Electronic scanning-slit fluorography.

Abstract

Scattered radiation degrades contrast and signal-to-noise ratio of an x-ray image. If an image intensifier is used as the image receptor, scattering of light photons and electrons within the image intensifier, optical system, and video camera produces veiling glare. anti-scatter grids, air gaps, and paired scanning slits have been used for rejection of scattered radiation. However, none of these methods is effective against veiling glare, because veiling glare is generated after the radiation has passed through any of these anti-scatter devices. In chapter 1 is introduced an innovative approach for highly efficient rejection of both scattered radiation and veiling glare in digital fluorography. This method has been named electronic collimation, and the x-ray imaging technique based upon it is called electronic scanning-slit fluorography. It involves replacing paired fore and aft slits for scatter rejection with only one beam-defining tantalum fore aperture. As this aperture scans across the portion of the patient to be imaged, pulsed x-ray exposures produce images which are digitized and stored in the computer memory. Since the video signal within the projection of the aperture on the image intensifier is much more intense than behind the tantalum, one can discriminate electronically between these two signals and thus eliminate the unwanted x-ray scatter and veiling glare. Such electronic collimation does not require synchronization between the slit scanning and detector readout, which makes it much simpler than alternative methods and potentially adaptable to any digital fluorography system. Theoretical considerations relevant for the construction and evaluation of a prototype unit for electronic scanning slit fluorography are presented in Chapter 2. This chapter consists of four sections. In the first section 'Principles of image detection' the concepts of quantum efficiency and detective quantum efficiency (DQE) are introduced as the most meaningful way to compare different methods of scatter rejection. The DQE is the fraction of incident photons that would have to be detected without additional noise to yield the same signal-to-noise ratio as is actually observed by the detector in question. The second section 'Effect of scatter' contains the derivation of the functional dependence of the image contrast and the signal-to-noise ratio on the scatter-to-primary ratio. This derivation yields the scatter degradation factor (SDF) which is the fraction of available primary beam contrast due to the presence of scatter.(ABSTRACT TRUNCATED AT 400 WORDS)