Radiographic grids

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Radiographic grids

  1. 1. RADIOGRAPHIC GRIDS SOURAV TALUKDER MBBS
  2. 2. INVENTED BY DR GUSTAV BUCKY IN 1913
  3. 3. GRIDS DEVICES THAT REDUCE THE AMOUNT OF SCATTERED RADIATION REACHING THE IMAGE RECEPTOR
  4. 4. FUNCTION TO IMPROVE IMAGE CONTRAST
  5. 5. SCATTER
  6. 6. FACTORS AFFECTING SCATTER PRODUCTION WITHIN THE PATIENT • KILOVOLTAGE • FIELD SIZE • THICKNESS OF IRRADIATED TISSUE SCATTER IS PRODUCED WITHIN THE PATIENT THROUGH COMPTON INTERACTION
  7. 7. GRID RADIOGRAPHY IS RECOMMENDED FOR: • ANATOMICAL PARTS > 10 cm • WITH HIGH kVp ( NOT ALWAYS—MAMMO) • SOFT TSSUE STRUCTURES TO INCREASE CONTRAST • STRUCTURES AFFECTED BY PATHOLOGICAL CONDITION THAT WOULD INCRESE SCATTER PRODUCTION
  8. 8. LOCATION BETWEEN PATIENT AND IMAGE RECEPTOR
  9. 9. GRIDS INCREASE PATIENT DOSE
  10. 10. CONSRUCTION  LEAD  INTERSPACE MATERIALS a. aluminium b. organic TYPICAL LEAD STRIPS-50 µms and INTERSPACE- 350 µms
  11. 11. COMPARISON BETWEEN TWO TYPES OF INTERSPACE MATERIALS ALUMINUM ORGANIC 1. STRUCTURALLY STRONGER 2. REMOVES MORE SECONDARY RADIATION- MORE IMAGE CONTRAST 3. REMOVES MORE PRIMARY RADIATION- MORE PATIENT EXPOSURE 1. STRUCTURALLY WEAKER 2. REMOVES LESS SECONDARY RADIATION 3. REMOVES LESS PRIMARY RADIATION
  12. 12. IDEAL GRID REMOVES 100% OF SCATTER RADIATION PASSES ALL PRIMARY RADIATION IN REALITY THERE IS NO SUCH IDEAL GRID
  13. 13. GRID RATIO • HEIGHT OF LEAD STRIPS(h) DIVIDED BY DISTANCE BETWEEN EACH LEAD STRIPS(D) GR =h/D HIGHER THE RATIO- a. INCREASED IMAGE CONTRAST b. MORE EXPOSURE 2 PATIENT c. MORE EFFECT OF GRID CUT-OFF
  14. 14. GRID RATIO
  15. 15. GRID PATTERN ORIENTATION OF LEAD STRIPS IN THEIR LONGITUDINAL AXIS a. LINEAR b. CROSSED GRID RATIO OF CROSSED GRIDS ═ SUM OF THE RATIOS OF THE TWO LINEAR GRIDS
  16. 16. GRID STYLES FOCUSSED GRID- LEAD STRIPS ARE ANGLED SLIGHTLY; THEY FOCUS IN SPACE • CANTING- TILTING OF THE LEAD STRIPS TO CREATE FOCUSSED GRID • MOST GRIDS ARE FOCUSSED • CONVERGENT LINE • CONVERGENT POINT • FOCAL DISTANCE
  17. 17. • FOCUSSING RANGE- WIDE FOR LOW-RATIO GRID AND NARROW FOR A HIGH RATIO GRID PARALLEL GRID- LEAD STRIPS ARE PARALLEL • USED WITH VERY SMALL X-RAY FIELD OR LONG TARGET-GRID DISTANCE
  18. 18. GRID FREQUENCY MEASURED BY LINES PER INCH Typical: 103 (NORMAL RANGE- 60 TO 200) 25.4 Lines per inch = ------------ W + w w = thickness of interspace (mm) W = thickness of lead strips (mm) w W
  19. 19. LEAD CONTENT • MORE LINES/INCH AT CONSTANT GRID RATIO MEANS LESS LEAD CONTENT AND LESS CONTRAST IMPROVEMENT
  20. 20. GRID PERFORMANCE THREE INDICATORS  PRIMARY TRANSMISSION  BUCKY FACTOR  CONTRAST IMPROVEMENT FACTOR
  21. 21. PRIMARY TRANSMISSION MEASUREMENT OF THE PERCENTAGE OF PRIMARY RADIATION TRANSMITTED THROUGH THE GRID P.T = (INTENSITY WITH GRID/ INTENSITY WITHOUT GRID)×100 P.T-IDEALLY 100%
  22. 22. PRIMARY TRANSMISSION • Typical values: 55 - 75% • Theoretic calculation: (fraction of grid that is interspace) Tp (%)= 100 X W / (W+w) where W = Interspace thickness w = lead strip thickness • actual transmission < theoretical – primary attenuated by interspace material – focusing imperfections w W W+w
  23. 23. BUCKY FACTOR RATIO OF THE INCIDENT RADIATION FALLING ON THE GRID TO THE TRANSMITTED RADIATION PASSING THROUGH THE GRID B.F= INCIDENT RADIATION/ TRANSMITTED RADIATION
  24. 24. • INDICATES ACTUAL INCREASE IN EXPOSURE DUE TO GRID’S PRESENCE • DUE TO ATTENUATION OF BOTH PRIMARY AND SECONDARY RADIATION • HIGHER GRID RATIO = HIGHER BUCKY FACTOR • TYPICAL VALUE : 3-6
  25. 25. CONTRAST IMPROVEMENT FACTOR ULTIMATE TEST OF GRID’S PERFORMANCE C.I.F= CONTRAST WITH A GRID/ CONTRAST WITHOUT A GRID HIGHER THE GRID RATIO, HIGHER THE C.I FACTOR
  26. 26. GRID CUT OFF LOSS OF PRIMARY RADIATION THAT OCCURS WHEN THE IMAGES OF THE LEAD STRIPS ARE PROJECTED WIDER THAN THEY WOULD BE WITH ORDINARY MAGNIFICATION
  27. 27. TYPES  FOCUSSED GRIDS USED UPSIDE DOWN  LATERAL DECENTERING  DISTANCE DECENTERING  COMBINED DECENTERING
  28. 28. FOCUSSED GRID UPSIDE DOWN DARK BAND OF CENTRAL EXPOSURE SEVERE CUT-OFF AT PERIPHERY . CROSSED GRID- SMALL SQUARE AT THE CENTRE IS EXPOSED
  29. 29. LATERAL DECENTERING RESULTS FROM THE X-RAY TUBE BEING POSITIONED LATERAL TO THE CONVERGENT LINE BUT AT THE CORRECT FOCAL DISTANCE PROBABLY MOST COMMON TYPE OF GRID CUT-OFF
  30. 30. LATERAL DECENTERING uniform loss of radiation over entire film uniformly light radiograph no recognizable characteristic (dangerous)
  31. 31. LATERAL DECENTERING • also occurs when grid tilted • Magnitude depends upon- grid ratio focal distance amount of decentering
  32. 32. LATERAL DECENTERING • Significant problem in portable radiography • Because exact centering not possible • minimizing lateral decentering – low ratio grids – long focal distances
  33. 33. DISTANCE DECENTERING • TARGET OF THE X-RAY TUBE IS CORRECTLY CENTERED TO THE GRID BUT POSITIONED ABOVE OR BELOW THE CONVERGENT LINE • FAR AND NEAR DECENTERING- CUTOFF GREATER IN NEAR • ALL PARALLEL GRIDS HAVE SOME DEGREE OF DISTANCE DECENTERING
  34. 34. • Far focus-grid decentering •Near focus-grid decentering •cutoff at periphery •dark center •cutoff proportional to •grid ratio •decentering distance
  35. 35. COMBINED LATERAL AND FOCUS DECENTERING  MOST COMMONLY RECOGNIZED • UNEVEN EXPOSURE- FILM IS LIGHT ON ONE AND DARK ON OTHER SIDE
  36. 36. MOVING GRIDS • INVENTED BY DR. HOLLIS E. POTTER IN 1920 • MOVE TO BLUR OUT SHADOWS CAST BY LEAD STRIPS • MOVES 1 TO 3 CMS BACK FORTH THROUGHOUT THE EXPOSURE
  37. 37. P-B DIAPHRAGM
  38. 38. PRECAUTIONS • GRID MUST MOVE FAST ENOUGH TO BLUR ITS LEAD STRIPS • TRANSVERSE MOTION OF THE GRID SHOULD BE SYNCHRONOUS WITH THE PULSES OF THE X-RAY GENERATOR
  39. 39. DISADVANTAGES • COSTLY • VIBRATION POTENTIAL • LIMITS MINIMUM EXPOSURE TIME • INCREASES PATIENT DOSE
  40. 40. GRID SELECTION • BALANCE BETWEEN CLEAN UP AND PATIENT EXPOSURE • BELOW 90 KVP- 8:1 GRIDS • ABOVE 90 KVP- 12:1 GRIDS • CROSSED GRID- HIGH SCATTER RADIATION AS IN BIPOLAR DSA
  41. 41. GRID INFO
  42. 42. AIR GAP TECHNIQUES • ALTERNATIVE METHOD • LOGIC • USE • IMAGE SHARPNESS AND FOCAL FILM DISTANCE • PATIENT EXPOSURE
  43. 43. ACKNOWLEDGEMENTS • CHRISTENSEN’S PHYSICS OF DIAGNOSTIC RADIOLOGY • GOOGLE FOR INTERNET SOURCES
  44. 44. THANK YOU

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