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Grids

Grids. RTEC 244. Dr. Gustave Bucky. Since Dr. Gustav Bucky built the first grid in 1913, his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known techniques to trap the scatter.

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Grids

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  1. Grids RTEC 244

  2. Dr. Gustave Bucky • Since Dr. Gustav Bucky built the first grid in 1913, • his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known techniques to trap the scatter CROSSHATCH GRID Potter-Bucky Diaphragm • Dr. Hollis Potter made improvements to the use of grids • Realigned lead strips to run in one direction • Moved grid during exposure to make lines invisible on image

  3. Transmission Responsible for dark areas Absorption Responsible for light areas Scatter Creates fog Lowers contrast Increases as kV increases Field size increases Thickness of part increases Z# decreases Creating the Image

  4. Patient Dose Exam Detail required Part thickness Desired technique (kVp) Equipment availability Indications for Grid Use Part thickness > 10 cm kVp > 60 EXCEPTIONS? Grid Selection

  5. Grid is placedbetween patient (behind table or upright bucky) & cassetteIf placed BACWARDS CAN CAUSE GRID ERRORS

  6. What are some factors that increase scatter radiation?

  7. Grids “clean up” scatter radiation • A high quality grid can attenuate 80 –90 % of scatter radiation 3 factors contribute to an increase in scatter • Increased kVp • Increased x-ray field size • Increased patient thickness

  8. Ideally, only those x-rays that do not interact with the patient should reach the IR…. • However, scatter radiation is a factor that must be managed • Proper collimation has the PRIMARY effect of reducing patient dose by _________ ? • Proper collimation also improved image contrast by reducing radiographic noise or fog caused by scatter

  9. How does increasing kVp affect patient dose?

  10. Basic Grid Construction • Radiopaque lead strips • Separated by radiolucent interspace material • Typically aluminum

  11. Grids • Allow primary radiation to reach the image receptor (IR) • Absorb most scattered radiation • Primary disadvantage of grid use • Grid lines on film

  12. CASSETTES W/ GRID CAPSSTATIONARY GRIDS • Stationary grids • Grids that can be attached to a cassette for use or a specially designed • Grid cassettes

  13. Height of lead strips divided by thickness of interspacing material • Grid ratio = h/D Grid Dimensions • h = the height of the radiopaque strips • D = the distance between the strips • the thickness of the interspace material • GRID RATI0 • The distance between lead strips may remain constant so the thickness of the grid must increase as grid ratios increase. It is possible to appreciate • the smaller angle of deflection of the x-ray photon that will pass through the 16:1 ratio grid. • Thus, high ratio grids usually "clean-up the beam," removing scatter radiation more effectively than low ratio grids.

  14. Grid Ratio • Higher grid ratio • More efficient in removing scatter • Typical grid ratio range is 5:1 to 16:1

  15. Grid Ratio Example • If a grid has an interspace of 0.5mm and lead strips that are 3mm high, what is it's grid ratio? • GR = 3mm/0.5mm GR = 6:1

  16. Grid Frequency • The number of lead strips per inch or cm • Frequency range • 60-200 lines/in • 25-80 lines/cm • Typically higher frequency grids have thinner lead strips • Higher frequency with the same interspace distance reduces the grid effectiveness

  17. The higher the ratio the straighter the photon must travel to reach the IR • Grid ratios range from 5:1 to 16:1 • Most common 8:1 to 10:1 • A 5:1 grid will clean up 85% • 16:1 clean up 97%

  18. Digital Imaging Systems • Very high-frequency grids • 103-200 lines/in • 41-80 lines/cm • Recommended for use with digital systems • Minimizes grid line appearance

  19. Air-Gap Technique • Or Air filtration • Increase OID by 10 to 15 cm • This reduces the amount of scatter reaching the IR because some scatter will miss the IR. • It is about the same as using an 8:1 grid • mAs is increased 10% for every cm of air gap • Increases magnification and reduction in detail. Has some selective uses with chest imaging and cerebral angiography

  20. Air-Gap Technique • An alternative to grid use • 10” air gap has similar clean-up of 15:1 grid • Problems: • Increased OID = increase in blur • Must increase SID • Motion due to lack of contact to IR • AIR GAP. • Drawing illustrates the tube, object, grid and film relationship in conventional radiography and the use of an air gap to decrease the effect of scatter radiation. Note that an increase in the FFD tends to decrease the magnification of the image on the film.

  21. Grid Patterns • Criss-cross or cross-hatched • Linear • Parallel • Focused

  22. Linear Grid • Lead strips run the length of cassette • Allows primary beam to be angled along the long axis of grid without obtaining “cut-off”

  23. Focused Linear Grids • Lead strips are angled to match divergence of beam • Primary beam will align with interspace material • Scatter absorbed by lead strips • Convergence line • Narrow positioning latitude • Improper centering results in peripheral cut-off • Only useful at preset SID distance • Higher ratio grids require careful alignment with tube

  24. All lead strips are parallel to one another Absorb a large amount of primary beam Resulting in some cut-off Parallel Linear Grids

  25. Grid Use & Movement • Potter-Bucky diaphragm • The Bucky • Mounts a 17” x 19” grid above cassette • Moves the grid during exposure • Reciprocating • Motor drives grid back and forth during exposure • Oscillating • Electromagnet pulls grid to one side • Releases it during exposure

  26. Grids and Exposure Factors • Whenever a grid is placed in beam to remove scatter • Density of radiograph will go down • Exposure factors must be increased to compensate for lack of density • Required increase in technique can be calculated • Grid conversion (GCF) or Bucky factor GCF = mAs with grid mAs without grid

  27. GRID CONVERSION FACTORS NO GRID 1 5:1 2 6:1 3 8:1 4 10/12:1 5 16:1 6 CHANGES IN MAS

  28. Give It a Try! • Original: 20mAs with an 8:1 grid • Find new mAs with a 12:1 grid • mAs2 = 20 mAs x 5 4 • mAs2 = 100 4 • mAs2 = 25

  29. Selectivity or ability to “clean up”the heavier the grid the more Pb it contains

  30. Selectivity “K” factor • Describes grid’s ability to allow primary radiation to reach image receptor and prevent scatter • Grids are designed to absorb scatter • Sometimes they do absorb primary radiation • Compares radiographic contrast of an image with a grid to radiographic contrast of an image without a grid • Typically ranges between 1.5 – 3.5

  31. Grid Errors • Off-level • Off-center • Off-focus • Upside-down • Moire effect • Proper alignment between x-ray tube and grid • Very important • Improper alignment will result in cut-off

  32. Grid Problems • Increased OID, especially with moving grids • The biggest problem with grids is misalignment GRID PROBLEMS RESULT IN: UNDEREXPOSED IMAGE OR UNDEREXPOSED EDGES OF IMAGE

  33. Grid Problems – Off Level CUT OFF MORE SEVERE ON ONE SIDE THAN THE OTHER

  34. Grid Problems – Off Center A problem with focused & crossed grids

  35. Grid Problems – Off Focus(wrong SID) CUT OFF EQUAL ON BOTH SIDES

  36. Grid Problems – Upside-Down A problem with focused grids = severe cut off on both edges

  37. STATIONARY GRID/NOT MOVINGGRID CUT OFF - EVENLY ACROSS

  38. GRIDS CAN LEAVE LINES ON THE IMAGE

  39. CR GRIDS

  40. REG GRID VS DR GRID What is this called? What causes this?

  41. Moire Effect • Digital systems • When grid lines are parallel to scan lines • High frequency grids can prevent this phenomenon

  42. Moire Effect • When grid lines are parallel to scan lines

  43. DR USES HIGER FREQUENCY GRIDS

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