Development focuses on amorphous silicon arraysAfter years of laboratory development, Varian Associates of PaloAlto, CA, has deemed amorphous silicon imaging ready for primetime, with a new imaging product group that will develop
After years of laboratory development, Varian Associates of PaloAlto, CA, has deemed amorphous silicon imaging ready for primetime, with a new imaging product group that will develop digitalx-ray/fluoroscopy imaging components based on the technology forOEM customers.
The new imaging products group was established Oct. 1, thestart of Varian's fiscal year, with the intention of possiblyfabricating its first prototype for viewing at the RadiologicalSociety of North America conference in December. The group ispart of Varian's healthcare systems division, which also managesits x-ray tube and radiation oncology product lines.
David Gilblom, formerly general manager of Sierra Scientific,was hired away from the digital equipment division of Picker Internationalto launch Varian's new product line. Four engineers and a technicianwho were working on amorphous silicon wafer technology at Varian'sEdward L. Ginzton Research Center were assigned to the new group.Components will be built at Varian's contract manufacturing facilityin Tempe, AZ. A sales and marketing team will be recruited thisfall.
The new group's mission, according to Gilblom, is to fill thegap between R&D companies that are perfecting amorphous silicontechnology and OEM vendors that plan to integrate components builtby Varian into future x-ray/fluoroscopy equipment designs.
"The news is that here's a longstanding laboratory programthat's ready for the market, so we're going to turn this intoa real business," he said.
On the R&D side, Varian has established nonexclusive supplyrelationships with dpiX, also of Palo Alto, and Optical ImagingSystems in Troy, MI, two of the three companies that manufactureamorphous silicon panels. Gilblom has also talked with the thirdfirm, EG&G Amorphous Silicon, in Santa Clara, CA, about supplyingpanels.
DpiX was organized in March by R&D lab Xerox PARC (PaloAlto Research Center) to drum up commercial demand for thin-filmtransistor (TFT) array technology (SCAN 3/13/96). A dpiX arrayconsists of a glass substrate on which is deposited a layer ofamorphous silicon, a semiconductor material that acts as a thin-filmtransistor switching element. The arrays can be customized foreither image-display or image-sensor applications.
Varian began working on amorphous silicon detector technologyin 1990, Gilblom said. Its efforts include an ongoing contractwith the Department of Defense's Advanced Research Projects Agency(ARPA) to develop a prototype of a diagnostic imaging system basedon amorphous silicon. Xerox PARC developed the detector panelsfor the project, while Varian contributed associated electronics.
Changing equipment design. Varian's component product lineitself will be developed over time to meet the specificationsof its undisclosed OEM customers. Digital x-ray upgrades for imageintensifiers and video cameras already in the field are an obviousmarket opportunity, according to Gilblom, but he is also lookingforward to working with OEM designers to explore how the flat-paneldetectors can alter digital R/F equipment design.
"Amorphous silicon is lighter and more compact than imageintensifiers or film cassettes, so you can change the imaginggeometry, alter patient positioning, and lighten the superstructure,"he said.
The amorphous silicon array technique overcomes inherent problemsthat have bedeviled charge-coupled device development, Gilblomsaid. Only two CCD designs have shown promise in providing arrayslarge enough for general radiological applications, accordingto Gilblom. One technique employs fiber-optic reducers that produce4 x 4-inch field-of-views large enough for breast biopsy. Expansionto larger formats is prohibitively expensive, he said.
Other designers, such as the engineers who designed Swissray'sAddOn-Bucky, joined an optical lens with the CCD array and a fluorescentscreen (SCAN 12/13/95). They demonstrated the practicality ofusing multiple CCDs, but it is probably not possible to collectenough light with a lens configuration to run at the quantum limit,where the signal-to-noise ratio is limited by the x-ray exposurerather than the collection efficiency of the CCD optics, Gilblomsaid.
Amorphous silicon devices can be much larger than CCD arrays,because the designer is basically working with a thin layer oftransistorized image detectors deposited on a sheet of glass,Gilblom said. Japanese electronics manufacturer NEC, for example,has shown active-matrix, liquid-crystal computer displays fabricatedfrom amorphous silicon that are as large as 21 inches in diameter,Gilblom said. The glass for some of these advanced devices is23 x 31 inches.
"You still have yield problems, and it tends to get kindof expensive, but you don't face technological limits that wouldprohibit you from eventually building a 14 x 17-inch device,"he said.
Varian's prototype will be a panel assembly with an 8 x 10-inchactive area generating a field-of-view slightly larger than thatproduced by a 9-inch image intensifier, Gilblom said. The unitwill be connected to a computer that controls the operating modesof the panel. It will generate real-time digital data that canbe channeled by Varian's OEM customers into their imaging processingelectronics.
"We've run all the tests we need to know that all thenumbers work out," Gilblom said. "It is now a matterof putting the pieces of equipment together."