Achieving Radiographic Quality: Image Formation

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Achieving Radiographic QualityImage Formation
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IMAGE FORMATION

• Radiographic usefulness limited by quality of
  image on recording surface
• Technicians must understand
• how to produce diagnostic radiographs
• proper radiographic terminology
• factors affecting radiographic appearance

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Terminology for Radiographic Appearance

• Subject (Patient)
• Density
• Thickness
• Contrast
• Radiographic (Film)
• Density
• Contrast
• Detail

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Describing Radiographic Appearance

• Subject (Patient) Density
• Ability of different bodily tissues to absorb
  x-rays
• Two factors determine subject density
• Average Atomic Number of tissue
• Thickness of tissue

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Describing Radiographic Appearance

• Subject Density Average Atomic Number
• Bone - calcium (atomic 20) phosphorus (atomic
  15) absorbs greater amount of x-rays
• Muscle - hydrogen (atomic 1) and nitrogen
  (atomic 7) absorbs lesser amount of x-rays

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Describing Radiographic Appearance

• Subject Density Average Atomic Number
• Finished radiographs contain white, black gray
  areas
• These differences caused by different tissue
  absorption rates

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Describing Radiographic Appearance

• Subject Density Atomic Number
• High Atomic NumberIncreased Density
• Increased DensityIncreased Absorption
• Increased AbsorptionWhite Appearance
• Low Atomic Decreased Density
• Decreased Density Decreased Absorption
• Decreased Absorption Black Appearance

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Describing Radiographic Appearance

• Subject Density Atomic Number
• Tissues from least density (low atomic) to most
  density (high atomic)
• Air Bubbles Black
• Fat Blubber Dark Gray-Light Black
• Muscle/Water Biceps Gray
• Bone Bone Light Gray-Light White
• Metal Bullet Bright White

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Describing Radiographic Appearance Subject
Density
Figure 2-2 p. 11 Han Hurd 3rd edition
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Describing Radiographic Appearance

• Subject Density Atomic Number
• Metal bone appear white/light on finished
  radiographs due to increased density, high atomic
  number increased absorption rate
• Air water appear black/dark on finished
  radiographs due to decreased density, low atomic
  number decreased absorption rate

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Describing Radiographic Appearance

• Subject Density Thickness vs. Absorption
• Thickness of subject densities also affects
  amount of x-rays absorbed
• Thicker subject densities more x-ray absorption
  whiter radiographic appearance
• Thinner subject densities less absorption
  darker radiographic appearance

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Describing Radiographic Appearance

• Subject Densities vs. Radiographic Contrast
• Radiographic variations in subject densities
  between adjacent areas
• High subject contrast-mostly blacks whites
  within adjacent areas (dark air in lung next to
  white heart tissue)
• Low subject contrast-mostly shades of gray within
  adjacent areas (light gray liver next to dark
  gray stomach)

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Describing Radiographic Appearance

• Radiographic Density
• Degree of blackness on finished radiograph
• Occurs on areas of film receiving exposure to
  x-rays
• Produced by deposits of black metallic silver in
  film emulsion layer

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Describing Radiographic Appearance

• Radiographic Density
• Increased (darkened) by increasing milliamperage
  (mA), exposure time (s)
• Decreased (lightened) by decreasing milliamperage
  (mA), exposure time (s)
• mAs primarily controls radiographic density

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Describing Radiographic Appearance

• Radiographic Density
• When an image contains excessive radiographic
  density (too much blackness resulting from too
  high of mAs) it is termed OVEREXPOSED
• When an image does not contain enough
  radiographic density (too much whiteness from too
  low of mAs) it is termed UNDEREXPOSED

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Describing Radiographic Appearance

• Radiographic Contrast
• Divided into
• Overall Radiographic Contrast
• Scale of Radiographic Contrast

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Describing Radiographic Appearance

• Overall Radiographic Contrast
• Controlled by mAs
• Degree of difference between blacks and whites
• High Overall Contrast large degree of difference
  between blacks whites
• Low Overall Contrast small degree of difference
  between blacks whites

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Describing Radiographic Appearance

• Scale of Radiographic Contrast
• Controlled by kVp which is the penetration power
  of the x-ray beam
• Percentage of blacks and whites verses percentage
  of shades of gray on finished radiograph
• Two types
• Short Scale
• Long Scale

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Describing Radiographic Appearance

• Short Scale of Radiographic Contrast or Decreased
  kVp
• Large percentage of blacks whites with small
  percentage of varying shades of grays
• Associated with lower kVp and less penetration
  power

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Describing Radiographic Appearance

• Long Scale of Radiographic Contrast or Increased
  kVp
• Small percentage of blacks and whites with large
  percentage of varying shades of grays
• Associated with higher kVp and more penetration
  power

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Describing Radiographic Appearance

• Scale of Radiographic Contrast - kVp
• When an image contains an excessive or too long
  of scale of contrast (too much kVp resulting from
  too high of penetration power) it is termed
  OVERPENETRATED
• When an image does not contain enough or too
  short of scale of contrast (too little kVp
  resulting from too low of penetration power) it
  is termed UNDERPENETRATED

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Describing Radiographic Appearance

• Ideal Radiographic Scale of Contrast
• For most radiographic studies a longer scale of
  radiographic contrast is preferred
• There should be many varying shades of grays
  some light, some dark and some medium shades
• There should also be a few blacks and whites
  mixed in as well
• The ideal scale should be longer but also must be
  BALANCED to be diagnostic

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Describing Radiographic Appearance

• Radiographic Detail
• Sharp, crisp well defined tissue interfaces on
  finished radiograph
• Diagnostic radiographs demonstrate excellent
  detail

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Factors Affecting Radiographic Appearance

• Subject Density
• Subject Thickness
• mAs Level
• kVp Level
• Film Contrast
• Film Scale
• Film Fogging

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Factors Affecting Radiographic Appearance

• Exposure Latitude
• Patient Motion
• Penumbra
• Focal Spot Size (FSS)
• Focal Film Distance (FFD)
• Object Film Distance (OFD)
• Geometric Distortion

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Factors Affecting Radiographic Appearance

• Subject Density
• Higher subject density whiter radiographic
  appearance
• Lower subject density darker radiographic
  appearance

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Factors Affecting Radiographic Appearance

• KVP Level
• Higher kVp
• Longer radiographic scale of contrast
• Fewer blacks whites - more grays
• Lower kVp
• Shorter radiographic scale of contrast
• More blacks whites - fewer grays

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Factors Affecting Radiographic Appearance

• Film Contrast
• X-Ray film has its own inherent contrast
• manufactured to produce either long or short
  scale of contrast
• Long film scale more grays
• Short film scale more blacks whites

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Factors Affecting Radiographic Appearance

• Film Scale
• Film producing long scale of contrast long
  latitude film
• Film producing short scale of contrast contrast
  film
• Long latitude film preferred for most general
  radiography

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Factors Affecting Radiographic Appearance

• Exposure Latitude
• Range of exposures that produces a film density
  of diagnostic quality
• Long scale of film contrast contains more
  exposure latitude
• Short scale of film contrast contains less
  exposure latitude
• Benefit minor exposure errors forgiven

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Factors Affecting Radiographic Appearance

• Film Fogging
• Loss of clarity lack of crispness
• Loss of tissue definition
• Lack of serosal surface detail
• Decreased sharpness between adjacent tissues on
  finished radiograph
• Overall dull grayed out appearance

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Factors Affecting Radiographic Appearance

• Film Fogging caused by
• low grade light leaks in darkroom
• heat humidity
• improper processing
• scatter radiation (to be defined in future
  lecture)

• Film Fogging results in greatly decreased ability
  to distinguish between differing densities of
  adjacent tissue interfaces on finished radiograph

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Imaging Terminology Film Fogging Example
Figure 2-3 p. 12 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Patient Motion
• Decrease in radiographic detail
• Fuzzy or blurry appearance
• Poorly defined tissue interfaces

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Imaging TerminologyPatient Motion Example
No Patient Motion
Patient Motion
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Factors Affecting Radiographic Appearance

• Penumbra
• Blurring or loss of definition at tissue
  interfaces
• Decreases radiographic detail
• Penumbra increased by
• Increased Focal Spot Size (FSS)
• Decreased Focal Film Distance (FFD)
• Increased Object Film Distance (OFD)

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Factors Affecting Radiographic Appearance

• Focal Spot Size Penumbra
• Why not just decrease focal spot size to decrease
  penumbra?
• Because smaller focal spot size less heat
  dissipation
• Heat build-up can damage equipment!

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Factors Affecting Radiographic Appearance

• Focal Spot Size Penumbra
• Large actual focal spot used to create small
  effective focal spot
• Small effective focal spot greatly reduces
  penumbra
• small as possible to create detailed films
• large enough to prevent overheating

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Factors Affecting Radiographic Appearance Example
Figure 2-6 p. 14 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Focal Film Distance Penumbra
• Distance from focal spot to film
• Average FFD - 36-40 inches
• Decreasing FFD increases penumbra
• Increasing FFD decreases penumbra

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Factors Affecting Radiographic Appearance

• FFD Penumbra Example

Figure 2-7 p. 14 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Focal Film Distance Penumbra
• Do NOT increase FFD above 3640 Increasing FFD
  decreases intensity of x-rays
• Decreased intensity fewer x-rays available to
  expose film
• Fewer x-rays decreased density non-diagnostic
  radiograph

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Factors Affecting Radiographic Appearance

• FFD, Penumbra Inverse Square Law
• Why cant intensity of x-ray beam be increased to
  overcome decreased density due to increased FFD?
• Unsafe to do so because of Inverse Square Law

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Factors Affecting Radiographic Appearance

• Penumbra-FFD Inverse Square Law
• Intensity of x-ray beam decreases at a rate
  inverse to square of distance
• If FFD doubled, then mAs must increase 4 times to
  maintain same radiographic density!

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Factors Affecting Radiographic Appearance

• Penumbra-FFD Inverse Square Law
• Remember increasing mAs also increases exposure
  time
• Increased time increased chance of patient
  movement
• Patient motion always a problem in veterinary
  radiography!

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Factors Affecting Radiographic Appearance

• Penumbra-FFD Inverse Square Law
• Therefore For most radiographic procedures, a
  focal film distance (FFD) of 3640 inches is
  sufficient to minimize penumbra effect, obey
  inverse square law and maintain proper
  radiographic density

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Factors Affecting Radiographic Appearance

• Penumbra Object Film Distance
• OFD - distance from recording surface to
  area/tissue being imaged
• Increasing OFD increases penumbra magnifies
  area being imaged
• Maintain as short/small OFD as possible

Figure 2-9 p. 15 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance FSS,
FFD, OFD Penumbra

• No practical way to totally eliminate penumbra
  effect
• However a combination of 11-20 degree FSS,
  36-40 inch FFD minimal OFD will minimize
  penumbra produce detailed diagnostic radiographs

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Factors Affecting Radiographic Appearance

• Image Geometry
• Tissue shape must be accurately recorded for
  diagnostic radiographic production
• Distortion of tissue shape by improper
  positioning can inaccurate interpretation
• Important to understand geometric projection of
  tissue shape onto recording surface

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Factors Affecting Radiographic Appearance

• Image Geometry Causes of Distortion
• Foreshortening
• Magnification
• False Narrowing

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Factors Affecting Radiographic Appearance

• Image Geometry Foreshortening
• Subject appears shortened on finished image
  true length not accurately represented
• Subject being imaged must be parallel to
  recording surface or foreshortening occurs
• Foreshortening most often seen when imaging long
  bones

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Factors Affecting Radiographic Appearance

• Example Foreshortening
• One end of subject further away from recording
  surface than other
• Subject appears shorter wider than true size

Figure 2-9 p. 15 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Image Geometry Magnification
• Subject must be as close to recording surface as
  possible
• Increased OFD increased size of image
  magnification
• Increased magnification increased penumbra
  blurring loss of detail

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Factors Affecting Radiographic Appearance
Magnification
Figure 2-10 p. 16 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Image Geometry False Narrowing
• Occurs most frequently in tissues with adjacent
  radiolucent radiodense areas - vertebrae
• Joint spaces appear narrower than true size

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Factors Affecting Radiographic Appearance

• False Narrowing
• Increased distance from center of primary beam
  cause x-rays to strike subject area at increasing
  angles
• False narrowing occurs as result of these angles
• These areas must be parallel to recording surface
  or false narrowing occurs

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Factors Affecting Radiographic Appearance

• False Narrowing Example
• Lateral recumbent cervical spine
• Naturally sags at mid-point
• Sagging creates false narrowing of joint spaces
  causing possible misinterpretation
• Add padding to mid cervical region, making
  vertebral area parallel to recording surface

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Factors Affecting Radiographic Appearance False
Narrowing
Properly Padded Joints
Falsly Narrowed Joints
Figure 2-11 p. 16 Han Hurd 3rd edition
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Factors Affecting Radiographic Appearance

• Image GeometryFalse Narrowing
• To Prevent false narrowing
• make several exposures of vertebral column
  centering over different areas
• center primary beam over joint of interest to
  maximize joint space