Developer of light-sensitive chips seeks PET camera improvements

Avalanche photodiodes may offer 1-mm resolution

On the strength of a $3 million private equity placement in July,Advanced Photonix of Camarillo, CA, is gearing up for productionof avalanche photodiodes (APDs), a new type of image detectorthat could improve PET camera design.

Advanced Photonix has worked on the detectors with PET developerCTI Services of Knoxville, TN, since 1992. Engineers from thetwo companies concluded that APDs may replace photomultipliertubes used in PET. Arrays comprising hundreds of PMTs are usedin conventional PET cameras to detect positronic energy. Angerlogic is used to localize their point of origin. The most sensitiveof these devices is capable of 3-mm resolution, said Shawn Fagen,commercial product manager.

Laboratory tests suggest that cameras based on APD detectortechnology may resolve objects as small as 1 mm, according toFagen. Photon-counting inefficiencies inherent in the Anger techniquecan be eliminated by using APDs to generate more signal.

"This can either mean shorter imaging times or lower radioisotopedoses," Fagen said.

Because positron-emitting radioisotopes emit two photons thattravel in opposite directions when the isotope decays, PET canbe far more precise than single-photon tomography. Time resolutionis important in PET in identifying those paired photons and theirpoint of origin. APDs are the first detectors in a laboratorysetting to achieve a time resolution of one nanosecond, Fagensaid.

Research that led to this technology was originally financedby the U.S. government's Strategic Defense Initiative in the 1980s.The light-sensitive integrated circuits were designed for a ground-basedmissile interceptor, Fagen said. The U.S. Air Force adopted APDsfor its advanced imaging test bed, a device capable of examiningsatellites from the ground, he said.

The small size and unique physical behavior of APDs are keysto their performance. APDs are monolithic silicon arrays consistingof at least 64 pixels on a 3-inch wide array. Only 100 micronsseparate the pixels from one another. A PET scanner could be equippedwith up to 1000 of these arrays and still be hundreds of poundslighter than PMT-based PET cameras, according to Fagen.

The arrays operate with the benefit of 80% quantum efficiencyat room temperature through the spectra of bismuth germanate orother high-energy scintillators. Photons detected by the APDsare directly converted into electrical energy that is immediatelyamplified by the detector's own internal gain, he said.

"Preamplifer noise becomes less relevant to system performance.Overall signal-to-noise is increased," Fagen said.

Research has not proceeded without setbacks. Fagen admits todisappointments as well as successes with the technology. He considersthe next nine months to be crucial to demonstrating the proofof concept.

"If it works, CTI is committed to buying large numbersof them to integrate into a future generation of PET scanners,"he said.

Advanced Photonix is a publicly traded company that makes custompin photodiodes. They are used as optical detectors in missile-guidancesystems, automotive cruise controls and airport baggage-inspectionsystems. The firm generated sales of $1.9 million in the quarterending July 2.

The $3 million private placement will be used to ramp up productionof the firm's first-generation large-area avalanche photodiodesand to support research activities, according to the company.