Computed radiography (CR) sometimes called “film replacement technology” uses cassette-based phosphor storage plates (Imaging Plates or IP). When the IP is exposed to X-ray radiation the phosphor layer in the plate stores the image. The IP is then scanned and converted to a digital signal which is transferred to a computer for processing and display. Digital radiography (DR) systems use active matrix flat panel detectors based on direct or indirect conversions of X-rays to charge, the image is converted to digital data in real-time and is available for review within seconds.
While both CR and DR have a wider dose range and can be post processed to eliminate mistakes and avoid repeat examinations, DR has some significant advantages over CR.
DR improves workflow because more images can be taken and processed in the same amount of time, allowing imaging facilities to handle more patients in a given period of time and consequently lowers the cost per image. DR systems house all components together and do not require any transfer of film or cassette. The time needed to produce a final DR image is 10 seconds or less.
CR involves more steps because cassette processing takes longer. CR requires the cassette be removed from the X-ray machine and then placed into a reader. This is a labour-intensive and time-consuming step that requires the technician to leave the patient and workstation with each imaging procedure.
Both GOS (gadolinium oxysulfide) and CsI (cesium iodide) based DR detectors have higher dose efficiency than CR. When DR with CsI is used, DR systems are two to three times more efficient at converting dose to signal than CR. This increased dose utilization means that DR can produce the same image quality as CR at a lower dose or that DR can produce higher contrast resolution images than CR using the same dose.
IP plates used in CR have a lower efficiency of detection compared to DR detectors, thus a higher radiation dose is needed to obtain adequate image resolution resulting in longer exposure times and the need for more powerful x-ray generators.
Flexibility, Portability and Cost
DR detector panels are now equipped with a data processing engine and carry their own calibration files, which allows the images to be corrected on the panel. With the calibration data stored on the panel, the DR panels are more portable between equipment and rooms. On-panel image corrections decrease the image transmission time and improve the wireless link robustness by reducing the number of images transmitted with each image acquisition. State-of-the art DR panels are now also equipped with the most advanced wireless networking standards increasing transmission speed and reliability.
The initial cost of CR is lower, however maintenance costs on CR are 80% higher than on DR. Less real estate is required with DR as the full system only consists of a panel (460x460x15mm) and desk top computer, whereas a full CR system consists of the CR unit (560x540x392mm), cassettes and desktop computer.