RADIATION PROTECTION IN DIAGNOSTIC RADIOLOGY

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RADIATION PROTECTION IN DIAGNOSTIC RADIOLOGY

• Chapter 3
• Part 1 Radiography

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Content

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

3
Overview

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

4

• 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
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Intensifying screen

• Layer of material used 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)

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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
• colorless thin film avoiding abrasions of
  fluorescent layer due to the use of screen

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Intensifying screen structure (II)
(Incident X-ray beam)
Supporting Base (240 ?m)
Screen
Reflecting layer (25 ?m)
Fluorescent layer (100 to 400 ?m)
Protective overcoat (20 ?m)
(Light-sensitive film)
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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)

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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)
• ? (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

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

• 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
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Topic 2 Screen film combination
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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

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Screen film combination performance

• Spatial Resolution capability of a screen film
  combination to display a limited number of line
  pairs per mm. It can be assessed by the Hüttner
  resolution pattern.
• 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

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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

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Topic 3 Radiographic film structure and
characteristics
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Radiographic film(structure and characteristics)

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

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Radiographic film structure
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Film Developing

• 1)  ??????? ?????? ??? AgBr ??? ? ??? Ag ?????
  ?? ???.
• 2)  ???? ???? ?? Developer ???? ???? ???Ag
  ???? ??? ????? Ag ?? ???? ??? ?? ????? ????
  ??????.
• 3) ???? ?? fixer ?? ????? ??? ????? ??? ???? ??
  ????? ???? AgBr ???? ??? ?? ???? ??? ??? ???? ???
  ??????.

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Optical Density

• ????? ????????? ???????? ??????? ??? ????.
• ???? ???? ?????? ??? ??? Viewing box ???? ??
  ???? Li ??? ??? ???? ??? ?? ???? ? Lt ??? ???
  ???? ???? ?? ???? ?? ???? ? ??????? D ?? ???????
  ???? ?????? ???? ?? ???.
• ??? ??? ??? ?? ???? ???? ??? ??????? ????? Log10
  1 ?? ???.
• ???? ??????? ?????? ??????? ??? 0.5 ??2 ??
  ?????.
•  
• ?????? ??? ??????? ?? ???? DD ???? ???????
  ?????????? ??????. ??? ??????? ???? ????? ???????
  ???? ?????????? ?????.

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Characteristic curve of a radiographic film
Optical Density (OD)
Saturation
D2
Visually evaluable range of densities
? (D2 - D1) / (log E2 - log E1)
?
The ? of a film the gradient of the straight
line portion of the characteristic curve
D1
Normal range of exposures
Base fog
Log Exposure (mR)
E1
E2
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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
• 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
Log Exposure (mR)
Log Exposure (mR)
Film A is faster than Film B
Film A and B have the same sensibility 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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Topic 4 Anti-scatter grid and grid performance
parameters
IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
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Anti-scatter grid (I)

• Radiation emerging from the patient
• primary beam contributing to the image
  formation
• scattered radiation not reaching the detector
  but 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

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Anti-scatter grid (II)
Source of X-rays
Patient
Scattered X-rays
Lead strip
Film and cassette
Useful X-rays
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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 total radiation without the anti-scatter
  grid to that with the anti-scatter grid placed in
  the beam for a similar density

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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

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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 (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
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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)
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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
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Grid focusing error due to lateral shift(leading
to 25 loss of X-ray beam)
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Topic 5 Darkroom and viewing box
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Darkroom characteristics

• Safelight
• number (as low as possible), distance from the
  table
• type and colors of filters
• bulb color (red)
• power (lt 25 W)
• External light tightness
• Room temperature lt 20
• Film storage conditions

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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
• 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

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Viewbox brightness

EXAMPLE OF MEASUREMENTS
CORRECT CONFIGURATION (cd/m2)
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Viewbox color and brightness

WRONG CONFIGURATIONS (cd/m2)
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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,
  ChapmanHall, 1998