MEDRADSC 2Y03 Lecture Notes - Lecture 15: Optical Fiber Cable, Digital Radiography, Flat Panel Detector
25 Oct 2019
School
Department
Course
Professor
Week 9 Lecture 1
November, 2018
Objectives
• Describe the CCD used in digital imaging and explain function
• Pre and post-processing for Digital Radiography
Charged-Coupled Device (CCD)
• CCD is a solid state device that converts visible light photons to electrons
• CCD is coupled to a screen that gives off light
• Excellent coupling is possible
• Fiber optic cables
• Little light is wasted
Good CCD Coupling
• Digital radiography not CR
• The dexels are related at the bottom with the CCD array
• The x-ray beam hits the phosphor and the light is
channeled down and collected by the CCD
• The fiber optic array allows the light to move down
Charged-Coupled Device (CCD)
• Most video cameras and in digital cameras
• In medical imaging
o Can be used either for still or dynamic (multiple frames per second) imaging
o Virtually all digital (and many analog) fluoroscopy and digital mammography
systems use CCD cameras
o Some direct-read systems
o Can detect a wider range of light at the lower end (when there
is smaller amount of it they can still find it)
CCD
• Helps form the electronic signal form the light
• Can have a silicon surface as its photosensitive layer
CCD
• Typical size = 2.5cm x 2.5cm
• Pixels = 1024 x 1024 or 2048 x 2048
• Light → silicon surface → electrons → collect in dexels
• Intensity of light → electrons are produced in dexels
CCD
• Has discrete dexels electronics and hold it
• Etched on to its surface
• Larger 8cm x 8cm chips have been produced → expensive
CCD
• Read out process:
• Bucket brigade system
• Signal goes from one to the next until its moves to the next
and then it moves over
CCD
• When it gets to the bottom it is read out
• Voltage is used to move the dexels
• It will dump it into a transistor which will help control the
flow in order to digitize the signal
CCD
• Read out process
• Quantization of array charge values
• The bottom row shifts entire electronic charge across
• Dumps it into a transistor
• Electronic signal is produced and digitized
• The flow is controlled by the transistor
Advantages of CCD
• 3 main advantages for medical imaging
o sensitivity
o dynamic range
▪ nice gray scale
▪ less patient dose then PMT
o size (dexel)
DR – Direct Digital Radiography
• direct acquisition systems directly convert incoming x-ray photons to an electronic
signal (x-ray to electronic signal)
• these systems use selenium and are sometimes referred to as direct radiography (DR)
systems
• the term flat panel detector is used to describe both the indirect amorpous silicon and
the direct amorphous selenium plates used in some DR systems
• NOTE: the phosphor type used in room 5 is cesium iodine with a 100% fill factor
o Want to keep fill factor small so there is more area for useful image
o Therefore, there is no limitations → whole thing is for the image
o Fill factor is the size of the PD (photodiode)
