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Why Does this Site Require Cookies?This site uses cookies to improve performance by remembering that you are logged in when you go from page to page. To provide access without cookies would require the site to create a new session for every page you visit, which slows the system down to an unacceptable level. What Gets Stored in a Cookie?This site stores nothing other than an automatically generated session ID in the cookie; no other information is captured. In general, only the information that you provide, or the choices you make while visiting a web site, can be stored in a cookie. For example, the site cannot determine your email name unless you choose to type it. Allowing a website to create a cookie does not give that or any other site access to the rest of your computer, and only the site that created the cookie can read it. The images shown above were obtained at 50 kV (left) and 125 kV (right) at table top (i.e., with no scatter removal Bucky). The low kV image has superior contrast because of reduced levels of scatter. As the kV is reduced, the average photon energy is reduced; accordingly, the proportion of photoelectric interactions increases and the proportion of scatter events is reduced. Scatter is generally lower at lower kV values, especially when imaging bone which contains a high atomic number material (Calcium, Z = 20). In general, extremity imaging may be performed at table top because two factors help to reduce the amount of scatter: (a) thin body parts are easier to penetrate and therefore permit the use of lower x-ray tube voltages (kV); (b) the presence of bone (high Z) means that most interactions are photoelectric (not Compton) and thus scatter is reduced. The image on the left (i.e., 50 kV), for a constant receptor radiation dose of 5 mGy (S ~ 200), required 15 mAs whereas the image on the right (125 kV) required only 0.5 mAs to achieve the same receptor dose. Increasing the x-ray tube voltage from 50 to 125 kV increases both the total x-ray tube output (air kerma) as well as the average photon energy (i.e., penetrating power). For both of these reasons, increasing the x-ray tube voltage by this amount required a 30 fold reduction in the mAs to maintain the same amount of radiation incident on the image receptor. The dynamic range of the image obtained at 50 kV was 160 (i.e., L = 2.2), whereas the dynamic range of the image obtained at 125 kV was markedly reduced to 50 (i.e., L = 1.7). In radiology, it is generally true that high x-ray tube voltages (i.e., increased photon energies) reduce the dynamic range in radiographic images, and vice versa. The AP skull radiographs shown above were also obtained at the table top using 65 kV (left) and 125 kV (right). As expected, the low kV image is markedly superior to the high kV image because of the substantially lower levels of scatter. To achieve the same receptor dose, the mAs for the 125 kV image on the right was reduced by a factor of about 15 - the 125 kV image was obtained using 0.66 mAs, whereas the 65 kV image was obtained using 10 mAs, and where both resulted in an image receptor radiation intensity of 5 uR (i.e., S ~ 200). The lateral skull radiographs shown above were also obtained at the table top using 60 kV (left) and 125 kV (right). Note that one can use a lower kV (60 kV) on the lateral skull than the AP (65 kV) because the lateral is generally thinner and therefore easier to penetrate. As expected, the low kV image is markedly superior to the high kV image because of the substantially lower levels of scatter.To achieve the same receptor dose, the mAs for the 125 kV image on the right was reduced by a factor of about ~25 - the 125 kV image was obtained using 0.5 mAs, whereas the 60 kV image was obtained using 12 mAs, with both resulting in an image receptor radiation intensity of 10 mR (i.e., S ~ 100). How does scatter affect the radiographic image?Scattered radiation reduces the level of contrast of a hidden X ray image, introduces additional quantum noise, and decreases image sharpness and increases background heterogeneity.
How does scatter radiation affect the contrast of radiographs?Scatter radiation will decrease the contrast of the radiograph. Factors that contribute to scatter radiation are increasing volume of tissue, tube kilovoltage, the density of matter, and field size. Ways to reduce scatter include close collimation, grids, or air gap technique.
What affects the contrast of the radiographic image?Subject Contrast
It is dependent on the absorption differences in the component, the wavelength of the primary radiation, and intensity and distribution of secondary radiation due to scattering. It should be no surprise that absorption differences within the subject will affect the level of contrast in a radiograph.
What is the relationship of scatter and contrast?Collimation and Contrast
As collimation increases, the quantity of scatter radiation decreases, and radiographic contrast increases; as collimation decreases, the quantity of scatter radiation increases, and radiographic contrast decreases.
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