image quality exposure/dose

10/27/2014

Exposure Feedback and Image Quality in Digital Radiography: New Developments

Ralph Schaetzing, Ph.D. Walter Streng

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Image Quality vs. Exposure/Dose

IMAGE QUALITY

? EXPOSURE/DOSE

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Image Quality: What Is It? Image quality depends on objective physical characteristics intrinsic to an imaging system that can be measured independently of an observer Image quality is whatever the observer says it is (i.e., a subjective image perception, “…eye of the beholder") Image quality defined by observer's ability to achieve an acceptable level of performance for a specific task using the imaging system

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Image Quality Triangle Perf.

Performance Metrics

? Subj.

?

Diagnostic Accuracy Diagnostic Error Rate Patient Management Patient Outcome

Subjective Metrics

Perf.

?

Obj.

Objective Metrics Subj.

Perception Confidence Preference

Physical Characteristics Laboratory Measurements

Obj.

Today, correlation between the 3 perspectives is weak!

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When Dose Wasn’t So Important…

All Images © Radiology Centennial, Inc. (www.xray.hmc.psu.edu/rci/)

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But, Dose is Important… Human exposure to ionizing radiation not without risk

• Deterministic effects – Direct, causal relationship to exposure

•

Stochastic (random) effects – Indirect relationship (causality not provable) – Time shifted (years, decades, …)

Exposure must be medically indicated (physician judgment: benefit vs. vs risk!)

1909 2009

© Radiology Centennial, Inc.

• Not all physicians/radiographers have dose awareness • Not all physicians/radiographers care • Those who do care often have difficulty understanding dose concepts/relationships

• ALARA – As Low As Reasonably Achievable 151 consecutive CT scans!

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Outline Why Radiographic Exposure Feedback? Radiographic Exposure Feedback – The Basic Problems Exposure Feedback – The Current Solutions Exposure Feedback – The New Solution (IEC 62494-1:2008) Implementation of IEC 62494 62494-1 1 Summary

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Exposure Feedback: How? What? Where? Feedback on exposure/dose available in multiple places, some directly measurable, some derived/calculated

• •

Air Kerma*

•

Entrance Skin Exposure (ESE) Entrance Surface Air Kerma

•

Organ Dose A Average Gl d l D Glandular Dose Absorbed Dose Effective Dose

•

Detector Entrance Exposure Detector Entrance Air Kerma Detector Absorbed Dose

X-Ray Tube, Filter, Collimator (kV, mA, s, anode, filter material, ripple,…)

Kerma-Area Product (Dose-Area Product)

Exposure Index

Inverse square law

Grid, AEC

Detector/Image Receptor *Kerma = Kinetic energy released in matter, or per unit mass

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Detector Response vs. Detector Exposure?

• Detector material characteristics

(e.g., x-ray absorption, kV/scatter sensitivity)

• Latent image keeping/fading (esp. CR) • Acquisition system characteristics

(e.g., linearity, electronics, A/D converter) System/Detector Response (CsI)

Detector Response (ADC C Counts)

Detector/Image Receptor as Dosimeter? 14000

System/Detector Response (Gd2O2S) 80 kVp, 0.5 mm Cu + 1.0 mm Al

12000 10000 8000 6000

Average

4000

Linear ( ) 2000 0 0

2

4

6

8

10

12

14

16

Detector Exposure (mR)

Mean Pixel Value

(1 mR = 8 8.73 73 µGy)

What’s the detector exposure at 1500? ~ 2.3 µGy if RQA 7 or RQA 9 beam ~ 2.9 µGy if RQA 5 beam ~ 5.2 µGy if RQA 3 beam Which is correct? ALL OF THEM!

Detector Exposure (µGy)

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Relative Detector Dose Need ded to Reach F Fixed Detector Respo onse

Energy Dependence of Detector Materials 3

50 kV

GOS – S/F (Kodak Lanex Med/T-Mat G) GOS – DR (Carestream Lanex Med/Varian) CsI – DR ((Trixell Pixium 4600)) αSe – DR (DRC DirectRay Array) BaFBrI – CR (Carestream CR850/GP-25)

2 70 kV 90 kV

120 kV

1

Calibration! Calibration! Calibration!

Normalized to 80 kV with ½ mm Cu + 1 mm Al 1

2

3

ISO Beam Condition

IImage receptors may not be the best dosimeters!

4

R. Van Metter, J. Yorkston, Proc. SPIE 6142 (2006);

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Exposure Feedback: Analogue Analogue screen/film situation easy: optical density (OD) changes with detector exposure/dose

• Too dark: detector (film) overexposed

Shoulder

(relative to nominal sensitivity)

• Too light: detector (film) underexposed (relative to nominal sensitivity)

Overexposure

Density, D D

•

Underexposure

Over intended operating range: direct, visual exposure feedback to radiographer

• In toe and shoulder, little response to

dose changes (threshold and saturation): no exposure feedback

Toe

• Exposure feedback accuracy not very high: D-logE curve/sensitivity determined under controlled laboratory conditions; field conditions more variable

0.0

0.5

1.0

1.5

2.0

2.5

Log E (Radiation Exposure)

• Image itself is a Quality Control tool! Image from U. Neitzel, Philips

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Exposure Feedback: Digital Digital System: thanks to image processing for display, relatively constant visual image impression, regardless of radiation exposure (the “two-edged sword” of digital)

• Exposure indicators less obvious (e.g., noise, SNR)

Multiple factors affect feedback

(log)) Detector Signal, S

• Potential for unrecognized dose variations

Dose Reduction?

• Exposure technique (esp. kV, filter) • Detector response • Signal normalization “Speed” 3200 1600 • Image processing “Dose (µGy)” 0.31 0.63 (esp. segmentation)

Excess Dose?

800 400 200 100 50 25 1.25 2.5 5.0 10.0 20.0 40.0

(detector, nominal)

Image from U. Neitzel, Philips

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“Speed” as Exposure Feedback Analogue S/F Speed, S defined • Exposure needed to reach defined, measurable signal level?

X-ray Sensitometry – Screen/Film

Dnet (Net Density vs. X-ray Exposure) 4 S/F S/F S/F S=1200 S=400 S=300

• Speed (“sensitivity”) defined* by detector

entrance exposure (Ks) giving net density of 1.0 under specific conditions (exposure, beam quality, geometry, processing,…) Ex.: S = 400, Ks = 1000/S = 2.5 µGy

3 1200

2

• Around “Speed Point”, each screen/film

system has a limited useful exposure range defined by gradient, Dmax of D-logE curve ( different films for different exposures!)

*ISO 9236-1 (2004) Photography – Sensitometry of screen/film systems for medical radiography – Part 1: Determination of sensitometric curve shape, speed and average gradient

S

1

1000 K s ( Gy )

ISO 9236-1

0 0.1

300

1.0 10.0 100.0 1000.0µGy Detector Exposure

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“Speed” as Exposure Feedback Digital “Speed” (no consistent definition!)

X-ray Sensitometry – CR/DR (Detector Signal vs. X-ray Exposure)

(depends on system characteristics)

• Linear, wide-latitude exposure response,

and variable detector kV-dependence make definition of “Speed” non-trivial and manufacturer-dependent

• “Speed/Exposure Indicators” provided by

different manufacturers are defined under different beam qualities, different exposure conditions, different beam geometries, and are mathematically different ( comparison practically impossible)

(log) Detector Signal, S

• What is “measurable signal” in detector?

0 0.1

CR/DR: pick a “speed"

1.0 10.0 100.0 1000.0µGy 4 decades of exposure

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Exposure Feedback: Common Steps Detector entrance exposure creates Latent Image in Detector/Image Receptor that is correlated with (usually proportional to) energy/photons absorbed by detector (Basic function of detector: record aerial image exiting patient) Latent Image can be interrogated/read out to produce recordable detector signal (system/process details are important!) Relationship between recorded detector signal (e.g., pixel gray levels) and detector entrance exposure can be measured under controlled conditions ( Detector Response Function under calibration conditions)

BUT, DETECTOR SIGNAL CANNOT BE USED TO DERIVE PATIENT DOSE (still need other dose metrics, e.g., DAP, ESE, effective dose)

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Exposure Feedback Today lg M

EI

S

Speed

Each receptor manufacturer has own techniques for exposure/dose feedback:

?

• Own calibration conditions/beam quality • Own measurement tools • Own calculation methods • Own image processing • Own names NOT an exact science i

• Rough estimate/indicator for radiographer • Comparison between manufacturers’ metrics practically impossible

• Even comparison within one manufacturer can sometimes be tricky (segmentation)

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Exposure Feedback Today Foreground (Collimation)

Segmentation

Background (Direct X-rays) Despite identical technique factors, exposure feedback may be different, depending on position of anatomy

Segmented Anatomy (ROI)

Important: exclude direct x-ray background and collimation from feedback calculation BUT, find anatomical ROI (segmentation)

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Carestream’s Exposure Index (EI) Indication of “speed as used”: measure of detector entrance exposure (K) needed to produce fixed image output (similar to S/F)

 K ( Gy )  EI  1000  log    2000  8.69  EI computed from mean 12-bit pixel value in diagnostically relevant region of interest

ISO S/F Speed: S 

1000 K s ( Gy )

K(µGy)

ISO Speed

Exposure Index

1.25

800

1300 - 1400

2.50

400

1400 - 1500

3.13

320

1500 - 1600

4.00

250

1600 - 1700

5.00

200

1700 - 1800

6.25

160

1800 - 1900

8.00

125

1900 - 2000

10.00

100

2000 - 2100

• ROI found by histogram-based segmentation (not gray-levels, but rather “busyness”/activity)

• ROI area (not necessarily contiguous) used by image processing algorithm to derive optimization parameters

EI (vs. K) calibration: 80kVp (+0.5 mm Cu, +1 mm Al)

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Agfa’s lgM Logarithm of Median detector entrance exposure (K) computed from diagnostically relevant lobe of image gray-level histogram EI depends on selected Speed Class, SC (scanner sensitivity, i.e., expected dose range) Target values, lgMref, allow per-image deviations to be quantified (lgMref – local medical decisions)

 K ( Gy )   SC  lg M  1.9607  log10   log10    2.5   400  lgM (vs. K) calibration: 75kVp (+1.5 mm Cu)

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Fuji’s S (Sensitivity) Indication of “speed as used”: measure of detector entrance exposure (K) needed to produce fixed image output (exposure that gets mapped into center of gray-level range when optimized image-processing tonescale is applied) S computed from mean 10-bit pixel value in diagnostically relevant region of interest

• ROI found by histogram-based segmentation (Exposure Data Recognizer, EDR)

S

200 1738  K (mR) K ( µGy )

S (vs. K) calibration: 80kVp (no added filtration)

K(µGy)

ISO Speed

S

2.50

400

695

3.13

320

555

4.00

250

436

5.00

200

348

6.25

160

278

8.00

125

217

10.00

100

174

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Detour: Calibration Conditions Formulae for Exposure Feedback (EI, lgM, S, etc.) valid under calibration conditions

1+1=? 1 + 1 =X 2

WRONG!

Calibration Conditions: binary number system, “+” means simple linear addition

1 + 1 = 10 Base 2 Don’t assume (don’t let others assume) things about the expected behavior of Exposure Feedback until you know the calibration conditions!

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Exposure Feedback Today: Variety! logarithmic

linear

10 µGy

2000

2.26

2.56

174

100

1520

5 µGy

1700

1.96

2.26

348

200

760

2.5 µGy

1400

1.66

1.96

695

400

380

1.25 µGy

1100

1.36

1.66

1390

800

190

Agfa lgM SC=400

Fuji S

Kerma

Carestream Agfa lgM EI SC=200

Philips EI Siemens EXI

Adapted from Neitzel: The Exposure Index and its Standardization, 2006

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IEC 62494-1:2008: Exposure Index…

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IEC 62494-1: New Vocabulary… Original Data: Raw Data to which allowed corrections have been applied

Value of Interest: central tendency of Original Data in the Relevant Image Region

Image Receptor Air Kerma: Detector Entrance Exposure (no backscatter)

Relevant Image Region: exam-specific area(s) containing diagnostically relevant information

Exposure Index: measure of detector response to radiation in the Relevant Image Region of an image acquired with a particular digital x-ray imaging system Target Exposure Index: expected value of Exposure Index when properly exposed Deviation Index: number that quantifies difference between Exposure Index and Target Exposure Index

Calibration Conditions: conditions under which Exposure Index is calibrated (flat-field exposure with specified x-ray beam quality) Calibration Function: Value of Interest as a function of Image Receptor Air Kerma (under Calibration Conditions) Inverse Calibration Function: Image Receptor Air Kerma as a function of Value of Interest (under Calibration Conditions)

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IEC 62494-1: Exposure Index Overview Unified exposure indicator for digital radiography manufacturer specific indicators currently used • Replaces variety of manufacturer-specific

(BUT, NOT for mammography, dental, image-intensifier-based systems)

Exposure feedback based on Detector Entrance Exposure (IMAGE RECEPTOR AIR KERMA)

• Single irradiation event (no multi-exposure CR, tomosynthesis, dual-energy, etc.) • Standardized exposure conditions (“Calibration Conditions”) Index defined by users users, based on body part • Target Values for Exposure Index, • “Deviation Index” gives difference between Exposure Index and Target Values BUT…

• NOT an indicator of patient exposure • NOT usable for comparisons to Diagnostic Reference Levels

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IEC 62494-1: Exposure Index Overview Find RELEVANT IMAGE REGION (Segment anatomic Region of Interest, ROI)

ORIGINAL DATA (corrected RAW DATA)

If DI  0, 0 Why? Compute DEVIATION INDEX (DI) from EXPOSURE INDEX and TARGET EXPOSURE INDEX* (log scale: DI ~ exposure points) *TARGET EXPOSURE INDEX set by facility/government

Use known relationship between pixel value and detector dose to Convert pixel values within ROI into an “Average” Detector Dose (VALUE OF INTEREST)

Calculate EXPOSURE INDEX (EI) from “VALUE OF INTEREST under CALIBRATION CONDITIONS (linear scale: EI  dose) Terms in SMALL CAPS are IEC terminology

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IEC 62494-1: Calibration Conditions Exposure Index must be calibrated under well-defined conditions of radiation quality and quantity (e.g., kV, mAs, anode, filtration)

• Exposure Index is dose metric, and dose varies with exposure technique  CALIBRATE • Calibrate over useful signal range (using homogeneous exposure, no backscatter) If diagnostic exposure conditions differ from Calibration Conditions, relationship between Detector Entrance Exposure and Detector Signal will be different “Calibration Calibration Conditions” Conditions defined in Annex C (normative):

• 70 kVp ± 4 kVp • Added filtration: 21 mm Al or 0.5 mm Cu + 2 mm Al • Half-Value Layer: 6.8 ± 0.3 mm (vary kV to achieve)

Close to RQA 5 (IEC)

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IEC 62494-1: Calibration Function Measured relationship between Detector Signal and Detector Entrance Exposure under Calibration Conditions

• Detector Signal: “Value of Interest” calculated in certain Region of Interest (ROI) – Measure flat-field exposures over defined operating range of detector (air kerma, no backscatter) – Central (statistical) tendency (mean/median/mode) of “Original Data*” in “Relevant Image Region”

• Region of Interest: “Relevant Image Region” – For clinical images: found by image segmentation (segmentation methods NOT defined by IEC – manufacturersupplied, documented)

*Original Data: Raw Data corrected for homogeneity, gain/offset, dead pixels, geometry. Original Data may be nonlinearly related to Detector Entrance Exposure

“Value of Interes st”

– For calibration: central 10% of detector area

Calibration Functions Valid for beam quality used under Calibration Conditions “Detector Entrance Exposure”

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Given a certain “Value of Interest”, V (e.g., mean pixel value in detector ROI), what value of “Image Receptor Air Kerma” (Detector Entrance Exposure) created it?

• Mathematical inverse, g(V), of measured

Calibration Function (manufacturer-supplied)

• Answer valid only for “Calibration Conditions” (uncertainty of 20% allowed)

• BUT: for beam qualities deviating from Calibration Conditions Conditions, relationship between Detector Entrance Exposure and Values of Interest will change

“Detector Entrance Exposure”

IEC 62494-1: Inverse Calibration Function Valid only for beam quality used under Calibration C diti Conditions

“Value of Interest”, V

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IEC 62494-1: Exposure Index, EI Exposure Index (EI) related to Value of Interest, V, as follows:

EI  c0  g (V ) 

100  g (V ) Gy

c0 = 100 µGy-1

where g(V) is Inverse Calibration Function (Detector Entrance Exposure at Value of Interest, V)

• Example: EI = 200  Detector Entrance Exposure, g(V) = 2 µGy EI sensitive to:

• Choice/calculation of “Relevant Image Region” • Choice of Central-Tendency metric (mean, mode, median, …) • Detector characteristics (e.g., energy dependence)  system comparisons still problematic

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IEC 62494-1: Deviation Index, DI Deviation Index quantifies difference between calculated Exposure Index of current image and Target Exposure Index, EIT, of exam type and application in question

 EI DI  10  log10   EI T

  

When EI = EIT (correct exposure), DI = 0* ~ -50% too low

~ -37% too low

~ -21% too low

EI Correct

~ 26% too high

~ 58% too high

~100% too high

-3

-2

-1

0

1

2

3

DI

Target Exposure Indices must be stored locally in digital imaging system for each relevant combination of detector/body part/exam type

• EIT values set by professional societies/responsible organizations *DI is logarithmic scale (ISO R10), like exposure points on an x-ray generator

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IEC 62494-1: What it does and doesn’t do It DOES: EI-arrows arrows point in same direction • Make all manufacturer EI Make EI vary linearly with Detector Entrance Exposure • • Provides reference values for constancy testing

It DOESN’T:

• Specify the algorithm to calculate EI (manufacturer) • Specify method to calculating Relevant Image Region (manufacturer) • Specify which central tendency metric to use (manufacturer) EI comparisons between different digital imaging systems can still be dangerous

• kV dependence of different detectors can lead to different EI behavior • Same exposure conditions on different systems can lead to different EI values • Different exposure conditions different systems can result in same EI value

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Implementation of IEC 62494-1 DICOM standard has added Tags/Values to support IEC 62494-1:

DICOM Tag

Tag Name

Console Value

0018,6ee1

IEC Exposure Index

IEC Exposure Index

0018,6ee2

IEC Target EI

IEC Target Exposure Index

0018,6ee3

IEC Deviation Index

IEC Deviation Index

Manufacturers starting to implement in new systems

• •

Some still also enable old exposure feedback number, or allow both (DICOM tags?) Generally not backwards compatible to older systems/software (e.g., calibration)

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IEC Exposure Index: How Precise? Experiment: in a flat-field exposure, a dosimeter placed on the detector gives a reading of 3.423 µGy. The IEC Exposure Index calculated from the Value of Interest in the Relevant Image Region is: EI = 342.31276?

EI = 342.3?

EI = 340?

EI = 300?

Recall precision of analogue S/F-Speed (S) definitions in ISO, DIN standards*: Continuous ….…200.…....….…..282…..…...………….………...562…..…....….800… Exposure (µGy) 5.00 3.56 1.78 1.25 ISO R20

180 200 220 250 280 320 360 400 450 500 560 630 710 800

ISO R10 Speed Class (DIN)

200

250

320

400

200

500

630

400 3.35

800 800

Factor of 2 in Dose!

1.68

*Graphic adapted from U. Neitzel

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IEC Exposure Index: How Precise? Reasonable approach: let Deviation Index define appropriate precision ~ -50% too low

~ -37% too low

~ -21% too low

EI Correct

~ 26% too high

~ 58% too high

~100% too high

-3

-2

-1

0

1

2

3

270

342

431

EI Range:

DI

No digits needed behind decimal point!

Practice:

• Under ideal conditions: variation of EI < 25% • Under suboptimal conditions (e.g., collimation/positioning differences affect segmentation): variation of EI < 50%

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CSH Implementation: IEC Exposure Index Diagnostically “Relevant Image Region,” ROI

• Use current image segmentation algorithm that finds clinical ROI for image processing (with slight modifications)

• “Relevant Image Region” can also be defined manually by Key Operator “Value of Interest:”

• Average (mean) over linear pixel data (IEC EI) or log pixel data (CSH EI) Inverse Calibration Function:

• Equivalent “Detector Entrance Exposure” under IEC Calibration Conditions calculated using CSH/Kodak “Values of Interest” and known conversion factor, C0, for detector of interest

Exposure Index (EI), Deviation Index (DI)

• Both EIs calculated (either/both can be displayed), also after image reprocessing • DI calculated for IEC EI only

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Image Quality vs. Exposure Index?

IMAGE QUALITY

Operating Range (DI)

Operating Point(s) (EIT)

EXPOSURE INDEX

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Summary IEC Exposure Index: new quality-control tool to monitor exam-specific, per-image dose and radiographic image quality in digital radiography systems Also useful for operational quality control (constancy testing) IEC Exposure Index functionality can be (is being) added to existing software

• •

Must take into account calibration standardization (vs. manufacturer variability) DICOM now contains required tags

Comparison between manufacturers still difficult, but better than before

•

Standard is silent or (too?) flexible on certain implementation issues (segmentation, central tendency metric, detector characteristics) – potential for variability, ambiguity

•

Success of IEC Exposure Index depends not only on acquisition system  PACS and other information/image management software must also adopt

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One can go still deeper…

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