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Xerox uses new silicon technologyto develop x-ray digitization sensors


Company will offer toolkit to developersA new solid-state detector technology under development at Xerox'sPalo Alto Research Center (PARC) in Palo Alto, CA, could providea means of building large-area image sensors for medical

Company will offer toolkit to developers

A new solid-state detector technology under development at Xerox'sPalo Alto Research Center (PARC) in Palo Alto, CA, could providea means of building large-area image sensors for medical imagingapplications. The sensors could be an improvement over existingtechnology used to digitize conventional x-ray film.

A number of medical imaging vendors are developing real-timedigital imaging systems to replace conventional x-ray film inapplications such as full-field-of-view digital mammography. Thesecompanies, however, have been forced to work around the size limitationsof digital sensors because sensors based on single-crystal siliconwafers are limited to sizes no larger than about 6 inches in diameter.Some companies tile together images from smaller sensors, whileothers use line-scanning techniques to provide a full-field digitalimage. Xerox has dispensed with the silicon wafer approach infavor of a method that allows large-area x-ray sensors to be fabricatedon a single glass substrate.

Xerox plans to fabricate image sensors on a newly establishedamorphous-silicon manufacturing line, which is used primarilyto make active-matrix liquid crystal displays. Major Japaneseelectronics manufacturers dominate the market for these displays,which are found in laptop computers. Leveraging the rapidly maturingmanufacturing capabilities in this industry could help spur developmentof image sensors for medical applications as well, providing ameans for eventually producing 14 x 17 image sensors.

To promote diffusion of its technology among product developers,Xerox PARC has developed an evaluation toolkit of its large-areax-ray image technology. The toolkit uses thin 7.7 x 9.6 inch solid-statesensors, although in the future Xerox will be able to make muchlarger sensors. The tool kit will include the sensor array, read-outcharge amplifiers, analog-to-digital (A/D) electronics, and ascintillation screen, which is used to convert x-rays to visiblefluorescence for imaging by the sensor. Xerox hopes to begin offeringthe toolkit by the end of the year.

Based on an internal Xerox design, the sensor will feature a1536 x 1920 pixel array arranged on a 127-micron pitch. The arrayplaces a transistor and photodiode at each pixel site, whereasdisplays only need a transistor at each pixel. The photodiodeconverts incoming light to an electrical signal.

Off-screen charge amplifiers will read out the signal and an8-bit A/D converter will provide 256 gray levels. The image canbe read out at 30 frames per second.

"The resolution is good enough for many diagnostic applications,"said Andy Anderson, product marketing manager at the electronicsand imaging lab at PARC. "Our aim is to get these kits intothe hands of potential developers so that we can determine thebest configurations for specific applications and products."

Resolution is everything. The resolution of the image sensoris a key determinant for its use in specific applications. Forexample, an image sensor providing a resolution of 2 line pairsper millimeter (about a 500-micron pitch), can be used for imagingduring high-intensity radiation therapy, improving the resolutionover alternative technologies, such as ion chambers. At somewhathigher resolutions, the image sensor may be used to replace fluoroscopicsensors or film that is used for imaging tumor locations.

Researchers at the University of Michigan in Ann Arbor are workingwith Xerox on this application. Fluoroscopy is an area where solid-statesensors could replace x-ray image intensifier tubes. These systemsuse a large-area phosphor screen to convert x-rays to electrons,which are accelerated, de-magnified, and imaged onto a CCD (charge-coupleddevice) camera. They are large, bulky, and require high voltage.Solid-state sensor arrays based on amorphous silicon are flat,require much less power, and should provide comparable or betterperformance.

The third area where Xerox sees uses for these sensor arraysis in conventional radiography. The idea is to replace film witha low-cost method for producing digital x-rays. Today, digitalx-rays can be produced in several ways, such as by digitizingconventional x-ray films with a laser or CCD scanner or usingcomputed radiography to expose the x-ray image onto a photoconductorplate. However, film scanners are slow, while computed radiographysystems are expensive, starting at $150,000. Amorphous siliconimage sensors could be a lower cost option.

The most demanding radiographic application is mammography. Debateas to the resolution needed in digitized x-ray images has centeredon a pixel measuring 50 microns as adequate. Anderson notes thatPARC has achieved a 90-micron pixel in its advanced display products,and although the flat-panel display processes are mature, producinga 50-micron pixel is still a challenge for large-area sensors.

Another area of concern for end users is the ability to supportthe 14 x 17 film format common in medical imaging. The glass substratesused for today's flat panel displays can accommodate displaysthat large, so image sensor arrays could also be produced. Butlarge-area sensors offer more opportunities for manufacturingdefects in image quality, and manufacturing processes must beperfected to eliminate defects, Anderson said.

But perhaps the most important near-term emphasis is on the optimizationof the readout electronics. Here the goal is to find the bestcharge readout scheme to improve the image signal-to-noise ratio.

Other technical approaches to solid-state x-ray image sensorsare under investigation. For example, x-ray sensors made fromsilicon wafers have been demonstrated, but their small size makesthem more suitable for small-area imaging. Larger area imagersare possible, but require expensive tiling of multiple arraysor fiber-optic tapers.

In addition to Xerox, EG&G Reticon of Sunnyvale, CA, is alsoworking on a large-area amorphous-silicon array, while LittonSystems Canada of Etobicoke, Ontario, is building sensors madewith selenium. Du Pont of Wilmington, DE, is working on a photoconductorapproach that eliminates the use of a phosphor screen, convertingx-rays directly to electrical signals (SCAN 2/15/95). Expect tosee a variety of new products over the next few years targetedfor specific applications.

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