Overview of Digital Detector Technology
J. Anthony Seibert, Ph.D. Department of Radiology University of California, Davis
Disclosure
• Member (uncompensated) – Barco-Voxar Medical Advisory Board – ALARA (CR manufacturer) Advisory Board
Learning Objectives
• Describe digital versus screen-film acquisition
• Introduce digital detector technologies
• Compare cassette and cassette-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 x-rays through patient Exposed film Film processor
Developer Fixer Wash Dry Gray Scale encoded on Film Intensifying Screens film x-rays → light
Digital x-ray detector 2. Display Digital Pixel Digital to Analog 1. Acquisition Matrix Conversion Transmitted x-rays through patient
Digital processing Analog to Digital Conversion
Charge X-ray converter collection x-rays → electrons device 3. Archiving
Analog versus Digital
Spatial Resolution MTF of pixel aperture (DEL)
1 100 µm 0.8
0.6 200 µm 1000 µm 0.4 Modulation 0.2
0 01234567891011 Frequency (lp/mm)
Sampling Detector Pitch Element, “DEL”
2 Characteristic Curve: Response of screen/film vs. digital detectors 5 Useless 4 10,000 Film-screen (400 speed) Digital 3 1,000 Overexposed Useless 2 100 Correctly exposed
1 10 intensity Relative Film Optical Density Film Optical Underexposed 0 1 0.01 0.1 1 10 100 Exposure, mR
20000 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 noise-limited contrast – Proper exposure potentially hidden
Image Processing
• Crucial for optimal image presentation • Flexibility adds potential advantage – Disease-specific image processing – Computer aided detection
3 Digital System Technologies Projection Radiography
• Computed Radiography (CR)
• CCD “Direct” • CMOS Radiography (DR) • Flat Panel (TFT) arrays
Consideration: “Cassette” vs. “Cassetteless” operation
Computed Radiography (CR) ...is the generic term applied to an imaging system comprised of:
Photostimulable Storage Phosphor to acquire the x-ray projection image CR Reader to extract the electronic latent image Digital electronics to convert the signals to digital form
Image Acquisition
CR QC
Patient information Workstation Latent image produced DICOM / PACS
CR Laser film printer Reader
Latent image extracted Display / Archive
4 CR: Photostimulated Luminescence
Conduction band
τ tunneling phonon τ recombination Energy 4f 6 5d Band BaFBr Laser F/F+ e- PSL stimulation 8.3 eV 3.0 eV 2.0 eV τ Eu
4f 7 Eu 3+ / Eu 2+ e Valence band Incident PSLC complexes (F centers) are x-rays created in numbers proportional to incident x-ray intensity
CR: Latent Image Readout
Reference detector f-θ Cylindrical mirror lens Laser Light channeling guide Source
Polygonal Output Signal Mirror ADC Laser beam: PMT Scan direction x= 1279 y=To 1333 image z=processor 500
Plate translation: Sub-scan direction
Phosphor Plate Cycle
PSP Base support x-ray exposure
plate exposure: reuse create latent image laser beam scan plate readout: extract latent image light erasure plate erasure: remove residual signal
5 CR Innovations
• High-speed line scan systems (<10 sec)
• Dual-side readout capabilities (increase DQE)
• Structured phosphors • Mammography applications ??
• Low cost table-top CR readers
DR: “Direct” Radiography
....refers to the acquisition and capture of the x-ray image without user intervention
– “Indirect” detector: a conversion of x-rays into light and then light into photoelectrons
– “Direct” detector: a conversion of x-rays to electron-hole pairs with direct signal capture
CCD detector systems
• Area Scintillator / CCD lens coupling
Scintillating Screen (Gd2O2S), CsI • Area Scintillator / fiberoptical coupling Array of CCD Fiber Optical Cameras Scintillating Coupling Screen (CsI) • Slot scintillator linear array fiberoptical coupling
6 CCD area detector 35 cm High fill factor ~ 100 % Good light conversion efficiency (~85%)
2.5 cm 5 cm 5 cm 43 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 budget-limited situations • Capable imaging systems
Scanning slot chest x-ray system • CCD array • Fiberoptic coupling • No grid • Reduced scatter • Low dose
• “Effective” DQE compares to flat-panel systems
7 CMOS
• “RAM” with photodiode converter
• Random access readout
• Low voltage operation (5V)
• ? NOISE ……
• Large FOV detector available (tiled CMOS)
CMOS Complementary Metal Oxide Semiconductor
PIXEL ARRAY DECODER ROW DRIVERS LATCHES COUNTER
COLUMN SIGNAL CONDITIONING VS_OUT
CLK VR_OUT RUN DECODER DEFAULT TIMING READ LOAD COUNTER ADDRESS AND FRAME DATA +5V CONTROL LATCHES
Array of tiled CMOS sensors
Xrays
Scintillator Fiberoptic plate
Microlens optics CMOS sensors
Controller electronics
7000x7000 element array, 17” x 17” FOV for one implementation
8 Thin-Film-Transistor Array
Laptop LCD display X-ray converter Photo-emitter Photo detector
TFT Active Matrix Array
TFT active matrix array
Amplifiers – Signal out Gate G1 switches Active Area Thin-Film Transistor Dead G2 Zone Storage Capacitor G3
Charge Collector D1 D2 D3 Electrode CR1 CR2 CR3 Analog to Data lines Charge Digital Amplifiers Converters
Thin-Film-Transistor Array • Indirect (CsI scintillator + photodiode) • Direct (a-selenium)
9 Indirect / Direct flat panel detector systems
Flat-panel Fluoroscopy / Fluorography
• Based upon TFT charge storage and readout technology
• Thin-Film-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 Narrow (TV camera limit) than conventional)
Distortion No Distortion Distortion from curved input surface of II
Detector Size Weight and thickness much Heavy, bulky detector lower
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
Saturation Limit
X-Ray Intensity X-Ray Intensity
Flat panel vs. Image Intensifier
• Electronic noise limits flat-panel amplification gain at fluoro levels (1-5 µr/frame)
• Pixel binning (2x2, 3x3) offers improvements
• Low noise TFT’s are slowly being produced; variable gain technologies on the horizon
• II’s will likely go the way of the CRT…….
11 Detector Characteristics
• MTF • NPS • DQE
““Pre-sampled”Pre-sampled” MTF 1.0 0.9 0.8 a-Selenium: 0.13 mm 0.7 0.6 0.5 CR: 0.05 mm Modulation Modulation 0.4 Screen-film 0.3 0.2 CsI-TFT: 0.20 mm 0.1 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 CsI - TFT
0.6 ) f
( a-Se - TFT 0.4
DQE 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 multi-film studies (scoliosis, etc) – Dental panorex – Operating room – Mammography – Radiation Therapy
Escape
Dental Panorex example Integration into “digital” paradigm often requires creative ideas, e.g., modification of cassette for CR
Technological Advances CR mammography 50 µm spot Thicker phosphor layer
front Transparent back phosphor base Line-scan CR array reader 2 light guides with “structured” phosphor
Resurgence of “slot-scan” systems Portable • No grid, great scatter rejection DR Device • Low patient dose
CR systems with DR form factor
13 Pros and Cons: CR vs. DR
• CR • DR – Flexibility* – Single room use • portables, multiple rooms, • Dedicated chest, bucky mammography -- direct • C-arm / U-arm replacement – 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 DQE * • Higher dose for same SNR • Better dose efficiency
– Integration/interfacing – Integration/interfacing * • PACS +++ • PACS +++ • x-ray system + • x-ray system +++
– Range of systems and * – Higher costs for detector costs to match needs and x-ray source $$$
Escape
Less distinction between CR / DR “cassette” vs “cassetteless” Portable CR Lower cost Portability Flexibility Low cost
DR Speed Ease of use Integrated High cost 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?
Tissue-selective Images Soft – tissue Only Bone Only
Nodule not in soft tissue image → 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 HL-7 “broker”) – Reduce technologist input errors • Technologist QC Workstation – Image manipulation and processing – “For Processing” vs “For 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 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 • Look-up-Tables, DICOM GSDF, Part 14 – Heterogeneous environment more difficult…. IHE?
Conclusions
• CR is the most flexible and cost-effective technology for digital acquisition
• Direct digital radiographic devices have advantages in efficiency and throughput
• Real-time imaging & advanced processing are clinically relevant considerations
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