MEDRADSC 2Y03 Lecture Notes - Lecture 16: Flat Panel Detector, Amorphous Silicon, Radiography
25 Oct 2019
School
Department
Course
Professor
Week 10 Lecture 1
November 13th, 2018
• Digital detectors
• Digital images
• Detectors
• Pre and postprocessing tools
Objectives
• Convolution
• Kernel
• Edge enhancement
• Smoothing
• WW
• WL
• Post-processing
Digital Radiography
• Scan projection radiography (CT)
Overview of Direct Read System and Indirect Read System
Indirect Read System
• X-ray photons (analog) → light → electrons (digital)
Direct read Systems
• X-ray photons → electrons
• Can use amorphous silicon or selenium
Generic Design of Flat Panel Detectors
• Amorphous selenium (Z=34) in direct read systems
o More popular because of its atomic number
• Amorphous silicon (Z=14) is deposited in multiple layers on glass substrate in indirect
read systems
• Conducting paths embedded within layers
Indirect Flat Panel Detectors
• Scintillating (x-ray to light) material is layered on the front surface of flat panel detector
• Light produced from the screen will strike the flat panel detector
Indirect Flat Panel Detector
• Much of the light produced from the scintillating material has to travel a relatively large
distance
• This will cause more blurring
• To improve this situation flat panel detectors may use CSI grown in “needles”
Cesium Iodide Needle Crystals
• Thick CSI layer has high QDE
• Needles act as light pipes, channeling light toward photodiode
• Minimal light spreading within phosphor
CSI/a-SI
• Csi
• A-SI
o Amorphous silicon
• High photoelectric capture
o This will create a better spatial resolution
• Z = 55 and 53
Digital Images
Indirect Detector Construction
• This is for indirect
• CCL → collects the light
Detector
• Detector elements could be called dexels
• Pixels have to have an electronic area, light senesitvie area and detector area
• The more light senitive areas the more useful the pixel is
Fill Factor
• High fill factor → areas that are taken up by other things
(storage or electronics) are smaller
• We want a high fill factor
• Want a portion of the sensitive pixel to the incoming pixel
→ better spatial resolution, higher fill factor
• 100% fill factor = great surface area sensitive to the
incoming signal → less dose to the patient
Digital Matrix
• Voxel is the depth
• Pixel is the picture element
