Control Of Scatter & Grid Flashcards
What is a beam restricting device?
Tools an RT can use to limit the amount of scatter radiation reaching the IR.
What is scatter?
X-ray photons that have changed direction after interacting with matter.
What are three factors that contribute to scatter?
Increased kVp
Increased X-ray field size
Increased pt thickness
Beam restricting devices reduce scatter by:
Decrease X-ray beam
Decrease amount of tissue irradiated
As scatter radiation increases, the radiograph loses contrast & appears:
Gray and dull due to fog.
Fog is:
Unintended optical density on a radiograph that reduces contrast because of light or chemical contamination.
Three effects of scatter:
Degrades visibility of detail or image detail
Degrades contrast resolution
Degrades spatial resolution
Spatial resolution is controlled by:
Focal spot size.
Contrast resolution is affected by:
Scatter radiation
Scatter can be controlled by:
Beam restricting devices, grids & collimation.
Collimation reduces _________ & improves _________.
Pt dose/contrast resolution
As X-ray energy increases (kVp) scatter (increases or decreases?)
increases
It is not possible to use low kVp on all of our exams because:
Fewer X-rays reach the IR at low kVp (Higher percentage of a low energy beam gets attenuated by the pt).
With larger pt’s, kVp must be ______ (high or low) to ensure adequate penetration.
high
With increased kVp comes increased _______ and decreased _______.
Scatter/contrast.
Because of reduction in patient dose, ____ kVp is preferred to ___ kVp.
High/low
As field size is increased, scatter radiation is _______.
Increased.
As we collimate, we reduce the amount of OD as well as the amount of X-rays reaching the IR, therefore we must ______ our technique.
increase.
Imaging thick parts of the body results in ___________ than when we X-ray thinner parts.
More scatter
Thicker body parts require (more or less) technique.
More
The angle of the scatter is greater in
the remnant beam.
A compression paddle can reduce the thickness of pt anatomy & bring the body closer to the IR, and therefore:
Improves spatial resolution.
Reasons to restrict the X-ray beam:
Only the part being examined should be exposed and large X-ray fields result in more scatter radiation.
Types of beam-restricting devices:
Aperture diaphragm
Cones
Cylinders
Collimators
An aperture diaphragm is:
a lead or lead-lined diaphragm with hole in it attached to the X-ray tube. IR receives an image 1 cm smaller on all sides than IR.
Disadvantages of aperture diaphragm:
Projected field size is not adjustable
Edges of image are blurry (penumbra)
Penumbra is reduced when beam restrictor is ______________ from the tube port.
further away
The further the beam restrictor from the port:
the sharper the edges of the exposed area.
Cones and cylinders are:
extended metal structures which constricts the useful beam to appropriate size.
Distal end of cones and cylinders determine:
field size.
Cones and cylinders attach how?
to the slots in the bottom of the collimator.
Disadvantages of cones:
If angle of cone is greater than divergent angle of primary beam then beam is not being restricted. Also, when cone and IR are not aligned, one side of radiograph may not be exposed (cone cutting).
Formula to determine field size:
SID x lower diameter of opening divided by distance from focal spot to bottom of aperture or cone.
The most common beam-restricting device is:
the collimator box
When X-rays are produced at a spot on the anode other than the focal spot, it is referred to as:
off-focus radiation.
Collimators are used to reduce the amount of
off-focus radiation.
Off–focus radiation results in images similar to
shadows of pt, beyond the exposed field of radiation.
Collimators consist of:
First stage entrance shutters, light and mirror, and second stage cross shutters.
Two sets of adjustable shutters within the collimator controls:
(top set:) off focus radiation and (bottom set:) penumbra.
Collimator lamp & mirror must be adjusted so that projected light coincides with X-ray beam or what may happen?
Collimator cutoff of anatomic structures may occur.
Collimator box is marked according to field size at:
fixed SID.
PBL devices
Positive beam limiting devices.
What are positive beam limiting devices?
Automatically cones down to appropriate field size.
Manual collimation is still necessary with PBL in order to:
more tightly cone down image and in doing so, reducing pt exposure & improving image quality.
PBL was mandated by:
US FDA in 1974
The X-ray beam should never exceed the size of:
the IR.
Additional collimator filtration may be necessary depending on:
tube potential.
Which thickness of filters are most common?
0, 1, 2 and 3mm of Al are most common.
Mammography does not have collimators because:
they result in increased filtration of X-ray beam. A special aperture is utilized and an exposure cannot be taken without the special aperture.
Ancillary devices:
Lead blocker
Lead mask
Lead blockers are helpful when
examining large patients…they absorb scatter. Used mostly for L5-S1 vertebral exams and for AP projection of shoulder.
Lead masks:
Used in cerebral angiography, are cut to correspond to particular field size and are secured to end of the collimator.
What is a grid?
Device for reducing scatter that takes over where beam restricting devices stop.
What does scatter do to the image?
Reduces image contrast & contrast resolution.
What is contrast resolution?
The ability to image adjacent similar tissues or the degree of difference in OD between areas of an image.
What is one of the most important characteristics of film quality?
Contrast
As kVp is lowered, scatter is reduced due to:
Enhanced differential absorption
What is differential absorption?
The different degrees of absorption in different types of tissue.
Beam restricting devices are not totally sufficient because:
They are positioned between the X-ray source and patient.
X-rays arriving at the IR consist of both ________ &. _________ X-rays.
Transmitted/scattered
The loss of contrast results from:
The presence of scattered X-rays.
Most of angled scatter are removed by grids thus:
Increasing contrast.
Unlike beam-restricting devices the grid is positioned:
Between the pt and the IR.
Together, beam restricting devices and grids _______ scatter radiation.
Reduce
What does a grid consist of?
Raidopaque material (grid strips) alternating with sections of radiolucent material (interspace material).
Who invented the grid?
Gustave Bucky in 1913
The lead strips in the grid are so thin they are sometimes referred to as:
Lead foil strips.
The material used in interspace of the grid can be:
Plastic or aluminum.
The surface of the grid is called:
The face of the grid.
A line called the center line is usually drawn on the face of the grid to inform the RT:
The direction of the grid line.
Using the grid in the wrong direction will result in:
improper exposure
The interspace material is made of either:
Aluminum or plastic fiber.
The interspace material should be radiolucent so as not to:
absorb the X-ray beam.
The purpose of the interspace material is to:
maintain a precise separation between the delicate lead strips.
Plastic is the preferred material for interspace material for grids because:
aluminum has a higher atomic number and therefore absorbs some of the primary beam, increasing pt dose.
Lead is the most widely used material for grid strips due to:
high atomic number and high mass density. It is also easy to shape and is inexpensive.
The strips and interspaces are bound together and covered by aluminum or plastic to:
give the grid strength & seal out moisture.
The grid is made to match the size of the cassette. True or false?
True.
Information about a grid’s construction is contained:
on a label placed on the tube side of the grid.
The information about a grid’s construction consists of:
type of interspace material
grid frequency and ratio
grid size and range of SIDs that can be used.
If the primary photons and the grid are aligned:
the primary photons will pass through the grid.
Scattered radiation travels in a changed direction and more likely to be:
absorbed by the lead strips of the grid.
Only X-rays traveling ________________, are transmitted through the grid.
in the direction of interspace.
When a primary photon bounces off a part of the pt’s body:
it loses energy as it is scattered.
Since scattered rays have lost energy:
it becomes easier for the lead strips to absorb the scatter.
High quality grids attenuate _______ % of scatter radiation.
80-90. This is considered “good cleanup.”
Grid ratio is:
the relationship between the height of the lead strips and the distance between the strips. (h/D)
Grid dimensions consist of:
thickness of grid strips
width of interspace
height of grid
High ratio grids are ______ _______ in cleaning up scatter radiation.
more effective
The angle of scatter allowed by high ratio grids is:
less than that permitted by low-ratio grids.
Higher ratio grids are difficult to:
manufacture.
Higher grid ratio gives higher radiation exposure to pt but:
improves contrast.
Grid ratios range from:
5:1 to 16:1.
Higher ratio grids are used in exams using:
high kVp.
5:1 grids clean up approximately ____ %
85
16:1 cleans up approximately ____%
97
As grid ratio increases, radiographic density:
decreases. (More lead is in the cassette, so technique must increase in order to maintain density.)
As grid ratio decreases, radiographic density
increases
Increasing the grid ratio requires additional:
mAs
Decreasing the grid ratio requires less:
mAs.
As grid ratio increases, patient dose:
increases. Because higher ratio grids have more lead to absorb X-rays, mAs must be increased.
As grid ratio decreases, patient dose:
decreases. Because lower ratio grids have less lead to absorb X-rays, mAs can be decreased.
The higher the grid ratio, the ____ positioning latitude of the grid.
less. If grid is not properly aligned with the tube then more X-rays will be absorbed.
The lower the grid ratio, the ____ the positioning latitude.
wider
Grid frequency is:
the number of grid strips of grid lines per inch or cm.
Grids with high frequency show less distinct grid lines compared to grid with low frequency because:
the higher the frequency of the grid, the thinner are its lead strips.
A grid with more grid lines per inch is ______ efficient at absorbing scattered radiation because:
more/ it has more lead in it.
The usual grid frequency range is:
60 - 110 lines per inch.
High frequency grids require _____ radiographic technique resulting in :
high/higher patient dose.
The principle function of a grid is to:
improve image contrast.
Contrast Improvement Factor (k) is:
the ratio of radiographic contrast with a grid to that without a grid.
Contrast Improvement Factor shows:
the ability of the grid to improve radiographic contrast.
Most grids have contrast improvement factors between:
1.5 and 2.5. This means the contrast is approximately double when grids are used.
A Contrast Improvement Factor of 1 indicates:
no improvement.
The Contrast Improvement Factor is higher for:
higher ratio grids.
When a grid is used, the radiographic technique is ______ in order to produce the same OD compared to a non-grid exposure.
increased.
The Bucky Factor is:
a number that can be used to determine the adjustment in mAs when changing from one grid to another or not using a grid at all.
The Bucky Factor, or grid conversion factor, or grid factor is calculated by:
mAs with grid/mAs without grid.
Bucky Factor chart:
No grid = 1
5: 1 = 2
6: 1 = 3
8: 1 = 4
12: 1 = 5
16: 1 = 6
Grid Conversion Formula
mAs1/mAs2 = GCF1/GCF2
Selectivity:
the ratio of primary radiation transmitted through the grid to the amount of scatter radiation transmitted through the grid. Identified by the Greek letter, sigma.
Selectivity is a function of the __________, not the X-ray beam.
construction characteristics of the grid
Selectivity formula:
primary radiation transmitted/scatter radiation transmitted.
What has the primary influence on selectivity?
The total lead content in the grid.
Grids can have the same grid ratio, but different amounts of lead because:
they have a different grid frequency.
The grid ratio is ________________. The grid frequency is:
height of the strip over the distance / Number of strips per inch.
The heavier the grid, the more lead contains and the ______ the selectivity.
higher
Grids that absorb a greater percentage of scatter than primary radiation are described as having a _____ degree of selectivity.
greater
The higher the selectivity the more ______ it is in cleaning up radiation.
efficient.
Grid types:
linear crossed focused parallel moving stationary short/long dimension.
A linear grid has lines the run:
in only one direction.
What grid pattern is most popular and why?
Linear, because they allow angulation of the tube along the length of the lead lines.
Cross-hatched has lead lines that run:
at right angles to one another.
Crossed grids remove more scattered photons than linear grids because:
they contain more lead strips oriented in two directions.
Use of crossed grids is limited since:
the tube cannot be angled in any direction without producing grid cutoff.
Grid cuttoff is:
undesirable absorption of primary X-rays by the grid.
What is the most common type of grid pattern?
Focused grid
How are the strips aligned in a focused grid?
The strips in the very center of the grid are parallel to each other, the strips at the sides of the grid are angled. The angle increases as the strips get closer to the sides of the grid.
Why are the strips in a focus grid aligned the way they are?
To match the way the primary photons emerge from the x-ray tube.
How do focused grids allow more transmitted photons to reach the IR than parallel grids?
Since scattered radiation travels in a changed direction from primary radiation, it is more probably that the scattered rays will be absorbed by the angled strips of the focused grid.
The SID used with a focused grid must fall within a certain range or the radiograph will display ________. The acceptable range is determined by:
grid cut off/the convergent point.
The convergent point is:
an imaginary point in space above the grid where the focused lead lines would meet or converge if they were extended.
The convergent line is:
the line that would be formed the the convergent points were connected along the length of the grid.
The convergent line and the convergent point determine the __________ of a focused grid.
focal distance
The focal distance is:
the distance between the grid and the convergent line or point.
The focal distance determines:
the focal range.
The focal range is:
the recommended range of SIDs that can be used with a focused grid. It is a tolerance range that extends a little bit above and below the focal distance.
If the xray tube is placed either above or below the focal range:
grid cut off can occur.
The convergent line or point always fall:
within the focal range.
Two common focal ranges are:
36 to 42 inches with a focal distance of 40 inches, and 66 to 74 inches with a focal distance of 72 inches.
What is the simplest type of grid, and what are some of it’s characteristics?
Parallel grids. Easiest to manufacture & undesirable due to grid cut off.
Grid cut off is more pronounced in a parallel grid under what conditions?
Short SID and large-area IR.
Grid cut off is undesirable, why?
it decreases the radiographic density (OD) by reducing the number of photons reaching the IR.
How does grid ratio have an effect on grid cutoff?
The higher the grid ratio, the more the grid cutoff. The more lead in the grid, the more cutoff there will be.
Four types of grid cut off errors:
Upside down focused grid
Off-level grid
Off-center grid
Off-focus
In order to avoid grid cutoff, the tube and IR must be:
properly aligned.
Upside down focused grid error occurs when:
a focused grid is placed upside down on the IR which results in the gird lines going opposite the angle of the diverging xray beam.
How does upside down focused grid error appear?
as a significant loss of density along the edges of the image.
How are focused grids labeled in order to avoid upside down errors?
“Tube side.”
When do off level grid errors occur?
when the xray beam is angled across the lead strips from either the IR being angled or the tube being angled.
What is the most common type of grid cut off?
Off-level grid cutoff.
What types of grids can off-level cut off occur with?
Focused and parallel.
How does off-focus cut off appear?
As an overall decrease in density on the radiograph.
Off center cut off occurs when:
CR is not aligned from side to side with the center of the focused grid. Also called lateral decentering.
Off center cut off happens due to:
the divergence of the primary beam does not match the angle of the lead strips.
Off center cutoff appears as:
an overall loss of density.
Off focus grid cutoff occurs when:
the SID is outside of the recommended focal range. Cutoff occurs if the SID is less than or greater than the focal range.
Off focus cut off appears as:
loss of density at the periphery of the film.
Off focus/off center grid cut off are very common and easily occurs during __________. It is easily recognized as _________.
mobile radiographs/ uneven exposure - appears dark on one side and light on the other.
It is possible to see the grid lines when using this type of grid:
Stationary grid
Types of stationary grids:
Wafer
grid cassette
grid cap
Wafer stationary grid:
matches size of cassette
Grid cassette :
IR that has a grid permanently mounted to its front surface.
Grid cap:
Contains permanently mounted grid - IR slides in behind it.
Moving grid is:
Slightly moving the grid during the xray exposure will blur out the grid lines.
Moving grids are part of the _____-_______ diaphragm commonly referred to as the _______.
Potter-Bucky/Bucky
Who was the Potter-Bucky diaphragm designed by?
Hollis Potter
Two kinds of moving grids:
Reciprocating (moves side to side) and Oscillating (moves in a circular motion.).
If the Bucky device fails to move the grid (or moves too slowly):
the grid lines may appear on the image. (called capturing the grid).
The Bucky device is located:
directly under the xray table.
The Bucky device consists of:
a focused grid and a motor to move the grid. It also has a tray where the cassette is placed.
Where is the grid in the Bucky?
positioned above the tray and cassette and usually cannot be seen.
Disadvantages of moving grids:
Increased distance OID (between pt and IR)
Motion (in cassette holding device causing blur)
Pronounced grid lines (in slower machines such as half & full wave generators)
Minimum exposure time (longer than stationary grid.)
RTs should select grid according to:
size of body part kVp being used amount of contrast required grid pattern likelihood of grid cutoff pt dose
Biggest rule of thumb when choosing to use a grid or not:
A grid should be used when the body part measures more than 10 cm.
More scatter is produced when body part is _______ because there are more atoms for the X-ray photons to collide with.
large
Grids should be used when the kVp is set above:
70 (some use them above 60 kVp.).
If using a kVp above 90, a grid ratio of at least _____ should be used.
8:1
If high contrast is necessary, then a _______ (high or low?) ratio grid should be used.
high
There is a greater chance of producing grid-cutoff with a _______ (higher or lower?) ratio grid.
higher
If misalignment of a grid is probable (portable radiography), then a ______ ratio grid should be used.
lower
Most X-ray rooms use _________ (grid ratio) which compromises between contrast improvement and pt dose.
12:1
Long/short dimension grids are useful when?
in mobile radiography.
Long dimension grids:
lead strips run parallel to long axis of grid.
Short dimension grids:
lead strips run perpendicular to long axis of grid.
Short dimension grids are helpful for exams when:
it is difficult to center the CR perpendicular without any grid cutoff, such as AP CXR.
Air gap technique is:
another alternative to prevent scatter and improving contrast.
How is the air gap technique performed?
by increasing distance between the pt and IR (increased OID) the scatter will miss the IR. The greater the gap, the greater the reduction of scatter.
With the air gap technique, the number of photons reaching the IR is reduced because:
less scatter reaches the IR.
When using the air gap technique, what must be increased?
mAs.
Larger OID produces increase in
magnification.
When using a large OID for the air gap technique, how does one compensate?
by increasing the SID. Density maintenance formula utilized to maintain film density.
The air gap technique is not as efficient for a:
large body part.
When and how is a lead blocker used?
For thick body parts on exposure where there is a large areas of unabsorbed primary radiation on a radiograph. (Lateral Lumbar)
Unabsorbed primary radiation produces:
scatter when the photons hit the X-ray table top. These photons can be absorbed by a lead blocker.
The Moire effect:
is a grid error that occurs with CR imaging plates when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers. Occurs with grids used in a stationary fashion.
To prevent the Moire effect:
a higher frequency grid is recommended when utilizing a stationary grid during CR imaging.
