8. Grid Flashcards
is a device used to improve the contrast of the
radiographic image
Grid
When an x-ray beam passes through the
body, one of three things will occur with the primary photons that originated at the target. They
will:
- ) Pass through the body unaffected
- ) Be absorbed by the body
- ) Interact and change direction
The photons that pass through the body ______ will
interact with the image receptor to create the image
unaffected
These differences exist because some photons pass through the body while others are
absorbed
Primary radiation that interacts and as a result of this interaction, changes direction is known as
scatter radiation
Since these photons _____ direction, they are no longer able to record exposures on the image receptor that
relate to the patient’s anatomy
change
add an overall density to the receptor
and as a result of this overall graying of the image,
contrast is lowered.
Scattered photons
Scatter increases with ______ in the volume of the tissue irradiated and decreases with ______ atomic number of the tissue
increases,
increased
The volume of tissue irradiated that is controlled by the ______ of the patient and the exposure field size
thickness
The amount of scatter radiation increases:
- Increases in patient thickness
- Larger field size
- Decrease atomic number of the tissue
General rule (Grid is employed):
- body part thickness exceeds 10 cm (6 in)
2. kVp is above 60
is a thin, flat, rectangular device made by placing
a series of radiopaque lead strips side by side and
separating the strips by an interspace material.
Grid
The very first grid was made in ____ by the
1913,
American Radiologist Gustav Bucky (1880-1963)
consisted of wide strips of lead spaced 2 cm apart and running in 2 directions, along the
the length of the image and across the image
Dr. Bucky’s first grid
In ______, ______ ,a Chicago radiologist
improved Dr. Bucky’s grid design
1920, Hollis Potter (1880-1963)
He realigned the lead strips so they would run in only 1 direction, made the lead strips thinner and therefore less obvious on the image, and then designed a new device (now known as the Potter-Bucky diaphragm) which allowed the grid to move during the exposure.
Hollis Potter (1880-1963)
Involves the selection of materials, grid ratio and grid frequency
Grid Construction
a grid is a series of _______ strips
which alternate with ______ interspace strips
radiopaque,
radiolucent
radiolucent
Interspace material
Interspace material Either \_\_\_\_\_\_ (commonly used since it easier to manufacture, durable & has a high atomic) and fiber
aluminum
higher atomic number than fiber, it can provide additional absorption of low-energy scatter
Aluminum
(higher atomic number) increases the
absorption of the primary photons & this is a
disadvantage especially with low kVp techniques where this absorption would be greater
Aluminum
is the ratio of the height of the lead strips to the distance between the strips
Grid Ratio
If the height is grid is constant, _______ the distance between the lead strips would result in an increase in the grid ratio
decreasing
If the height of the grid is a constant, ______ the
distance between the lead strips would result in a decrease grid ratio
increasing
An ______ relationship exists between the distance of the lead strips and grid ratio when the height of the grid strips remain the same
inverse
allow less scatter radiation to pass through their interspace material to reach the image receptor
Higher grid ratios
are more effective at removing
scatter
Higher grid ratios
defined as the number of grid lines per inch or centimetre
Grid Frequency
Grids are made with a range in frequency from
60-200 lines/inch (25-80 lines/cm)
Most commonly used grids have a frequency of
85-103 lines/inch (33-41 lines/cm)
Very high frequency grids of approximately ________ are recommended for
stationary grids used with digital image receptor systems to minimize the possibility of seeing the grid lines on the image
103-300 lines/inch (41-80 lines/cm)
In general, lead content is greater in a grid that has a ______ grid ratio and _____ grid frequency
higher,
lower
As the lead content of a grid increases, the ability of the grid to remove scatter and improve contrast ______
increases
Grid with lead strips running in only one direction are called
linear grids
Grid Patterns
Linear Parallel,
Linear Focused,
Criss-Cross
Grids are also made by placing 2 linear grids on top of
one another so the grid lines are running at right angles
Criss-cross or cross-hatched
are more commonly used in clinical practice because they can be used when performing procedures that require tube angulation
Linear grids
Angulation across the long axis would result in the
primary beam being directed into the
lead strips
Results If the primary beam is angled into the lead and the lead will absorb an undesirable amount of primary radiation
Grid cut-off
GRID TYPES
Parallel Grids,
Focused Grids
are made with the lead and interspace
strips running parallel to one another
Parallel Grids
are designed so that the central grid strips are parallel and as the strips move away from the central axis they become more and more inclined
Focused Grids
If these lead strips were extended, the strips would
intersect along a line in space known as the
convergence line
The distance from the face of the grid to the points of
convergence of the lead strips is called the
grid radius
For the grid to be _______, the x-ray tube must be located along the convergence line
properly focused
Focused grids with _____ grid ratios allow for _____ latitude in the alignment of the tube with the grid.
lower,
greater
With _____ grid ratio, proper alignment of the grid with the tube is more critical
higher
are less commonly employed than focused because the strips do not try to coincide with the divergence of
the x-ray beam, some grid cut off will occur along the lateral edges, especially when the grid is employed at the short SIDs
Parallel grids
A grid is used either in a _______ or _______
in a Potter-Bucky diaphragm to move it during
exposure
stationary position,
mounted
are used primarily in portable procedures or for upright or horizontal beam views
Stationary grids
Stationary grids are used primarily in portable
procedures or for upright or horizontal beam views.
• This design is called as the
Grid cassette
The most common use of the grid is for procedures using the
Potter-Bucky diaphragm
Potter-Bucky diaphragm usually called the
Bucky
Device is mounted below the tabletop of radiographic and
radiographic/fluoroscopic tables and holds the cassette in place below the grid
Potter-Bucky diaphragm
These grids are approximately ________,
large enough to cover a _______ cassette
placed either lengthwise or crosswise in the cassette tray
17” x 19” (43 x 48 cm),
14” x 17” (35 x 43 cm)
Potter-Bucky diaphragm
The lead strips of the grid run along the ____ axis of the table
long
To blur the head lines, the grid must move at a ______ to the direction of the lines
right angle
There are 2 movement mechanisms used today : the movements are described as
reciprocating and oscillating
a motor drives the grid back and forth during the exposure for a total distance of no more
than 2-3 cm
Reciprocating grid
an electromagnet pulls the grid to one
side and then releases it during exposure
Oscillating grid
Once a grid is selected and mounted in the Potter-Bucky Diaphragm, it is
not easily changed
are generally made by the
department administrator in collaboration with the
radiologist.
Purchase decisions
The more efficient a grid is at absorbing scatter, the _____ exposure will be received by the image receptor
less
Increasing ____ to compensate but will result in greater patient dose
mAs
The better the grid cleans up scatter, the _____ will be the dose given to the patient to achieve an adequate
exposure
greater
The ______ of a grid in cleaning up or removing
scatter can be quantitatively measured
efficiency
defines 2 criteria for
measuring a grid’s performance: Selectivity and Contrast
Improvement Ability
International Commission on Radiologic Units and
Measurements (ICRU) Handbook 89
ICRU
International Commission on Radiologic Units and
Measurements
Although grids designed to absorb
scatter, they also absorb some primary radiation
Selectivity
Grids that absorb a greater percentage of scatter than
primary radiation are described as having a _____ degree of selectivity
greater
The greater the grid is at removing scatter, the greater
will be the selectivity of the grid. This means that a grid with a higher lead content would have a ______
selectivity
greater
The best measure
of how well a grid functions is its ability to improve
contrast in the clinical setting
Contrast Improvement Factor
It is dependent on the amount of scatter produced,
which is controlled by kVp and volume of tissue
irradiated
Contrast Improvement Factor
As the amount of scatter radiation increases, the ______ will be the contrast and the _____ the contrast
improvement factor
lower,
lower
Poor images can result from ______ use of the grid
improper
occurs when the tube is angled across the
long axis of the grid strips
Off-level
can occur with a focused grid and it is the only positioning error possible with a parallel grid
Off-level grid error
The x-ray tube must be centered along
the central axis of a focused grid to prevent an
off-center (off-axis or lateral decentering) grid error
The result is a decrease in exposure across the entire image
Off-center
The greater the degree of lateral decentering, the _____ the grid cut-off
greater
is made to be used at very
specific distances as identified as the focal range labelled on the front of the grid
focus grid
If the SID is not within the focus range of the grid, _____ will happen with the focused grids
grid cut off
result in grid cut-off along the peripheral edges of the image
Off-focus errors
a focused grid has an identified tube side
based o the way the grid strips are angled
Upside-down
If the grid is used ______, severe peripheral grid cut-off will occur
Upside-down
is a grid error that occurs with digital
image receptor systems when the grid lines are captured and
scanned parallel to the scan lines in the imaging plate readers
Moire Effect
AN ALTERNATE SCATTER REDUCTION METHOD
AIR-GAP TECHNIQUE
This is best done by restricting the primary beam
AIR-GAP TECHNIQUE
Collimating to the size of the are being examined is
_____ to image quality.
critical
is an alternative to the use of a grid
AIR-GAP TECHNIQUE
It has primary applications in magnification
radiography and to a lesser extent, in chest radiography
AIR-GAP TECHNIQUE
The technique involves placing the patient at a ______ object image receptor distance (OID), thus creating an air gap between the patient and the image receptor
greater
While the same amount of scatter will be created during the exposure, less of the scatter will reach the image receptor if the patient is moved
farther away
The primary disadvantage of the air-gap technique: _______ that results from the OID
loss of sharpness
