Terahertz Pulsed Imaging to Play a Role in Oncologic Imaging
By Dana Hinesly
Terahertz radiation lies between the infrared and microwave regions of the electromagnetic spectrum, ranging from ~300 GHz (which is a wavelength of 1 mm) to 10 THz (which is a wavelength of about 30 ?m). Terahertz pulsed imaging (TPI) is a noninvasive technique that uses this type of radiation and, in the future, might offer clinicians a nonionizing imaging alternative. To learn more about the technology and its potential imaging applications, Medical Imaging conferred with Eric R. Mueller, PhD, director of engineering for Coherent Inc, Santa Clara, Calif, which provides laser-based solutions to the commercial and scientific communities.
MI: What type of imaging is possible with this technology?
|Eric R. Mueller, PhD|
Mueller: A pulsed terahertz system uses a very, very short pulse of light, and the pulse is so short that it contains many different frequencies. In many of the existing TPI systems, the energy is concentrated at the low end of the THz spectrum, near 200 to 300 GHz. Although that yields the best penetration through tissue, it gives the least-quality resolution. The most interesting work I?ve seen done has concentrated not on trying to image through the body?which really is not practical?but rather, imaging either on the surface or internally (either with the use of a catheter or during surgery).
MI: What potential application could that have?
Mueller: It?s important to understand that this technology is still really new, but what?s been found to this point is that the reflection of terahertz light off cancerous tissue is different from the reflection off noncancerous tissue?and the contrast appears to be pretty noticeable. So with skin cancer, for example, when surgeons remove cancerous tissue, they stop cutting based on experience. They then take their last cut and send it to histology, which tells them whether they?ve cut deep enough. In some cases, patients must undergo a second surgery to remove the entire cancer. The interest in this case is that since it appears there is very good contrast between the cancerous and noncancerous tissue, surgeons could scan the area during the procedure and know to stop cutting, because they would know whether they got all the cancer. Another example would be a patient undergoing breast-cancer surgery. Again, doctors cut based on experience, because the visual contrast between cancerous and noncancerous breast tissue is pretty poor. But by looking at the tissue with terahertz images, you are able to see a much higher contrast between the two.
MI: Will TPI replace existing modalities?
Mueller: It is not a replacement for x-rays, it?s not a replacement for MRI, and it is not going to replace mammography. There are people who say it is going to do all of those things, but it?s just not true, because the water content in the body is so large that it just absorbs all of the terahertz. My sense is that it?s going to provide a capability that doesn?t exist now versus displacing an existing technology. What we?re talking about is enabling a different mode of treatment. If the surgeon can tell the difference between cancerous and noncancerous tissue, they don?t cut as deep. The mode of treatment changes, and it changes in such a way that the patient benefits.
MI: Why isn?t terahertz more widely available?
Mueller: Terahertz has been worked on for many years, but accessed only by experts because you had to build everything; it wasn?t like buying a microscope?nobody makes it. So, the only people who did any work with it were experts in terahertz technology itself, and they spent a huge amount of their time, thought, energy, and focus on building a detector and/or a source. If someone wanted to use it, either they had to collaborate with an expert, or they had to become an expert?and if you?re a surgeon, that?s a fairly unlikely path. Right now, the instruments are sold to researchers who want to be able to do all kinds of things with it, not just one thing. The next step is going to involve driving both the cost down and the availability up. The only way to do that is to have a narrowly defined application and design an instrument solely for that application, carving as much cost out as you can.
MI: Are there other benefits to TPI?
Mueller: The nonionizing nature makes it safe for everyone, including the people in the operating room, which, in some cases, is a bigger issue, because they?re subjected to radiation all day, every day. There also is some work starting in terahertz imaging that is not pulsed; it?s called terahertz continuous-wave imaging (TCW). With TCW, the radiation comes in a steady stream, rather than pulsed, and it is not covering a broad spectrum of wavelengths; rather, it just covers one. TCW imaging can get the data much faster and a much higher dynamic range is available, meaning you can penetrate through more absorbing tissue, making it possible to see things that are not directly on the surface of the tissue. I don?t think either TPI or TCW will completely supplant the other. The most likely outcome is that there may be some areas where TPI is better and others where TCW is better. Ultimately, the requirements need to drive the technology.
MI: What research is being done?
Mueller: A mix of work is taking place, with some research tailored toward specific applications, but the vast majority of what?s being done is what I call ?prospecting,? where people are looking at potential applications and really just scratching the surface. It is still a field in its infancy, and although people are answering questions, I think more of what they?re doing is figuring out the right questions to ask.
So, while some experiments show there is good contrast for basal cell carcinoma, for example, someone else will see that and start asking questions: What do you get when you look at another type of skin cancer? There is a difference in terahertz reflectivity, but what?s the spectral nature of that? Is the best place to do that imaging 1 THz or 3 THz? Those kinds of questions are pretty basic, but those are still the kinds of questions being answered at this time.
Dana Hinesly is a contributing writer for Medical Imaging. For more information, contact .