IEEE Spectrum - North American - March 2014 - 20

nEwS

the MeDicAl
X-rAy's new PhAse
Phase-contrast imaging could show
more with less radiation
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 phasecontrast 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
20

MaR 2014

nORTh aMERICan

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 warehousesize 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 VelásquezGarcia, principal research scientist at MIT's
Microsystems Technology Laboratories.

SPECTRUM.IEEE.ORG

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-tomicrochip 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." -m a r k a nder son

mICrosysTems TeChnoLoGy LaboraTorIes/mIT

exTremely Tiny x-rAy: An electron-emitting
chip is at the heart of a new type of X-ray source. It
can replace large particle accelerators in phasecontrast imaging.

http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2014