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The Exposure Index for
Digital Radiography
(IEC 62494-1 and AAPM
Report 116)
S. Jeff Shepard, MS, DABR, FAAPM
Imaging Physics Department
Diagnostic Imaging Division
The University of Texas M. D. Anderson Cancer Center
Houston, Texas
Acknowledgement:
Michael Flynn, PhD
AAPM Spring
Clinical Meeting
2015
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Henry Ford Hospital
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System
Learning Objectives
• Recognizing good technique in DR - Noise
• Understand causes and solutions to “exposure
creep”
• Understand IEC/TG116 Exposure Index
• Verifying EI calibration in the clinic
• Be aware of additional recommendations in TG116
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Identifying Correct Clinical Technique
Recognizing Bad Images
• In the film-screen world, under- and
over- exposures were easily recognized
by the appearance of the recorded image
– Repeat at a higher or lower technique
based on optical density
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Identifying Correct Clinical Technique
Recognizing Bad Images
• With Digital Radiography, under- and
over-exposures are not so easily
recognized
– Adequate images over a much wider range of
exposure
– Post-processing can hide mistakes
– Excellent dynamic range may have down-sides
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Exposure Indices
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Exposure Indices
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Exposure “Creep”
• Under-Exposure
– Higher noise
– Detail visibility suffers
• Over-Exposure
– Lower noise (“Pretty” – improved SNR)
– High patient dose
• Radiologists may complain about noise, but
not usually about over-exposure
– Technologists learn quickly how to avoid criticism
– If no one is paying attention, exposures will
“creep” up.
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Exposure “Creep”
Widely known problem that’s been around
for a long time
• Freedman M, Pe E, Mun SK, Lo SCB, Nelson M,
“The potential for unnecessary patient exposure
from the use of storage phosphor imaging
systems,” SPIE 1897:472-479 (1993).
• Gur D, Fuhman CR, Feist JH, Slifko R, Peace B,
“Natural migration to a higher dose in CR imaging,”
Proc Eighth European Congress of Radiology,
Vienna Sep 12-17, 154 (1993).
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Vendor-Specific Indices
Manuf.
Index
Lin or
Log
Exposure
relation
Std kV/Filter
Cal
Agfa
lgM
Log
Direct
75/1.5 mm Cu
1.96 bel @ 2.5 uGy
Alara
EIV
Log
Direct
70/7.1 mm HVL
2000 mbel @ 10 µGy
Canon
EXP
Linear Direct
80/8.2 mm Al
2000 @ 10 µGy
Canon
REX
Linear Direct
--
106 @ 10 µGy*
Carestream EI
Log
80/0.5 Cu + 1 Al
3000 @ 1 mR
Fuji
Linear Inverse
80/3 mm Al
“Total”
200@1 mR
2.85 @ 0.5 mAs
S
Direct
GE
UDExp
Linear Direct
80/21 mm Al
Imaging
Dynamics
SE
Linear Inverse
80/1 mm Cu
200 @ 10 uGy
Konika
S
Linear Inverse
80/3 mm Al
“Total”
200@1 mR
70/7.1 mm HVL
Philips
EI_s
Linear Inverse
Siemens
EXI
Linear Direct
70/0.6
mm Cu
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400 @ 2.53 µGy
100 @ 1 uGy
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Exposure Indices
IEC 62494-1 & AAPM TG116
• Recommendation for a standard detector
exposure index for all digital radiography
• Published 2008
• Mike Flynn and Jeff Shepard represented the
USA (USNC TAG members) and AAPM on the
IEC working group
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Exposure Index – IEC
Exposure Index (EI):
• Index is proportional to the air Kerma that the
detector would have received under standard
beam conditions for the same raw pixel value in
the relevant image region.
• Calibrated for the imaging system over the
specified operating range of image receptor air
kerma such that:
EI = (100 uGy-1) * KCAL
where KCAL is the image receptor air kerma in μGy
under the calibration conditions.
• For Krel = 10 uGy, EI = 1000
• For Krel = 2.5 uGy, EI = 250
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“Relevant” Image Region
• Gray histogram is for the
entire image.
• Black histogram is for the
relevant anatomic region
or Values of Interest
(VOI)
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“Relevant” Image Region
• Gray histogram is for the
entire image.
• Black histogram is for the
relevant anatomic region
or Values of Interest
(VOI)
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“Relevant” Image Region
• AAPM TG116: “The median is recommended
rather than the mean or mode because it is
less affected by data extremes and outliers.”
• IEC 62494-1: “The [indicator] shall be
calculated using the mean, median, mode,
trimmed mean, trimean, or other recognized
statistical method for the description of
central tendency of the [values] in the
relevant image region.”
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Region to assess signal indicator
Systems vary in
the region used to
assess the signal
for an image.
• Full Image
• Regular regions
• Anatomic regions
(segmentation)
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Deviation Index
Both standards also call for a “Deviation Index”
DI = 10 × Log10{EI/EIT(b,v)}
• EIT (b,v) is a table of target values stored
by body part (b) and view (v)
DI = 0 is a perfect exposure
DI = +1 means exposure was high/low by about 28%
(one density or mAs step)
• EIT tables to be customized for each site
• If not customized, default value of DI is 0.
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Deviation Index
• Exposure indices are saved in the DICOM header
– 0018,1411 (EI)
– 0018,1412 (EIT)
– 0018,1413 (DI)
– 0018,1405 (EI) and 0018,6000 (Sensitivity) are old and
no longer recommended
• DI Format:
– AAPM: Decimal string with one decimal place (tracking
and trend management)
– IEC: Integer
• Both indices change with VOI modification by
the tech
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Values of interest – VOI
Number of Pixels
Pixel values to be filtered/re-scaled for
presentation
EI and DI
calculated from
median pixel value
EI = EIT
DI = 0.0
Values of Interest
Pixel Value
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VOI Recognition Failure
• Gonadal shields, prosthetics, surgical mods
• False EI & DI reported
EI =≠ EITT
-1.3
DI = 0.0
Number of Pixels
EI and DI calculated
from incorrect median
pixel value
Correct
Values of Interest
Incorrect Values of Interest
Pixel Value
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Manual Correction of VOI Failure
Tech manually returns the VOI to the proper position
by reprocessing
Number of Pixels
Correct EI and DI
calculated from new
median pixel value
EI = EIT
DI = 0.0
Correct
Values of Interest
Pixel Value
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Standard Beam
RQA5 is the standard beam condition
•
•
•
•
RQR5 pre-filtered beam (70 kVp, 2.6 mm Al
HVL)
70 kVp
21 mm added pure Al filtration
6.8 mm Al HVL
But …
•
•
•
Pure Aluminum is impractical for field
measurements (expensive, heavy, hard to find).
An alternative is to use copper (cheap, portable
and widely available).
Valid substitution?
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Beam Quality – spectral shape
Report 116 reported equivalent spectral shape with
RQA5 conditions and Cu/Al filtration
HVL
6.8 mm Al
Normalized Spectra,
Cu/Al spectrum is
about 2X that of
RQA5
72.59 kVp
3.02 mm Al pre-filtered HVL
• 21 mm type 1090 Al (99.9%)
• Filtered HVL : 6.80 mm Al
+ + + + +
• 0.5 mm Cu + 2.8 mm Al
• Filtered HVL : 6.80 mm Al
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Standard Beam
Report 116 illustrated how the addition of different Al
filters to a Cu filter could compensate for differences in
the unfiltered beam quality.
Approximate Al to add to
0.5 mm Cu to achieve 6.8
mm HVL at 70 kVp.
Averaged from about 25
systems tested.
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Beam Quality – IEC vs Report 116
AAPM Report 116
•
•
•
•
HVL = 6.8 mm Al
Adjust Al and (if necessary) kVP to get HVL
70 ± 4 kVP
0.5 mm Cu + (0 – 4) mm Al (type 1100)
– Brass acceptable as a Cu substitute
– 21 mm pure Al acceptable.
IEC Beam condition
•
•
•
•
70 kVP
0.5 mm Cu + 2 mm Al
HVL = 6.8 +/- 0.3 mm Al
Assumes 2.9 mm HVL unfiltered beam
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Beam Quality – IEC vs Report 116
AAPM Report 116
•
•
•
•
HVL = 6.8 mm Al
Adjust Al and (if necessary) kVP to get HVL
70 ± 4 kVP
0.5 mm Cu + (0 – 4) mm Al (type 1100)
– Brass acceptable as a Cu substitute
– 21 mm pure Al acceptable.
IEC Beam condition
•
•
•
•
70 kVP
0.5 mm Cu + 2 mm Al
HVL = 6.8 +/- 0.3 mm Al
Assumes 2.9 mm HVL unfiltered beam
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Verifying EI calibration
Verifying EI Calibration
•
•
•
•
•
•
Check generator kV calibration/reproducibility
Set up the Standard Beam HVL (add copper and
adjust Al filtration)
Establish a reference dosimeter away from the CR
Obtain grid and table-top attenuation factors from
the manufacturer
Set up for a 10 µGy exposure to the detector (EI =
1000)
Compare to EI that the system reports
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Beam setup – step 1
Prior to any measurements verify that the
x-ray source has acceptable exposure
reproducibility (coefficient of variation <
0.03) and kV accuracy (within ± 3%) at
the standardized condition.
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Beam setup – step 2
Collimate the x-ray beam to only
cover the ion chamber with no
more than 1 inch margins.
Add 0.5 mm copper filtration at
the face of the collimator.
For DR systems, the detector
should be covered with a lead
apron or similar barrier when
making the exposures for HVL
determination and adjustment.
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Beam setup – step 3
Measure the HVL of the filtered
beam
Adjust the kVP and/or aluminium
filtration within the limits
specified to obtain a HVL as
close as possible to 6.8 mm Al.
Aluminum for HVL
70 kVP + 4
0 – 4 mm Al
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Beam setup – step 4 (DR)
The detector should be placed as far from the
x-ray source as is practical, at least 100 cm.
Collimate to the edges of the detector.
If present, remove the grid and any other
components between the ion chamber and the
image detector. If any components cannot be
removed, obtain the attenuation factors from
the DR system or component vendor.
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Beam setup – step 4 (CR)
Source
Collimator
For CR, the cassette should be
separated from any surface
that may increase
backscatter from that
surface, as recommended in
AAPM Report 93 (TG 10).
Use lead behind the plate to
further reduce backscatter.
Added Filtration
Source to
Detector
Distance
(maximum
possible)
If the detector is not square,
the long axis of the detector
should be perpendicular to
the x-ray tube A-C axis.
Lab Jack
(CR only)
> 25 cm
(CR only)
Lead (CR only)
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Beam setup – step 5
Place a calibrated ion
chamber at the center of
the beam approximately
midway between the
source and detector
(Position A).
All distances should be
measured from the focal
spot as indicated on the
x-ray tube housing.
Detector housing
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Beam setup – step 6
Source
Collimator
Added Filtration
Use lead to protect a DR detector.
Source to
Chamber
Distance
Make an exposure. Using an inverse
square correction and applying
the grid and table attenuation
factors (if present) determine
the air kerma at the detector
(KCal).
Source to
Detector
Distance
(maximum
possible)
Ion chamber
(position A)
Chamber to
Detector
Distance
Change the mAs setting to obtain a
Kcal value that is in the middle of
the detector’s response range
(suggest 10 uGy).
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Beam setup – step 7
Source
Collimator
Move the ion chamber perpendicular to
the tube axis such that it is at the
edge of the field of view (Position
B).
Added Filtration
Ensure that the entire ion chamber is
in the radiation beam and is not
shadowed by a collimator blade.
Source to
Chamber
Distance
Ion chamber
(position B)
Make an exposure using the mAs found
earlier and determine the ratio of
the air kerma at Position A to that
at Position B.
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Source to
Detector
Distance
(maximum
possible)
Chamber to
Detector
Distance
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Beam setup – step 8
Remove the protective lead and
expose the detector again.
Verify the resulting KCal by
monitoring the exposure with the
chamber at position B.
Compare the corrected reading to
the Exposure Index reported by
the system/detector. EI should
equal KCal *100 if properly
calibrated,
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EI Calibration
EI calibration should be verified
• At acceptance
• Annually thereafter
• After service events and software
upgrades
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Rules for repeats
Little clinical information on reject
thresholds in literature
• Van Metter and Yorkston
– Chest and abdominal imaging
– Wide range of patient thicknesses
– Most AEC controlled images fell within
the range
of DI = ± 1.2.
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Rules for Repeats
Emulating film/screen limits
ΔOD = γ*Log10 {E2/E1}, and
DI = 10*Log10 {E2/E1}
So: Δ DI = 10*ΔOD / γ
γ (film gamma): slope of H&D at 1.0 above B+F
For an OD range of +0.3 (0.6 OD total) and γ = 2.5:
Δ DI = 10 * {0.6/2.5} = 2.4
Acceptable DI range is + 1.2
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DI action levels (Table 2)
DI
EI
Action
Between -1.0 79% < exp < 126%
and +1.0
Check for noise and
clipping (always)
< -1.0
< 79% of target
Check for noise and
consult with
radiologist/management
on need for repeat,
investigate cause.
Between +1.0
and +3.0
126% – 200%
of target
Repeat only if relevant
anatomy is clipped or
“burned out”
> 3.0
> 200% of
target
Repeat only if relevant
anatomy is clipped or
“burned out”, require
immediate management
follow-up
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DI Action Levels
Sites reporting excessive follow-up.
Action levels based on screen/film emulation is
admittedly arbitrary.
Little or no published literature.
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DI Action Levels
AAPM Report 232
To investigate the current state of practice
for CR/DR Exposure and Deviation Indices
based on AAPM Report 116 and IEC 62494, for
the purpose of establishing achievable goals
(reference levels) and action levels in digital
radiography. The products of this task group
will be a brief report and an updated version of
Table 2 from AAPM Report #116.
Jaydev Dave and Kyle Jones, Co-Chairs
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Other TG116 Recommendations
• VOI Histograms and graphical pixel
overlays
• Repeat/Reject logs
• Import/Export of EIT tables
• Export of for-processing image data
• Dependence of EI on kV
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AEC and Technique Charts
Still important in controlling patient
exposure
• AEC may need to be adjusted for CR and
add-on DR
– Energy dependence may not be the same as
GdO2S
• Technique charts are important for
portables and extremities
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AEC Calibration Using EI
EI should remain consistent with:
•
•
•
•
Varying
Varying
Varying
Varying
energy
phantom thickness
AEC cell
dose rate (mA)
• EI should be reproducible over
multiple exposures
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What the Exposure Index is NOT for
Patient dose estimation
– Need beam HVL, pt. thickness, SSD and
SID, grid atten, AEC pickup atten, detector
assy. input atten …
– If you have all these, you don’t need EI!
System intercomparisons
– Index says nothing about detector energy
dependence, efficiency.
- VOI recognition strategy will dramatically
affect indices.
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Exposure Index Monitoring
QC programs based on exposure indices
are successful
– Seibert JA, Shelton DK, and Moore EH,
“Computed Radiography X-ray Exposure
Trends,” Academic Radiology 3, 313-318
(1996).
Katie Hulme will cover this next.
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Summary
Exposure Indices
• Noise, not density
• Exposure creep
• Exposure indices
– AAPM Report 116
– IEC 62494-1
• Calibration
• Rules for repeats
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THANK YOU!
jshepard@di.mdanderson.org
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References:
Fredman M, Pe E, Mun SK, Lo SCB, Nelson M, The
potential for unnecessary patient exposure from
the use of storage phosphor imaging systems, SPIE
1897:472-479 (1993).
Gur D, Fuhman CR, Feist JH, Slifko R, Peace B,
Natural migration to a higher dose in CR imaging,
Proc. Eighth European Congress of Radiology,
Vienna,Sep 12-17, 154 (1993).
Yorkston J. Flat-panel DR detectors for radiography
and fluoroscopy. In: Specifications, Performance
Evaluations, and Quality Assurance of Radiographic
and Fluoroscopic Systems in the Digital Era,
Goldman LW and Yester MV eds. Madison, WI:
Medical Physics Publishing (2004)177-228.
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References:
Willis CE, Thompson SK and Shepard SJ. Artifacts
and Misadventures in Digital Radiography. Applied
Radiology 33(1):11-20, January 2004.
R.E. Alvarez, J.A. Seibert, and S. K. Thompson,
Comparison of dual energy detector system
performance, Medical Physics31(3), 556-565
(2004).
JA Seibert, DK Shelton, and EH Moore, Computed
Radiography X-ray Exposure Trends, Academic
Radiology 3, 313-318 (1996).
J A Seibert, et al, AAPM Report #93, “Acceptance
Testing and Quality Control of Photostimulable
Storage Phosphor Imaging Systems: Report of
AAPM Task Group 10.” AAPM (2006)
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References:
Richard S., Siewerdsen J. H., Jaffray D. A., Moseley
D. J. and Bakhtiar B., Generalized DQE analysis of
radiographic and dual-energy imaging using flatpanel detectors, Med. Phys. 32 (5), May 2005,
1397 – 1415
Lehman L. A., Alvarez R. A., Macovski A., Brody W. R.,
Pelc N. J., Reiderer S. J., and Hall A., Generalized
image combinations in dual KVP digital radiography,
Med. Phys. 8 (5), Sept/Oct 1981, 659 – 667
Shepard S.J., et al, AAPM Report 116, An Exposure
Index for Digital Radiography (Executive
Summary), Med. Phys., 2009
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References:
IEC 62494-1, Medical electrical equipment- Exposure
index of digital X-ray imaging systems Part 1:
Definitions and requirements for general
radiography, International Electrotechnical
Commission, 2008
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