Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

2:' 2.5 ---'w c~ 2.0~ 1.5~ Q.o 1.00.00.1 0.3 1.0 3.0 10.0 30.0Exposure (mR)FIGURE 6-17. Hurter and Driffield (H & D)curves are shown for two different screenfilmdetectors, system A and system B. A lateralshift in H & D curve indicates a changein the speed (or sensitivity) between systems.The horizontal dotted line correspondingto OD = 1.0 + base + fog indicatesthe point at which the speed is evaluated.The x-ray exposures required to reach thisOD level are indicated for each system (verticalarrows). System A requires less exposurethan system B to achieve the same OD;therefore, it is the faster, more sensitive ofthe two screen-film systems.detail work, typically bone radiographs of the extremities. Screen-film systems inthe 600-speed class are used in some radiology departments for specialty applications(e.g., angiography) where short exposure times are very important. To put thesensitivity of typical screen-film systems in more concrete terms, a 400-speed, 35-X 43-cm cassette (requiring about 0.6 mR to achieve an 00 of 1.0 + base + fog),producing a uniform gray of about 1.1500, is exposed to about 2.4 X 10 10 (i.e.,24 billion) x-ray photons, corresponding to about 156,000 x-ray photons persquare millimeter.Whereas a horizontal shift between two H & 0 curves (see Fig. 6-17) demonstratesthat the systems differ in speed, systems with different contrast have H & 0curves with different slopes (Fig. 6-18). Contrast is desirable in screen-film radiography,but compromises exist. Screen-film system A in Fig. 6-18 has higher contrastthan system B, because it has a steeper slope. A disadvantage of high contrast is3.53.0AB~2.5C/)c~ 2.0Q) FIGURE 6-18. A Hurter and DriffieldOJ~ 1.5c (H & D) curve illustrates screen-filmIII~ •.. systems A and B, which differ in theirQ.::l contrast. The slope of system A iso 1.0 -Q)rnsteeper than that of system B, andtherefore system A has higher con-0.5 ~ trast. The latitude of a screen-film systemrefers to the range of exposuresthat produce acceptable optical densi-0.00.1 0.3 1.0 3.0 10.0 ties. The compromise of a high-contrastscreen-film system (i.e., system A)is that it has reduced latitude.Exposure (mR)t