RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

L15.1: Optimization of protection in radiography: technical aspects

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 15.1: Optimization of protection in radiography: technical aspects 2

Topics

Intensifying screen structure and characteristics

Screen-film combination

Radiographic film structure and characteristics

Antiscatter grid

Film processor

Darkroom and View Box

Image parameters


Overview

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


Part 15.1: Optimization of protection in radiography

Topic 1: Intensifying screen structure and characteristics



Primary beam attenuation and latent image

Bone

Air

Soft tissue

X

Primary collimation

Film, fluorescent screenor image intensifier

Beam intensityat detector level

« Latent »radiological

image

Antiscatter Grid

Scatteredradiation


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 light-sensitive emulsion of the radiographic film (X Ray light photons)

• Reduce the patient dose needed to achieve a given level of film quality

• Reduce the exposure time as well as the power required from the X Ray generator (cost savings)

• Increase photoelectric effect better use of the beam energy (image formation)


Intensifying screen structure (I)

• Supporting Base (mainly polyester material) • chemically neutral, resistant to X Ray exposure, flexible

• Reflecting layer (Titanium dioxide - TiO2)• a crystalline compound reflecting photons toward

sensitive emulsion

• Fluorescent layer (polymer)• crystals dispersed in a suspension of plastic material

• Protective overcoat • colourless thin overcoat to help avoid abrasions of

fluorescent layer due to the use of screen


Supporting Base (240 m)

Reflecting layer (25 m)

Fluorescent layer (100 to 400 m)

Protective overcoat (20 m)

(Incident X Ray beam)

(Light-sensitive film)

Screen

Intensifying screen structure (II)


Intensifying screen structure (III)

• The fluorescent layer

• 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:Tb and Gd2O2S:Tb more sensitive and effective than CaWO4


Intensifying screen characteristics (I)

• IF (Intensification 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)


Intensifying screen characteristics (II)

Conversion efficiency—

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 wave length 90%


250 300 350 400 450 500 550 600

CaWO4

BaSO4:Pb

BaSO4:Eu,Sr

YTaO4:Nb

Sensitivity of a Conventional Film

UV Blue Green

Rel

ativ

e S

ensi

tivi

ty o

f F

ilm

Intensifying screen characteristics (III)


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

050 60 70 80 90 100 110 120

CaWO4

LaOBr

Gd2O2S

kV

Inte

nsi

fyin

g f

acto

r



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 combination performance

• Spatial Resolution: capability of a screen film combination to record and display a test pattern specified in cycles/mm. 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: Noise as a function of frequency• Quantum Detection Efficiency (QDE): Measure of

combined effect of signal and noise performance as a function of frequency


Screen film combination performance

• Assure that screen emission spectrum matches sensitivity of film being used

• Screen film contact • loss of spatial resolution

• blurred image

• Cleanliness

• Inter cassette sensitivity


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 exposure film ~ 30 c/mm; 200 speed screen-film system ~ 10 c/mm; 400 screen-film system ~ 6 c/mm; mammography system ~ 15 c/mm



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



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


Radiographic film structure

Emulsion (~5-20 µm thick)

Base (~200 µm thick)

Supercoat

Adhesive layer

Single Emulsion Film

Anti-curl,anti-halation layer


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


Radiographic film structure

Emulsion

Base

Supercoat

Emulsion

Adhesive layer

Double Emulsion Film

Adhesive layer

Supercoat


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


Processing

• Development• Converts latent image to metallic silver

• Fixing• Dissolves unexposed silver halide crystals,

leaving only metallic silver, creating a permanent image


Steps in image formation


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) light

• Safelights must not affect film


Spectral response of film


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


Crossover


Optical density

Optical Density = log10 I0 / It

Film

I0It

Transmitted light intensity

Incident light intensity

e.g. 10% transmission = 1.00 1% transmission = 2.00


Characteristic curve of a radiographic film

Optical Density (OD)

Visually evaluablerange of densities

Base + fog

Saturation

Log Exposure (mR)

Normal range of exposures

The of a film: the gradient of the «straight line» portion of the characteristic curve

OD2

OD1

E1 E2

= (OD2 - OD1) / (log E2 - log E1)


Average gradient

• The straight line portion of the characteristic curve is difficult to determine (and there may not be one), so the average gradient is measured between optical densities of 0.25 and 2.00


Film sensitometry parameters

• Base + fog: The optical density of a film due to its base density plus any action of the developer on the unexposed silver halide crystals usually 0.15 -0.30.

• Sensitivity (speed): The reciprocal of the exposure value needed to achieve a film net optical density of 1.00

• Gamma (contrast): The average gradient of the characteristic curve

• Latitude: The range of exposures that can be recorded and visualized on the film.


Comparison of characteristic curves

Log Exposure (mR)

Film A

Film B

Film A and B have the same

contrast

Film A is faster than Film B

OD OD

Film A and B have the same sensitivity but

different contrast

Film B

Film A

Log Exposure (mR)

1+B+Fog


Sensitometric strip1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Sensitometry: A method of exposing a film by means of a light sensitometer and assessing its response to exposure and development


Sensitometric strip


Latitude

Film B has higher latitude(range of useful exposures)than film A, but has lowercontrast (slope of the curve)



Topic 4: Anti-scatter grid and grid performance parameters



Anti-scatter grid (I)

• Radiation emerging from the patient• primary beam: contributing to the image formation

• scattered radiation: reduces contrast

• 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

Lead stripScattered X Rays

Useful X Rays Film and cassette

Patient


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 total radiation without the anti-scatter grid in a

specified radiation beam to that with the anti-scatter grid placed in the beam


Grid performance parameters (II)

• Strip number• The number of attenuating lead strips per cm

• Grid focusing distance• Distance between the front of a focused grid and the line formed

by the converging attenuating lead strips of the grid


Example of anti-scatter grids (grid ratio)

h

D

h D

Grid ratio: r =1 tg=

Grid: A Grid: B

Grid: C

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

5 < r < 16


Grid selectivity(I)

Grid: A Grid: B

Grid: C


Grid selectivity (II)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

100

90

80

70

6055504540353025201510

50

5%3.8%

r

% o

f s

catt

ere

d b

eam

tra

ns

mit

ted

• A grid with r = 12 transmits 5% of scattered radiation

• A grid with r = 16 transmits 3.8%

30% difference in patient dose


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)

GRIDCHARACTERISTICS

Shortestdistance

Longestdistance

Focalization(cm)

Ratior

(cm) (cm)

80 7 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

Grid

Film andcassette

Grid shadow

Lateral shift


Grid focusing error due to lateral shift(leading to 25% loss of X Ray beam)

GRIDCHARACTERISTICS

MAXIMUMLATERAL SHIFT

Focalization(cm)

Ratior

(cm)

80 7 2.8

80 10 2

100 10 2.5

100 14 1.8

150 13 2.9



Topic 5: Film processor



The automatic film processor


Automatic processors

• Constant temperature

• Constant processing time

• Automatic replenishment of chemicals

• Drying of films

BUT

• Can introduce artifacts


Film processor QC

• Most important QC features:• proper film storage

• darkroom cleanliness

• cassette and screen care

• processor chemical care

• Sensitometry and processor quality control

• artifacts

• processor cleanliness


Sensitometry (I)

• Sensitometer and densitometer required• Essential - to keep film processing under control• To be performed daily• Values to be controlled:

• base + fog• mid-density• density difference


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

IAEA

Processor quality control

• The base-plus-fog level, mid density, and density difference should be plotted on control charts (Reference Gray, et al., ACR Mammography Quality Control Manual)

15.1: Optimization of protection in radiography: technical aspects 57


Manual Processing

• There are many places where X Ray films are processed manually, in open tanks

• Manual processing can be very effective, BUT there can be many quality problems

• It is essential that the developer temperature be controlled and that the development time be selected based on the temperature


Dark room conditions in some hospitals


Film Processing

• Film processing includes:

• developer

• water wash

• fixer

• water wash

• Washing is very important to avoid chemical contamination, and to assure archival properties of the image


Basic Film Processing Requirements

• Temperature - constant and optimum (recommended by the film manufacturer)

• Time – measured, based on developer temperature

• Developer activity (chemical condition) - Properly replenished developer and fixer


Temperature (I)

• The temperature of the developer should be as recommended by the manufacturer

• Use a thermometer to check the temperature before processing


Maintaining temperature

• Ideally both developer and fixer containers should be surrounded by a water bath (as a thermal jacket)

• This water bath should maintained at the temperature specified by the film manufacturer


Manual processing tanks

Water bathsurroundingtanks (not filled here)


Chemical activity

• The correct chemicals for manual processing must be used

• Chemicals should be replenished daily based on the film (chemical) manufacturer’s instructions


Chemical activity

• 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.


• As films are developed, the developer and fixer chemicals become depleted

• Compensate for this by proper replenishment of the chemicals

• Also, air will oxidise the developer (making it turn brown)

• Both will cause underdevelopment and poor quality X Ray films

Chemical activity (IV)


• 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

• Step wedge

• Uniform block of acrylic at least 20 cm thick

Measuring Chemical Activity (I)


• Procedure

• Start with properly mixed, fresh chemicals at the temperature specified by the manufacturer

• 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 (using correct temperature and processing time) and use as reference

• compare processor check film with standard film to check chemical activity

Measuring Chemical Activity (II)


• Signs that developer activity is low• Loss of film contrast

• Loss of overall film density

• Replace developer and fixer (at the same time) if developer activity is low

Measuring Chemical Activity (III)


• Signs that fixer activity is low

• films take longer to ‘clear’

• Replace fixer if activity is low

Measuring Chemical Activity (IV)


• 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)

• There should be a continuous flow of water through the wash tank at the same temperature as the developer and fixed

Washing



Topic 6: Darkroom and view box



Darkroom characteristics

• Safelight • number (as low as possible),

correct distance from the table

• type and colours of filters

• power ( 15 W or less)

• No light leaks from outside

• Room temperature < 20°-22°

• Film storage conditions


Since the viewing conditions are essential for a good interpretation of the diagnostic images, the viewing conditions must be optimal

• Cleanliness of external and internal surface• Brightness (luminance)

• homogeneity of different viewing boxes: 1300 - 2000 cd/m2

• homogeneity within the same viewing box• Colour

• colour mismatch must be avoided• Environment (illuminance)

• ambient light level: 50 lux maximum

Viewbox characteristics


Viewing box brightness

EXAMPLE OF MEASUREMENTS

CORRECT CONFIGURATION

(cd/m2)

5700 5810

6200

5610 6110

6130

5920 5860 6090 5920


Viewing box color and brightness

WRONG CONFIGURATIONS

(cd/m2)

WHITECOLOR

BLUECOLOR

5700 5810

6200

3510 3870

4160

5920 5860 2150 3110


Measurement of Luminance

Units: cd.m-2


Measurement of Illuminance

Units: lux


Example of poor viewing box



Topic 7: More image quality characteristics



Image quality characteristics

• Density

• Contrast

• Resolution

• Unsharpness

• Noise

• Distortion

• MTF


Factors characterising film quality

Film Geometry Subject

Density,contrast,speed,latitude

Distortion,magnification,

blur (unsharpness)

Contrast(thickness,

density,atomic number)

Processing Motion


• The difference between the optical density in two parts of a radiographic image

• 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 (e.g., screen-film system and processing)

Contrast


Subject Contrast (1)


• 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)

Subject Contrast (2)


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

Subject contrast (3)


• 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- or barium-containing contrast agents in the patient

Subject contrast (4)


Detector contrast


• Spatial resolution (or image blur) is the ability to distinguish closely spaced objects

• Resolution is measured in a number of ways, but most commonly as cycles per millimeter (c/mm)

• The higher the cycles per millimeter, the better the resolution

Resolution and unsharpness


• Resolution is affected by a number of factors:

• focal spot size

• type intensifying screen

• motion

• image noise

Spatial resolution


Focal spot

Object

Effect of focal spot on resolution


• The fluctuation of optical density in the image over very small distances

• Some noise is inherent in the imaging system, some is controllable

Noise (1)


• Noise is mostly caused by:1. The number of X Ray photons used

in the image (quantum mottle) - most important component

2. The limited absorption efficiency of X Rays by the screen (structure mottle)

3. The crystal size and distribution in film (film graininess)

Noise (2)


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

Object

Image

Magnification = image size/object size= SID/SOD

SOD

SID

Magnification


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 (view box)


References

• The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011

• The physics of diagnostic imaging, Dowsett et al, Hodder Arnold, 2006

• Quality Control in Diagnostic Radiology, Gray JE. et al. http://diquad.com/QC%20Book.html

• Mammography quality control: Radiologic technologists manual. American College of Radiology, Reston, VA. 1999

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

Documents

optimization of protection

technical aspectsiaea15

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