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
