What Potential Does Terahertz Technology Have in Medical Imaging?
Every few weeks for the last decade or so, newspapers and trade publications have announced impending “revolution” in terahertz technology, promising huge — perhaps paradigm-shifting — breakthroughs in medicine, security, manufacturing, and communication. In biomedical imaging, these stories predicted “t-rays” would soon be blazing new trails in the detection and treatment of cancers, severe burns, and more.
But Peter H. Siegel, Ph.D., today offers a more conservative appraisal — and less rosy timetable. As a senior terahertz researcher at the California Institute of Technology and the Jet Propulsion Laboratory, Siegel would be delighted to see all these media predictions come true, but says claims of huge, “generational gains” on the terahertz front are propelled more by hype than by hard evidence.
“Terahertz is where MRI was 25 years ago,” he says. Impressive gains are possible, perhaps even likely, but a scientific revolution on par with CT or MRI may never be in the offing. “Terahertz technology is an exciting possibility, but requires a lot more research and investigation. It's definitely not ready for prime time.” Chuckling, he adds, “Most media stories on terahertz are way overblown.”
Terahertz Primer
Terahertz technologies harness submillimeter-wave radiation (or “t-rays,” “t-waves,” or “t-light”) at frequencies from 0.1 to 10 terahertz, coinciding with the spectrum between the infrared and microwave bands. These pulses of light cycle on and off at an astonishing one trillion times per second, but have held the attention of scientists ever since the late 1970s.
Many scientists regard t-rays as the last great frontier of the electromagnetic spectrum, but finding “killer” applications outside the traditional niches of radio astronomy, Earth and planetary remote sensing, and molecular spectroscopy—particularly in biomedical imaging — has been relatively slow. It doesn’t help, Siegel says, that U.S. researchers lag far behind their European counterparts in funding.
Residing at the lower end of the electromagnetic spectrum, t-rays behave like radio waves. When excited, Siegel says, “they propagate and focus via traditional quasi-optical techniques, but utilize lenses typically made of low-loss plastics or crystals rather than the glasses prevalent at optical wavelengths.”
Radiologists find this area of study so attractive because t-rays are non-ionizing, which suggests no harm is done to tissue or DNA. They also offer the possibility of performing spectroscopic measurements over a very wide frequency range, and can even capture signatures — admittedly very broad ones — from liquids and solids. In some non-biomedical applications, t-rays have already yielded impressive gains:
· Airport Security. T-rays facilitate the search for explosives, as they can penetrate opaque objects (clothing, plastics, paper, and more) with such clarity that some critics liken them to a “virtual strip search.” T-rays can also detect, and in special cases classify, unknown powders hidden inside mailed envelopes, including weaponized anthrax.
· Valuable Art. In early February, University of Michigan researchers announced that “non-destructive, non-invasive” t-rays allowed art historians to study murals and pencil sketches hidden beneath several coats of paint, plaster, or oil paint.
· Space Flight. After two destroyed space shuttles, terahertz scanners may offer NASA the ability to detect small, potentially dangerous, surface cracks in their orbiter fleet. This application might avert future mishaps, researchers believe.
· Communication. Scientists hope to harness terahertz technology to develop blazing-fast computers and wireless communications.
Siegel notes that t-rays have traditionally been used to detect lightweight molecules and atoms — be they in low-pressure, ultra-cold interstellar space; around planets and planetary bodies; or in the Earth’s stratosphere. To date, nearly a dozen spaceflight instruments have measured these signatures, which are critical tracers for such processes as ozone depletion, global warming, and pollution monitoring, as well as in furthering research in basic astrophysics, planetary composition, and cosmology.
Terahertz radiation has key strengths, but also limitations. Most notably, t-rays cannot penetrate water or metal. Some terahertz frequencies can penetrate fatty tissue a few millimeters thick, Siegel notes, leading some researchers to speculate about their use in detecting epithelial cancer.
Other Medical Applications
Determining the speed and evolutionary direction of emerging technologies is tricky business, admits Marvin Nelson, M.D., Radiologist-in-Chief at Children’s Hospital, Los Angeles, and Professor of Radiology at the USC Keck School of Medicine. Still, he sees terahertz as having “more dermatologic applications than anything else in medicine.”
Nelson sees t-rays helping physicians map out microscopic changes in cutaneous neurofibromas, evaluate metabolic changes in cutaneous lesions, and endoscopically examine mucosal surfaces in the gastrointestinal tract. “Those are all possibilities, but no one can discount the possibility of a revolutionary application that opens new horizons,” he says.
Recent headlines may claim that terahertz technology will sharply reduce the need for breast cancer surgery, but Siegel says he doesn’t “buy” it. “I would say terahertz could be an effective tool — along with OCT, ultrasound, and MRI — to help radiologists determine tumor margins on skin surfaces. OCT (optical coherence tomography) is a good competitor, but you would be looking at small areas and getting lots of scattering.”
To date, terahertz research in biomedical applications has focused on basal cell carcinomas and, to a much lesser extent, on breast tumors. Siegel is hoping a National Institutes of Health grant will someday broaden these studies to include other skin ailments and lesions, but NIH denied his most recent funding proposal in this area. And even if eventually approved, he says, “this application is a TBD — to be determined.” To underscore the infancy of terahertz research, Siegel notes that all but one clinical study to date has been in vitro.
Looking at the larger picture, he comments, “What's really important is this: When you’re doing a surgical procedure on a tumor and want to get all the infestation, you're not going to settle for a system that gives you a 50 percent margin. You've got to be able see where the tumor completely disappears.”
Even terahertz’s main biomedical selling point — it being non-ionizing — isn’t a clear slam dunk, he says. “Because it is non-ionizing, there aren’t a heck of a lot of things that you can do with [terahertz] that are particularly unique, especially in terms of penetration. X-ray technology is really good and it’s been around a long, long time. In fact, these terahertz applications don't do the kinds of things that MRI did immediately, like penetrating the whole body, and being able to pull out tumor signatures deep inside tissue.”
Terahertz today remains a largely unknown quantity to the medical community. Clinical instrumentation is non-existent and what instrumentation does exist is prohibitively priced — upward of $500,000 per unit. “I think it will take quite a number of years for that instrumentation to make its way into general use,” Siegel says. “Until we have some solid studies, it will be very hard to convince radiologists this is something they should be looking to buy.”