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Grids. RTEC 244. Dr. Gustave Bucky. Since Dr. Gustav Bucky built the first grid in 1913, his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known techniques to trap the scatter. - PowerPoint PPT Presentation
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1
Grids
RTEC 244
2 Dr. Gustave Bucky
Since Dr. Gustav Bucky built the first grid in 1913,
his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known techniques to trap the scatter
CROSSHATCH GRID
Potter-Bucky Diaphragm
Dr. Hollis Potter made improvements to the use of grids Realigned lead strips
to run in one direction Moved grid during
exposure to make lines invisible on image
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Creating the Image Transmission
Responsible for dark areas Absorption
Responsible for light areas Scatter
Creates fog Lowers contrast
Increases as kV increases Field size increases Thickness of part
increases Z# decreases
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Grid Selection Patient Dose Exam Detail required Part thickness Desired technique
(kVp) Equipment availability
Indications for Grid Use
• Part thickness > 10 cm• kVp > 60• EXCEPTIONS?
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Grid is placedbetween patient (behind table or upright bucky) & cassetteIf placed BACWARDS CAN CAUSE GRID ERRORS
6What are some factors that increase scatter radiation?
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8Grids “clean up” scatter radiation
A high quality grid can attenuate 80 –90 % of scatter radiation
3 factors contribute to an increase in scatter
Increased kVp Increased x-ray field size Increased patient
thickness
9Ideally, only those x-rays that do not interact with the patient should reach the IR….
However, scatter radiation is a factor that must be managed
Proper collimation has the PRIMARY effect of reducing patient dose by _________ ?
Proper collimation also improved image contrast by reducing radiographic noise or fog caused by scatter
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How does increasing kVp affect patient dose?
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Basic Grid Construction• Radiopaque lead strips• Separated by
radiolucent interspace material– Typically aluminum
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Grids Allow primary
radiation to reach the image receptor (IR)
Absorb most scattered radiation
Primary disadvantage of grid use Grid lines on film
13CASSETTES W/ GRID CAPSSTATIONARY GRIDS
Stationary gridsGrids that can be attached to a cassette for use or a specially designedGrid cassettes
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Grid Dimensions• h = the height of the
radiopaque strips• D = the distance
between the strips – the thickness of the
interspace material
Height of lead strips divided by thickness of interspacing material
Grid ratio = h/D
GRID RATI0 The distance between lead strips may remain
constant so the thickness of the grid must increase as grid ratios increase. It is possible to appreciate
the smaller angle of deflection of the x-ray photon that will pass through the 16:1 ratio grid.
Thus, high ratio grids usually "clean-up the beam," removing scatter radiation more effectively than low ratio grids.
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Grid Ratio Higher grid ratio
More efficient in removing scatter
Typical grid ratio range is 5:1 to 16:1
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Grid Ratio Example If a grid has an
interspace of 0.5mm and lead strips that are 3mm high, what is it's grid ratio?
GR = 3mm/0.5mm
GR = 6:1
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Grid Frequency The number of lead
strips per inch or cm Frequency range
60-200 lines/in 25-80 lines/cm
Typically higher frequency grids have thinner lead strips
Higher frequency with the same interspace distance reduces the grid effectiveness
18The higher the ratio the straighter the photon must travel to reach the IR Grid ratios range from 5:1
to 16:1 Most common 8:1 to 10:1 A 5:1 grid will clean up
85% 16:1 clean up 97%
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Digital Imaging Systems Very high-frequency grids
103-200 lines/in 41-80 lines/cm
Recommended for use with digital systems Minimizes grid line appearance
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Air-Gap Technique Or Air filtration Increase OID by 10 to 15 cm This reduces the amount of scatter reaching the
IR because some scatter will miss the IR. It is about the same as using an 8:1 grid mAs is increased 10% for every cm of air gap Increases magnification and reduction in detail.
Has some selective uses with chest imaging and cerebral angiography
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Air-Gap Technique
An alternative to grid use 10” air gap has similar
clean-up of 15:1 grid
Problems: Increased OID = increase in blur Must increase SID Motion due to lack of
contact to IR
AIR GAP. Drawing illustrates the tube, object, grid
and film relationship in conventional radiography and the use of an air gap to decrease the effect of scatter radiation. Note that an increase in the FFD tends to decrease the magnification of the image on the film.
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Grid Patterns Criss-cross or cross-hatched Linear
Parallel Focused
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Linear Grid Lead strips run the length of cassette Allows primary beam to be angled along the
long axis of grid without obtaining “cut-off”
24Focused Linear Grids Lead strips are angled
to match divergence of beam Primary beam will align
with interspace material Scatter absorbed by lead
strips
Convergence line Narrow positioning latitude
Improper centering results in peripheral cut-off
Only useful at preset SID distance
Higher ratio grids require careful alignment with tube
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Parallel Linear Grids All lead strips are
parallel to one another Absorb a large amount
of primary beam Resulting in some cut-
off
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Grid Use & Movement Potter-Bucky diaphragm
The Bucky Mounts a 17” x 19” grid above cassette Moves the grid during exposure
Reciprocating Motor drives grid back and forth during
exposure Oscillating
Electromagnet pulls grid to one side Releases it during exposure
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28Grids and Exposure Factors Whenever a grid is placed in beam to
remove scatter Density of radiograph will go down Exposure factors must be increased to
compensate for lack of density
Required increase in technique can be calculated Grid conversion (GCF) or Bucky factor
GCF = mAs with grid mAs without grid
29GRID CONVERSION FACTORSNO GRID 15:1 26:1 38:1 410/12:1 516:1 6
CHANGES IN MAS
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Give It a Try! Original: 20mAs with an 8:1 grid Find new mAs with a 12:1 grid
mAs2 = 20 mAs x 5 4 mAs2 = 100 4 mAs2 = 25
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Selectivity or ability to “clean up”the heavier the grid the more Pb it contains
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Selectivity “K” factor Describes grid’s ability to allow primary
radiation to reach image receptor and prevent scatter
Grids are designed to absorb scatter Sometimes they do absorb primary radiation
Compares radiographic contrast of an image with a grid to radiographic contrast of an image without a grid
Typically ranges between 1.5 – 3.5
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Grid Errors
Off-level Off-center Off-focus Upside-down Moire effect
Proper alignment between x-ray tube and grid Very important
Improper alignment will result in cut-off
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Grid Problems Increased OID, especially with moving
grids The biggest problem with grids is
misalignment
GRID PROBLEMS RESULT IN:UNDEREXPOSED IMAGEOR UNDEREXPOSEDEDGES OF IMAGE
36Grid Problems – Off Level
CUT OFF MORE SEVERE ON ONE SIDE THAN THE
OTHER
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Grid Problems – Off Center
A problem with focused & crossed grids
38Grid Problems – Off Focus (wrong SID)
CUT OFF EQUAL ON BOTH SIDES
39Grid Problems – Upside-Down
A problem with focused grids = severe cut off on both edges
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STATIONARY GRID/NOT MOVINGGRID CUT OFF - EVENLY ACROSS
41
GRIDS CAN
LEAVE LINES
ON THE IMAGE
42
CR GRIDS
43
REG GRID VS DR GRIDWhat is this called?
What causes this?
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Moire Effect Digital systems
When grid lines are parallel to scan lines
High frequency grids can prevent this phenomenon
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Moire Effect
PMT
Beam deflector
LaserSource
Light channeling guide
Plate translation: Sub-scan direction
Laser beam: Scan direction
Output Signal
Reference detec tor
Beam splitter
Cylindrical m irrorf-thetalens
Amplifier
ADC
To imageprocessor
When grid lines are parallel to scan lines
46DR USES HIGER FREQUENCY GRIDS
47