that I had confirmed in my biophysics course, in which we worked out that the mutation rate should be 1 in 10^5 copies of a single base pair of your DNA using simple thermodynamics, but is actually closer to 1 in 10^10. This is because your cells have nanomachinery that compares the protein sequence in your DNA to the other base in the base pair sequence and makes them match properly. Each cell contains numerous self repair mechanisms that unzip your DNA and repair damage. As a result, your statement about Russian Roulette is down right wrong. Don't take my word for it, consult any undergraduate entry level textbook on biochemistry. It's fascinating stuff.

Allison then further goes on to note that damage is cumulative across many different types of harm. So, for example, he considered the effects of radioactive alpha particle emitting gases from regions rich in naturally occurring ground Uranium, meaning the South West of England. He concluded that, although the gases didn't cause any extra statistically significant deaths compared to the rest of the country, when added with smoking, the radiation did kill people. The radiation by itself was not dangerous, only dangerous when cumulatively added to smoking.

As a further personal note, I will say I've seen the particle tracks of naturally occurring atmospheric muons going through a proper particle detector. It's not pretty. Yet hold out your hand and five are going through your palm per second. And apparently we're still alive... That, more than anything else, brought home to me just how radiation hardened life on Earth is, and by extension human beings.
I bet the resulting theoretical probability to get the fatal cell injury while being scanned is still less than the very real probability to die in car accident while crossing a street
Yes, although if you go there, it's been pointed out that even the most favorable numbers put the machines at more dangerous than terrorists.


Has anyone looked at this from a different angle...

Let me state up-front - I opt out of the scanner in San Diego every week due to concerns of radiation and skin cancer. I have a really bad spot on my scalp from a BAD sunburn 4 years ago that destroyed hair follicles and still oozes if I graze it with a comb. I won't tempt fate.

Now, forgive my ignorance but if the Rapiscan machines are using x-rays to scan, then doesn't that mean they have to have a radiation source inside? You know - the kind that the government was worried about being stolen from hospitals and being used for a dirty bomb. Just how much nuclear material is inside of those machines?

Here's the point: Since the scanners are by definition on the perimeter of the secured area, anyone could walk up to one with anything strapped on their body. What if a suicide bomber walked up, assumed the position, and when the scanner started he detonated his bomb? Not only would he injure/kill a fair amount of people in the long security lines, but wouldn't that be enough to scatter radiation? Granted it would be confined to that gate/checkpoint area and maybe that whole terminal with good HVAC, but that might be enough. The shock/scare factor would be high, and that terminal would be out of commission for quite some time - right?

Re: Has anyone looked at this from a different angle...

No, don't worry about that. X-ray sources are basically analogous to old-style CRT tubes, with a few pieces missing but running at much higher voltages (OK, I know this is an oversimplification, but it's good enough to get the feel of it). Basically, when the power is off, there is no radioactivity whatsoever, so if someone blew up one of the scanners, it wouldn't constitute a dirty bomb, any more than blowing up a random old TV set might.
Whilst I don't approve of the groping, I would not wish to see TSA people dying from cancer in 20 years either.

And I'm sure no one in the administration of the TSA much cares, since the TSA workers are relatively low-skill, majority-POC workers who aren't allowed to unionize, so there's no one available to speak for them.
There are union workers in private screening companies (at KSFO, IIRC) who are required to follow TSA protocols.
Fuck these things so hard. I will never, ever step foot in one.

Luckily I'm a small white female so I probably never will be forced to, either.

I'd much rather get my grope on by some uninterested TSA agent anyway.
I wouldn't be sure of that. I've seen a number of reports of only the small white female types being passed through the scanners, totally randomly of course.
thank you for this.

so many people think I'm alarmist for being wary of these scans. and while originally my problem with the machines was on a privacy/principle of the matter level, I've recently learned that I'm expecting my first child and there is no way I'm taking that kind of risk. Especially after learning that the IAEA and WHO (among others) want further study to the point that they advise children and pregnant women be excluded. So the fact that the FDA gave it's blessings is great and all, but for me the jury is still out and that's not dice I'm willing to roll.

here are some articles I found, in case you had not already seen them:
Am an xray tech for 30+ years and was in surgery for a month. This month the radiation safety person came to the xray dept and shreaked "Lisa,What's going on?!",when she got my radiation readings. I received a pretty high dose for a tech.I had an abdominal CAT Scan this month.(It was for horrible pain in my diaphragm-gall bladder region and an Ultrasound was negative. IT WAS SHINGLES!) And when coming home from Hawaii in June they had me walk thru the xray scanner 3x! I kept telling them it was the metal "Bling" on my shirt.They finally frisked me...I didn't know this was an option! Before leaving for Hawaii,I had joked with my tech. friends whether I should sneak my film badge(Reads the radiation) on my person to find out what the real "zap" is, on a trip from Milwaukee to Hawaii.Really wished I had!(But I would have caught "Hell" from boss.It would have been obvious.)
I'm a small white female.
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