Military roentgenology / War Department.

Date:
[1944]
Catalogue details

Licence: Public Domain Mark

Credit: Military roentgenology / War Department. Source: Wellcome Collection.

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    the first two densities again, if they be in close approximation, the contrast will be greater than if they be separated by intervening steps of density. Differences in densities in the light exposure range, below 0.3 density units, and in the heavy exposure range, above 1.8 density units, are difficult to dis- tinguish. Therefore, a density difference as con- sidered above in the range 1.0-1.5 density units is much more evident than in the range 1.8-2.4 density units; that is, contrast is less in the latter situation. Mathematically, contrast can be expressed as, C = D, — Ds, where D, and Dy are the densities of two different areas of the film. Contrast, in this sense, generally refers to the entire over-all film contrast. Roentgenographic contrast is affected by— Kilovoltage—the lower the kilovoltage the greater the contrast. It is conceivable that for any one part there may be a kilovoltage productive of X-rays capa- ble of penetrating certain tissues (soft tissues) and not penetrating certain other tissues contained (that is, bone). With such conditions there would be obtamed the highest contrast. Thus a certain kilovoltage may accomplish a desirable contrast for roentgenography of the hand but when used for roent- genography of the pelvis, it would result In excessive contrast. Exposure holder—cassettes with intensifying screens Increase contrast. The greater the “‘speed”’ of the screen, the greater the contrast obtained. Comparing the use of cassettes with cardboard hold- ers, less contrast 1s obtamable with the latter. (See fig. 82.) Miscellaneous—development solutions— warm development or excessive devel- opment time will increase contrast. Auxiliary devices—use of filters, cones, diaphragms, cylinders and grids will Increase contrast by mmimization of secondary radiation; film emul- sion characteristics also influence the contrast. 130. DISTORTION. Distortion is the perversion of the true shape of the part as represented on the roentgenogram. The true shape cannot be exactly reproduced since all the structural features of the body are represented in a single plane. Thus, dis- tortion is present in every roentgenogram. If the image is distorted to such an extent that it appears enlarged beyond its true size, then the distortion is referred to as magnification. Distortion 1s affected by (fig. 83)— Part-film distance—the less the part-film distance, the less the distortion (less magnification of the true size). Focal-film distance—the greater the focal- film distance, the less the distortion (less magnification of the true size). Tube alignment—improper alignment of the tube with respect to the part and film under consideration will result in variations in the part-film distances concerned with various portions in any plane and therefore result in un- symmetrical image representation. Focal-spot size—the smaller the focal spot, the less the distortion (less magnifica- tion of the true size). SECTION II. EXPOSURE FACTORS 131. GENERAL. Frequently, there arises the neces- sity to change the established exposure factors recommended for a given technique. The intensity of radiation varies inversely as the square of the focal-film distance. (See fig. 84.) Since density is affected by milliamperage, time, kilovoltage, and the distance, It Is Important to understand the mathe- matical rules which will govern their variation and control. It may be desired to increase or decrease the kilovoltage. Circumstances may require the use or removal of certain auxiliary devices such as diaphragms, cones, or grids. Intensifying screens may be desirable other than direct exposure with cardboard holders. In all of the foregoing situations, knowledge of the adequate compensations in the technique is essential. 132. MILLIAMPERAGE-DISTANCE RELATION- SHIP. The milliamperage (ma) requirements are directly proportional to the square of the focal-film distance (d). This relationship may be expressed in the followmg formula: Mai: Maz: :D,?: D2 Where Ma, and D,? are the original milliamperage and distance, respectively and conversely, and Maz and D,? are the new milliamperage and distance. conditions may arise where it is desired to change the milliamperage. It may be desired to increase the milliamperage to provide for shorter exposure time in order to minimize the effects of motion or the capacity of the tube may require a reduction in milliamperage. On the other hand, the quality of the roentgenogram might be increased by sharpness of the detail. This may be accomplished by an increase in the focal-film distance. Any change in the miliamperage or distance will affect the film density and, if one factor is changed, the other must be appropriately compensated. For an example, suppose the technique recommends for a P-A view of a skull, 76 kvp, 20 ma, 314 sec at 30-inch focal-film distance (D;). If it is desired to increase the sharp- ness of the detail the distance may be increased to
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