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RADIATION PROTECTION IN
DIAGNOSTIC RADIOLOGY
Chapter 3
Part 1 : Radiography
Content
 Intensifying screen structure and
characteristics
 Screen film combination
 Radiographic film structure and
characteristics
 Anti scatter grid
 Darkroom
 Viewing Box
Overview
• To become familiar with basic knowledge
of the component that form the
radiographic chain.
Topic 1 : Intensifying screen structure and
characteristics
Primary beam attenuation and
latent image
Film, fluorescent screen
or image intensifier
Scattered
radiation
Bone
X
« Latent »
radiological
image
Soft
tissue
Air
Primary
collimation
Antiscatter Grid
Beam intensity
at detector level
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
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
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)
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) (LaOBr:Tm) (Gd2O2S:Tb)
 more sensitive and effective than (CaWO4)
Intensifying screen
characteristics (I)
• IF (Intensifying Factor): ratio of exposures giving the
same film optical density, with and without screen
• 50 < IF < 150 (depending on screen material and X-ray beam energy)
• QDE (Quantum Detection Efficiency): fraction of photons
absorbed by the screen
• 40% for CaWO4 < QDE < 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 <  < 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%
Relative Sensitivity of Film
Intensifying screen
characteristics (II)
Sensitivity of a Conventional Film
BaSO4:Eu,Sr
YTaO4:Nb
BaSO4:Pb
CaWO4
250
300
UV
350
400
Blue
450
500
550
Green
600
Intensifying screen
characteristics (III)
Intensifying factor: ratio of exposures giving the same
film optical density, with and without screen
175
150
Gd2O2S
125
100 LaOBr
75
50
CaWO4
25
0
50 60 70 80 90 100 110 120
kV
Topic 2 : Screen film combination
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
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
 Quantum
noise, Screen noise, Granularity
Detection Efficiency (QDE): the quotient of
signal to noise ratios (SNR) of radiographic image and
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

Topic 3 : Radiographic film
structure and characteristics
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
Radiographic film structure
Protective layer
Film emulsion
Binding layer
Anti cross-over (optional)
Filter
Supporting Base
Single emulsion film
‫‪Film Developing‬‬
‫‪ )1‬نورباعث يونيزه شدن ‪ AgBr‬شده و يون ‪ +Ag‬ايجاد مي‬
‫كند‪.‬‬
‫‪ )2‬وقتي فيلم در ‪ Developer‬قرار گرفت يون‪ +Ag‬احيا شده‬
‫وذرات ‪ Ag‬كه سياه رنگ مي باشند بدست میآیند‪.‬‬
‫‪ )3‬فيلم در ‪ fixer‬كه محلول سخت كننده است قرار مي گيرد‪،‬‬
‫ذرات ‪ AgBr‬شسته شده تا ديگر نور روي فيلم اثر نگذارد‪.‬‬
‫‪Optical Density‬‬
‫• سياهي امولسيـون فتوگرافي دانسيته نام دارد‪.‬‬
‫• وقتي فيلم اكسپوز شده روي ‪ Viewing box‬قرار مي گيرد ‪ Li‬شدت نور‬
‫وارد شده به فيلم و ‪ Lt‬شدت نور عبور كرده از فيلم مي باشد و دانسيته ‪ D‬از‬
‫لگاريتم نسبت انتقال بدست مي آيد‪.‬‬
‫‪L t  Li 10  D‬‬
‫‪‬‬
‫‪D  Log  L t /Li ‬‬
‫• اگر شدت نور ازفيلم عبور كند دانسيته برابر ‪ Log10 =1‬مي شود‪.‬‬
‫• اغلب اطالعات تشخيص ي دانسيته بين ‪ 0.5‬تا ‪ 2‬را دارند‪.‬‬
‫•‬
‫• اختالف بين دانسيته دو نقطه‪ DD‬بنام كنتراست راديوگرافي ميباشد‪ .‬اين‬
‫كنتراست باعث تشخيص تغييرات توسط راديولژيست ميشود‪.‬‬
Characteristic curve of a
radiographic film
Optical
Density (OD)
D
Saturation
2
Visually evaluable
range of densities

D1
Base
+ fog
 = (D2 - D1) / (log E2 - log E1)
The  of a film : the
gradient of the «straight
line» portion of the
Normal range characteristic curve
of exposures
E1
E2
Log Exposure (mR)
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
Comparison of characteristic
curves
(OD)
(OD)
Film
A
Film
B
Log Exposure (mR)
Film A is faster
than Film B
Film A and B
have the same
contrast
Film
A
Film
B
Log Exposure (mR)
Film A and B
have the same
sensibility but
different
contrast
Sensitometric strip
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Sensitometry: A method of exposing a film by
means of a light sensitometer and assessing
its response to exposure and development
Topic 4 : Anti-scatter grid and grid
performance parameters
IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources
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
Anti-scatter grid (II)
Source of X-rays
Patient
Scattered X-rays
Lead strip
Useful X-rays
Film and cassette
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
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
Example of anti-scatter grids
(grid ratio)
Grid : C
Grid : A
Grid : B
D
h


1
h
Grid ratio : r = =
tg
D

5 < r < 16
• Grid A and B have the same strip number
• Grid B and C have the same interspace between the lamella
Grid selectivity (II)
% of scattered beam transmitted
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
5%
3.8%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
r
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)
Grid focusing error
(leading to 25% of beam loss)
GRID
CHARACTERISTICS
Shortest
distance
Longest
distance
Focalization
(cm)
80
Ratio
r
7
(cm)
(cm)
68
96
80
10
72
91
100
10
87
116
100
14
91
110
150
13
130
180
Grid out of center
(virtual deformation of grid shadow)
X-Ray source
Lateral shift
Film and
cassette
Grid
Grid shadow
Grid focusing error due to lateral shift
(leading to 25% loss of X-ray beam)
GRID
CHARACTERISTICS
MAXIMUM
LATERAL SHIFT
Focalization
(cm)
80
Ratio
r
7
(cm)
80
10
2
100
10
2.5
100
14
1.8
150
13
2.9
2.8
Topic 5 : Darkroom and viewing box
Darkroom characteristics
• Safelight
– number (as low as possible),
distance from the table
– type and colors of filters
– bulb color (red)
– power (< 25 W)
• External light tightness
• Room temperature < 20°
• Film storage conditions
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
Viewbox brightness
5700
5810
5610
6200
5920
EXAMPLE OF
MEASUREMENTS
6110
6130
5860
6090
5920
CORRECT
CONFIGURATION
(cd/m2)
Viewbox color and brightness
5700
5810
3510
6200
5920
3870
BLUE
COLOR
4160
5860
2150
3110
WRONG
CONFIGURATIONS
(cd/m2)
WHITE
COLOR
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