Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

educed latitude. The shaded region on Fig. 6-18 illustrates the useable range ofoptical densities, and the latitude is the range of x-ray exposures that deliver ODsin the usable range. Latitude is called dynamic range in the engineering world. SystemA in Fig. 6-18 has higher contrast but lower latitude. It is more difficult to consistentlyachieve proper exposures with low-latitude screen-film systems, and thereforethese systems contribute to higher retake rates. In the case of chest radiography,it may be impossible to achieve adequate contrast of both the mediastinum and thelung fields with a low-latitude system.The film emulsion should be sensitive to the wavelengths of light emitted by thescreen. Ca W04 emits blue light to which silver halide is sensitive. Native silverhalide is not as sensitive to the green or red part of the visible spectrum. Consequently,in the days before the introduction of rare earth screens, darkrooms hadbright red or yellow safelights that had little effect on the films of the day. The emissionspectrum of GdzOzS:Tb is green, and to increase the sensitivity of silver halideto this wavelength sensitizers are added to the film emulsion. Modern film emulsionsthat are green-sensitized are called orthochromatic, and film emulsions that aresensitized all the way into the red region of the visible spectrum (e.g., color slidefilm) are called panchromatic. Because matching the spectral sensitivity of the filmto the spectral output of the screen is so important, and for other compatibility reasons,the screens and films are usually purchased as a combination-the screen-filmcombination or screen-film system. Combining screens from one source and filmfrom another is not a recommended practice unless the film manufacturer specifiesthat the film is designed for use with that screen.The reciprocity law of film states that the relationship between exposure andOD should remain constant regardless of the exposure rate. This is equivalent tosaying that the H & D curve is constant as a function of exposure rate. However,for very long or very short exposures, an exposure rate dependency between exposureand OD is observed, and this is called reciprocity law failure (Fig. 6-19). At long~1.0enct3 0.8~ 0.6:ao 0.4ReciprocityLaw Obeyance =:\_-~-~~ ~~ ~_L0.001 0.01 0.1 1 10Exposure Time (see)FIGURE 6-19. The concept of reciprocitylaw failure is illustrated. At all points alongthe curve, the screen-film system is exposedexactly the same but at different exposurerates in the time indicated on the x-axis. Atshort and long exposure times, a reductionin the efficiency or speed of the screen-filmsystem occurs. If the reciprocity law were tohold, the curve would appear asthat shownas the dashed line.