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The Grid n Kyle Thornton n DMI 50B n Kyle Thornton n DMI 50B.

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Presentation on theme: "The Grid n Kyle Thornton n DMI 50B n Kyle Thornton n DMI 50B."— Presentation transcript:

1 The Grid n Kyle Thornton n DMI 50B n Kyle Thornton n DMI 50B

2 What Is A Grid? n Invented in 1913 by Gustaf Bucky –Consisted of a framework containing lead foil strips standing on edge, parallel and equidistant to each other n In 1920, Hollis Potter invented a mechanism for suspending the grid in a framework that moved between the patient and film –The motion eliminated the grid lines in the image n The grid is the most effective way to remove secondary radiation from large radiographic fields

3 What Does A Grid Do? n A grid is an important radiographic tool n A grid absorbs scatter radiation before it reaches the film n A grid improves contrast on the film n A grid has a special composition and many different types n Used properly, the grid is a technologist’s best friend

4 Grid Construction n Grid ratio n Grid frequency n Interspace material n Lead strips

5 Grid Ratio n Three important dimensions of a grid –Grid thickness - T –Interspace material thickness - D –Grid height - h n Grid ratio is the height divided by interspace material thickness –Grid ratio = h/D

6 Why Is Grid Ratio Important? n Grid ratio determines how scatter radiation is “cleaned up” n The higher the grid ratio, the more cleanup n Grids of higher ratios require more technique n This results in a higher patient dose n Ratios range from 5:1 - 16: 1 n Mammo grids have very low ratios

7 Grid Ratio Equation n The distance between each grid strip is 200  m and the height is 2.4 mm. What is the grid ratio? n Hint: Ratio = h/D n Step 1 – Identify h n Step 2 – Identify D n Step 3 – ???? n Step 4 - ????

8 Grid Frequency n The number of strips or lines per inch or centimeter is grid frequency n Higher frequencies display less lines n Higher frequencies affect patient dose n Higher frequencies are generally associated with higher ratios n Most grid frequencies are 60-110 lines/inch n Mammo grids have very high frequencies, but low ratios

9 Interspace Material n The material between the grid strips n Maintains a precise separation between the strips n Generally constructed from aluminum or plastic fiber n Aluminum has definite advantages over fiber

10 Grid Strips n Should be very thin and have high scatter absorption properties n Lead is best n The entire grid is encased in aluminum for protection n Sometimes it is further encased in plastic for more protection

11 Grid Performance n Contrast improvement factor n Bucky factor n Selectivity

12 Contrast Improvement Factor n Grids remove scatter radiation before it reaches the film n Therefore it improves contrast n Contrast improvement factor compares contrast improvement with a grid to that without a grid

13 Contrast Improvement Factor Equation n K = Radiographic contrast with grid Radiographic contrast without grid n Most grids have a contrast improvement of 1.5 - 2.5 n Contrast improvement is higher with higher ratio grids n Lead content also determines contrast improvement

14 Bucky Factor n Also called grid factor n This compares the increased technique necessary for grid use n Bucky factor will increase with with increasing grid ratio n It will also increase with increasing kVp n B = Incident remnant radiation Transmitted remnant radiation n The amount of radiation hitting the grid will always be greater than the amount hitting the film

15 Grid Selectivity n Related to grid construction itself n The total lead content of the grid has an influence on selectivity n The more lead, the more cleanup n  = Primary radiation transmitted through grid Scatter radiation transmitted through grid

16 General Rules Of Grid Characteristics n High ratio grids have high contrast improvement factors n High frequency grids have thin strips of interspace material and low contrast improvement factors n Heavy grids have high selectivity and high contrast improvement factors

17 Grid Types n Linear parallel n Crossed n Focused n Moving grids –Single stroke –Reciprocating –Oscillating

18 Linear Parallel Grid n Simplest to construct n The grid strips are parallel n Most latitude

19 Crossed Grid n Two linear grids at right angles to each other n Was used primarily for pneumoencephalography n Used for high contrast studies n Very high cleanup n Not used very much n Must be centered exactly n Must be directly perpendicular to grid

20 Focused Grid n The strips run on one axis and are tilted n Strips are parallel to the primary x-ray path across entire film n Must use within a proscribed distance

21 Moving Grids Single Stroke n Antiquated n Grid had to be cocked with a spring mechanism n Worked in synch with exposure time n The mechanism moved once throughout exposure n Had to be reset for each exposure

22 Reciprocating Grid n Moves back and forth during exposure n Motor driven n Does not have to be reset for each exposure

23 Oscillating Grid n Similar to a reciprocating grid n Moves in a circular motion as opposed to back and forth

24 Grid mounted within Bucky Tray

25 Advantages And Disadvantages Of Moving Grids n Advantages –No grid lines –Problems occur infrequently n Disadvantages n Mechanical problems may occur n Very infrequently, motion is detected on radiograph

26 Grid Cutoff n A big problem with linear and crossed grids n Less of a problem with focused grids n The primary beam has been absorbed n Has a negative effect on image detail, density, and contrast

27 Grid Errors n Off-center –Beam is not centered to center of grid n Upside down –Focused grid only –Causes severe grid cutoff in periphery of film

28 More Grid Errors n Off-focus error –Focusing distance not observed –Focused grid only n Using the incorrect focal distance results in grid cutoff in the periphery of the image

29 Yet another grid error n Off-center – off- focus –Partial grid cut-off occurs over the entire image

30 Even more grid errors… n Off-level –Beam is not perpendicular to grid Or –Grid is not perpendicular to the beam Either way, you’re repeating that film

31 How many grid errors can there be? n The classic grid error –Focused grid placed upside down

32 Demo of Focused Grid Used Upside Down n DPt0Gjz0&feature=related DPt0Gjz0&feature=related

33 Grid errors from the beam and grid’s perspective

34 Summary of grid errors and associated results n Off-level –cutoff across image; underexposed, light image n Off-center –Grid cutoff across image; underexposed, light image n Off-focus –Grid cutoff toward edge of image n Focused Grid Placed Upside-down –Severe grid cutoff toward edge of image n Off-center, off-focus –Grid cutoff on one side of image

35 Grid Selection n Depends upon body part to be radiographed n Chest radiography uses high kVp n 8:1 ratio can be used for most general work –Up to about 90 kVp n Focused grids are generally superior n Lower ratio grids offer more positioning latitude

36 Grids And Patient Dose n Patient dose increases with increasing grid ratio n High ratio grids are generally used for high kVp studies n Patient dose decreases with higher kVp use n Less radiation is absorbed in tissues with higher kVp

37 Suggested Grid Conversion Factors

38 Alternatives To Grid Use n Air-gap technique n OID is increased n Equal to approximately 8:1 grid n Increases magnification n Distance must be increased to overcome magnification n Not effective with high kVp

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