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What is the purpose of the grid?
- improves radiographic contrast
- absorbs scattered radiation before it reaches the IR.
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Device used to improve the contrast of the radiographic image.
Grid
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Is absorption or transmission responsible for the light areas of the radiograph?
absorption
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When an x-ray beam passes through the body, what 3 things will occur with the primary photons that originated at the target? They will:
- pass through the body unaffected (transmission)
- be absorbed by the body (absorption)
- interact and change direction (scatter)
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What is the result of photoelectric absorption?
The complete absorption of the primary photon and, a a result of this interaction, changes the direction of a secondary photon. The secondary radiation created by this interaction is, with few exceptions, very weak and is quickly absorbed in surrounding tissues.
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Primary radiation that interacts and, as a result of this interaction, changes direction is known as_______. This interaction is known as__________.
- scatter
- compton interaction
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Scattered photons add an overall exposure to the IR and, as a result of this overall graying of the image,___________.
contrast is lowered
-
Field size can be kept to a minimum by what?
collimation
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The greater the atomic number, more or less scatter will be created?
less
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Scatter increases as:
- kVp increases
- field size increases
- thickness increases
- atomic number decreases (only indirect relationship)
-
What is the ONLY thing that is indirect to scatter?
atomic number
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As kVp decreases, scatter?
decreases
-
As field size decreases, scatter?
decreases
-
As thickness decreases, scatter?
decreases
-
As atomic number decreases, scatter?
increases
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A grid should be used if part thickness is over what?
10 cm (as part thickness increases; scatter production increases)
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When should a grid be used pertaining to kVp?
if kVp is over 60 (as kVp increases, the production of scatter increases)
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What is the construction of the basic grid?
A grid is made by placing a series of radiopaque lead strips side by side and separating the strips by an interspace material that is radiolucent
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What is a basic grid typically made of?
- aluminum
- an example is of the cut grid in class to see the aluminum strips and interspaces. Very thin strips are in the grid.
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Who created the gridr?
- radiologist Dr. Gustav Bucky
- 1913
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Tell about Dr. Bucky's first grid.
It consisted of wide stips of lead spaced 2 cm apart and running in two directions, along the length of the image and across the image. This design created an image of the grid that wa superimposed onthe patient's image. Despite having to view the anatomy through this checkboard pattern, the origional grid did remove scatter and improve image.
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What was the primary disadvantages of Bucky's grid?
- Increased dose to patients
- Grid lines on film
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Who made improvements to Bucky's grid? What was it then called?
- Dr. Hollis Potter
- Potter-Bucky Diaphragm
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How did the Potter-Bucky diaphragm improve the grid?
- Realigned lead strips to run in one direction and also made the strips thinner (less obvious on the image)
- Moved grid during exposure to make lines invisible on image
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What significantly improved with the Potter-Bucky diaphragm?
Contrast improved without impairing the view of the pts anatomy
-
h=
the height of the radiopaque strips
-
D=
distance between the stips (the thickness of the interspace material)
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What is the Grid Ratio Formula?
- h/D
- height of lead strips divided by thickness of interspacing material
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Is a higher or lower grid ratio is more effiecient in removing scatter?
higher
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What is the typical grid ratio range?
5:1-6:1
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If a grid has an interspace of .5mm and lead strips that are 3mm high, what would go on top when looking for grid ratio?
- 3mm(height)
- 3mm/.5mm=6:1grid ratio?
-
What is the number of grid lines(lead strips) per inch or cm?
grid frequency
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What is grid frequency measured by?
lines/inch or lines/cm
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What is the grid frequency range?
- 60-200 lines/in or
- 25-80 lines/cm
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Typically _______ frequency grids have thinner lead strips.
higher
-
A KUB would produce a ________ greater amount of scatter as opposed to a hand image.
greater
-
A hand would have less scatter production than a KUB due to what?
the low kVp selection needed to the hand.
-
When are photons are responsible for creating contrast on an image?
when they pass through the body unafected and then interact with the IR to create an image.
-
How do differnces in contrast exist?
they exist because some photons pass through the body while others are absorbed.
-
Absorption of photons occurs as the result of what?
photoelectric absorption
-
What is the interaction that produces scatter radiation?
Compton scatter
-
What is a result of scattered photons adding an overall exposure to the IR?
graying of the image(contrast is lowered)
-
Compton scatter increases as kVp___________.
increases
-
Note: scatter increases and contrast is further impaired as kVp increases.
-
The pt is also the source of scatter that degrades the image, it is logical to assume that the pr will also control the _________of the scatter produced.
quantity
-
What is the volume of the pt that is irradiated controlled by?
thickness of the patient and the exposure field size.
-
Why is less scatter is produced in bone than in soft tissues?
bone absorbs more photons photoelectrically. This is the result of changes in the number and types of atoms that are present for interaction.
-
Because a grid is designed to absorb the unwanted scatter, it is necessary to use a grid with what?
thicker, larger body parts and with procedures that require higher kVp techniques.
-
How is the grid made/set up?
- typically aluminum
- radiopaque lead strips seperated by radiolucent interspace material
-
Are the lead strips radiopaque or radiolucent?
radiopaque
-
Are the interspaces radiopaque or radiolucent?
radiolucent
-
The first grids design created an image of the grid that was ____________ on the patient's image.
superimposed
-
Despite having to view the first grids anatomy through this checkboard pattern, it did remove_________ and improve__________.
- remove scatter
- improve contrast
-
All of Dr. Potters improvements resulted ina practical grid device, which significantly improved________ without impairing the view of the pt's anatomy.
contrast
-
The interspace material should be radiolucent. Why?
to allow radiation to pass easily through it
-
When are very high-frequency grids used?
digital imaging systems
-
What is the range of high-frequency grids?
- 103-200 lines/in
- 41-81 lines/cm
-
What does a very high frequency grid do?
minimizes grid line appearance
-
What is the most important factor in a grids efficiency?
its lead content
-
How is lead content measured?
g/cm2 (mass per unit area)
-
There is more lead content in a grid with _____ ratio and ________ frequency.
-
As lead content increases, what happens to scatter and contrast?
the removal of scatter increases and therefore contrast will also increase (direct relationship)
-
What are the two types of grid patterns?
- cross-hatched (criss-cross)
- linear (parallel or focused)
-
Describe the cross-hatched grid.
- has horizontal and vertical lead strips
- the primary beam must be centered perpendicular to the grid
- the grid must remain flat
-
Describe a linear grid.
- lead strips run the length of the cassette
- allows primary beam to be angled along the long axis of the grid without obtaining "cut-off"
-
What are the two types of linear grids?
-
On a focused grid how are the lead strips set up?
they are angled to match the divergence of the beam
-
On a focused linear grid, where is the primary beam?
its aligned with the interspace material
-
For a linear grid to be properly focused, the x-ray tube must be located where?
along the convergence line
-
On a focused grid, where is the scatter absorbed?
scatter is absorbed by the lead strips
-
A focused linear grid is only useful where?
at a preset SID distance
-
Focused grids with lower grid ratios allow for what?
greater latitude in the alignment of the tube with the grid
-
(Lower or higher grid ratios)proper alignment of the grid with the tube is more critical with linear focused grids.
higher grid ratios
-
The distance from the face of the grid to the points of convergence of lead strips is called?
grid radius
-
What is most common, focused or parallel grids?
focused
-
In a parallel linear grid, what will occur becuse the strips do not try to coincide with the divergence of the x-ray beam?
some grid cutoff will occur along the lateral edges, especially when the grid is employed at short SIDs.
-
Where is the parallel grid best employed and why?
the parallel grid is best employed at long SIDs because the beam will be straighter, more perpendicular
-
In what type of grid are the lead strips paralell to one another?
parallel linear grids
-
What do parallel linear grids absorb?
they absorb a large amount of the primary beam, rusulting in some cut-off
-
2 types of grids
- stationary grid
- mounted in a Potter-Bucky diaphragm to move it during exposure
-
When grids are used in a stationary grid, grid lines on the image will usually be noticed on close nispection. This is especially true with ____-frequency grids. ______-frequency grids, like those used for digital imaging, have a minimum visual effect.
-
Stationary grids are used primarily in what?
portable procedures or for upright or horizontal beam views
-
What is the most common use of the grid?
procedures that use the potter-bucky diaphragm (the Bucky)
-
The Potter-Bucky diaphragm mounts a _____x________ grid above the cassette.
17in. x 19in
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What does the Potter-Bucky diaphragm do during an exposure? Why?
The grid moves during the exposure, so that grid lines will be blurred and therefore not evident on the image
-
What are the two types of grid movement? Where is it used?
- reciprocating and oscillating
- the bucky
-
How does the image blur when using a bucky?
to blur the lines, the grid must move at a right angle to the direction of the lines. This means that it will be moving back and forth across the table and not from top to bottom.
-
What happens during reciprocating?
a motor drives the grid back and forth during the exposure for a total distance of no more than 2-3cm
-
What happens during oscillating?
an electromagnet pulls the grid to one side and then releases it during the exposure. The grid oscillates in a circular motion within the grid frame.
-
Parallel grids function better at _____ SID.
long
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In the focused grid, the strips are designed ________________.
to match the divergence of the x-ray beam
-
What is a convergence line?
the invisable lines that would extend and intersect in space at this point
-
What is the grid radius?
the distance from the face of the grid to the points of convergence of the lead strips
-
For the grid to be properly focused, the x-ray tube must be located where?
along the convergence line
-
At long SIDs the beam will be _______
straighter and more perpendicular.
-
When a grid is placed in a beam to remove scatter:
- density of the radiograph will decrease
- the exposure factors must be increased to compensate for the lack of density
- dose to pt will increase due to the compensations made in technical factors.
-
The more effieienct a grid is at absorbing scatter, the ________exposure will be recieved by the IR.
less
-
When using a grid, _______ must be increased. What does this do to the pts dose?
-
The better the grid cleans up scatter, the ______ will be the dose given to the pt to achieve an adequate exposure.
greater
-
How can you find out how much mAs is needed when adding a grid?
- use the grid conversion factor (GCF)
- mAs with grid/ mAs without grid
-
When converting from one grid ratio to another what formula should be used?
- the grid conversion formula
- mAs1 = GCF1mAs2 GCF2
-
What is the GCF for no grid?
1
-
What is the GCF for a 5:1 grid?
2
-
What is the GCF for a 6:1 grid?
3
-
What is the GCF for a 8:1 grid?
4
-
What is the GCF for a 10:1 grid?
5
-
What is the GCF for a 12:1 grid?
5
-
What is the GCF for a 16:1 grid?
6
-
GCF and mAs have what kind of relationship?
direct relationship
-
When there is an increase in grid ratio and an increase in GCF, what happens to radiation absorption?
there is more absorption, less x-rays reach the IR
-
When there is more radiation absorption and less x-rays reach the IR, what happens to density and brightness? What must you do to your technical factors?
- density decreases
- brightness increases
technical factors will need to be increased (increase in mAs)
-
The efficiency of a grid in cleaning up or removing scatter can be quantitatively measured by the __________.
International Commission on Radiologic Units and Measurements (ICRU)
-
What is the criteria in which the ICRU evaluates grid performance?
- selectivity
- contrast improvement ability
-
If the height of the grid is a constant, decreasing the distance between the lead strips would result in an _________ in the GR.
increase
-
If the height of the grid is a constant, increasing the distance between the lead strips would result in a ________ in GR.
decrease
-
The distance between the lead strips and grid ratio are what kind of relationship?
- inverse
- (GR-height of the grid strips remain the same)
-
Scattered photons have to be more closely aligned to the direction of the primary photons in order to reach the IR, this means...
that higher GR are more effective at removing scatter. For the same reason, higher GR's require greater accuracy in their positioning and are more prone to grid errors
-
If you increase the distance between the lead strips, what are you doing to GR?
decrease (indirect relationsip)
-
If the distance between the lead strips decreased, what would happen to the GR?
increase
-
If the GR increased, what would happen to the distance between the lead strips?
they would decrease (indirect relationship)
-
_____ GR's allow less scatter radiation to pass through their interspace material to reach the IR.
higher
-
Higher GR's are more or less effective at removing scatter?
more effective
-
The best measure of how well a grid functions is its ability to ________.
improve contrast
-
What is selectivity?
the percent of primary radiation transmitted vs. the percent of scatter radiation
-
A grid with a higher lead content would have a greater or lesser selectivity?
higher
-
What is the K factor (contrast improvement) formula?
K=contrast with grid/contrast without the grid
-
The ______ the K factor, the greater the contrast improvement.
higher
-
Describes grid's ability to allow primary radiation to reach the IR and prevent scatter
selectivity
-
Highly selective grids are....
- highly selecteive grids are better at removing scattered radiation
- high lead content grids are more selective because it is able to absorb more scatter
-
What are the contrast (k factor) ranges?
1.5-3.5
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