Principles of Radiographic Imaging CH 18: The Grid
Determining mAs when using grid
Grid conversion factor (GCF) = mAs with the grid/mAs without the grid. This formula is the Bucky factor, too.
How do grids work?
Photons going at an angle hit the lead strips and are absorbed.
For the grid to be properly focused
the x-ray tube must be located along the convergence line.
When an off-level grid error occurs
there is an undesirable absorption of primary radiation, resulting in a decrease in exposure across the entire image
If a grid ratio is higher,
photons have to go in a straighter path in order to reach the IR
Grid-conversion factors increase
with higher grid ratios and increasing kVp.
Distance from the face of the grid to the convergence line is called the
grid radius
K factor measures a
grid's ability to improve contrast Grids with a high ratio have high K factors
amount of scatter radiation increases with
increases in pt thickness larger field sizes decreases in atomic # of the tissue and increases in kVp
Scatter increases with
increases in the volume of the tissue irradiated and decreases with increased atomic number of the tissue, and increases when kVp is increased
What does scatter do to the image
increases radiographic density or IR exposure Decreases (lowers) contrast Descreases visibility of detail
Relationship b/w the distance between the lead strips and grid ratio when height of the grid is constant
inverse relationship
What is the grid's impact on visibility of detail
it will improve visibility of detail
As lead content increases
its ability to remove scatter and increase contrast increases
A 10" air-gap has the same degree of clean-up of scatter as a
15:1 grid for a 10 cm body part
What is the highest ratio for a grid
16:1
Parallel grid types are more or less commonly employed?
less - because of an increased grid cut-off at the edges where the beam is most divergent
Higher grid ratios allow
less scatter to pass through their interspace material to reach the IR
Parallel grids are best used at
long SIDs - when the most perpendicular portion of the beam is being used
First grid
1913 - Gustav Bucky - wide strips of lead 2 cm apart and in 2 directions.
As kVp increases
scatter increases and contrast is further impaired
grid cut-off
undesirable absorption of primary radiation - excessive loss of density
Most commonly used grids
85-103 lines/inch (33-41 lines/cm)
Most grids have contrast improvement factors b/w
1.5 and 3.5 - means that contrast is 1.5-3.5 times better when using the grid.
What is the disadvantage to aluminum as an interspace material
Absorbs some primary photons - an issue at lower kVps
Potter-Bucky diaphragm
Bucky - mounted below the tabletop and holds the cassette in place below the grid. It can move the grid so grid lines will be blurred. About 17x19" in order to cover a 14x17" cassette when placed either lengthwise or crosswise.
K factor
Contrast improvement ability
Why is lead the preferred material for the opaque strips?
High atomic # - for absorption inexpensive easy to shape into thin foil
To keep the Moire effect from happening
High-frequency grids of 103 lines per inch or greater should be used for digital IR systems when a stationary grid is needed.
Grid ratio increases if
Lead strip height is constant and distance b/w strips decreases Distance b/w lead strips is constant and grid height increases
What effect do grids have on patient dose
Pt dose is increased because mAs is usually adjusted when grids are in use
On focused grids,
SID must be set at the convergence line - grids are rated for SID ranges
What does the interspace material do?
Since it is radiolucent, it allows remnant photons to pass - but will still attenuate some
Selectivity
a criteria used to measure a grid's performance - if a grid is said to have a greater degree of selectivity, that grid absorbs a greater % of scatter vs. primary radiation. Selectivity = % primary radiation transmitted / % scatter radiation transmitted
Contrast improvement ability
a criteria used to measure a grid's performance - the best measure of how well a grid functions is its ability to improve contrast. Depends on the amt of scatter produced (controlled by kVp) and the volume of irradiated tissue. As the amt of scatter radiation increases, the lower will be the contrast and lower the contrast improvement factor. Formula: K = Radiographic contrast with the grid / radiographic contrast without the grid
The Moire effect
a grid error that occurs with digital IR systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers. Occurs with grids in stationary fashion for exams like portable radiography or translating hip images. In order for the Moire pattern to demonstrate, grid lines must be running in the same direction as the movement of the laser beam that is scanning the imaging plate.
Reciprocating grid
a motor drives the grid back and forth during the exposure for a total distance of no more than 2-3 cm
Generally, grids are
about one inch larger than the IR size.
Higher ratio grids
absorb more scatter decrease radiographic density or IR exposure increase contrast require greater positioning accuracy more prone to grid errors
Alternate scatter reduction method
air-gap technique
focused grids with lower grid ratios
allow for greater latitude in the alignment of the tube with the grid
Why is fiber preferred at lower kVp
allows more photons to pass - doesn't attenuate as much as aluminum Also lowers the patient dose
air-gap technique
an alternative to using a grid. Primary applications in magnification radiography and, to a lesser extent, in chest radiography. The technique involves placing the pt at a greater OID, creating an air gap b/w the pt and the IR. The amt of scatter reaching the IR will be reduced.
The encasing is
an aluminum cover to protect the lead strips and interspace material - to protect them from damage
Oscillating grid
an electromagnet pulls the grid to one side and then releases it during exposure. Moves in a circular motion within the grid frame
Grid cut-off results in
an excessive loss in density
Where are grids placed?
b/w the patient and the IR
A focused grid has a tube side and a receptor side
based on the angulation of the grid strips. Proper tube/grid alignment is essential to prevent undesirable absorption of primary radiation - grid cut-off
Why does mAs need to be increased when using grids
because density is decreased
A grid is employed when
body part thickness is over 10 cm kVp is above 60 - because scatter becomes enough of an issue to degrade the image
If one grid is 8:1, 103 lines per inch, and another is 12:1 ratio and 103 lines per inch,
both have the same frequency - the 12:1 means the lead strips are higher
What radiation do grids absorb
both scatter and some transmitted photons
What angulation is OK on linear grids
cephalic or caudal - parallel to grid lines in the Bucky system
long focal range grids (60-72")
chest radiography
field size can be kept to a minimum by
collimation
total quantity of lead in the grid
combo of the grid ratio and frequency - it is the grid's lead content that is most important in determining the grid's efficiency at cleaning up scatter.
Adding grids decrease
density
Focused grids
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. The strips would finally intersect at a point in space known as the convergence line.
Grid
device used to improve the contrast of the radiographic image by absorbing scatter radiation before it can reach the IR.
GCF and mAs have a
directly proportional relationship
An off-level grid error can occur with a
focused grid and it is the only positioning error possible with a parallel grid.
Which grid has a front and back
focused grids
Which grid is designed for a particular SID range
focused grids
Errors are more common with
focused grids - due to the decreased positioning latitude
Off-focus
focused grids are used at very specific distances identified as the focal range labeled on front of the grid. When a grid is used at a distance other than specified, an off-focus error results. The lead strips no longer match the divergence of the beam. Results in grid cut-off along the peripheral edges of the image.
short focal range grids (14-18")
for use in mammography
How is grid total lead content measured
grams/cm - determines its efficiency in removing scatter
Higher ratio grids require
greater accuracy in their positioning and are more prone to grid errors
Higher grid ratios require
greater positioning accuracy to prevent grid cut-off
Focused grids with low grid ratios allow
greater positioning latitude - means lead strips are further apart or are shorter - photons don't have to be perfectly aligned. Can be less accurate.
Angling across the grid strips results in
grid cut-off
Off-focus errors result in
grid cut-off along the peripheral edges of the image.
Grid lines can be seen when
grids are used in a stationary fashion - especially with low-frequency grids
criss-cross or cross-hatched grids
grids that place 2 linear grids on top of one another at right angles. No tube angulation is permitted since angulation would result in grid cut-off.
Linear grids
grids with lead strips running in only one direction. More common in clinical practice - can be used when performing procedures that require tube angulation ALONG the LONG AXIS of the grid.
Lead content is greater in a grid that
has a higher grid ratio and lower grid frequency
Grids with higher grid ratio and higher lead content
have greater selectivity
positioning latitude
how perfectly does the pt have to be so there's no grid cut-off
Direction in which the grid moves
important if it is to accomplish the job of blurring the grid lines. Must move at a right angle to the direction of the lines. Back and forth across the table and not top to bottom
1920, Hollis Potter
improved Bucky's grid design. Realigned the strips in just one direction, thinner strips, and then designed it to move during exposure.
What will an increase mAs do for the patient
increase pt dose
parallel grids
made with the lead and interspace strips running parallel to one another - they would never intersect.Less commonly employed than focused grids. Some grid cut-off will occur along the lateral edges, especially at short SIDs, because the strips don't try to coincide with the divergence of the x-ray beam. Best employed at long SIDs because beam will be straighter, more perpendicular beam.
Grid ratio
major influence on the ability of the grid to improve contrast. It is the ratio of the height of the lead strips to the distance between the strips. Grid ratio = h/d. If height is constant, decreasing the distance b/w the strips results in an increase in the grid ratio.
Moving the grid
makes the lead strips blur and are no longer visible on the image.
Lead content measure is
mass per unit area - or grams per square cm.
Focused grids with high grid ratios require
more positioning accuracy to avoid grid cut-off. Lead strips are closer together or have taller strips - photons may be cut-off. Need more positioning accuracy.
High grid ratio, high grid frequency and high lead content (grams/cm) means
more scatter absorption less density increased contrast
Mounted Bucky grids
move - reciprocating or oscillating - upon hitting exposure grid lines run parallel to the long axis of the table grid lines are blurred due to motion - moved at rt. angle to lines
Less scatter will reach the IR if the pt is
moved farther away - results in improved contrast without the use of the grid - air-gap
Grids used for flim/screen mammography studies
moving grids
Grid choice
must be made in the equipment purchase phase of quality control. Not easily changed. Generally made by the department administrator in collaboration with the radiologist.
Do grids prevent all scatter?
no
What effect do grids have on the production of scatter
no impact - they can only have an impact on scatter that is already produced
if K = 1
no improvement in contrast has occurred
Off-level grid error
occurs when the tube is angled across the long axis of the grid strips - improper tube or grid positioning. Common problem in portable procedures when detents do not ensure alignment Possible with both focused and parallel grids The ONLY error possible with parallel grids Results in loss of density/exposure across the entire image
what is the only grid positioning error that can occur with parallel grids
off-level error
what grid positioning errors are possible with focused gridgs
off-level, off-center, off-focus and upside-down
Grid types
parallel grids focused grids
Stationary grids are used for
portable exams grid lines will be evident
higher grid ratios
proper alignment of the grid with the tube is more critical
If mAs must be increased,
pt dose will increase
The efficiency of a grid in cleaning up or removing scatter can be
quantitatively measured.
Interspace material
radiolucent - aluminum and fiber. Aluminum is more common - easier to use in manufacturing and is more durable; higher atomic # than fiber, so it absorbs more low-energy scatter. Also can absorb primary photons (disadvantage with low kVp techniques). Fiber is preferred when using low kVp techniques - pediatric radiography.
What's in a grid
radiopaque lead strips side by side and separated by an interspace material which is radiolucent - usually aluminum
Grid materials
radiopaque strips of a dense material of high atomic #. Lead - inexpensive and easy to shape into very thin foil.
Exposure latitude
range of exposure factors that can be used for good diagnostic image.
Movements of grids
reciprocating oscillating
Very high-frequency grids (103-200 lines/inch)
recommended for stationary grids used with digital IR systems to minimize the possibility of seeing the grid lines on the image
ICRU Handbook 89 defines 2 criteria to measure a grid's performance
selectivity and contrast improvement ability
If a grid is used at a distance other than specified
severe grid cut-off will occur.
Upside -down
severe peripheral grid cut-off will occur. Radiation will pass thru the grid along the central axis where the grid strips are most perpendicular and radiation will be increasingly absorbed away from the center. Tech should check the tube side prior to using a focused grid. Divergence of the lead strips oppose the divergence of the beam. Drastic loss of density.
Grid cut-off is most pronounced at a short or long SID?
short
high frequency grids are recommended for
stationary grids to minimize grid lines on the image
grid uses
stationary position or mounted in a Potter-Bucky diaphragm
As the lead content of a grid increases
the ability of the grid to remove scatter and improve contrast increases
Grid radius
the distance from the face of the grid to the point of convergence of the lead strips
The higher the K factor
the greater the contrast improvement
The higher the K factor,
the greater the contrast improvement
the greater the degree of lateral decentering
the greater the grid cut-off
The better a grid is at removing scatter
the greater will be the selectivity of the grid
Grid cassette
the grid is built into the cassette - requires reloading b/w exposures using the grid. Inconvenient when multiple images are needed.
The greater the atomic number of the tissue
the less will be the scatter produced
Primary disadvantage of the air-gap techique
the loss of sharpness (recorded detail, resolution) that results from an increased OID
The patient is the source of
the majority of scatter
The higher the grid ratio,
the more mAs will need to be increased
grid frequency
the number of grid lines per inch or cm. A range in frequency of 60-200 lines/inch (25-80 lines/cm).
In order to pass through the interspace material in a grid with higher grid ratios
the scattered photon would have to be more closely aligned to the direction of the primary photon in order to reach the IR
The volume of tissue that is irradiated in controlled by
the thickness of the pt and the exposure field size
Scatter is bad because
they change direction in a random fashion and does not correspond to the patient's anatomy
interspace material is
thicker and usually made of aluminum
A grid's shape
thin, flat rectangular
Grids with higher grid frequencies have
thinner lead strips
High frequency grids have
thinner lead strips
Purpose of the grid
to improve (increase) contrast by absorbing scatter, thus decreasing radiographic density or digital IR exposure
What is the grid's main purpose
to improve contrast
What is the purpose of the interspace material
to keep the lead strips apart
Poor images can result from improper
use of the grid - occur mainly with grids that have a focused design because they are made to coincide with the divergence of the x-ray beam. Tube must be centered to the grid and aligned at the correct distance.
When converting from one grid ratio to another
use this formula: mAs1/mAs2 = GF1/GCF2 - need to use a table to determine GCF
Lead strips are
very thin foil
grid cut-off
when the primary beam is angled INTO the lead, the lead will absorb an undesirable amount of primary radiation resulting in this problem.
improper grid positioning most commonly occurs
with stationary grids in mobile procedures or decub views. eg. - a pt's weight is not evenly distributed on the grid - so the grid may not be properly aligned to the tube.
A grid with higher lead content
would have a greater selectivity
Off-center
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. Most of the perpendicular portion of the x-ray beam will not correspond to the most perpendicular portion of the grid - decrease in exposure across the entire image