1 / 28

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

brigit
Télécharger la présentation

RA 111 RADIOGRAPHIC IMAGING II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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?

More Related