AAPM 2006–Digital Imaging Continuing Education Design and Performance Characteristics of Computed Radiographic Acquisition Technologies Ralph Schaetzing, Ph.D. Agfa Corporation Greenville, SC, USA
Digital Radiography: Acquisition Technologies in General Digital Image CONVERT Latent Image INTERACT Aerial X-ray Image (Image-in-Space) 2
Digital Radiography: Acquisition Technologies in Context Operational Operational Technical Technical Treatment Diagnosis Digital Image Reproduce CONVERT Distribute Process PATIENT Latent Image OUTCOME Store INTERACT Acquire Economic Economic Aerial X-ray Image (Socio-) (Socio-) Clinical Clinical (Image-in-Space) Referral Exam 3
Digital Radiography: A Taxonomy • Many dimensions along which to classify DR technologies • Direct vs. Indirect x-ray-to-signal conversion • Scanned (e.g., point, line) vs. Full-field } Related • Beam geometry/Detector geometry • Detector type/material • Dynamic vs. Static • … 4
Digital Radiography: A Taxonomy (x-ray interaction/detector*, signal extraction) Photoconductor Screen/Film Scintillator Storage Phosphor Storage Phosphor Photoconductor Screen/Film Scintillator Storage Phosphor Storage Phosphor + + + + + point scan point scan point scan point scan point scan Computed Radiography Scintillator Storage Phosphor Storage Phosphor Screen/Film Scintillator Storage Phosphor Storage Phosphor Screen/Film + + + + line/slot scan line scan line scan line scan Scanned X-ray quanta X-ray quanta Read-out Direct Indirect intermediate(s) “Full-field” meas. output signal meas. output signal Read-out Screen/Film Screen/Film Scintillator Scintillator + + video chain video chain Photoconductor Photoconductor Scintillator Scintillator + + flat-panel array flat-panel array * Other detectors (e.g., pressurized gas, Si/metal strips) have also been used 5
Historical Context Installed Base: 1 Full-field (incl. x-ray) Price: $1,200,000 imaging with R&D on SP Size: ∼ 10 m 2 PSL intermediates scanning systems Speed: 40 plates/hr (1842 - 1936) "Commercial Era" "Commercial Era" 1900 1920 1940 1960 1980 2000 Full-field Installed Base: ∼ 20,000+ Price: ∼ 10x lower night-vision Size: ∼ 10x smaller "cameras" Speed: ∼ 2-4x faster (IR/heat stim. SP) Kodak CR: the most widespread form of DR! 6
Learning Objectives • Describe the form and function of today’s computed radiography (CR) systems • Identify the main factors that influence the image quality of CR systems • Compare modern CR systems to other acquisition technologies • Describe the latest and future developments in CR 7
Computed Radiography Technologies • Basics • System Design • Screens • Scanners • Imaging Performance • Input/Output Relationship • Spatial Resolution • Noise • New CR Developments 8
Basics CR Characteristics High-energy S Detector is SP screen (PSL � aerial IMAGE (Image) P screen, Imaging Plate, IP, …) exposure Down-Conversion S (e.g., x-rays) C Screen can absorb, and store � Low-energy R Low- -energy energy Low (partially) as a latent image, E (visible) UNIFORM UNIFORM E emission IMAGE incoming high-energy N stimulation stimulation ( λ e ) λ s ( λ electromagnetic radiation ) ( s ) Exposure to low-energy � stimulating radiation ( λ s ) High-energy (Image) S causes screen to emit the UNIFORM P Up-Conversion exposure previously stored energy at S (e.g., x-rays, UV) a (shorter) wavelength ( λ e ) Low-energy C R (visible) Low- -energy (e.g., IR) energy (e.g., IR) Low in the visible – λ s , λ e must E emission IMAGE aerial IMAGE aerial IMAGE E be sufficiently different, or ( λ e ) N stimulation stimulation λ s ( λ no CR possible ) ( s ) 9
Basics: CR: Digital Alternative to Screen/Film • BOTH systems • use phosphor screens as x-ray absorbers • use screens with similar structures (small phosphor particles dispersed in a binder) • emit light promptly on x-ray exposure (x-ray luminescence) • use screens that can be exposed thousands of times • ONLY storage phosphors • can retain a portion of the absorbed x-ray energy (as a latent image of trapped electrons, e - ) • can be read out at a later time, (destructively, i.e., latent image is erased as it is read) 10
Basics: CR vs. Screen/Film - Advantages of CR • Extended Exposure Latitude (10000:1 vs. ∼ 40:1) • High exposure flexibility with 1 detector (retakes � ) • Reusable Detector • Reduction in consumables (film, chemistry) costs (but, full impact only with softcopy interpretation) • Compatibility/Scalability/Workflow/Productivity • No major changes to equipment/rooms/technique • Flexible reader placement (centralized and/or distributed architectures) • Digital Data • Gateway for projection radiography into PACS 11
Computed Radiography Technologies • Basics • System Design } • Screens A System! • Scanners • Imaging Performance • Input/Output Relationship • Spatial Resolution • Noise • New CR Developments 12
Design: Storage Phosphor Screens • Support (flexible, rigid) coated with tiny (3-10 µm) SP particles dispersed in binder • Screen is turbid (white) • Many materials tested, Phosphor Phosphor only a few successful ∼ 100-250 µm • SrS:Ce, Sm Support Support • RbBr:Tl • BaFX: Eu 2+ (where X=Br, I) • CsBr: Eu 2+ (new) Screen Structure (ideal) • SP mechanisms/processes at micro (quantum) level still subject of active research! 13
Design: Storage Phosphor Screens • Manufacturer-specific layers to optimize mechanical, optical, electrical performance, e.g., • Wear, handling layer Protective Overcoat • Electrostatic discharge layer Phosphor Phosphor • Optical coupling layer Backing • reflective backing Layers – direct more emitted Support Support light to surface/photodetector Anti-static Layer • absorbing backing, dyes, filters Screen Structure (real) – reduce spread/transmission of stimulating light (sharpness) • X-ray backscatter control layer (lead) 14
Design: Three-step Imaging Cycle x-ray ( λ s ) ( λ s ) aerial image Stored Signal Prompt Emission Stimulated Emission Remnant Prompt Emission Stimulated Emission of Light ( λ e ) ∼ 50% of Light ( λ e ) of Light ( λ e ) ∼ 50% of Light ( λ e ) (trapped e - ) Signal Phosphor Phosphor Phosphor Phosphor Phosphor Phosphor Support Support Support Support Support Support Read Out Expose (INTERACT) (CONVERT Latent Image) (Create Latent Image) Erase Lamps Erase Lamps Erase Lamps "Fresh" Screen Phosphor Phosphor Phosphor Phosphor Support Support Support Support Erase (Reset/Reinitialize) (Remove Residual Latent Image) 15
Design: The Flying-Spot CR Scanner Beam Laser Shaping • Components Source Beam Deflection Mech. (x-direction) • IP transport stage Intensity • Beam deflector Control IP Transport Stage • Laser + intensity control Optical (y-direction) • Beam shaping/control Opt. Filter • Collection optics Imaging Plate Light Light (IP) • Optical filter Collection Optics Collection Optics Photo- Photo- • Photodetector detector detector Elec. • Analog electronics Analog Electronics Analog Electronics Control • A/D Converter (signal conditioning) (signal conditioning) Computer Comp. • Image buffer • Control computer Analog- -to Digital Conversion to Digital Conversion Analog Image Buffer • (Erase station) (Sampling+Quantization) (Sampling+Quantization) 16
Design: The Flying-Spot CR Scanner Laser Source + Intensity Control • � Efficient, rapid, accurate read-out of latent image • Power: high-power light source = laser (gas, solid-state) compact, efficient, reliable, tens of mW over ∼ 100 µm Ø • Wavelength, λ s : choice depends Laser on energy needed to stimulate Source λ s latent image electrons out of traps Intensity (typically reddish), and emission Control spectral range ( λ e , typically bluish) • Constancy: laser power must be constant during scan to avoid artifacts/noise (fluctuation tolerance as low as ∼ 0.1% - active control with feedback loops) 17
Design: The Flying-Spot CR Scanner Beam Shaping Optics • Problem: laser point source and beam deflector cause size, shape, and speed of beam at IP surface to change with beam angle (similar to flashlight beam moving along wall) • Signal output and Beam Shaping resolution depend on Beam Deflection beam position - BAD (x-direction) • Special scanning optics keep beam size/shape/speed largely independent Imaging Plate of beam position (IP) 18
Design: The Flying-Spot CR Scanner Beam Deflector • Scans beam in one direction across IP surface (transport stage handles orthogonal direction) • Desired scan speed/throughput Beam Deflection determines deflector type (x-direction) • rotating drum (slow) • galvanometer/mirror (shown) IP Transport Stage (y-direction) • rotating mirrored polygon (fast) "Fast-scan" • Beam placement Imaging Plate direction accuracy is critical (IP) to avoid artifacts (edge jitter, waviness) • error tolerance: fractions of the pixel dimension 19
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