Computed Radiography (CR) is a foundational digital imaging method within X-ray technology. It functions as a two-step process, utilizing specialized plates to capture X-ray energy and then converting that stored energy into a digital picture. CR served as a technological bridge, allowing healthcare facilities to transition from older, film-based systems to digital workflows without completely replacing their existing X-ray equipment.
How Computed Radiography Captures Images
The physical component responsible for initial image capture in CR is the Imaging Plate (IP), which is housed inside a cassette that looks much like a traditional film cassette. This plate is coated with a layer of photostimulable phosphors (PSP), most commonly barium fluorohalide compounds. When the plate is exposed to X-rays, the energy from the radiation excites electrons within the phosphor layer.
These excited electrons are then trapped within the crystal lattice of the phosphor material, holding the X-ray image information as a kind of stored energy. This stored energy pattern is known as the latent image. Because the cassette system is similar, CR allows existing X-ray machines to be retrofitted for digital imaging without modification.
The Processing Workflow
After the X-ray exposure, the cassette containing the latent image on the Imaging Plate must be taken to a specialized machine called a CR reader. A finely focused, high-intensity laser beam is directed across the plate in a raster pattern.
The energy from the laser stimulates the trapped electrons within the phosphor layer, causing them to immediately release their stored energy as visible light. This emitted light, often violet or blue in color, has an intensity directly proportional to the original X-ray exposure received at that point on the plate. A highly sensitive light detector, known as a photomultiplier tube (PMT), captures this light. The PMT converts the light signal into a proportional electrical current, which an Analog-to-Digital Converter (ADC) then turns into discrete digital values, forming the final digital image. Once the entire plate is scanned, the plate is exposed to a bright white light to erase any residual energy, making it ready for immediate reuse.
CR vs. Traditional Film Radiography
Computed Radiography advanced traditional film-screen radiography by eliminating the need for wet chemical processing. Film-based systems required a darkroom, specialized chemicals, and a multi-step developing process, which was time-consuming and labor-intensive. CR digitized this process.
Unlike film, which is single-use, the photostimulable phosphor plates in CR systems are reusable. This reusability significantly reduced the long-term cost of consumables and minimized the environmental impact associated with chemical disposal. CR images are immediately available for digital viewing on a monitor, which allows for post-processing adjustments that were impossible with film. Technologists can digitally manipulate image brightness, contrast, and magnification, which can enhance diagnostic details and often prevents the need for a repeat X-ray exposure.
Common Uses and Current Role
While CR systems were once the standard for digital X-ray imaging, they have largely been superseded by newer Direct Radiography (DR) technology in major hospitals and high-volume centers. However, CR continues to hold a significant place in the modern imaging landscape due to its versatility and cost-effectiveness. The technology’s cassette-based nature allows it to be easily integrated into existing X-ray rooms without expensive modifications to the generator or table.
Today, CR systems are frequently found in smaller clinics, private practices, and mobile X-ray units where portability is a priority. They are also a common choice in developing countries or remote locations where a lower initial investment is a primary consideration. Although CR requires an extra step of manually taking the cassette to the reader, making it slower than the nearly instantaneous image capture of DR, it remains an affordable digital solution for various clinical applications, including general orthopedic and chest X-rays.