

X-Ray Backscatter Imaging Safety From Basic Principles - robhu
http://compilerbitch.livejournal.com/218216.html

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RK
Here's my take on the post from a radiation physics point of view.

I'm not convinced that these pose a great radiation risk. I would be mostly
concerned about people with genetic conditions, who could not normally undergo
x-rays, but then again flying itself might be worse from a radiation point of
view. I'm much more concerned about the civil liberties issue.

 _Firstly, I'll say one thing: there is no such thing as a truly safe
radiation dose._

The problem with this statement is that there is very poor data for low dose,
long term exposure. And as you can imagine, it's not exactly easy to set up
controlled experiments. The generally accepted model is the linear, no
threshold model, which agrees with his statement. But there is some evidence
for a threshold limit (below which cancer doesn't occur) and slight evidence
that small amounts of radiation is actually beneficial (hormesis).

 _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._

Actually, for photons (x-rays/gamma rays), most damage is done by so called
indirect damage, where the photon creates a free radical (in this case OH.),
which then interacts with DNA.

 _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._

This is just plain hyperbole. You almost certainly need multiple radiation
interactions to produce DNA damage that will result in fatal cancer. Most DNA
damage is repaired by the cell, most persistent DNA damage is benign, and most
cancer is killed off by the body. That's not even to say anything about the
fact that once a cancer gets to the point of discoverability that most are
survivable (you still don't want cancer though!).

 _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_

This was in reference to the hydrogen bomb. You can not (generally) change the
energy of x-rays. You can "harden" an x-ray beam energy distribution by
filtering out the lower energy x-rays, but not the other way around. I assume
he really meant neutrons.

The main mistake he (and others) makes is seeming to not understand the nature
of radiation dose and how that relates to the body. Dose is simply the amount
of energy deposited by radiation per unit mass. It can be thought of as a
density like quantity. The related term equivalent dose is used for setting
radiation exposure limits and takes into account the biological effects of
different radiation types (and energies) and different tolerances of organs
and tissues.

When people see that low energy radiation is mostly absorbed near the bodies
surface, this seems to concern them a lot. What is more important is what the
actual dose is. From a general carcinogenesis point of view, it's actually
considered better that the dose is not to the entire body

What's generally not understood is that limit of dose to a specific part of
the body (e.g. skin) is higher than the limit of the total body dose. This is
because your body is better at dealing with localized damage, than it is with
systemic damage.

~~~
berntb
>>But there is some evidence for a threshold limit (below which cancer doesn't
occur) and slight evidence that small amounts of radiation is actually
beneficial (hormesis).

Interesting, but one question.

That part about a threshold sounds a bit strange?

Are you ignoring alpha (because you have to ingest/breath it) -- or do mean
that there can't be unrepairable DNA damage from a single alpha particle?

~~~
RK
According to my radiobiology text (Hall), "mammalian cells on average
experience over 100,000 DNA lesions per day as a result of replication errors,
chemical decay of their bases, attack by reactive oxygen species, or exposure
to ionizing radiation. However, the mutation rate in mammalian cells is quite
low owing to the development of DNA repair pathways that handle each type of
damage."

Basically the threshold hypothesis assumes that the body can successfully
repair up to a point, beyond which the risk of cancer increases with the
increasing number DNA lesions.

Alphas (and other charged particles heavier than electrons) act in a slightly
different way. They can do more direct DNA damage. They can also occur
externally, from cosmic rays or accelerator beams. I only addressed photons
because we were talking about the body scanners.

~~~
berntb
OK.

I only read a compendium for a course I didn't take. :-) Was a cool subject.

------
nubian
Someone needs to go through these with a dosimeter. On a regular basis.

Taking the manufacturer's word for it isn't even good enough for software or
gas mileage; we rely on third-party benchmarking.

And where's the independent safety auditing to check for hardware dose
limiting in case of faulty software, for example?

<http://en.wikipedia.org/wiki/Therac-25>

You couldn't roll out a medical device like this, but foist it on the public
at large and suddenly the testing requirements are very lax.

Field testing is essential. You can't just use a model that the manufacturer
"kindly" provides for free. (To do proper Consumer Reports, you have to buy a
unit on the open market incognito.) And you can't test for wear-and-tear
malfunction by using a brand-new model.

I found this report:

[http://publicintelligence.net/nist-rapiscan-
secure-1000-reda...](http://publicintelligence.net/nist-rapiscan-
secure-1000-redacted-radiological-safety-report/)

Which doesn't mention who provided the unit, and seems content to test it in
black-box mode, focused on the externalities and not the actual hardware or
software internals.

What about possibly buggy software updates?

iPhone apps get more auditing trying to release a new version than this thing.

~~~
danielharan
A medical device would also have to be safe for its operators.

~~~
Semiapies
Yes. I suspect, if anything develops, we're going to hear about illnesses
among the TSA workers standing beside these things.

------
santry
> 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.

Could this component be a fogbank
([http://www.thewaythefutureblogs.com/2010/11/what-fogbanks-
co...](http://www.thewaythefutureblogs.com/2010/11/what-fogbanks-cost/)) just
discussed on HN (<http://news.ycombinator.com/item?id=1928233>)?

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icegreentea
Not disagreeing with the math or anything... but does anyone see the stomach
and lungs and stuff are? I mean, since we don't have a true global source of
x-rays there's bound to be some shadowing (ass cheeks for an example), and all
the stuff going on in the chest looks like a combination of shadowing,
artifacts, and human mind gone wild?

If we can only see the shin bones cause there's like half a centimeter of
flesh there, how can we see the lungs without seeing the rib cage???

~~~
andrewbadera
I would imagine it has to do with variable penetration and/or a form of
constructive interference or resonance, due to factors I couldn't specifically
identify offhand.

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raquo
Um... you could go into the scanner wearing a radiation-measuring device on
you that sends data live to your server. I even doubt that would be illegal,
although you'll probably end up on a couple lists.

------
tybris
How about tin foil underpants?

~~~
jdp23
turns out somebody's actually marketing them: <http://gawker.com/5695677/>

