Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

screenx-rays1x-rays1x-rays1emulsionfilm baseemulsion/\FIGURE 6-7. A cross-section of a screen-film cassette with x-ray interaction events isshown. A: X-rays incident on and absorbed by a thick screen-film detector producelight photons in the screen matrix that propagate a great distance. The thickerintensifying screens absorb a greater fraction of the incident x-rays, but a greaterlateral spread of the visible light occurs, with a consequent reduction in spatial resolution.B: A thin radiographic screen-film detector results in less x-ray absorption,but less lateral spread of visible light and better spatial resolution is achieved. C: Forx-ray imaging procedures requiring maximum spatial resolution, a single-screen single-emulsionradiographic cassette is used. For such systems, the x-rays first traversethe film emulsion and base and then strike the intensifying screen. Because most ofthe interactions occur near the top of the intensifying screen, this geometry resultsin less light spread and better spatial resolution.shown in Fig. 6-7C therefore results in a shorter diffusion path and higher resolutionthan if the screen were placed on top of the film. The geometry in Fig. 6-7Cis possible only because x-rays easily penetrate the film.For dual-phosphor dual-emulsion systems, it is very important that the lightfrom the top screen not penetrate the film base and expose the bottom emulsion;? 60o'-'"u.I-~ 40.................................... ] .Film1 234 5 6 7 8Spatial Frequency (cycles/mm)FIGURE 6-8. The modulation transferfunction (MTF) as a function ofspatial frequency is illustrated forintensifying screens of differentthicknesses. The thicker screen correspondsto a 120 mglcm 2 thicknessof Gd202S, and the thinner screenis for a 34 mg/cm 2 thickness ofGd202S.Thicker screens have betterabsorption properties but exhibitpoorer spatial resolution. The MTFof film only is illustrated, and filmby itself has far better spatial resolutionthan that of intensifyingscreens. In a screen-film cassette,the resolution properties of thescreen determine the overall spatialresolution.