Flat panel detectors Flat panel detectors (FPDs) are the most common kind of direct digital detectors. They are classified in two main categories: 1.
Indirect FPDs Amorphous silicon (a-Si) is the most common material of commercial FPDs. Combining a-Si detectors with a
scintillator in the detector’s outer layer, which is made from
caesium iodide (CsI) or
gadolinium oxysulfide (Gd2O2S), converts X-rays to light. Because of this conversion the a-Si detector is considered an indirect imaging device. The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal. The digital signal is then read out by
thin film transistors (TFTs) or fiber-coupled CCDs. 2.
Direct FPDs. Amorphous
selenium (a-Se) FPDs are known as “direct” detectors because X-ray
photons are converted directly into charge. The outer layer of the flat panel in this design is typically a high-voltage
bias electrode. X-ray photons create
electron-hole pairs in a-Se, and the transit of these electrons and holes depends on the potential of the bias voltage charge. As the holes are replaced with electrons, the resultant charge pattern in the selenium layer is read out by a TFT array, active matrix array, electrometer probes or microplasma line addressing.
Other direct digital detectors Detectors based on
CMOS and
charge-coupled device (CCD) have also been developed, but despite lower costs compared to FPDs of some systems, bulky designs and worse image quality have precluded widespread adoption. A high-density line-scan solid state detector is composed of a photostimulable barium fluorobromide doped with europium (BaFBr:Eu) or caesium bromide (CsBr) phosphor. The phosphor detector records the X-ray energy during exposure and is scanned by a laser diode to excite the stored energy which is released and read out by a digital image capture array of a CCD.
Phosphor plate radiography Phosphor plate radiography resembles the old analogue system of a light sensitive film sandwiched between two x-ray sensitive screens, the difference being the analogue film has been replaced by an imaging plate with photostimulable phosphor (PSP), which records the image to be read by an image reading device, which transfers the image usually to a
picture archiving and communication system (PACS). (not to be confused with
computed tomography which uses computer processing to convert multiple projectional radiographies to a
3D image). After X-ray exposure the plate (sheet) is placed in a special scanner where the
latent image is retrieved point by point and digitized, using
laser light scanning. The digitized images are stored and displayed on the computer screen. Since there is no physical printout, and after the readout process a digital image is obtained, CR has been known as an indirect digital technology, bridging the gap between x-ray film and fully digital detectors. ==Industrial usage==