A breakthrough
toward digital PET

Philips Brings Light Sensors Into the Digital Age

October 09, 2009
by Brendon Nafziger, DOTmed News Associate Editor
The analog light-detecting sensors that help PET scans work might go the way of vacuum tubes in computers, if Royal Philips Electronics technology takes off.

The Netherlands-based company issued a statement Thursday saying it had successfully completed proof-of-concept trials on its fully digital silicon photomultiplier.

Photomultipliers, usually made of glass tubes, are involved in a variety of technologies. In PET imaging, they are used in combination with scintillator material to detect the high-energy gamma rays decaying from radioactive tracers.

But because these vacuum-tube photomultipliers have to turn an analog signal into one read by a bank of electronics, they're not as fast as they could be. That's where solid-state technology comes in.

Digital dawn

"Speed, sensitivity, robustness," says Steve Klink, a spokesperson for Philips, when asked to describe the benefits of the all-digital photomultiplier.

First, it's fast, because the detector and reader live together on the same chip. It both detects the photons, the "quanta of light," as Klink puts it, and counts them without needing any time-consuming signal conversion.

In fact, according to Carsten Degenhardt, the R&D head of the project in Germany, the timer installed on the device can detect the arrival of photons with a range of uncertainty of only eight picoseconds. (A picosecond is one-trillionth of a second.)

It's also more accurate than an analog device. "We don't deal with any analog signals, so we have better control of the noise," says Degenhardt. It can therefore pick up very faint signals.

And it's tough. "Vacuum devices are made out of glass. They can break easily and are influenced by electromagnetic interference," adds Degenhardt. They're also sensitive to fluctuations in temperature. But not solid-state electronics: the device, which can fit in the palm of the hand, like most chips, is quite rugged.

Next steps

The movement from analog to digital is a "trend happening everywhere else," says Klink. "If you look at the compact disc to the mp3 players, or in lighting where you go from a light bulb to LED, it's the same thing we envision to happen with photomultipliers in medical imaging systems."

The technology, which Philips started working on in 2005, can so far only resolve one pixel. The next step is to create a laboratory-based model, says Klink, with a larger array for PET imaging. He expects a prototype to be finished by the end of this year. But, what with the hoops a medical device has to go through with the FDA, Philips can't speculate on when such a silicon-photomultiplier-powered PET scanner could be available to the public.

Other applications for the device could come out sooner, though, provided Philips finds the right partner. Philips is actively searching for someone to bring to market other technologies using this device. Contenders include night-vision goggles or tools that perform DNA sequencing by picking up weak light fluctuations.

As the device relies on CMOS electronics -- just like many digital cameras -- it should be easy to manufacture, Degenhardt says, "It's mainstream high-volume technology. You can make the chips just like other integrated circuits."

"We've shown proof of principle," adds Klink. "The technology works, and this was a step we had to solve. There are no fundamental issues going forward, but there is obviously a lot of engineering involved."