Computed Radiography (CR) system
Computed Radiography Image Receptors (CR IRs) are made by fixing them on portable or table and upright X-ray units. The CR IR has a cassette, which has an imaging plate (IP) attached to it. There is a phosphor on this IP which reacts with radiation coming through the patient. Some part of the energy absorbed by the phosphor is emitted in the form of visible light (Luminescence) and the remaining energy is stored in the phosphor as a latent image.
Image receptor (IR) is mainly made up of Support, phosphor and protective layer. It contains the main phosphor barium fluorohalide crystals which are dopped with europium. [BaFBr (85%) and BaFI (15%): Eu] Here is called europium activator, it is added in very small amounts, it is responsible for the photostimulable luminescence (PSL) property of phosphor. There are two stages of image acquisition in the CR imaging system:
1. Image capture in IP
2. Image readout
Image capture in IP
When the exit radiation from the patient falls on the cassette, the latent image is formed due to the radiation being absorbed by the phosphor. The activator causes defects in the crystal, which is called the F-center. These can trap the F-center electron. In this process, the first is the ionization of europium atom due to the photoelectric effect by absorbing energy from radiation and an electron is emitted in the valence band.
These electrons move from the valence band to the conduction band, whereby they trap in the F-center of the forbidden zone. At these F-centers, electrons can stay for a period of time. Therefore, in an exposure millions of electrons trap in the F-center. The number of these electrons is proportional to the absorbed radiation energy.
In the long run, these trapped electrons automatically come to the ground state. But by exposing the phosphor through a red light souce, the process of returning these electrons to the ground state can be increased. This is why it is called photo stimulable phosphor.
When the imaging plate is exposed by a red laser light, the F-center absorbs the energy and gives it to the electron. The electron reaches the conduction band from where it reaches the valence band and emits blue green light whose energy is more than red laser light. The electron connects to Eu+3 to form Eu+2.
All trapped electrons cannot be released from phosphor, so the IP is exposed by a very bright light source, which causes the most trapped electrons to be released. Now IP can be used again for radiation exposure.
imaging plate
Phosphor can be made as a flexible screen (0.5 mm) which is enclosed in a hard cassette called the imaging plate (IP). The first imaging plate was made by Fuji, Japan in 1983, which is similar to the screen-film cassette. Cassette is available in different sizes, with 14 "X17", 14 "X 14", 10 "X 10" and 8 "X 10" main. It is also available for different varieties like general radiography, mammography, dental radiography etc. The spatial frequency is 2-3 lp / mm for general radiography and 10 lp / mm for mammography.
