Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Flat panel detector systems make use of technology similar to that used in a laptopcomputer display, and much of this has to do with wiring the huge number of individualdisplay elements. Instead of producing individual electrical connections toeach one of the elements in a flat panel display, a series of horizontal and verticalelectrical lines are used which, when combined with appropriate readout logic, canaddress each individual display element. With this approach only 2,000 connectionsbetween the imaging plate and the readout electronics are required for a 1,000X 1,000 display, instead of 1,000,000 individual connections. For a flat panel display,the wiring is used to send signals from the computer graphics card to eachdisplay element, whereas in a detector the wiring is used to measure the signal generatedin each detector element.Indirect Detection Flat Panel SystemsIndirect flat panel detectors are sensitive to visible light, and an x-ray intensifYingscreen (typically GdzOz5 or CsI) is used to convert incident x-rays to light, whichis then detected by the flat panel detector. The term "indirect" comes from the factthat x-rays are absorbed in the screen, and the absorbed x-ray energy is then relayedto the photodetector by visible light photons. This indirect detection strategy isanalogous to a screen-film system, except that the electronic sensor replaces thelight-sensitive film emulsion. Dual-emulsion film is thin and x-rays penetrate it easily;therefore, in a screen-film cassette, the film is sandwiched between screens toreduce the average light propagation path length and improve spatial resolution.Flat panels are thicker than film and do not transmit x-rays well; consequently, asandwiched design is not possible. Instead, the intensifYing screen is layered on thefront surface of the flat panel array. This means that the light emanating from theback of the intensifYing screen strikes the flat panel, and much of the light that isreleased in the screen has to propagate relatively large distances through the screen,which results in more blurring. To improve this situation, most flat panel detectorsystems for general radiography use CsI screens instead of GdzOz5. CsI is grown incolumnar crystals (as described in Chapter 9), and the columns act as light pipes toreduce the lateral spread of light.A typical configuration for a flat panel detector system is shown in Fig. 11-7.The flat panel comprises a large number of individual detector elements, each onecapable of storing charge in response to x-ray exposure. Each detector element hasa light-sensitive region, and a small corner of it contains the electronics. Just beforeexposure, the capacitor, which stores the accumulated x-ray signal on each detectorelement, is shunted to ground, dissipating lingering charge and resetting thedevice. The light-sensitive region is a photoconductor, and electrons are released inthe photoconductor region on exposure to visible light. During exposure, chargeis built up in each detector element and is held there by the capacitor. After exposure,the charge in each detector element is read out using electronics as illustratedin Fig. 11-8.A transistor is a simple electronic switch that has three electrical connectionsthegate, the source, and the drain. Each detector element in a flat panel detectorhas a transistor associated with it; the source is the capacitor that stores the chargeaccumulated during exposure, the drain is connected to the readout line (vertical