AAPM 2005-Continuing Education Course in Radiographic and Fluoroscopy Physics and Technology
A Comparison of Screen/Film and Digital Imaging: Image Processing, Image Quality, and Dose
Ralph Schaetzing, Ph.D. Agfa Corporation Greenville, SC This presentation…
O does…
O focus on salient characteristics of two projection-radiography acquisition technology classes
O Analog: screen/film (S/F)
O Digital: generic
O does not…
O focus on technology details (see other presentations)
O Specific technologies used as examples only
O address alternatives to projection imaging (i.e., x-sectional)
O cheerlead
O No technology-class recommendations - too many other issues
1 Digital Image Acquisition Technologies X-ray quanta X-ray quanta Ð intermediÐ ate(s) Direct Indirect Ð meas. output signal meas. output signal
Screen/Film Storage Phosphor + + point scan point scan
Screen/Film Storage Phosphor + + line scan line scan Commonly called Silicon Strip Pressurized Gas Scintillator Computed + + + Radiography line/slot scan line/slot scan line/slot scan (CR)
Photoconductor Scintillator Scintillator + + + flat-panel array area sensor(s) flat-panel array Commonly called Digital Radiography (DR)
2 The Medical Imaging Chain (Analog, Digital)
Human
ANALOGUE WORLD Acquire (visible) Reproduce
Preprocess ProcessProcess (Optimize) Process for Output
Store
Distribute
DIGITAL WORLD 3 (hidden) The Bigger Picture
Producers/Competitors
Clinical Technical
A R e c p Observers q r u S D e o P t r s i r o s d r o e u e c c e Regulators/Standardizers
Operational Economic
Consumers
4 Imaging and Information System Environment Outline of Presentation
O Dose and Image Quality in S/F and Digital Systems
O Dose requirements: the speed limit
O Image quality as a dose metric
O Does image quality really matter?
O Image Processing in S/F and Digital Systems
O Image processing for display optimization
O Image processing for decision support
O Wrap-up and Conclusions
5 Outline of Presentation
O Dose and Image Quality in S/F and Digital Systems
O Dose requirements: the speed limit
O Image quality as a dose metric
O Does image quality really matter?
O Image Processing in S/F and Digital Systems
O Image processing for display optimization
O Image processing for decision support
O Wrap-up and Conclusions
6 When x-ray dose wasn’t so important…
All Images © Radiology Centennial, Inc. 7 When x-ray dose became important…
O Biological effects became clearer quickly
© Radiology Centennial, Inc. O Dose: a metric by which to compare systems (and facilities – e.g., FDA’s NEXT Survey)
O ALARA Principle: As Low As Reasonably Achievable
8 Comparing Dose Requirements: Speed
O Analog S/F O Digital
O ISO 9236-1 (2004) O Linear, wide-latitude Photography -- Sensitometry response, and variable of screen/film systems for detector kV-dependence medical radiography -- Part makes definition non-trivial, 1: Determination of manufacturer-dependent sensitometric curve shape, O Confusion and frustration speed and average gradient O Efforts underway to create 1000 S = standardized definition of Ks (µGy) speed (AAPM, DIN, IEC, manufacturers) O Speed defined by incident air
kerma (Ks) giving net density of 1.0 under specific conditions (exposure, processing, etc.)
9 Comparing Dose Requirements: Speed
X-ray Sensitometry - Screen/Film and CR O Linear, wide-latitude response (Density or CR Signal vs. X-ray Exposure) of digital systems still confused 4 S/F S/F S/F with dose efficiency S/F S/F S/F S=1200S=1200 S=400S=400 S=300S=300 O Latitude ≠ Dose Reduction! (Can always find S/F system 3 that operates at same dose 12001200 CR level as digital system) "pick a speed"
O Multiple, different S/F systems 2 needed to cover same exposure range as one digital system 1 O Question: even if signal levels matched, is S/F image quality (e.g., sharpness, noise) level 300300 0 comparable to that of digital? 0.1 1.0 10.0 100.0 1000.0 µGy
10 4 decades of exposure Comparing Dose Requirements: Detective Quantum Efficiency (DQE)
Signalin Signalout Input Recorded Detector Exposure Noisein Noiseout Image
(Noiseout > Noisein)
(Measured) Noise from ideal detector* DQE = Measured Noise from real detector* *Noise values must be expressed in same units = 1.0 for ideal detector
2 2 ∝ (Contrast or Gain) x (Sharpness) DQE Measured Noise
11 spatial-frequency- and exposure-dependent, i.e., a SURFACE! Comparing Dose Requirements: Reported DQE(0) Values for S/F and Digital
1.0 Ideal Detector 0.9 0.8 R&D 0.7 R&D Needle scint. 0.6 + TFT Needle IP-CR Photoconductor 0.5 + TFT (gen. rad.) Powder IP-CR 0.4 Powder scint. (dual-sided) 0.3
(@ f = 0 cy/mm) + TFT Powder IP-CR 0.2 Powder scint. Screen/Film +CCD 0.1
Detective Quantum Efficiency Detective Quantum X-ray film 0.0 Indirect Direct
12 Comparing Dose Requirements: DQE: Caveat Emptor…
O Standard exists…
O IEC 62220-1 (2003) My DQE is bigger Medical electrical equipment than your DQE! - Characteristics of digital X- ray imaging devices - Part 1: Determination of the detective quantum efficiency
O But…
O Make sure that standard was followed!
13 Which image has the highest quality?
AC
B D
14 Image Quality: What is it?
O Image quality depends only on intrinsic, objective physical characteristics of an imaging system, and can be measured independently of an observer
O Image quality is whatever the observer says it is (i.e., it is a subjective perception of the image, "in the eye of the beholder")
O Image quality is defined by an observer's ability to achieve an acceptable level of performance for a specified task
15 Image Quality: What is it?
Perf. Performance Perf. ? Measures Diagnostic Accuracy Subj. ? Diagnostic Error Rate ? Obj. Patient Management Patient Outcome
Subjective Objective Measures Measures Subj. Perception Physical Confidence Characteristics Preference Obj.
16 Radiographic Image Quality: Does it matter?
Evolution of Projection Photon Counting Evolution of Radiologist Error Rate Radiography and DR (needle scint., TFT) (unaided) Image Quality DR (photocond., TFT) DR (scint., CCD) CR (needle IP, line scan) CR (powder IP, point scan) DR (II, CCD) DIGITAL 1895 ANALOG 2005 II/TV (Fluoroscopy) S/F (UV) S/F (Green) S/F (Blue) Direct Film Exposure (no screen) 1895 2005 Significant image quality changes! Very little change in performance…
17 Significant dose changes: 50-100x Radiographic Image Quality: Does it matter?
O Imaging system manufacturers Evolution of Radiologist Error Rate are wasting their time trying to (unaided) maintain or improve image If the imaging system quality because end users can't is the limiting step… or don't use it clinically New acquisition, image processing Limiting factor: radiologist systems with unprecedented objective image quality
O Radiologists are so skilled that ? they can locate the relevant clinical information consistently, If the radiologist independent of image quality is the limiting step… (at least, to date) Decision-support systems: Computer-Aided Detection/Diagnosis Limiting factor: imaging system 1895 2005
18 Outline of Presentation
O Dose and Image Quality in S/F and Digital Systems
O Dose requirements: the speed limit
O Image quality as a dose metric
O Does image quality really matter?
O Image Processing in S/F and Digital Systems
O Image processing for display optimization
O Image processing for decision support
O Wrap-up and Conclusions
19 Medical Image Processing – Many Goals
O Optimize for human vision O Optimize for storage
O Signal contrast O Memory requirements
O Latitude O Storage media requirements
O Dynamic range O Cost (acquired vs. displayed) O Optimize for distribution O Sharpness O Network efficiency O Noise (bandwidth) O Optimize for machine vision O Transmission/delivery time O CAD (e.g., teleradiology)
O CADx O Access time (read/write)
O Cost
20 Image Processing for Display: Optimizing for Human Vision
Can you find the 20 “blob” targets in this image?
Agfa MUSICATM
21 MUSICA = MUlti-Scale Image Contrast Amplification Image Processing for Display: Optimizing for Human Vision in S/F
O Detector choices O Chemical processing choices
O Screen O Chemistry type, activity, seasoning, replenishment O Film
O Exposure choices O Time
O Temperature O Technique (kVp, mA, s)
O Adjacency effect O Hypersensitization
O Edge enhancement (unsharp O Uniform pre-exposure to light nucleates sub-image masking!) through control of that becomes developable developer dynamics with further exposure O Viewing choices
O Improves speed, SNR O Viewbox luminance, quality, spectrum, masking, magnif.
22 Image Processing for Display: Optimizing for Human Vision in Digital
O Wide latitude + post-processing O Contrast (Gradation/Tone Scale) flexibility of most digital systems O Look-up Tables (LUTs) enables HUGE variety of O Histogram-based optimization techniques O Contrast/Latitude dilemma O Manual vs. … O Sharpness (Edge Enhancement) O Semi-automatic (exam- dependent) processing vs. … O Unsharp masking
O Multi-scale techniques O Fully automatic (hands-free) processing (custom contrast at each of a set of frequency bands)
O Noise (Reduction/Smoothing)
O Used sparingly (?effect on signals?)
23 Image Processing for Decision Support: Optimizing for Machine Vision (CAD) Computers can be trained to do this well and consistently (for well-defined targets)
O Sources of radiologist error (chest*) Radiologist CAD Radiologist alone alone with CAD O Search/Scanning: 15% Missed Missed
O Recognition: 37% Missed Total NumberofCases
O Perceptual Decision: 48% O Interpretive Decision:
O Performance evaluation nontrivial Detected Marked Detected
O Pathology/target-dependent
O Operating point? (TP, FP)
O Best schemes can outperform radiologists (target-dependent) Missed The Theory of CAD
*Kundel H, Visual search, object recognition and reader error in radiology, 24 Proc. SPIE 5372 (2004), pp1-11 (SPIE Press, Bellingham WA) Image Processing for Decision Support: CAD Implementation Issues
O Detection performance sensitive O Proc. protocol (image vs. study)
to acquisition system details O Testing/Scoring/Training (“truth”)
O Contrast resolution O Productivity/Workflow/Recalls
O Wide/narrow latitude O Case load (screening?, volume?)
O Linear/nonlinear response O Satisfaction of Search O Spatial resolution (faith vs. blind faith) (e.g., pixel size) O Observer experience/knowledge O Noise O Clinical Efficacy O Operational issues O Radiologist Acceptance O S/F: extra digitization step O Cost/reimbursement O Consistency of results?
25 Outline of Presentation
O Dose and Image Quality in S/F and Digital Systems
O Dose requirements: the speed limit
O Image quality as a dose metric
O Does image quality really matter?
O Image Processing in S/F and Digital Systems
O Image processing for display optimization
O Image processing for decision support
O Wrap-up and Conclusions
26 Analog or Digital Technologies – Quo vadis? S/F or Digital Dose ( ) Image Quality Physical ( ) Subjective ( ) S/F Performance-based S/F Digital Digital
Image Processing Display Optimization
Decision Support (e.g., CAD) 27 Back to the Bigger Picture: The Path to System Equilibrium…
Convenience Producers/Competitors Consistency
Clinical Technical
A R e c p Observers q r Flexibility u S D e o Control P t r s i r o s d r o e u e c c e Regulators/Standardizers
Operational Economic Interfaces Consumers Technolust (e.g., EPR) 28 Thank You for Your Attention