Machines that can see through objects and inside the human body in real time have been around for decades. But because of their bulk and cost, they’re mostly found in airports, where they’re used for screening, or medical buildings, where MRI facilities —comprised of multiple rooms—can cost upwards of $3 million.
But a collaborative effort between scientists at Sandia National Laboratories, Rice University and the Tokyo Institute of Technology is aiming to make this kind of imaging much more portable and affordable—a change that could have major implications for medical imaging, passenger screening and even food inspection.
The technique, detailed in the journal Nano Letters, uses terahertz radiation (also known as submillimeter waves, due to the size of their wavelengths), which falls between the smaller-wavelengths typically used for electronics and the larger waves used for optics. The waves are emitted by a transmitter, but unlike in larger machines, are intercepted by a detector made from a thin film of densely packed carbon nanotubes, making the imaging process less complex and bulky.
Somewhat similar technology is already used in large airport screening devices. But according to Sandia Lab’s François Léonard, one of the authors of the paper, the new technique uses even smaller wavelengths—between 300 gigahertz and 3 terahertz, instead of the standard 30 to 300 gigahertz frequency of millimeter waves.
The smaller wavelength size could be helpful for security purposes, Léonard says: Some explosives that aren't as visible in the millimeter range can be seen with terahertz technology. So not only could these detectors allow for quicker screenings, thanks to their smaller size, but they could be better suited to the task of stopping potential terrorists, as well.
It’s been a challenge for those in the industry to find materials that can not only absorb the energy at such low frequencies efficiently, but also convert them into a useful electronic signal—which is why it’s the detection technology that’s the real innovation. Because carbon nanotubes (long, thin cylindrical tunes of carbon molecules) excel at absorbing electromagnetic light, researchers have long been interested in their use as detectors. But in the past, because terahertz waves are large compared to the size of the nanotubes, they’ve required using an antenna, which adds to a device’s size, cost and power requirements.
“[Previous] nanotube detectors used only one or a few nanotubes,” Léonard says. “Because nanotubes are so small, the terahertz radiation had to be funneled to the nanotube to improve the detectivity.”
Now though, researchers have found a way to combine several nanotubes together in a densely packed thin film, combining both metallic nanotubes, which absorb the waves, and semiconducting nanotubes, which help turn the waves into a useable signal. Léonard says achieving this density using other types of detectors would be extremely difficult.
According to the researchers, this technique doesn’t require extra power to operate. It can also operate at room temperature—a big win for certain applications like MRI machines, which have to be bathed in liquid helium (achieving temperatures around 450 degrees below zero Fahrenheit) to achieve high-quality images.
This video gives a behind-the-scenes look at what the procedure looks like:
Rice University physicist Junichiro Kono, one of the paper’s other authors, thinks the technology can also be used to improve security screenings of passengers and cargo as well. But he also believes terahertz technology could one day replace bulky, expensive MRI machines with a device that’s much smaller.
“The potential improvements in size, ease, cost and mobility of a terahertz-based detector are phenomenal,” Kono said in a Rice University story on the research. “With this technology, you could conceivably design a handheld terahertz detection camera that images tumors in real time with pinpoint accuracy. And it could be done without the intimidating nature of MRI technology.”
Léonard says it’s too soon to tell when their detectors will make their way from the lab to actual devices, but he says they may first be used in portable devices to inspect food or other materials without damaging or disturbing them. For the moment, the technique is still in its infancy, confined to the lab. We'll likely have to wait until prototypes are produced before we know exactly where these terahertz detecors will work best.