Download Attenuation

Principles of Imaging Science I (RAD119)
Attenuation
Radiographic Technique
Attenuation
• When x-ray photons interact with matter, the
quantity is reduced from the original x-ray beam
• Attenuation is the result of interactions between
x-ray and matter that include absorption and
scatter
• Photoelectric absorption
• Compton scattering
• Coherent scattering
• Differential absorption increases as kVp
decreases
Differential Absorption
Three types of xrays are important
to the making of a
radiograph: those
scattered by
Compton
)
interaction ((A);
those absorbed
photoelectrically
(B); and those
transmitted through
the patient without
interaction (C).
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Interaction of xrays by absorption
and scatter is
called attenuation.
In this example,
the x-ray beam
has been
attenuated 97%;
3% of the x-rays
have been
transmitted.
Attenuation
• Contingent upon the thickness of the body part,
the atomic number, and density
• Thicker body parts attenuate more x-ray
photons than the same body part that is
thinner
• Higher atomic number structures absorb more
x-ray photons than lower atomic number
structures
• Due to higher # of electrons
• Denser structures absorb more x-ray photons
as compared with less dense structures
(kg/m3)
Human Body Tissue
Substance
Atomic #
Density
(kg/m3)
Fat
6.3
910
Soft Tissue
Water
Muscle
7.4
7
4
7.5
1000
1000
7.6
1000
Bone
13.8
1850
2
Contrast Material
Contrast Agent
Atomic #
Density (kg/m3)
Air
7.6
1.3
Iodine
53
4930
Barium
56
3500
Radiographic Demonstration
• Air
– Easily penetrated
– Increased density (dark)
• Fat
– Harder to penetrate than air
– Lower atomic # and density than muscle
– Easier to penetrate than muscle
– Decreased density (grey)
Radiographic Demonstration
• Muscle
– Harder to penetrate than fat
– Higher atomic # and density compared to fat
– Decreased density (grey)
• Bone
– H
Hardest
d tb
body
d substance
b t
tto penetrate
t t
– Highest atomic # and density
– Decreased density (white) due absorption of x-ray photons
• Subject contrast is achieved due to differences in
photon attenuation
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Conventional Radiography
•
•
•
•
Method is film-based.
Method uses intensifying screens.
Film is placed between two screens.
Screens emit light when x-rays strike
them.
• Film is processed chemically.
• Processed film is viewed on lightbox.
Digital Imaging
• Broad term first used medically in 1970s in
computed tomography (CT).
• Digital imaging is defined as any image
q
p
process that p
produces an
acquisition
electronic image that can be viewed and
manipulated on a computer.
• In radiology, images can be sent via
computer networks to a variety of
locations.
Computed Radiography
•
•
•
•
•
Uses storage phosphor plates
Uses existing equipment
Requires special cassettes
Requires a special cassette reader
Uses a computer workstation and
viewing station and a printer
• Method was slow to be accepted by
radiologists.
• Installation increased in the early
1990s.
• More and more hospitals are replacing
film/screen technology with digital
systems.
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Digital Radiography
• Cassetteless system
• Uses a flat panel detector
or charge-coupled device
(CCD) hard-wired to
computer
• Requires new installation
of room or retrofit
Digital / Conventional
Radiography
RADIOGRAPHIC DENSITY
Conventional Radiography
• One of the photographic properties that
determines visibility of detail
• Overall blackness or darkness of the entire
radiographic image or a specific area
• When evaluating an image for proper
radiographic density, the density of the
entire image is considered
• Optical density vs Radiographic density
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Optical Density Measurement
Densitometer
• Formula: OD = log10 (Io/It)
– OD = Optical density
– Io = Intensity of the original (incident) light
– It = Intensity of the transmitted light
Exposure Indicators
Digital Imaging
• The amount of light given off by the imaging plate is a
result of the radiation exposure that the plate has
received.
• The light is converted into a signal that is used to
calculate the exposure indicator number, which is a
different number from one vendor to another.
• The base exposure indicator number for all systems
designates the middle of the detector operating range.
Exposure Indicators
• For Fuji, Phillips, and Konica systems, the
exposure indicator is known as the S, or
sensitivity, number.
– The higher
g
the S number with these systems,
y
, the lower the
exposure.
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Exposure Indicators
• Kodak uses exposure index, or EI, as the
exposure indicator.
– An EI number plus 300 (EI + 300) is equal to a doubling of
exposure, and an EI number of minus 300 (EI − 300) is equal to
a halving of exposure. (Direct relationship)
Exposure Indicators
• The term for exposure indicator in an Agfa
system is the lgM, or logarithm of the median
exposure.
– Each step of 0.3 above or below 2.6 equals an exposure
factor of 2.
Exposure Indicators
Summary
• S, EI, and lgM are terms used by
manufacturers to indicate the amount of
exposure.
• The exposure range numbers represent
the maximum to minimum diagnostic
exposures.
• The middle value in that range represents
the S, EI, or lgM number.
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CONTRAST
• The second photographic property that
determines visibility of detail
– Subject Contrast
– Film Contrast
CONTRAST
• Ensures visibility of detail
• Dependent upon adequate density
• Density
y difference between adjacent
j
structures
• Changes in density affect image contrast
CONTRAST
• HIGH CONTRAST
–
–
–
–
Low kVp
Black & White
Short scale contrast
Used for skeletal
anatomy
• LOW CONTRAST
–
–
–
–
High kVp
Shades of gray
Long scale contrast
Used for Chest, KUB,
or as warranted by
M.D.
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Digital Image Receptor Systems
• There is no substitute for proper kilovoltage peak and
milliampere-second settings. Images cannot be created
from nothing; that is, insufficient photons, insufficient
penetration, or overpenetration will result in loss of
g
information that cannot be manufactured by
y
diagnostic
manipulation of the image parameters.
• Exposure latitude is slightly greater with digital imaging
than that of film/screen imaging because of the wide
range of exposures recorded with digital systems.
DENSITY
• CONTROLLING FACTOR: mAs
•
•
•
•
mAs = mA X time (sec)
mA = mAs/time
time = mAs/mA
Reciprocity Law
– The same radiographic film density will result from
different mA and time selections, provided that the
mAs totals are equal
Calculations
mAs
mA
Time
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Calculations
Density Influencing Factor
• kVp
– Affects the penetrability of x-ray photons
through the patient
– Affects the quality of the x-ray beam based
upon the emission spectrum
– Whole number increments (Major/Minor)
Density Influencing Factor
• SID
– Based upon Inverse
Square Law
• Film/Screen
Combination (RSS)
– Slow, Medium, High
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Density:
INFLUENCING FACTORS
• kilovoltage
I1
==
I2
I1:
Beginning Intensity
I2 :
New Intensity
kVp12
kVp22
kVp1: Beginning kilovoltage
kVp2 : New kilovoltage
Density:
kilovoltage calculations
Density: kilovoltage calculations
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Density:
INFLUENCING FACTORS
• Source - Image Distance
mAs1
mAs2
mAs1:
=
D12
D22
OR mAs 2 = mAs1 D22
D12
Beginning mAs
mAs2 :
New mAs
D1:
Beginning distance
D2 :
New distance
Density: Distance Calculations
Density: Distance calculations
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General Rules
New Distance
(inches)
mAs Change by General Rule
Formula
mAs change
30
40
60
72
80
96
0.56
1.0
2 25
2.25
3.24
4.0
5.76
½
1
2X
3X
4X
6X
mAs – Round to tenth location when using seconds
kVp - Utilize whole numbers
Density:
INFLUENCING FACTORS
• Film/Screen Combination
mAs1
mAs2
==
RSS2
RSS1
mAs1:
Beginning mAs
mAs2 :
New mAs
RSS1:
Beginning Film/Screen Speed
RSS2 :
New Film/Screen Speed
Density: RSS calculations
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Contrast
• 15% Rule
– A 15% increase in kilovoltage will double the
exposure. This is comparable to doubling the
mAs, exposure time, or mA.
– A 15% decrease in kilovoltage will halve the
exposure. This is comparable to halving the
mAs, exposure time, or mA.
– Kilovoltage should not be the primary factor
used to change density
– 2nd Semester: Applied to maintain density
while altering contrast
15% Rule: Calculations
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