Lecture 5 Intro to digital and CR by J63o2xk8

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									      FRCR: Physics Lectures
 Diagnostic Radiology

             Lecture 5
An introduction to digital imaging and
    Computed Radiography (CR)

            Dr Tim Wood
           Clinical Scientist
                 Overview
• What is a digital image?
  – Pixels and bit depth
  – The advantages of digital
  – The disadvantages
• Computed Radiography (CR)
  – The theory of CR
  – The advantages and disadvantages
• Digital Radiography (DR)
  – The theory of DR
  – The advantages and disadvantages
              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 is a dying medium for X-ray imaging…
Digital imaging
      What is a digital image?
• A digital image can be thought of as an array of
  pixels (or voxels in 3D imaging) that each take a
  discrete value
• The value assigned is dependent on the X-ray
  intensity striking it
• Depending on its value, each pixel is assigned a
  shade of grey
• Pixel size may determine the limiting spatial
  resolution of the system
What is a digital image?
              Binary numbers
• Our conventional number system is ‘decimal’
  – There are 10 base numbers (including 0)
• Binary system has only two base numbers (1
  and 0)
• Computer memory holds a voltage signal that
  can either be a 1 or 0
  – This is known as a bit
  – With 4 bits, 16 (24) possible values can be stored e.g.
    0000, 0001, …, 1111
  – With 8 bits, 256 (28) possible values can be stored
• Computer memory is described in terms of the
  number of bytes (B)
  – 1 B = 8 bits
      What is a digital image?
• The value is stored in binary format, and the
  maximum value is related to the bit depth
  – Greater bit depth = more levels of grey in the image =
    greater potential contrast in the image
  – e.g. 12 bit = 4096 (212) levels of grey
• Bit depth and number of pixels determines the
  size of the final image on the hard-drive
  – e.g. 512 x 512 image with 4096 levels of grey
    requires 0.38 MB, but 1024 x 1024 image requires
    1.54 MB!
      What is a digital image?
• Image size is important for archiving and
  transferring to and from the PACS
• Bit depth is primarily determined by the dynamic
  range of the detector
  – Typically, 12-bit or 14-bit sufficient for DR


• Image processing, display, artefacts, terminology
  and PACS will be dealt with by Craig
      Why bother with digital?
• Film has been used since the beginning, so why
  are we changing to digital techniques?
  – Increased latitude and dynamic range
  – Images can be accessed simultaneously at multiple
    workstations
  – Viewing stations can be set up in any location
  – Uses digital archives rather than film libraries
  – Images quicker to retrieve and less likely to be lost
  – Post processing
  – Softcopy reporting – lower cost if do not print
  – No need for dangerous processing chemicals
      Disadvantages of digital
• Initial cost
• Problems with interconnectivity
• Lack of information and set up of automatic
  exposure control (AEC)
• Lack of link between exposure and brightness
  – Potential for dose creep (see following slides)
• Human error in choosing exam type and speed
  class
• Generally poorer limiting spatial resolution when
  compared with film
        Dynamic Range - Film
• With conventional film,
  too low a dose will
  results in a ‘thin’ film
• Too high a dose results
  in a very dark film
• Fixed and limited
  dynamic range – must
  match exposure
  parameters to the film
  being used
  – Gives a measure of
    control over patient dose!
          Dynamic Range - Digital
• With digital, too low a dose will
  still produce a recognisable
  image (just a bit noisy!)
• Similarly, too high a dose will
  produce a recognisable image
  (but with very little noise!)
• Consequences:
   – Less retakes = GOOD
   – Dose creep = BAD – must pay
     special attention to digital imaging
     to ensure doses are optimised
       Digital Imaging Techniques
•   Computed Tomography (CT)
•   Radionuclide imaging
•   Film scanner
•   Computed radiography (CR)
•   Direct digital radiography (DR)
•   Flat panel fluoroscopy
•   Electronic portal imaging device (EPID)
Computed Radiography
  Computed Radiography (CR)
• The most common technique for producing
  digital images
• Was the first digital technique available
  commercially
• Exploits storage phosphors which emit light that
  is proportional to the intensity of the X-rays that
  hit it, when they are stimulated by a laser
  beam
• Primary reason for being the most common
  technique is that it is the cheapest (at least in the
  short term)
   – Old X-ray sets used for film-screen radiography can
     be used, provided exposure factors and AECs are
     adjusted for the new type of detector
CR Components
Physical Principles of Computed
      Radiography (CR)
• Fluorescence describes the immediate release
  of low energy light photons after the absorption
  of X-ray photons (exploited in traditional film-
  screen radiography)
• Phosphorescence describes the delayed
  release of light photon energy. This is the
  principle of CR
   CR Stage 1: Image Capture
• Image receptor is a laser stimulable phosphor,
  known as an image plate (IP)
• Capture image by irradiating an IP in the same
  way as conventional film
  – Does not need a new X-ray system when replacing
    film-screen (just make sure automatic exposure
    controls are re-calibrated)
• Typically ~40% of X ray photons are absorbed
• IPs retain majority of absorbed X-ray energy as
  a pattern of electrons in meta-stable energy
  states
  – The spatial distribution of stored electrons is
    equivalent to the pattern of absorbed x rays – latent
    image
            The image plate
• Combination of barium fluorohalide dosed
  with europium (BaFX:Eu)
• Halide (X) is 85% Bromide and 15% Iodide
• Powdered phosphor mixed with binder and
  laid down on a base with a protective layer
  on top (prevent physical damage to the
  phosphor layer)
• Similar to intensifying screen
• Held in a light tight cassette similar to film-
  screen radiography
Fraction of X ray photons absorbed
      as a function of energy

                              Discontinuous
                             increase due to
                                K edge of
                                 Barium
               The image plate


 Protection layer     Protects against mechanical/chemical influences

 Phosphor layer       Phosphor atoms

 Adhesive layer       Improves mechanical stability
Anti halation layer   Blocks laser beam
   White PET          Absorption of scattered light photons

  Support layer       Support layer
Between one and two
hundred electrons          These traps are
become trapped per X-ray   artificial faults
photon absorbed            in the crystal
                           where halogen
                           vacancies exist



                              They are
                              known as
                                ‘F’ or
                               Colour
                               Centres
           Electron Trapping
• Photon is absorbed by
  an electron
• Electron can move
  through conduction
  band
• They can then be
  trapped in Colour
  Centres which forms
  our latent image
            Electron Trapping
• Electrons can
  remain trapped in         The fading
  an IP for many            characteristic
                            is shown
  hours after the
                            here
  exposure
  CR Stage 2: Image Read Out
• Electrons are
  actively stimulated
  to release their
  stored energy
• This is done by
  scanning the IP
  with an intense
  laser beam
  CR Stage 2: Image Read Out
• A red Laser is used as this matches the energy
  gap between Colour Centre and conduction
  band
• Light in the blue end of the visible spectrum is
  emitted
• Hence, optical separation of input and output
  light photons
  – Means a colour filter can be used to prevent laser
    photons contaminating the output signal
• Blue light photons are collected via a
  photomultiplier tube and digital image is
  produced
                  Read Out
• Stimulation of IPs
  with laser causes
  trapped electrons
  to transfer to
  conduction band
• These then relax to
  the ground state,
  emitting blue light
  photons
               How Does CR work?
  Conduction band




                                   Electron
                                   traps




X Rays are
absorbed




         Valence band
    Photo stimulated luminescence
  Conduction band




Red laser light

                         Blue
                         light




          Valence band
       The read/erasure cycle
• The image plate is removed from the cassette
  inside the CR reader
• Scanning or laser achieved with a rotating mirror
• The light guide (with optical filter) directs the
  emitted blue light to a photomultiplier tube,
  which measures the intensity of the light
  (proportional to the number of X-rays absorbed)
• Whilst repeatedly scanning the plate, it is moved
  through the laser beam
• Once scanned, the residual signal is removed by
  exposing the plate to a very bright light source
  (erasure cycle)
• Takes about 30-45 s to read and erase an image
  plate
CR Reader cycle




                  Analogue
                  output
                             Dynamic Range
• Film dynamic
  range defined by
  characteristic
  curve  10:1
• CR dynamic range
  > 10,000:1
• Linear relationship
  between
  log(signal) and
  log(dose)
Körner M et al. Radiographics 2007;27:675-686
             Dynamic Range
• The linear relationship would give a very ‘flat’
  image with minimal contrast if signal converted
  directly to greyscale
• Hence, need to process the image before
  displaying to the reader
• Apply various processing stages (Craig will deal
  with this in more detail), and use an optimised
  look-up table (LUT) to map the signal intensity to
  an appropriate grey level
• However, the reader is able to adjust the window
  and level to optimise the presentation of each
  individual image
            Window and Level
• Window width controls the gradient (gamma) of
  the transition from black to white
   – Narrow width will change from black to white over a
     very limited range of exposure levels – increases
     contrast in that range, but everything outside this
     range will be completely black or white
• Level determines the (typically) central value of
  exposure over which the window width is
  defined
• (ImageJ for examples?)
                    Image quality
• Pixel size limits spatial resolution in CR
   – For small plates (detail required) ~5.5 lp/mm
   – Large plates (detail not essential) ~3.5 lp/mm
   – (Film-screen ~8-12 lp/mm)
• Other limits to resolution in CR;
   – Scattering of laser light in the phosphor layer results
     in detected light from a larger area than expected
   – Divergence of light emitted before detection
      • Increases with thickness of phosphor
      • Some phosphors have needle-like structure to guide light
        (like an optical fibre), but quite brittle so not for general use
CR image quality
              Image quality
• Image processing (e.g. edge enhancement) may
  improve visibility of fine detail
• Contrast is determined by the image processing
  and LUT that is applied (and the window/level
  the user decides upon)
 Detector Dose Indicators (DDI)
• As discussed, the restricted latitude of film gives
  a clear indication of dose
   – Too dark a film = overexposure
   – Too light a film = underexposure
• Film has ‘in-built’ quality control of exposure
• CR (and all other digital) images will be
  presented with the greyscale optimised no
  matter what dose is given
   – Will always see a recognisable image, but the noise
     will vary
• Can result in dose creep
   – Increased dose (lower noise) is not punished by the
     detection medium, so tendency to go for slowly
     increasing image quality – NOT ACCEPTABLE!
                        DDI
• The DDI has been introduced for digital imaging
  as an indication of the level exposure on a broad
  region of the detector
• Analogous to the OD of film
• The definition of DDI is manufacturer specific!
  –   Some manufacturers have high DDI = underexposure
  –   Some the other way round
  –   Some are a function of the log of dose
  –   Some are linear…
• Manufacturers will provide an indication of
  acceptable range of DDI, but local departments
  must validate these – DRLs and OPTIMISATION
• Operators should monitor DDI of patient
  exposures to ensure doses remain acceptable

								
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