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LECTURE 1 The Radiographic Image
Prepared by:- KAMARUL AMIN ABDULLAH @ ABU BAKAR
UiTM
Faculty of Health Sciences Medical Imaging Department
11/23/2011 KAMARUL AMIN (C) 1
Lesson Objectives
At the end of the lesson, the student should able to explain:-
• The definition of density, contrast, unsharpness, and distortion.
• The factors affecting them.
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DENSITY
• Represents the degree of darkening of an exposed x-ray film (white to black).
• The overall density of the film affects the diagnostic value of the film (ideal density, too light, too dark).
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Cont’d..
• Black areas on a developed radiograph are produced by deposits of black metallic silver in the film emulsion that result from exposure to x-rays and their subsequent processing.
• Degree of blackness on a radiograph depends on the amount of x-rays reaching the film.
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Cont’d..
• Also known as the logarithm of opacity or optical density (OD).
• OD: the ratio of the amount of light incident on the film to the amount of light transmitted through the film. 1. Light incident – the light striking the radiograph from the back
(viewing box). 2. Light transmitted – the light which is seen coming through the
radiograph while being viewed either by human eye or densitometer.
OD = LOG10 light incident
light transmitted
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Factors Affecting Density
Exposure Factors Patient Size
Distance Object Density Film Processing
Film/Screen Combination Collimation
Grids Anode Heel Effect
Filtration
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Cont’d..
• Exposure Factors (mAs, kVp).
• An unnecessary increase in any of these factors results in an increase in film density.
A. mAs
– The chief controlling factor of exposure and density.
– Directly control the total number or amount of x-ray photons reaching the films.
– Increasing mA or time increases the density and Decreasing mA or time will decrease the density.
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Cont’d..
B. kVp
– kVp affects density differently than mAs. In order for there to be a significant increase in density a 15% change in kVp must be made.
– Directly control the penetration or quality of x-ray photons.
– There is a peak or optimal kVp for each body part.
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Cont’d..
• Patient Size: the larger the patient’s body, the more x-rays that are needed to produce an ideal film density.
i.e.: Increase in thickness, means that the patient can absorb more x-rays which will results in a lighter image.
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Cont’d..
Distance • Distance is inversely related to density.
• A decrease of distance of the source of x-rays to film increases the density and vice versa.
• Known as the Inverse Square Law.
• e.g.:If distance is doubled, density decreases four times.
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Cont’d..
• Object Density: determined by type of material (metals, structure (e.g. bone, tissue), composite, etc.) and by amount of material.
• Types of tissue affects density. For example, higher density tissues will cause less x-rays to reach film, therefore have lighter areas on the film at that place.
• Density (darkness) decreases when object density increases.
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RADIOLUCENT
Dark Gray to Black
Low object density results in high film density OR
allows x-ray to easily penetrate (e.g. air, soft tissue)
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RADIOPAQUE
White to Light Gray
High object density results in low film density OR
x-rays do not easily penetrate (e.g. bone, metals)
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Activities..
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Instruction: Please indicate which parts are the radiolucent and radiopaque for each images.
Cont’d..
Film Processing: due to light, mishandling and automatic film processor.
The developing time – increase time will increase density
The temperature of the developer – increase temperature will increase density
Mishandling and light can occur due to wet hands, open cassettes in light areas, high beam safelight, leakage, etc.
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Cont’d..
Film/Screen Combination • Screens were used to enhance the radiation so
that fewer x-ray photons (less dose) / more light photons are used to create a radiographic image with optimum density.
• The color of the glow of the intensifying screen must match the color sensitivity of the film (spectral matching).
• Good film/screen combination = GOOD Density
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Cont’d..
Collimation • Limiting the size or increase collimation will
decrease density.
• i.e. it reduces the scattered radiation that will adds density to the film.
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Cont’d..
Grids • Grids also absorb scattered radiation before
reaching the film.
• It will reduce the density.
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Cont’d..
Anode Heel Effect
• Density is greater near cathode end.
• Density is lesser near the anode end.
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Cont’d..
Filtration • Improve quality of beam by absorbing low
photon energies.
• Good quality of beam has high penetration and improve the density.
• More penetration = more interaction with matter = more x-ray photons interact with film.
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CONTRAST
• Defined as visible difference between two adjacent radiographic densities.
• Primary function is to visible image details.
• Is divided into
Radiographic Contrast
Subject Contrast
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Radiographic Contrast
• Density between two adjacent areas on a radiograph.
Many black and white areas means you have contrast.
If a radiograph has many grays and a small density difference between 2 adjacent areas, then it has low contrast.
Need right amount of contrast. (Include grays, blacks and whites so eye can easily see details).
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Subject Contrast
• Defined as the difference in density and mass between two adjacent anatomic structures.
• Depends on:
Thickness of the anatomic part.
Density of the anatomic part.
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Factors Affecting Contrast
• Controlling Factor: kVp Level
• Grids
• Beam Restriction
• Filtration
• Subject Contrast
• Film Processing
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Cont’d..
kVp LEVEL • The controlling factor for contrast.
• kVp affects energy (penetrating ability) of x-rays.
• The penetrability will control the differential absorption of x-ray.
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Cont’d..
• As HIGH kVp = more penetration = more scatter = low contrast = long scale
• As LOW kVp = less penetration = less scatter = high contrast = short scale
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Cont’d..
Grids • Reduce the amount of scattered radiation
reaching the film.
• And this will improve the contrast as less density has been added to the film.
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Cont’d..
• Increase Scattered Radiation = Low Contrast = HIGH kVp = High Penetrability
• Decrease Scattered Radiation = High Contrast = LOW kVp = Low Penetrability
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Cont’d..
Beam Restriction • It limits the area being irradiated.
• Close or Increase collimation will reduce scattered by reducing Compton’s Interaction.
• Less scattered reduces the number shades of gray, thereby increasing contrast.
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Cont’d..
Filtration • As filtration is increased, the penetrability of
the beam will also increase.
• The increase of high average penetration/energies will produce low contrast as many gray tones will produce.
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Cont’d..
Subject Contrast • Results from varying object densities within
patient.
• For example, BONE will have more white on the film than SOFT TISSUE.
Bone HIGH contrast low kVp
Soft tissue LOW contrast high kVp
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Cont’d..
Film Processing • Inadequate processing degrades the
radiographic contrast.
• Optimum temperature, time, replenishment chemicals, water flow and etc. will produce GOOD contrast.
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RECORDED DETAILS
• The distinct representation of an object’s true borders or edges.
• It is often called sharpness of detail, definition or resolution.
• Terms used to describe image sharpness, clarity, distinctness, and perceptibility.
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Factors Affecting Details
Motion
Focal Spot Size
Source-Image-Distance (SID)
Object-Image-Distance (OID)
Film/Screen Combination
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Cont’d..
Motion • The most common cause.
Voluntary motion • Motion caused by the movement of the patient and
equipment.
• Best controlled by good patient instructions
Involuntary motion • Motion caused by uncontrolled motion of the body
such as the heart beat or peristalsis
• Best controlled by short exposure times
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Cont’d..
Focal Spot Size A small focal spot is used when fine detail is needed.
A large focal spot can cause unsharpness of recorded details. However, using of large focal spot size can lengthen the x-ray tube life.
It is because the x-ray produced by larger focal spot will cause bigger divergence of the beam.
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Cont’d..
Source-Image-Distance (SID) • As SID increases detail increase.
• Longer SID can reduce penumbra effect.
• Penumbra
A fuzzy border of an object that is obscure.
• Umbra
The true border.
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Cont’d..
Object Image Distance (OID) • Distance from anatomical part to the film.
• Decrease the OID will increase details.
• Increase the OID will decrease the details.
• Increase of OID cause the magnification to occur.
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Target 16”
Target 16”
Increase OID, Increase magnification, Decrease Details.
Decrease OID, Decrease magnification, Increase Details
OID
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Cont’d..
Film-Screen Combination • Slow Speed = Increase Details
• High Speed = Decrease Details
• Speed primarily affected by Phosphors Size of Intensifying Screen and Crystals Size of Film.
Larger Size = Poor Details
Smaller Size = Greater Details
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DISTORTION
• Any misrepresentation of anatomic structure on an image that has changing of its size or shape.
• There are TWO (2) types of distortion :
A) Size
B) Shape
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Factors Affecting Distortion
A) SIZE
• Magnification
• Caused by excessive OID
• Caused by insufficient SID
• Causes anatomic structures to appear larger from an original size.
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Cont’d..
B) SHAPE
Caused by:-
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1. Film-Object relationship
- Improper structure with the film.
2. Beam alignment
- Improper angulations of tube with the structure/part.
Cont’d..
• It has TWO (2) types:-
1. Elongation
Causes anatomic structure to appear longer than original size.
2. Fore shortening
Causes anatomic structure to appear shorter than original size.
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