TABLE OF CONTENTS (E-mail: taibi@fe.infn.it)
The sensitivity of X-ray film to direct X-ray exposure is low. To avoid large patient doses it is therefore desirable to use a more efficient imaging detector. This is accomplished by converting the X-ray image into light by means of a scintillating screen, also called X-ray intensifying screen, and then recording visible photons on film. The screen absorbs a large fraction of the incident X-rays and also provides signal amplification. The imaging properties of screen-film systems are reviewed in this chapter. Since the understanding of the physics of diagnostic radiology has evolved in parallel with the development of screen-film detectors this chapter also serves to introduce the fundamental physical parameters which define the quality of a radiographic image.
A scintillating screen is composed of high Z phosphors which emit visible or near-visible light under X-ray irradiation. X-ray-to-light conversion is a multi-step process and a burst of light photons are emitted per X-ray interaction. The scintillation mechanism in inorganic materials is determined by the crystal nature of the phosphor [1]. The photoelectric effect is the dominant type of X-ray interaction within the screen and, due to the transferred energy, a photoelectron has the potential to release many electron-hole pairs, i.e. to move up electrons from the valence band to the conduction band. The return of an electron to the valence band with the emission of...
