Nanotube-Powered X-Rays

Tiny electron emitters inside an x-ray generator could improve medical imaging and cancer therapy.

By Prachi Patel

Carbon nanotubes are at the heart of a new x-ray machine that is slated for clinical tests later this year at the University of North Carolina (UNC) Hospitals. The machine could perform much better than those used today for x-ray imaging and cancer therapy, say the UNC researchers who developed the technology. They have shown that it speeds up organ imaging, takes sharper images, and could increase the accuracy of radiotherapy so it doesn't harm normal tissue.

Capturing the heart: In a new scanner, carbon nanotubes fire electrons instantly to generate x-rays. This gives sharp, high-resolution pictures, such as this one of a fast-beating mouse heart. Credit: Otto Zhou, University of North Carolina

Conventional x-ray machines consist of a long tube with an electron emitter, typically a tungsten filament, at one end and a metal electrode at the other. The tungsten filament emits electrons when it is heated to 1,000 degrees Celsius. The electrons are accelerated along the tube and strike the metal, creating x-rays.

Instead of a single tungsten emitter, the UNC team uses an array of vertical carbon nanotubes that serve as hundreds of tiny electron guns. While tungsten requires time to warm up, the nanotubes emit electrons from their tips instantly when a voltage is applied to them.

The researchers presented work on their nanotube scanner at the meeting last week of the American Association of Physicists in Medicine.

Physics and materials science professor Otto Zhou cofounded a company called Xintek in Research Triangle Park, NC, to commercialize the technology. Xintek has teamed with Siemens Medical Solutions to form a joint-venture company, XinRay Systems, which has developed the prototype system that will be clinically tested this year.

Taking clear, high-resolution x-ray images of body organs is much easier with the new multi-beam x-ray source, Zhou says. Conventional computerized tomography (CT) scan machines take a few minutes to create clear 3-D images using x-ray. "Because the radiation is coming from one point in space, the machine has to move the [electron] source and detector around the object," Zhou says. The x-ray emitter fires while the tube moves. The motion of the heart and lungs can blur images, so a CT scanner takes hundreds of pictures that are synthesized to reconstruct a 3-D image.

The new machine, by contrast, turns multiple nanotube emitters on and off in sequence to take pictures from different angles without moving. Because the emitters turn on and off instantaneously, says Daniel Kopans, director of breast imaging at Massachusetts General Hospital, the system should be able to take more images every second. This faster exposure, Kopans says, should reduce blur, much as a high-speed camera captures ultrafast motion. Zhou and his colleagues have been able to take breast images at nearly twice the resolution of commercial scanners, using 25 simultaneous beams in a few seconds.

Fast, real-time imaging will in turn improve cancer treatment. "State-of-the-art radiation therapy is highly image-based," says Sha Chang, a professor of radiation oncology at the UNC School of Medicine who is working with Zhou. Pictures of the tumor area are taken so that radiation can be focused on the tumor, sparing the normal tissue surrounding it. But since today's scanners are slow, Chang says it isn't possible to take 3-D images and treat the patient at the same time. "Using the [nanotube] x-ray imaging device allows [us] to collect 3-D imaging while we're treating the patient, to make sure high-dose radiation and heat [are] delivered to the right place," she says.

The clinical test results will determine if Xintek can enter the medical-imaging market. Meanwhile, the company is also selling its nanotube emitters to display manufacturers. Companies such as Samsung and Motorola are making displays based on nanotube emitters that promise to consume less power than liquid-crystal displays or plasma screens while providing the brightness and sharpness of bulky cathode-ray-tube TVs because they work on the same principle: shooting electrons at a screen coated with red, green, and blue phosphors.

Xintek's imaging technology is also proving useful for research on laboratory animals. It can take sharp cardiac images of mice, which is hard because of their rapid heartbeats. Zhou says that biomedical researchers at UNC are already using the system and are installing a second unit at the medical-school research facility.

http://www.technologyreview.com/biomedicine/23107/