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Technology for people
Amorphous Silicon Pixel Detectors for Radiography
Gerhard RoosVarian Imaging Products
Presentation Outline
Background
Operation
Typical Specifications
Current Applications and Trends
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Background
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Definition
Large area integrating image detectors
Deposition of amorphous silicon structures on glass
Inverse flat panel display
Mostly used for medical radiographic imaging
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Flat Panel History
1985 to 90 – Theoretical studies and early research-Xerox PARC, Varian, GE, Thomson, Philips, U. Mich. etc.
1991 – First small area functional prototypes1993 – First full size (40 x 64 cm) prototypes1997 – First commercial products announced1998 – First production shipments1998 – Cone Beam CT application published 2003 – First commercial CBCT products2006 – Annual volume >8000 imagers
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
a-Si as image detector
Advantages:Low cost per area, large area per deviceRadiation hardGood quantum efficiency
Disadvantages:Time responseUniformitySimple, relatively large pixel structures onlyComplex system implementation
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
PaxScan 4030 Imager
Receptor:14” x 16” x 1.5”- 2.5”Fiber-optic data cable24V dc power cablePassive cooling
Command Processor:10” x 11” x 2.5”Real-time normalizationRecursive filterFrame accumulationX-ray generator control signals16 bit digital video output
Power Supply:10” x 11” x 2.75”5V/10A, 24V/3A
Operation
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
x-ray photons
a-Si arraybase plate
Driver boardReadout board
Digitalboard
Readout board
Readout or DriverIC in TAB package
scintillator
Indirect Conversion Imagers
Indirect Conversion Imagers- Sensor Schematic
Row 1
Row 2
Row 3
Vbi
as
Ga
te D
riv
er
Readout ASIC
Indirect Conversion Imagers- Actual pixels
Row Line
Data Line Bias Line
Photodiode
TFT Switch
One Pixel
Commonly Used Scintillator Screens
Phosphor Cesium Iodide
Layer Structure
LightSpreading
x-ray photons
photoconductora Si -TFT array
base plateReadout board
Driver board
ADCboard
Driver board
Top electrode
Direct Conversion Imagers
Direct Conversion Imagers- Sensor Schematic
Row 1
Row 2
Row 3
Vbi
as
Ga
te D
riv
er
Readout ASIC
Typical signal readout
sample
reset switch
TFTCdata2
Rdata
Cfb
pindiode Cpix
Coverlap
_
+
Vbias
Cdata1
Vref
Vgate
Preamp
i2
vin2
a-Si array readout chip
Cin
Dynamic gainswitch
Image Normalization Requirements
Thresholding
Offset & Gaincorrection
RecursiveFilter &
Accumulation
DefectivePixel
Replacement
Specifications
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Typical Specifications for Medical Flat Panels
Entrance exposure: 5 nGy/frame to 50 µGy/frameHigh exposure DQE:
Diagnostic ~70% at 80 kV and ~40% at 125 kVTherapy Portal Imaging ~2% at 6MV
Photodiode quantum efficiency: ~80% (500 to 600 nm)Spatial resolution (10% MTF): 3.5 to 10 lp/min12 to 17 bit A/D conversionPixel time constant (Cpix x RTFT): 1 to 10 usPixel size: 65 µm to 500 µmPixel capacity: 0.5 to 50 pCReadout rate: >32 µs/line; 0.1 to 100 frames per secondDark current: <1 pA/mm2
Active area: 200 x 200 mm to 430 x 430 mm
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Flat panel dynamic range
Flat panelnoise ~1000 ē
Saturation ~ 60M ē
60k range at 1x1
120k range at 2x2
Charge amplifiernoise ~1700 ē
Saturation ~ 12M ē
7k range at 1x1
10k range at 2x2
A/D converter< 16k range
Multiple gain ranging flat panel read out
Signal-to-noise performance
1.0
10.0
100.0
1000.0
0.1 1.0 10.0 100.0 1000.0 10000.0
Dose rate (uR/frame)
Sign
al-to
-Noi
se ra
tio
DQE in Low Dose Fluoroscopy
PVD HgI2 imager #37
Image lag
First frame lag
4.7%
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 5 10 15 20
Time after exposure (s)
Frac
tion
of e
xpos
ure
(%)
Applications
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Cardiac Imaging- Radiography and fluoroscopy
DSA Images of the Liver
85kVp, 200mA, 80msec 85kVp, 200mA, 100msec
Image intensifierFlat panel imager
• Better contrast for imaging small blood vessels, at 20% less dose
• Image quality is uniform out to the edges
Radiographic images
Radiographic images- direct vs. indirect conversion
Radiation Therapy
Therapy Portal Imaging
Orthogonal Radiography and fluoroscopy
Cone beam CT
CBCT scanof a human head
MV Portal Images
Also:
Mamography and CT Mamography
Wireless radiography
Dental radiography and CT
Security applications
Industrial non-destructive testing
Scientific research
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Alternatives
Amorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image DetectorsAmorphous Silicon Flat Panel Image Detectors
Improved MV Sensitivity- CsI array scintillator
CsI
Flat Panel Sensor
CsI array Scintillator
CsI array Scintillator
Prototype MV Imager
Comparison
Conventional CsI Array Scintillator
Dose = 0.092 cGy Dose = 0.046 cGy
Fast readout
Strip Detectors:Strip Detectors:64 x 5763.3 x 30 cm520 µm pixel1700 fps 1x13000 fps 2x2Photon counting mode
Phase-2 Concept