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FRCR: Physics Lectures
Diagnostic Radiology
Lecture 4
Film-screen radiography
Dr Tim Wood
Clinical Scientist

Overview
• Film-screen radiography
• Processing
• Intensifying screens and the film cassette
• The characteristic curve and sensitivity
• Image quality

The story so far…
• We know how X-rays are made in the X-ray tube and
how they interact with the patient
• We know how we control the quality and intensity of the
X-ray beam, and hence patient dose, with kVp, mAs,
filtration and distance
• We discussed the main descriptors of image quality
– Contrast
– Spatial Resolution
– Noise
• Discussed ways to improve contrast by minimising
scatter and using contrast agents
• Remember, there is always a balance between patient
dose and image quality – fit for the clinical task!

Film-Screen Imaging
• Traditionally, all X-ray image capture has been
through X-ray film
Film base
Emulsion
Emulsion
Adhesive
layer
Protective
layer

Film
• Polyester film base gives mechanical strength to
the film – does not react to X rays
• Emulsion consists of silver halide grains (AgBr)
– The image is formed by the reaction of AgBr grains to
X-ray photons
– The sensitivity of the film depends on number of grains
– Must be evenly distribution
– Typically each crystal is about 1 μm in size
• larger grains = more sensitive (contrast),
• smaller grain = better resolution
• Adhesive layer ensures emulsion stays firmly
attached to base
• Protective layer prevents mechanical damage

Film
• Film is actually much more sensitive to visible
light and UV than it is to X-rays
– Hence, use a fluorescent screen to convert X-ray
photons to light photons
– Enables lower patient dose!
• A latent image is formed upon exposure, which
cannot be seen unless the film undergoes
chemical processing
– Mobile silver ions are attracted to electrons liberated
by light photons, forming a speck of silver metal on
the surface

Processing
• The invisible latent image is
made visible by processing
• There are three stages to this
process;
– Development
– Fixing
– Washing

Processing
• First stage is development:
– Film is immersed in an alkaline solution of a reducing
agent (electron donor)
– Reduces positive silver ions to metallic grain of silver
(black specks)
– Unexposed crystals are unaffected by the developer –
bromide ions repel the electron donor molecules
– However, given sufficient time, the developer will
penetrate the unexposed crystals
– The amount of background fog is dependent upon the
time, strength and temperature of the developer

Processing
• Second stage is fixing:
– If the film is exposed to light after the first stage, the
whole film becomes black
– To ‘fix’ the film, unaffected grains are dissolved by an
acid solution, leaving the X-ray image in the form of
black silver specks
• Final stage is washing:
– The film is washed in water and dried with hot air
– Inadequate washing would result in a brown/yellow
film over time (from excess acid) and smell

Processing
• Automatic processors use a roller system to
transfer the film through the different solutions
• Regular Quality Assurance of the processor is
vital for producing good quality radiographs
• Image is then viewed by transmission of light
from a light box with uniform brightness
– Dark = lots of X-rays
– Light = relatively few X-rays e.g. through bone

Production of a Radiograph
Process Time What Happens
1. Manufacture Crystals of a suitable size
are made and suspended
in gelatine
2. Exposure 0.01 – 10 sec Latent image created
3. Wetting 10 sec Wet film so that
subsequent development
is uniform
4. Development 3 – 10 min Convert latent image to
silver
5. (Acid) wash 1 min Stop development and
remove excess developer
6. Fixing and hardening 10 – 30 sec Dissolve out remaining
AgBr and harden gelatine
7. Washing 30 sec Remove products of
developer and fixer
8. Dry 30 sec Remove water

Logarithms
• A logarithm is an exponent – the exponent to
which the base must be raised to produce a given
number
– 104 = 10x10x10x10 = 10,000
– = log1010000 = 4
– i.e., 4 is the logarithm of 10000 with base 10
• Seen in many applications
– Richter earthquake scale
– Sound level measurements (decibels = dB)
– Optical Densities blackness on film (OD)
• Written as log10x or if no base specified in physics
texts as log x it is interpreted as the same

Properties of logs
• log101 = 0
• log1010 = 1
• log10xy = log10x + log10y
• log10x/y = log10x - log10y

Optical Density
• Optical Density: the
amount of blackening in
the film
• Defined as the log of the
ratio of the intensities of
the incident and
transmitted light
– log is used as the eyes
response is logarithmic

Optical Density
• Optical density can be measured with a
densitometer
• From the definition, if 1% of light is transmitted,
D = 2.0
• If 10% is transmitted, D = 1.0
• The density of an area of interest on a
properly exposure film should be about 1.0
– Lung field may be ~2.0
• Areas with D>3.0 too dark to see any detail on a
standard light box

Contrast
• Contrast is the difference in optical densities
Contrast = OD1 – OD2
• High contrast - e.g. black and white
• Low contrast – e.g. grey and grey!

Intensifying screens
• Film is relatively insensitive to X-rays directly
– Only about 2% of the X-rays would interact with the
emulsion
– Requires unacceptably high doses to give a
diagnostic image
• An intensifying screen is a phosphor sheet the
same size as the film, which converts the X-rays
to a pattern of light photons
• The intensity of the light is proportional to the
intensity of X-rays
• The pattern of light is then captured by the film
– One exception is intraoral dental radiography, where
screens are not practical

• Modern intensifying screens use rare earth
materials, which emit light that is matched to the
sensitivity of the film being used
– Spectral match between the emission of the screen
and the absorption in the film e.g. blue or green
– K-edges clinically relevant (39-61 keV)
• Rare earth screens used as they very efficient at
converted absorbed X-ray energy into light
– Results in a ‘faster’ (more sensitive) system
• The sensitive emulsion of the film must be in
close contact with the screen

• General radiography film usually double coated
with emulsion on each side of the base
• The front screen absorbs ~1/3 of X-rays and
~1/2 light travels forward and is absorbed by
front layer of emulsion
• Rear screen absorbs ~1/2 of X-rays transmitted
through the front and exposes the rear emulsion
• ~2/3 of total X-ray fluence absorbed in screens
• Mammography only uses a single screen to
maximise spatial resolution (more on this later)
• Screen materials chosen to have no
phosphorescence (delayed fluorescence) to
avoid ghost images

The film-cassette
• Flat, light tight box with
pressure pads to ensure
film in good contact with
the screen(s) mounted on
the front (and back)
• The tube side of the
cassette is low atomic
number material (Z~6) to
minimise attenuation
• Rear of cassette often lead
backed to minimise back
scatter (not in mammo)

The characteristic curve
• Plotting OD against
log exposure gives
the Characteristic
Curve of the X-ray
film
• Different types of film
– subtle differences
but all basically the
same
Log exposure
Optical
density
Fog
Linear region,
gradient = gamma
Saturation
Solarisation

• Depends on type of film, processing and storage
• Fog: Background blackening due to
manufacture and storage (undesirable)
– Generally in the range 0.15-0.2
• Linear portion: useful part of the curve in which
optical density (blackening) is proportional to the
log of X-ray exposure
• The gradient of the linear portion determines
contrast in an image and patient exposures
must lie within this region
– Need to match this to the clinical task!
• Hence, film suffers from a limited and fixed
dynamic range

• Gradient of linear region =
Gamma,  = OD2 – OD1
log E2-log E1
• Gamma depends on
– Emulsion
– Size and distribution of
grains
– Film developing
• Gamma ~ Contrast
• Latitude = useful range of
exposures
Linear
region
Latitude
Log exposure
Optical
density

• Gamma and latitude are inversely related
– High gamma = low latitude
– Wide latitude (low gamma) for chests
– High gamma (low latitude) for mammography
• At doses above the shoulder region, the curve
flattens off at D~3.5
– Saturation, whereby all silver bromide crystals have
been converted to silver
• At extremely high exposures density will begin to
fall again due to solarization
– Not relevant to radiography

Film Speed
• Definition: 1 / ExposureB+F+1
• Reciprocal of Exposure to cause an OD of 1
above base plus fog
• Speed of film = sensitivity = amount of radiation
required to produce a radiograph of standard
density
• Speed shifts H-D curve left and right
• Fast film requires less radiation (lower patient
dose)
• Speed is generally used as a relative term
defined at a certain OD; one film may be faster
than another at a certain point on the curve

Factors affecting speed
• Size of grains – larger means faster
– This is the main factor and conflicts with the need for
small crystals to give good image sharpness.
– Fast films are grainier but reduce patient dose
• Thickness of emulsion
– Double layers of emulsion give faster films
• Radiosensitisers added
• (X-ray energy)

Effect of developing conditions
• Increasing developer temperature, concentration
or time increases speed at the expense of fog
• Developer conditions should be optimised for
maximum gamma, and minimum fog
• Automatic processor has temperature controls
and time maintained by roller speed
• Concentration is controlled by automatic
replenishment of the chemicals

Film-screen sensitivity
• Intensification factor
– Each X-ray photon generates ~1000 light photons
– Just under half of these will reach the film
– ~100 light photons to create a latent image
– Hence, more efficient process
– Intensification factor is the ratio of air KERMA to
produce D = 1 for film alone, to that with a screen
– Intensification factor typically 30-100
• Speed class
– Most common descriptor of sensitivity
– Speed = 1000/K, where K is air KERMA (in μGy) to
achieve D = 1
– Typically 400 speed (K = 2.5 μGy)

Image quality
• Contrast
– Contrast in film-screen radiography is due to both
subject contrast, scatter and gamma
– Remember, high gamma = high contrast = low
latitude (and vice-versa)
– Contrast is fixed for any given film and processing
conditions
– Image detail may be lost if contrast is too high as it
may be lost in the saturated or fog regions
– Hence, vital to match gamma to the clinical task
– Ambient light conditions and viewing box uniformity
may also impact on the subjective contrast presented
to the Radiologist
• Use a darkened room, mask off unused areas of lightbox, etc

Image quality
• Screen-unsharpness
– The film-screen system has inherent unsharpness
additional to geometric, motion and absorption
– Only partly due to finite size of the emulsion crystals
– Most significant effect is due to spread of light from
the point of X-ray absorption in the phosphor, to
detection by the film
– Depends on the point in the phosphor where the
interaction occurs
– Thicker phosphor layers more sensitive (absorb more
X-rays), but result in more blurring – allow lower
patient doses

Screen-unsharpness
Film
Phosphor
Object

Screen unsharpness
• Speed class should be chosen carefully to
match the application
– e.g. 400-speed (thick phosphor) for thick sections of
the body (abdo/pelvis),
– e.g. 100-150-speed (thin phosphor) for extremeties
(require detail)
• Also may have reflective layer on top of
phosphor to increase sensitivity (reflect light
photons back to the film) at the expense of
resolution
• Colour dyes to absorb light photons at wider
angles (longer path lengths) – at the expense of
sensitivity

Screen unsharpness
• Crossover – light photons from the front screen
may be absorbed by the rear emulsion (and
vice-versa)
– Crossover is a significant contributor to overall
unsharpness
– Reason for only using one screen in mammography
where resolution is critical
• Minimise screen-unsharpness by ensuring good
contact between the screen and film
– Poor contact may result from damage to the film
cassette

Film-screen in clinical practice
• Kilovoltage: Increased kV gives…
– Increased penetration = lower patient dose
– Increased exposure latitude = larger range of tissues
displayed, BUT lower radiographic contrast
– Reduction in mAs = shorter exposures = less motion
blur
• mAs
– Correct mAs must be chosen to ensure the
correct level of blackening on the film – avoid
under or overexposing the film
• Too much = saturation, too little = ‘thin’ image
– Produce standard protocols that can be adapted for
patient size

Exposure Control
• For an acceptable image, require a dose at the
image receptor of about 3 μGy for film-screen
radiography
• This is the exit dose from the patient after
attenuation
• Entrance surface dose (ESD) is much higher
than this;
– ~10 times greater than exit dose for PA chest
– ~100 times greater for skull
– ~1000 times greater for AP pelvis
– ~5000 times greater for lateral lumbar spine

Automatic Exposure Control (AEC)
• Limited latitude of film makes it difficult to
choose correct mAs – skill and experience of
radiographer
• Alternative is to use an AEC to terminate the
exposure when enough dose has been delivered
to the film
• AEC is a thin radiation detector (ionisation
chamber) behind the grid, but in front of the film
(though in mammo it is behind to avoid imaging
the chamber on the film)
• Usually three chambers that can be operated
together or individually

Automatic Exposure Control (AEC)
• When a predetermined level of radiation is
detected, the exposure terminates
• Choice of chambers determined by clinical task
– e.g. left and right for lungs in PA chest, but central if
looking at spine
• Also has a density control that can increase or
decrease exposure where necessary
• AEC limited to exposures in the Bucky system

Modern Day
• Film is dying out
• Across most (but not all) of the country film
is no longer used for General X-ray
imaging
• Only mammography (breast imaging),
where very high resolution specialist film is
used
– This Trust no longer uses film for
mammography, and is on the verge of being
fully digital…

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