Download Attenuation

Principles of Imaging Science I (RAD119)
Attenuation
Radiographic Technique
Study without reflection is a waste of time; reflection without study is dangerous.
Confucius
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).
1
Video
Video
Video
2
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
7.5
1000
1000
7.6
1000
Bone
13.8
1850
3
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
– Hardest body substance to penetrate
– Highest atomic # and density
– Decreased density (white) due absorption of x-ray photons
• Subject contrast is achieved due to differences in
photon attenuation
4
Radiograph of bony structures results from the
differential absorption between bone and soft tissue.
Radiographic Technique
• Conventional Radiography
• Digital Imaging
– Computed Radiography (CR)
– Direct Radiography (DR)
• AKA Digital Radiography
5
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.
Video
Digital Imaging
• Broad term first used medically in 1970s in
computed tomography (CT).
• Digital imaging is defined as any image
acquisition process that produces an
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.
6
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.
Video
Direct (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
7
Video
Digital / Conventional
Radiography
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.
– For Fuji, Phillips, and Konica systems, the exposure indicator is
known as the S, or sensitivity, number.
– The higher the S number with these systems, the lower the
exposure.
– For example, an S number of 400 is half the exposure of an S
number of 200, and an S number of 100 is twice the exposure of
an S number of 200. (Inverse Relationship)
8
Exposure Indicators
• For Fuji, Phillips, and Konica systems, the
exposure indicator is known as the S, or
sensitivity, number.
– The higher the S number with these systems, the lower the
exposure.
• An S number of 400 is half the exposure of an S number of 200,
and an S number of 100 is twice the exposure of an S number of
200. (Inverse Relationship)
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)
• The numbers for the Kodak system have a direct relationship to the
amount of exposure so that each change of 300 results in change in
exposure by a factor of 2.
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.
• An lgM of 2.9 equals twice the exposure of 2.6 lgM, and an lgM of 2.3
equals an exposure half that of 2.6. (Direct Relationship)
9
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.
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
diagnostic information that cannot be manufactured by
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.
IMAGING COMPARISONS
Dynamic Range
Film
Dynamic Range
Digital
10
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
Radiographic Density Evaluation
Radiographic Density Evaluation
11
Optical Density Measurement
Densitometer
Diagnostic Quality
Images: Optical Density
Low: 0.25 – 0.5
High: 2.0 – 3.0
Amount of light transmitted through a radiograph is
determined by the optical density (OD) of a film. The
step-wedge radiograph shows a representative range
of OD.
Radiographic Density
A, Overexposed radiograph of the chest is too black to be diagnostic. B,
Likewise, underexposed chest radiograph is unacceptable because there
is no detail to the lung fields.
12
Optical density is
determined principally
by the mAs value, as
shown by these
phantom radiographs
of the abdomen taken
at 70 kVp. A, 10 mAs.
B, Plus 25%, 12.5 mAs.
C, Plus 50%, 15 mAs
Changes in the mAs value have a
direct effect on OD. A, The original
image. B, The decrease in OD
when the mAs value is decreased
by half. C, The increase in OD
when the mAs value is doubled
IMAGING COMPARISONS
Dynamic Range
Film
Dynamic Range
Digital
13
CONTRAST
• The second photographic property that
determines visibility of detail
– Subject Contrast
– Film Contrast
CONTRAST
• Ensures visibility of detail
• Dependent upon adequate density
• Density difference between adjacent
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.
14
This vicious guard dog
posed to demonstrate
differences in contrast.
A, Low contrast. B,
Moderate contrast. C,
High contrast.
Images of a step wedge exposed at low kVp (A) and
high kVp (B) illustrate the meaning of short scale and
long scale of contrast, respectively.
Contrast
60 kVp vs 80 kVp
15
Contrast
Digital Imaging – Post Processing
The image on the left shows a lower contrast, or more shades of gray, due to a wide window
width. When a narrow window width is displayed, the image will have higher contrast, or fewer
shades of gray, as seen in the image on the right.
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
mAs Reciprocity
60 kVp, 3.2 mAs
80 mA, 40 ms
60 kVp, 3.2 mAs
160 mA, 20 ms
60 kVp, 6.4 mAs
16
Calculations
mAs = mA X time
mA = mAs/time
Time = mAs/mA
mAs
mA
Time
Calculations
Calculations
17
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
Density:
INFLUENCING FACTORS
• kilovoltage
I1
==
I2
I1:
Beginning Intensity
I2 :
New Intensity
kVp12
kVp22
kVp1: Beginning kilovoltage
kVp2 : New kilovoltage
18
Normal chest radiograph
taken at 70 kVp (B). If the
kilovoltage is increased
15% to 80 kVp (A),
overexposure occurs.
Similarly, at 15% less, 60
kVp (C), the radiograph is
underexposed.
Calculations
Calculations
19
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
Normal chest
radiograph taken
at 100 cm
source-to-image
receptor distance
(SID). B, If the
exposure
technique factors
are not changed,
a similar
radiograph at 90
cm SID (A) will be
overexposed and
at 180 cm SID
(C)
underexposed.
Calculations
20
Calculations
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
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
21
Calculations
Calculations
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
22
15% Rule: Calculations
• A radiograph of elbow is obtained using 56
kVp, 6.2 mAs, 40” SID. What kVp is
needed to double the density?
• 64 kVp
• A radiograph of the clavicle is obtained
using 78 kVp, 18.5 mAs, 10:1 grid, 40”
SID. What kVp will halve the exposure?
• 66 kVp
23