Film Imaging and Processing Flashcards by Rachel Sladden

Image receptor - film (LOOK AT SLIDES)

Protective coat is transparent layer (usually gelatine) to protect emulsion
Emulsion has silver halide (usually silver bromide) crystals held in gelatine
Adhesive layer sticks emulsion to the base
Base is translucent blue plastic to make structure sturdy

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Image receptor - screen (LOOK AT SLIDES)

Protective layer is a transparent layer
to protect the phosphor
Phosphor can be one of several
different materials
All convert
x rays to light
Reflective layer reflects light back
towards film
Base is a strong plastic sheet to make structure sturdy

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Image receptor - screen interaction (LOOK AT SLIDES)

X-ray photon will likely interact
in phosphor layer above/
below film converting its
energy to many (~1000) light
photons
Photons are emitted in all
directions. Those that head
towards film may activate
silver halide in emulsion.
As photons spread out from
interaction point they
cause larger area of blackening
< resolution of
system

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Fluorescent materials

Calcium tungstate
Rare earth phosphors e.g. gadolinium & lanthanum
Yttrium (non-rare earth but similar properties)
-diff screens emit diff colours of light &
diff films are most sensitive to diff colours.
Thus films & screens must be matched to work
efficiently

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Speed depends on

Thickness of phosphor layer in screen
Crystal size in film emulsion
Conversion efficiency
Light absorbing dyes within screen
-rare earth screens up to 50x faster than calcium tungstate

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Thickness of phosphor layer in screen

Thicker phosphors have more chance of interaction with x-rays

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Crystal size in film emulsion

Larger crystals give more blackening but worse resolution

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Conversion efficiency

More light photons out for each x-ray photon interaction

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Light absorbing dyes within screen

Prevent the light from spreading too far increasing resolution but
decreasing speed

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Latent image formation

Film emulsion made up of silver halide crystals suspended in gelatine (>90% AgBr, <10% AgI)
When x-ray photon hits crystal in emulsion an e- from Br ion gets enough energy to escape
e- moves to defect in crystal or to Ag2S impurity known as sensitivity speck

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Latent image formation after e- moves to sensitivity speck

This becomes -ively charged and attracts mobile silver ion from within crystal to form silver atom
Ag atom then acts as trap for a second e- and
process is repeated
In this way small deposits of silver will have formed in those
crystals hit by an x-ray photon
These crystals will then be more sensitive to developer and
whole crystal will be blackened

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Developer

An alkali and acts as reducing agent
Converts remaining Ag+ ions to silver atoms by addition of e-
Process begins at sensitivity specks
-crystals which have been sensitised affected first
If film developed for too long all AgBr crystals will be converted to silver (black film)

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Intermediate washing

Stops developer working on film

Removes developer from process

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Fixing

Fixer is an acid

Removes unreacted silver halide crystals

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Fixing duration

About twice the time taken to clear

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Final wash

Fixer soln (containing silver and bromine compounds) still slightly light sensitive
Must be removed otherwise it slowly reacts with silver image producing AgS (brown stain)

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Stages

Latent image formation
Developer
Intermediate wash
Fixing
Final wash
Dry it

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Latent image fading

Silver atoms may, in time, revert to silver ions
Optical density (OD) may fall by 10-20% in 24 hours
Has implications for mobile units

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Factors affecting radiographic image

Contrast (subject and film)
Characteristics of x-ray beam
Image geometry
Image sharpness and resolution

Factors affecting subject contrast

Different target/ filter combinations
Reducing beam penetration / tube voltage
Using contrast media
Use of scatter rejection grids or air gap

Film - contrast (LOOK AT GRAPH)

film contrast dependent on film's sensitivity to x-rays
OD of 0 = 100% transmission
OD of 1 = 10% transmitted
Optical density of 2= 1% transmitted
(log scale of relative exposure)

Film - fog and gamma (LOOK AT GRAPH)

Always some level of background fog

To reduce fog keep in fridge

Film - speed (LOOK AT GRAPH)

Amount of dose it takes to blacken film to OD of 1 above base + fog (letters A-F)

Fog and scatter (LOOK AT SLIDE)

Background fog > with age of film and inappropriate storage
Scatter within px produces unwanted non-specific film density
Fog reduces contrast ratio

Ideal characteristics of x-ray beam

Parallel - to produce distortion Produced from point source to reduce penumbra (unsharpness) Sufficiently penetrating to pass through varying degree

Film faults

Dark Pale films Film back to front Opaque film

Dark (film fault)

``` Overexposed Overdeveloped ->time, > temp, > conc. Fogging -storage (radiation, light, > temp), old film, faulty cassettes, faulty darkroom/ safelights ```

Dark (film fault)

``` Overexposed Overdeveloped ->time, > temp, > conc. Fogging -storage (radiation, light, > temp), old film, faulty cassettes, faulty darkroom/ safelights ```

Pale films (fault)

Under exposed Under developed (< time, < temp, < conc, old developer, fixer contaminating developer) Film back to front

Pale films (fault)

``` Under exposed Under developed (< time, < temp, < conc, old developer, fixer contaminating developer) ```

Opaque film

Inadequate fixation or exhausted solution (AgBr remains) Inadequate final washing (becomes brown) Partially developed due to incomplete immersion in developer (hand developing)

Marks on film

``` Bending Finger marks Static Chemical Roller marks Dirty screens ```

Marks on film

``` Bending Finger marks Static Chemical Roller marks Dirty screens ```

Digital radiography

Is replacing film Can be Computed Radiography (CR) or Digital Radiography (DR) Produces softcopy images, not normally printed

Computed radiography

Requires both image plate and reader Used wherever film/ screen currently used Photo-stimulable phosphors instead of film

CR - expusure stage

An image plate stores energy from x-ray photon by trapping e-s in crystal defects

CR - readout stage

A red laser scans image, releasing e-s and energy as blue light Lightguide delivers blue light too detector and output recorded for each laser position to create an image

Digital radiography

'Expose and see' systems - very quick readout Different kind of detectors -amorphous silicon (a-Si) -amorphous selenium (a-Se)

The digital image

Greater latitude than film/screen Resolution mainly determined by pixel size/density of the CR/DR system 10 bits = 1024 grey levels (grey levels = 2n where n = no of bits) Use Look Up Tables (LUTs) to process image

Quality assurance aims

``` To produce radiographs of consistently high standard To > repeats To determine source of errors To > efficiency To < costs To < dose to pxs and staff ```

Reject analysis

Collect all rejected films and record date, nature of error, suspected cause, action taken Number of repeats Total number of films taken

Quality assurance - dark room

``` Cleanliness Light-tightness (inspection, coin test) Safelights -correct filter -distance 4ft/1.2m from bench -<25w bulb -coin test ```

Coin test

Flash exposure to make film more sensitive Place coin on film In dark (to check light-tight) or with safe Light - repeat at different times

Quality assurance - chemicals

``` Follow manufacturers instructions Correct temp Replenish every fortnight Monitor deterioration - step wedge - tooth phantom -->expose when chemicals are fresh then repeat everyday ```

Quality assurance - cassettes

``` Clean regularly Check light-tightness -place in window -develop Check film/ screen contrast -graph paper ```