Principles of further imaging

  • Created by: Emmatjies
  • Created on: 12-01-20 12:32

Learning Objectives

  • Understand the image acquisition process for both types of digital imaging
  • Understand how a digital image is formed.
  • Be familiar with the basics of imaging systems.
  • Differences in workflow with CR and DR.
  • Recognise the advantages and disadvantages of CR anf DR
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Digital Imaging

Conventional -Produces analogue images, where the light waves are recorded exactly how they are.

DR- when the analogue signal is converted to digital signal (numbers).

There are two types of Digital imaging:

  • Phosphostimulable storage phosphor plate (PSP)- Cassette image receptors.
  • Flat Panel detectors (FPD), indirect and direct (no cassettes)

Silver halide crystals within the intensifying screens emit light when struck by x-rays.

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Photostimulable storage phosphor plate

  • Protective layer
  • Phosphor layer- traps electrons
  • Light reflective layer- Allows light to move forward through the detector, prevents light from escaping.
  • Conductive layer- Reduces static electricity
  • Support layer
  • Colour layer - Further absorb stimulating light
  • Backing and barcode
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Image Acquisition

  • Beam interacts with electrons in phosphor crystals
  • Transfers energy to electron.
  • Trapping electrons in phosphor centre
  • Plate is exposed to light from reader
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Types of readers

Point scan- Only one single point of imaging plate is exposed to the laser at one time.

Line scan - Simultaneous exposure by the laser of the imaging plate crystals one light at a time. Multiple lasers scan simultaneously.

Red laser light is used as necessary extra energy.

TFT (thin film transistor) array - photosensitive arangement of pixels or detector elements containing photoiodide.

FET (Field effect transistor)- Detects the signal from each detector element and acts like a switch to send the signal to the imag eprocessor.

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Processing PSP

  • The barcode is scanned to assign the image to the pt.
  • Cassette is inserted into the reader.
  • Cassette is opened and imaging plate is removed to move through the reader.
  • The imaging plate is exposed to light. Laser lights scan in a raster pattern.
  • Red lasers are used for extra energy.
  • This excites the electrons allowing them to excape from the phosphor centre.
  • The inscreased energy causes an emission of visible blue light.
  • The visible blue light is recieved through a charged coupled device linear array photodetector.
  • The light signal is sent to the ADC (analogue to digital converter) where it is transferred into digital signal.
  • The imagine plate is then erased by flooding it with bright white light.
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Image acquisition of FPD

Direct Image aquisition:

  • X-ray photons are absorbed by the photoconductor and immediately converted into electrons.
  • Electrons are stored in the detector elements of the TFT array.
  • They are absorbed by the photoiodide which then genreates an electrical charge.
  • An FET detects the signal and sends the signal to the image processor.

Indirect image acquisition:

  • Scintillators absorb x-ray photons and convert energy into light.
  • Light is stored in the detector in elements of the TFT array. 
  • Light is absorbed by the photoiodide which then creates an electrical signal.
  • FET detects signal from each detector element and acts like a switch to send the signal to the image processor.
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Post processing functions

  • Annotation
  • Window level
  • Orientation
  • Shuttering
  • Magnification
  • Image stiching
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  • Cracks
  • Grid lines
  • Hair 
  • Dirt
  • Inappropriate processing algorithm
  • Tape residue
  • Double exposure
  • Dead pixels
  • Miscalibration
  • Image lag
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PSP advantages and disadvantages


  • No chemicals
  • low running costs
  • dont require new rooms
  • no lost films
  • simulatneous viewing
  • Post processing image manipulation
  • Reject analysis


  • needs regular secondary erasure
  • Convenience of post processing manipulation could increase radiation exposure for pt.
  • Image acquisition is slower than FPD
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FPD advantages and disadvantages


  • Quicker
  • better spatial resolution
  • Reject analysis
  • No need to leave the room to process
  • Simultaneous viewing
  • Post processing features
  • Grid software - lead to less dose to pt and more comfortable for pts.


  • Higher initial cost
  • Cant retrofit room
  • Detects are very expensive if broken
  • Weight of detectors
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