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 <Lung> <Soft Tissue> <Spine> Video Signal Video Signal Noise Floor Detector Signal Detector Signal Noise Floor 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