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).
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Video
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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 7.5
1000 1000
7.6
1000
Bone
13.8
1850
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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.
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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.
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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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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.
IMAGING COMPARISONS
Dynamic Range Film
Dynamic Range Digital
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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
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.
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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
IMAGING COMPARISONS
Dynamic Range Film
Dynamic Range Digital
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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 X time mA = mAs/time Time = mAs/mA mAs
mA
Time
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
==
I2 I1:
Beginning Intensity
I2 :
New Intensity
kVp12 kVp22
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 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
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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
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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
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