Digital Imaging Flashcards
(45 cards)
how many pixels/matrix in average image
256x256=65536 pixels
what is a pixel
image made up of pixels (picture elements)
each pixel has specific grey shade (black to white)
pixels arranged in a matrix of fixed dimension
define image sharpness
how well you can distinguish 2 close together objects
how is spatial resolution measured
line pairs per mm (lp/mm)
spatial resolution of
a. conventional film
b. human eye can resolve
c. current dental systems use
a. conventional film: 22lp/mm
b. human eye can resolve:8
c. current dental systems use:between 6 and 22
what is pixel shade
each pixel assigned a shade of grey depending on amount of radiation that has reached the sensor
what is number of grey shades dependent on
bit depth of image file
how to calculate bit depth
2 to power n
2 types of dental digital imaging technologies and name examples
- direct: virtually instant image. Charge couple devices (CCDs), complementary metal oxide semi-conductor (CMOS)
- indirect: plate must be read –> several seconds before image available. computer radiography plates
process of direct digital imaging
- scintillation layer converts x rays to light
2. semi-conductor based pixel array converts light to electron charge
compare process of CCD and CMOS
plate exposed to light –> energy released
CCD: light passed along pixels and read at the end
CMOS: light read from each pixel individually
adv/disadvantages of CCD and CMOS
CCD: -expensive to manufacture (have to transport charge across chip without converting)
- create high quality low noise images
- higher fill factor
- better pixel uniformity
explain fill factor and apply to CCD/CMOS
how much of pixel converts light to charge.
CCD: whole pixel catches light and converts it to charge –> high fill factor
CMOS: space on each pixel for microcircuitry for detection so less efficient conversion of light energy (lower fill factor)
explain pixel uniformity and apply to CCD/CMOS
same amount of amplification from each pixel
CCD: same in all pixels because light is passed between pixels
CMOS: all individual detection –> poor pixel uniformity (bad)
how digital x rays are seen on pc stages
scintillation layer –> pixels –> wiring (voltage) –> analogue digital converter (ADC) –> converted to discreet numbers for digital signal –> image seen on PC
matrix number for
a. black
b. white
a. black: 0
b. white: 255
why are wireless sensors good
use less radiofrequency transmission (less xray exposure)
why are all in one digital imaging devices bad 2
- have to hold it –> unsteady, blur
- operator has to stand v close to x ray –> exposure to x rays
2 alternative names for indirect digital imaging
- photostimulable phosphor plates PSPs
- computed radiography CR
how do PSPs work
- plastic plate coated with storage phosphor material sensitive to x rays
- phosphor stores x ray energy (like latent image in photographic film)
- plate placed in laser reader/computer which releases photon energy as light
what is a phosphor
anything that converts one energy to another ie x rays –> light
what way round to put in phosphor plates and why
magnet side away from machine, otherwise would get image of magnet
advantages of PSPs over CCD/ CMOS 5
- larger variety of imaging areas
- no bulky connecting cable
- slimmer imaging plate (no electricity built in)
- cost: PSP £20-50 per plate, CCD/CMOS £1000+ per sensor
- has linear response to xrays over a wider range than most CCD/CMOS (no blooming)
explain blooming
overload of pixels and bleeding in to adjacent pixels –> sensor does not respond in linear fashion to areas of high radiolucency eg pharynx
