Chest Imaging

Storage phosphor radiography

(also called computed radiography (CR)), a digital technique that uses photostimulable phosphor screens to produce images. Instead of a filmscreen combination, a photostimulable phosphor screen is used as the X-ray receptor. The system is cassette-based and is therefore compatible with existing X-ray equipment including bedside radiography. Dedicated chest systems using storage phosphor screens are also available. In contrast to intensifying screens where excited electrons release the absorbed energy immediately after X-ray exposure, phosphor screens capture an important part of this absorbed energy. This energy is stored as a "latent image" and can be read out later. During this read-out process the storage phosphor is stimulated by a scanning red laser, its electrons return to a more stable energy level and emit blue light (photostimulated luminescence). This light, which is proportional to the absorbed X-ray intensity, is collected by a light guide and a photomultiplier tube. The output of this photomultiplier is amplified and digitized (Fig.1).

The major advantages of the storage phosphor detector in comparison to conventional filmscreen systems are its wide latitude and its linear response to radiation. These features are especially interesting in the chest because exposure errors become less critical and both high and low density areas can be visualized simultaneously and accurately in one exposure. Bedside chest radiography has benefited in particular from the constant image quality of CR. The wide latitude, on the other hand, guarantees simultaneous adequate imaging of the mediastinum and the lungs. The spatial resolution of storage phosphor systems is lower than that of screen-film systems, even when "high resolution" storage phosphor systems providing a higher number of pixels per mm by increasing the digital sampling rate are used. However, in clinical practice, this has not turned out to be a prohibitive problem. Small linear and nodular opacities can usually be visualized, as well as with storage phosphor systems as with conventional film-screen systems. This is related to the fact that image processing can considerably improve image quality.

Today, the numerous studies that have been performed to investigate the clinical utility of storage phosphor chest radiography confirm that the technique is suited for clinical use and provides important advantages when compared to conventional filmscreen systems.

JV

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Fig.1

Diagram showing the different steps necessary to produce a digital image in computed radiography.
Storage phosphor radiography, Fig.1