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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

Three types of x-

rays 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).

Differential Absorption

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Video

Video

Video

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Interaction of x-

rays 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)

Substance Atomic # Density

(kg/m3)

Fat 6.3 910

Soft Tissue

Water

7.4

7.5

1000

1000

Muscle 7.6 1000

Bone 13.8 1850

Human Body Tissue

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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

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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

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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.

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.

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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

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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)

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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)

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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.

Dynamic Range

Film

Dynamic Range

Digital

IMAGING COMPARISONS

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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

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Optical Density Measurement

Densitometer

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.

Diagnostic Quality

Images: Optical Density

Low: 0.25 – 0.5

High: 2.0 – 3.0

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.

Radiographic Density

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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

Dynamic Range

Film

Dynamic Range

Digital

IMAGING COMPARISONS

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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 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.

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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

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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

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Calculations

mAs

mA Time

mAs = mA X time

mA = mAs/time

Time = mAs/mA

Calculations

Calculations

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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 kVp12

==

I2 kVp22

I1: Beginning Intensity

I2 : New Intensity

kVp1: Beginning kilovoltage

kVp2 : New kilovoltage

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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

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Density:

INFLUENCING FACTORS

• Source - Image Distance

mAs1 = D12

mAs2 D22

mAs1: Beginning mAs

mAs2 : New mAs

D1: Beginning distance

D2 : New distance

OR mAs 2 = mAs1 D22

D12

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

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Calculations

General Rules

New Distance

(inches)

mAs Change by

Formula

General Rule

mAs change

30 0.56 ½

40 1.0 1

60 2.25 2X

72 3.24 3X

80 4.0 4X

96 5.76 6X

mAs – Round to tenth location when using seconds

kVp - Utilize whole numbers

Density:

INFLUENCING FACTORS • Film/Screen Combination

mAs1 RSS2

==

mAs2 RSS1

mAs1: Beginning mAs

mAs2 : New mAs

RSS1: Beginning Film/Screen Speed

RSS2 : New Film/Screen Speed

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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

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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