Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Film has excellent spatial resolution, far beyond that of the screen (see Fig. 6-8).Film used for microfiche, for example, is capable of spatial resolutions approaching100 cycles per millimeter. The more interesting property of film in the context ofscreen-film radiography, therefore, is how film responds to x-ray or light exposure.If a sheet of fresh unexposed film is taken out of the box and processed, it willhave an 00 in the range from about 0.11 to 0.15. This 00 is that of the film base.The polyester film base is usually slightly tinted to hide faint chemical residue fromthe processing, which can appear as an unattractive brown. Although gray-tintedfilm base is available, a blue tint is considered by many to be pleasing to the eye andis used quite commonly. Film that has been stored for a long period or exposed toheat or to background radiation can develop a uniform fog level. In this case, anotherwise unexposed sheet of film after processing may have an 00 in the 0.13 to0.18 range. The 00 resulting from the combination of background fogging andthe tinting of the film base is referred to as the "base + fog" 00. Base + fog levelsexceeding about 0.20 are considered unacceptable, and replacement of such filmshould be considered.When film is exposed to the light emitted from an intensifYing screen in ascreen-film cassette, its response as a function of x-ray exposure is nonlinear (Fig. 6-14). Hurter and Oriffield studied the response of film to light in the 1890s, and thegraph describing 00 versus the logarithm (base 10) of exposure is called the H &D curve, in their honor. A more generic term is the characteristic curve. Notice thatthe x-axis of the H & 0 curve (see Fig. 6-14) is on a logarithmic scale, and this axisis often called the log relative exposure. The 00 (the value on the y-axis) is itself thelogarithm of the transmission, and therefore the H & 0 curve is a 10glO-loglOplotof optical transmission versus x-ray exposure.Salient features of the H & 0 curve are shown in Fig. 6-14. The curve has asigmoid shape. The toe is the low-exposure region of the curve. Areas of low exposureon a radiograph are said to be "in the toe" of the H & 0 curve. For example,the mediastinum on a chest radiograph is almost always in the toe. The toe regionextends down to zero exposure (which cannot actually be plotted on a logarithmic~2.5(J)c~ 2.0~ 1.5:;:;c..o 1.00.00.1 0.3 1.0 3.0 10.0 30.0Exposure (mR)FIGURE 6-14. A Hurter and Driffield (H &D) curve is a plot of a film's optical density(OD) as a function of the log of exposure.The regions of the H & D curve include thetoe, the linear region, and the shoulder. Thebase + fog level corresponds to the OD ofunexposed film.