Chapter 11:Full Flashcards
1
Q
What factors contribute to the production of scatter radiation?
A
kVp, field size, and patient thickness.
2
Q
What is one method to control scatter radiation?
A
Using beam restrictors.
3
Q
How does scatter radiation affect image quality?
A
It impacts image contrast.
4
Q
What is the purpose of radiographic grids?
A
To improve contrast and reduce scatter radiation.
5
Q
What is the Bucky factor?
A
A measure of the grid’s ability to improve contrast.
6
Q
What are the types of grids mentioned?
A
Parallel grid, crossed grid, focused grid, and moving grid.
7
Q
What are common grid problems?
A
Off level, off center, off focus, and upside down grid.
8
Q
What considerations are important for grid selection?
A
Patient dose and air-gap technique.
9
Q
What are remnant x-rays?
A
X-rays that exit from the patient.
10
Q
What are image-forming x-rays?
A
X-rays that exit and interact with the image receptor.
11
Q
What factors influence the production of scatter radiation?
A
Kilovoltage, field size, and patient thickness.
12
Q
What is the importance of proper collimation?
A
It helps control scatter radiation.
13
Q
What happens to the number of Compton interactions as x-ray energy is increased?
A
The absolute number of Compton interactions decreases.
14
Q
How does the decrease in photoelectric interactions compare to Compton interactions as x-ray energy increases?
A
The number of photoelectric interactions decreases much more rapidly than Compton interactions.
15
Q
What happens to the relative number of x-rays undergoing Compton scattering as x-ray energy increases?
A
The relative number of x-rays that undergo Compton scattering increases.
16
Q
What is the effect of increased photoelectric absorption on patient radiation dose?
A
It results in a considerable increase in patient radiation dose.
17
Q
What are the relative contributions of photoelectric effect and Compton scatter to the radiographic image?
A
They contribute differently, with photoelectric effect increasing absorption and Compton scatter affecting image contrast.
18
Q
What is the significance of kilovoltage in skull radiographs?
A
It affects the quality of the radiographs and the resultant patient exposures.
19
Q
How does field size influence scatter radiation?
A
Field size affects the level of scatter radiation and is controlled by the radiologic technologist.
20
Q
What is the relationship between field size and optical density?
A
Field size can influence scatter radiation, which in turn affects optical density.
21
Q
What is the recommended technique for lumbar spine radiography?
A
Collimation of the beam to the vertebral column.
22
Q
What is the effect of the full-field technique on image contrast?
A
It results in reduced image contrast.
23
Q
How does patient thickness affect scatter radiation?
A
Imaging thick parts of the body results in more scatter radiation than imaging thin body parts.
24
Q
What types of tissue contribute to the production of scatter radiation?
A
Muscle, fat, and bone.
25
How does the size of body parts influence scatter radiation production?
Larger body parts have more tissue to interact with photons, resulting in greater scatter production.
26
What should be done to decrease scatter radiation?
Use the smallest possible field size.
27
How does patient thickness affect x-ray scattering?
More x-rays are scattered with increasing patient thickness.
28
What can be used to reduce scatter radiation to the image receptor?
Devices such as a compression paddle.
29
What are the benefits of compressing anatomy during radiography?
Improves spatial resolution, contrast resolution, and lowers patient radiation dose.
30
What is contrast in a radiographic image?
The degree of difference in optical density (OD) between areas of the image.
31
What does contrast resolution refer to?
The ability to image and distinguish soft tissues.
32
What happens to image contrast when only transmitted, unscattered x-rays are used?
The image would be very sharp, resulting in high contrast.
33
What is the result when only scatter radiation reaches the image receptor?
The image would be dull gray with very low contrast.
34
What do image-forming x-rays consist of?
Both transmitted and scattered x-rays, resulting in moderate contrast.
35
What are the two types of devices that reduce scatter radiation reaching the image receptor?
Beam restrictors and grids.
36
What is the simplest beam-restricting device?
Aperture diaphragm.
37
What is an aperture diaphragm?
A lead or lead-lined metal diaphragm attached to the x-ray tube head.
38
What is the design of the opening in an aperture diaphragm?
It usually covers just less than the size of the image receptor used.
39
What is the main disadvantage of the aperture diaphragm?
It does not provide a sharp demarcation of the edge of the x-ray beam.
40
How does a diaphragm affect edge penumbra?
With a diaphragm, off-focus is not controlled, resulting in a large area of penumbra at the edge of the exposed film area.
41
What advantage does a cylinder cone have over a diaphragm?
A cylinder cone controls off-focus radiation much better and decreases edge penumbra considerably.
42
What are radiographic extension cones and cylinders considered?
Modifications of the aperture diaphragm.
43
What shape is the useful beam produced by an extension cone or cylinder?
Usually circular.
44
What is a major drawback of using cones in radiography?
A different cone is required for each different field size and for every SID, making it impractical.
45
What issue arises from the circular cross-section of the x-ray beam when using cones?
It cannot be fitted to a rectangular film, leading to cone-cutting.
46
What is a major drawback of using cones in radiography?
It is impossible to correctly align the x-ray beam with the anatomical area of interest due to the lack of a centering light.
47
What does misalignment of the x-ray beam with the film receptor lead to?
It leads to poor image quality.
48
What is the most commonly used beam-restricting device in radiography?
The light-localizing variable-aperture collimator.
49
How does collimation benefit patients during radiography?
It reduces the patient radiation dose and improves contrast resolution.
50
What should the size of the x-ray beam be in relation to the image receptor?
The x-ray beam should not exceed the size of the image receptor.
51
What is a radiographic grid made of?
Alternating sections of radiopaque material (grid strip) and radiolucent material (interspace material).
52
What is the primary function of a radiographic grid?
To reduce the level of scatter radiation that reaches the image receptor.
53
Where is the grid positioned during radiographic imaging?
Between the patient and the image receptor.
54
Who first demonstrated the technique of using grids to reduce scatter radiation?
Gustave Bucky in 1913.
55
What does the grid design allow in terms of x-ray transmission?
It transmits only x-rays whose direction is on a straight line from the x-ray tube target to the image receptor.
56
How does the grid affect scatter radiation?
Scatter radiation is absorbed in the grid material.
57
Who made the Bucky grid practical and how?
Dr. Hollis Potter by moving it during the radiographic exposure to blur the grid lines out of the image.
58
What is the correct NEMA term for the grid developed by Dr. Hollis Potter?
Potter-Bucky diaphragm.
59
What happens to X-rays that strike the radiopaque grid strips?
They are absorbed and do not reach the image receptor.
60
How does a grid affect film density?
It decreases film density because it absorbs a small portion of the primary beam and most of the scattered radiation.
61
What must be done to compensate for the decreased film density caused by a grid?
Increase the amount of incident radiation.
62
What are the three important dimensions of a grid?
Thickness of the grid strip (T), width of the interspace material (D), and height of the grid (h).
63
How is the grid ratio calculated?
By dividing the height of the grid by the interspace width.
64
If a grid is constructed with 50-μm strips and a 350-μm interspace, what is being questioned?
What percentage of X-rays incident on the grid will be absorbed by its entrance surface?
65
What is the grid ratio when the lead strips are 2.4 mm high and separated by 0.2 mm?
The grid ratio is 12:1.
66
Why are high-ratio grids more effective in reducing scatter radiation?
Because the angle of scatter allowed by high-ratio grids is less than that permitted by low-ratio grids.
67
What is the general range of grid ratios?
Grid ratios range from 5:1 to 16:1.
68
What grid ratio is frequently used with general-purpose x-ray imaging systems?
An 8:1 to 10:1 grid.
69
How much scatter radiation does a 5:1 grid reduce?
Approximately 85%.
70
How much scatter radiation can a 16:1 grid reduce?
As much as 97%.
71
What effect do high-ratio grids have on patient radiation dose?
They increase the patient radiation dose.
72
What is grid frequency?
The number of grid strips per centimeter.
73
How do high-frequency grids affect the appearance of grid lines on a radiographic image?
They show less distinct grid lines than low-frequency grids.
74
What happens to the interspace of a grid as its frequency increases while keeping strip width constant?
The interspace must be thinner and the grid ratio must be higher.
75
What is the effect of high-frequency grids on patient radiation dose?
They require high radiographic technique and result in a higher patient radiation dose.
76
Why does increasing grid frequency lead to a higher patient radiation dose?
Because more grid strip is available to absorb x-rays, requiring a higher radiographic technique.
77
What is the purpose of the interspace material in grids?
To maintain a precise separation between the delicate lead strips of the grid.
78
What materials are commonly used for interspace material in grids?
Aluminum or plastic fiber.
79
What is one advantage of using aluminum over plastic for grid interspace material?
Aluminum has a higher atomic number (Z), providing selective filtration of scattered x-rays.
80
How does aluminum affect the visibility of grid lines on radiographs?
It produces less visible grid lines compared to plastic.
81
What is a consequence of aluminum's properties at low kVp?
It increases the absorption of primary x-rays in the interspace, resulting in higher mAs and a higher patient dose.
82
What is a nonhygroscopic property of aluminum?
It does not absorb moisture like plastic fiber does.
83
Why is aluminum easier to manufacture compared to plastic fiber?
Aluminum is easier to form and roll into sheets of precise thickness.
84
What are the ideal characteristics of a grid strip?
It should be infinitely thin and have high absorption properties.
85
Why is lead the most widely used material for grid strips?
Because it is easy to shape, relatively inexpensive, has a high atomic number, and high mass density.
86
What materials have been tried for grid strips besides lead?
Tungsten, platinum, gold, and uranium.
87
What does the contrast improvement factor reveal?
The ability of the grid to improve image contrast.
88
How does the contrast improvement factor vary with grid ratio?
It is higher for high-ratio grids.
89
What does a contrast improvement factor of 1 indicate?
No improvement in contrast.
90
What is the typical contrast improvement factor range for most grids?
Between 1.5 and 2.5.
91
How much does image contrast approximately increase when grids are used?
It is approximately doubled.
92
What is the contrast improvement factor for a 12:1 grid if the average gradient without a grid is 1.1 and with the grid is 2.8?
The contrast improvement factor is 2.5.
93
What must be increased when a grid is used in radiographic technique?
The radiographic technique must be increased to produce the same image receptor signal.
94
What is the Bucky factor also known as?
The grid factor.
95
Who is the Bucky factor named after?
Gustave Bucky, the inventor of the grid.
96
What does the Bucky factor attempt to measure?
The penetration of primary and scatter radiation through the grid.
97
How does the grid ratio affect the Bucky factor?
The higher the grid ratio, the higher the Bucky factor.
98
What happens to the penetration of scatter radiation as grid ratio increases?
It becomes less likely, causing the Bucky factor to increase.
99
How does increasing kVp affect the Bucky factor?
The Bucky factor increases with increasing kVp.
100
Why does the Bucky factor increase at high voltage?
More scatter radiation is produced, which has a more difficult time penetrating the grid.
101
What are the two types of linear/parallel grids?
Focused and non-focused.
102
What is a key characteristic of parallel grids?
Lead strips run parallel to one another in one direction only.
103
What must the X-ray beam be aligned with when using a parallel grid?
The center of the long axis.
104
What is a common issue associated with parallel grids?
Grid cutoff, which is the undesirable absorption of primary X-rays by the grid.
105
When is grid cutoff most pronounced?
When the grid is used at a short SID and with a large-area image receptor.
106
What types of grids can be either moving or stationary?
All parallel grids.
107
What is the easiest type of grid to manufacture?
Linear/parallel grid.
108
Can angulation with the long axis be performed with parallel grids?
Yes, angulation with the long axis is possible.
109
What is a characteristic of linear/parallel grids?
They do not coincide with the divergence of the x-ray beam, leading to some grid cutoff along the lateral edges.
110
When is the parallel grid best employed?
At long SIDs, as the beam will be straighter and more perpendicular.
111
What defines a cross-hatch/cross grid?
It consists of two sets of lead strips superimposed and running at 90° to one another.
112
What is a key requirement when using a cross-hatch grid?
The beam must be aligned with the center of the grid
113
How do cross-hatch grids compare to parallel grids in terms of efficiency?
Cross-hatch grids are more efficient in cleaning up scatter radiation.
114
What is a significant disadvantage of using crossed grids?
Positioning the grid is critical
115
What is a consequence of using crossed grids regarding patient radiation dose?
The exposure technique required is substantial, resulting in a higher patient radiation dose.
116
What are the two types of grid focus?
Focused and Non-Focused.
117
How are the lead strips arranged in a focused grid?
They are inclined inward, focusing at a predetermined point above the grid.
118
What is the purpose of a focused grid?
To minimize grid cutoff.
119
What does the convergence line represent in a focused grid?
The line in space where extended lead strips would intersect.
120
What is the grid radius?
The distance from the face of the grid to the points of convergence of the lead strips.
121
How are the lead strips arranged in a non-focused grid?
They are parallel and uniform to one another throughout.
122
What are the three possible motions of reciprocating grids?
1. Single-stroke (one-way), 2. Reciprocating (forward & backward), 3. Catapult.
123
What usually causes an off-level grid?
An improperly positioned x-ray tube.
124
When does an off-level grid occur during radiography?
When the grid tilts during horizontal beam radiography or when the image receptor sinks into the patient’s bed.
125
What usually causes an off-level grid?
An improperly positioned x-ray tube.
126
When does an off-level grid occur during radiography?
When the grid tilts during horizontal beam radiography or when the image receptor sinks into the patient’s bed.
127
What is the result of a lateral shift of the grid?
Grid cutoff across the entire radiograph, producing lower optical density (OD).
128
What is the term for the error caused by lateral shift of the grid?
Lateral decentering.
129
What major problem arises when using a focused grid at unspecified SIDs?
Grid cutoff becomes more severe the farther the grid is from the specified focal distance.
130
How does grid cutoff vary across the image receptor?
It is more severe at the edges.
131
What major problem arises when using a focused grid?
It occurs when radiographs are taken at SIDs unspecified for that grid.
132
What happens to grid cutoff as the distance from the specified focal distance increases?
The grid cutoff becomes more severe.
133
Is grid cutoff uniform across the image receptor?
No, it is more severe at the edges.
134
What effect does an upside-down focused grid have on a radiographic image?
It shows severe grid cutoff on either side of the central ray.
135
What is the primary function of grids in radiography?
Grids absorb scatter radiation, which adds exposure to the image receptor.
136
How does the efficiency of a grid affect exposure to the image receptor?
The more efficient a grid is at absorbing scatter, the less exposure will be received by the image receptor.
137
What compensations must be made when using an efficient grid?
Compensations must be made to increase exposure, generally accomplished by increasing mAs.
138
What is the relationship between grid efficiency and patient dose?
The better the grid cleans up scatter, the greater the dose given to the patient to achieve adequate exposure.
139
What formula is used to calculate the amount of mAs needed when using a grid?
Grid Conversion Factor (GCF), also known as the Bucky factor.
140
If a satisfactory chest radiograph is produced using 5 mAs without a grid, what is needed for a second image using a 12:1 grid?
The mAs needed can be calculated using the Grid Conversion Factor.
141
What is a major disadvantage of using radiographic grids?
Increased patient radiation dose.
142
How much more radiation may a patient receive when using a grid compared to not using one?
Several times more radiation.
143
What is the approximate increase in patient radiation dose when using a moving grid instead of a stationary grid?
Approximately 15% more.
144
What is the air gap technique?
An alternative to the use of radiographic grids that reduces scatter radiation and enhances image contrast.
145
How far is the image receptor moved from the patient when using the air gap technique?
10 to 15 cm.
146
How much should the mAs be increased for every centimeter of air gap in the air gap technique?
Approximately 10%.
147
How does the patient dose associated with the air gap technique compare to that of a grid technique?
It is higher than that associated with the nongrid technique and approximately equivalent to that of an intermediate grid technique.
148
What is one disadvantage of the air-gap technique?
Image magnification with associated focal-spot blur.
