Download Radiographic Image Production

Principles of Imaging Science I (RAD 119)
Film, Screens, and Cassettes
Radiographic Image Production
 X-ray photons emitted from the x-ray tube interact with the
body, exit the patient (exit beam) and interact with the image
receptor (image forming x-rays)
 PE Absorption
 Compton Scatter
Radiographic Image Production
 Image receptor converts the exit beam into a latent image
through fluorescence of intensifying screen phosphors
 X-ray film is one medium that captures the diagnostically
useful information to render a diagnosis
 Image intensifier and associated TV monitor
 Laser imaging (CT, MRI, CR)
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Photosensitive Film
 First image receptor chosen by Röntgen
 No longer most common image receptor because of
predominance of digital imaging in 21st century
 Knowledge of film/screen technology still required of
radiographers
Radiographic Film Construction
 Base
 Adhesive layer
 Emulsion
 Supercoat (overcoat)
Radiographic Film Base
 Foundation of radiographic film
 Flexible, yet durable
 Uniformly radiolucent
 Film must retain its size and shape throughout use and
processing
 Thickness ranges from 150-200 micrometers
 Modern base is thin sheet of polyester
• Dimensional stability to minimize distortion
• Semi-rigid for viewbox placement
 Blue tint
 Reduces eyestrain
 Increases diagnostic accuracy
 Crossover
 Light from one screen exposed opposite emulsion
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Film Emulsion (3-5 um)
• Latent image produced by energy deposited in silver halide
crystals
• Distribution of exposed and unexposed crystals in
undeveloped film
• Double layer
Film Emulsion
• Gelatin
– Provides support for silver halide crystals
– Permits crystals to be suspended evenly
– Radiolucent, low Z#
– Allows processing chemicals to penetrate through
• Silver halide crystals
–
Silver (+), Bromine (-), Iodine (-) combine in crystal lattice structure
• Silver Bromide (@98%)
• Silver Iodide or Silver Chloride (@2%)
– Energy deposited in silver halide crystals represents anatomy
• Processing
– Latent image converted to manifest image
LATENT IMAGE FORMATION
 Latent image: Invisible image; distribution of exposed and
unexposed crystals in undeveloped film
 Gurney-Mott Theory
 X-rays cause ionization of the of AgBr crystal
 Free electrons gravitate to the sensitivity speck
 Positive Ag+ crystals attracted to negatively charged sensitivity
speck
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LATENT IMAGE FORMATION
Supercoat/Overcoat
 Protective layer of gelatin
 Provides sturdiness to unexposed radiographic film
 Antistatic
 Reduces damage from scratches, pressure, or contamination
during storage, handling, and processing
Film Storage and Handling
 Prior to exposure:
 Storage
 On end
 Age
 Expiration dates
 Rotate stock
 Heat
 Below 68ºF (20ºC)
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Film Storage and Handling
 Prior to exposure:
 Humidity
 30-60%
 Light
 Radiation
Film Identification
 Each exposure must include medical record
information
 Date of exposure
 Full name of patient
 Name of institution
 Name of ordering physician
 Patient identification number
 Type of exam
Screen Film
 Most widely used type of film
Standard Screen Film Sizes
English Unit
(inches)
 Placed between intensifying
screens in a cassette
 Other Types
18 X 43
8 X 10
 Direct Exposure
 Mammography
SI Unit
(cm)
20 X 25
24 X 30
 Laser
10 X 12
28 X 35
 Duplicating
14 X 14
35 X 35
14 X 17
35 X 43
 Spot
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Screen Film Characteristics
 Speed based upon sensitivity of x-ray & light for Image Receptor
(IR)
 Film speed has limitations, due to ideal thickness of emulsion
layer. Light is absorbed quickly on outermost layer and will not
expose portion closest to base if too thick
 Double emulsion increases speed
 Contrast
 High, Medium, Low
 High: Sm silver crystals, uniform size
 Low: Lg silver crystals, non-uniform size
 Multiple Latitude available (inversely related to contrast)
Screen Film Characteristics
 Spectral Matching
 Response to spectrum of light emitted by screen
 Rare earth phosphors Z# 57 – 71
 Emit UV, blue, green and red
 Screen film responds to UV & blue, not green, yellow,
or red unless spectral dye is added
 Radiographic film is either blue-sensitive or green
sensitive
 Proper matching avoids technique error
Spectral Matching
 Blue Sensitive Film
 Blue or UV emitting screens
 If used with Green emitting
screens, IR speed is reduced
 Green Sensitive Film
 Green emitting screens
Radiographic films are either bluesensitive or green-sensitive and they
require amber-and red-filtered
safelights, respectively.
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Screen Film Characteristics
 Safelight Filters
 Used in darkroom processing to prevent exposure to
white light
 15 watt bulb, > 5 feet from work surface
 Provides sufficient illumination to process film
 Incandescent bulb with colored filter
 Amber filter: Blue sensitive film
 Fog a green sensitive film
 Red filter: Blue or green sensitive film
(orthochromatic)
Screen Film Characteristics
 Crossover
 Light from intensifying screens crosses the base and exposes the
opposite side emulsion.
 Image blur results
Screen Film Crossover
 Minimized by:
 Tablet or cubic silver halide crystals
 Crossover control layer between base and
emulsion
 Absorbs crossover light
 Remains as separate, distinct layer
 Removed during processing
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Screen Film Characteristics
• Reciprocity Law
– Short or long exposure times reduce speed due to
exposure from intensifying screen light
– Increases patient exposure due to increase in mAs needed
– Relative speed ratings
• 1 ms =
95%
• 10 mS = 100%
• 100 mS = 100%
• 1 sec = 90%
• 10 sec = 60%
Intensifying Screens
• Used to reduce patient radiation dose
without compromising anatomic detail
• Converts image forming x-ray photon
energy to light energy forming the
latent image through amplification
 99% of image comes from light conversion
 1% of image comes from direct interaction
between x-rays and film
• Screen-film is placed between two
intensifying screens within the cassette
– Cassette is light tight film holder
– Open 2 – 3” to retrieve or replace film
• Screen Artifacts
Construction
 Base
 Phosphor layer
 Reflective layer
 Protective coat
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Screen Construction
• Protective Coating
• Thin plastic outer layer
• Protects phosphor layer from
abrasion
• Resists abrasion, eliminates static
build-up
• Artifact on film
• Phosphor Layer
– Emits light when exposed with x-ray energy
– Active layer
– High atomic # (Y, La, Gd)
• Reflective Layer
- Magnesium oxidide, titanium dioxide
– Redirects isotopic light photons back to film
• Base
– Polyester support inner layer
– Supports phosphor layer
Intensifying Screen Reflective Layer
• Screens without reflective layers are not as efficient as
those with reflective layers because fewer light
photons reach the film.
 Allows for reduction of patient dose
Luminesence
 Luminescence occurs when an outer-shell electron is raised to an
excited state and returns to its ground state with the emission of a light
photon
 Types
 Fluorescence
 Visible light is emitted only while the phosphor is stimulation
 Phosphorescence
 Visible light continues to be emitted after stimulation
 Screen lag or afterglow
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Screen Characteristics
 Speed
 Quantifiable value used to denote the conversion efficiency of x-ray
energy to light energy
 Phosphor type
Calcium Tungstate (RSS 50 – 200)
 5% conversion efficiency
o Blue emission
 Rare Earth (RSS 80 – 1200)
 15% - 20% conversion efficiency
o Ytrium, lanthanum, gadolinium
o Blue or green emission

 Phosphor size
 Phosphor layer thickness
 Reflective layer
CONVERSION EFFICIENCIES OF RARE
EARTH AND CALCIUM TUNGSTATE
Beam Absorption
 Calcium tungstate
 Absorb 20-40% of beam
 Rare earths
 Absorb 50-60% of beam
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Film/Screen Contact
Cassette
 Front surface (x-ray tube

side) made of low atomic #
material (Carbon fiber Z#6)
 Back surface made of high
atomic # material to absorb
backscatter radiation
Cross-sectional view of cassette containing front and
back screens and loaded with double-emulsion film.
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