Digital Radiography Flashcards
Construction of digital images
Made up of pixels
each pixel has specific grey shade
pixels arranged in matrix in fixed dimension
Spatial resolution
Sharpness
Line pairs per mm
Human eye 8 lp/pmm, dental system 6-22 lp/pmm
Pixel shade
Each pixel is a certain shade depending on amount of radiation reaching sensor.
No. of grey shades dependent on bit depth.
Direct digital imaging stages
- Scintilltion Layer
Xray - light
2: Semiconductor based pixel array
Light - electron charge
Indirect digital imaging
Photostimulable phosphor plates
Plastic plate coated with storage phosphor material sensitive to X-rays
Phosphor stores Xray energy
PSP plate placed in laser reader - releases Photon energy as light
Scintillation Layer stage
Charge coupled device (Expensive as maintain charge across chip with no distortion, high quality and low noise, better pixel uniformity.)
Semiconductor based pixel array
Complementary metal oxide semicounductor - active pixel sensor Lower power than CCD Easier to manufacture Noisier Cheaper
PSPs Computed Radiography
Phosphor material: Europium activated barium fluorohalide
Red light from neon/helium laser makes stored energy into light
Released light detected by photomultiplier and converted to electrical charge
Electrical signal converted to digital signal by analogue to digital converter.
After scanning, image erased by exposure to intense light - plate can be reused.
ADV of PSP over CCD/CMOS
No bulky connecting cable
Slimmer imaging plate
Cost: £20-50 PSP, £1000 CCD/CMOS
PSP DISADV
Need to process plates within 8 hours
Separate processing increases time
Careful handling required to prevent scratches
Blooming
More of a problem in direct digital devices.
Overload of pixels and bleeding of signals into adjacent pixels.
Manipulation of images
- Brightness
- Contrast
- Image noise
B: Altering brightness raises or lowers all grey levels by same amount
C: Different levels of X-ray exposure
IN: Decreases with increased X-ray exposure
