Flat panel detector

Last revised by Henry Knipe on 21 Nov 2023

Flat panel detector (FPD) is the most common detector type used in direct digital radiography (DR). The x-rays are converted to electrical charges, either directly or indirectly (x-rays first converted to visible light, then to charges). The charges are then read out using a thin film transistor (TFT) array.

TFT active-matrix array is consisting of a transistor, charge collector electrode, and a storage capacitor with amorphous silicon substrate 2.


Indirect flat panel detector

Construction of an indirect flat panel detector:

  • scintillator layer made of cesium iodide (CsI)

  • photodiode made of amorphous silicon (a-Si)

  • TFT readout array

X-ray photons encounter a cesium iodide (CsI) scintillator and are converted to visible light. The needle-like CsI structure acts to minimize scatter at this step. The light then reaches a low-noise amorphous silicon (a-Si) photodiode array where it is converted into an electrical charge. Each photodiode represents a single pixel, and each produces an electrical charge that is read out digitally by the TFT array layer underneath before finally being sent to the image processor 1.

A similar process of coupling a scintillator with a visible light detector can be implemented using charge-coupled device (CCD) or complementary metal–oxide–semiconductor (CMOS) detectors. These are not classified as flat panel detectors. Their implementation is limited due to restricted size of the detectors 4,5.

Direct flat panel detector

Construction of a direct flat panel detector:

  • high voltage electrode

  • photoconductor layer made of amorphous selenium (a-Se)

  • TFT readout array

This technique employs a semiconductor material which produces electron-hole pairs in proportion to the incident x-ray intensity. The most commonly used semiconductor is amorphous selenium (a-Se) 1,3.

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