Flat panel detector

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Flat panel detectors (FPD) are used in direct digital radiography (DDR) for the conversion of x-rays to light (indirect conversion) or charge (direct conversion) which is read out using a thin film transistor (TFT) array.

Types

indirect conversion FPDs

  • outermost layer is scintillator: phosphor screen (cesium iodide or gadolinium oxysulfide)
  • detector: based on amorphous silicon photodiode
  • convert x-rays to light and then to charge (hence called indirect FPDs)

direct conversion FPDs

  • outermost layer: high voltage bias electrode
  • detector: based on amorphous selenium 
  • convert x-rays to charge directly (hence called direct FPDs)

It is important to note that indirect conversion can also be detected using charge-coupled devices (CCDs). These are small and require optical coupling.