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C HAPTE R 1 Terminology, Positioning, and Imaging Principles
CONTRAST Definition
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Radiographic contrast is defined as the difference in density between adjacent areas of a radiographic image. When the density difference is large, the contrast is high and when the density difference is small, the contrast is low. This is demonstrated by the step wedge and by the chest radiograph in Fig. 1-126, which shows greater differences in density between adjacent areas; thus, this would be high contrast. Fig. 1-127 shows low contrast with less difference in density on adjacent areas of the step wedge and the associated radiograph. Contrast can be described as long-scale or short-scale contrast, referring to the total range of optical densities from the lightest to the darkest part of the radiographic image. This is also demonstrated in Fig. 1-126, which shows short-scale/high-contrast (greater differences in adjacent densities and fewer visible density steps), compared with Fig. 1-127, which illustrates long-scale/ low-contrast. Contrast allows the anatomic detail on a radiographic image to be visualized. Optimum radiographic contrast is important, and an understanding of contrast is essential for evaluating image quality. Low or high contrast is not good or bad by itself. For example, low contrast (long-scale contrast) is desirable on radiographic images of the chest. Many shades of gray are required for visualization of fine lung markings, as is illustrated by the two chest radiographs in Figs. 1-126 and 1-127. The low-contrast (long-scale contrast) image in Fig. 1-127 reveals more shades of gray, as evident by the faint outlines of vertebrae that are visible through the heart and the mediastinal structures. The shades of gray that outline the vertebrae are less visible through the heart and the mediastinum on the high-contrast chest radiograph shown in Fig. 1-126.
Fig. 1-126 High-contrast, short-scale 50 kV, 800 mAs.
Controlling Factors
The primary controlling factor for contrast in film-based imaging is kilovoltage (kV). kV controls the energy or penetrating power of the primary x-ray beam. The higher the kV, the greater is the energy, and the more uniformly the x-ray beam penetrates the various mass densities of all tissues. Therefore, higher kV produces less variation in attenuation (differential absorption), resulting in lower contrast. kV is also a secondary controlling factor of density. Higher kV, resulting in both more numerous x-rays and greater energy x-rays, causes more x-ray energy to reach the IR, with a corresponding increase in overall density. A general rule of thumb states that a 15% increase in kV will increase film density, similar to doubling the mAs. In the lower kV range, such as 50 to 70 kV, an 8- to 10-kV increase would double the density (equivalent to doubling the mAs). In the 80- to 100-kV range, a 12- to 15-kV increase is required to double the density. The importance of this relates to radiation protection because as kV is increased, mAs can be significantly reduced, resulting in absorption of less radiation by the patient. Other factors may affect radiographic contrast. The amount of scatter radiation the film-screen receives influences the radiographic contrast. Scatter radiation is radiation that has been changed in direction and intensity as a result of interaction with patient tissue. The amount of scatter produced depends on the intensity of the x-ray beam, the amount of tissue irradiated, and the type and thickness of the tissue. Close collimation of the x-ray field reduces the amount of tissue irradiated, reducing the amount of scatter produced and increasing contrast. Close collimation also reduces the radiation dose to the patient and the technologist. Irradiation of thick body parts produces a considerable amount of scatter radiation, which decreases image contrast. A device called a grid is used to absorb much of the scatter radiation before it hits the IR.
Fig. 1-127 Low-contrast, long-scale 110 kV, 10 mAs.
