State-of-the-Art Full Field Digital
Mammography – Detector Technology
used in FDA Approved FFDM Systems
and Newly Emerging Detector Technology
AAPM 2004 Annual Meeting
CE: Mammography Physics and Technology – 8
Jerry A. Thomas, M.S., DABR, CHP, DABSNM
Assistant Professor of Radiology
Department of Radiology and Radiological Sciences
Uniformed Services University
Bethesda, MD 20814
Image Quality Advantages
Film-Screen vs. Digital
Film Screen
Digital
Higher Spatial Resolution
Higher Image Contrast
Higher DQE (SNR, Dose)
Contrast & Dynamic Range
Screen - Film
Characteristic
Curve
Low
Skin Line
High
Average Tissue
Very
Low
Dense Tissue
Log Signal
Optical Density
Contrast
Digital Detector
Characteristic
Curve
.
Low Med. High
Log Relative Exposure
Log Relative Exposure
3-4 x Improvement in Dynamic Range
Spatial Resolution for Mammography
GE Panel
Kodak Min-R
Contrast transfer function
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
Spatial Frequency (line pairs/mm)
10
Detective Quantum Efficiency (DQE)
•
DQE of a detector
SNR2 at detector output
DQE =
SNR2 at detector input
SNR = signal-to-noise ratio
•
Measures transfer of both signal and noise.
Detective Quantum Efficiency (DQE)
•
Input SNR2 proportional to patient dose
DQE ∝
Image Quality
Patient Dose
• Noise is a limiting factor for detectability, image
processing, and advanced applications
• Doubling DQE means:
• Same output SNR (“image quality”) at half the dose
• 40% improvement in SNR at same dose
DQE and Noise
Control DQE & noise by:
•increasing x-rays reaching detector
•efficient detection of x-rays
- efficient conversion to signal
- efficient scatter rejection
•efficient coupling between elements in
the detection chain
•Low electronic noise
Applications of Digital
Mammography
• Large Field Systems
– Diagnostic application
– 20 x 30 Field-of-View
– 2000 - 4000 pixels
– 50 - 100 um pixel size
Approaches to Digital
Mammography
•Scanning slot
•Large amorphous silicon arrays with CsI
•Direct conversion/selenium plates
•Photostimulated Luminescence
•Silicon microstrip detectors
FFDM Systems
Technology
Detector
Vendor
Single Sided Reading
Agfa
Dual Sided Reading
Fugi
CR
CsI - CCD
Fischer
Silicon Microstrip
Sectra
Scanning
Slot
Flat Panel
a-Silicon
Panel
CsI
GE
a-Selenium
Lorad
Siemens
Agfa
Planmed
Dose-Efficiency for Digital Mammography Sensors
Indirect Sensors
9X-rays converted into light
9Light converted to analog signal
9Analog converted to digital signal
Noise Source(s)
9Conversion of x-rays to light/analog/digital signals
9Detector dark current
9Electronic noise
Examples:
™Film screen
™CsI to CCD’s or photo-diodes (a-Si panels)
Dose-Efficiency for Digital Mammography Sensors
Indirect Sensors
Direct Sensors
9X-rays converted to analog signal
9Analog converted to digital signal
Noise Source(s)
9Conversion of x-rays to analog/digital
signal
9Detector dark current
9Electronic noise
Example:
™a-Selenium
Dose-Efficiency for Digital Mammography Sensors
Indirect Sensors
Direct Sensors
Direct Photon Counting Sensors
9X-rays detected/counted individually
Noise Source(s)
9Eliminated by using thresholding
Example:
™Silicon microstrip
SCANNING MOTION OF DETECTOR
Fischer Imaging Corporation
Fischer Imaging
SenoScan FFDM System
Requirements of the Phosphor for
Scanning Slit Detector
• High DQE - detector quantum efficiency
• Fast rise time
• Short persistence (low afterglow)
• Low radiation memory
Composition of
Cesium Iodide
Senoscan® Detector
m
m
221
Fiber Optic
Plate
4 CCD’s
m
10
m
Fischer Slot Scan FFDM
Detector:
• 1.0cm x 22 cm (scans 30 cm)
• 4 CCD chips (each 405 x 2048 pixels)
• 27 Micron pixel at CCD
• Use 2 x 2 electronic averaging
• 54 x 54 micron pixels
• Matrix 4096 x 5600
• 60 micron dead zone between CCD’s
Fischer Slot Scan FFDM
Scintillator:
•CsI
•0.150 mm thick
•85% absorption at 20 kev
Fischer Slot Scan FFDM
Scan time:
•5 sec total scan
•200 mA
•200 ms exposure to tissue (40mAs)
•1200 mAs
•22 x 30cm image area (standard)
Fisher Slot Scan FFDM
X-ray Tube:
• 4 inch Tungsten-Rhenium target
• 7 degree target angle
• 4 cm spacing
• Axis perpendicular to scan
• Use 32 – 34 KVp ( SNR Contrast)
• Collimate to 1.2 cm width
Fischer Slot Scan FFDM
•High Resolution mode
- ½ field length and
width (11x15)
- ½ scan velocity
- 400 ms tissue exposure (80mAs)
- 27 x 27 micron pixels
- same matrix and scan time
- double dose
Time Delay Integration
Transfer
Gate
Charge Transfer
4 3 2 1
Column Vector
Velocity Vector
SR
CCD
Time Delay Integration
Accuracy dependent upon:
•Synchronism of detector motion and charge
transfer rates
•Alignment of detector motion vector and pixel
matrix
•Efficiency of charge transfer between pixels
MTF
Fischer System MTF
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Slot 27 u Pixel
Slot 54 u Pixel
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Frequency (cycles/mm)
a-Silicon Array with CsI
Amorphous
Amorphous
Contact
Contact Fingers
FingersSilicon
Silicon Array
Array
Contact
Contact Leads
Leads
For
For Read-Out
Read-Out
Electronics
Electronics
Glass
Glass Substrate
Substrate
Scintillator
Scintillator
(CsI)
(CsI)
Opticlad Reflector
Graphite Cover
Scintillator
a-Si Array
Epoxy Seal
Glass
Substrate
CsI a-Silicon Array
Scintillator & Reflector
Reflector
300 to 400 µ CsI
X-Ray Photons
Light photons
guided via CsI
needle
structure
10µ needle width
preamp
preamp
preamp
preamp
Photo D
Drivers
Scintillator
Photo D/TFT
preamp
preamp
preamp
preamp
TFT
Flat Panel-Light Sensor
Very
High Fill
Factor
Sensitive Area
Fill Factor =
Pitch x Pitch
data line
Pitch
Pitch
FET
scan line
Flat Panel-Light Sensor
data line
scan
electronics
(DRIVERS)
scan line
FET
Photodiode
• Operation - single element
– FET is turned on --diode is
charged
– FET is turned off
– X-rays exposure creates
electron-hole pairs in diode
causing it to partially discharge
– FET is turned on--diode is recharged--amount of charge
needed is measured
readout
electronics
(DATA COLLECTION)
Vbias
GE Senographe 2000D
a-Selenium Technology
Photons
Selenium
Electrons
Amorphous Silicon Panel
Electrons
Read Out Electronics
Digital Data
X-rays
Top electrode(A)
_
V
_
+
+
a-Se
Field effect transistor channel
Gate(A1)
Glass substrate
Si02
Storage Capacitor Cij
Pixel electrode (B)
Ground
High Efficiency of a-Se
•Selenium provides almost 100% charge collection efficiency due to
electrostatic field effects in the photoconductor
•Almost complete absorption of the x-rays is achieved within ~200µm
layers
•Selenium allows high fill factor for small pixel size required in
mammography
-HV
top bias electrode
a-Se
+
-
trapped
image
charge
- - - -
- - - -
- - -
- - - - -
Blocking layer
Pixel electrodes
- - - -
- -
MULTIPLEXER
Digitizer computer
SCANNING CONTROL
Gate line (i+1)
Storage capacitor
Gate line (i)
i,j-1
i,j
i,j+1
Thin film transistor
Gate line (i-1)
Pixel electrode (B)
(j-1)
(j)
(j+1)
Data (source) lines
a-Se Flat Panel Detector
Detective Quantum Efficiency vs. Exposure
Lazzari,et.al
SPIE 2003
MTF of a-Se detector
1
0.9
0.8
0.7
MTF
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
Frequency, lp/mm
5
6
a-Selenium Detector
•Hologic/Lorad product approved: 24x29cm
* 70 micron pixel size
•Siemens (OEM detector from Anrad)
•Agfa (OEM detector/unit from Hologic/Lorad)
•Instrumentarium evaluating 17 x 24 prototype
* 85 micron pixel size
•Planmed (OEM detector from Anrad)
*Same detector specifications as Instrumentarium
Amorphous-Selenium Detector
Detector images to chest wall edge
Selenia
HTC grid retracts
Small foot print
5 cm at chest wall
Siemens
Taylored solutions for
different needs
High
Volume
FFDM FD
(aSe detector)
FFDM CR
(syngo-based &
workflow-optimized)
FFDM CR
(multi-organ)
syngo
syngo
Low
Volume
NonDedicated
Dedicated
Agfa Digital Mammography Unit
Plus Agfa workstation with
Musica Image Processing Software.
Instrumentarium Detector Specifications
• Detector size:
– 174 x 239 mm
– Larger size will be
available in the future
• Pixel size
– 85µm
• Matrix size:
– 2048 x 2816
• Image size:
– 11Mb
CR Technology
Protective Layer
Photo-stimuable phosphor layer
Support
Fuji Mammo CR
•Pixel size 50 micron
•Use dual sided reading to DQE
•Imaging Plate has thicker phosphor layer
•Results in 40 - 50% increase in NEQ (DQE)
•Post processing enhances image
(e.g. filtering to recognize and enhance
calcifications)
Protective
layer
Photo-detector
Laser beam
Phosphor
layer
Optical
mirror
guide
TransImaging plate
emission
Parent
support
Optical guide
Photo-detector
Front side
Spatial Filter; 1-t
Image 1
Image 1’
Addition
Back side
Addition
Image
Spatial Filter; t
Image 2
Image 2’
t;addition ratio
Addition Image = (1-t) x Image1 + t x Image2
1.2E+05
Dependency of NEQ on
Addition Ratio at Each
1.0E+05
Spatial Frequency
0.5c/mm
8.0E+04
6.0E+04
1.5c/mm
4.0E+04
2.5c/mm
2.0E+04
Back
Side
image
Addition Ratio (front back)
1.0F0.0
0.8F0.2
0.7F0.3
0.6F0.4
0.5F0.5
0.4F0.6
0.3F0.7
0.2F0.8
0.0E+00
0.0F0.9
3.0c/mm
0.0F1.0
The maximum image
quality with this new
method may be obtained
when optimal addition
ratio is simultaneously
used for each frequency.
2.0c/mm
0.9F0.1
NEQ
1.0c/mm
Front
Side
image
1.0
Response
t
1-t
0
Spatial Frequency
Fuji
CR Mammography
Reader
Fuji Clearview Reader
• Four cassette stacker
• Processes 20 screening exams/hour
(or 100 CR plates/hour)
• Multi-objective Frequency Processing (MFP)
(enhances dense/peripheral tissue)
• Pattern Enhancement Processing for
Mammography (PEM)
(enhances calcifications)
Agfa CR Mammography Cassette/Plate
Agfa Mammography CR Plate
100 micron pixel size
Agfa CR Plate Reader
Sectra
TM
MicroDose
Mammography
• Equal or better image quality using only 1/5 of the
radiation dose compared to modern
film-screen
systems
• One patient every 4-5 minutes
• A digital solution that support the entire workflow
from examination to a documented diagnose
Detector Technology - Conversion steps
CsI+aSi flat panel
aSe flat panel
Photon Counting
X-ray Photon
X-ray Photon
X-ray Photon
Scintillator
aSe
amplification
X-ray into electrons
Light
Electrons
Capacitor
Capacitor
charge storage
charge storage
Analogue signal
A/D - Converter
Digital signal
Analogue signal
A/D - Converter
Digital signal
MicroDose
Detector
5
4
3
2
1
Digital signal
5 (00000000000101)
Digital Mammography Unit
Analogue
Film Cassette
Digital
Digital Detector
What Decides the DQE on System Level?
DQE on System Level
• Detector technology
– Noise in the electronics and the detector
– Absorption efficiency in the detector
– Energy Weighting - How the information in the photons are transferred to the
image as a function of their energy
• Scattered radiation
– Reduction of scattered radiation
• X-ray spectrum
– The spectrum of the X-ray beam
Detector Technology - Photon Counting
Silicon microstrip detectors
•Silicon p+n junction
•Reverse bias
•Edge-on exposure
•3 mm deep – 95% absorption to 20 kev
•Counts individual photons
-digitized upon detection
-suppresses noise
Silicon microstrip detector
•Dead volume layer on top
•Detector several cm long
•Detection rate 5 x 106 x-rays /sec/pixel
•Slit-scanning, 5 sec. scan time
•Very low dose
Detector Technology - Photon Counting
Detector Technology - Photon Counting
Detector Technology - Photon Counting
Detector Technology - Photon Counting
Detector Technology - Photon Counting
Photon Counting - No Electronic Noise
Energy
X-ray photon
Lowest Photon Energy
Threshold
Electronic Noise
1
2 3
Time
Scattered Radiation – Scanning Technology
Scattered Radiation - Scanning Technology
X-ray photons
Scanning technology with
slit detector
• 97% reduction
• High SNR
• Minimal dose
What decides the DQE on System level?
Generally
Sectra MDM
•
Noise in the electronics and the
detector
9 No noise
•
Absorption efficiency in the detector
9 More than 90% absorption
•
How the information in the photons
are transferred to the image as a
function of their energy
9 Every X-ray photon is processed
individually
•
Reduction of scattered radiation
•
The quality of the X-ray beam
9 Scattered radiation is reduced
by 97%
9 The beam quality from the
Tungsten anode is optimal for
the MicroDose detector
Digital Detector - Facts
•
Photon counter
– Counts every photon
•
Superb image quality
– 50 µm pixels (0.05 mm), 10 lp/mm
– Image matrix 4800 x 5200 pixels
– 14 bits contrast resolution
– Large dynamic range
– Excellent DQE
•
Dose - efficient
•
Large FOV, 24 x 26 cm
Digital Mammography image
DQE on System Level - Comparison
Picture of review station - #10
Benefits of Digital Mammography
•
•
•
•
•
•
•
•
Reduced radiation dose
Post-acquisition image processing
Rapid image display
Improved imaging of dense breasts
Improved sensitivity and specificity
Simplified archival, retrieval, and transmission
Potential for computer-aided diagnosis
Potential for telemammography
Digital Detector
Advanced Applications
•
•
•
•
•
•
Computer-Aided Detection
Tele-Radiology
Tomosynthesis
Dual-Energy Subtraction
Digital Subtraction Angiography
Stereo / Computer Aided Localization
Comparison of ACR
Phantom Scores
6
5
4
3
Digital System
Film System
2
1
0
Fibers
Calcs
Objects
Mass
Conclusions
• Digital mammography has numerous
potential advantages over film screen
imaging
• Full field systems approved, more expected
• Present unique challenges to assess
– trade-offs of technologies
– performance assessment
– QC/QA procedures
Acknowledgements
Thanks for input from:
• Agfa Gaevert
• Instrumentarium Imaging
• Fischer Imaging
• Fujifilm Medical Systems
• GE Medical Systems
• Hologic/LORAD
• MTMI
• Planmed
• Sectra Imtec AB
• Siemens Medical Solutions
THANK YOU