2011 Issue

41 T HESEDEVICESCAN also display hidden materials in containers that may otherwise be harmful to people. The quality policy at Varian Medical Systems: Through our dedicated people, Varian Medical Systems will provide qual- ity products and services that improve healthcare, and save lives. Among some of its products, Varian X-Ray Products in Salt Lake City produces a line of PaxS- canTM amorphous silicon flat-panel im- aging devices for the medical diagnostic and industrial inspection industries. Varian: Early Pioneers In the early 1990s, researchers at Varian’s Ginston Technology Center in California began development work to bring to- gether several new technologies combin- ing speed, image quality, compactness and ease of use. In 1998, Varian became the first and only company in the world to deliver amorphous silicon flat panel systems capable of both fluoroscopic and radioscopic digital imaging. Varian has continued to refine and improve our products and competencies in real time flat panel image receptor technology. Varian has over 20,000 receptors installed in various applications spanning the medical and industrial imaging markets. Real-time X-Ray Imaging Systems Varian’s PaxScan™ products meld the best in amorphous silicon sensor panels, radiation-converting materials, low-noise analog and high-speed digital electron- ics, custom ASIC control and processing electronics, and compact packaging. As new materials, processes, circuits and techniques emerge which promise improvements in performance or ex- tensions to new applications, they will become part of our imaging technology. Today, our technology includes: • Amorphous silicon sensor panels • X-Ray conversion • Real-Time Imaging Amorphous Silicon Sensor Panels The purpose of the sensor panel is to Imaging For A Better World John E Richardson PE Flat Panel imaging helps save lives. Accompanied with an x-ray tube system, flat panel imaging devices will display needed information to make life saving diagnosis on patients. State of the art x-ray imaging devices can detect some cancers 1 to 3 years before they can be felt. accumulate charge generated by the absorption of x-rays and to provide it row by row during scanning to the charge amplifiers. The charge storage device is a capacitor in photoconductor imagers or a photodiode in panels used with scintillators. The switch used to permit the charge to flow out can be a single diode, a diode pair or a thin-film transis- tor. All possible combinations of these storage devices can be made to work but each has a specific set of advantages and disadvantages. Varian products use the photodiode TFT combination because of its simplicity of use, commercial avail- ability and flexibility of design. Photomicrograph of an amorphous silicon sensor panel In the array pictured here, the switch is a thin-film transistor (TFT) much like the switches used in active-matrix liquid crys- tal displays. An important goal in panel design is maximizing the area of the im- ager that is taken up by the photodiode (a high fill factor ) so that a minimum amount of arriving light is wasted. The signals are carried by thin metal lines. The pixel center-to-center distance in this sensor panel is 127 microns and the fill factor is 35%. Panels with higher fill factor are used in the current Varian products. Circuit diagram of an amorphous silicon sensor panel In operation, the photodiodes are reverse-biased by an external volt-