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RA 111 RADIOGRAPHIC IMAGING II

RA 111 RADIOGRAPHIC IMAGING II. CONTROL OF SCATTER RADIATION. SCATTER RADIATION. DEFINITION: THREE INTERACTIONS PASSES THROUGH ABSORBED SCATTERS. INFLUENCING FACTORS. VOLUME OF TISSUE Volume = tissue thickness and area exposed Increase thickness = increase scatter

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RA 111 RADIOGRAPHIC IMAGING II

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  1. RA 111 RADIOGRAPHIC IMAGING II CONTROL OF SCATTER RADIATION

  2. SCATTER RADIATION • DEFINITION: • THREE INTERACTIONS • PASSES THROUGH • ABSORBED • SCATTERS

  3. INFLUENCING FACTORS • VOLUME OF TISSUE • Volume = tissue thickness and area exposed • Increase thickness = increase scatter • Increase area = increase scatter • BEAM ENERGY • Increase KV = increase scatter • Decrease KV = decrease scatter

  4. EFFECTS OF SCATTER • DENSITY • Scatter puts exposure on the film • Effect on density? • CONTRAST • Scatter exposes areas of the image that are supposed to be light • Effect on contrast?

  5. METHODS FOR CONTROLLING SCATTER BEAM RESTRICTION GRIDS

  6. BEAM RESTRICTION • PURPOSE? • SECONDARY BENEFIT? • DEVICES • Aperture diaphragm • Cones and cylinders • Variable collimators

  7. APERTURE DIAPHRAGM • LEAD SHEET • Opening the size of the area needed for the exposure • Must be manually changed to change beam size

  8. CONES AND CYLINDERS • EXTENSIONS OF THE APERTURE DIAPHRAGM • Circular beam • Must be manually changed

  9. VARIABLE APERTURE COLLIMATOR • ALLOWS A VARIATION IN THE SIZE AND SHAPE OF THE BEAM • Controlled manually or electronically (PBL) • Light indicates beam • Light must be aligned with beam • More effective than the other methods

  10. GRIDS • PURPOSE? • STRUCTURE • Vertically placed lead strips • Radiolucent material between them

  11. GRID FUNCTION • PRIMARY RADIATION WILL PASS BETWEEN THE LEAD STRIPS • SCATTER RADIATION WILL BE ABSORBED BY LEAD STRIPS • Why? • Significance?

  12. GRID RATIO • DEFINITION • Ratio of height of lead strips to width of interspace (Ratio = H/D) • Ratio controlled by either height of lead strips or width of interspace • Common ratios = 5:1, 6:1, 8:1, 12:1, 16:1

  13. GRID RATIO • EFFECT ON SCATTER ABSORPTION • High ratio grids absorb more scatter (+reverse) • Why? • EFFECT ON IMAGE CONTRAST • High ratio = short scale (+reverse) • Why?

  14. GRID RATIO • EFFECT ON IMAGE DENSITY • High ratio = lower density (+reverse) • Why? • EFFECT ON PATIENT EXPOSURE • High ratio = greater pt. exposure (+reverse) • Why?

  15. SELECTION OF GRID RATIO • DESIRED CONTRAST FOR EXAM • PT. SIZE AND CONDITION • KV REQUIRED FOR EXAM • CIRCUMSTANCES IN WHICH THE EXAM IS BEING DONE • PATIENT EXPOSURE

  16. GRID FREQUENCY • DEFINITION: • Number of lines per inch or cm • ADVANTATE OF HIGH FREQUENCY • Thinner lines – less visible on image • DISADVANTAGES • Require more exposure • Why?

  17. GRID TYPES • LINEAR • Strips running one direction • CROSSED/RHOMBIC • Strips running two directions • PARALLEL • Strips parallel to each other • FOCUSED • Strips parallel to x-ray beam

  18. FOCUSED GRIDS • GRID RADIUS • Distance from the grid at which the strips would meet if extended • FOCUSING DISTANCE • SID that must be used with focused grids • Equal to the grid radius • About 10% error allowable

  19. GRID TYPES • GRIDS CAN BE: • PARALLEL / LINEAR • PARALLEL / CROSSED • FOCUSED / LINEAR • FOCUSED / CROSSED

  20. ADVANTAGES DISADVANTAGES • LINEAR vs CROSSED • Crossed pick up more scatter • Why? • Crossed do not allow tube angles • Why? • Linear grids pick up less scatter • Why? • Linear grids allow angles parallel to grid strips but not across • Why?

  21. ADVANTAGES DISADVANTAGES • PARALLEL vs FOCUSED • Parallel grids cut off radiation at edges of image • Why? • Parallel grids allow for a variety of SIDs • Why? • Focused grids reduce cut-off at edges • Why? • Focused grids must have an exact SID • Why?

  22. MOVING GRIDS(Potter-Bucky Diaphragm) • PURPOSE: • Make grid lines invisible • GRID LINES • Visible image of lead strips • OPERATION • Grid moves during exposure • DISADVANTAGES • Increased OID • Table / IR vibration • Longer minimum exposure time

  23. GRID PERFORMANCE • SELECTIVITY • Ratio of transmitted primary radiation to transmitted scatter • Related to lead content and grid ratio (p253) • CONTRAST IMPROVEMENT • Ratio of contrast with a grid to contrast without a grid • Higher for high ratio grids

  24. BUCKY FACTOR • GRIDS REQUIRE AN INCREASE IN EXPOSURE FACTORS • GRID FACTOR IS A MULTIPLIER TO INDICATE REQUIRED EXPOSURE WHEN GRIDS ARE ADDED OR CHANGED • BASED ON GRID RATIO

  25. BUCKY FACTOR • No grid = 1 • 5:1 = 2 • 6:1 = 3 • 8:1 = 4 • 12:1 = 5 • 16:1 = 6 • FORMULA?

  26. GRID ERRORS • GRID CUTOFF • Removal of excess image-forming radiation • Reduction in image density • Can occur with all grids • Parallel grids susceptible with short SID and/or large field size • Focused grids susceptible when used improperly

  27. GRID ERRORS • OFF-LEVEL GRID • Cutoff across entire image • OFF-CENTER GRID • Cutoff across entire image • OFF-FOCUS GRID • Cutoff at edges of image • UPSIDE-DOWN GRID • Cutoff at sides of image

  28. AIR GAP TECHNIQUE • USING A LARGE OID TO REDUCE SCATTER ON IMAGE • SCATTER TRAVELING AT ANGLE WILL BYPASS THE IMAGE • CRID MAY NOT BE NEEDED TO GET APPROPRIATE CONTRAST • PROBLEMS?

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