RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

1
RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• L15.1 Optimization of protection in radiography
  technical aspects

2
Topics

• Intensifying screen structure and characteristics
• Screen film combination
• Radiographic film structure and characteristics
• Anti scatter grid
• Film processor
• Darkroom and Viewing Box
• Image parameters

3
Overview

• To become familiar with basic knowledge of the
  component that form the radiographic chain.

4
Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 Intensifying screen structure and
  characteristics

5
Primary beam attenuation and latent image
Film, fluorescent screen or image intensifier
Scattered radiation
 Latent  radiological image
Bone
X
Soft tissue
Air
Primary collimation
Antiscatter Grid
Beam intensity at detector level
6
Intensifying screen

• Layer of material placed immediately adjacent to
  film in conventional radiography to
• Convert the incident X Rays into radiation more
  suitable for the radiation-sensitive emulsion of
  the radiographic film (X Ray ? light photons)
• Reduce the patient exposure needed to achieve a
  given level of film blackening
• Reduce the exposure time as well as the power of
  the X Ray generator (cost savings)
• Increase photoelectric effect ? better use of the
  beam energy (image formation)

7
Intensifying screen structure (I)

• Supporting Base (mainly polyester material)
• chemically neutral, resistant to X Ray exposure,
  flexible, perfectly flat
• Reflecting layer (Titanium dioxide - TiO2)
• a crystalline compound reflecting backward
  photons to sensitive emulsion
• Fluorescent layer (polymer)
• crystals dispersed in a suspension of plastic
  material
• Protective overcoat
• colourless thin film avoiding abrasions of
  fluorescent layer due to the use of screen

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Intensifying screen structure (II)
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Intensifying screen structure (III)

• The fluorescent layer (luminophor crystals)
  should
• be able to absorb the maximum quantity of X Rays
• convert the X Ray energy into light energy
• match its fluorescence with the film sensitivity
  (color of emitted light)
• Type of material
• Calcium tungstate (CaWO4) (till 1972)
• Rare earth (since 1970) (LaOBrTm) (Gd2O2STb)?
  more sensitive and effective than (CaWO4)

10
Intensifying screen characteristics (I)

• IF (Intensifying Factor) ratio of exposures
  giving the same film optical density, with and
  without screen
• 50 lt IF lt 150 (depending on screen material and X
  Ray beam energy)
• QDE (Quantum Detection Efficiency) fraction of
  photons absorbed by the screen
• 40 for CaWO4 lt QDE lt 75 for rare earth
  (depending on crystal material, thickness of
  fluorescent layer and X Ray spectrum)

11
Intensifying screen characteristics (II)

• ? (Rendering coefficient) ratio of light energy
  emitted to X Ray energy absorbed ()
• 3 for CaWO4 lt ? lt 20 for rare earth
• C (Detection Coefficient) ratio of energy
  captured and used by the film to energy emitted
  by the crystal ()
• C is maximum for screens emitting in UV color
  wave length ? 90

12
Intensifying screen characteristics (III)
Sensitivity of a Conventional Film
BaSO4Eu,Sr
YTaO4Nb
Relative Sensitivity of Film
BaSO4Pb
CaWO4
250
300
350
400
450
500
550
600
UV
Blue
Green
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Intensifying screen characteristics (IV)

• Intensifying factor ratio of exposures giving
  the same film optical density, with and without
  screen

175 150 125 100 75 50 25 0
Gd2O2S
LaOBr
Intensifying factor
CaWO4
kV
50 60 70 80 90 100 110 120
14
Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Screen film combination

15
Screen film combination

• Sensitivity (screen film) The quotient K0/Ka,
  where K0 1 mGy and Ka is the air kerma
  free-in-air for the net density D 1.0, measured
  in the film plane
• Screen film system A particular intensifying
  screen used with a particular type of film
• Sensitivity class Defined range of sensitivity
  values of a screen film system
• Single emulsion film One coated film used with
  one intensifying screen
• Double emulsion film A double coated film used
  with a couple of intensifying screens
• Screen film contact ? ? ? ? Quantum mottle

16
Screen film combination performance

• Spatial Resolution capability of a screen film
  combination to objectivate a limited number of
  line pairs per mm. It can be assessed by the
  Hüttner resolution pattern which should contain
  several cycles at each frequency in order to
  simulate the periodicity
• Modulation Transfer Function (MTF) description
  of how sinusoidal fluctuations in X Ray
  transmission through the screen film combination
  are reproduced in the image
• Noise spectrum component of noise due to
  intensifying system (screen film)
• Quantum noise, Screen noise, Granularity
• Quantum Detection Efficiency (QDE) the quotient
  of signal to noise ratios (SNR) of radiographic
  image and latent image

17
Screen film combination performance

• Identification of screen by type and format
• type mismatch (use of different types of screens)
  FOR THE SAME FORMAT is not ADVISABLE
• Screen film contact
• loss of spatial resolution
• blurred image
• Cleanliness
• Inter cassette sensitivity

18
Effect of screen on resolution

• Screen resolution is dependent on the crystal
  size and thickness of screen
• Direct exposure radiography has better resolution
  than screen-film (but requires around 40 times
  the radiation exposure)
• Direct exp. - 50 lp/mm, normal screens 10
  lp/mm, fast screens 6 lp/mm, mammo. systems 15
  lp/mm

19
Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Radiographic film structure, image
  formation and processing characteristics

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Radiographic film(structure and characteristics)

• Protective layer (outer surface)
• Sensitive layer (20µ)
• Base material (transparency and mechanical
  resistance) (170µ)
• Binding (base-sensitive layer) or anti cross-over
  layer
• Filtering layer
• Sensitivity class

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Radiographic film structure
Supercoat
Emulsion (5-20 µm thick)
Adhesive layer
Base (200 µm thick)
Anti-curl, anti-halation layer
Single Emulsion Film
22
Film construction

• Supercoat - prevents scratching
• Base
• provides relatively thick, semi-rigid structure
  to film, but still allowing flexibility
• almost (but not completely) transparent
• Emulsion
• image layer, composed of gelatine and silver
  halide (Br, I) crystals in ionic form
• speed,contrast, resolution varied in emulsion

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Radiographic film structure
Supercoat
Emulsion
Adhesive layer
Base
Adhesive layer
Emulsion
Supercoat
Double Emulsion Film
24
Silver halide reaction

• Latent image (invisible) formed by interaction of
  a light photon from screen, with a halide ion
  within the crystals, which
• releases an electron,
• which in turn reacts with silver ion,
• forming atomic silver within the crystal

25
Processing

• Development
• Converts latent image by converting the silver
  ions of exposed silver halide crystals to
  metallic silver
• Fixing
• Dissolves unexposed silver halide crystals,
  leaving only atomic silver, and creating a
  permanent image

26
Steps in image formation
27
Spectral response and spectral matching

• The variation in film sensitivity to the various
  colours of light
• Film is usually blue or blue-green sensitive
  (orthochromatic)
• Screens emit blue (e.g.. calcium tungstate) or
  green (rare earth screens)
• Safelights must not affect film

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Spectral response of film
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Crossover

• In double emulsion film, light emitted by one
  screen can cross over through the adjacent
  emulsion, and the base and expose the second
  emulsion
• This will reduce the resolution of the image
• Is prevented with a light-absorbing dye layer

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Crossover
31
Optical density
Transmitted light intensity
Incident light intensity
It
I0
OD log10 I0 / It
Film
e.g. 10 transmission OD 1 1
transmission OD 2
32
Characteristic curve of a radiographic film
Optical Density (OD)
Saturation
OD2
Visually evaluable range of densities
? (OD2 - OD1) / (log E2 - log E1)
?
The ? of a film the gradient of the straight
line portion of the characteristic curve
OD1
Normal range of exposures
Base fog
Log Exposure (mR)
E1
E2
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Average gradient

• The straight line portion of the characteristic
  curve is difficult to determine, so the average
  gradient is measured between net ODs of 0.25 and
  2.0
• OD 2.0 is used because at this level, only 1 of
  the light is transmitted, and in a normal
  lightbox, little light would be seen
• OD 0.25 is used because the eye can only detect
  contrast differences of 10, and there is
  insufficient contrast available below 0.25

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Average gradient
Average gradient is the slope of the line drawn
between OD 0.25 and OD 2 above (basefog)
35
Average gradient --gt Contrast

• Contrast is usually measured as the average
  gradient
• An alternative is to measure the difference in OD
  from 2 steps above the speed measurement step (or
  the step closest to an OD of 1.2 i.e. net OD of
  1.0), and the OD 2 steps below.

36
Film sensitometry parameters

• Base fog The OD of a film due to its base
  density plus any action of the developer on the
  radiographically unexposed emulsion usually 0.15
  -0.25.
• Sensitivity (speed) The reciprocal of the
  exposure value needed to achieve a film net OD of
  1.0
• Gamma (contrast) The gradient of the straight
  line portion of the characteristic curve
• Latitude Steepness of a characteristic curve,
  determining the range of exposures that can be
  transformed into a visually evaluable range of OD

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Comparison of characteristic curves
OD
OD
Film A
Film A
Film B
Film B
1BFog
Log Exposure (mR)
Log Exposure (mR)
Film A is faster than Film B
Film A and B have the same sensitivity but
different contrast
Film A and B have the same contrast
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Sensitometric strip
Sensitometry A method of exposing a film by
means of a light sensitometer and assessing its
response to exposure and development
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Sensitometric strip
40
Latitude
Film B has higher latitude (range of useful
exposures) than film A, but has lower contrast
(slope of the curve)
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Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 4 Anti-scatter grid and grid performance
  parameters

42
Anti-scatter grid (I)

• Radiation emerging from the patient
• primary beam contributing to the image formation
• scattered radiation reaching also the detector
  but reducing contrast and also contributing to
  the patient dose
• the grid (between patient and film) eliminates
  most of scattered radiation
• stationary grid
• moving grid (better performance)
• focused grid
• Potter-Bucky system

43
Anti-scatter grid (II)
Source of X Rays
Patient
Scattered X Rays
Lead strip
Film and cassette
Useful X Rays
44
Grid performance parameters (I)

• Grid ratio
• Ratio of the height of the strips to the width of
  the gaps at the central line
• Contrast improvement ratio
• Ratio of the transmission of primary radiation to
  the transmission of total radiation
• Grid exposure factor
• Ratio of the indicated value of rate of total
  radiation without the anti-scatter grid in a
  specified radiation beam to that with the
  anti-scatter grid placed in the beam

45
Grid performance parameters (II)

• Strip number
• The number of attenuating lamella per cm
• Grid focusing distance
• Distance between the front of a focused grid and
  the line formed by the converging planes which
  include the attenuating lamella of the grid

46
Example of anti-scatter grids (grid ratio)
Grid C
Grid A
Grid B
D
h
?
?
?
1
h
Grid ratio r

5 lt r lt 16
?
tg
D

• Grid A and B have the same strip number
• Grid B and C have the same interspace between
  the lamella

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Grid selectivity(I)
Grid C
Grid A
Grid B
?
?
?
48
Grid selectivity (II)
100 90 80 70 60 55 50 45 40 35 30 25 20 15 10
5 0

• A grid with r 12 transmits 5
• of scattered radiation
• A grid with r 16 transmits 3.8
• N.B. slight difference

of scattered beam transmitted
5
3.8
r
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16
49
Grid focusing error(virtual increasing of grid
shadow)
X Ray source (too far)
X Ray source (too close)
Grid
Film and cassette
grid shadow deformation (applicable to both
cases)
50
Grid focusing error(leading to 25 of beam loss)
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Grid out of center(virtual deformation of grid
shadow)
Lateral shift
X Ray source
Film and cassette
Grid
Grid shadow
52
Grid focusing error due to lateral shift(leading
to 25 loss of X Ray beam)
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Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 5 Film processor

54
The automatic film processor
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Automatic processors

• Constant temperature
• Constant processing time
• Automatic replenishment of chemicals
• Drying of films
• BUT
• Can introduce artifacts

56
Film processor QC

• Most important QC features
• proper film storage
• cassette and screen care
• processor chemical care
• sensitometry
• artifacts
• processor cleanliness

57
Sensitometry (I)

• Sensitometer and densitometer required
• Essential - to keep film processing under control
• To be performed daily
• Main parameters investigated
• base fog
• speed
• gradient (gamma)
• contrast

58
Sensitometry (II)

• Use a sensitometer to expose a film to light
  through the special step wedge
• Ensure that the emulsion side of the film (if
  single emulsion) is toward the light source
• Select the correct light colour (green, blue) on
  the sensitometer (if selectable), and expose
  until the signal shows the exposure is complete
• Process the film immediately

59
Sensitometry (III)

• Before measuring the optical densities of the
  step-wedge, a visual comparison can be made to a
  reference strip to rule out a procedure fault,
  such as exposure with a different colour of
  light, or exposure of the base instead of the
  emulsion side

60
Sensitometry (IV)

• Plot the step densities on graph paper
• From the characteristic curve (the graph of
  measured optical density against the exposure by
  light) the values of base and fog, maximum
  density, speed and mean gradient can be derived.

61
Manual Processing

• There are many places where X Ray films are
  processed manually, in open tanks
• Sometimes in very back conditions
• Manual processing can be very effective, BUT
  there can be many quality problems

62
Dark room conditions in some hospitals
63
Film Processing

• Remember that film processing has the following
  stages
• developer
• water wash
• fixer
• water wash
• Washing is very important to avoid chemical
  contamination, and for a good X Ray picture

64
Basic Film Processing Requirements

• Temperature - constant and optimum
• Time - measured
• Developer activity (chemical condition) - fresh
  and unoxidised

65
Temperature (I)

• The temperature of the developer should be about
  20oC (or as recommended by the manufacturer)
• Use a thermometer regularly to check the
  temperature
• If a thermometer is not available, the developer
  should be a comfortable temperature for the skin

66
Temperature (II)

• Maintaining the right temperature is ESSENTIAL.
• If developer is too cold processing will not
  occur
• If developer is too hot processing will be too
  fast and hard to control.

67
Maintaining temperature

• Ideally both developer and fixer containers
  should be surrounded by a water bath (as a
  thermal jacket)
• This water bath should be heated (or cooled) to
  20oC
• The best method used an immersion heater with a
  thermostat
• However hot (or cold) water can be added to the
  bath to keep it at a constant temperature
• Requirements sometimes IMPOSSIBLE to fulfill
  Africa, Asia,

68
Manual processing tanks
Water bath surrounding tanks (not filled here)
69
Development time (I)

• If the developer temperature is constant and
  known, a standard time for processing should be
  used.
• Ideally this is about 3 minutes.
• Exact time should be calculated from a time
  versus temperature graph.
• A large clock or watch (visible in low light)
  should be used.

70
Development time (II)

• Reasonable processing may be established (with
  experience) by looking at the film near the
  safelight towards the end of development time.
• However, this creates high fog levels
• May cause excessive chemical splashing and
  increase films accidentally dropped in the
  developer tank

71
Chemical activity (I)

• The correct chemicals for manual processing must
  be used
• Correct developer density can be checked with a
  hydrometer
• pH paper can give an indication of the chemical
  condition - Developer pH 10 Fixer pH 4.
• Silver content paper indicates if fixer is
  exhausted

72
Chemical activity (II)

• As chemicals are lost through splashing and
  dripping new chemicals must be added
• Draw a line on the developer container to show
  the proper level of chemical required - fill to
  this line every few hours.

73
Chemical activity (III)

• Films must be agitated every 20 seconds during
  development and fixing.
• Once film is developed the film is washed in
  clean water before being put in the fixer.
• Never put films from the fixer back in the
  developer
• Avoid splashing fixer into the developer
  container.

74
Chemical activity (IV)

• As films are developed, the developer and fixer
  chemicals get used up and become stale - this
  is often called low chemical activity
• Also, air will oxidise the developer (making it
  turn brown)
• Both will cause poor quality X Rays

75
Measuring Chemical Activity (I)

• Use of a sensitometer is preferred with the use
  of a densitometer
• However, much can be done with a standard
  phantom and viewing box
• Standard phantom could be a
• stepwedge
• any familiar object (e.g. clock)

76
Measuring Chemical Activity (II)

• Procedure
• Expose object at a set kVp, mAs and focus to film
  distance
• record these factors for future use
• always use the same factors for test film
• Process film and use as reference
• compare processor check film with standard film
  to check chemical activity

77
Measuring Chemical Activity (III)

• Signs that developer activity is low
• longer exposure times than expected
• Loss of film contrast
• Loss of high densities on the film
• Replace developer if activity is low

78
Measuring Chemical Activity (IV)

• Signs that fixer activity is low
• films take longer to clear
• silver content paper reading high gt 5
• Replace fixer if activity is low

79
Washing

• Films must be rinsed briefly but thoroughly
  between developer and fixer,
• And washed for 30 minutes following fixing, to
  clear all traces of fixer (which can degrade the
  X Ray over time)
• Wash water must be clean and changed frequently
  to remove all developer and fixer

80
Viewbox characteristics

• Since the viewing conditions are essential for a
  good interpretation of the diagnostic images, the
  viewing conditions must be optimal
• Cleanliness of external/internal surface
• Brightness (luminance)
• homogeneity of different viewing boxes 1300 -
  2000 cd/m2
• homogeneity within the same viewing box
• Colouring
• colour mismatch must be avoided
• Environment (illuminance)
• ambient light level 50 lux maximum

81
Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 6 Darkroom and viewing box

82
Darkroom characteristics

• Safelight
• number (as low as possible), distance from the
  table
• type and colours of filters
• bulb color (red) or adapted to film
• power (lt 25 W)
• External light tightness
• Hygrometry (30 - 60)
• Room temperature lt 20
• Film storage conditions

83
Viewing box characteristics

• Since the viewing conditions are essential for a
  good interpretation of the diagnostic images, the
  viewing conditions must be optimal
• Cleanliness of external/internal surface
• Brightness
• homogeneity of different viewing boxes 1300 -
  2000 cd/m2
• homogeneity within the same viewing box
• Coloring
• color mismatch must be avoided
• Environment
• ambient light level 50 lux maximum

84
Viewing box brightness

EXAMPLE OF MEASUREMENTS
CORRECT CONFIGURATION (cd/m2)
85
Viewing box color and brightness

WHITE COLOR
BLUE COLOR
WRONG CONFIGURATIONS (cd/m2)
86
Measurement of Luminance
Units cd.m-2
87
Measurement of Illuminance
Units lux
88
Example of poor viewing box
89
Part 15.1 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 7 More image parameters

90
Image parameters

• Density
• Contrast
• Resolution
• Unsharpness
• Noise
• Distortion
• MTF

91
Factors affecting film quality
92
Contrast

• The difference between OD in two parts of a
  radiographic image (see Topic 3)
• Made up from two sources
• 1. Subject contrast the different amounts of
  radiation exiting different parts of the body
• Affected by tissue density, atomic number and
  density, X Ray energy (kVp), scatter
• 2. Detector contrast made up of the properties
  of the detector (ex film/screen system and
  processing)

93
Subject Contrast (1)
94
Subject Contrast (2)

• Apart from the patient, the important factors are
  kVp and scatter
• High kVp means higher penetration and less
  variation in absorption in body tissues, and thus
  lower contrast
• Low kVp gives more differential absorption and
  thus high contrast (we use low kVp for
  mammography)

95
Subject contrast (3)

• Scattered radiation can significantly reduce
  contrast, and is reduced with a grid

96
Subject contrast (4)

• Grid performance can be described by the
  radiographic contrast improvement ratio k
• k (Image contrast with grid)/(contrast without
  grid)
• k is normally between 1.5 and 2.5
• Subject contrast can be improved by using iodine-
  of barium-containing contrast agents in the
  patient

97
Detector contrast
98
Resolution and unsharpness

• Spatial resolution (or image blur) is the ability
  to distinguish two closely spaced objects
• Resolution is measured in a number of ways, but
  most commonly as line pairs per mm (lp/mm)
• The higher the lp/mm, the better the resolution

99
Spatial resolution

• Resolution is affected by a number of factors
• focal spot size
• use of an intensifying screen
• motion
• image noise

100
Effect of focal spot on resolution
101
Noise (1)

• The fluctuation of OD in the image over very
  small distances
• Some noise is inherent in the imaging system,
  some is controllable

102
Noise (2)

• Noise is mostly caused by
• The number of X Ray photons used in the image
  (quantum mottle) - most important component
• The limited absorption efficiency of X Rays by
  the screen (structure mottle)
• The crystal size and distribution in film (film
  graininess)

103
Noise (2)

• Noise is mostly caused by
• The number of X Ray photons used in the image
  (quantum mottle) - most important component
• The limited absorption efficiency of X Rays by
  the screen (structure mottle)
• The crystal size and distribution in film (film
  graininess)

104
Magnification

• The larger the gap between the object and the
  image receptor, the more the image will be
  magnified

105
Summary

• The main components of the radiography chain and
  their respective role are explained
• conventional film and screen-film combination
  characteristics
• required conditions for film processing
  (darkroom) and image viewing (viewing box)

106
Where to Get More Information

• Physics of diagnostic radiology, Curry et al, Lea
  Febiger, 1990
• Imaging systems in medical diagnostics, Krestel
  ed., Siemens, 1990
• The physics of diagnostic imaging, Dowsett et al,
  Chapman Hall, 1998