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Control of Scatter Radiation

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Presentation on theme: "Control of Scatter Radiation"— Presentation transcript:

1 Control of Scatter Radiation
Bushong Ch. 14

2 Objectives Begin discussing factors that influence image detail or visibility of detail Spatial & Contrast resolution Radiographic Noise Scatter Radiation Ways to reduce scatter & improve image quality Primary beam restriction & Grids Technique adjustments when using grids

3 What are some factors that increase scatter radiation?

4 3 factors contribute to an increase in scatter
Increased kVp Increased x-ray field size Increased patient thickness

5 2 principal characteristics of any image are Spatial & Contrast Resolution
Spatial resolution Resolution is the ability to image two separate objects and visually distinguish one from the other Spatial resolution is the ability to image small objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung nodules) Determined by focal-spot size and other factors that contribute to blur Diagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm. (CT measured in cm)


7 SMPTE Test Pattern

8 Contrast Resolution Determined by scatter radiation and other sources of radiographic noise Radiographic noise (image fog) = A uniform signal produced by scattered x-rays Digital imaging the grainy or uneven appearance of an image caused by an insufficient number of primary x-rays



11 Image-forming x-rays Two kinds of photons are responsible for the OD and contrast on an image: Photons that pass through without interacting and those that are scattered though Compton. X-rays that exit the patient are remnant and those that interact with the IR are image-forming.

12 However, scatter radiation is a factor that must be managed
Ideally, only those x-rays that do not interact with the patient should reach the IR…. However, scatter radiation is a factor that must be managed Proper collimation has the PRIMARY effect of reducing patient dose by _________ ? Proper collimation also improved image contrast by reducing radiographic noise or fog caused by scatter

13 Fog or Noise

14 Contrast changes with the use of a grid
Less scatter radiation & less radiographic noise – shorter scale = “better contrast” With Grid No Grid

15 kVp As x-ray energy increases Photoelectric and Compton interactions decrease. Explain? At 50 kVp 79% photoelectric, 21% Compton & less than 1% transmission At 80 kVp 46% photoelectric, 52% Compton & 2% transmission Pg. 225

16 How does increasing kVp affect patient dose?

17 Patient Thickness Imaging thick parts of the body results in more scatter radiation than thin parts IMAGE TEST TOOL

18 Is patient thickness something the radiographer can control?

19 Patient thickness Normally, No
Compression devices improves spatial resolution by reducing patient thickness and bringing the object closer to the IR. Compression also reduces patient dose and contrast resolution

20 Compression Improves spatial resolution Reduces OID
Reduces patient dose Improves contrast resolution (reducing fog or noise)

21 Compression

22 Field Size As field size increases, intensity of scatter radiation also increases rapidly. Especially during fluoroscopy

23 Compare images: What do you think about radiographic contrast & image noise?

24 Control of Scatter Radiation
Technologists routinely use two types of devices to reduce the amount of scatter radiation reaching the IR Beam restrictors Grids

25 3 Types of beam-restricting devices
Aperture Diaphragm Cones or Cylinders Variable aperture collimator

26 Aperture Diaphragm The simplest of all beam-restricting devices
Lead or lead-lined metal diaphragm attached to the x-ray tube head The opening in the diaphragm is usually designed to cover just less than the IR used

27 Diaphragm Fixed lead opening Fixed image receptor size Constant SID
Source-to-diaphragm distance = SDD

28 Cones & Cylinders Are modifications of the aperture diaphragm
Alignment is one difficulty when using cones Now mostly used with spines teeth & heads

29 Improved contrast resolution of the frontal sinuses

30 Variable Aperture Collimator
The most common beam-restricting device is the light-localizing variable aperture collimator The first part of the collimator serves to control off-focus radiation. What is off-focus radiation?

31 Off - focus Radiation X-ray tubes are designed so that the projectile e- interacts with the target. However, some of the e- bounce off the target and land on other areas This caused x-rays to be produced out side the focal spot

32 Extrafocal Radiation These rays can also be called off-focus radiation
Extrafocal radiation is undesirable because it extends the size of the focal spot, increases patient skin dose & reduces image contrast

33 Off-focus radiation

34 Fixed diaphragm in the tube housing
Using a grid does not reduce extrafocal radiation

35 First-stage entrance shuttering device
Has multiple collimator blades protruding from the top of the collimator into the tube housing

36 The second-stage collimator shutters
Pb leaves are at least 3 mm thick They work in pairs and are independently controlled

37 The collimator lamp & mirror
Must be adjusted so that the projected light field coincides with the x-ray beam Misalignment of the light field and beam can result in collimator cutoff of anatomic structures

38 Always keep the collimated area smaller than the size of the cassette
What is a PBL You should always see a 4 sided borders.

39 Total Filtration Filtration review…
The collimator assembly is usually equivalent to approximately _______ mm Al filtration. Minimum filtration for tubes that can operate about 70 kVp is _______ mm Al or equiv.

40 The Grid

41 Contrast & Contrast Resolution
Two devices are used to reduce Compton effect beam-restricting devices and radiographic grids Beam-restricting devices effects what reaches the patient. Grids effect the remnant beam

42 Contrast & Contrast Resolution
Contrast = the comparison of areas of light, dark and shades of gray on the image Contrast Resolution = the ability to image adjacent similar tissues

43 Beam-restricting devices
Are helpful to improve contrast resolution however the inherent problem is they are placed between the source and the patient. Even under the most favorable conditions, most if the remnant x-rays are scattered. Table pg. 237

44 Effects of Scatter Radiation on Image Contrast
Contrast is the degree of difference in OD between areas of an image If you could only capture transmitted, unscattered x-rays, the image would be very sharp The corresponding bone-soft tissue interface, would be very abrupt, and therefore the image contrast would be high

45 Grids Are very effective device for reducing scatter radiation
The grid is a series of sections of radiopaque material (grid strips) alternating with sections of radiolucent material (interspace material) The grid is designed to transmit only x-rays that are traveling in a straight line from the source to the IR

46 Grids “clean up” scatter radiation
A high quality grid can attenuate 80 – 90 percent of scatter radiation

47 Grid Strips Should be very thin and have high photon absorption properties Lead is most common Tungsten, platinum, gold, and uranium have been tried but Pb is still most desirable

48 Interspace Material Used to maintain precise separation between the delicate lead strips Aluminum or Plastic Fiber Grid Casing = covered completely by thin aluminum to provide rigidity and to seal out moisture. Yuck!

49 Grid Ratio 3 important dimensions on a grid: The thickness of the grid strips, the width of the interspace material, and the height of the grid The grid ratio is the HEIGHT of the grid divided by the INTERSPACE WIDTH: Grid ratio = h D

50 h = height of the grid, T = thickness of the grid strip, D = width of the interspace material

51 Grid Ratio High-ratio grids are more effective in cleaning up scatter radiation than low-ratio grids The angle of deviation is smaller for high-ratio grids. (the photon must be traveling in a straighter line to make it through the grid) However, the higher the ratio the more radiation exposure necessary to get a sufficient number of x-rays through the grid to the IR

52 The higher the ratio the straighter the photon must travel to reach the IR
Grid ratios range from 5:1 to 16:1 Most common 8:1 to 10:1 A 5:1 grid will clean up 85% 16:1 clean up 97%

53 Grid Frequency The number of grid strips or grid lines per inch or centimeter The higher the frequency the more strips and less interspace material and the higher the grid ratio As grid frequency increases, patient does is increase because more scatter will be absorbed

54 Grid Frequency Some grids reduce the thickness of the strips to reduce the exposure to the patient, this over all reduces the grid clean up Grids have frequencies in the range of 25 to 45 lines per centimeter (60 to 110 lines per inch)

55 Higher frequency with the same interspace distance reduces the grid effectiveness

56 Grid Performance The principal function of a grid is to improve image contrast Contrast Improvement Factor (k) = the ratio of the contrast of a radiograph made with a grid to the contrast of the radiograph made without a grid. A contrast improvement factor of 1 indicates no improvements The higher the grid ratio & frequency the higher the k

57 Bucky Factor Using grids require more patient dose. Why is this?
When a grid is used technique must be increased to maintain OD The amount of increase is given by the Bucky factor (B) or grid factor

58 Bucky Factor or grid factor
The higher the grid ratio or frequency the higher the bucky factor The Bucky factor increases with increasing kVp Pg 235: We will use the average values for calculations.

59 Selectivity or ability to “clean up” the heavier the grid the more Pb it contains

60 Grid Types Parallel Grid – simplest type of grid
All the lead strips are parallel Only clean up scatter in one direction (along the axis of the grid) Easy to make, however can cause grid cutoff with short SID’s.

61 Grid cutoff Distance to cutoff SID Grid ratio With decreasing
SID more potential for grid cutoff IR size will also Influence grid cutoff

62 Grid Cutoff – Parallel grid

63 Crossed Grid Have lead strips running along the long and short axes of the grid Made by placing two parallel grid on top of each other

64 Crossed Grid Have twice the grid ratio as linear grids
However, CR vs grid placement is critical. The CR must align with the center of the grid and the grid and CR must be exactly parallel or grid cutoff will occur

65 Focused Grid Designed to minimize grid cutoff
Lead strips are aligned with the divergence of the x-ray beam Each focused grid must be identified with the appropriate SID Wrong SID = Grid cutoff

66 Focused grid have a little SID latitude (eg
Focused grid have a little SID latitude (eg. 100cm grid could be used at 90cm – 110cm)

67 Moving Grids All stationary grids will give you grid lines on your radiograph. Thinner Pb strips will give you less noticeable lines. However, thinner strips have less Pb content not “cleaning up” as well Grid Lines are made when primary x-rays are absorbed in the grid strips.

68 Focused grids are usually used as moving grids
The grid is placed in a holding mechanism that begins moving just before the x-ray exposure and continues moving after the exposure ends 2 types of movement Reciprocating & Oscillating

69 Grid Motion Reciprocating = moves several times about 2cm back and forth during the exposure Oscillating = moves several times about 2 – 3 cm in a circular pattern Most grids are moving. Except for portable imaging

70 Grid Problems Increased OID, especially with moving grids

71 Grid Problems – Off Level

72 Grid Problems – Off Center
A problem with focused & crossed grids

73 Grid Problems – Off Focus (wrong SID)

74 Grid Problems – Upside-Down
A problem with focused & crossed grids

75 Grid Selection Patient Dose Exam Detail required Part thickness
Pg 241 – mAs changes Exam Detail required Part thickness Desired technique (kVp) Equipment availability

76 Questions….?

77 Questions …? Technique adjustments problems

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