DR

• Digital radiography


  Imaging technique that uses digital sensors to capture and process radiographic images instead of traditional photographic film

  
• Acceleration to all digital imaging continues because of its undeniable advantages
• Several approaches might be used to produce digital radiographs

  
• It is not clear, yet which one is the best one
• Digital imaging vocabulary is not yet standard or universally accepted

  
• Use of digital radiography


  • Helps to eliminate a lot of limitations that were present with screen film technology

  
• Capture element


  Where the x-rays are captured

  
• Coupling element


  Where the x-ray-generated signal transferred to the collection element

  
• Collection element


  Where the x-ray generated signal collected and sent out via electronics for digitalization and processing

  
• Scanned Projection Radiography (SPR)


  1. Patient is positioned on CT couch and is driven through the gantry

  2. Tube and IR (array) are stationary

  3. Result is digital radiograph

  
• Scanned Projection Radiography (SPR)


  • Collimation to the fan x-ray beam with associated scatter radiation rejection
  • Improvement of the image contrast

  
• Scanning motion required several seconds, resulting in patient motion blur
• At present time, SPR is regaining its importance with some modifications as an adjunct to digital radiographic tomosynthesis (DRT)
• The purpose of all forms of tomography is to improve image contrast

  
• Charge-Coupled Device (CCD)


  • Light-sensing element for most digital cameras
  • High sensitivity
  • Wide dynamic range
  • Small size
• CCD


  • Silicon based semiconductor
  • Sensitivity is the ability to detect and respond to very low levels of visible light
  • Size is very small, making it highly adaptable to DR in its various forms
• Dynamic range


  Ability of the CCD to respond to a wide range of light intensity, from very dim to very bright
• CCD has high sensitivity to X-ray exposure and a very wide dynamic range

  
• CCD radiation response is linear
• With use of CCD, image contrast is unrelated to image receptor X-ray exposure
• CCD system response to very low X-ray exposure, leading to possibility of lowering patient dose
• CCD


  • Photosensitive receptor and electronics embedded in a substrate material in a silicon chip
  • Incident light from a scintillator strikes the detector, and electron–hole pairs are produced in the silicon
  • The amount of electron–hole pairs is related to the amount of light absorbed
  • The electrons are held by electrostatic forces in the array until the charge is read out to form the image
• Cesium Iodide/Charge Coupled device


  • Can be tiled to receive the light from an area x-ray beam as it interacts with scintillation phosphor
  • Tiled CCD receives light from a scintillator and allows the use of an area x-ray beam, so that exposure time is short
  • The scintillator light is efficiently transmitted through the fiber-optic bundles to the CCD array
  • Results in a high x-ray capture efficiency and good spatial resolution (up to 5 lp/mm)
• Cesium Iodide/Charge Coupled device


  • CCD is indirect DR process: 1) x-ray converted to light, 2) light converted into electronic signal
  • Assembly of multiple CCDs to view the area that x-ray beam presents creates a challenge to provide fidelity image at the edges of each CCD
  • This challenge is overcome by the interpolation of pixel values at each tile interspace

  
• Scintillator


  Determines how many of the incident x-ray photons are absorbed, how much light is produced, and the wavelength or color of the light
• Scintillator types


  • Structured (CsI) - needle-like crystals focus light onto a narrow area, reducing light spread and allowing thicker scintillators
  • Unstructured (Gd2O2) - powder-like grains produce more light spread, resulting in decreased efficiency of the detector
• Quantum efficiency


  The number of electrons produced related to the incident light from the scintillator
• Detective Quantum Efficiency (DQE)

  Ratio of output signal to signal-to-noise ratio, measure of the efficiency and fidelity with which detector can perform its task
• DQE for a perfect digital detector is 1 or 100%, but since some amount of noise is always present it's impossible to have 100% DQE
• Focus light


  • Onto a very narrow area
  • Reducing light spread
  • Allowing the use of thicker scintillators
  • Minimizing the loss of spatial resolution
• Gd2O2


  Considered unstructured because of its powder-like grains
• Turbid phosphors


  • Produce more light spread
  • Resulting in decreased efficiency of the detector
• Quantum efficiency


  Represents the absolute efficiency of the light collection and signal created in the chip
• Quantum efficiency


  Affects detective quantum efficiency (DQE)
• DQE


  The ratio of output signal to signal-to-noise ratio
• DQE


  A measure of the efficiency and fidelity with which a detector can perform its task
• DQE for a perfect digital detector is 1 or 100%, that would mean that there is no loss of information however, since some amount of noise is always present it's impossible to have 100% DQE
• CCD


  • Built to be as efficient as possible
  • The polysilicon layer must be transparent enough so that light passes through to the deeper substrate storage area but not so deep that it cannot be captured in the potential well
  • The spectrum sensitivity must match the spectral output of the phosphor
  • The less sensitive the CCD to the light spectrum of the scintillator, the less efficient the CCD
• TFT flat-panel array


  First devices to move beyond the cassette into detectors that would reside in the table or wall stand
• These detectors are no longer permanent fixtures and can be used portably as a wireless device