Digital Systems

Digital Radiography, also called flat-panel detectors, is truly a direct digital system with no cassettes and therefore none of the time consuming steps needed to process the plates. Digital radiography refers to devices in which the digitization of the x-ray signal occurs within the detector itself, providing an immediate full fidelity image on a softcopy display monitor. Images are ready for viewing in seconds instead of minutes, and DR produces excellent resolution and high quality images.

Computed Radiography is a cassette based digital system using photo stimulated storage phosphor image plate inside a cassette that is exposed in a manner similar to the traditional screen-film cassette. The cassette like screen-film can be used, on the tabletop, in the bucky, and is portable. Like the conventional intensifying screen used with film, CR plates produce light in response to x-rays at the time of exposure. However, storage phosphor plates have the additional property of being capable of storing some of the absorbed x-ray energy as a latent image. Once exposed, the cassette is taken to a reader where the plate is removed from the cassette, scanned to create the digital image, erased and placed back into the cassette for reuse.

Charge-coupled device detector is comprised of a large Field of View (e.g., 43 cm by 43 cm) scintillator that converts absorbed X-ray energy into light. It also includes an optical lens assembly to focus the light onto the photosensitive CCD array, and a CCD camera to integrate, scan and output the corresponding light image. CCD-based detector is typically comprised of a single-compound optical lens and a high-resolution CCD camera comprised of 9-16 million pixels. The photosensitive area of the CCD chip is actually quite small, on the order of 2.5 cm × 2.5 cm to 4.0 cm × 4.0 cm, which is required to maintain extremely high charge-coupling efficiency and low-noise operation during the readout of the image. Thus, there is a large optical demagnification that is necessary to focus the full FOV light image onto the CCD sensor. Image quality is determined by the demagnification factor, conversion efficiency, luminance and directionality of the light emission.

Film Digitizers are still necessary even in the all-digital or filmless facilities so that film images from outside referrals without digital capabilities can be input into the system and viewed digitally. Film digitizers convert the continuous optical density value of film into a digital image by sampling at discrete, evenly spaced locations and quantizing the transmitted light from a scan of the film into digital numbers. Commonly used scanners are CCD using transmitted light onto a linear CCD detector or the laser scanner using a gas laser or a solid-state diode laser source directed by mirror deflection and its intensity detected by a photomultiplier tube.