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Computers in Radiography

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					Computers in Radiography

    Computed Radiography
     Digital Radiography
             Tools
        Film (“Old Faithful”)
• An x-ray film is simultaneously:
  – the image detector,
  – the image display, and
  – the storage medium.
• Tried and true system
  – Produces excellent image quality under most
    conditions (motivation for change is low)
  – Large field of view and high spatial resolution
               Why change?
• Other imaging modalities are intrinsically digital
  or have made the change to digital imaging in the
  1970s
• Desire to communicate images between remote
  facilities
• Cost and space savings can be realized by
  eliminating darkrooms and film libraries
• Technical advantages of digital detectors/displays
    Computed radiography (CR)
• CR is a marketing term for photostimulable
  phosphor (PSP) detector systems
• Much of the x-ray energy absorbed by a PSP is
  trapped in the screen and can be read out later
• CR imaging plates are made of BaFBr and BaFI
  (often called barium fluorohalide)
• CR plate is a flexible screen, enclosed in a cassette
  similar to a screen-film cassette
          CR plate processing
1. Exposed cassette moved into reader unit;
   imaging plate is mechanically removed from the
   cassette
2. Imaging plate is translated across a moving stage
   and scanned by a laser beam
3. Laser light stimulates the emission of trapped
   energy in the imaging plate, releasing visible
   light
4. Light released from the plate is collected by a
   fiber optic light guide, strikes a photomultiplier
   tube (PMT), and an electronic signal is produced
Computer radiography (CR) plate reader
    CR plate processing (cont.)
5. The electronic signal is digitized and
   stored
6. The plate is exposed to bright white light
   to erase any residual trapped energy
7. The imaging plate is returned to the
   cassette and is ready for reuse
        Spectra of light in CR
• A red laser light is used to stimulate the
  phosphor to release trapped energy
• When the trapped energy is released, a
  broad spectrum of blue-green light is
  emitted
• An optical filter, placed in front of the PMT,
  prevents detection of the red laser light
Spectra used in a computed radiography (CR) system
   Latent image formation in CR
• When the x-ray energy is absorbed by the
  phosphor, it excites electrons associated with a
  europium impurity
   – Excited electrons become mobile
   – Some fraction interact with a so-called F-center
• The F-center traps electrons in a higher-energy,
  metastable state, where they can remain for days
  to weeks, with some fading over time
• Number of trapped electrons per unit area is
  proportional to intensity of x-rays incident at each
  location during the exposure
X-ray exposure and readout of photostimulable phosphor
             CR plate readout
• When the red laser light scans the exposed plate,
  the red light is absorbed at the F-center, where the
  energy is transferred to the trapped electron
• Electron gains enough energy to become mobile
  again
• Many of these electrons release blue-green light as
  they become reabsorbed by the europium atoms
• Plate erasure requires exposure to a very bright
  light source to flush almost all of the metastable
  electrons to their ground state
Latitude of CR compared to that of a screen-film system
Charge-coupled devices (CCDs)
• CCDs form images from visible light
• They are used in most modern video
  cameras and in digital cameras
• Silicon surface of the CCD is photosensitive
  – As visible light falls on each pixel, electrons are
    liberated and build up a charge in the pixel
  – More electrons are produced in pixels that
    receive greater light intensity
                CCD readout
• The readout process is similar to a bucket brigade
• Along one column of the CCD, the electronic
  charge is shifted pixel by pixel
• The charge from each pixel in the column is
  shifted simultaneously, in parallel
• The charges on each column are shifted onto the
  bottom row of pixels, that entire row is read out
  horizontally, and then the next charges from all
  columns are shifted down one pixel, and so on
Charge-coupled device (CCD) readout
             CCD applications
• CCD cameras are commonly used for fluoroscopy
  and digital cinecardiography
   – Light from the image intensifier focused onto the CCD
     chip
• If the field of view is only slightly larger than the
  CCD, such as in digital biopsy systems for
  mammography, a fiberoptic taper is placed
  between the intensifying screen and the CCD chip
• Large fields of view require a lens to focus the
  light, most of which is lost
Example couplings of CCD chips
         Flat panel detectors
• Flat panel systems use technology similar to
  that used in laptop computer displays
  – Much of this has to do with wiring the large
    number of individual display elements (any one
    of which may be accessed at random)
• Only 2,000 connections between the
  imaging plate and the readout electronics
  are required for a 1,000 x 1,000 display,
  instead of 1,000,000 individual connections
Flat panel detector array
Readout process for a flat panel detector array
              Spatial resolution
• The size of the detector element on a flat panel
  largely determines the spatial resolution of the
  detector system
   – A panel with 125- x 125-m pixels has a maximum
     resolution of 4 cycles/mm
• Small detector elements are needed for high
  spatial resolution
• The electronics of each detector take up a fixed
  amount of space
   – The light collection efficiency decreases as the detector
     elements get smaller
Light collection efficiency of detector elements
              Indirect detection
• Indirect flat panel detectors are sensitive to visible
  light
   – An x-ray intensifying screen (typically Gd2O2S or CsI)
     is used to convert incident x-rays to light
   – Light is then detected by the flat panel detector
• Analogous to a screen-film system
• Thickness of screen can lead to blurring
   – CsI screens used most often to reduce lateral spread of
     light
Indirect detection flat panel system




     Direct flat panel detector
              Direct detection
• With direct detectors, the electrons released in the
  detector layer from x-ray interactions are used to
  form the image directly
• An electric field is applied to make the electrons
  travel directly to the detector elements, virtually
  eliminating blurring
• Electric field lines can be altered at each detector
  element so that the sensitive area of the detector
  element collects electrons that would otherwise
  reach insensitive areas, increasing the effective fill
  factor
        Digital mammography
• Mammography presents challenges for
  digital detectors because of the high spatial
  resolution required to detect and
  characterize microcalcifications
• Full-field digital mammography systems
  based on CCD cameras make use of a
  mosaic of CCD systems
  – Increases the area of the light sensor and keeps
    the demagnification factor low
Digital biopsy and mammography applications
            Slot-scan systems
• Another technology used for full-field digital
  mammography
• Uses a long, narrow array of CCD chips (about 4
  mm x 18 cm )
• Breast remains stationary and under compression
  while the x-ray beam is scanned across it
• Little scattered radiation detected due to narrow
  beam geometry
• X-ray tube uses tungsten target operated at 45 kVp
  for efficiency and heat loading during exposures
  lasting 1 to 2 seconds
Readout logic of a time delay and integration (TDI) system, which
           is used for slot-scan digital mammography
     Time delay and integration
• Because the slot width (4 mm) is much wider than
  one pixel (0.050 mm), scanning the system with
  the detector would result in image blurring if the
  detector motion were not compensated for
• As the slot scans in one direction, the CCD chip is
  read out in bucket brigade fashion in the direction
  opposite to the scan motion, at the same velocity
• Increases the sensitivity of the detector system
  (and reduces heat loading) by the ratio of the slot
  width to the pixel width (a factor of 80)
  Implementation of digital imaging
• CR is often the first digital radiographic system
  installed in a hospital because it works with the
  existing x-ray systems and the plates are relatively
  inexpensive
• One CR reader can typically handle the workload
  of three examination rooms
• Flat panel detectors are expensive; can only be in
  one place at one time
• Useful in suites that have high patient throughput,
  such as a dedicated chest room
      Digital image processing
• One of the advantages of having an image
  in digital form is that its appearance can be
  modified and sometimes improved using a
  computer to manipulate the image
• Image processing is a vast subject on which
  many books have been written
      Digital image correction
• Most digital radiographic detector systems
  have an appreciable number of defects and
  imperfections
• Under most situations the blemishes on the
  image can be corrected using software
• Subtle differences in the sensitivity of each
  detector element can also be corrected
Raw and corrected images from a flat panel detector array
   Example of image processing
• In the following images, a CR image is enhanced
  for viewing by applying a sigmoid look-up-table
  to map image pixel values to displayed shades of
  gray (this mimics film response)
• Edges in the image are then enhanced to bring out
  more detail
• Finally, alternative color mappings are applied to
  illustrate the ability to radically change the
  appearance of a digital image
Original CR image
Sigmoid curve look-up table applied
Gaussian blurring applied to image
Original image subtracted from blurred image
Difference image added to contrast-enhanced image
Different sigmoid LUT applied to emphasize lung contrast
Inverse image with logarithmic LUT applied
False color, sigmoid LUT applied (hot iron)
Alternative false color LUT applied

				
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