GRIDS.pptx GRIDS.pptxGRIDS.pptxGRIDS.pptx

GRIDS.pptx

1. GRIDS BY Dinesh Sekar Asst Professor Dept of medical imaging Tomch&rc

2. INDEX INTRODUCTION GRID RATIO TYPES OF GRIDS EVALUATION OF GRID PERFORMANCE PRIMARY TRANSMISSION BUCKY FACTOR CONTRAST IMPROVEMENT FACTOR GRID CUTOFF AIR GAP TECHNIQUES 12-08-2025 2

3. INTRODUCTION The radiographic grid was invented by Dr. Gustave Bucky in 1913. Gustave Bucky is credited with the development of the antiscatter grid in 1913, and Hollis Potter with the moving grid 7 years later. Together, the moving grid assembly has been described as the Potter–Bucky grid. It consists of a series of lead foil strips separated by x-ray-transparent spacers. Primary radiation is oriented in the same axis as the lead strips and passes between them to reach the film. Scatter radiation arises from many points within the patient and is multidirectional, so most of it is absorbed by the lead strips and only a small amount passes between them. The inters paces of grids are filled either aluminum or some organic compound. 12-08-2025 3

4. GRID RATIO Grid ratio is defined as the ratio between the height of the lead strips and the distance between them. Ratios usually range from 4:1 to 16: l. Grid pattern refers - orientation of the lead strips in their longitudinal axis. The two basic grid patterns are linear and crossed. 12-08-2025 4

5. LINEAR GRID In a linear grid the lead parallel to each other in their longitudinal axis Major advantage is they allow us to angle the x-ray tube along the length of the grid without loss of primary radiation from grid cutoff. 12-08-2025 5

6. FOCUSED GRID A focused grid is a grid made up of lead strips that are angled slightly so that they focus in space. A focused grid may be either linear or crossed, because the focusing refers to the cross-sectional plane of the lead strips. 12-08-2025 6

7. CROSSED GRID A 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. They cannot be used with oblique techniques requiring angulation of the x-ray tube. 12-08-2025 7

8. 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. The focusing range is fairly wide for a low-ratio grid and narrow for a high-ratio grid. 12-08-2025 8

9. PARALLEL GRID A parallel grid is one in which the lead strips are parallel and focused at infinity, so they do not have a convergent line. These grids can only be used in either very small x-ray fields or long target-grid distances. They are frequently used in fluoroscopic spot film devices. 12-08-2025 9

10. LINES PER INCH Lines per inch is the number of lead strips per inch of grid. Calculated by adding the thickness of the lead strips and interspaces and dividing this sum into l. The thickness of the lead strips and interspaces is expressed in millimeters, the answer must be multiplied by 25.4, which is the number of mm/in. 12-08-2025 10

11. EVALUATION OF GRID PERFORMANCE Three methods of evaluating performance: Primary transmission ( tp ) Bucky factor (b) Contrast improvement factor (k) 12-08-2025 11

12. PRIMARY TRANSMISSION Primary transmission is a measurement of the percentage of primary radiation transmitted through a grid. The x-ray beam is collimated to a narrow pencil of radiation, and the phantom is placed a great distance from the grid. With this arrangement, no scatter radiation reaches the grid. A small amount is produced in the phantom, but it diffuses out of the beam in the large air-gap between the phantom and the grid. 12-08-2025 12

13. The first measurement is made with the grid in place to determine the intensity of the radiation transmitted through the grid, and the second measurement is made after removal of the grid to determine the intensity of the radiation directed at the grid. 12-08-2025 13

14. The measured primary transmission is always less than the calculated primary transmission. The dimensions for grid number 8, is a 15:1 grid with an experimentally determined primary transmission at 64%, lead strips are 60 1-1 thick and its interspaces are 390 1-1 wide. The calculated primary transmission is 87%, which is 23% more than the measured primary transmission. 12-08-2025 14

15. BUCKY FACTOR Bucky factor is the ratio of the incident radiation falling on the grid to the transmitted radiation passing through the grid. Bucky factor indicates the absorption of both primary and secondary radiation. It is determined with a large x-ray field and a thick phantom. 12-08-2025 15

16. The transmitted radiation is measured with the grid in place, and the incident radiation is measured after the grid has been removed. High-ratio grids absorb more scatter radiation and have larger Bucky factors than low-ratio grids. The size of the Bucky factor also depends on the energy of the x-ray beam. High-energy beams generate more scatter radiation. Higher the Bucky factor, the greater the exposure factors and radiation dosage to the patient. If the Bucky factor for a particular grid-energy combination is 5, then exposure factors and patient exposure both increase 5 times over what they would be for the same examination without a grid. 12-08-2025 16

17. CONTRAST IMPROVEMENT FACTOR The contrast improvement factor (K) is the ratio of the contrast with a grid to the contrast without a grid. Ultimate test of grid performance because it is a measure of a grid's ability to improve contrast. The contrast improvement factor depends on k V p, field size, and phantom thickness. 12-08-2025 17

18. Larger the quantity of scatter radiation, the poorer the contrast, and lower the K. To permit comparison of different grids, the K is usually determined at 100 kVp with a large field and a phantom 20 cm thick, as recommended by the International Commission on Radiologic Units and Measurements 12-08-2025 18

19. LEAD CONTENT The lead content of a grid is expressed in g/cm2. If the grid ratio remains constant and the number of lines per inch is increased, the lead content must decrease. The only ways to increase the number of lines per inch is by decreasing the thickness of either the lead strips or interspaces. If the lead strips are made thinner, the number of lines per inch increases without affecting grid ratio, because the thickness of lead strips is not considered in determining grid ratio. 12-08-2025 19

20. When the lead strips are made thinner, however, there is only a small increase in the number of lines per inch at a cost of a large decrease in lead content. If the number of lines per inch is increased by decreasing the width of the interspaces. The lead strips must be made shorter to keep the grid ratio constant and the lead content of the grid decreases. This puts a limitation on the number of lines per inch a grid may have and still be effective. 12-08-2025 20

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22. GRID CUTOFF Grid cutoff is the loss of primary radiation that occurs when the images of the lead strips are projected wider than they would be with ordinary magnification . It is the result of a poor geometric relationship between the primary beam and the lead foil strips of the grid. Cutoff 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. 12-08-2025 22

23. The resultant radiograph will be light in the area in which the cutoff occurs. With linear grids there may be uniform lightening of the whole film, one edge of the film, or both edges of the film, depending on how the cutoff is produced. The amount of cutoff is always greatest with high-ratio grids and short grid-focus distances. 12-08-2025 23

24. Situations that produce grid cutoff: Focused grids used upside down Lateral decentering (grid angulation) Focus-grid distance decentering Combined lateral and focus-grid distance decentering 12-08-2025 24

25. UPSIDE DOWN FOCUSED GRID When a focused grid is used upside down, there is severe peripheral cutoff with a dark band of exposure in the center of the film and no exposure at the film's periphery. The higher the grid ratio, the narrower the exposed area. When a crossed grid is used upside down, only a small square in the center of the film is exposed. 12-08-2025 25

26. LATERAL DECENTERING Lateral decentering results from the x-ray tube being positioned lateral to the convergent line but at the correct focal distance. Three factors affect the magnitude of cutoff from lateral decentering: grid ratio, focal distance, and the amount of decentering. The amount of cutoff increases as the grid ratio and decentering distance increase, and cutoff decreases as the focal distance increases. 12-08-2025 26

27. The x-ray tube was centered at the convergent line for the film strip on the left, and then laterally decentered 1, 2, and 3 in. for the next three strips. The films become progressively lighter as the amount of lateral decentering increases, but the exposure is still uniform. The center and both edges of the film are equally exposed, and it's impossible to recognize the cutoff from inspection of the film. 12-08-2025 27

28. OFF-LEVEL GRIDS When a linear grid is tilted, as it frequently is in portable radiography, there is a uniform loss of primary radiation across the entire surface of the grid . The effect on the film is the same as the effect of lateral decentering. 12-08-2025 28

29. FOCUS-GRID DISTANCE DECENTERING In 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. If the target is above the convergent line, it is called far focus-grid distance decentering. If the target is below the convergent line, it is called near focus-grid distance decentering. The results are the same, but they differ in magnitude. The cutoff is greater with near than with far focus-grid distance decentering. 12-08-2025 29

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31. COMBINED LATERAL AND FOCUS-GRID DISTANCE DECENTERING Combined decentering causes an uneven exposure, resulting in a film that is light on one side and dark on the other side. There are two kinds of combined decentering, depending on whether the tube target is above or below the convergent line. The amount of cutoff is directly proportional to the grid ratio and decentering distance, and inversely proportional to the focal distance of the grid. 12-08-2025 31

32. The projected images of the lead strips are broader on the side opposite the tube target than on the same side, and the film is light on the far side. Cutoff is least on the side under the x-ray tube. 12-08-2025 32

33. The projected images of the lead strips directly below the tube target are broader than those on the opposite side, and the film is light on the near side. Cutoff is greatest on the side directly under the x-ray tube. 12-08-2025 33

34. MOVING GRIDS Moving grid is called a Potter Bucky grid. Grids are moved to blur out the shadows cast by the lead strips. They are reciprocating, which means they continuously move 1 to 3 cm back and forth throughout the exposure. They start moving when the x-ray tube anode begins to rotate. Moving grids are advantageous because they eliminate grid lines from the film. 12-08-2025 34

35. If you are using a moving grid and want to avoid grid lines, you must take two precautions. First - the grid must move fast enough to blur its lead strips Second - the transverse motion of the grid should be synchronous with the pulses of the x-ray generator. When this happens, the shadow of each lead strip is superimposed on the shadow of its neighbor. Disadvantages of moving grids are costly, subject to failure, may vibrate the x-ray table, put a limit on the minimum exposure time because they move slowly & increase the patient's radiation dose. 12-08-2025 35

36. AIR GAP TECHNIQUES Scatter radiation arising in the patient from Compton reactions disperses in all directions, so the patient acts like a large light bulb. The name "air filtration" has been applied to air gap techniques, but this is a misnomer. Scatter radiation decreases not from filtration but from scattered photons missing the film. Negligible quantities of radiation are absorbed in the gap, and the beam is not appreciably hardened, so the name "air filtration" should be discarded. 12-08-2025 36

37. B and C compares the distribution of scattered radiation arising from superficial tissue blocks on opposite sides of the patient. More radiation reaches the film from scattering near the exit surface because the film intercepts a larger angle of the scattered beam. The angle of interception is smaller for scattering occurring on the input side of the patient because a natural "gap" separates the film from the scattering site. 12-08-2025 37

38. Many scattered photons from the input surface in C are absorbed during their long journey through the patient, whereas those originating near the exit surface in B have only a short escape distance. The air gap is most effective in removing scatter radiation when the scatter originates close to the film. 12-08-2025 38

39. OPTIMUM GAP WIDTH Guidelines should be used to select a gap width: 1. The thicker the part, the more advantageous a larger gap. 2. The first inch of air gap improves contrast more than any subsequent inch. The increase from a 1- to a 2-in. gap improves contrast more than an increase from a 14- to a 15-in. gap. 12-08-2025 39

40. 3. Image sharpness deteriorates with increasing gap width unless the focal-film distance is increased to compensate for the greater magnification. An increase in focal film distance from 6 to 10 ft is customary in chest radiography to compensate for this loss of sharpness. A 50% increase in distance doubles the exposure factors. 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. 12-08-2025 40

41. EXPOSURE FACTORS WITH AIR GAPS The exposure factor, patient exposure, and transmitted primary radiation are all assigned a value of one for the grid technique, which is used as the standard for comparison. In this example, the primary transmission of the grid is 65%, and the experimental design was chosen to produce films of equal density and contrast. The x-ray tube exposure must be increased for the air gap technique because of the larger focal-film distance. 12-08-2025 41

42. With the grid technique 1.54 primary photons must pass through the patient for each one reaching the film the difference (35%) is absorbed in the grid. Only 1.19 primary photons need pass through the patient with the air gap technique to produce a comparable concentration per unit area of film; the difference is lost as a consequence of the inverse square law. The air gap loses less primary radiation, so the patient's exposure is less. 12-08-2025 42

43. MAGNIFICATION WITH AIR GAPS Two factors determine the amount of magnification, the object-film distance and the focal-film distance. Magnification is greatest with a short focal-film distance and a long object-film distance. As a general rule, image sharpness deteriorates with magnification. The objective with an air gap technique is to preserve image sharpness by lengthening the focal-film distance until magnification returns to pre-air-gap levels. 12-08-2025 43

44. The objective is not accomplished with a 120-in. focal-film distance, at least not for a 10-in. air gap and a reasonably sized (10-in. thick) patient. Magnification is less with the 72-in. focal-film distance for all object-film distances up to 15 in. The longer focal-film distance produces a more uniform magnification from the front to the back of the patient, a desirable characteristic. 12-08-2025 44

45. With a 5-in. gap, the curve would maintain its slope but would slide down the magnification scale to cross the 72-in. curve at a shorter object film distance. The air gap technique is utilized in magnification mammography to aid in the visualization of  microcalcifications  that may not be clearly distinguished. 12-08-2025 45

46. REFERENCES Christensen's Physics of Diagnostic Radiology. Farr’s Physics for Medical Imaging. Stewart E, Murphy A, Air gap technique (mammography). Reference article, Radiopaedia.org (Accessed on 09 Jun 2024) https://doi.org/10.53347/rID-50102 12-08-2025 46

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