MEDICAL IMAGING

Digital imaging methods using a variety of sensor technologies are maturing rapidly and could replace current phosphor screen/film-based imaging. In a promising approach under development at Nanoptics Inc. (Gainesville, FL), a novel plastic fiberoptic bundle acts as a scintillator screen. This scintillating fiber plate converts x-rays into visible photons, which are imaged onto a charge-coupled-device (CCD) camera.

MEDICAL IMAGING

Scintillating fiber plates aid x-ray imaging

Digital imaging methods using a variety of sensor technologies are maturing rapidly and could replace current phosphor screen/film-based imaging. In a promising approach under development at Nanoptics Inc. (Gainesville, FL), a novel plastic fiberoptic bundle acts as a scintillator screen. This scintillating fiber plate converts x-rays into visible photons, which are imaged onto a charge-coupled-device (CCD) camera.

The ability to achieve better resolution, near-real-time imaging, and lower x-ray exposures are the major benefits that the new digital x-ray technologies offer over screen/film combinations. One area drawing a lot of attention is digital mammography. Next year, new systems that produce large-area, digital images are expected from Fischer Imaging (Denver, CO), Lorad (Danbury, CT), and Bennett X-Ray Technologies (Copiague, NY).

While these systems are being developed in a variety of configurations, all have a similar technical approach for acquiring a digital image. As with screen/film combinations, these first-generation digital systems still use a phosphor screen to convert x-rays into photons.These photons are then collected and imaged onto a high-resolution CCD by means of a lens or a glass fiberoptic taper. The fast read-out CCD converts the photons into an electrical signal that is displayed as an x-ray image on a monitor.

However, phosphor screens can limit performance of these systems by reducing contrast, signal-to-noise ratio, and image resolution. When an x-ray is absorbed in the scintillating material, the light it produces disperses, spreading light to adjacent pixels. Thinner phosphor screens can be used, but their lower absorption efficiency, due to the reduced thickness, can mean higher x-ray exposures for the patient. The addition of dyes helps preserve absorption efficiency and reduce lateral spreading, but because of increased quantum-dependent noise, it sharply lowers system sensitivity at high spatial frequencies. In addition, the long decay time prohibits use of these phosphors in fast-scanning applications.

The scintillating fiber plate (SFP) developed at Nanoptics is designed to overcome some of the shortcomings of conventional (rare-earth) phosphor screens. Millions of plastic optical microfibers are used to construct the SFP (see figure). The fibers are doped with tin to enhance x-ray absorption and a scintillating dye that emits light at about 550 nm. The visible light is then guided by the same microfibers to a CCD. This structure eliminates lateral spreading of the light, leading to much better image resolution and contrast. The SFP can also be thick to optimize absorption efficiency while maintaining high spatial resolution. In addition, the dye decay time is only a few nanoseconds, permitting its use in fast scanning applications.

Conventional scintillators have about a 12% conversion efficiency (x-rays to photons), but SFPs only have about 6%. That is still about twice as good as previous plastic scintillators have achieved. To increase the number of generated photons that exit the SFP, other features have been incorporated. For example, a reflective coating on the x-ray input side redirects photons back toward the CCD, thereby almost doubling the light yield. Such an option is not feasible with conventional phosphor scintillators because they are opaque to background light. The result is that the number of photons arriving at the CCD from the SFP and from conventional systems is quite similar.

Resolution on increase

Nanoptics president Dr. James Walker says that he hopes to show x-ray images with a prototype SFP at November`s Radiological Society of North America show in Chicago, IL. "We expect to achieve a detector quantum efficiency (DQE, an overall measure of device quality) of about 80%. Screen/film combinations have a DQE of about 10%-30% and first-generation digital mammography systems are 55%-70%," says Walker. "Our resolution will be at least 16 line pairs per mm (lp/mm), but could go as high as 20 lp/mm."

By next summer, Nanoptics hopes to integrate eight modules with a long slot detector that will be compatible with the Fischer Imaging slot-scanning system. This system mechanically scans a 20-cm detector over a 30-cm range to acquire a 8000 ¥ 10,000-pixel full-breast image. "We hope these tests prove the benefits of our design so that it can form the basis for second-generation digital x-ray mammography systems," adds Walker.

Chris Chinnock

CHRIS CHINNOCK is a technical writer based in Norwalk, CT.

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