Chapter 8: Image Characteristics

Chapter 8

1. Dental X-Ray
Image Characteristics
Chapter 8
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2. LEARNING OBJECTIVES
LESSON 8.1: DENTAL X-RAY
IMAGE CHARACTERISTICS
1.Define the key terms associated with film image
characteristics.
2.Differentiate between radiolucent and radiopaque areas on a
dental image.
3.Describe a diagnostic dental image.
4.List the two visual characteristics of the radiographic image.
5.List the factors that influence density and contrast.
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3. LEARNING OBJECTIVES
LESSON 8.1: DENTAL X-RAY
IMAGE CHARACTERISTICS (CONT.)
6.Discuss the difference between high contrast and low
contrast.
7.Describe film contrast and subject contrast.
8.Describe the difference between short-scale contrast and
long-scale contrast.
9.Identify images of high contrast, low contrast, no contrast,
short-scale contrast, and long-scale contrast.
10.Describe a stepwedge and explain its function.
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4. LEARNING OBJECTIVES
LESSON 8.1: DENTAL X-RAY
IMAGE CHARACTERISTICS (CONT.)
11.List the three geometric characteristics of the radiographic
image.
12.List the factors that influence sharpness, magnification, and
distortion.
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5. INTRODUCTION
•Purpose
•To describe in detail the visual image
characteristics of film density and contrast
•To define the geometric image characteristics of
sharpness, magnification, and distortion
•To discuss how influencing factors alter these
image characteristics
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6. DENTAL X-RAY
IMAGE CHARACTERISTICS
•A dental radiograph appears as a black and white
image or picture with varying shades of gray.
•Radiolucent
•The portion of the processed radiograph that is dark or
black
•A structure that appears black on film lacks density.
•Radiopaque
•The portion of the processed radiograph that appears
white
•A structure that appears white on film is dense and
absorbs or resists passage of the x-ray beam.
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7. RADIOLUCENT AND RADIOPAQUE
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8. DENTAL X-RAY
IMAGE CHARACTERISTICS
(CONT.)
•In a diagnostic radiograph the images:
•Have proper density and contrast
•Have sharp outlines
•Are of the same shape and size as the object
radiographed
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9. VISUAL CHARACTERISTICS
•Density
•Contrast
•Density and contrast are the two visual
characteristics that influence the quality of a dental
radiograph.
•It is important to know how to alter radiographic
variables to change the contrast, density, and image
detail
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10. DENSITY
•The overall darkness or blackness of a dental
radiograph
•The relative transparency depends on distribution of black silver
particles in the emulsion (part of the film)
•Influencing factors
•Kilovoltage
•Milliamperage
•Exposure time
•Subject thickness
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11. VISUAL CHARACTERISTICS
AND INFLUENCING
FACTORS
Visual Characteristics Influencing Factors Effect of Influencing Factors
Density mA mA = density
mA = density
kV kV = density
kV = density
Time Time = density
Time = density
Subject Thickness Thickness = density
Thickness = density
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12. CONTRAST
•Contrast is the difference in degrees of
blackness between adjacent areas.
•“A film that is a compromise between low contrast and
high contrast is preferred.”
•The overall contrast of a dental image is determined by
the film contrast and the subject contrast.
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13. CONTRAST (CONT.)
•Influencing factors
•Increasing the kilovoltage affects image contrast by
increasing the mean or average energy of the x-rays
and by producing higher energy x-rays.
•More areas of varying tissue density are recorded on
the image and appear as shades of gray.
•Remember, kilovoltage controls contrast
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14. A.When kV is low, result is low density and high contrast
B.Optimal image
C.kV is high, the result is high density and low contrast

15. VISUAL CHARACTERISTICS
AND INFLUENCING
FACTORS
Visual
Characteristics
Influencing
Factors
Effect of Influencing Factors
Contrast kV kV = long-scale contrast, low contrast
kV = short-scale contrast; high contrast
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16. CONTRAST (CONT.)
•High contrast
•A dental image that has very dark areas and very light
areas
•Low contrast
•A dental image that does not have very dark and very
light areas but instead has many shades of gray
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17. CONTRAST (CONT.)
•The overall contrast is determined by:
•Film contrast
•The inherent qualities of the film and film
processing
•Subject contrast
•The characteristics of the subject that influence
radiographic contrast
•Kilovoltage controls contrast
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18. CONTRAST (CONT.)
•Scales of contrast
•The range of useful densities
•Short-scale contrast (high contrast)
•A radiograph with only two densities, black and white
•Occurs with machines functioning at low kVp
•Long-scale contrast (low contrast)
•A radiograph with many densities, many shades of gray
•Occurs with machines functioning at high kVp
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19. THE EFFECT OF
KILOVOLTAGE ON
CONTRAST
KilovoltageContrast Scale of
Contrast
Example
High Low Long-scale A
Low High Short-scale B
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20. CONTRAST (CONT.)
•Identify images of:
•High contrast (an image that has very dark areas and
very light areas)
•Low contrast (an image that has many gray areas)
•No contrast
•Short-scale contrast (an image that shows only two
densities)
•Long-scale contrast (an image that exhibits many shades
of gray)
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21. CONTRAST (CONT.)
•An aluminum stepwedge can demonstrate short-
scale and long-scale contrast.
•The stepwedge consists of uniform layered thicknesses of an x-
ray absorbing material.
•The stepwedge is used to demonstrate corresponding film
densities and contrast scales.
•Different densities appear on the dental radiograph.
•A typical stepwedge is constructed of aluminum steps in 2-
mm increments.
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22. STEP WEDGE
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23. GEOMETRIC
CHARACTERISTICS
•Sharpness
•A lack of sharpness can change a diagnosis dramatically. It may
cause a problem to be undetected at an early stage when the
prognosis would have been more favorable.
•Magnification
•Magnification is especially important when measuring the length
of a tooth during root canal therapy.
•Distortion
•occurs when a radiograph is taken at a vertical angle that is
greater than or less than necessary, resulting in a film image that
appears either stretched out or shorter than it really is.
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24. SHARPNESS
•Sharpness is the capability of the x-ray
receptor to reproduce the distinct outlines
of an object
•Also knows as detail, resolution, and definition
•A certain lack of image sharpness is
present in every radiograph; it is called the
penumbra.
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25. TABLE 8-3 PAGE 77
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26. SHARPNESS (CONT.)
•Influenced by three factors
•Focal spot size
•The smaller the focal spot, the sharper the image
appears.
•Film composition
•The emulsion of faster film contains larger crystals
that produce less image sharpness.
•Movement
•Movement causes a loss of image sharpness.
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27. THE SMALLER THE FOCAL
SPOT THE SHAPER THE
IMAGE
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28. THEORETICAL ‘POINT SOURCE’ SHAPE
IMAGE WITHOUT PENUMBRA
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29. THE INFLUENCE OF
MOTION IN IMAGE
SHARPNESS
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30. IMAGE OF A PATIENT WHO
MOVED DURING X -RAY
EXPOSURE
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31. MAGNIFICATION
•Results from the divergent paths of the x-
ray beam as they radiate from the focal spot
•Influencing factors
•Target-receptor distance
•A longer PID and target-receptor distance results in
less image magnification.
•Object-receptor distance
•An increase in object-receptor distance results in an
increase in image magnification.
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32. MAGNIFICATION AS A RESULT OF THE
DIVERGENT PATHS OF THE
X-RAY BEAM
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33. LONGER PID (16 INCHES) AND
TARGET-RECEPTOR DISTANCE
RESULTS IN LESS IMAGE
MAGNIFICATION
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34. OBJECT-RECEPTOR DISTANCE; THE
CLOSER THE TOOTH IS ON THE
RECEPTOR, THE LESS
MAGNIFICATION IS SEEN OF THE
IMAGE
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35. DISTORTION
•A variation in the true size and shape of the object
being radiographed
•Results from unequal magnification of different parts of the same
object because of improper receptor alignment or angulation of the x-
ray beam; can be too long or too short
•Influencing factors
•Object-film alignment
•The object and film must be parallel to each other or there will be
distortion.
•X-ray beam angulation
•The x-ray beam must be directed perpendicular to the tooth and
film.
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36. FORESHORTENING
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37. ELONGATION
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