Dose Considerations in DR
S. Jeff Shepard, MS, DABR, FAAPM
Imaging Physics Department
Diagnostic Imaging Division
The University of Texas M. D. Anderson Cancer Center
Houston, Texas
AAPM Annual
Meeting 2007
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Acknowledgements:
A. Kyle Jones, PhD
Rai Polman
Outline
Dose Considerations
• Noise, not optical density for
appropriateness
• Exposure creep
• Exposure Indices
• Rules for repeats
• TG116 and IEC WG43
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Dose Considerations
Recognizing Bad Images
• In the filmfilm-screen world, underunder- and
overover- 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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1
Dose Considerations
Recognizing Bad Images
• With Digital Radiography, underunder- and
overover-exposures are not so easily
recognized
– Adequate images of a much wider range of
exposure
– Excellent dynamic range may have downdown-sides
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Dose Considerations
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Dose Considerations
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2
Dose Considerations
“Exposure creep”
creep”
• UnderUnder-exposure
– Higher noise
– Detail visibility suffers
• OverOver-Exposure
– Lower noise (“
(“Pretty”
Pretty” – improved SNR)
– High patient dose
• Radiologists may complain about noise, not
usually about overover-exposure
– Technologists learn quickly how to avoid criticism
– If no one is paying attention, exposures will
“creep”
creep” up.
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Dose Considerations
Widely known problem that’
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,”
systems,” SPIE 1897:4721897:472-479
(1993).
• Gur D, Fuhman CR, Feist JH, Slifko R,
Peace B, “Natural migration to a higher
dose in CR imaging,”
imaging,” Proc Eighth European
Congress of Radiology, Vienna Sep 1212-17,
154 (1993).
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Dose Considerations
Solution – Exposure Index
• Numerical indicator related to dose to
detector (noise content in the image)
• QC programs based on exposure indices
are successful
– Seibert JA, Shelton DK, and Moore EH,
“Computed Radiography XX-ray Exposure
Trends,”
Trends,” Academic Radiology 3, 313313-318
(1996).
• Not all manufacturers provide indices.
• No standardization
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3
Dose Considerations
Approximate Exposure Indicator Values
vs. Receptor Exposure
Manufacturer
Symbol
50 µGy
100 µGy
200 µGy
REX
50
100
200
Canon
(Brightness = 16,
Contrast = 10)
IDC (ST = 200)
f#
-1
0
1
Philips
EI
200
100
50
Fuji, Konica
S
400
200
100
Kodak (CR, STD)
EI
1700
2000
2300
Seimens
EI
500
1000
2000
10
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Dose Considerations
Approximate Exposure Indicator Values
vs. Receptor Exposure
Manufacturer
Symbol
50 µGy
100 µGy
200 µGy
REX
50
100
200
Canon
(Brightness = 16,
Contrast = 10)
IDC (ST = 200)
f#
-1
0
1
Philips
EI
200
100
50
Fuji, Konica
S
400
200
100
Kodak (CR, STD)
EI
1700
2000
2300
Seimens
EI
500
1000
2000
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Dose Considerations
AAPM TG116
• Recommendation for a standard detector exposure
index for all digital radiography
• Index (KIND) is the air Kerma that the detector
would have received under standard beam
conditions for the same raw pixel value
• Standard beam
–
–
–
70 + 4 kVp
0.5 mmCu + [0 – 3] mmAl filtration
6.8 mm (+
(+ 0.2 mm) Al HVL
• Calculated for every image from the
forfor-processing pixel value in the VOI
recognized by the system
• Final approval expected by RSNA
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4
Dose Considerations
AAPM TG116 also calls for a Deviation Index
DI = 10 × Log10{KIND/KTGT(b,v)}
• KTGT(b,v)
(b,v) is a table of target KIND values stored
by body part (b) and view (v)
DI = 0 is a perfect exposure
DIL = +1 means exposure was high/low by about 28%
(one density or mAs step)
• KTGT tables to be customized for each site
• Format:
– TG116: Decimal string with one decimal place
– WG43: Integer
• Both indices saved in the DICOM header (Tag TBD)
• Both indices change with VOI modification by the
tech
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Dose Considerations
Number of Pixels
Both indices change with VOI
modification by the tech
KIND and DI
calculated from
this pixel value
KIND = KTGT
DI = 0.0
Values of Interest
Pixel Value
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Dose Considerations
VOI recognition algorithm fails
Number of Pixels
• Gonadal shields, prosthetics, surgical mods
• False DI reported
KIND ≠ KTGT
KIND and DI
incorrectly calculated
from this pixel value
DI = -1.3
Correct
Values of Interest
Incorrect Values of Interest
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Pixel Value
15
5
Dose Considerations
Tech returns VOI to proper position
manually
Number of Pixels
KIND and DI
calculated from
this pixel value
KIND = KTGT
DI = 0.0
Correct
Values of Interest
Incorrect Values of Interest
Pixel Value
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Dose Considerations
International Electrotechnical
Commission (IEC) standard
•
•
•
Subcommittee 62b, WG43
Definition only
Current draft parallels AAPM TG116
– EI: A unitless value (KREC * 100 µGy-1)
• Intent is to avoid confusion with patient exposure
– Similar standard beam
– Deviation Index (DI) same as DI (integer
value vs decimal string with one decimal)
• Committee draft for vote (CDV)
expected this fall
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Dose Considerations
AAPM
TG116
IEC
WG43 (SC 62b)
Exposure
Index
AirAir-kerma at the receptor
KIND = Krec (uGy)
uGy)
EI = Krec *100 µGy-1
(unitless)
unitless)
Calibration
Energy
RQARQA-5 Equivalent
RQARQA-5 Equivalent
70 + 4 kVp
70 + 4 kVp
Calibration
Filtration
RQARQA-5 Equivalent
RQARQA-5 Equivalent
0.5 mm Cu (+ 00-3 mmAl)
mmAl)
6.8 + 0.2 mm A1 HVL
or 21 mm Al
0.5 mm Cu + 2 mm Al or
21 mm Al
6.8 + 0.3 mm A1 HVL
Deviation
Index
Deviation Index
DI = 10*log10(KIND/KTGT)
Deviation Index
DI = 10*log10(EI/ETGT)
Index format
Signed decimal string with
3 significant Fig’
Fig’s
Unspecified
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Dose Considerations
What the index is NOT for:
• Patient dose estimation
– Need beam HVL, output, pt. thickness, SSD,
SID grid atten,
atten, AEC pickup atten,
atten, detector
input atten
– If you have all these, you don’
don’t need KIND!
• System intercomparisons
– Index says nothing about detector energy
dependence or efficiency
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Dose Considerations
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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Dose Considerations
Rules for repeats
• Emulating film/screen limits
⎛E ⎞
⎛E ⎞
∆OD = λ * Log10 ⎜ 2 ⎟ and ∆DI = 10 * Log10 ⎜ 2 ⎟
E
⎝ 1⎠
⎝ E1 ⎠
⎛ ∆OD ⎞
Combining: ∆DI = 10 * ⎜
⎟
⎝ γ ⎠
For an OD range of ± 0.3 OD (0.6 OD total) and γ = 2.5 :
⎛ 0.6 ⎞
∆DI = 10 ∗ ⎜
⎟
⎝ 2.5 ⎠
= 2.4
or DI = ±1.2
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7
Dose Considerations
KIND
DI
Action
Between -1.0
and +1.0
79% < exp < 126%
< -1.0
< 79% of target
Between +1.0
and +3.0
126% – 200% of
target
> 3.0
> 200% of
target
Check for VOI
recognition failure,
accept if ok.
Check for noise and
consult with
radiologist on need
for repeat,
investigate cause.
Check for VOI
recognition failure,
accept if ok.
Investigate cause.
Check for saturation
and consult with
radiologist on need
for repeat,
investigate cause
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Dose Considerations
Exposure Index monitoring
• Collect KIND or DI for every image and analyze
–
–
–
–
–
–
by tech
technique factors
by xx-ray system
by plate scanning unit (CR)
by processing unit (CR)
by anatomical view
• Longitudinal studies
– Track performance over time
– Mean & Standard Deviation
– Watch for trends upward (“
(“Creep”
Creep”) and
increasing std. dev.
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S-Number
Histogram
DI-D
Dose
Considerations
Exposure Index Tracking
300
800
250
700
• Use histogram analysis to illustrate
problem areas
Frequency
600
Mean = 187.5
Std Dev = 158.5
500
150 400
Mean
300
Std Dev
100 200
100
S
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00
00
00
50
30
10
0
0
80
90
0
0
70
0
Nov-06
Month
60
50
0
0
0
30
Oct-06
40
20
0
Sep-06
0
0
0
50
10
Index Value
200
Dec-06
24
8
Dose Considerations
Exposure Index monitoring
• Many systems have no index at all
• Many systems do not log the indices
• For those that do,
– multiple machines with logs that are not
accessible over the network
– inconsistent file formats
• Must be performed manually
– Paper logs
– Manual electronic log downdown-load
– Manual conversion to common file format
Impractical! Too much work!
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Dose Considerations
Exposure Index monitoring
• HomeHome-made solution:
– A batch file on every IIP runs every
night and copies the Fuji SS-number
log file to a location that is accessible
to their FTP service
– Unix program contacts each IIP and
extracts the log file (ftp), cleans up
the data and parses it into a
database.
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Dose Considerations
Data from any month can be downdownloaded over the intranet for analysis
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9
Dose Considerations
AEC calibration and technique charts
are still important in controlling
patient exposure
• AEC may need to be adjusted for CR and
addadd-on DR
– Energy dependence may not be the same as
GdO2S
• Technique charts are important for
portables and extremities
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Summary
Dose Considerations
• Noise, not density
• Exposure creep
• Exposure indices
– AAPM TG116
– IEC WG43
• Exposure index logs
• Rules for repeats
AAPM Radiographic and Fluoroscopic
SubSub-Committee is always open to new
ideas
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THANK YOU!
jshepard@di.mdanderson.org
10
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
1897:4721897:472-479 (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 1212-17, 154 (1993).
Yorkston J. FlatFlat-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(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):1133(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), 556556-565
(2004).
JA Seibert, DK Shelton, and EH Moore, Computed
Radiography XX-ray Exposure Trends, Academic
Radiology 3, 313313-318 (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.”
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 dualdual-energy imaging using flatflatpanel 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
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