Conventional X-ray tubes contain a metal filament that's heated to over 1000°C until it emits electrons. These electrons are accelerated into a metal target, which then emits X-rays. The researchers are working on an alternative: "cold" X-ray tubes, in which the filament is replaced with a bundle of sharp-tipped carbon nanotubes. When a voltage is applied, electrons are instantly emitted from the tips of the nanotubes. No heating is required and the emission current can be controlled and modulated instantaneously by the external field.
"Think of each nanotube as a lightning rod on top of a building. The high electric field at the tip of the lightning rod draws the electric current from the cloud. Carbon nanotubes emit electrons using a similar principle," explained Sha Chang, who presented this work at the AAPM meeting (abstract 11909).
Chang and colleagues have built a miniature X-ray scanner based on the carbon nanotube technology and used it to image the interior anatomy of small laboratory animals. While it can be difficult for existing X-ray technologies to compensate for the animal's fast breathing, the nanotubes can be turned on and off instantaneously. This makes it relatively easy to synchronize their X-ray emission with equipment that monitors the animal's breathing or heart rate.
The carbon nanotube-based devices could also prove of benefit within clinical diagnostic imaging systems. For example, CT scanners currently used for breast imaging rely on a rotating X-ray source. As each nanotube produces its own focal spot, they can be arranged into an array of individually-addressable X-ray sources. Such an array could surround the patient, enabling imaging in a few seconds by electronically turning on and off each source, and without any mechanical motion.
This high-speed, gantry-free scanning can improve patient comfort and boost image quality by reducing motion blur. Using 25 simultaneous beams, Chang's team has produced images of growths in breast tissue, with nearly twice the resolution of that achieved by commercial scanners.
This summer, the team will conduct clinical tests of a first-generation, carbon nanotube-based imaging system for high-speed, image-guided radiotherapy. The research system is being developed by XinRay Systems - a joint venture between Siemens and UNC start-up Xintech.