Performance Assessment of
DR Systems
Director of Symposium
JA Seibert, Ph.D.
UC Davis Medical Center
Sacramento, CA
Digital System Technologies
Projection Radiography
• Computed Radiography (CR)
• CCD cameras
• CMOS detectors
“Direct”
Radiography
(DR)
• TFT Flat Panel arrays
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Speakers for DR symposium
• Robin Winsor, Chief Technical Officer
– Imaging Dynamics Company, Calgary, AB
Canada
• Richard Aufrichtig, X-ray Detector Engrg
– General Electric Medical Systems,
Santa Clara, CA
• Nikos Gkanatsios, Scientist
– Hologic Medical Imaging, Danbury, CT
DR: “Direct” Radiography
....refers to the acquisition and capture of the
x-ray image without user intervention
ACQUISITION
“Indirect” detector: a conversion of xx-rays into
light and then light into photoelectrons
“Direct” detector: a conversion of xx-rays to
electronelectron-hole pairs with direct signal capture
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Large FOV Radiography Systems
• Computed radiography
• CCD linear array and CCD camera systems
• Thin Film Transistor indirect acquisition
• Thin Film Transistor direct acquisition
CCD detector
35 cm
High fill factor ~ 100 %
Good light conversion
efficiency (~85%)
5 cm
5 cm
2.5 cm
43 cm
2.5 cm
Larger CCD
arrays
Scintillator
Optical de-magnification
Lens efficiency?
Secondary Quantum Sink
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Light emission & Optical coupling
Scintillator
Light
Large loss of light!!!
Optical coupling inefficiency
can contribute to
“secondary quantum sink”
Lens
X-rays
CCD / CMOS
Detector
Demagnification
Lens design is crucial!
Optically Coupled CCD cameras
Direct acquisition
Optical design potentially suffers from poor lens collection
efficiency --“secondary
--“secondary quantum sink”
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Scanning-slot Digital Mammo System
Fiberoptic taper – CCD array detector
~8000 x 128 channels
Front View
Side View
Display
Pb
Shield
Phosphor
Fiber Optic
CCD
Direction
of Scan
Computer
A/D A/D
DMA
Large FOV Radiography Systems
• Computed radiography
• CCD linear array and CCD camera systems
• Thin Film Transistor indirect acquisition
• Thin Film Transistor direct acquisition
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Thin-Film-Transistor Array
Laptop display
Photo-emitter
X-ray converter
Photo detector
TFT Active Matrix Array
Amorphous Silicon
TFT active matrix array
Amplifiers – Signal out
G1
Active
Area
G2
Dead
Zone
Gate
switches
ThinThin-Film
Transistor
Storage
Fill Factor = Active area ÷ (Active area + Dead Zone) Capacitor
G3
Ideal detector: 100%
Large pixels: ~ 70% …… ~ 150 µm
Small pixels: ~ 30 % …… ~ 70 µm
Charge
D1
Data lines
CR1
D2
CR2
D3
CR3
Collector
Electrode
Charge
Amplifiers
Analog to
Digital
Converters
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Indirect detector:
a-Si TFT/ CsI phosphor
X-ray
Structured XX-ray
phosphor (CsI)
Light
Source
Gate
Drain
G
S
+
D
Adjacent gate line
TFT
Charge
Photodiode
Storage capacitor
X-rays to light to electrons to electronic signal
Direct detector:
a-Se / TFT array
Incident xx-rays
High
voltage
+
-
- - -
-
+
-
+
+
-
+
+
+
++ +
+
-
Top electrode
Dielectric layer
Selenium photoconductor
Charge collection electrode
(pixel size)
ThinThin-FilmFilm-Transistor
Storage capacitor
Glass substrate
Stored charge
X-rays to electrons to electronic signal
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Direct / Indirect flat panel
detector systems
TFT digital mammo systems
Indirect detector mammo system
Direct detector mammo system
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Digital pre and post processing
• Pre-processing
– Bad pixel corrections
– Shading / nonnon-uniformity corrections
• Post-processing
– Dynamic range compression
– Contrast enhancement
– Spatial resolution enhancement
Flat-field Correction
• Linear response required
• Periodic flat field acquisitions needed
Uncorrected
Contrast Enhanced
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Digital Radiography and Dose
• Flexibility is a doubledouble-edged sword with DR
–
–
–
reduced retakes
variable speed (tailor exposure to exam)…. but
more difficult to correctly use
• All digital detectors should monitor exposure levels
• Good image quality and appropriate SNR are more
important than low radiation dose
• Dose for a given SNR inversely depends on DQE
Digital Radiography QC
• Information?
– Task group #10 AAPM on Computed Radiography
– Draft: email to jaseibert@
jaseibert@ucdavis.
ucdavis.edu
– DMIST digital mammography trial
• Automated daily tests
–
–
Specified tests must verify adequate performance
Deficiencies must be corrected before imaging
• Weekly, Monthly, SemiSemi-Annually
– More complete, quantitative evaluation
– Continuous analysis: correct potential problems
before they occur
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The Bottom Line
• Digital detectors will replace screenscreen-film
• The system is only as good as the weakest link
–
–
–
–
Secondary quantum sink
FlatFlat-fielding / Image processing
Continuing optimization and Quality Control
Soft copy display
• There are several viable “digital” pathways to
the filmless, digital radiology department
Now to the speakers……
• Robin Winsor:
Winsor: CCDCCD-based digital
radiography system
• Richard Aufrichtig:
Aufrichtig: Indirect TFT detector
radiography system
• Nikos Gkanatsios:
Gkanatsios: Direct TFT detector
mammography system
• Followed by a 30 minute panel discussion, so
get your questions ready……
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