Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, Varian, like some other flat-panel detector manufacturers, takes an indirect approach to creating electrical charge carriers. X rays first impinge upon scintillators made from either gadolinium oxysulfide or cesium-iodide. The scintillators absorb the X rays and convert them into visible light photons that then pass onto the photodiode array. Because cesium-iodide is such an excellent absorber of X rays, and converts them to visible light photons at energies that amorphous silicon is best able to convert to charge carriers, the combination of these two materials has the highest-rated Detective Quantum Efficiency (DQE) in use today. DQE is the yardstick by which the performance of photoconductors is measured. A high DQE rating means that superior images can be obtained with low dosages of X rays.

Real-time imaging
Varian’s PaxScan® flat-panel detectors can acquire high-resolution radiographs at up to seven frames per second (FPS) and moving fluoroscopic images at up to 30 FPS. The PaxScan 4030A also has been incorporated into Varian’s radiotherapy products, including its Acuity™ and PortalVision™ systems (see IMRT: Targeting Cancer) that enable radiation oncology teams to properly position patients, target tumors, and verify treatment accuracy. Acuity’s flat-panel imager, for example, can be used during brachytherapy procedures to image cancer patients, develop treatment plans, and precisely place radioactive isotopes within tumors.
      As a new technology, the flat-panel imagers remain relatively expensive compared to the more traditional X-ray imaging systems. However, the technology can be expected to become more cost-competitive as more users move to take advantage of the substantial cost and time savings offered by digital X-ray imaging.
      Varian’s engineers have been working with several imaging equipment manufacturers to incorporate flat-panel detectors into a variety of different imaging systems. The PaxScan panel has already been incorporated into commercially available systems for gastrointestinal and vascular diagnostic procedures. Varian’s flat-panel detectors are also being investigated for use in orthodontic applications. It is hoped that they will play a role during surgery by generating images that can help to guide doctors as they work. The flat-panel imagers also have potential as a noninvasive means of evaluating the structural integrity of bridges, rocket motors, and shipping containers, as well as the quality of multilayer microchips.
      New market opportunities for flat-panel detectors have grown significantly in the past year, according to Chuck Blouir, marketing manager for Varian’s imaging products. “We are still finding new applications for our digital flat-panel technology in medical, industrial, and homeland security markets where speed, image quality, and cost-efficiency are essential. I fully expect this to become the new standard in X-ray imaging.”

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