Direct Digital Radiography or Direct Capture Radiography

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Direct Digital Radiographyor Direct Capture
Radiography

• Bushong Ch. 26 29
• Carter Ch. 6

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Late 1990s

• A new approach to imaging appeared
• DR or DDR or Direct Capture imaging
• At the moment departments cant go all DR they
  must still have F/S or CR. Do you know
  why?

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Directed Digital Radiography(DDR)

• Directed digital radiography, a
• term used to describe total
• electronic imaging capturing.
• DR is hard-wired to the image processing system.
  Eliminates the need for an image plate
  altogether.

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DDR Systems
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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

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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

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CR vs DR

• CR
• imaging plate
• processed in a Digital Reader
• Signal sent to computer
• Viewed on a monitor

• DR
• CCD, TFT or photodiode receiver (like bucky)
• directly into digital signal
• seen immediately on monitor

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Digital Radiography
DDR
CR
Direct Capture
Indirect Capture
Computed Radiography (CR) - PSL
Direct-to-Digital Radiography (DDR)-Selenium
Direct-to-Digital Radiography Silicon Scint.
Laser Scanning Digitizers
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Digital Radiography
Fundamentals of Digital Radiography

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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.

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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?

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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.

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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-.

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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.

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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

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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.

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Amorphous Selenium

• No scintillation phosphor is involved
• The image-forming x-ray beam interacts directly
  with amorphous selenium (a-Se),
• producing a
• charged pair.

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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

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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

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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).

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CCD Array with a scintillation phosphor
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Direct vs Indirect DR
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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.

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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.

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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

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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!

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Coupling Element

• Transfers the x-ray signal to the collection
  element.
• Ex A lens or fiber optic assembly, a contact
  layer, or amorphous selenium.

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DEL Digital Value

• Digital Value depends on
• Charge collected by DEL.
• Bit depth
• 10 bit 1 1024
• 12 bit 1 - 4096

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DEL collects x-raysignal
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Spatial Resolution
Should be best with DR. DR is limited by pixel
size
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Image Resolution
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Image Resolution (how sharply is the image
seen)

• CR DR
• 4000 x 4000
• image only as good a monitor
• 525 vs 1000 line
• more pixels more memory needed to store
• resolution dependent on pixel size

• DR 4 lp/mm
• CR 6 lp/mm
• RAD 8 lp/mm
• Mammo 15 lp/mm
• IMAGE APPEARS SHARPER BECAUSE CONTRAST CAN BE
  ADJUSTED BY THE COMPUTER
• (DIFFERENCES IN DENSITY)

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Pixel Pitch

• Spatial resolution determined by pixel pitch.
• Detector element (DEL) size
• 140 µm 3.7 lp/mm
• 100 µm 5.0 lp/mm

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Fill Factor
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F/S DDR imaging systems
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Unlike CR plates, only the exposed pixels
contribute to the image data base.

• One exposure Detector Readout

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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

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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

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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

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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

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Patient Dose

• Important factors that affect patient dose
• DQE when using CsI systems
• Both systems fill factor
• The percentage of the pixel face that contains
  the x-ray detector.
• Fill factor is approximately 80

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Viewing the Digital Image

• Ch. 29
• Review pg 34 in carter

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Photometry

• The science of the response of the human eye to
  light
• The basic unit of photometry is the lumen (lm).

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Illuminance

• describes the intensity of light incident on a
  surface
• Luminance intensity is a property of the source
  of light, such as a viewbox or a digital
• display device

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Cosine Law

• Is important when one is describing the luminous
  intensity of a digital display device. When a
  monitor is viewed straight on, the luminous
  intensity is maximum. When a monitor is viewed
  from an angle, the contrast and the luminous
  intensity are reduced.

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When a digital display device is viewed from the
side, illumination and image contrast are
reduced.
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Hard CopySoft Copy
Radiology 1895
Radiology 2001
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Soft copy viewingdigital cathode ray tube (CRT)
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active matrix liquid crystal display (AMLCD)
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Active matrix liquid crystal displays are
superior to cathode ray tube displays.

• LCD design
• reduces ambient light
• Better contrast resolution
• Less noise

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Spatial Resolution

• improves with the use of higher-megapixel digital
  display devices
• A 1-megapixel display will have a 10001000-pixel
  arrangement. A high-resolution monitor will have
  a 5-megapixel display, or a 20002500-pixel
  arrangement

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LCD Disadvantage

• The principal disadvantage of an AMLCD is the
  angular dependence of viewing
• Ergonomic design of digital workstations is
  critical

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Digital Image Postprocessing

• Process
• Annotation
• Window and level Magnification
• Image flip
• Image inversion
• Subtraction

• Results
• Label the image
• Expand the digital grayscale to visible
• Improve visualization and spatial resolution
• Reorient image presentation
• Make white-black and black-white
• Improve image contrast

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Postprocessing

• Region-of-interest
• Edge enhancement
• Pan, scroll, and zoom

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DDR has all the advantages of CR imaging
techniques

• Post processing PACS

Questions?