Direct vs. Indirect Conversion

Direct-readout electronic X-ray detectors can be divided into two classes. They either use a direct technique (direct-conversion detectors) or an indirect technique (indirect-conversion detectors) for converting X-rays into an electric charge. "Direct-conversion" should not be confused with "direct-readout", which is a capability of all electronic detectors.

• Direct-conversion detectors have an X-ray photoconductor, such as amorphous selenium, that directly converts X-ray photons into an electric charge.
• Indirect-conversion detectors, on the other hand, have a scintillator that first converts X-rays into visible light. That light is then converted into an electric charge by means of photodetectors such as amorphous silicon photodiode arrays or CCDs. Thin-film transistor (TFT) arrays may be used in both direct- and indirect-conversion detectors.

In both direct- and indirect-conversion detectors, the electric charge pattern that remains after the X-ray exposure is sensed by an electronic readout mechanism, and analog-to-digital conversion is performed to produce the digital image.

DR is a self-scanning, direct-readout X-ray image acquisition technology which makes use of CCD detectors or flat panel digital detectors based on amorphous selenium or amorphous silicon.

Direct-conversion flat panel detectors based on amorphous selenium

The selenium-based technology uses an amorphous selenium-coated thin-film-transistor (TFT) array to capture and convert X-ray energy directly into digital signals. It is a direct-conversion technology that converts X-ray beams into electric charges directly. No light-emitting materials as with CR systems, no intermediate steps or additional processes as with silicon-based systems or CCD systems are required to capture and convert the incident X-ray energy. Under a bias voltage applied across the detector structure, incident X-rays directly generate electron-hole pairs in the selenium layer. These charges are collected by individual storage capacitors associated with each detector element for readout by customized electronics within the array.

Indirect-conversion flat panel detectors based on amorphous silicon

A second type of digital sensor is based on amorphous silicon. This type of flat-panel sensor uses thin films of silicon integrated with arrays of photodiodes. These photodiodes are coated with a crystalline cesium iodide scintillator or a rare-earth scintillator (Terbium-doped Gadolinium dioxide sulfide). When these scintillators are struck by X-rays, visible light is emitted proportional to the incident X-ray energy. The light photons are then converted into an electric charge by the photodiode arrays. Unlike the selenium-based system, this type of indirect-conversion detector technology requires a two-step process for X-ray detection, i.e. the scintillator converts the X-ray beams into visible light, and light is then converted into an electric charge by photo-detectors, such as amorphous silicon photodiodes.

Indirect-conversion CCD detectors

The oldest indirect-conversion DR system is based on charge-coupled devices (CCDs). These arrays record light created by the impact of X-rays on a scintillation material, such as photostimulable phosphors. The most salient characteristic of CCDs is that they are relatively small - typically 2-4 cm2, which is much smaller than typical projected X-ray areas. Because of this, CCD-based systems require optical coupling to reduce the size of the projected visible light image and transfer the image to the face of one or more CCDs. Although CCD-based detectors require optical coupling and image demagnification, they are attractive because they are both widely available and relatively low in cost.