Digital processing Flashcards
Why go digital?
> dynamic range of images Digital processing Digital storage, retrieval and transport (PACS) Potential for -Dose reduction to pxs Low running costs Computer aided detection
Image receptors and capture
Image first captured using sensor
Then transformed into series of binary numbers (0s and 1s)
Typically has 12-16 bit accuracy, before being displayed on monitor
2 main types of sensors used in first step: computed radiography (CR) - cheaper- and CCD/CMOS (like camera in phone)
Computed radiography
Most common method for digital dental radiography as it can be retrofitted to existing equipment
Uses storage phosphor that stores energy of x-ray photons, then releases it as light in response to stimulation with laser
CR commonly referred to as
Photostimulable phosphor (PSP)
Most common PSP
Barium fluorohalide doped with Europium (BaFX:Eu)
Halide (X) is a mixture of bromide (85%) and iodide (15%)
How is PSP made
Phosphor in powdered form is mixed with adhesive and laid down as base coat 0.3mm thick
Imaging receptor PSP
Similar in appearance to conventional film
How does PSP work
Some energy from x-ray photons that pass through px transferred to e-s in PSP, which get trapped by ‘F centres’
Exposed image receptor is scanned by red laser beam in CR reader, which releases trapped energy as visible light
Array of optical fibres direct emitted light (blue) to 1 or more photomultiplier tubes to measure its intensity
Intensity of light released from PSP is directly related to intensity of x-ray photons
Receptor scanned sequentially across width as it moves progressively through scanning beam
Higher bit accuracy
More accurate image
More storage needed
Digital radiography sensors
Solid state devices
-CCD: charge coupled devices
-CMOS: complementary metal oxide semiconductors
Cannot be manufactured in sizes bigger than ~5cm^2, limiting their use to intra-oral radiography
Slightly bigger than conventional film, but much thicker (up to 10mm)
Most attached to docking station with wire cable, but wireless sensors that use WiFi now available
How do CCD/CMOS sensors work?
Use indirect conversion of x-ray pattern into electronic signal using light emitting phosphor (caesium iodide)
Silicon of receptor acts as photodiodide and converts light emitted by phosphor to electrical charge
Charge pattern formed from pixels in sensor matrix forms radiographic image
How do CCD/CMOS sensors work? (image on slide)
Use indirect conversion of x-ray pattern into electronic signal using light emitting phosphor (caesium iodide)
Silicon of receptor acts as photodiodide and converts light emitted by phosphor to electrical charge
Charge pattern formed from pixels in sensor matrix forms radiographic image
Comparison of film, CR and CCD/CMOS
Resolution (lp/mm) -8-15, 3.5-5.5, 4-5 Image quality -good, better, better Processing -10 min, 10-30s, 5s Cost -cheapest, cheap, expensive
Main advantage of digital imaging
Ability to manipulate image in order to > diagnostic value Variety of tools available to do this -edge enhancement -noise reduction -windowing
Bits & bytes
Image divided into matrix of pixels (usually 512x512)
Each pixel assigned numerical value to intensity of signal in that part of image
Value stored in each pixel is in binary format (series of 0s & 1s)
In radiology, images typically have 12 bit depth (2^12), equivalent to 4096 levels of signal (grey level) between black and white
Thus single 512x512 image with 12 bit depth requires 512x512x12/8=0.375MB of memory to store it
If pixel had 1 bit depth
it could either have value of 1 (black) or 0 (white)
If pixel had 2 bit depth
there could be 4 possible pixel values (2^2)
-00,01,10,11
Windowing
4096 levels of pixel intensity can only be displayed over 256 shades of grey on computer monitor
If all 4096 units were displayed on single image it would have very poor contrast (ability to distinguish different regions of image)
Instead 256 grey scale spread over narrow range of pixel values (the window) and it is centred around a level
New technology, new problems
Laser malfunction
Underexposure
Edge enhancement algorithm (thin black line around metal filling)
Receptor back to front (copper dot visible on image)
Bends in PSP (crease in receptor white lines on angles)
PSP delaminating (edge opens first, blobby on one edge)
Processing problems that affected wet films
Developer strength
Temp
Fixation time
Washing
Hospital radiology systems
Introduction of of picture archiving and communication and communication system (PACSs). Advantages:
- images instantly available in any location, not just hospital
- images viewed simultaneously at different locations
- images cannot get lost
- film stores eliminated
- imaging integrated with other electronic records
PACS
Repository (store) of images, but also controls flow of info to the repository (new images deposited into it) and from store (for display on monitors)
Image data from various modalities (x-ray, ultrasound, CT, MRI etc.) are all in same format that can be recognised and used by system - current standard is DICOM (Digital Imaging and Communications in Medicine)
PACS and RIS
Pacs integrated with RIS (Radiology Info System), which stores data regarding previous imaging investigations
Monitors
Images accessed via monitors connected to standard PCs
Reporting monitors (used by radiologists) are high-quality and incorporate wide range of software tools for image manipulation
It is essential that monitors are calibrated to DICOM standard, so greyscale rendering of image optimised to performance of human eye
-can be checked using standard test image developed by Society of Motion Picture and Television Engineers
Elsewhere, in clinics etc. there are review workstations with lower specification monitors and limited imaging software
Images displayed on these monitors not as good as those on reporting workstations, and caution needed when interpreting images
