|
The revolutionary impact of computerized tomography has caused radiologists and imaging scientists to re-examine the utility of conventional projection radiography. in addition, the improved x-ray technology which has accompanied computerized tomography shows great promise in being applied to new versions of projection radiography with dramatic improvements in performance. Projection radiography has a distinct advantage in that an entire volume of interest is presented in a single image. In CT this same volume requires an array of cross-sectional images. Certain structures, such as blood vessels, are much more readily visualized in the projection mode. It would be very cumbersome to study the stenosis of a vessel by viewing successive crosssectional images. It would indeed be fortuitous if the vessel remained in a single CT section for any considerable extent. Projection radiography, however, measures the line integral of the attenuation coefficient rather than the attenuation coefficient itself as is done in CT. This property of CT has enabled the use of the very powerful display technique of windowing where a particular small range of attenuation coefficients are observed. in this presentation, a subtle change can be studied where the visualization is limited solely by the counting statistics and dose. In general, this windowing property has not been available in projection radiography so that radiographic images have tended to be limited by contrast, rather than signal-to-noise ratio. This is an undesirable situation which must be remedied if projection radiography is to significantly improve. Two important factors are required for improved projection radiography, improved detection and image processing. The existing detection systems have relatively low quantum efficiency, significant scatter and poor dynamic range. Recently, projection systems have been used with CT detection systems, such as the GE Scout View system. These essentially provide all of the desired properties with somewhat reduced resolution. In addition, a variety of digital image processing activities have been initiated including various subtraction modes and spatial filtering which provide visualization of structures which were otherwise contrast limited.
|
