Infographics / Digital Radiography

Flat-Panel DR Detector Construction (TFT Active Matrix)

Two layered cross-sections. Left, indirect conversion: incident x-rays strike a caesium iodide scintillator that converts them to visible light, which spreads laterally down to an amorphous-silicon photodiode that makes charge, read out by a thin-film-transistor active matrix. Right, direct conversion: a metal top electrode and a kilovolt bias set a field across an amorphous-selenium photoconductor, which turns x-rays straight into charge that drifts down a narrow path to the same TFT array, giving less lateral spread and better resolution.

Indirect vs direct conversion flat-panel DR detector construction Two layered cross-sections. Left, indirect conversion: incident x-rays strike a caesium iodide scintillator that converts them to visible light, which spreads laterally down to an amorphous-silicon photodiode that makes charge, read out by a thin-film-transistor active matrix. Right, direct conversion: a metal top electrode and a kilovolt bias set a field across an amorphous-selenium photoconductor, which turns x-rays straight into charge that drifts down a narrow path to the same TFT array, giving less lateral spread and better resolution. Indirect conversion x-rays → light → charge Direct conversion x-rays → charge (no light) incident x-rays CsI:Tl scintillator columnar · x-rays → light a-Si photodiode light → charge metal top electrode + a-Se photoconductor x-rays → charge, directly E kV bias (≈10 V/µm) holes TFT active matrix a-Si:H · read one row at a time TFT active matrix a-Si:H · read one row at a time Each pixel = sensor + capacitor + TFT switch; the array is addressed one gate line at a time. Direct has no light-spread stage, so charge stays narrow → sharper image, better resolution.
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