1. FRCR: Physics Lectures Diagnostic Radiology
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
Emulsion
Protective layer
Adhesive layer
Film base
Emulsion

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 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

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: Final stage is washing:
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 hardening
10 – 30 sec
Dissolve out remaining AgBr and harden gelatine
7. Washing
30 sec
Remove products of developer and fixer
8. Dry
Remove water

12. Logarithms
A logarithm is an exponent – the exponent to which the base must be raised to produce a given number
104 = 10x10x10x10 = 10,000
= log = 4
i.e., 4 is the logarithm of 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

13. Properties of logs log101 = 0 log1010 = 1 log10xy = log10x + log10y

14. 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

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 = OD1 – OD2
Contrast is the difference in optical densities
Contrast = OD1 – OD2
High contrast - e.g. black and white
Low contrast – e.g. grey and 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
Optical density
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
Saturation
Linear region,
gradient = gamma
Solarisation
Hurter-Driffield Curve, first published 1890, England
Exposure (mAs)
An increase in log exposure of 0.3 = a doubling of the exposure
Fog
Log exposure

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
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

23. The characteristic curve
Optical density
Gradient of linear region =
Gamma,  = OD2 – OD log E2-log E1
Gamma depends on
Emulsion
Size and distribution of grains
Film developing
Gamma ~ Contrast
Latitude = useful range of exposures
Linear region
Large gamma = contrasty film (uniform crystals)
Small gamma = low contrast film (different sized crystals)
Fast film, large grains, likely to be various sizes therefore low contrast
Latitude – range of exposures to give the linear response region
High gamma = small film latitude
Low gamma = greater film latitude
Darker films further to right than lighter films but contrast may be the same
Latitude
Log exposure

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 / 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

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
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 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

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
Object
Phosphor
Film

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 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…