The X-ray that imaged your father’s broken leg when he was a kid isn’t fundamentally different from the one at your doctor’s office today—other than that it’s now digital rather than film-based. But the technology is on the verge of a big leap forward that will let doctors see more with less light.
To take an X-ray today of some part of the body other than the bones, a patient needs to ingest a contrast agent like iodine or barium that binds to the tumor or other structure the doctor wants to examine. But contrast agents can be hard on the kidneys, enough to harm or even kill some already weakened patients. And some kinds of X-ray imaging today, like computed tomography (CT) scans, involve such heavy X-ray doses that the patients risk an increased cancer rate just to provide good pictures for their doctors.
Enter a technique called X-ray phase-contrast imaging (XPCI), in which instead of measuring the X-ray’s intensity, you measure the change in an X-ray’s phase. X-rays move slower when they pass through muscle mass than through blood, for instance. So a chest X-ray wave front that passes through a lung sac (mostly air and blood vessels) will arrive sooner than a wave front that passes through the heart.
XPCI can therefore image not just bones and cartilage but also internal organs, internal bleeding, tumors, and plenty more. Moreover, XPCI involves no toxic contrast agents and a substantially lower X-ray dosage.
“People are getting more and more CT scans. The doctors love them because they can get a better diagnosis, but the radiation dose is getting to be too high,” says Mark Eaton, president and CEO of the X-ray company Stellarray, based in Austin, Texas. “Phase-contrast imaging is fundamentally different. What you’re detecting is the phase change, not the absorption. It is a profound level of dose reduction, by one to two orders of magnitude.”
The catch to date is that XPCI has been available only from extremely powerful, laserlike X-ray sources such as warehouse-size particle accelerators.
Traditional X-ray machines shoot out a range of different X-ray wavelengths from various points and in various directions. XPCI is more demanding. “What you really need is basically something like a laser, but for X-rays,” says Luis Fernando Velásquez-Garcia, principal research scientist at MIT’s Microsystems Technology Laboratories.
For XPCI X-ray sources, particle accelerators are best. But just as some particle accelerators have been miniaturized onto chips, X-ray sources that were formerly based on particle accelerators have been shrinking too. One of the more promising XPCI developments is a tabletop chip-based system being developed by Velásquez-Garcia and his colleagues at MIT and Massachusetts General Hospital, in Boston.
The researchers have fabricated a chip that contains some 12 100 microscopic electron emitters, each of which spits electrons out of a nanometer-sharp metal tip. The resulting beams of electrons are accelerated by a high-voltage electric field and crash into a thin gold film that emits a nearly monochromatic beam of X-rays.
Stellarray is developing its own XPCI imaging device, which is not unlike the MIT technology, says Eaton, but he would not disclose the device’s specifics.
Mark Evans is the CEO of Radius Health, an X-ray company based in Oxford, England, and Los Angeles. He says the vacuum-tube-to-microchip size reduction of next-generation X-ray devices is just as important as the dose reduction. Today’s X-ray machines are the size of mainframe computers. XPCI machines could get down to PC size or smaller.
“Most ICUs and ERs are busy places, so positioning this quarter-ton monster takes a lot of time and effort, sometimes half an hour,” Evans says. “If you could make it smaller and lighter, you could save a lot of the radiographer’s time.”
Heinz Busta, of Chicago-based Prairie Prototypes, chairs the steering committee of the International Vacuum Nanoelectronics Conference, which is the leading conference in XPCI and miniaturized X-ray sources. He says he’s seen a rapid increase in interest in XPCI recently, and that engineers in China, Europe, Japan, and South Korea are working on shrinking XPCI sources as well. “For nondestructive testing, sterilization, homeland security, medical imaging, there are huge applications,” he says, “and not only new applications but safer applications.”
This article originally appeared in print as “The Medical X-ray’s New Phase.”