Overview of Digital Detector Technology

Transcription

1 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

2 Conventional screen/film detector 1. Acquisition, Display, Archiving Transmitted x-raysx through patient Exposed film Film processor Developer Fixer Wash Dry Film Intensifying Screens x-rays light Gray Scale encoded on film Digital x-ray x detector 1. Acquisition Transmitted x-raysx through patient Digital Pixel Matrix 2. Display Digital to Analog Conversion 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) Modulation µm 200 µm 1000 µm Frequency (lp/mm) Sampling Pitch Detector Element, DEL 2

3 Film Optical Density Characteristic Curve: Response of screen/film vs. digital detectors 5 Film-screen (400 speed) Useless Useless Underexposed Digital Overexposed Correctly exposed ,000 1, Relative intensity 1 Exposure, mr 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 specific image processing Computer aided detection 3

4 Digital System Technologies Projection Radiography Computed Radiography (CR) CCD CMOS Flat Panel (TFT) arrays Direct Radiography (DR) 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 x projection image CR Reader to extract the electronic latent image Digital electronics to convert the signals to digital form Image Acquisition Patient information Latent image produced DICOM / PACS CR QC Workstation CR Reader Laser film printer Latent image extracted Display / Archive 4

5 CR: Photostimulated Luminescence Conduction band PSL 3.0 ev phonon 4f 6 5d F/F + τ Eu τ tunneling e - τ recombination Laser stimulation 2.0 ev Energy Band BaFBr 8.3 ev 4f 7 Eu 3+ / Eu 2+ Incident x-rays e Valence band PSLC complexes (F centers) are created in numbers proportional to incident x-ray x intensity CR: Latent Image Readout Reference detector Laser Source Polygonal Mirror Laser beam: Scan direction f-θ lens Cylindrical mirror Light channeling guide PMT Output Signal ADC x= 1279 y= To 1333 image z= processor 500 Plate translation: Sub-scan direction Phosphor Plate Cycle PSP reuse x-ray exposure laser beam scan light erasure Base support plate exposure: create latent image plate readout: extract latent image plate erasure: remove residual signal 5

6 CR Innovations High-speed line scan systems (<10 sec) Dual-side readout capabilities (increase DQE) Structured phosphors Mammography applications?? Low cost table-top 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 x into light and then light into photoelectrons Direct detector: a conversion of x-rays x to electron-hole pairs with direct signal capture CCD detector systems Area Scintillator / lens coupling CCD Scintillating Screen (Gd 2 O 2 S), CsI Area Scintillator / fiberoptical coupling Scintillating Screen (CsI) Slot scintillator linear array fiberoptical coupling Fiber Optical Coupling Array of CCD Cameras 6

7 35 cm CCD area detector High fill factor ~ 100 % Good light conversion efficiency (~85%) 2.5 cm 5 cm 43 cm 2.5 cm 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 x system CCD array Fiberoptic coupling No grid Reduced scatter Low dose Effective DQE compares to flat-panel systems 7

8 CMOS RAM with photodiode converter Random access readout Low voltage operation (5V)? NOISE Large FOV detector available (tiled CMOS) CMOS Complementary Metal Oxide Semiconductor LATCHES COUNTER DECODER ROW DRIVERS PIXEL ARRAY COLUMN SIGNAL CONDITIONING VS_OUT CLK RUN DEFAULT LOAD ADDRESS DATA +5V TIMING AND CONTROL DECODER COUNTER LATCHES VR_OUT READ FRAME 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

9 Thin-Film Film-Transistor Array Laptop LCD display Photo-emitter X-ray converter Photo detector TFT Active Matrix Array TFT active matrix array Amplifiers Signal out Active Area Dead Zone G1 G2 G3 Gate switches Thin-Film Transistor Storage Capacitor Data lines D1 CR1 D2 CR2 D3 CR3 Charge Collector Electrode Charge Amplifiers Analog to Digital Converters Thin-Film Film-Transistor Array Indirect (CsI scintillator + photodiode) Direct (a-selenium)( 9

10 Indirect / Direct flat panel detector systems Flat-panel Fluoroscopy / Fluorography Based upon TFT charge storage and readout technology Thin-Film 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

11 Flat vs. Fat Dynamic Range Digital Flat Panel Very Wide (5-10 times more than conventional) Conventional II 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 Detector Signal Noise Floor <Lung> <Soft Tissue> <Spine> Video Signal Saturation Limit 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

12 Detector Characteristics MTF NPS DQE Modulation Screen-film CsI-TFT: 0.20 mm Pre-sampled Pre-sampled MTF CR: 0.10 mm CR: 0.05 mm a-selenium: 0.13 mm Frequency (lp/mm) Detective Quantum Efficiency, Radiography 0.8 CsI - TFT DQE( f ) a-se - TFT CR dual-side Screen-film CR Conventional Spatial Frequency (cycles/mm) 12

13 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 CR mammography front back 2 light guides Technological Advances 50 µm m spot Thicker phosphor layer Transparent phosphor base Line-scan CR array reader with structured phosphor Portable DR Device Resurgence of slot-scan systems No grid, great scatter rejection Low patient dose CR systems with DR form factor 13

14 Pros and Cons: CR vs. DR CR Flexibility* portables, multiple rooms, mammography -- direct replacement Proven technology* 2 decades of experience Screen-film paradigm extra steps for processing DR Single room use Dedicated chest, bucky C-arm / U-armU New technology experience is expanding Acquire and display* 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 PACS +++ x-ray system + Integration/interfacing * PACS +++ x-ray system +++ Range of systems and * costs to match needs Higher costs for detector and x-ray x source $$$ Escape Less distinction between CR / DR cassette vs cassetteless Portable CR Lower cost Portability Flexibility Low cost DR Speed Ease of use High cost Integrated High speed 14

15 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 selective Images Soft tissue Only Bone Only Nodule not in soft tissue image Nodule calcified 15

16 Digital Tomosynthesis: reduce structured noise Left Right Shift images to select plane Add to create tomogram 3 cm above detector 9 views, + to x dose Tomographic ramp Niklason, L.T. et.al. Radiology 205: 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

17 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 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 17