Digital X-Rays
Digital radiography (DR) is an advanced form of x-ray inspection which produces a digital radiographic image instantly on a computer. This technique uses x-ray-sensitive plates to capture data during object examination, which is immediately transferred to a computer without the use of an intermediate cassette. The incident x-ray radiation is converted into an equivalent electric charge and then to a digital image through a detector sensor.
Compared to other imaging devices, flat panel detectors, also known as digital detector arrays (DDAs) provide high-quality digital images. They can have a better signal-to-noise ratio and improved dynamic range, which, in turn, provides high sensitivity for radiographic applications.
Flat panel detectors work on two different approaches, namely, indirect conversion and direct conversion. Indirect conversion flat panel detectors have a scintillator layer which converts x-ray photons to photons of visible light and utilises a photodiode matrix of amorphous silicon to subsequently convert the light photons into an electrical charge. This charge is proportional to the number and energy of x-ray photons interacting with the detector pixel and therefore the amount and density of the material that has absorbed the x-rays.
Direct conversion flat panel detectors use a photoconductor like amorphous selenium (a-Se) or Cadmium telluride (Cd-Te) on a multi-micro electrode plate, providing the greatest sharpness and resolution. The information on both types of detectors is read by thin film transistors.
In the direct conversion process, when x-ray photons impact the photoconductor, like amorphous Selenium, they are directly converted to electronic signals which are amplified and digitised. As there is no scintillator, the lateral spread of light photons is absent here, ensuring a sharper image. This differentiates it from indirect construction.
When a flat panel detector is coupled with an appropriate manipulator and image processing software, it can be used for x-ray Computed Tomography (CT) inspection, producing a 3D image of the external and internal structure of the test object.
Another digital x-ray detection media type is linear detector arrays (LDAs). These consist of a single row of x-ray detection pixels, rather than a matrix. The LDA or the object under inspection must move relative to each other to formulate a 2D radiographic image. LDAs are suited to the inspection of objects moving on a conveyor belt.
