FRCR: Physics Lectures Diagnostic Radiology

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FRCR Physics LecturesDiagnostic Radiology

• Lecture 4
• Film-screen radiography
• Dr Tim Wood
• Clinical Scientist

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Overview

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

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The story so far
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What is image quality

• Image quality describes the overall appearance of
  the image and its fitness for purpose
• Remember There is always a play-off between
  image quality and patient dose
• We only need images that are of diagnostic
  quality (fit for purpose) not pretty pieces of
  art!
• The main factors to consider are
• Contrast
• Spatial resolution
• Noise

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Image contrast

• The final contrast in the image will depend on a
  number of factors, such as
• Subject contrast an inherent property of the
  patient being imaged that will depend on the
  attenuation coefficients of the tissues (or
  contrast media), the thickness of structures, the
  nature of any overlapping tissues and the
  incident X-ray spectrum (kVp, filtration, etc
  discussed previously)
• Detector properties film and digital detectors
  each have different implications for the contrast
  in the final image (these will be discussed in
  the next lectures)
• Scattered radiation scatter can degrade image
  contrast if it reaches the detector as it conveys
  no information about where it came from. Scatter
  rejection techniques may be used to remove this.

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Contrast and scatter

• Primary radiation carries the information to be
  imaged
• Scatter obscures this as it carries no
  information about where it came from
• The amount of scatter (S) may be several times
  the amount of primary (P) in the same position
• The ratio S/P depends on thickness of patient
• For typical PA chest 41 (20 of photons carry
  useful information)
• For typical lateral pelvis 91 (10 of photons)

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Contrast and scatter

• Scatter is generally quite uniform across the
  image
• Reduces contrast that would otherwise be produced
  by the primary by a factor of (1S/P) up to 10
  times!

Intensity
SP
10
contrast reduced
S
8
Scatter 4 x Primary (PA Chest)
SP contrast (10-9)/10 x 100 10
Primary contrast (2-1)/2 x 100 50
P
2
Position
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Scatter reduction

• Scatter and patient dose may be reduced by
• Reducing the field area with collimation (reduce
  volume of tissue generating scatter)
• Compress the tissue to minimise overlying
  structures (reduce volume of tissue generating
  scatter)
• Scatter may be reduced at the expense of
  increased patient dose by
• Reducing kVp less forward scatter is produced
  and is much less penetrating (less scatter
  produced)
• Use an anti-scatter grid to remove scatter from
  the X-ray beam
• Use an air-gap to reduce the intensity of scatter
  reaching the detector

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Spatial Resolution

• Spatial resolution describes the ability to see
  fine detail within an image
• Fine detail is clearer when the contrast is high
• e.g. microcalcifications
• May be expressed as the smallest visible detail,
  but most common descriptor is the highest
  frequency of lines that can be resolved in a
  high-contrast bar pattern

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Limiting factors

• Geometric Unsharpness due to finite focal spot
  size (discussed previously)
• Movement Unsharpness imaging moving structures
  may result in blur e.g. heart, lungs, etc
• Minimise by immobilisation e.g. compression in
  mammography
• Breath hold techniques
• Use shortest possible exposure times (may be at
  the expense of high mA)
• Absorption Unsharpness gradual changes in
  absorption near tapered/rounded structures
• Minimise by careful patient positioning
• Detector resolution will be discussed in
  following lectures

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Noise

• Noise is a random, usually unwanted, variation in
  brightness or colour information in a visible
  image
• Noise is one of the most important limiting
  factors to contrast and spatial resolution
• The most significant source is quantum noise (or
  mottle) due to the low levels of radiation used
  to form an image
• Other sources include film grain and electronic
  noise in the image receptor

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Noise

• Image noise is most apparent in image regions
  with low signal level, such as shadow regions or
  underexposed images
• Noise gives a grainy, mottled, textured or snowy
  appearance to an image
• Noise can mask fine detail in a radiograph
• Noise reduces visibility of parts of a
  radiographic image
• Noise is particularly an issue with image details
  that are already of low contrast

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So what does this all really mean?
Something with lots of contrast is easier to see
than
Signal on detector
something with little contrast
Position on detector
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So what does this all really mean?
But there is some loss of sharpness due to
various process, so the sharp edge
Signal on detector
actually becomes blurred, which may not matter
for large structures
Position on detector
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So what does this all really mean?
Signal on detector
but if the structure is small
Position on detector
Signal on detector
it may start to disappear into the background
Position on detector
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So what does this all really mean?
Up to now we havent considered the third
component, noise
which may not be a problem if there isnt too
much
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So what does this all really mean?
but if there is lots of noise in the system,
the detail may be obscured
Signal on detector
whilst the low level of noise may be
problematic if contrast is low to start with
Position on detector
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So what does this all really mean?
or resultion is poor
Signal on detector
Position on detector
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Pause
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Film-screen radiography
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Film-Screen Imaging

• Traditionally, all X-ray image capture has been
  through X-ray film

Emulsion
Protective layer
Adhesive layer
Film base
Emulsion
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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

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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

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Processing

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

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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

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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

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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

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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
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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

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Properties of logs

• log101 0
• log1010 1
• log10xy log10x log10y
• log10x/y log10x - log10y

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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

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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 Dgt3.0 too dark to see any detail on a
  standard light box

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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!

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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

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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

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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

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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 (Z6) to minimise attenuation
• Rear of cassette often lead backed to minimise
  back scatter (not in mammo)

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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
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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

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The characteristic curve

• 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

Optical density
Linear region
Latitude
Log exposure
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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 D3.5
• 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

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Film Speed

• Definition 1 / ExposureBF1
• 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

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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)

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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

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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)

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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

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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

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Screen-unsharpness
Object
Phosphor
Film
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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

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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

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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

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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

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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

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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

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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