A certain density, for example, 1.5, is selected as the basis for the preparation of the chart. Wherever this density occurs on the stepped-wedge radiographs, there are corresponding values of thickness, milliampere-minutes, and kilovoltage. It is unlikely that many of the radiographs will contain a value of exactly 1.5 in density, but the correct thickness for this density can be found by interpolation between steps. Thickness and milliampere-minute values are plotted for the different kilovoltages in the manner shown in Figure 44.
Another method, requiring fewer stepped wedge exposures but more arithmetical manipulation, is to make one step-tablet exposure at each kilovoltage and to measure the densities in the processed stepped-wedge radiographs. The exposure that would have given the chosen density (in this case 1.5) under any particular thickness of the stepped wedge can then be determined from the characteristic curve of the film used (See "The Characteristic Curve"). The values for thickness, kilovoltage, and exposure are plotted as described in the figure above.
Note that thickness is on a linear scale, and that milliampere-minutes are on a logarithmic scale. The logarithmic scale is not necessary, but it is very convenient because it compresses an otherwise long scale. A further advantage of the logarithmic exposure scale is that it usually allows the location of the points for any one kilovoltage to be well approximated by a straight line.
Any given exposure chart applies to a set of specific conditions. These fixed conditions are:
1. The x-ray machine used
2. A certain source-film distance
3. A particular film type
4. Processing conditions used
5. The film density on which the chart is based
6. The type of screens (if any) that are used
Only if the conditions used in making the radiograph agree in all particulars with those used in preparation of the exposure chart can values of exposure be read directly from the chart. Any change requires the application of a correction factor. The correction factor applying to each of the conditions listed previously will be discussed separately.
1. It is sometimes difficult to find a correction factor to make an exposure chart prepared for one x-ray machine applicable to another. Different x-ray machines operating at the same nominal kilovoltage and milliamperage settings may give not only different intensities but also different qualities of radiation.
2. A change in source-film distance may be compensated for by the use of the inverse square law or, if fluorescent screens are used, by referring to the earlier table. Some exposure charts give exposures in terms of "exposure factor" rather than in terms of milliampere-minutes or milliampere-seconds. Charts of this type are readily applied to any value of source-film distance.
3. The use of a different type of film can be corrected for by comparing the difference in the amount of exposure necessary to give the same density on both films from relative exposure charts such as those shown in Figure 47.
For example, to obtain a density of 1.5 using Film Y, 0.6 more exposure is required than for Film X.
This log exposure difference is found on the L scale and corresponds to an exposure factor of 3.99 on the D scale. (Read directly below the log E difference.) Therefore, in order to obtain the same density on Film Y as on Film X, multiply the original exposure by 3.99 to get the new exposure. Conversely, if going from Film Y to Film X, divide the original exposure by 3.99 to obtain the new exposure.
You can use these procedures to change densities on a single film as well.
Simply find the log E difference needed to obtain the new density on the film curve; read the corresponding exposure factor from the chart; then multiply to increase density or divide to decrease density.
4. A change in processing conditions causes a change in effective film speed. If the processing of the radiographs differs from that used for the exposures from which the chart was made, the correction factor must be found by experiment.
5. The chart gives exposures to produce a certain density. If a different density is required, the correction factor may be calculated from the film's characteristic curve (See "The Characteristic Curve").
6. If the type of screens is changed, for example from lead foil to fluorescent, it is easier and more accurate to make a new exposure chart than to attempt to determine correction factors.
Sliding scales can be applied to exposure charts to allow for changes in one or more of the conditions discussed, with the exception of the first and the last. The methods of preparing and using such scales are described in detail later on.
In some radiographic operations, the exposure time and the source-film distance are set by economic considerations or on the basis of previous experience and test radiographs. The tube current is, of course, limited by the design of the tube. This leaves as variables only the thickness of the specimen and the kilovoltage. When these conditions exist, the exposure chart may take a simplified form as shown in Figure 45, which allows the kilovoltage for any particular specimen thickness to be chosen readily. Such a chart will probably be particularly useful when uniform sections must be radiographed in large numbers by relatively untrained persons. This type of exposure chart may be derived from a chart similar to Figure 44 by following the horizontal line corresponding to the chosen milliampere-minute value and noting the thickness corresponding to this exposure for each kilovoltage. These thicknesses are then plotted against kilovoltage.
Figure 45: Typical exposure chart for use when exposure and distance are held constant and kilovoltage is varied to conform to specimen thickness. Film X (Figure 47 for example), exposed with lead foil screens to a density of 1.5. Source-film distance, 40 inches; exposure, 50 mA-min.
