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1. GRIDS

2. Grids: It was invented by Dr. Gustave Bucky in 1913 The radiographic grid consists of a series of lead foil strips separated by x-ray-transparent spacers. Most effective way of removing scatter radiation

3. Primary radiation is oriented in the same axis as the lead strips and passes between them. Scatter radiation arises within the patient and is multidirectional, so most of it is absorbed by the lead strips

4. Consists of Lead and interspaces. The interspaces of grids are filled either with aluminum or some organic compounds. There is no clear-cut differentiation as to which is better.

5. TERMINOLOGIES: Grid ratio is defined as the ratio between the height of the lead strips and the distance between them . It signifies grids ability to remove scatter radiation.

6. TERMINOLOGIES: Grid pattern refers to the orientation of the lead strips in their longitudinal axis. According to the pattern of the grid that we see from as a view from the top: I. LINEAR GRID II. CROSSED GRID III. PARALLEL GRID IV. FOCUSSED GRID

7. Linear grid the lead strips are parallel to each other in their longitudinal axis Most x-ray tables are equipped with linear grids. Advantage : they allow us to angle the x-ray tube along the length of the grid without loss of primary radiation from grid "cutoff.“

8. Crossed grid is made up of two superimposed linear grids that have the same focusing distance The grid ratio of crossed grids is equal to the sum of the ratios of the two linear grids disadvantage. cannot be used with oblique techniques requiring angulation of the x-ray tube

9. Focused grid: Grid made up of lead strips that are slightly angled so that they focus in space . May be either linear or crossed . Linear focused grids converge at a line in space called the convergent line. Crossed grids converge at a point in space called the convergent point. FOCAL DISTANCE : Perpendicular distance between the grid and the convergent line or point .

10. Parallel grid is one in which the lead strips are parallel when viewed in cross section. They are focused at infinity, so they do not have a convergent line. Can only be used effectively with either very small Xray fields or long target-grid distances .

11. Focusing range In practice, grids have a focusing range that indicates the distance within which the grid can be used without a significant loss of primary radiation. The focusing range is fairly wide for a low-ratio grid and narrow for a high-ratio grid.

12. TERMINOLOGIES: Grid frequency: is the number of lead strips per inch of grid. It can be calculated by adding the thickness of the lead strips and interspaces and dividing this sum into l. Because the thickness of the lead strips and interspaces is usually expressed in millimeters, the answer must be multiplied by 25.4, which is the number of mm/in. The final equation is:

13. EVALUATION OF GRID PERFORMANCE: Grids are used to improve contrast by absorbing secondary radiation before it reaches the film. The "ideal grid" would absorb all secondary radiation and no primary radiation. The price of better film contrast is increased patient exposure. 3 methods of evaluating performance: l. Primary transmission ( Tp ) 2. Bucky factor (B) 3. Contrast improvement factor (K)

14. 1. Primary transmission ( Tp ) Primary transmission is a measurement of the percentage of primary radiation transmitted through a grid. (ideal = 100%) 2 measurements made: First: is made with the grid in place to determine the intensity of the radiation transmitted through the grid (Ip) Second is made after removal of the grid to determine the intensity of the radiation directed at the grid ( I’p ).

15. 2. Bucky Factor (B) The Bucky factor is the ratio of the incident radiation falling on the grid to the transmitted radiation passing through the grid.

16. It is a practical determination, because it indicates how much we must increase exposure factors when we change from a nongrid to a grid technique. It also tells us how much the patient's exposure dose is increased by the use of a grid.

17. B vs Tp ? The Bucky factor is similar to primary transmission except for one difference. Primary transmission indicates only the amount of primary radiation absorbed by a grid, whereas the Bucky factor indicates the absorption of both primary and secondary radiation.

18. High B? Advantage: High film quality Disadvantage: High exposure factors High radiation dosage to the patient

19. 3. Contrast improvement factor (K) It is the ratio of the contrast with a grid to the contrast without a grid. Measure of the grid’s ability to improve contrast which is its primary function. It depends on : 1. kVp 2. Field size 3. Phantom thickness More closely related to the Lead content of the grid than any other factor.

20. Lead content: If we cut a grid up into 1-cm squares and then weight one square, its weight in grams is the lead content of the grid . The lead content of a grid is expressed in g/cm2.

21. 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 . Result of a poor geometric relationship between the primary beam and the Lead foil strips of the grid . Cut off is complete and no primary radiation reaches the film when the projected images of the Lead strips are thicker than the width of the interspaces. Amount of cut off is always greatest with high ratio grids and short grid focus distances .

22. 4 SITUATIONS THAT PRODUCE GRID CUT OFF : 1. FOCUSED GRIDS USED UPSIDE DOWN 2. LATERAL DECENTERING (GRID ANGULATION ) 3. FOCUS GRID DISTANCE DECENTERING 4. COMBINED LATERAL AND FOCUS-GRID DISTANCE DECENTERING .

23. 1. FOCUSED GRIDS USED UPSIDE DOWN When a focused grid is used upside down, there is severe peripheral cut off with a dark band of exposure in the center of the film with no exposure at the periphery . The higher the grid ratio, narrower the exposed area .

24. 2. LATERAL DECENTERING (GRID ANGULATION ) When the X-ray tube is positioned lateral to the convergent line but at the correct focal distance. Uniform loss of radiation over the entire surface of the grid, producing a uniformly light radiograph . 3 factors affect the magnitude of cut off from lateral decentering : - Grid ratio Amount of decentering Focal distance

25. OFF LEVEL GRIDS When a linear grid is tilted, there is uniform loss of primary radiation across the entire surface of the grid . Effect on the film being same as that of lateral decentering .

26. 3. FOCUS GRID DISTANCE DECENTERING The target of the X-ray tube is correctly centered to the grid , but it is positioned above or below the convergent line . Cut off is greater with near than far focus –grid distance decentering The central portion of the film isn’t affected but the periphery is light. The loss of primary radiation is directly proportional to the grid ratio and the distance from the center line.

27. 4. COMBINED LATERAL AND FOCUS-GRID DISTANCE DECENTERING Causes an uneven exposure resulting in a film that is light on one side and dark on the other side. Directly proportional to the grid ratio and decentering distance and inversely proportional to the focal distance of the grid .

28. Moving Grids: Invented by Dr. Hollis E. Potter in 1920 . Also called a Potter Bucky grid Moved to blur out the shadows cast by the lead strips . Continuously moves 1 to 3 cm back and forth throughout the exposure .

29. Moving Grids: PRECAUTIONS : The grid must move fast enough to blur the lead strips The transverse motion of the grid should be synchronous with the pulses of the Xray generator . DISADVANTAGES : Costly Subject to failure May vibrate the Xray table Places a limit on the minimum exposure time because they move slowly INCREASES THE PATIENT RADIATION DOSE.

30. Grid selection: The price of increased “cleanup” with high ratio grids is that patient exposure is considerably increased and that Xray tube centering becomes critical . 8:1 grids will give adequate results below 90 kVp Above 90 kvp,12:1 grids are preferred

31. Air gap techniques: Scatter radiation arising from the patient from Compton reactions disperses in all directions , so the patient acts like a large light bulb. The closer the patient is to the film, the greater the concentration of scatter per unit area . With an air gap, the concentration decreases due to more photons missing the film in the gap . Used Magnification radiography Chest radiography .

32. Guidelines for selection of gap width: 1. Thicker the part, the more advantageous a larger air gap 2. The first inch of any air gap improves contrast more than any subsequent inch 3. Image sharpness deteriorates with increasing gap width unless the focal film distance is increased to compensate for the greater magnification. 4. If the gap is widened by moving the patient away from the film with a fixed focal film distance, the patient is closer to the X-ray tube and his exposure increases.

33. Magnification with airgap factor determining magnification Object film distance(max with long distance) Focal film distance(max with short distance) sharpnesss is maintained by lenghthenning the focal film distance

34. Exposure factor with air gap xray tube exposure must be increased as focal film distance is increased. exposure is to be increased by 2.8 times as per inverse square law . Patient expsure is usually less than grid as grid absorbs primary photon as well

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