Charge-coupled device detector

Charge-coupled device (CCD) detectors are used in digital radiography for the indirect conversion of x-ray photons into an electric charge (indirect because the x-ray photons are first converted into light via a scintillating screen).

Structure

A charge-coupled device can either be an area array (square) or linear array (rectangular or 'one-dimensional'). It comprises discrete detector elements (dexels) that have a photosensitive area made of amorphous silicon, and an electronic component. The space taken up by these electronics results in decreased absorption efficiency as this portion of the dexel will not be photosensitive. Charge-coupled device chips are small, for example, 40-60 mm, when compared with a standard radiographic field of view, and the demagnification of the image contributes to the inefficiency of the system.

Fill factor

= (photosensitive area of detector element / total area of detector element) x 100

Function

The charge-coupled device requires optical coupling to focus the large field of view onto the small chip. This is not an efficient process and a large fraction of photons are lost. Each dexel releases and stores electrons when struck by light. Therefore at the end of an exposure, each dexel will have a given electric charge determined by how much light it received during the exposure.

There are charge amplifiers at the end of each column of the matrix that converts the charge to a voltage. The charges in the dexels move down the columns so that the matrix is read row by row. The information is then digitized to give a greyscale value for each corresponding pixel.