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Direct Digital Radiography

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					 Direct to Digital Radiography
or Direct Capture Radiography

      Bushong Ch. 26 & 29
               &
          Carter Ch. 6
Late 1990’s
   A new approach to imaging appeared

   DR or DDR or Direct Capture imaging

   NEW - Wireless DR Detector
   Directed Digital Radiography
               (DDR)
Directed digital radiography, a
  term used to describe total
  electronic imaging capturing.
DR is hard-wired or Wi-Fi to the
    image processing system.
  Eliminates the need for an
     image plate altogether.
DDR Systems
IMAGE CAPTURE
CR
        PSP – photostimulable phosphor plate
        REPLACES FILM IN THE CASSETTE


DR – NO CASSETTE – LIGHT or E-
        Captured directly
        On to a transistor, photodiode or charge-coupled
         device
        Sent directly to a monitor
DIRECT RADIOGRAPHY
   uses a TFT, CCD, or photodiode to
    receive image data (like bucky)
   that captures and converts x-ray energy
   directly into digital signal
   seen immediately on monitor
   then sent to PACS/ printer/ other
    workstations FOR VIEWING
CR vs DR
CR                        DR
 imaging plate            CCD, TFT or
                            photodiode receiver
   processed in a          (like bucky)
    Digital Reader
                              directly into digital
   Signal sent to            signal
    computer

   Viewed on a monitor      seen immediately on
                              monitor –
     DDR                  Digital
                                                 CR
                        Radiography


               Direct                 Indirect
              Capture                 Capture



Direct-to-Digital                            Computed
  Radiography                               Radiography
(DDR)-Selenium                               (CR) - PSL


Direct-to-Digital                              Laser
 Radiography                                 Scanning
 Silicon Scint.                              Digitizers
  Digital
Radiography
   Fundamentals
          of
Digital Radiography
Flat-Panel Detectors
   Flat-panel detectors consist of a
    photoconductor, amorphous selenium
    (a-Se), which holds a charge on its
    surface that can then be read out by a
    TFT. This category also includes silicon
    and CCD detectors.
Capture Element
   Where the remnant photons are
    captured.
   DR = Cesium iodide (CsI), Gadolium
    oxysulfide (GdOS), or Amorphous
    selenium (a-Se).

   And for CR? What is the name of the
    compound?
Direct vs Indirect Conversion
   In direct conversion, x-ray photons
    are absorbed by the coating material
    and immediately converted into an
    electrical signal. The DR plate has a
    radiation-conversion material or
    scintillator, typically made of a-Se. This
    material absorbs x-rays and converts
    them to electrons, which are stored in
    the TFT detectors.
Collection element
   Collects converted x-ray signal.

   Types: Photodiode, A charge-coupled
    device (CCD), or A thin-film transistor
    (TFT).
   Photodiode & CCD collect light. TFT is
    charge sensitive and collects E-.
TFT
   The thin-film transistor (TFT) is a
    photosensitive array made up of small
    (about 100 to 200μm) pixels. Each pixel
    contains a photodiode that absorbs the
    electrons and generates electrical
    charges.
Active Matrix Array (AMA)
Pixels are read sequentially, one at a time

   Each TFT or CCD
    detector represents
    a pixel

   DEL = charge
    collecting detector
    element
DR
   A field-effect transistor (FET) or
    silicon TFT isolates each pixel element
    and reacts like a switch to send the
    electrical charges to the image
    processor.
Amorphous Selenium
   No scintillation phosphor is involved
   The image-forming x-ray beam
    interacts directly with amorphous
    selenium (a-Se),
    producing a
    charged pair.
Amorphous Selenium
   The a-Se is both the capture element
    and the converting element.
   a-Se is a direct DR process by which x-
    rays are converted
    to electric signal
DDR only using amorphous
selenium (a-Se)
   The exit x-ray photon interact with the
    a-Si (detector element/DEL). Photon
    energy is trapped on detector (signal)

   The TFT stores the signal until readout,
    one pixel at a time
Indirect Conversion
   Indirect conversion is a two-step process:
    x-ray photons are converted to light, and
    then the light photons are converted to an
    electrical signal.
   A scintillator converts x-rays into visible light.
    The light is then converted into an electric
    charge by photodetectors such as amorphous
    silicon photodiode arrays or charge-coupled
    devices (CCDs).
CCD Array with a scintillation
phosphor
Direct vs Indirect DR
Charge-Coupled Device
   CCD, which is the light-sensing
    element.
   The CCD is a silicon-based
    semiconductor
   has three principal advantageous
    imaging characteristics: sensitivity,
    dynamic range, and size.
Sensitivity


   is the ability of the CCD to detect and
    respond to very low levels of visible
    light
   This sensitivity is important for low
    patient radiation dose in digital imaging.
Coating for DR
   AMORPHOUS SILICON (indirect)

   AMORPHOUS SELENIUM (direct)

   See pag 372 in Carltons
Dynamic range
 is the ability of the CCD to respond to a
  wide range of light intensity, from very
  dim to
  very bright
 DR should

lower patient
dose
Size
   A CCD is very small, and this makes it
    highly adaptable to uses in radiology

   The CCD itself measures approximately
    1 to 2 cm, but the pixel size is an
    exceptional 100 × 100 μm!
Coupling Element
   Transfers the x-ray signal to the
    collection element.

   Ex: A lens or fiber optic assembly, a
    contact layer, or amorphous selenium.
DEL Digital Value
   Digital Value depends on:
       Charge collected by DEL.
       Bit depth
       10 bit = 1 – 1024
       12 bit =1 - 4096
DEL collects x-ray
signal
F/S & DDR imaging systems
Unlike CR plates, only the
exposed pixels contribute to the
image data base.

   One exposure = Detector Readout
TFT Array Detectors
   Detector is refreshed after
    exposure
   If no exposures are produced. . .
    detector refreshed every 30 – 45
    sec
   Built in AEC, An ion chamber
    between grid and detector
Advantages/Disadvantages
   CsI phosphors have high detective
    quantum efficiency (DQE) = lower
    patient dose
       DQE = % of x-rays absorbed by the
        phosphors
   a-Se only: there is no spreading of light
    in the phosphor = better spatial
    resolution
Dynamic range
 is the ability of the CCD to respond to a
  wide range of light intensity, from very
  dim to
  very bright
 DR should

lower patient
dose
DR
   Initial expense high
   very low dose to pt – due to the high
    DQE over CR and F/S. Fewer photons
    required to produce and image.
   image quality of 100s using a 400s
    technique
   Therfore ¼ the dose needed to make
    the image
Patient Dose
   Important factors that affect patient
    dose
   DQE: when using CsI systems
   Both systems “fill factor”
Digital Image Postprocessing
   Process               Results
   Annotation            Label the image
   Window and level      Expand the digital
    Magnification          grayscale to visible
                          Improve visualization
                           and spatial resolution
   Image flip            Reorient image
                           presentation
   Image inversion       Make white-black and
                           black-white
                          Improve image contrast
   Subtraction
Postprocessing
   Region-of-interest

   Edge enhancement

   Pan, scroll, and zoom
DDR has all the advantages of
CR imaging techniques
   Post processing & PACS
DIGITAL IMAGE STORAGE
Redundant Array of
Inexpensive Disk
(RAID)
DR Image Quality
   Pixel Pitch
   Fill Factor
   Amorphous Selenium vs Amorphous
    Silicon
   (RAID) Redundant Array of
    Inexpensive Disk
   DICOM
   PACS
   HISTOGRAM
   ALGORYTHM
   HIS/RIS
IMAGE TRANSMISSION
PACS




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posted:9/29/2012
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