Direct digital radiography

Last revised by Dr Jeremy Jones on 20 Sep 2021

Direct digital radiography (DDR) refers to direct digital registration of the image at the detector with no intermediate processing step required to obtain the digital signals as in computed radiography (CR).

There are two primary methods of conversion, either indirect or direct:

Indirect conversion

Indirect conversion is so named because this technique still uses a scintillator to convert x-rays to light before conversion to an electrical charge for subsequent readout.

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

Direct conversion

Direct conversion is so named because this technique directly converts the absorbed x-ray into a proportionally sized electrical charge with no intermediate scintillating step.

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) 1,2.

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