POI CHAPTER 3 PDF - Factors Affecting Radiographic Contrast

## FACTORS AFFECTING RADIOGRAPHIC CONTRAST ### II. Contrast Media - Has a higher atomic number and will increase attenuation of the beam, resulting in increased subject contrast. - "CONTRAST MEDIA CAN BE USED TO INCREASE SUBJECT CONTRAST." ### III. Collimation - The collimator is a box-like stru...

## FACTORS AFFECTING RADIOGRAPHIC CONTRAST ### II. Contrast Media - Has a higher atomic number and will increase attenuation of the beam, resulting in increased subject contrast. - "CONTRAST MEDIA CAN BE USED TO INCREASE SUBJECT CONTRAST." ### III. Collimation - The collimator is a box-like structure attached to the port of the x-ray tube for the purpose of restricting the x-ray beam. Inside the collimator are matching pairs of lead shutters resembling leaded plates. - The lower shutters are movable to allow for varying field sizes. - The upper pair of shutters is located nearest the focal spot and is known as the port or entrance shutters. The entrance shutters are fixed to serve much like an aperture diaphragm. The lower shutters are furthermost from the focal Spot and adjust in the longitudinal and transverse directions. - When the shutter pairs are completely closed, they should meet in the center of the field. - Another characteristic of the collimator is the beam centering device. - Centering of the primary beam is extremely critical to prevent distortion of the image. - Collimators have a light housed inside the structure that is used for centering the beam. - When the light is illuminated, it is deflected by a mirror that is mounted in the middle of the collimator. - The mirror is directly in the path of the x-ray beam and rests at a 45° angle. - The light rays hit the mirror and are reflected through the open shutters to represent the borders of the beam. - The shutters can be adjusted to change the field size of the beam by using the light to guide the shutter placement. - The lower portion of the collimator is usually covered with clear plastic with a crossbar in the middle. - As the light passes through the crossbar, the center of the beam is marked. This is extremely helpful for accurate centering of the primary beam. - The centering device, which includes both the mirror and light, is the most vulnerable part of the collimator and may need frequent testing and adjustment. - The distance from the focal spot to the midpoint of the mirror must be the same as the distance from the bulb to the midpoint of the mirror. - This relationship is important for the light to truly represent the beam size. If the focal spot-to-mirror distance is greater than the bulb-to-mirror distance, the x-ray beam will be smaller than the light beam indicates. - The main advantages include its effectiveness in reducing penumbra and the variety of field sizes. - The collimator permits rectangular field sizes, which are more effective because most recording devices are rectangular in shape. - The ability to produce rectangular or square shapes provides greater and more effective beam restriction. - Collimators also add to the total inherent filtration of the x-ray beam. The collimator is the equivalent of approximately 1.0 mm aluminum, which is primarily a result of the mirror that is placed in the path of the beam. ### Automatic Collimators - The automatic collimator, also known as a positive beam limiting device (PBLD) movement of the shutters, which is accomplished with electric motors instead of manual selection. - Electronic sensing devices are attached to the cassette tray. - As the cassette is placed in the tray and latched, the sensors are activated to automatically adjust the shutters to the size of the film. - Most automatic collimators are accurate to within 2 to 3%. ### Beam Alignment Test for Collimators - The test is performed using a 14 x 17 cassette. - The light beam is adjusted to leave a 2-inch unexposed border around the edge of the cassette. - The corners of the light beam are marked with paper clips or other metal markers placed on the cassette. - The center of the beam is also marked along with a reference point using an R or L marker. - Make the exposure using approximately 2.5 milliampere seconds (mAs) and 55 kilovoltage peak (kVp). - The test radiograph is examined to determine if the light beam, as marked by the metal objects, is identical to the primary beam. ### EXPOSURE FACTORS AND FILM QUALITY-BEAM RESTRICTION - As the size of the primary beam decreases, the amount of scatter produced during the exposure will also decrease. - Decreasing the scatter will result in decreased density and increased contrast. - Radiographs made with a small field size demonstrate fewer grays, with a more black and white appearance, which means a shorter scale of contrast. - Because beam restriction decreases the amount of scatter, increased restriction of the beam (field size becomes smaller) will result in decreased density recorded on the film, with a shorter scale of contrast. - Beam restriction improves the visibility of detail. - As the field size decreases: scatter radiation will decrease, density will decrease, and contrast will increase (shorter scale) - Increased beam restriction decreases scatter and therefore decreases density. - To compensate for the decreased density, one must adjust the mAs. ### CALCULATING FIELD SIZE - When using field size adjustments, one must know the relationship between the aperture or shutter opening and the field size. - Aperture opening = width or diameter of field x distance from focal spot to aperture focal-film distance