2011 Issue

54 x-rays. About 20-25 electron volts are needed to generate each light photon. When doped with thallium, CsI emits at about 550 nm, just at the peak of the spectral sensitivity of amorphous silicon. The combination of CsI and amorphous silicon has the highest DQE of all materials in production today. Real-time X-Ray Imaging Systems In the past, real-time x-ray imaging (fluoroscopy or radioscopy) has usually involved a television camera combined with a device to convert incoming x-rays into light visible to the camera. Until recently, cameras with image tubes were common but new systems use CCD models almost exclusively. CCDs (and other related solid-state imaging devices) have advantages over im- age tubes in stability, geometric accuracy, signal uniformity and size but these advantages are substantially surrendered when the x-ray conversion facility is added. Sensor-panel imagers bring these advantages back supplemented with a few of their own. The illustrations show why. CCD with X-ray Image Intensifier This combination provides real-time imaging with low x- ray flux over reasonably large areas. Geometric distortion and susceptibility to image burn are high. Because the intensifier relies on electron acceleration for gain, it is suscep- tible to external magnetic fields and requires high voltage. Lens-coupled CCD Since the optical collection efficiency of the lens is very low, this combination re- quires high flux or an inten- sified camera for real-time operation. The mirror moves the camera from the primary x-ray beam. Changing the field of view or the energy band is as easy as changing the converter screen. CCD with Fiber-optic Reducer This combination provides a simple solution for small areas. Geometric distor tion and uniformity are good. Various screens provide adaptability to various energy bands. At higher energies, a right-angle reducer may be needed to move the camera from the primary beam. Sensor-panel Imager The simplicity of this imager avoids most of the opportuni- ties for degradation in other imagers. Dynamic range, con- trast, and geometry are im- proved. Converter selection is provided. At higher energies, only the scan and read elec- tronics need be positioned out of the primary beam. These state of the art flat panel imaging devices have improved the health care and security of our society. The technology is still somewhat new and will continue to provide innovation to imaging in many fields and industries. It has already made its mark in improved visualization and identification of certain diseases and identification of harmful substances. Mr. Richardson graduated from the University of Utah with Mechanical Engineering and Engineering Administration degrees. He is a registered professional engineer. As an engineering manager at Varian Medical Systems,he has been actively involved in design of x-ray tubes over four decades, and holds nine patents. He is also active with the American Society of Mechanical Engineers. Imaging for a Better World — continued from page 42 Richardson