A Comparison of Screen/Film and Digital Imaging:
Image Processing, Image Quality, and Dose
Ralph Schaetzing, Ph.D.
Agfa Corporation
Greenville, SC
AAPM 2005-Continuing Education Course in
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)
Ocheerlead
Storage Phosphor Storage Phosphor + line scan Storage Phosphor Storage Phosphor + point scan Screen/Film Screen/Film + point scan Photoconductor Photoconductor + flat-panel array Scintillator Scintillator + area sensor(s) Scintillator Scintillator + flat-panel array Silicon Strip Silicon Strip + line/slot scan Scintillator Scintillator + line/slot scan Pressurized Gas Pressurized Gas + line/slot scan Storage Phosphor Storage Phosphor + line scan Storage Phosphor Storage Phosphor + point scan Commonly called Computed Radiography (CR) Photoconductor Photoconductor + flat-panel array Scintillator Scintillator + area sensor(s) Scintillator Scintillator + flat-panel array Silicon Strip Silicon Strip + line/slot scan Scintillator Scintillator + line/slot scan Pressurized Gas Pressurized Gas + line/slot scan
Commonly called Digital Radiography (DR)
Digital Image Acquisition Technologies
Direct
Direct
X-ray quantaÐmeas. output signal
Indirect
Indirect
X-ray quanta
Ð
intermediate(s)
Ð
meas. output signal
Screen/Film
Screen/Film
+ line scan
The Medical Imaging Chain (Analog, Digital)
HumanDIGITAL WORLD
Distribute Store ProcessProcess (Optimize) Process for Output Preprocess
ANALOGUE WORLD
(visible)
The Bigger Picture
Regulators/Standardizers Observers Producers/Competitors Consumers Clinical Technical Economic Operational o P r c s e s A qc u e i r t S o re D R p e r c o d u eOutline of Presentation
O
Dose and Image Quality in S/F and Digital Systems
O
Dose requirements: the speed limit
OImage 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
OImage processing for decision support
Outline of Presentation
O
Dose and Image Quality in S/F and Digital Systems
O
Dose requirements: the speed limit
OImage 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
OImage processing for decision support
When x-ray dose wasn’t so important…
All Images © Radiology Centennial, Inc.
When x-ray dose became important…
O
Biological effects became clearer quickly
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
Comparing Dose Requirements:
Speed
O Analog S/F
O ISO 9236-1 (2004)
Photography -- Sensitometry
of screen/film systems for
medical radiography -- Part
1: Determination of
sensitometric curve shape,
speed and average gradient
O Digital
O Linear, wide-latitude
response, and variable detector kV-dependence makes definition non-trivial, manufacturer-dependent
O Confusion and frustration O Efforts underway to create
standardized definition of speed (AAPM, DIN, IEC, manufacturers)
)
(
1000
Gy
K
S
sµ
=
O Speed defined by incident air
kerma (Ks) giving net density of 1.0 under specific conditions (exposure, processing, etc.)
Comparing Dose Requirements:
Speed
0 1 2 3 4X-ray Sensitometry - Screen/Film and CR
(Density or CR Signal vs. X-ray Exposure)
S/F S=1200 S/F S=1200 S/F S=400 S/F S=400 S/F S=300 S/F S=300 CR "pick a speed" 300 300 1200 1200 O Linear, wide-latitude response
of digital systems still confused with dose efficiency
O Latitude ≠ Dose Reduction!
(Can always find S/F system that operates at same dose level as digital system)
O Multiple, different S/F systems needed to cover same exposure range as one digital system
O Question: even if signal levels matched, is S/F image quality (e.g., sharpness, noise) level comparable to that of digital?
Comparing Dose Requirements:
Detective Quantum Efficiency (DQE)
DQE =
(Measured) Noise from ideal detector*
Measured Noise from real detector*
*Noise values must be expressed in same units
Detector
Signalin Signalout
Input
Exposure
Recorded
Image
Noisein Noiseout
(Noiseout > Noisein)
DQE
∝
(Contrast or Gain)
Measured Noise
2x (Sharpness)
2Comparing Dose Requirements:
Reported DQE(0) Values for S/F and Digital
Direct
Indirect
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 Photoconductor + TFT (gen. rad.) Powder scint. +CCD Needle scint. + TFT Powder scint. + TFT Needle IP-CR Powder IP-CR (dual-sided) Powder IP-CR R&D R&DDetective Quantum Efficiency
(@ f = 0 cy/mm)
Ideal Detector
Screen/Film X-ray film
Comparing Dose Requirements:
DQE: Caveat Emptor…
My DQE is bigger
than your DQE!
O Standard exists…
O IEC 62220-1 (2003)
Medical electrical equipment
- Characteristics of digital
X-ray imaging devices - Part 1:
Determination of the
detective quantum efficiency
O But…
O Make sure that standard was
Which image has the highest quality?
A
C
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
Image Quality: What is it?
Objective Measures Physical Characteristics Subjective Measures Perception Confidence Preference Performance Measures Diagnostic Accuracy Diagnostic Error Rate Patient Management Patient Outcome?
?
Obj. Subj. Obj. Perf.?
Subj. Perf.Radiographic Image Quality: Does it matter?
2005 1895
Direct Film Exposure (no screen) S/F (Blue)
S/F (Green) S/F (UV)
Significant image quality changes! II/TV (Fluoroscopy)
DR (II, CCD) CR (powder IP, point scan) CR (needle IP, line scan) DR (scint., CCD) DR (photocond., TFT) ANALOG DIGITAL DR (needle scint., TFT) Photon Counting Evolution of Projection Radiography and Image Quality
Evolution of Radiologist Error Rate (unaided)
2005 1895
Radiographic Image Quality: Does it matter?
Evolution of Radiologist Error Rate (unaided) 2005 1895
?
2005 1895If the imaging system is the limiting step…
New acquisition, image processing systems with unprecedented
objective image quality
If the radiologist is the limiting step…
Decision-support systems:
Computer-Aided Detection/Diagnosis
O Imaging system manufacturers are wasting their time trying to maintain or improve image
quality because end users can't or don't use it clinically
Limiting factor: radiologist
O Radiologists are so skilled that they can locate the relevant
clinical information consistently, independent of image quality (at least, to date)
Outline of Presentation
O
Dose and Image Quality in S/F and Digital Systems
O
Dose requirements: the speed limit
OImage 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
OImage processing for decision support
Medical Image Processing – Many Goals
O Optimize for human visionO Signal contrast O Latitude O Dynamic range (acquired vs. displayed) O Sharpness O Noise
O Optimize for machine vision O CAD
O CADx
O Optimize for storage
O Memory requirements
O Storage media requirements O Cost
O Optimize for distribution O Network efficiency
(bandwidth)
O Transmission/delivery time
(e.g., teleradiology)
O Access time (read/write) O Cost
Image Processing for Display:
Optimizing for Human Vision
Can you find the
Can you find the
20 “blob” targets
20 “blob” targets
in this image?
in this image?
Agfa MUSICA Agfa MUSICATMTMImage Processing for Display:
Optimizing for Human Vision in S/F
O Detector choicesO Screen O Film
O Chemical processing choices O Chemistry type, activity,
seasoning, replenishment
O Time
O Temperature
O Adjacency effect
O Edge enhancement (unsharp
masking!) through control of developer dynamics
O Viewing choices
O Viewbox luminance, quality,
spectrum, masking, magnif. O Exposure choices
O Technique (kVp, mA, s) O Hypersensitization
O Uniform pre-exposure to
light nucleates sub-image that becomes developable with further exposure
Image Processing for Display:
Optimizing for Human Vision in Digital
O Wide latitude + post-processingflexibility of most digital systems enables HUGE variety of
optimization techniques
O Manual vs. …
O Semi-automatic
(exam-dependent) processing vs. …
O Fully automatic (hands-free)
processing
O Contrast (Gradation/Tone Scale) O Look-up Tables (LUTs)
O Histogram-based
O Contrast/Latitude dilemma O Sharpness (Edge Enhancement)
O Unsharp masking
O Multi-scale techniques
(custom contrast at each of a set of frequency bands) O Noise (Reduction/Smoothing)
O Used sparingly
Image Processing for Decision Support:
Optimizing for Machine Vision (CAD)
O Sources of radiologist error (chest*) O Search/Scanning: 15% O Recognition: 37% O Perceptual Decision: O Interpretive Decision: 48% Detected Radiologist alone Missed Missed Marked Missed CAD alone Missed Detected Radiologist with CAD
The Theory of CAD
Total Number of Cases
Computers can be trained to do this well and consistently (for well-defined targets)
O Performance evaluation nontrivial O Pathology/target-dependent O Operating point? (TP, FP)
O Best schemes can outperform
Image Processing for Decision Support:
CAD Implementation Issues
O Detection performance sensitive to acquisition system details
O Contrast resolution
O Wide/narrow latitude
O Linear/nonlinear response
O Spatial resolution
(e.g., pixel size)
O Noise
O Operational issues
O S/F: extra digitization step O Consistency of results?
O Proc. protocol (image vs. study) O Testing/Scoring/Training (“truth”) O Productivity/Workflow/Recalls O Case load (screening?, volume?) O Satisfaction of Search
(faith vs. blind faith)
O Observer experience/knowledge O Clinical Efficacy
O Radiologist Acceptance O Cost/reimbursement
Outline of Presentation
O
Dose and Image Quality in S/F and Digital Systems
O
Dose requirements: the speed limit
OImage 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
OImage processing for decision support
Analog or Digital Technologies – Quo vadis?
S/F
Digital
Image Quality
Image Processing
S/F
Digital
S/F or Digital
Physical Subjective Performance-based Display OptimizationDecision Support (e.g., CAD)
( )
( )
( )
Back to the Bigger Picture:
The Path to System Equilibrium…
Regulators/Standardizers Observers Producers/Competitors Consumers Clinical Technical Economic Operational o P r c s e s A qc u e i r t S o re D R p e r c o d u e
