Primary Exposure Factors IV By Professor Stelmark Film-Screen Characteristics The visibility of the anatomic structures and the accuracy of their structural lines recorded determine the overall quality of the radiographic image. For film-screen, visibility of the recorded detail refers to the photographic properties of the image. Visibility of the recorded detail is achieved by the proper balance of radiographic density and radiographic contrast. Radiographic Density Radiographic film that has been exposed to radiant energy and chemically processed is composed of minute deposits of black metallic silver visualized as density. The varying densities on the processed film represent the attenuation properties of the anatomic part imaged. Radiographic density is the amount of overall blackness produced on the processed image. A radiograph must have sufficient density to visualize the anatomic structures of interest. The radiographer must evaluate the overall density on the image to determine whether it is sufficient to visualize the anatomic area of interest. He or she then decides whether the radiograph is diagnostic or unacceptable. If a radiograph is deemed unacceptable, the radiographer must determine what factors contributed to the density error. Knowledge about the factors that affect the density on a radiographic image is critical to developing effective problem-solving skills Optimal Insufficient Density Excessive Density Exposure Intensity and Optical Density Increasing the exposure intensity to the film-screen image receptor increases the optical density, whereas decreasing the exposure intensity to the filmscreen image receptor decreases the optical density. Radiographic Contrast Producing a radiographic image with diagnostic densities is important to visualize the anatomic area of interest. To differentiate among the anatomic tissues, there must be density differences. Density differences are a result of the tissues’ differential absorption of the x-ray photons. Differential absorption is the difference between the xray photons that are absorbed versus those that penetrate the body. It is called differential because different body structures absorb x-ray photons to different extents. Anatomic structures such as bone are denser and absorb more x-ray photons than structures filled with air such as the lungs. Radiographic contrast is the degree of difference or ratio between adjacent densities. The ability to distinguish between densities enables differences in anatomic tissues to be visualized. An image that has a diagnostic density but no differences in densities appears as a homogeneous object. When the absorption characteristics of an object differ, the image presents with varying densities. Radiographic images are typically described by their scale of contrast or the range of densities visible. A radiograph with few densities but great differences among them is said to have high contrast. This is also described as shortscale contrast . A radiograph with a large number of densities but little differences among them is said to have low contrast. This is also described as long-scale contrast. High Contrast Low Contrast Radiographic contrast is the combined result of multiple factors associated with the anatomic structure, quality of the radiation, and the capabilities of the film. Subject contrast refers to the absorption characteristics of the anatomic tissue radiographed along with the quality of the x-ray beam. Differences in tissue thickness, density, and effective atomic number contribute to subject contrast. Low Contrast The radiographer must evaluate the composition of the anatomic structure to be radiographed and determine the factors to manipulate to produce the desired level of radiographic contrast. Achieving the desired level of contrast that best visualizes the anatomic area of interest maximizes the amount of information visible for a diagnosis. High Contrast Low Contrast Milliamperage and Exposure Time The quantity of radiation reaching the patient affects the amount of remnant radiation reaching the IR. The product of milliamperage and exposure time has a direct proportional relationship with the quantity of x-rays produced. As the mAs is increased, the quantity of radiation reaching the IR is increased. As the mAs is decreased, the amount of radiation reaching the IR is decreased. Changes in mAs have a direct effect on density mAs is the main controlling factor of density When using a film-screen IR, radiographers need to assess the level of density produced on the processed image and determine whether the density is sufficient to visualize the anatomic area of interest. When the radiograph is deemed unacceptable it may need to be repeated. The radiographer must decide how much of a change in mAs is needed to correct for the density error. In general, for repeat radiographs necessitated by density errors, the mAs is adjusted by a factor of 2; therefore a minimum change involves doubling or halving the mAs. If the radiograph necessitates an adjustment greater than a factor of 2, the radiographer should multiply or divide the mAs by 4. ¼ of required mAs 4 x required mAs Radiographs that have sufficient but not optimal density usually are not repeated. If a radiograph must be repeated because of another error, such as positioning, the radiographer may also use the opportunity to make an adjustment in density to produce a radiograph of optimal quality. Making a visible change in radiographic ensity requires that the minimum amount of change in mAs be approximately 30% (depending on equipment, this may vary between 25% and 35%). Radiographic images generally are not repeated to make only a slight visible change. A radiographic image repeated because of insufficient or excessive density requires a change in mAs by a factor of at least 2. The mAs does not have a direct effect on image brightness when using digital IRs. During computer processing, image brightness is maintained when the mAs is too low or too high. A lower-than-required mAs produces an image with increased quantum noise and a higher-than-needed mAs exposes the patient to unnecessary radiation. To best visualize the anatomic area of interest, the mAs selected must produce a sufficient amount of radiation reaching the IR, regardless of type. Excessive or insufficient mAs adversely affects image quality and affects patient radiation exposure. Kilovoltage The area of interest must be adequately penetrated before the mAs can be adjusted to produce a quality radiographic image. When adequate penetration is achieved, further increasing the kVp results in more radiation reaching the IR. Unlike mAs, the kVp affects the amount of radiation exposure to the IR and radiographic contrast. kVp is the main controlling factor of radiographic contrast. A high kVp results in less absorption and more transmission in the anatomic tissues, which results in less variation in the x-ray intensities exiting the patient (remnant), producing a low-contrast (long-scale) image. A low kVp results in more absorption and less transmission in the anatomic tissues, but with more variation in the x-ray intensities exiting the patient, resulting in a high-contrast (short-scale) image. Low kVp High Contrast High kVp Low Contrast Increasing the kVp increases IR exposure and the density produced on a film image. For film-screen IRs, kVp has a direct relationship with density; however, the effect of the kVp on density is not equal throughout the range of kVp (low, middle, and high). A greater change in the kVp is needed when operating at a high kVp (greater than 90) compared with operating at a low kVp (less than 70 Kilovoltage is not a factor typically manipulated to vary the amount of IR exposure in film-screen imaging because the kVp also affects contrast. However, it is sometimes necessary to manipulate the kVp to maintain the required exposure to the IR. For example, using portable or mobile x-ray equipment may limit choices of mAs settings and therefore the radiographer must adjust the kVp to maintain sufficient exposure to the IR. Maintaining or adjusting exposure to the IR can be accomplished with kVp by using the 15% rule. The 15% rule states that changing the kVp by 15% has the same effect as doubling the mAs, or reducing the mAs by 50%; for example, increasing the kVp from 82 to 94 (15%) produces the same exposure to the IR as increasing the mAs from 10 to 20. Kilovoltage and Digital Image Quality Assuming that the anatomic part is adequately penetrated, changing the kVp does not affect the digital image the same as a film-screen image. Image brightness and contrast are primarily controlled during computer processing.