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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….

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frcr physics lectures diagnostic radiology

FRCR: Physics LecturesDiagnostic Radiology

Lecture 4

Film-screen radiography

Dr Tim Wood

Clinical Scientist

overview
Overview
  • Film-screen radiography
  • Processing
  • Intensifying screens and the film cassette
  • The characteristic curve and sensitivity
  • Image quality
the story so far
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
Film-Screen Imaging
  • Traditionally, all X-ray image capture has been through X-ray film

Emulsion

Protective layer

Adhesive layer

Film base

Emulsion

slide5
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
slide6
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
Processing
  • The invisible latent image is made visible by processing
  • There are three stages to this process;
    • Development
    • Fixing
    • Washing
processing1
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
processing2
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
processing3
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
logarithms
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
Properties of logs
  • log101 = 0
  • log1010 = 1
  • log10xy = log10x + log10y
  • log10x/y = log10x - log10y
optical density
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 density1
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
  • 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
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
intensifying screens1
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
intensifying screens2
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
the film cassette
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
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

Fog

Log exposure

the characteristic curve1
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 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
the characteristic curve2
The characteristic curve

Optical density

  • 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

the characteristic curve3
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
film speed
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 radiationrequired 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
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
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
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
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 quality1
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
Screen-unsharpness

Object

Phosphor

Film

screen unsharpness1
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 unsharpness2
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
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
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
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 aec1
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
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…