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SCATTER RADIATION

SCATTER RADIATION. By: SUSAN A. OLAVIDEZ, M.A.Ed., R.R.T. SCATTER RADIATION. X-rays that travel in a different direction after exiting the patient body. Scatter radiation is primarily a product of Compton interaction.

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SCATTER RADIATION

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  1. SCATTER RADIATION By: SUSAN A. OLAVIDEZ, M.A.Ed., R.R.T.

  2. SCATTER RADIATION • X-rays that travel in a different direction after exiting the patient body. • Scatter radiation is primarily a product of Compton interaction. • Scatter radiation produces fog on the x-ray film which tends to dull the image. It increases density but reduces contrast on the radiograph. Susan A. Olavidez

  3. FACTORS AFFECTING SCATTER RADIATION • KILOVOLTAGE - as x-ray energy increases, the relative number of photons that undergo Compton interaction also increases. Thus increasing kilovoltage, increases the production of scatter radiation. However, the use of low kV technique would require an increase in mAs, thus increasing patient dose. • FIELD SIZE - As field size increases, the production of scatter also increases. • PATIENT THICKNESS - The thicker the part to be examined the higher the production of scatter radiation. Susan A. Olavidez

  4. CONTROL OF SCATTER RADIATION • use of beam restricting devices. This limits the field size to smaller area thus the production of scatter radiation also decreases. • use of grids. Grids absorbs scatter radiation before it reaches the film • use of air-gap technique. Gap between the patient and the film decreases the amount of scatter radiation that will reach the film. Susan A. Olavidez

  5. BEAM RESTRICTORS • Beam restrictors are devices that limits the field size to a small area of interest. • THREE TYPES OF BEAM RESTRICTORS • aperture diaphragm. The simplest type of beam restrictor, and is made up of lead or lead-lined metal sheet attached to the x-ray tube head. One diaphragm is used for each particular size of film • cones and cylinders. These are modifications of the aperture diaphragm. It has an extended metal structure which produces a circular image. Susan A. Olavidez

  6. Variable-aperture collimator. Most common beam-restricting device use in radiography. It can be used for any field size. • DEVELOPMENT • light localization. Accomplished with a small lamp and mirror. • collimating leaves like the lens diaphragm of a photographic camera producing circular fields. • Collimator shutter leaves that work in pairs and are independently controlled. This produces both rectangular and square fields. • Positive beam-limiting devices- automatically adjust the field size according to the size of cassette used. Susan A. Olavidez

  7. GRID • A device that absorbs scatter radiation before it reaches the film. • A device made from carefully fabricated series of radiopaque material (lead) alternating with sections of radiolucent material (aluminum or plastic) • Developed by Dr. Gustave Bucky in 1913 Susan A. Olavidez

  8. Characteristic of Grid Construction • GRID RATIO - the height of grid strip divided by the thickness of interspace material. Grids with high grid ratio are more effective in cleaning up scatter radiation. However it will require higher exposure factor and therefore high patient dose Susan A. Olavidez

  9. GRID FREQUENCY - the number of grid strips or grid lines per inch or per centimeter. Grid with high frequencies show less distinct grid lines on the x-ray films. The higher the grid frequency , the higher the radiographic technique required and the greater the dose to the patient. Susan A. Olavidez

  10. Measuring Grid Performance • CONTRAST IMPROVEMENT FACTOR - ratio of the contrast of an x-ray film taken with the grid to the contrast of an x-ray film taken without a grid. • BUCKY FACTOR - the ratio of the incident remnant radiation to the transmitted remnant radiation. It measure how much of an increase in technique factor will be required compared with non-grid exposure. • SELECTIVITY - the ratio of transmitted primary radiation to transmitted scatter radiation, Susan A. Olavidez

  11. Types of Grid • LINEAR GRID - the simplest type of grid. It has grid strips running along the axis of the grid. • CROSSED HATCHED GRID - are grids that are made by putting together two linear grids with lead strips perpendicular to one another. Susan A. Olavidez

  12. PARALLEL GRID - grid strips are parallel to each other. • FOCUSED GRID - grid strips are angulated so that they lie on imaginary radius lines from the center of a circle. Susan A. Olavidez

  13. MOVING GRID - grid moves during exposure. Dr. Hollis Potter in 1920 conceptualized the idea of moving grid. Other names are “Potter-Bucky Diaphragm”, “Bucky diaphragm”, and “Bucky grid”. Three Basic Types of Moving Grid Mechanisms • single-stroke grid • reciprocating grid • oscillating grid Susan A. Olavidez

  14. Errors in the Use of Grid • OFF LEVEL GRID - central axis of the beam is not perpendicular to the grid. • OFF-CENTER GRID - when focused grid is not positioned off center. • OFF-FOCUSED - when focused grid is not used at the proper focal distance. • UPSIDE-DOWN GRID - focused grid positioned upside-down. Susan A. Olavidez

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