Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 14
• The Radiographic Image

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Contrast

• difference in density between areas on the
  radiograph
• Contrast depends on
• subject contrast
• film contrast
• fog and scatter

3
Subject Contrast

• difference in x-ray intensity transmitted through
  various parts of subject
• Depends on
• thickness difference
• density difference
• atomic number difference
• radiation quality (kVp, HVL)

I
IS
IL
Subject Contrast IS / IL
4
Subject Contrast Radiation Quality

• high kVp lower subject contrast
• long scale contrast (less difference between
  areas receiving varying amounts of radiation)
• low kVp high subject contrast
• short scale contrast (more black white more
  difference between areas receiving varying
  amounts of radiation)
• low kVp increases patient dose

5
Exposure Latitude

• range of incident radiation intensities which
  produce desired film density
• Latitude contrast vary inversely
• high contrast low latitude
• low contrast high latitude

6
Speed Contrast

• Contrast controls slope of characteristic curve

Whites whiter, blacks blacker
7
Exposure Latitude
Low Contrast High Latitude Higher kVp
High Contrast Low Latitude Lower kVp

• For low contrast film
• shallow slope
• greater exposure latitude
• wider mAs range produces proper film density
• increasing kVp causes
• decreased contrast (slope)
• increased latitude

8
Exposure Latitude

• Film/screen latitude must match exam
• chest requires high latitude
• mammography requires lower latitude
• kVp affects latitude
• kVp must match latitude required by exam

9
High Latitude System

• limits contrast
• less sensitive to technique changes

10
Increasing kVp

• decreases contrast
• increases latitude
• increases film darkening for same mAs
• Rules of thumb
• 10 kVp increase at 50-60 kVp has same effect as
  doubling mAs
• 15 kVp increase at 85 kVp has same effect as
  doubling mAs

11
Film Latitude

• Contrast affected by
• film type
• film density (see next slide)
• optical density differences of .04 (10)
  discernible by human eye

12
Contrast vs. Density

• Contrast depends on density
• best contrast at mid-densities
• Slope of H D curve changes with density
• slope lower at toe and shoulder
• slope greatest at mid densities
• Too light or too dark film loses contrast

Optical Density
log relative exposure
13
Fog and Scatter

• Fog and Scatter reduce contrast

14
Fog and Scatter

• Fog and Scatter reduce contrast
• Scatter
• produces unwanted density
• mostly a result of Compton interactions
• increases with
• kVp
• part thickness
• field size
• collimation reduces scatter

15
Fog

• Development of film grains not exposed to light
  or x-rays
• produces unwanted density
• lowers radiographic contrast

16
Determining Fog

• run half sheet of film through developer, fixer,
  wash, dryer
• run other half through all but developer
• compare densities
• difference is fog

17
Exposure Fog

• also called fog but different from development
  fog
• refers to accidental exposure to radiation

Wall
Darkroom Wall
18
Development Fog

• Development of unexposed grains
• Aka true fog
• Sources of optical density increase
• Storage
• high temperature
• high humidity
• chemistry contamination
• excessive developer time
• excessive developertemperature

19
Fog and Scatter

• Alter characteristic curve
• Reduce contrast at clinical densities
• less effect at higher densities

20
Image Quality

• ability of film to record each point of image as
  point on film
• Influenced by
• radiographic mottle
• also called noise
• sharpness
• resolution

21
Radiographic Mottle

• Appearance
• irregular density variations in mid-density areas
  exposed to uniform x-ray fields
• Components
• Screen mottle
• structure mottle
• film graininess
• only visible under magnification
• not significant in radiology
• quantum mottle
• Random nature of radiation

22
Structure Mottle

• Caused by defects in intensifying screens
• thickness variations
• physical imperfections
• usually not a consideration for good quality
  screens

23
Quantum Mottle

• Cause
• random x-ray emission
• statistical fluctuations in of quanta / unit
  area absorbed by intensifying screen
• Math
• standard deviation related to square root of
  total number of photons interacting with screen

24
Quantum Mottle

• Math (cont.)
• fractional fluctuation greatest when of photons
  is smallest
• 10 100 ---- gt
  --------- (.1 gt .01) 100
  10,000
• throw a dice 12 times or 12,000 times variation
  from expected 1/6 for each face will probably be
  more for 12 throws!

Numerator is square root of denominator
25
Quantum Mottle kVp

• Raising the kilovoltage while maintaining the
  same optical density results in
• lower patient exposure
• lower mAs for same density
• less x-ray photons exposing film
• higher quantum mottle

26
Quantum Mottle Visibility

• Best visualized on good-quality high contrast
  radiograph
• Poor detail (blurring) may mask quantum mottle
• can be caused by
• thick screens
• poor screen/film contact

27
Noise Speed

• Cause of noise (quantum mottle)
• statistical fluctuation in of x-ray photons
  absorbed by intensifying screen to form image
• fast systems can result in unacceptable noise
• high contrast images are sharpness limited
• low contrast images are noise limited
• most important diagnostic information here

28
Noise Speed

• high noise reduces visibility of low contrast
  objects
• Look through dirty window
• Can tell person from horse
• Harder to tell one person from another

29
Noise Speed

• high noise reduces visibility of low contrast
  objects
• Look through dirty window

30
Exposure Steps

1. Photon must interact with screen
2. Screen must produce light
3. Light must reach film
4. Light must expose film

31
Quantum Mottle Speed

• faster screen / film system results in
• less patient exposure
• fewer x-ray photons contributing to image
• quantum mottle
• ??? (sometimes)
• Stay tuned

32
Exposure Steps

1. Photon must interact with screen
2. Screen must produce light
3. Light must reach film
4. Light must expose film

33
Increasing System Speed by using Screens with
increased photon absorption

• Greater fraction of incident photons absorbed
• Assume equal film density
• Same photon interactions
• same quantum noise
• Less incident photons
• Less patient exposure

100 photons in
50 absorbed
50 photons interact
Screen
50 photons unattenuated
75 photons in
50 photons interact
Screen
67 absorbed
25 photons unattenuated
34
Increase System Speed by using Thicker Screens

• Assume equal film density
• Same photon interactions
• same quantum noise
• Noise less visible for thicker screens
• Less sharp
• Fewer incident photons
• Less patient exposure

100 photons in
50 absorbed
50 photons interact
Screen
50 photons unattenuated
75 photons in
50 photons interact
Screen
67 absorbed
25 photons unattenuated
35
Exposure Steps

1. Photon must interact with screen
2. Screen must produce light
3. Light must reach film
4. Light must expose film

36
Increase System Speed by using more efficient
Screen Phosphor

• More efficient higher x-ray to light conversion
  efficiency
• more light per photon
• Less photons required to achieve same film
  density
• increased quantum noise

50 absorbed
100 photons in
50 photons interact
Screen
Same amt of light
50 photons in
25 photons interact
Screen
50 absorbed
37
Exposure Steps

1. Photon must interact with screen
2. Screen must produce light
3. Light must reach film
4. Light must expose film

38
Increase System Speed by Increasing Film Speed

• more film darkening per photon
• less photons required to interact with screen
• increased quantum noise

50 absorbed
100 photons in
50 photons interact
Screen
50 photons in
25 photons interact
Screen
50 absorbed
39
Speed and Noise Summary

• Increase speed with no change in noise
• increase phosphor layer thickness
• higher x-ray absorption phosphor
• Increase speed with increased noise
• phosphor with higher x-ray to light conversion
  efficiency
• faster film

40
Sharpness

• Ability of film to define an edge
• Sharpness and Contrast
• unsharp edge much easier to detect under
  conditions of high contrast
• sharp edge are less visible under conditions of
  low contrast
• One cause of unsharpness
• Penumbra
• Shadow caused by finite size of focal spot
• Stay tuned for chapter 15

41
Reduce Unsharpness by Adding Light-Absorbing Dye
to Screen

• Less light spread in screen
• Decreases screen speed
• less screen light reaches film
• less quantum noise
• more photons must interact with screen for same
  density film

Screen
Film
42
Minimizing Geometric Unsharpness

• minimize focal spot size
• maximize source to image distance
• minimize object to image distance

Minimize
maximize
minimize
43
Sources of Unsharpness

• Geometry
• Motion
• minimized by short exposure times
• Absorption
• absorber may not have sharp edges
• round or oval objects

44
Sources of Unsharpness

• screen
• light diffusion in screen phosphor layer
• poor screen-film contact
• parallax
• viewing images on 2 emulsions

Screen
Film
45
Total Unsharpness

• combination of all the above
  BUTnot the sum!
• larger than largest component
• largest component controls unsharpness
• improvement in smaller components dont help much

46
Sharpness Resolution

• Sharpness
• ability of imaging system to record sharply
  defined margins or abrupt edges
• Resolving Power (Resolution)
• ability to record separate images of small
  objects very close together

47
Resolution

• Units
• lines or line pairs per distance
• such as lead bars separated by equally wide
  spaces
• Expresses limiting resolution
• Limiting resolution implies high contrast
  situation
• does not indicate how well system preserves
  contrast

1 mm
4 lines (line pairs) per mm
48
Line Spread Function

• Measures unsharpness at edges
• Measurement
• narrow slit (10 microns) in contact with
  film/screen
• ideal system would provide 10 micron wide image
• slit width measured with microdensitometer
• plot density vs. distance
• Film exhibits almost no spread

X-Ray Tube
Lead
Typical
Cassette
49
Modulation Transfer Function (MTF)

• Calculated from line spread function
• Compares information recorded to information
  available
• function of line pairs/mm
• quantifies decrease in contrast at higher
  frequencies

50
MTF

• as sharpness decreases so does contrast
• less sharp system blurs dark light areas
  together
• maximum density decreases
• minimum density increases
• at very high line pairs per mm film will be
  uniform gray

51
Modulation Transfer Function (MTF)
100 (1)
80 (0.8)
Lowest Frequency
40 (0.4)
0 (0.0)
Highest Frequency
Fraction of contrast reproduced decreases at
increasing frequency because lines and spaces
blur into one another
52
MTF

• Combines concepts
• sharpness
• resolution
• contrast

1
MTF
0
Frequency
53
MTF

• Equation
• exposure amplitude outputMTF
  --------------------------------------
  exposure amplitude input
• Basic Concept
• information recordedMTF
  -------------------------------
  information available
• Example
• MTF of .6 at 3 lines per mm means that 60 of
  contrast is reproduced at this frequency

54
System MTF

• System MTF calculated by multiplying MTFs of
  each component at each frequency
• System MTF no better than the MTF of the poorest
  component
• MTFs lt 1

55
Wiener Spectrum

• Also called power spectrum
• Noise measured as a function of frequency content
  
• Types of noise measured
• grain noise
• independent of object frequency
• quantum noise
• affected by MTF just like the rest of the image
• screen noise
• operationally negligible