Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulation with visible light, to produce a luminescent signal. X-rays may induce such an energy storage. A plate based on this mechanism is called a photostimulable phosphor (PSP) plate and is one type of X-ray detector used in projectional radiography. Creating an image requires illuminating the plate twice: the first exposure, to the radiation of interest, "writes" the image, and a later, second illumination (typically by a visible-wavelength laser) "reads" the image. The device to read such a plate is known as a phosphorimager (occasionally spelled phosphoimager, perhaps reflecting its common application in molecular biology for detecting radiolabeled phosphorylated proteins and nucleic acids).
Structure and mechanism
On photostimulable phosphor (PSP) plates, the phosphor layer is typically 0.1 to 0.3 mm thick. After the initial exposure by short-wavelength (typically, X-ray) electromagnetic radiation, excited electrons in the phosphor material remain 'trapped' in 'colour centres' ("F-centers") in the crystal lattice until stimulated by the second illumination. For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5 micrometers, and is described as "barium fluorobromide containing a trace amount of bivalent europium as a luminescence center". Europium is a divalent cation that replaces barium to create a solid solution. When Eu2+ ions are struck by ionizing radiation, they lose an additional electron to become Eu3+ ions. These electrons enter the conduction band of the crystal and become trapped in the bromine ion empty lattice of the crystal, resulting in a metastable state that is higher in energy than the original condition.
Energy release and digitalization
A lower-frequency light source that is insufficient in energy to create more Eu3+ ions can return the trapped electrons to the conduction band. As these mobilized electrons encounter Eu3+ ions, they release a blue-violet 400 nm luminescence. This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal. It can be collected often by a photomultiplier tube, which is clocked at a specific resolution or pixel capture frequency. The light is thereby converted to an electronic signal and significantly amplified. The electronic signal is then quantized via an ADC to discrete (digital) values for each pixel and placed into the image processor pixel map.
Afterwards, the plates can be "erased," by exposing the plate to room-intensity white light. Thereby, the plate can be used over and over again. Imaging plates can theoretically be re-used thousands of times if they are handled carefully and under certain radiation exposure conditions. PSP plate handling under industrial conditions often results in damage after a few hundred uses. Mechanical damage such as scratches and abrasions are common, as well as radiation fatigue or imprinting due to high energy applications. An image can be erased by simply exposing the plate to a room-level fluorescent light - but more efficient, complete erasure is required to avoid signal carry-over and artifacts. Most laser scanners automatically erase the plate (current technology uses red LED lighting) after laser scanning is complete. The imaging plate can then be re-used.
Reusable phosphor plates are environmentally safe but need to be disposed of according to local regulations due to the composition of the phosphor, which contains the heavy metal Barium.
Medical X-ray Imaging
Projectional radiography using a photostimulable phosphor plate as an X-ray detector can be called "phosphor plate radiography" or "computed radiography" (not to be confused with computed tomography which uses computer processing to convert multiple projectional radiographies to a 3D image).
The imaging plate is housed in a special cassette and placed under the body part or object to be examined and the x-ray exposure is made. The imaging plate is then run through a special laser scanner, or CR reader, that reads and converts the image to a digital radiograph. The digital image can then be viewed and enhanced using software that has functions very similar to other conventional digital image-processing software, such as contrast, brightness, filtration and zoom. CR IPs can be retrofitted to existing exam rooms and used in multiple x-ray sites since IPs are processed through a CR reader (scanner) that can be shared between multiple exam rooms.
Differences from Direct Radiography
PSP plate radiography is often distinguished from Direct Radiography (DR). Direct radiography usually refers to image capture onto an amorphous silicone or selenium plate, the data being directly passed electronically to the processing computer. PSP plate radiography instead uses a cassette with the plate, which stores the image until it is loaded into the computer.
PSP plate radiography and DR should not be confused with fluoroscopy, where there is a continuous beam of radiation, and the images appear on the screen like on a TV. This is the system many people are familiar with, where the image of the article being x-rayed is viewed in real time on a monitor or display. Many people think airports use fluoroscopes for baggage inspection, when in fact LDAs (Linear Diode Arrays) are universally used to generate static images of luggage content. LDAs are also used in a wide variety of other screening and imaging applications, and are also presented in a digital format. Fluorosopes until recently have used a device called an image intensifier to enhance the analog output of the real time x-ray image from a fluorescent screen, viewing the I.I output with a video or CCD camera and digitally enhancing the video to reduce the noise inherent in the system; the latest fluroscopes now use flat detectors read at up to 60 frames per second to yield a real-time image via a dedicated imaging computer.
Image plates were pioneered for commercial use by Fuji in the 1980s.