A field effect transistor (FET) or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor ( Figure 5-2). Each pixel contains a photodiode that absorbs the electrons and generates electrical charges. The TFT is a photosensitive array made up of small (about 100 to 200 µm) pixels, also called a detector element (del) in these TFT arrays. This material absorbs x-rays and converts them to electrons, which are stored in the TFT detectors ( Figure 5-1). The flat-panel detector has a radiation-conversion material or photoconductor, typically made of amorphous selenium (a-Se) that is about 500 µm thick for radiography and 200 to 250 µm thick for mammography. In direct conversion, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The choice of x-ray absorption material determines whether the detector is direct conversion or indirect conversion. The 2-D array functions as a very efficient device that measures x-ray absorption rather than counting or measuring the actual x-rays. The absorption material is attached to the surface of this array either electrically, as in the case of the photoconductor, or physically, as in the case of the phosphor material. An AMFPI detector measures the response of these materials to x-ray absorption and is a large area two-dimensional (2-D) array of pixels fastened to a thin glass backing, or substrate. Scintillators are phosphors that produce light when absorbing x-rays. Photoconductors are materials that absorb x-rays, resulting in an electrical charge.
The two main types of x-ray absorption materials currently being used are photoconductors and scintillators. This chapter will explore these devices and differentiate between the two major types of x-ray signal conversion: indirect and direct.Īctive-matrix flat-panel imagers (AMFPI) consist of a flat-panel array with an x-ray absorption material. With flat-panel detectors, the materials used for detecting the x-ray signal and the sensors for recognizing that signal are permanently enclosed inside a rigid protective housing. Today these detectors are no longer permanent fixtures in these two pieces of equipment but can be used portably as a wireless device. These were the first devices to move beyond the cassette into detectors that would reside in the table and wall stand. The first thin-film transistor (TFT) flat-panel amorphous silicon and amorphous selenium detectors were introduced in 1995. Describe the cause of image lag and the process of correcting.Relate the design of a TFT flat-panel detector to its performance measurements.Describe a thallium doped cesium iodide (CsI) detector.Describe a gadolinium oxysulphide (Gd 2O 2S) detector.Differentiate between direct and indirect image capture.Describe the construction of direct and indirect TFT flat-panel detector systems.Define a thin-film transistor (TFT) flat-panel digital image detector.
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