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ages were obtained with computed tomography (CT) systems and dedicated line scan systems. Line scan images,2which are used routinely to provide localization images prior to CT, benefit from the low scatter associated with the linear collimation employed.

The emergence of digital (as opposed to film) techniques for x-ray projection imaging, combined with the fact that several other diagnostic modalities (e.g., nuclear medicine, CT, and more recently ultrasound and magnetic resonance imaging) provide digital data, has led to increased efforts focused on the possibility of completely digital radiology departments. The movement toward this has been slowed by the complexity of the task and also by the lack of a universally acceptable digital detector for x-ray projection images. While image-intensifier television systems have been adequate for digital fluoroscopy applications, a substitute for film has not yet been found that provides comparable diagnostic accuracy over a wide range of applications.

The development of photo-stimulable phosphor plates in the mid 1980s provided a detector with several attractive properties, including a digital format and greatly increased dynamic range. This modality compares well with film in all but a few applications and appears superior for bedside radiography because of its improved dynamic range. However, it does not have sufficient resolution for mammography and has the disadvantage, shared by film, that cassettes must be handled and processed.

The availability of rapidly obtained digital images has led to investigations of dual energy techniques for chest radiography and cardiac imaging.3Although most of these investigations have been carried out using conventional x-ray sources, there are ongoing investigations of alternative sources for producing narrow band radiation for applications of this type.

While it is recognized that film is unique in its ability to serve as detector, display mechanism, and archival storage medium, the availability of digital techniques offers the possibility of separately optimizing these functions. Significant current research activity revolves around the development and optimization of detectors, sources, and related imaging apparatus and procedures for several imaging applications. The remainder of this chapter summarizes these challenges within the context of some of the most significant areas of clinical applications of projection imaging.

2Produced by sequentially scanning a linear (one line of data at a time) detector over the area of interest.

3Dual-energy techniques involve acquiring two images from the same area using different x-ray spectra (by changing the tube voltage) in order to exploit differences in the attenuation coefficient of different body tissues as a function of the x-ray energy.

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