Light rather than X-Rays

Random thought, but I know that light can shine through the human body (for instance, your hand). Can we make a light bright enough to yield an equivalently useful image as an x-ray scan, without frying the target?

Clever question. One of the best examples of lateral thinking I’ve seen. But unfortunately the answer is no. For the same reason that we can’t see through snow despite the fact that air and ice are both transparent.

Every time light interacts with a new medium it refracts a little. Too much refraction and it loses coherence. So when visible light passes from skin to fat to blood to muscle to bone to muscle to blood to skin and back out it loses all coherence. Even though it’s bright enough to see we can’t form an image out of it.

Damn clever idea though.

Actually, it IS possible with some tissues, if they are thin and translucent enough. For example, scroll down here for a side by side comparison of X-ray and optical mammography. And I think I’ve seen images of body parts like hands scanned in a similar fashion, but my google skills fail me.

This is NOT true!
Research is currently being done on just such a system. The idea is as follows: If you have a bright enough light, some photons will make it through even a torso-width amount of tissue. The problem is, the scattered photons tend to blur the image (as you mentioned). The solution: have an electronic shutter synchronized with the light pulse that only allows the very first photons (the ones that took the straightest path) to create your image. Obviously, huge technological hurdles need to be overcome before that is a practical system, but the concept is workable.

Here is one hit that I found on such a system (warning :.DOC file).

Researchers are looking into using ballistic or quasi-ballistic light. Briefly, ballistic photons are photon that aren’t scattered by what you’re shining them through. This will be some fraction of all the photons that get through. Shining them through your hand, for example, the ballistic photons will give an image dependent on how much the different tissues scatter, similar to X-rays. You need a way to capture only the ballistic photons.

In both links they use a collimated source of light. In the first, they’re sending a short pulse, and receiving for only a short time, so they’re able to obtain better image contrast. In the second, they capture only photons that are very close to parallel to the source. These include all the ballistic ones, and few enough of the scattered photons, to get clear images. What I first read about them, IIRC, they were receiving on the far side of the object, like in the second link, but using time gating like in the first link (the scattered photons took a longer path, so were delayed relative to the ballistic ones).

Here’s the Wikipedia page on ballistic photons.

On preview, beowulff covered this, but I already had a nice post with different links written…

You can also use a coherent laser beam with a very short pulse to help filter out the light that didnt scatter from the light that did scatter and make a hologram out it.

That’s pretty cool.

You can also use light of lower frequencies than visible, rather than higher. I’ve heard of a prospective cancer treatment that uses an infrared laser to activate chemotherapy agents, so as to localize them precisely to where the tumor is: They can work through something like 5-10 cm of tissue and still be strong enough to activate the chemo, without doing significant harm to the intervening tissue.

Terry Pratchett has an IR helmet that focusses IR radiation on to the brain in an attempt to treat his Alzheimers - I am pretty sceptical about that particular application, but I have no doubt that the Virulite (from the same people) reduces the intensity and frequency of my Cold Sore outbreaks. I was just surprised at the claimed depth of IR penetration through the skull.


If you go down even further THz and microwave have been looked at for medical imaging e.g. see (half way down), though resolution and penetration are problems

I once volunteered for a study that used some of these optical imaging techniques to observe localized blood flow in the brain. The technique is known as Diffuse Optical Imaging (DOI). It works with the same basic principle as an fMRI, using blood flow as a proxy for brain activity, though with much lower resolution and limited depth. But, on the flipside, if DOI makes it to the clinic it will be a hell of a lot cheaper and simpler than fMRI. The hospital would just need one or two specialists with five or six figures worth of equipment, instead of a whole team of technicians in a purpose-built room housing seven or eight figures worth of MRI equipment.

In the study I was specifically participating in, the group was looking to see if the technique could be used to distinguish between brain activity for ordinary sensation and pain. Longer term, they want to use it on anaesthetized patients undergoing surgery, and possibly be able to tell if a nonresponsive but conscious patient is feeling pain.

See… thats the problem with bright ideas… someone always beats you to them.

har har har