Download Beam-restricting Devices & Grids

Control of Scatter
Radiation
Objectives
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Begin discussing factors that influence image
detail or visibility of detail
Spatial & Contrast resolution
Radiographic Noise
Scatter Radiation
Ways to reduce scatter & improve image quality
Primary beam restriction & Grids
Technique adjustments when using grids
What are some factors that increase
scatter radiation?
3 factors contribute to an increase in
scatter
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Increased kVp
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Increased x-ray field size
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Increased patient thickness
2 principal characteristics of any image
are Spatial & Contrast Resolution
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Spatial resolution
Resolution is the ability to image two separate
objects and visually distinguish one from the other
 Spatial resolution is the ability to image small objects
that have high subject contrast (eg. bone-soft tissue
interface, calcified lung nodules)
 Determined by focal-spot size and other factors that
contribute to blur
 Diagnostic x-ray has excellent spatial resolution. It is
measured in line pairs per mm. (CT measured in cm)
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SMPTE Test Pattern
Image-forming x-rays
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Two kinds of photons are responsible for the
OD and contrast on an image: Photons that pass
through without interacting and those that are
scattered though Compton.
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X-rays that exit the patient are remnant and
those that interact with the IR are imageforming.
Ideally, only those x-rays that do not interact
with the patient should reach the IR….
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However, scatter radiation is a factor that
must be managed
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Proper collimation has the PRIMARY effect of
reducing patient dose by _________ ?
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Proper collimation also improved image
contrast by reducing radiographic noise or
fog caused by scatter
Fog or Noise
Contrast changes with the use of a grid
Less scatter radiation & less radiographic
noise – shorter scale = “better contrast”
With Grid
No Grid
kVp
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As x-ray energy increases Photoelectric and
Compton interactions decrease. Explain?
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At 50 kVp 79% photoelectric, 21% Compton &
less than 1% transmission
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At 80 kVp 46% photoelectric, 52% Compton &
2% transmission
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Pg. 225
How does increasing kVp affect
patient dose?
Patient Thickness
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Imaging thick parts of the body results in more
scatter radiation than thin parts
IMAGE TEST TOOL
Is patient thickness something the
radiographer can control?
Patient thickness
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Normally, No
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Compression devices improves spatial resolution
by reducing patient thickness and bringing the
object closer to the IR. Compression also
reduces patient dose and contrast resolution
Compression
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Improves spatial
resolution
Reduces OID
Reduces patient dose
Improves contrast
resolution (reducing
fog or noise)
Compression
Field Size
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As field size increases, intensity of scatter
radiation also increases rapidly. Especially during
fluoroscopy
Compare images: What do you think about
radiographic contrast & image noise?
Control of Scatter Radiation
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Technologists routinely use two types of devices
to reduce the amount of scatter radiation
reaching the IR
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Beam restrictors
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Grids
3 Types of beam-restricting devices
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Aperture
Diaphragm
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Cones or Cylinders
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Variable aperture
collimator
Variable Aperture Collimator
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The most common beam-restricting device is
the light-localizing variable aperture collimator
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The first part of the collimator serves to control
off-focus radiation. What is off-focus radiation?
Off - focus Radiation
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X-ray tubes are designed so that the projectile einteracts with the target. However, some of the
e- bounce off the target and land on other areas
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This caused x-rays to be produced out side the
focal spot
Extrafocal Radiation
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These rays can also be called off-focus radiation
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Extrafocal radiation is undesirable because it
extends the size of the focal spot, increases
patient skin dose & reduces image contrast
Off-focus radiation
Fixed diaphragm in the tube
housing
Using a grid
does not reduce
extrafocal
radiation
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First-stage entrance shuttering device
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Has multiple
collimator blades
protruding from the
top of the collimator
into the tube housing
The collimator lamp & mirror
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Must be adjusted so
that the projected
light field coincides
with the x-ray beam
Misalignment of the
light field and beam
can result in
collimator cutoff of
anatomic structures
Total Filtration
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Filtration review…
Total Filtration = _______________ +
_____________?
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The collimator assembly is usually equivalent to
approximately _______ mm Al filtration.
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Minimum filtration for tubes that can operate
about 70 kVp is _______ mm Al or equiv.
The Grid
Contrast & Contrast Resolution
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Two devices are used to reduce Compton effect
beam-restricting devices and radiographic grids
Beam-restricting devices effects what reaches
the patient. Grids effect the remnant beam
Contrast & Contrast Resolution
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Contrast = the comparison of areas of light,
dark and shades of gray on the image
Contrast Resolution = the ability to image
adjacent similar tissues
Grid Ratio
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High-ratio grids are more effective in cleaning
up scatter radiation than low-ratio grids
The angle of deviation is smaller for high-ratio
grids. (the photon must be traveling in a
straighter line to make it through the grid)
However, the higher the ratio the more radiation
exposure necessary to get a sufficient number of
x-rays through the grid to the IR
Grid Ratio
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3 important dimensions on a grid: The thickness
of the grid strips, the width of the interspace
material, and the height of the grid
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The grid ratio is the HEIGHT of the grid
divided by the INTERSPACE WIDTH:
Grid ratio = h
D
h = height of the grid, T = thickness of the
grid strip, D = width of the interspace material
The 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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Grid Frequency
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The number of grid strips or grid lines per inch
or centimeter
The higher the frequency the more strips and
less interspace material and the higher the grid
ratio
As grid frequency increases, patient does is
increase because more scatter will be absorbed
Grid Frequency
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Some grids reduce the thickness of the strips to
reduce the exposure to the patient, this over all
reduces the grid clean up
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Grids have frequencies in the range of 25 to 45
lines per centimeter (60 to 110 lines per inch)
Higher frequency with the same interspace
distance reduces the grid effectiveness
Grid Performance
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The principal function of a grid is to improve
image contrast
Contrast Improvement Factor (k) = the ratio of
the contrast of a radiograph made with a grid to
the contrast of the radiograph made without a
grid. A contrast improvement factor of 1
indicates no improvements
The higher the grid ratio & frequency the higher
the k
Bucky Factor
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Using grids require more patient dose. Why is
this?
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When a grid is used technique must be increased
to maintain OD
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The amount of increase is given by the Bucky
factor (B) or grid factor
Bucky Factor or grid factor
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The higher the grid ratio or frequency the higher
the bucky factor
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The Bucky factor increases with increasing kVp
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Pg 235: We will use the average values for
calculations.
Selectivity or ability to “clean up”
the heavier the grid the more Pb it contains
Grid Types
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Parallel Grid – simplest type of grid
All the lead strips are parallel
Only clean up scatter in one direction (along the
axis of the grid)
Easy to make, however can cause grid cutoff
with short SID’s.
Grid cutoff
Distance to cutoff
SID
Grid ratio
 With decreasing
SID more potential
for grid cutoff
 IR size will also
Influence grid cutoff
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Grid Cutoff – Parallel grid
THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL
Crossed Grid
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Have lead strips running along the long and
short axes of the grid
Made by placing two parallel grid on top of each
other
Crossed Grid
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Have twice the grid ratio as linear
grids
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However, CR vs grid placement is critical. The
CR must align with the center of the grid and
the grid and CR must be exactly parallel or grid
cutoff will occur
Focused Grid
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Designed to minimize grid cutoff
Lead strips are aligned with the divergence of
the x-ray beam
Each focused grid must be identified with the
appropriate SID
Wrong SID = Grid cutoff
Focused grid have a little SID latitude (eg.
100cm grid could be used at 90cm – 110cm)
Moving Grids
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All stationary grids will give you grid lines on
your radiograph. Thinner Pb strips will give you
less noticeable lines. However, thinner strips
have less Pb content not “cleaning up” as well
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Grid Lines are made when primary x-rays are
absorbed in the grid strips.
Focused grids are usually used as
moving grids
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The grid is placed in a holding mechanism that
begins moving just before the x-ray exposure
and continues moving after the exposure ends
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2 types of movement Reciprocating &
Oscillating
Grid Motion
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Reciprocating = moves several times about 2cm
back and forth during the exposure
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Oscillating = moves several times about 2 – 3
cm in a circular pattern
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Most grids are moving. Except for portable
imaging
Grid Problems
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Increased OID, especially with moving grids
The biggest problem with grids is misalignment
GRID PROBLEMS
RESULT IN:
UNDEREXPOSED IMAGE
OR UNDEREXPOSED
EDGES OF IMAGE
Grid Problems – Off Level
Grid Problems – Off Center
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A problem with focused & crossed grids
Grid Problems – Off Focus (wrong SID)
Grid Problems – Upside-Down
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A problem with focused & crossed grids
Grid Selection
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Patient Dose
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Pg 241 – mAs changes
Exam
Detail required
Part thickness
Desired technique (kVp)
Equipment availability
Questions….?