file000259.ppt DIAGONSTIC RT NDTTEACHING

NDT

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

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

3. 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!

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

5. 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

6. 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 imageis 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

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

8. 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

9. 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

10. 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

11. 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 hardening10 –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

12. Logarithms
•A logarithm is an exponent –the exponent to
which the base must be raised to produce a given
number
–10
4
= 10x10x10x10 = 10,000
–= log
1010000 = 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 log
10x or if no base specified in physics
texts as log x it is interpreted as the same

13. Properties of logs
•log
101 = 0
•log
1010 = 1
•log
10xy = log
10x + log
10y
•log
10x/y = log
10x -log
10y

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

15. 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

16. Contrast
•Contrast is the difference in optical densities
Contrast = OD
1
–OD
2
•High contrast -e.g. black and white
•Low contrast –e.g. greyand grey!

17. 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

18. Intensifying screens
•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

19. Intensifying screens
•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

20. 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)

21. The characteristic curve
•Plotting OD against
log exposure gives
the Characteristic
Curveof 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

22. The characteristic curve
•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
contrastin 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

23. The characteristic curve
•Gradient of linear region =
Gamma, = OD
2
–OD
1
log E
2-log E
1
•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

24. The characteristic curve
•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

25. Film Speed
•Definition: 1 / Exposure
B+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

26. 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)

27. 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

28. 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)

29. 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 fixedfor any given film and processing
conditions
–Image detail maybe 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

30. 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

31. Screen-unsharpness
Film
Phosphor
Object

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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…