Sensors and Digital Imaging pt. 2 Flashcards
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how does it work?
cesium iodide scintillator converts x-rays into visible light
photosensitive pixels convert scintillators light into electrical signals. on-chip circuitry turns electrical signals into digital output
digital data is sent to a computer
digitization
CCD, CMOS types
x ray photons cause ionization of pixel silicone; ejected electrons’ are trapped in the pixel similar to the charge in a battery
simplified sensor concept
each pixel is like a tiny battery with its charge equal to the number of electrons trapped inside. the computer records the charge in each pixel and converts it into a gray value
— shades of gray
256
255- light
0- dark
To be able to manipulate the image, the
computer will turn the voltage signal to
numbers and then to shades of gray
All images consist of ‘pixels’ (picture
elements), including (3)
human vision,
insect vision and photographs
Each pixel is characterized by
its (2)
location and intensity.
Digital computer technology allows for enhancement through the
manipulation of the
binary bits of each individual pixel !
011000100011100101001000011110101…
Sensor thickness “sweet spot” is
4-
6mm i.e.
— source preferred i,e, not
limited to proprietary software
Open
Cost – is related to (2)
buying price +
warranty
Size – most come in sizes equivalent
to
dental film
Shape – (3)
square, rounded or cut (See
product examples)
types of sensors (2)
CMOS vs CCD
Resolution
Pixel size -
~ 15 μM (10-6 M)
~ 15 microns (10-6 M)
~ 15 10-3 mm
Line pair resolution
Actual – visible
Theoretical – calculated
- > 20 lp/mm
- > 26 lp/mm
CCD
(3)
Older technology
Require an additional
power via an adjunct box
Need additional power to
convert the photon energy
to an electronic digital
signal
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principle of CCD detector
X-ray → scintillating material → light photos → silicon → electrons
deposited in electron wells → transferred in a sequential manner
(charge-coupling) → read-out amplifier → images on monitor
first CCD system
1988
Radiovisiography
Trophy X-Ray
Vicennes, France
Invented by
Dr. Francis Mouyen
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principle of CMOS detector
X-ray → scintillating material → light photos → silicon → electrons
deposited in electron wells and converted to voltage in each pixel→
smoother signal digitization → software processing → images on
monitor
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CCD-CMOS receptor comparison
Power Consumption
Sensitivity to X rays
Cost to manufacture
Readout
Pixel Signal
Chip output
400 mW, 40 mW
High, low
High, medium
Complex, simple
Electrons, Voltage
Voltage (analog), Bit (digital)
CMOS
(3)
Newer technology
Require less power
Superior image quality
bit depth
the number of colors (or gray shades) that a pixel is able to show
2 (bit) = number of shades of gray
1 bit (21) = 2 shades of gray
2 bits (22) = 4 shades of gray
3 bits (23) = 8 shades of gray
4 bits (24) = 16 shades of gray
8 bits (28) = 256 shades of gray
16 bits (216) = 65,536 shades of gray
24 bits (224) = 16.7 million shades of gray
