X-ray ink?

Does it have to be invisible or just unreadable? You could do something like use a lead-based (or other metal-based) paint and then cover it with a fake tattoo or other paint. It's going to be obvious that there's something there, but the important part will be hidden.

As a weird (but clear) alternative, what about a substance that shows up under those fluorescent crime-scene lights?

Siana gives me more ideas - use some flesh-colored makeup that matches the skin tone. Make two batches, one with a lead or other metal powder added which is used to write the message, and the other as-is which lightly goes over the area with the writing. It may take a lot of work (or an expert makeup artist) to make it look natural, depending on how well it's going to be scrutinized. Maybe add some sort of spray coating to keep it from smearing or rubbing off. I obviously don't know much about this, but there's some ideas, and so far we have no idea if these things would work in your story.

There are tattoos which only show up under blacklight, but those take ages and ages to heal (I think it is in the region of a year until the scars fade on large designs) so this would leave the character unable to show the affected skin for a significant amount of time. If it only something applied to the skin as a temporary measure then it would need to be substantial to show up on an x-ray, so would probably be visible to others.

ETA: I just thought - there's a liquid that you can take if you are getting a stomach x-ray (and possibly other x-rays?) which shows up. Not sure what it is made of, or if you can go out and get some / make some without a medical reason, but the character might be able to make use of something like that. Just a thought...

In radiology, "contrast media" injected into an artery before making an X-ray or CT scan are used to make contrast images of the vessels.

Certain iodine-based contrast substances (iohexol, iodixanol, ioversol) in prepared solutions are colorless.

Gadolinium is a colorless contrast substance used in Magnetic Resonance Imaging (MRI).

I guess all above substances become visible when they dry up, but if you mix then with some oil-based cosmetic or some sort of movie-type skin-masking substance, they might not dry up...

Titanium dioxide or zinc oxide? I'm wondering about physical sunscreens and what they could do in an x-ray.

BigWords said:

ETA: I just thought - there's a liquid that you can take if you are getting a stomach x-ray (and possibly other x-rays?) which shows up. Not sure what it is made of, or if you can go out and get some / make some without a medical reason, but the character might be able to make use of something like that. Just a thought...

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i believe you're thinking of the barium liquid (looks like kaopectate color/texture) that can be ingested before a contrast series of x-rays. i don't know enough about tattoos to know if this would work as a tattoo ink, but the barium solution is definitely in veterinary clinics, which is where i used it (as an employee there). maybe a pharmacy would have some but probably would require a prescription.

hope that helps clarify.

Substances used in radiology today work because they either fills cavities in the body (intestinal tract CT scans) or agglomerates in sufficient density (MR or PET scans). The skin is just too thin to be a substrate of those chemicals.

X-rays are extremely penetrating. To deflect xrays off a surface the thickness of paper would require a very dense substance. Like a metal.

Maybe phosphorescent paint might glow under an X-ray beam with sufficient intensity to be noticed. A bit like blacklight does.

-cb

Substances used in angiography (pictures) can clearly show the arteries thinner than one millimeter. Those substances are greatly diluted twice - first in the solution for the injection and then in the blood.

boron said:

Substances used in angiography (pictures) can clearly show the arteries thinner than one millimeter. Those substances are greatly diluted twice - first in the solution for the injection and then in the blood.

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Those substances are injected in bolus at the point of measure. The photographs are impressive, but in reality they last a fraction of a second. And the x-ray machine is designed to emit rays with a specific range of energy for this specific application, and that range is different from that of airport scanners, for example. A coat of latex paint is 0.1mm thick, or about the thickness of laser printer paper, hence my skeptism of seeing anything with any old x-ray machine.

But the OP can imagine any scenario with a contrast agent which - by some litterary miracle - happens to stick to the skin and flare up under any x-ray beam like Times Square on New Year's Eve. As long as it doesn't become the central focus of the plot.

Someone mentionned tattoo ink and black light. A quick search uncovered the following:

http://www.amazon.com/dp/B001QALRUA/?tag=absowrit-20

-cb

veinglory said:

I need a substance that would be invisible when used to write on skin, but would show up clearly in an x-ray. Preferably something that would not easily rub off once dry. Any ideas?

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I've been retired long enough that I don't remember the brand names or concentrations, but the radiographic contrast that we used for retrograde pyelograms was transparent to vision. I never tried painting it on the outside to see if it would dry and show up on X-Ray, but if anything would do that, my guess is that would. It's a very concentrated version designed to light up tiny things in the collecting system

Just thinking about it, the thing that I see as trouble would be the technique of the X-Ray technician. They can alter the amount of radiation and have the plate at varrying distances such that if the tech knew to look for something it would be easier to explain than just a random find. Another option would be metal filings, basically splinters of metal embedded lightly enough that a cursory look at the skin fails to detect it, but the filings would easily show up on an X-ray.

To the OP: What type of X-ray would be used? What would be the time from writing on the skin to the X-ray image?

cbenoi1 said:

Those substances are injected in bolus at the point of measure. The photographs are impressive, but in reality they last a fraction of a second. And the x-ray machine is designed to emit rays with a specific range of energy for this specific application, and that range is different from that of airport scanners, for example. A coat of latex paint is 0.1mm thick, or about the thickness of laser printer paper, hence my skeptism of seeing anything with any old x-ray machine.

Click to expand...

I agree it should be a strong/concentrated contrast substance to show up on X-ray. The original contrast solution could be made more concentrated by allowing some solvent to evaporate and the contrast substance mixed with some translucent cream to allow for thicker writing.