Digital Radiography uses X-ray digital detectors instead of traditional film or Phosphor Plates (also known as Computed Radiography or CR). DR yields immediate and superior quality X-ray images at minimum time on target, with minimal radiation levels. A DR system is comprised from a control unit (typically a tablet) that acts as the major display, a source (either x-ray or gamma) and a chaser or detector that goes behind the object we want to scan.
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.