2011 Issue

42 age applied to them all. While the TFT switches are off, charge generated by light from the scintillator accumulates on the diodes. When readout is wanted, a row line is energized to turn on the switches in that row. The charge from all of the photodiodes in the selected row flows out through all of the data lines simultaneously. In large arrays: this pro- duces several thousand signals that must all be read at the same time. Varian has developed a custom 128-channel low- noise device with high charge capacity for this purpose to accommodate the wide dynamic range of amorphous silicon sensor panels. X-ray Conversion Methods Three common methods to conver t incoming x-rays into charge for elec- tronic readout can be implemented in amorphous silicon. They are the Intrinsic, the Photoconductor, and the Scintillator methods. Each method has its perfor- mance advantages and disadvantages and each has certain limitations on its use in practical x-ray imagers. In all three methods, the charge is accumulated for a frame period before being read out. Gamma cameras, in contrast, count each x-ray photon as it arrives. That technique is generally not used for x-ray imaging because the x-ray photon arrival rates are too high to permit counting. The Intrinsic Method Arriving x-rays are captured by the amorphous silicon diode where hole- electron pairs are generated. An applied bias separates the charge to prevent recombination. Because a charge pair is generated for about each 5 electron volts of x-ray energy, the signals are high. Un- fortunately, the x-ray absorption of silicon is very low so the photodiode needs to be 10 to 20 mm thick. Fabricating such devices of amorphous silicon is not fea- sible. Intrinsic devices have been made from crystalline silicon but only arrays of one or two lines are practical and even these are expensive. The Photoconductor Method Photoconductive materials with higher x-ray absorption than silicon can be coated on an array of conductive charge collection plates, each supplied with a storage capacitor. These also produce hole-elec tron pairs when x-rays are absorbed but the charge generated must be stored out of the layer to avoid lateral crosstalk. The applied field not only separates the charge but directs it towards the collector plate directly below to maintain image sharpness. Currently, the only photoconductor in production, selenium, also has relatively low x-ray ab- sorption and requires about 50 electron volts to produce a hole-electron pair. These restrict both the minimum dose needed and the size of the signal gener- ated. Other materials with lower energy requirements and higher x-ray absorp- tion are under development. The Scintillator Method A scintillator is a compound that absorbs x-rays and converts the energy to visible light. A good scintillator yields many light photons for each incoming x-ray photon; 20 to 50 visible photons out per 1 kV of incoming x-ray energy are typical. Scin- tillators usually consist of a high-atomic number material, which has high x-ray absorption, and a low-concentration activator that provides direct band transi- tions to facilitate visible photon emission. Scintillators may be granular like phos- phors or crystalline like cesium iodide. Structure of a phosphor scintillator Phosphors are materials which glow when exposed to x-rays. For maximum bright- ness, the phosphors used in x-ray imag- ing are made of rare-earth oxysulfides doped with other rare earths. The most common are gadolinium and lanthanum oxysulfides doped with terbium. These typically emit blue to green light which is well-matched to film sensitivity. Various grain sizes and chemical mixtures are used to produce a variety of resolution and brightness varieties. In use, these are mixed with a glue binder and coated on to plastic sheets. These were designed to be pressed against x-ray film to im- prove sensitivity but they may also be pressed against arrays of amorphous silicon photodiodes to make electronic x-ray detectors with sensitivity at least as good as that of film. Tens of electron volts are needed to produce each visible photon in a phosphor screen and x-ray absorption is good. Light scatter can be a problem if the layers must be thick to stop higher-energy x-rays. Structure of a cesium iodide scintillator For a better combination of resolution and brightness, cesium iodide is used. CsI has the useful property that it grows as a dense array of fine needles (10 to 20 micrometers in diameter) under the proper evaporation conditions. This produces crystals which act as light pipes for the visible photons generated near the input side of the layer allowing very thick (up to 1 mm) layers to be used with excellent retention of resolution. Because cesium has a high atomic number, it is an excellent x-ray absorber so this material makes very efficient use of the incoming continued on page 54