- Created by: Emmatjies
- Created on: 12-01-20 12:32
- 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
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.
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
- Beam interacts with electrons in phosphor crystals
- Transfers energy to electron.
- Trapping electrons in phosphor centre
- Plate is exposed to light from reader
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.
- 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.
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.
Post processing functions
- Window level
- Image stiching
- Grid lines
- Inappropriate processing algorithm
- Tape residue
- Double exposure
- Dead pixels
- Image lag
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
FPD advantages and disadvantages
- 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