Question

Why does the laser light in the CR reader need to be a different wavelength from the light given off by the imaging plate?

- It's about digital systems.

Answer 1

The PSP detector "system" is comprised of 2 main components. The detector is usually a cassette-based storage phosphor that absorbs X-ray energy transmitted through the patient and temporarily stores the X-ray latent image as a 2-dimensional array of electrons trapped in semistable energy wells. The imaging plate "reader" is comprised of a scanning laser beam to stimulate the trapped electrons and produce "photostimulated luminescence" of a different wavelength that is optically separable from the stimulation wavelength. The reader also includes a light guide and photomultiplier assembly that extracts and processes the stored X-ray content to a sampling resolution on the order of 100 microns (0.1 µm), digital electronics to create the corresponding digital image, and an erasure stage to eliminate any residual signal and prepare for the next exposure. From the reader, all images proceed to a quality control workstation for image evaluation, annotation and transfer to PACS . Most often, the storage phosphor is layered on a flexible or solid substrate in a cassette enclosure, which allows for the ability to directly replace a screen-film cassette in a conventional radiography room. Thus there is the flexibility and portability of a cassette with digital radiography acquisition capability using existing X-ray equipment; this is CR's greatest asset. CR cassettes of various size and number, together with a high-speed imaging plate reader can service multiple X-ray rooms, resulting in a relatively low initial acquisition cost. However, the technologist must handle the cassettes and process the imaging plates in a manner similar to film, which can reduce patient throughput in a busy clinical room and increase labor costs, as the time to handle the exposed imaging plate to the reader and output of the X-ray image can often take several minutes (about 45 to 60 seconds to "read" the plate with the moving laser beam). Other expenses to consider are the need for high-frequency (e.g., 170 lines per inch) antiscatter grids for stationary (non-Bucky) applications such as portable bedside imaging and fixed grid cassette holders, and for readjusting phototimer sensitivity to account for the lower detection efficiency of the conventional PSP imaging plate compared to 400 speed screen-film detectors. Long-term costs include cassette and imaging platelongevity, maintenance and cleaning of the imaging plates and reader assembly, replacement of damaged detectors, and continuous oversight with a quality control program.