The Radiographic Image

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Chapter 14 The Radiographic Image
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Image Clarity (?????)
Image clarity is used to describe the visibility
of diagnostically important detail in the
radiography.
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Image Clarity (?????)
Image Clarity
Image Quality
Radiographic Contrast
Subject Contrast
Film Contrast
Radiographic Mottle
Resolution
Fog and Scatter
Sharpness
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Subject Contrast
Subject contrast depends on 1. thickness
difference 2. density difference 3. atomic
number difference 4. radiation quality (kVp)
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Subject Contrast Thickness Difference
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Subject Contrast Density Difference
The greater the density of a tissue, the greater
is its ability to attenuate x rays. For example,
water is about 9 more dense than ice (because
ice floats in water). For equal thickness
of water and ice, it will demonstrate subject
contrast to X-ray beam.
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Subject Contrast Atomic Number Difference
Effective atomic number
Tissues
Bone
13.8
Muscle
7.4
Fat
5.9
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Subject Contrast Radiation Quality
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Fog and Scatter
Scatter radiation that reaches the x-ray film
or film-screen combination produces un-wanted
density, as a result, it reduces radiographic
contrast.
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Image Quality
Image Quality
Radiographic Mottle
Resolution
Sharpness
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Image Quality - Radiographic Mottle
Radiographic Mottle
Screen Mottle
Film Graininess
Quantum Mottle
Structure Mottle
size of silver halide crystal
varying thickness or physical imperfections in
the phosphor layer of an intensifying screen.
statistic fluctuation in number of x-ray
photons per unit area absorbed by the
intensifying screen.
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Image Quality - Sharpness
Sharpness is the ability of the x-ray film or
film-screen system to define an edge. Line
spread function provides a way to measure the
sharpness.
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Image Quality - Resolution
Resolving power is the ability to record separate
images of small objects that are placed very
close. Unit for the resolving power is line
pairs per mm.
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Measurement of Image Quality Modulation
Transfer Function (MTF)
The modulation transfer function (MTF) is a way
to measure the combined effects of sharpness and
resolution.
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Measurement of Image Quality MTF (cont.)
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Measurement of Image Quality MTF (cont.)
Fourier Transform
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Measurement of Image Quality MTF (cont.)
The 10 response on the MTF curve corresponds
roughly to the resolving power of the imaging
system.
For this imaging system, the resolving power is
about 7 lines per mm.