The acronym CR in radiology stands for Computed Radiography, a digital method for capturing X-ray images. This technology was a significant advancement, allowing a transition away from traditional chemical-based film processing toward a digital workflow. CR uses specialized equipment to convert the energy deposited by X-rays into a digital file that can be viewed and stored on a computer. Its purpose is to produce high-quality diagnostic images without the physical limitations and time delays associated with darkroom procedures.
Defining Computed Radiography
Computed Radiography uses a reusable imaging plate housed within a cassette, resembling older film cassettes. This plate contains a layer of Photostimulable Phosphor (PSP), the core component responsible for capturing the X-ray image. The PSP material stores energy from the incident X-ray beam, forming a latent image. This latent image is not yet visible but is proportional to the X-ray exposure received across the plate’s surface.
Introduced in the 1980s, CR systems served as a technological bridge between conventional film-screen radiography and fully electronic digital systems. Using existing X-ray equipment with CR cassettes made it a cost-effective upgrade for many healthcare facilities. Unlike film, the PSP plate captures a wider range of X-ray intensities, providing a broader exposure latitude that makes image acquisition more forgiving. This reduces the need for repeat exposures, helping to lower the radiation dose to patients.
The CR Imaging Process
The CR image creation process begins when the X-ray beam passes through the patient and strikes the PSP plate. X-ray photons excite electrons in the phosphor layer, trapping them in higher energy states. The distribution of these trapped electrons corresponds to the tissue densities, creating the invisible latent image.
After exposure, the cassette is placed into a specialized CR reader unit to make the image visible. Inside the reader, a focused laser beam scans the PSP plate in a precise raster pattern. The laser stimulates the trapped electrons, causing them to drop back to a lower energy state and release stored energy as blue-violet light, known as photostimulated luminescence. The intensity of this emitted light is proportional to the amount of X-ray energy originally absorbed.
A light guide collects the released light, directing it toward a photomultiplier tube (PMT), which converts the weak light signal into an electrical current. This analog signal is fed into an analog-to-digital converter (ADC), which samples the signal and assigns a numerical value to each point, creating the digital image pixels. Finally, the plate is exposed to a bright white light to erase any remaining trapped electrons, preparing the plate for reuse.
CR’s Position in Modern Radiology
While Computed Radiography was a major step forward, its role in high-volume radiology departments has largely been overtaken by Direct Radiography (DR) technology. DR systems use flat-panel detectors to convert X-ray energy directly into a digital signal, eliminating the separate reading step and making image acquisition almost instantaneous. This workflow difference means DR offers significantly faster image throughput and produces images with higher spatial resolution and better contrast.
Despite DR’s advantages, CR remains a relevant and practical tool in many medical settings, primarily due to its lower initial investment cost. CR allows facilities to upgrade to digital imaging without replacing existing X-ray generators and stands. CR cassettes are robust and highly portable, making them suitable for mobile X-ray units, remote clinics, or bedside examinations where a fixed DR panel would be impractical.
Maintenance of a CR system is relatively simple, focusing on the reader unit and the reusable PSP plates. These plates are less expensive to replace than the sophisticated flat-panel detectors used in DR units. CR continues to be a reliable and economical option for facilities with lower patient volumes or those prioritizing ruggedness and cost-efficiency over the speed of instant image display.