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Using lasers instead of x-rays

Updated Thursday, 24th February 2011

You might think lasers are quite dangerous things - but for looking inside a body, they might be much safer than x-rays.

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Chris Smith: So, to a new way to see inside the body in detail, but without needing to resort to X-rays, which can be harmful and even cause cancer. Now, a US-based scientist, Mark Niedre is investigating a new technique called early photon tomography or EPT. He sends very short but powerful pulses of harmless, near infrared, laser light through tissue and then he collects the first light particles called photons that come out the other side. The idea is that the first photons to come out must have made the shortest journey through the tissue without being bounced about on the way and therefore they can give you the most detailed picture of what the tissue looks like on the inside.

A laser being used in medical treatment
Lasers are already being used in a lot of medical procedures, as here - but could they replace x-ray machines?

Mark Niedre: So what we’re really talking about here is imaging in biological tissues, ie small animals and hopefully in people, with light. And, as I'm sure you’re aware, the issue with imaging with light in biological tissues is the fact that light scatters like crazy in tissue. So if you can think of taking a laser pointer and shining it at your finger, you see that light really diffuses and is really scattering a lot. And the problem with imaging in biological tissue is this has actually obscuring the feature that you might be interested in.

Chris Smith: So, in other words, where your whole finger lights up with the laser pointer rather than getting a clear image of the tissues in the finger, you’re seeing just a blob. And you’re saying we need to try and resolve this so we can get more detail?

Mark Niedre: Exactly, so we want to think more like, for example, an X-ray which essentially goes straight through biological tissue. It either goes straight through or it’s absorbed so you get a much sharper image as opposed to light.

Chris Smith: And what are you trying to do to solve that?

Mark Niedre: So our approach was the idea of taking extremely high speed pulse laser so this is a femtosecond laser, so this is 10 to the minus 15 second laser pulse, and then shooting that into biological tissue and then on the other side of the tissue using a camera that’s also a very high speed, gated camera so this is taking images on the picosecond so 10 to the minus 12. And so what the idea is, is to catch the photons that come out of the tissue on the other side first, and the idea is that the ones that came through first are the ones that had to have taken the shortest path through the tissue and therefore have got straighter and therefore should contain more spatial information than more diffusive later arriving photon.

Chris Smith: And how does the laser light when it goes through the tissue actually discriminate different features and structures that you then pick up with the detector?

Mark Niedre: So that’s a very good point. What we’re actually imaging here, there’s two approaches. One is to use just the light and use the native contrast of the tissue so scattering and absorption of the tissue, so different features, for example, bones and different tissues in the animal. In this case, what we’re actually looking at is fluorescently labelled targets.

Chris Smith: Oh I get it, so what you’re saying is you can target some kind of signal molecule to a specific tissue or structure. It locks on to that selectively and the laser then shows you where that is, and this means you get very good quality resolution of those structures?

Mark Niedre: It's a good way to put it, yes.

Chris Smith: And how did you prove that your technique actually works? What have you done in terms of imaging real live, living tissue to show that this is feasible?

Mark Niedre: Right, so the work we did was actually in mice with a lung tumour model, and our choice for that was driven largely by the fact that lung scatters light a lot, even by the standards of biological tissue. So we did a number of studies. So one was just the image with our system and then we do sort of correlative images, so one is with an X-ray CT so X-ray computed tomography which is a more conventional approach, a high resolution approach where you can see the tumour.

Interestingly, what we found is that using our technique, the tumour itself was fluorescent but in addition to that, the lung tissue and adjacent lobes of the lung were actually fluorescent as well, and this is something that we actually didn’t quite expect and this is one of the big results of the work. And that’s the idea that we’re actually imaging biochemical changes in the adjacent lung that are associated with the presence of the tumour but this isn't directly visible on something that’s more structural as opposed to biochemical, like an X-ray CT.

Chris Smith: And when you use this technique to image tissues, what sort of level of detail can you get with this? One would assume that because you can focus those reporter molecules very tightly on certain tissue types or certain chemicals, you could get really quite high levels of detail?

Mark Niedre: That’s correct. Now we have to be a little careful because this technique allows us to do much better than more conventional optical techniques using what you would call a continuous wave or constantly on laser as opposed to using pulse lasers like we’re doing here. So we can see resolutions down to a millimetre and possibly submillimetre scale. The critical point is that you can actually use targeted fluorescent probes targeted molecular probes which are showing very specific molecular information, and that’s what’s really exciting about the combination of the two techniques. One is that the development of targeted fluorescent probes and the other is advanced optical imaging techniques.

Chris Smith: Now one of the criticisms levelled at existing imaging techniques, like X-rays, is that they can damage tissues. Now you’re not cooking your mice with these lasers presumably?

Mark Niedre: That is correct. No, this is a very safe level of optical exposure, and in fact that’s one of the big advantages of using optical techniques is that optical radiation is non-ionising and it's extremely safe, provided it's below a certain threshold, where you start to have heating effects, and we’re well below that, so it's a very safe technique.

Chris Smith: Mark Niedre of Northeastern University in Boston shedding some light on a new way to see inside the body.

Extracted from an edition of Breaking Science originally broadcast November 2008. Listen to the full episode online.


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