Production and control of scatter radiation (beam

production and control of scatter radiation

1. Production and control of scatter radiation Presenter: Sujan Karki B.Sc.MIT 2 nd year NAMS,BIR HOSPITAL

2. Topics included Scatter radiation -Introduction Interaction of x-ray with matter Beam limiting devices Radiation grids Axillary method of controlling scatter radiation Conclusion References

3. Introduction When a photon beam interacts with matter, some of its part get absorbed, some gets deflected to new direction and rest of it is transmitted to produce a radiographic image. The part which is deflected from its original path to a new direction is known as scatter radiation . The secondary radiation which makes no favorable contribution to the formation of image and produce an overall blackness on the film thus reducing the image contrast. Fig: Scatter radiation

4. Cont.. The scattered X-ray photons -isotropic in direction at diagnostic energies. Thus, these scattered X-rays cannot be completely removed by the use of anti scatter grids or energy filters.

5. Interaction of x-ray with matter When x-rays completely penetrate the body, there is no interaction with matter, and no scatter or scattered radiation is formed as a result. When x-rays are absorbed in the body, however, their energy is “scattered,” or converted into new scatter x-rays. Three types of interactions occur when radiation is absorbed by matter: coherent scattering, Compton effect, and photoelectric effect. The result of either coherent scattering or the Compton effect is termed scatter radiation or simply scatter. Radiation produced by the photoelectric effect is correctly referred to as secondary radiation . Since more than one type of interaction takes place during radiography and the resulting radiation is so similar, the terms are often used interchangeably. The interactions that produce scatter radiation in radiography occur primarily within the patient. Some scattering also occurs as a result of interactions between the x-ray beam and the tabletop and image receptor (IR), and any other matter that happens to be within the radiation field.

6. Coherent Scattering Coherent scattering is also known as Thompson scatter. This type of interaction takes place at relatively low energy levels (below 10 keV). Figure shows the path of the x-ray photon during this interaction. Because coherent scattering occurs in the very low energy ranges, and outside the usual range for diagnostic imaging, this interaction has no significance to our daily work. It is mentioned here only to demonstrate that at very low kilovolts peak ( kVp ) levels there is an interaction in the body.

7. Cont.. It is of two types Thompson Scattering : single electron involved in the interaction. Rayleigh Scattering: Co-operative interaction of all the electrons of an atom . No Ionization --- why??? Because ,no energy transfer .only change of direction . Coherent Scattering accounts for less than 5% of all photons interactions and is of minor concern in diagnostic radiology. Coherent scattered photons travel in a forward direction. At 70 kVp , a few percent of the x-rays undergo coherent scattering, which contributes slightly to image noise, the general graying of an image that reduces image contrast.

8. Compton scattering The Compton effect occurs at energy levels throughout the diagnostic x-ray range of kVp . The incoming x-ray photon interacts with an outer orbital electron of an atom, removing it from the atom (ionization), and then proceeds in a different direction. The majority of the photon’s energy is converted into a new photon of scatter radiation. This new photon has less energy than the incoming primary beam photon and therefore a longer wavelength.

9. Direction and effects of Compton scattering

10. Cont.. Compton scatter travels in all directions.(including 180 degrees from the incident x-ray). If it is directed back toward the x-ray tube, it is termed backscatter. Most of the photons that are scattered will scatter in a more forward direction. As the kVp is increased, Compton interactions are increased.

11. Photoelectric effect In a photoelectric interaction, the incoming photon from the primary beam collides with an inner orbital electron of an atom. The photon is totally absorbed in the process and creates an absorbed dose in the patient. The electron’s departure leaves a “hole” in the orbit, which is filled by an electron from an outer shell. The difference in binding energy between the two shells is emitted as a new x-ray photon. This photon is referred to as a characteristic photon and is considered secondary radiation because it is radiation actually produced in the body. Its energy will be less than that of the primary photon. Photoelectric interactions are less prevalent in the diagnostic energy range than Compton interactions.

12. Cont … The likelihood of a photoelectric interaction is determined by both the kVp level and the electron-binding energy of the atom in which the interaction occurs. Because no part of the energy of the incoming photon exits the atom, photoelectric interactions are sometimes referred to as true absorption. In this text, references to scatter also apply to secondary radiation formed by the photoelectric effect. As kVp is increased, photoelectric effect is decreased. Note, this is the opposite of the Compton effect. In the diagnostic range of kVp used (50 to 100  kVp ) the majority of radiation interactions with the body are Compton interactions.

13. Radiographic effect of scatter radiation The production of scatter radiation during an exposure results in fog on the radiograph. Fog is unwanted exposure to the image. It does not strike the IR in a pattern that represents the subject, and it contributes nothing of value to the image. This fog produces an overall increase in radiographic density. The result is also a reduction in radiographic contrast Although increased density in the darker areas of the image is scarcely noticeable, areas that would otherwise be bright or white will instead be gray because of the fog. In other words, scatter radiation creates fog that reduces both contrast and the visibility of detail.

14. Causes of increase in scatter radiation Increase in kvp Increase in patient thickness Increase in field size

15. KVp Affects the penetrability of the beam. Higher kVp , more photons go through patient to the IR, less absorbed by patient, higher scatter and less contrast on image Lower the kVp , increase in dose absorbed by patient, less fog on film, more contrast image. As x-ray energy increases, the relative number of x-rays that undergo Compton Scattering increases. The absolute number of the Compton interactions decrease with increasing energies but the number of photoelectric interactions decreases more rapidly.

16. cont At 50 kVp 79% photoelectric, 21% Compton & less than 1% transmission. At 80 kVp 46% photoelectric, 52% Compton & 2% transmission Fig: The relative contribution of photoelectric effect and Compton scattering to the radiographic image.

17. Field size Has a significant impact on scatter radiation. Field size is computed in square inches or square cm As field size increases, intensity of scatter radiation also increases rapidly. Especially during fluoroscopy The relative intensity of the scatter varies more when the field size is small than when the field is large. When the field size is reduced, the resulting reduction in scatter will reduce the density on the image.

18. Cont.. The mAs must be increased to maintain density. The reduced scatter will improve contrast resolution resulting in improved image quality. To change from a 14” x 17” to a 10” x 12” increase mAs 25%. To change from a 14” x 17” to a 8” x 10” increase mAs 40%. Fig: Collimation of the x-ay beam results in less scatter radiation.

19. Part or patient thickness The relative intensity of scatter radiation increases with increasing thickness of the anatomy. There will be more scatter for a lumbar spine film compared to a cervical spine film. As tissue thickness increases, more of the rays go through multiple scattering. Normally body thickness is out of our control but we can change the method of imaging to improve image quality.

20. Part or patient thickness

21. Does patient thickness affects the image contrast…? Fig a Fig b

22. Beam restriction and scatter radiation In addition to decreasing patient dose, beam-restricting devices reduce the amount of scatter radiation that is produced within the patient, reducing the amount of scatter the IR is exposed to, and thereby increasing the radiographic contrast. The relationship between collimation (field size) and quantity of scatter radiation is illustrated in figure. As stated previously, collimation means decreasing the size of the projected field, so increasing collimation means decreasing field size, and decreasing collimation means increasing field size.

23. Cont.. It is the responsibility of the radiographer to limit the x-ray beam field size to the anatomic area of interest. Beam restriction serves two purposes: 1) limiting patient exposure and 2) reducing the amount of scatter radiation produced within the patient.

24. Types of Beam-Restricting Devices Several types of beam-restricting devices, which differ in sophistication and utility, are available. All beam-restricting devices are made of metal or a combination of metals that readily absorb x-rays. Also known as beam limiting device and includes 1) aperture diaphragm 3) cones and cylinders 4) various aperture collimator

25. Aperture Diaphragms The simplest type of beam-restricting device is the aperture diaphragm. An aperture diaphragm is a flat piece of lead (diaphragm) that has a hole (aperture) in it. Commercially made aperture diaphragms are available, or hospitals make their own for purposes specific to a radiographic unit. Aperture diaphragms are easy to use. They are placed directly below the x-ray tube window. An aperture diaphragm can be made by cutting rubberized lead into the size needed to create the diaphragm and cutting the center to create the shape and size of the aperture.

26. Aperture Diaphragms Typical trauma radiographic imaging system

27. Cont.. Although the size and shape of the aperture can be changed, the aperture cannot be adjusted from the designed size, and therefore the projected field size is not adjustable. In addition, because of the aperture’s proximity to the radiation source (focal spot), a large area of unsharpness surrounds the radiographic image. Although aperture diaphragms are still used in some applications, their use is not as widespread as other types of beam-restricting devices.

28. Cones and Cylinders Cones and cylinders are shaped differently but they have many of the same attributes. A cone or cylinder is essentially an aperture diaphragm that has an extended flange attached to it. The flange can vary in length and can be shaped as either a cone or a cylinder. The flange can also be made to telescope, increasing its total length . Similar to aperture diaphragms, cones and cylinders are easy to use. They slide onto the tube, directly below the window. Cones and cylinders limit unsharpness surrounding the radiographic image more than aperture diaphragms do, with cylinders accomplishing this task slightly better than cones as shown in Figure .

29. Cont.. However, they are limited in terms of available sizes, and they are not interchangeable among tube housings. Cones have a particular disadvantage compared with cylinders. If the angle of the flange of the cone is greater than the angle of divergence of the primary beam, the base plate or aperture diaphragm of the cone is the only metal actually restricting the primary beam. Therefore, cylinders generally are more useful than cones. Cones and cylinders are almost always made to produce a circular projected field, and they can be used to advantage for particular radiographic procedures

30. Improved contrast resolution

31. Collimators The most sophisticated, useful, and accepted type of beam-restricting device is the collimator. Collimators are considered the best type of beam-restricting device available for radiography. Beam restriction accomplished with the use of a collimator is referred to as collimation. The terms collimation and beam restriction are used interchangeably. note(Collimator misalignment should be less than 2% of the SID used, and the perpendicularity of the x-ray central ray must be less than or equal to 1 degree misaligned.)

32. A collimator has two sets of lead shutters. Located immediately below the tube window, the entrance shutters limit the x-ray beam much as the aperture diaphragm would. These shutters consist of longitudinal and lateral leaves or blades, each with its own control. This design makes the collimator adjustable in terms of its ability to produce projected fields of varying sizes. The field shape produced by a collimator is always rectangular or square, unless an aperture diaphragm, cone, or cylinder is slid in below the collimator. Collimators are equipped with a white light source and a mirror to project a light field onto the patient. This light is intended to indicate accurately where the primary x-ray beam will be projected during exposure.

33. Positive beam limiting devices Misalignment of the light field and x-ray beam can result in collimator cutoff of anatomical structures. Today nearly all light-localizing collimators manufactured in the United States for fixed radiographic equipment are automatic. They are called positive beam–limiting (PBL) devices . When a film-loaded cassette is inserted into the Bucky tray and is clamped into place, sensing devices in the tray identify the size and alignment of the cassette. A signal transmitted to the collimator housing actuates the synchronous motors that drive the collimator leaves to a precalibrated position, so the x-ray beam is restricted to the image receptor in use.

34. Is there any difference in these two pictures regarding radiographic contrast and image noise? Fig a Fig b

35. Two factors determine the amount of energy retained by deflecting photon Initial energy Deflecting photon

36. Filters Filtration is basically a process of shaping the x-ray beam to 1) increase the amount of useful photons 2) decrease the low energy photons This also decreases the patient dose and occupational dose

37. How filtration works When exposure is done both high and low energy xrays are produced When these xrays photons interact with the human body only the high energy photons penetrates the body, while low energy photons are absorbed in the body, hence increasing the patient radiaition dose. Filtration basically absorbs the low energy photons from the beam and hence increases the image contrast.

38. Comparison of the radiograph

39. Levels of filtration Inherent filtration Added filtration

40. Inherent filtration The absorption of the low energy x-ray photons by the x-ray tube components itself is known as the inherent filtration. Glass housing, metal enclosure and the assembly oil is responsible for inherent filtration. Inherent filtration of a tube is measured in ALUMINIUM EQUIVALENT . It is basically the amount of aluminum required to absorb the same radiation which the tube material was absorbing. Inherent filtration is generally between 0.5-1.00mm of aluminum equivalent.

41. Diagram of inherent filtration Position indicating device

42. Added filtration Added filtration is a result of any beam absorber which is placed in the path of the xray beam, this absorber absorbs the low or high energy photons. Ideally an added filter should absorb all the low energy photons and let the high energy photons pass through it. But no such material exists. SOLUTION- we always use added filters mostly in a group of Aluminium (13) + Copper(29).

43. Cont.. They are arranged as the high at. No.(COPPER) element faces the x-ray tube while the other(ALUMINIUM) one faces the patient. Most of the filtration is done by copper. They can't be used separately. Patient radiation dose as well as the image contrast is reduced. This combination of two layers of filters is also known as COMPOUND FILTERS. It also increases the tube loading.

44. Added filter

45. Wedge filter Wedge filters are mostly used at places where the body part to be radiographed varies greatly in densities.(thick from one side and thin from another side) Wedge filters are like the shape of a wedge , the thin part is placed under thick body part while thick part is placed under thin body part. Result is that ,beam attenuation by thick part is more hence less radiation reaches the part and beam attenuation by thin part is less hence more radiation reaches thick part. Therefore a radiograph of uniform density is achieved.

46. Sources of scatter radiation Patient himself Glass walls of x-ray tube Tabletop Cassette Back scatter from floor Back scatter from wall

47. Characteristics of scatter radiation More oblique in nature Produced by matter in all direction Less energy than primary beam With increase in energies energy of primary radiation more scatter in forward direction Amount of scatter depends on 1) volume of tissue 2) thickness of patient 3) kvp used

48. Effects of scatter radiation Increase overall density of the film which is not useful in production of image thus produces of fog Reduces contrast Reduce light transmitting ability of film Result in the formation of noise

49. Solution to decrease scatter radiation Various devices to reduce scatter Filters Aperture diaphragm Cones and cylinders Collimators Radiographic grids

50. Control of scatter radiation Generally two types of devices can be used to reduce scatter radiation reaching image receptor Before Patient On Patient After Patient Beam Restrictors Compression Grids

51. Axillary methods to control scatter radiation There are two auxiliary methods to control the scattered radiation : 1)Air Gap Technique 2)Compression Technique In conventional film screen system, the filtration effect of screen against the scatter and the harder, primary photons are intensified more than the softer, scattered ones. Being the film itself more or less equally sensitive to both scattered and primary photons, this leads to a reduction on the relative effect of scattered radiation on the film. The lead lined at the back of the conventional as well as modern cassette reduces the effect of back scattered radiation from the Bucky and ground to the film or IR.

52. Compression bands It reduces the amount of scatter by diminishing the volume of tissue through which x-ray passes It displaces the adipose tissue side ways thus reducing the volume of the part to be x-rayed and lowers the kilovoltage to be used. Advantage 1) It acts as a immobilizing device. 2) helps in the reduction of the scatter radiation thus improving the contrast.

53. Compression Reduces OID Improves spatial resolution Improves contrast resolution (reducing noise and fog) Reduces patient dose

54. Is patient thickness something the radiographer can control? Normally no But compression devices can be used in certain situations which reduces patient thickness and bringing the object closer to the IR, and therefore results in better spatial resolution and contrast resolution and also lesser exposure factors and patient dose

55. Air gap technique The air gap technique is an alternate scatter reduction method and can be used instead of a grid. It uses an increased OID to reduce scatter reaching the image receptor. This technique is used in lateral C-Spine and chest radiographs. Since the patient is the source of scatter and the increased OID causes much of the scattered radiation to miss the image receptor. This reduces scatter and to improve the contrast of the image. A major disadvantage of the air gap technique is the loss of sharpness, which results from the increased OID. The air does NOT filter out the scattered x-rays.

56. Air gap technique An OID of at least 6 in (15 cm) is required for effective scatter reduction. A longer SID is utilized with a small focal spot size to reduce the magnification., With the increased SID, the mAs must be increased to maintain radiographic density. When air gap technique is used the mAs is increased approximately 10% for every centimeter of air gap. The technique factor usually are about the same as those for an 8:1 grid ratio.

57. Cont.. The air gap technique technique is not normally as effective with high kVp radiography, in which the direction of the scattered x-ray is more forward. At the tube potentials below approximately 90kVp, the scatter x-rays are directed more to the side ,therefore they have a high probability of being scattered away from the image receptor.

58. Scatter removal by air gap technique

59. Radiation grids Rapid adaption of new technology and device. Dr. Gustave Bucky in 1913 demonstrated scatter radiation and it remedy by using grids, working in the field of radiology. It is a radiographic accessory which is designed to minimize the effect of scatter radiation reaching the film. Grid is composed of thin strips of lead which are separated from each other by interspace material which is penetrable by primary beam. Grids are commonly used in radiography, with grid ratio available in even numbers, such as 4:1, 6:1, 8:1, 10:1 or 12:1.  breast ( mammography ): uses 4:1 grid ratio Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique . Fig: X-RAY GRID PARALLEL 40 LINES RATIO 8:1 ,3 mm thick

60. The appropriate acquisition techniques, when using digital systems, depends on many factors including the image quality performance of the detector, grid performance, the scattering characteristics of the patient, imaging geometry, image processing, and the degree to which grid alignment can be assured. Appropriate choice of the relevant “digital” Bucky factor is necessary to obtain the best quality diagnostic image at the lowest possible patient dose. Image quality was improved for 80% of patients when their images were captured with the grid without increasing the patient dose.( Foos , 2012)

61. Radiographic grid is recommended for : Anatomical part more than 10 cm With high kvp (Not always mammo ) Soft tissue structures to increase contrast Structure affected by pathological condition that would increase scatter production

62. Ideal grid It is the which absorbs all scatter radiation and allows the primary radiation to pass through, but no grid is ideal. Uses: 1) abdomen 2) skull 3) spine 4) pelvis 5) penetrating chest x-rays

63. Cont … There are two basic types of grids: 1)Focused 2)Unfocused 3)Moving grid A grid is unfocused when all the lead strips are parallel to each other and perpendicular to the grid surface, provides more freedom i.e. distances can be varied. A grid is focused when all the lead strips except those in the center are inclined at an angle towards the center (they are actually on radius of a circle whose center is the focal spot of the tube when the correct distance is used). Moving grids (also known as Potter-Bucky grids): eliminates the fine grid lines that may appear on the image when focussed or parallel grids are used

64. Types of grids and their working principle Fig:Focused grid Fig:Parallel grid and grid cut- off of scattered as well as primary radiation.

65. High transmission cellular grid (HTC) Reduces scatter radiation in the two directions Grid strips-copper Interspace material-air Physical dimensions-3.8:1

66. Parameters of grid Grid ratio Grid frequency Nature of interspace material Lead contents

67. Grid ratio It is defined as the ratio between the height of lead strips and distance between two adjacent strips. It is the parameter which determines the ability of a grid to remove scatter radiation. It measures the narrowness of the slits. It determines the extent to which obliquely moving scatter radiation is absorbed.

68. Grid frequency Number of grid lines per inch or centimeter Usually range from 25 -45 lines per cm In mammographic grids have frequencies of 80 lines/cm 4:1 or 5:1 grid ratio of grid High frequency grids More grid strips High radiographic technique Higher patient dose No significant grid lines on the image

69. Radiographic Grids The higher the grid ratio the more exposure is required.

70. Comparison between two types of interspace material

71. Advantages and disadvantages of aluminum Advantages 1) Non-hydroscopic – does not absorb moisture as plastic fiber does. 2)Easy to manufacture as it can be roll into sheets of precise thickness. 3)Provide selective filtration of scattered x-rays not absorbed in grid strips. 4)Produce less visible grid lines on the radiograph . Disadvantages 1)Increase the absorption of primary x rays (low kVp ) 2)Higher mAs and higher patient dose 3)For low kVp , patient dose may be increased by 20%

72. Dose reduction by carbon fibers Typically the dose can be reduced by 3-15% by changing the table top, 6-12% by changing the front of the film cassette and 20-30% by using a grid with carbon fibre covers and fibre interspace. The higher cost of carbon fibre components can normally be justified by such dose savings. An indication of the absorption of all such components should be provided by manufacturers.( A P H ufton March 1986 BJR)

73. Bucky factor Ratio of the incident radiation falling on the grid to the transmitted radiation passing though the grid. Indicates actual increase in exposure due to grids presence Due to attenuation of both primary and secondary radiation Higher the grid ratio =higher bucky factor. Bucky factor(B) is also known as image factor.

74. Contrast improvement factor Grid ratio, however, does not reveal the ability of the grid to improve image contrast. This property of the grid is specified by the contrast improvement factor (k). A contrast improvement factor of 1indicates no improvement. Most grids have contrast improvement factors of between1.5 and 2.5. In other words, the image contrast is approximately doubled when grids are used.

75. Grid cutoff Loss of primary that occurs when the image of the lead stripes are projected wider than they would be with ordinary magnification Types of grid cutoff Focused grids used upside down Lateral decentering Distance decentering Combined decentering

76. Image quality dur to the misalignment of grid Mobile radiograph of proximal femur and hip, showing comminuted fracture of left acetabulum.  A,  Poor-quality radiograph resulted when grid was transversely tilted far enough to produce significant grid cutoff.  B,  Excellent-quality repeat radiograph on the same patient, performed with grid accurately positioned perpendicular to central ray.

77. Focussed grid upside down Dark band of central exposure Severe cut-off at periphery Crossed grid-small square at the centre is exposed

78. Lateral descentering 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 cutoff Uniform loss of radiation over entire film Uniformly light radiograph Not recognizable characteristics

79. Lateral descentering Also occurs when grid is tilted Magnitude depends upon 1) grid ratio 2) focal distance 3) amount of descentering Lateral decentering is the significant problem in portable radiography Becouse exact centering is not possible Minimizing lateral descentering 1) low ratio grids 2) long focal distances

80. Far focus-grid decentering and near focus-grid centering Cut off at periphery Dark centre Cutoff proportional to 1) grid ratio 2)decentering distance

81. Combined lateral and focus decentering Most commonly recognized Uneven exposure Film is light on one and dark on other side

82. Grid information

83. Artifacts in grid (moiré effect) vertical banding pattern artifact

84. Radiation safety Whenever possible, the radiographer should stand at least 6 ft (2 m) from the patient and useful beam. The lowest amount of scatter radiation occurs at a right angle (90 degrees) from the primary x-ray beam. A , Note radiographer standing at either the head or the foot of the patient at a right angle to the x-ray beam for dorsal decubitus position lateral projection of the abdomen. B , Radiographer standing at right angle to the x-ray beam for AP projection of the chest. IR, image receptor.

85. Conclusion Controlling the amount of scatter radiation produced in a patient and ultimately reaching the image receptor (IR) is essential in creating a good-quality image. Scatter radiation is detrimental to radiographic quality because it adds unwanted exposure (fog) to the image without adding any patient information. Digital IRs are more sensitive to lower-energy levels of radiation such as scatter, which results in increased fog in the image. Increased scatter radiation, either produced within the patient or higher-energy scatter exiting the patient, affects the exposure to the patient and anyone within close proximity. Beam-restricting devices decrease the x-ray beam field size and the amount of tissue irradiated, thereby reducing the amount of scatter radiation produced. It should be noted that grids do nothing to prevent scatter production; they merely reduce the amount of scatter reaching the IR.

86. References Sehlawi : Rad 206 p11 Fundamentals of Imaging - Control of Scatter Radiation The use of carbon fibre material in table tops, cassette fronts and grid covers: Magnitude of possible dose reduction Article   in   British Journal of Radiology   MYTH BUSTER: “DOSE INCREASES WITH CR & DR SYSTEM GRID ALIGNMENT” July 24, 2012 Radiologic Science for Technologist (Physics, Biology and Protection) by Stewart C. Bushong , 10th Edition.

87. Questions How does the scatter radiation effects the radiographic image? What are the factors that contribute to an increase in scatter radiation ? What do you understand by positive beam limiting device? What are the axillary method to control scatter radiation? Why is filter used in x-ray tube and what are its type?