Tt

• Digital Radiography (DR)


  All-digital imaging that provides several significant advantages over screen-film radiography
• Image receptor types


  • Screen film (chemical process)
  • Computed radiography CR
  • Digital radiography DR

  
• Computed radiography CR


  • Thermo luminescent dosimetry (TLD)
  • Optically stimulated luminescence (OSL)

  
• Screen-film radiographic images require chemical processing, relatively long time that can delay completion of the examination
• After an image has been obtained on film, little can be done to enhance the information content
• Images are available in the form of hard copy film that must be catalogued, transported, and stored for future review
• Such images can be viewed only in a single place at one time
• Digital imaging


  Any image acquisition process that produces an electronic image that can be viewed and manipulated on a computer

  
• Scanned Projection Radiography (SPR)


  Shortly after the introduction of third-generation computed tomography (CT), SPR was developed by CT vendors to facilitate patient positioning
• Scanned Projection Radiography (SPR)

  1. Patient is positioned on the CT couch and then is driven through the gantry while the x-ray tube is energized

  2. The x-ray tube and the detector array do not rotate but are stationary, and the result is a digital radiograph

  
• Scanned Projection Radiography (SPR)

  • Collimation to a fan x-ray with associated scatter radiation rejection and improvement in image contrast
  • The x-ray beam is collimated to a fan by pre-patient collimators
  • Post-patient image-forming x-rays likewise are collimated to a fan that corresponds to the detector array (grid) - a scintillation phosphor, usually Sodium Iodide (NaI) OR Cesium Iodide (CsI) - and is married to a linear array of CCDs (charge-coupled device) through a fiber optic light path

  
• Scanned Projection Radiography (SPR) was not very successful because chest anatomy has high subject contrast, so scatter radiation rejection is not all that important

  
• The scanning motion required several seconds, resulting in motion blur
• Tomosynthesis


  A method for performing high-resolution limited-angle tomography at radiation dose levels comparable with projectional radiography

  
• Charge-Coupled Device (CCD)


  A highly light sensitive device developed in the 1970s for military use, now found major application in astronomy and digital photography
• Charge-Coupled Device (CCD)


  • Sensitivity - ability to detect and respond to very low levels of visible light
  • Dynamic range - ability to respond to a wide range of light intensity, from very dim to very bright
  • Size - very small, highly adaptable to DR in its various forms

  
• At very low x-ray exposure, the response of a CCD system is greater than that of screen film, resulting in lower patient dose during DR
• Cesium Iodide/Charge-Coupled Device (CsI/CCD)

  A successful approach to DR where the scintillation light from a CsI phosphor is efficiently transmitted through fiber optic bundles to the CCD array, resulting in high x-ray capture efficiency and good spatial resolution

  
• The assembly of multiple CCDs for the purpose of viewing an area x-ray beam presents the challenge to create a seamless image at the edge of each CCD, which is accomplished by interpolation of pixel values at each tile interface

  
• Spatial resolution


  The ability of an imaging system to differentiate between two near-by objects, depends on the size of the pixel used
• Spatial resolution


  The ability of an imaging system to differentiate between two near-by objects
• Spatial resolution in digital imaging

  Depends on the size of the pixel used
• A large pixel size will be unable to resolve two near-by structures as compared to a small pixel size
• Spatial resolution


  Measured in line-pairs per millimeters
• Spatial resolution


  The size of one pixel on the ground
• Temporal resolution


  How often data of the same area is collected, typically referred to as Revisit Time
• Sensors can trade spatial resolution for temporal resolution


  It is difficult to maximize both
• As spatial resolution increases


  Temporal resolution decreases
• CsI (Cesium Iodide)


  Used to capture the x-ray in digital radiography

  
• Amorphous silicon (a-Si)


  A semiconductor that is not crystalline but a fluid that can be painted onto a supporting surface
• CsI phosphor


  • Available in the form of filaments to improve x-ray absorption and reduce light dispersion

  
• Photoelectric capture


  High in CsI due to the atomic numbers of cesium (55) and iodine (53)

  
• High x-ray interaction with CsI results in low patient radiation doses

  
• DR image receptor


  • Fabricated into individual pixels, each with a light-sensitive face of a-Si, a capacitor, and a TFT embedded

  
• Digital radiographic image production

  1. rays are converted to light by CsI phosphor, then to electric signal by the Active Matrix Array (AMA) of silicon photodiodes

  
• CsI/a-Si


  An indirect DR process where x-rays are converted first to light and then to electric signal

  
• A photomicrograph shows an active matrix array–thin-film transistor (AMA-TFT) digital radiography (DR) image receptor with a single pixel highlighted

  
• Fill factor


  The percentage of the pixel face that is sensitive to x-rays
• The fill factor is approximately 80%, therefore 20% of the x-ray beam does not contribute to the image

  
• As pixel size is reduced


  Spatial resolution improves but at the expense of the patient radiation dose