Download CHARACTERISTICS of MEDICAL IMAGES

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Our Goal
Obtain the best possible
image
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Best Possible Image?
What is our goal?
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A Diagnostic
Image Using the
Least Radiation
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A prettier picture does not provide benefit to
our patient
Prettier pictures often involve additional
radiation
Contrast
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difference in density between areas on
radiograph
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Your ability to see contrast depends on
Subject contrast
 Contrast sensitivity of receptor
 Scatter radiation
 Monitor / Viewing conditions
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Subject Contrast
difference in x-ray
intensity transmitted
through various parts of
subject
Depends on
thickness difference
 density difference
 atomic number difference
 radiation beam energy
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Subject Contrast & Radiation Quality
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high energy beam = lower
subject contrast
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low energy beam = high
subject contrast
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long scale contrast (less difference between
areas receiving varying amounts of radiation)
short scale contrast (more black & white;
more difference between areas receiving
varying amounts of radiation)
low energy beam increases
patient dose
Photon Phates
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Nothing
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Absorbed
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Photon passes unmolested
completely removed from beam
ceases to exist
Scattered
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change in direction
no useful information carried
source of noise
X
Primary vs Scatter
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Primary = Good photon
Scatter = Bad photon
Focal
Spot
“Good”
photon
Patient
X
Receptor
“Bad”
photon
Image Noise
Caution!
Image
Noise
Noise
– covers valid information with distracting or
obscuring garbage
Image Noise
– covers valid
information with
distracting or
obscuring garbage
Caution!
Image
Noise
Another Example
Painting Image One Dot at a
Time
Noise makes it hard to see low
contrast differences