بسم الله الرحمن الرحيم

1. بسم الله الرحمن الرحيم يكاد زيتها يضيء و لو لم تمسسه نار نورً على نور صدق الله العظيم

2. THE GRID By Ahmad Mokhtar Hamed Abodahab

3. No doubt that scattered radiatuon is a great problem disturbing the quality of radiographic image . • THE GRID • IS a great solution for this problem . • SO ….. What is the grid … & how it work ?

4. Introduction : • Scattered X ray produced from the patient by Compton effect • It is not useful , really it is a harmful factor to the process of diagnosis . • The intensity of scattered radiation is relativly rallated to : such as * Kv * Beam size * Patient thickness

5. Steps of solutions of this problem: • As scattered radiation has lower energy than the primary beam , & radiographic film is sensitive to both  increase film sensitivity to the 1 ry beam is the solution . • Using of intensifying screens  improving of image quality , BUT not due to the previous cause .

6. Selective filter passing 1 ry beam , was not effective • The axtremly effective device for elemenation of scattered X ray was THE GRID .

7. Structur of the grid : • In 1913 it is invented by Dr.Gustave Bucky • It is a board like device , composed of alternating strips of : * Radiopaque material ( Grid material ) & * Radio translucent material (interspace material )

8. This structures are encased in an Alumenium case , for : * Rigidity * Protection from moisture . • In Typical grid : • grid strips thickness 50 um , interspace strips 350 um • Up to 12.5% of all photons falling on grid absorbed .

9. HOW A GRID WORK : • All primary beam photons are passing when falling on interspace strips . • Scattered beams may or may not pass according there angle , if a beam falling by an angle . It will strike grid strip by side  absorbed & not passing to the film .

10. *1ry beam *Scattered

11. If a grid is remove about 80 : 90 % of scattered radiation = it has good clean up . • The percentage of absorbed radiation = width of grid strips width of (grid+iterspace ) strips

12. Characteristics of grid construction: • Certain characteristics are related to the construction of grid identifying its radiographic characters , these are : • 1- GRID RATIO • 2- GRID FREQUENCY • 3- GRID MATERIAL • 4- INTERSPACE MATERIAL

13. 1- GRID RATIO : • = h or Height of grid strips . D Thickness of interspace • High ratio  less angle of deviation is allowed  more effective clean up .

15. Increase of Grid ratio is made by : 1- Increase grid height 2- Reduce width of inter space or both • Grid ratio generally 5 : 1 to 16 : 1 • Disadvantages of high ratio : 1- Difficult to be manufactured 2- Need higher exposure to get satisfied image

16. 2- GRID FREQUENCY : • It is number of grid strips / inch or cm . * Grids with high frequency  less distinct grid lines in film . • Most grids have frequency 60:110/inch i.e. 24: 42 / cm • Disadvantage : Increase frequency  more dose needed , this can be overcomed by decrease thickness of grid strip . ( but this will decrease its clean up )

17. 3- GRID MATERIAL : • The grid strip should be thin & highly absorber to x ray . • Lead Is the most widely used material due to : * High atomic number * Easy to be shaped * Relatively inexpensive • Other substances : tungsten , platinum , gold & uranium . • Non of these materials is better than lead .

18. 4- Interspace material : • Aluminum or plastic fibers , are the main substances . • Advantages of aluminum : 1- High atomic number some selective filtration of scattered rays . 2- Give less visible grid lines 3- It doesn't absorb moisture . • Disadvantages : More dose is need with it .

19. * MEASURING GRID* PERFORMANCE • The principle function of grid is : eliminating of scattered radiation  improve contrast . • Thus to measure grid performance = measure of contrast improvement . • .: grid performance = CONTRAST IMROVEMENT FACTOR ( k )

20. Contrast improvement factor ( k ) : it is the ratio between contrast with & with out using a grid . k = Radiographic contrast with grid Radiographic contrast without grid • Most grids has (k) 1.5 : 2.5 k= Radiographic contrast with grid Radiographic contrast without grid

21. Factors affecting it : • High ratio  high k • Heavy grids  high k • High frequency  low k • K is generally measuring at 100 kv

22. SELECTIVITY (LEAD CONTENT) • In an ideal grid , all 1ry beams transmitted , & all scattered absorbed . • SELECTIVITY ( ) : is the ratio of transmitted 1ry radiation to scattered radiation . • Segma ( ) = 1ry radiation transmitted scattered rad transmitted

23. Total mass of lead is the main factor affecting selectivity . But proper arrangment is an important factor , if lead strips are adherent to each other  no image will be formed MORE LEAD  HIGHER SELETIVITY

25. * TYPES OF GRIDS * • Linear grids • Crossed grids • Focused grids • Moving grids

26. 1- linear grid • It is the simplest grid . • In which all lead strips are parallel . • It is the easiest to be manufactured . • Disadvantage : Grid cutoff ??

28. 2- Crossed grid : • In which the lead strips running parallel to both axes CROSSING each others . • it is made by sandwiching 2 linear grids , with each strips perpendicular to each others .  it is not too difficult to be manufactured . • Advantages : 1- much more efficient . 2- Easy made , not very expensive . 3- .

29. Disadvantages : ????

30. 3-Focused grid : • STRUCTURE : lead strips are angulated in a manner as parts of radiuses of a circle * The source of X ray should be at the center of this imaginary circle , if this not occur  excess cutoff will occur ( so it called FOCUSED ) • Advantage : elemenating of grid cutoff . • Disadvantages : * Difficult manufactured . * Must be focused well . *Producing grid lines ( in each of the previous types )

32. 4-Moving grids : • Grid lines : Are the shadow lines of the grid strips appearing in the radiograph . It occur due to absorbtion of some 1ry beams by grid strips . • The more wide grid strips more obvious lines . • More frequency  Less obvious lines .

33. At 1920 , Dr. Hollies potter suggesting to make the grid moving during radio graphing the film, thus a moving grid invented . • By adding moving mechanism to a linear grid , it will become MOVING GRID .

34. Types of moving grid mechanisms: • Single stroke grid . • Reciprocating grid • Oscillating grid 1- single stroke grid * In which the grid is loaded on a spring & loaded manually • Disadvantages : • Difficult to be used due to the manual cooking of the spring .

35. 2-Reciprocating grid : It is a grid moving by a motor in each time of exposure . It is moving in to & fro manner . Its main advantages : it is not requiring resetting after each exposure . • 3-Oscillating grid : it is moving in circular manner .

36. Disadvantages of moving grids : • It require bulky mechanism , which able to be failed . • More distance is need between Film & patient , this give false magnification in the film . • Increase of exposure time . • The movement of the grid , may vibrate the film blurred image . • Stroboscopic effect .

37. Precautions in using of the grid : Precautions in using of the grid : • Improper positioning is the most frequent problem in using of a stationary grid . • An important 4 situations must be prevented in using of grid : • OFF-LEVEL GRID • OFF-CENTER GRID • OFF-FOCUS GRID • UPSIDE-DOWN GRID

38. 1- OFF-LEVEL GRID • For proper function of grid , the axis of central beam must be perpendicular on the grid . • If it angulated , the primary beams will absorbed in the grid strips reduced density  OFF-LEVEL GRID {the word meaning cut OFF radiation due to improper LEVEL of grid}

39. OFF LEVEL

40. 2-OFF-CENTER GRID : • It occur with focused grid . • If focused grid displaced laterally , & central beam is not in the most central part of the grid  cutoff

41. T F OFF-CENTER

42. 3-OFF-FOCUS GRID : • This occur when focused grid used on non proper distance ( as this grid looks like a lens, when it used at improper distance --> improper image ) • With higher grid ratio , more accuracy in distance is needed , any small change in this distance --> off focus .

43. 4- Upside dowen grid • It occur when focused grid used in an inverted manner ( i.e. It is upper surface looking down ward ) ==> sever cutoff

44. AIR-GAP Technique • It is a technique used for reducing the scattering radiation . • The film is present at 4 : 6 inches distance from the radio graphed part  scattered beams ( which has low energy not reaching the film) • Some degree of magnification occur with this technique , which can be considered & accepted .