Overview of Digital
Detector Technology
J. Anthony Seibert, Ph.D.
Department of Radiology
University of California, Davis
Disclosure
• Member (uncompensated)
– BarcoBarco-Voxar Medical Advisory Board
– ALARA (CR manufacturer) Advisory Board
Learning Objectives
• Describe digital versus screenscreen-film acquisition
• Introduce digital detector technologies
• Compare cassette and cassettecassette-less operation
in terms of resolution, efficiency, noise
• Describe new acquisition & processing techniques
• Discuss PACS/RIS interfaces and features
1
Conventional screen/film detector
1. Acquisition, Display, Archiving
Transmitted xx-rays
through patient
Film processor
Exposed film
Developer
Fixer
Wash
Dry
Gray Scale
encoded on
film
Intensifying Screens
Film
x-rays → light
Digital xx-ray detector
2. Display
Digital to Analog
Conversion
Digital Pixel
Matrix
1. Acquisition
Transmitted xx-rays
through patient
Digital
processing
Analog to Digital
Conversion
Charge
collection
device
X-ray converter
x-rays → electrons
3. Archiving
Analog versus Digital
Spatial Resolution
MTF of pixel aperture (DEL)
1
100 µm
Modulation
0.8
200 µm
0.6
1000 µm
0.4
0.2
0
0
1
2
3
4
5
6
7
8
9
10 11
Frequency (lp/mm)
Sampling
Pitch
Detector
Element,
“DEL”
DEL”
2
Characteristic Curve:
Response of screen/film vs. digital detectors
5
Useless
10,000
FilmFilm-screen
(400 speed)
Digital
1,000
3
2
Overexposed
Useless
100
Correctly exposed
1
10
Relative intensity
Film Optical Density
4
Underexposed
0
0.01
20000
0.1
1
10
100
1
Exposure, mR
2000
200
20
2
Sensitivity (S)
Analog versus digital detectors
• Analog
–
–
–
–
Coupled acquisition and display
Higher resolution
Limited dynamic range, fixed detector contrast
Immediate exposure feedback
• Digital
–
–
–
–
Separated acquisition and display
Lower resolution
Higher dynamic range and noisenoise-limited contrast
Proper exposure potentially hidden
Image Processing
• Crucial for optimal image presentation
• Flexibility adds potential advantage
– DiseaseDisease-specific image processing
– Computer aided detection
3
Digital System Technologies
Projection Radiography
• Computed Radiography (CR)
• CCD
“Direct”
Radiography
(DR)
• CMOS
• Flat Panel (TFT) arrays
Consideration: “Cassette”
Cassette” vs. “Cassetteless”
Cassetteless” operation
Computed Radiography (CR)
...is the generic term applied to an imaging
system comprised of:
Photostimulable Storage Phosphor
to acquire the xx-ray projection image
CR Reader
to extract the electronic latent image
Digital electronics
to convert the signals to digital form
Image Acquisition
CR QC
Workstation
Patient information
Latent image produced
CR
Reader
Latent
image
extracted
DICOM / PACS
Laser film printer
Display / Archive
4
CR: Photostimulated Luminescence
τ
phonon
Conduction band
tunneling
τ recombination
Energy
Band BaFBr
4f 6 5d
Laser
stimulation
F/F+ e-
PSL
3.0 eV
8.3 eV
2.0 eV
τ Eu
4f 7
/
Eu 3+ Eu 2+
e
Incident
x-rays
Valence band
PSLC complexes (F centers) are
created in numbers proportional to
incident xx-ray intensity
CR: Latent Image Readout
Reference
detector
f -θ
lens
Cylindrical mirror
Light channeling guide
Laser
Source
Output Signal
Polygonal
Mirror
ADC
PMT
Laser beam:
Scan direction
x= 1279
image
y=To1333
z=processor
500
Plate translation:
SubSub-scan direction
Phosphor Plate Cycle
PSP
x-ray exposure
Base support
plate exposure:
create latent image
reuse
laser beam scan
plate readout:
extract latent image
light erasure
plate erasure:
remove residual signal
5
CR Innovations
• HighHigh-speed line scan systems (<10 sec)
• DualDual-side readout capabilities (increase DQE)
• Structured phosphors
• Mammography applications ??
• Low cost tabletable-top CR readers
DR: “Direct”
Direct” Radiography
....refers to the acquisition and capture of the
x-ray image without user intervention
– “Indirect”
Indirect” detector: a conversion of xx-rays into
light and then light into photoelectrons
– “Direct”
Direct” detector: a conversion of xx-rays to
electronelectron-hole pairs with direct signal capture
CCD detector systems
• Area Scintillator /
lens coupling
CCD
Scintillating Screen
(Gd2O2S), CsI
• Area Scintillator /
fiberoptical coupling
Scintillating
Screen (CsI)
Array of
CCD
Fiber Optical Cameras
Coupling
• Slot scintillator linear array
fiberoptical coupling
6
CCD area detector
35 cm
High fill factor ~ 100 %
Good light conversion
efficiency (~85%)
5 cm
5 cm
43 cm
2.5 cm
2.5 cm
4 to 16 megapixels
Optical de-magnification
Lens efficiency?
Secondary Quantum Sink
Optically coupled CCD systems
• Technology improvements are overcoming
quantum sink issues (lens / phosphor)
• Low cost systems for budgetbudget-limited situations
• Capable imaging systems
Scanning slot chest xx-ray system
•
•
•
•
•
CCD array
Fiberoptic coupling
No grid
Reduced scatter
Low dose
• “Effective”
Effective” DQE compares to flatflat-panel systems
7
CMOS
• “RAM”
RAM” with photodiode converter
• Random access readout
• Low voltage operation (5V)
• ? NOISE ……
• Large FOV detector available (tiled CMOS)
CMOS
DECODER
LATCHES
COUNTER
ROW DRIVERS
Complementary Metal Oxide Semiconductor
PIXEL
ARRAY
COLUMN SIGNAL CONDITIONING
CLK
RUN
DEFAULT
LOAD
ADDRESS
DATA
+5V
DECODER
TIMING
AND
CONTROL
COUNTER
VS_OUT
VR_OUT
READ
FRAME
LATCHES
Array of tiled CMOS sensors
Xrays
Scintillator
Fiberoptic plate
Microlens optics
CMOS sensors
Controller electronics
7000x7000 element array, 17”
17” x 17”
17” FOV for one implementation
8
ThinThin-FilmFilm-Transistor Array
Laptop LCD display
PhotoPhoto-emitter
X-ray converter
Photo detector
TFT Active Matrix Array
TFT active matrix array
Amplifiers – Signal out
G1
Active
Area
G2
Dead
Zone
G3
D1
Data lines
CR1
D2
CR2
D3
CR3
Gate
switches
ThinThin-Film
Transistor
Storage
Capacitor
Charge
Collector
Electrode
Charge
Amplifiers
Analog to
Digital
Converters
ThinThin-FilmFilm-Transistor Array
• Indirect (CsI scintillator + photodiode)
• Direct (a
(a-selenium)
9
Indirect / Direct flat panel
detector systems
FlatFlat-panel Fluoroscopy / Fluorography
• Based upon TFT charge storage and
readout technology
• ThinThin-FilmFilm-Transistor arrays
– Proven with radiography applications
– Now available in fluoroscopy
• CsI scintillator systems (indirect conversion)
• a-Se systems (direct conversion)
Flat panel vs. Image Intensifier
Flat
panel
II
Field coverage / size advantage to flat panel
Image distortion advantage to flat panel
10
Flat vs. Fat
Digital Flat Panel
Conventional II
Dynamic Range
Very Wide (5-10 times more
than conventional)
Narrow (TV camera limit)
Distortion
No Distortion
Distortion from curved input
surface of II
Detector Size
Weight and thickness much
lower
Heavy, bulky detector
Image Area
41 cm x 41 cm square
Round area is more than
20% smaller area for same
diameter
Image Quality
Good resolution, high DQE
Good resolution, high DQE
Dynamic range
Digital Flat Panel
II/TV
Saturation Limit
<Soft Tissue>
<Spine>
Saturation Limit
Video Signal
Detector Signal
<Lung>
Noise Floor
Noise Floor
X-Ray Intensity
X-Ray Intensity
Flat panel vs. Image Intensifier
• Electronic noise limits flatflat-panel amplification
gain at fluoro levels (1(1-5 µr/frame)
• Pixel binning (2x2, 3x3) offers improvements
• Low noise TFT’
TFT’s are slowly being produced;
variable gain technologies on the horizon
• II’
II’s will likely go the way of the CRT……
CRT……..
11
Detector Characteristics
• MTF
• NPS
• DQE
““Pre-sampled”
PrePre-sampled”
sampled” MTF
1.0
0.9
a-Selenium: 0.13 mm
0.8
Modulation
0.7
0.6
CR: 0.05 mm
0.5
0.4
ScreenScreen-film
0.3
CsICsI-TFT: 0.20 mm
0.2
0.1
CR: 0.10 mm
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5 4.0
4.5 5.0
Frequency (lp/mm)
Detective Quantum Efficiency, Radiography
0.8
DQE( f )
CsI - TFT
0.6
a-Se - TFT
0.4
CR “dual-side”
Screen-film
0.2
CR Conventional
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Spatial Frequency (cycles/mm)
12
Equipment considerations
• Specific applications
– Fluoroscopy
– Pediatrics
– Trauma and ED
– Orthopedics multimulti-film studies (scoliosis, etc)
– Dental panorex
– Operating room
– Mammography
– Radiation Therapy
Escape
Dental Panorex example
Integration into “digital”
digital” paradigm often requires
creative ideas, e.g., modification of cassette for CR
Technological Advances
CR mammography
50 µm spot
Thicker
phosphor
layer
front
back
Transparent
phosphor base
2 light guides
LineLine-scan CR array reader
with “structured”
structured” phosphor
Portable
DR Device
Resurgence of “slotslot-scan”
scan” systems
• No grid, great scatter rejection
• Low patient dose
CR systems with DR form factor
13
Pros and Cons: CR vs. DR
• CR
• DR
– Flexibility*
Flexibility*
– Single room use
• portables, multiple rooms,
mammography -- direct
replacement
• Dedicated chest, bucky
• C-arm / UU-arm
– New technology
– Proven technology*
• experience is expanding
• 2 decades of experience
– Screen-film paradigm
– Acquire and display*
• extra steps for processing
• no extra steps
• ↑ patient throughput
Pros and Cons: CR vs. DR
• CR
• DR
– Limited DQE
• Higher dose for same SNR
– Integration/interfacing
• PACS
+++
• x-ray system +
– Range of systems and *
costs to match needs
– Higher DQE *
• Better dose efficiency
– Integration/interfacing *
• PACS
+++
• x-ray system +++
– Higher costs for detector
and xx-ray source $$$
Escape
Less distinction between CR / DR
“cassette”
cassette” vs “cassetteless”
cassetteless”
Portable
Lower cost
CR
Portability
Flexibility
Low cost
DR
Speed
Ease of use
High cost
Integrated
High speed
14
Advanced Acquisition & Processing
Techniques
• Dual energy imaging
– Tissue selective imaging
– Differential attenuation with energy
• Digital tomosynthesis
– Acquisition from several projection angles
– Reconstruction of tomographic slices
Dual Energy Image Pair
Low kVp
High kVp
? Nodule?
TissueTissue-selective Images
Soft – tissue Only
Nodule not in soft tissue image
Bone Only
→
Nodule calcified
15
Digital Tomosynthesis: reduce structured noise
Left
Right
Shift images to select plane
Add to create tomogram
•
3 cm above detector
•
9 views, + to - 30°
•
1.4 x dose
Tomographic ramp
Niklason, L.T. et.al. Radiology 205:399-406
QC tools, phantoms, exposure data
• Consider systems with a simple yet robust QC
phantom and automated analysis
• Look for a system having exposure information
with database mining capabilities
• Find out about preventive maintenance and
unscheduled maintenance procedures
• Provide for adequate quality control support!!
• QC Workshop: Wednesday, Room 608
Escape
CR / DR Implementation
• PACS and DICOM
– Digital Imaging COmmunications in Medicine
– Provides standard for modality interfaces,
storage/retrieval, and print
• Modality Worklist (from RIS via HLHL-7 “broker”
broker”)
– Reduce technologist input errors
• Technologist QC Workstation
– Image manipulation and processing
– “For Processing”
Processing” vs “For Presentation”
Presentation”
– VOI LUT
16
CR/DR implementation
• Robust PACS/Network System
– Image Size: Storage Needs
• 8 - 32 Mbytes uncompressed
– 10 Pixels/mm
– 4300 x 3560 x 2 Bytes
• 3 - 13 Mbytes: 2.5:1 Lossless Compression
• Lossy compression???
– Network Transmission
• 100 Mbit/sec
Mbit/sec minimum (diagnostic workstations)
CR/DR implementation
• Uniformity Among CR/DR images and Display Monitors
– Acceptance Testing
• Measurement of Performance
• Correction of Substandard Performance
– Calibration of CR/DR Response
– Calibration of Monitors
• Maximum brightness
• LookLook-upup-Tables, DICOM GSDF, Part 14
– Heterogeneous environment more difficult…
difficult…. IHE?
Conclusions
• CR is the most flexible and costcost-effective
technology for digital acquisition
• Direct digital radiographic devices have
advantages in efficiency and throughput
• RealReal-time imaging & advanced processing
are clinically relevant considerations
17