8/2/2011
Imaging Educational Course - Radiography III
Understanding Exposure and Deviation
I di
Indices
David L. Leong1,2
Marie-Louise Butler2
1
2
Analogic Corporation, Peabody, MA, USA
UCD School of Medicine and Medical Sciences, University College Dublin, Dublin,
Ireland
AAPM 2011 WE-A-110-1
Learning Objectives
1. Understand AAPM recommendations,
2. Understand the IEC standard,
3. Understand factors that affect Exposure and
Deviation Index values.
4. Understand DICOM support for Exposure and
Deviation Index values.
2
1
8/2/2011
State of Confusion
• Manufacturer Specific Indices
–
–
–
–
–
–
–
–
Rex
S
lgM
EI
DEI
EXI
EVI
f#
• A range of units and relationships to exposure.
• Calibration conditions ranged from 70 to 80 kVp
with different filtration selections
3
AAPM Task Group 116
• TG116 was started in July 25, 2004
• Report published June 2009
• 29 individuals participated in the Task Group
• Full report available on AAPM Website
– http://www.aapm.org/pubs/reports/
• Executive summary published in Medical Physics
– S. J. Shepard et al. "An exposure indicator for digital
radiography:
di
h AAPM Task
T kG
Group 116 (Executive
(E
ti
Summary)," Med. Phys. 36, 2898-2914 (2009).
– http://www.aapm.org/pubs/reports/
4
2
8/2/2011
IEC Working Group 43/62b
• Preparatory work in the German (DIN)
standardization organization 2004-2006
• Proposal for standard by the German National
Committee June 2006
• WG43 was started in September 2006
• Experts from Germany, Belgium, Canada, China,
Japan, Netherlands, United States
– 16 members,
members 6 people overlapped with TG116
• First meeting October 2006 in Hamburg, Germany
• IEC 62494-1 published August 2008
• Stability date: 2012
5
Keys Parts
• Exposure Index
• Target Exposure Index
• Deviation Index
• Calibration Condition
6
3
8/2/2011
Exposure Index
IEC 62494
AAPM TG116
Name
Exposure Index
(EI)
Indicated Equivalent Air Kerma
(Kind)
Units
Unitless
μGy
Not specified
3 significant
Precision
• Different definitions, but substantially the same.
• Kind typically less than 1 as it is μGy
• EI is designed to be greater than 1
7
Exposure Index Formulas
• IEC:
VC AL  f  K C AL 
K CAL  f
1
E I  c 0  g V   100μG y -1  f
V C A L 
1
g V   f
1
V 
V 
• TG116:
K

 K

Q K  1000  log 10  ST D   1000  log 10  ST D 
 0.001 
 K0 
K IND  K 0 10
Q
1000
, where K 0  0.001 Gy
8
4
8/2/2011
Target Exposure Index
• Target values: KTGT (TG116) and EIT (IEC)
• EIT – expected value of EI when the detector is
properly exposed
• Manufacturer / Site Specified
– Needs to be adjustable by user
– Can be established by professional societies
• Body part and view specific targets
(recommended by TG116)
9
Deviation Index
IEC 62494
AAPM TG116
Name
Deviation Index (EI)
Deviation Index (EI)
Units
Unitless
Unitless
Not specified
1 decimal place
Precision
• TG116 and IEC are the same:
 EI 
D I  10  log 

 EIT 
• More important clinically than EI
10
5
8/2/2011
Action Levels (TG116)
Table 2. Exposure Indicator DI Control Limits for Clinical Images
DI
Range Action
> +3
+3.0
0
Excessive patient radiation exposure
Repeat only if relevant anatomy is
clipped or “burned out”
Require immediate management followup.
+1 to +3.0
Overexposure:
Repeat only if relevant anatomy is
clipped
pp
or “burned out”
−0.5 to +0.5
Target range
Less than −1.0
Underexposed:
Consult radiologist for repeat
Less than −3.0
Repeat
11
Calibration
IEC
Based on RQA5 from IEC 61267
AAPM
• kVp: 66 – 74
• kVp: 70 ± 4
• HVL: 6.8 ± 0.3 mm Al
• HVL: 6.8 ± 0.25 mm Al
– Added filter of either 21
mm aluminum or 0.5
mm copper and 2 mm
aluminum
– Adjustment of the kVp
from 66 to 74 is
permitted to achieve the
target HVL
– Added filter of either 21
mm aluminum or 0.5
mm copper and 0 to 4
mm aluminum*,
– Adjustment of the kVp
and filter permitted to
achieve the target HVL
*Type 1100
Document the added filter and X-ray tube
voltage used for calibration
12
6
8/2/2011
Calibration - Recommendation
• Start as close to 70 kVp as possible
– Measure non-invasively without added filtration
– If your meter support Practical Peak Voltage (PPV), use it
• Use at least 1 meter Source to Detector distance
• Adjust added filtration to achieve 6.8 mm HVL
– No less than 0.5 mm Cu or 18 mm Al
– No more than 0.5 mm Cu + 4 mm Al or 23 mm Al
– Raise or lower kV as a last resort
• Record the final filter thickness and generator kVp
– On repeat calibrations, start with the previous filter
thickness
13
Summary
IEC 62494
AAPM TG116
Exposure Indicator Name
Exposure Index (EI)
Indicated Equivalent Air
K
Kerma
(Kind)
Exposure Indicator Units
Unitless
μGy
Not specified
3 significant
Exposure Indicator
Precision
Deviation Index formula
Deviation Index Units
Deviation Index Precision
Beam Conditions
 EI 
D I  10  log 

 EIT 
K 
DI  10*log  IND 
 KTGT 
Unitless
Unitless
Not specified
1 decimal place
6.8 ± 0.3 mm Al
6.8 ± 0.25 mm Al
14
7
8/2/2011
Factors that affect EI
• Calculation of EI based on anatomical region of
interest. (ROI) - Relevant Image Region
• Method to determine ROI and area of ROI affect
EI value
• Either define a specific procedure or method
15
ROI and Exposure Variation
16
8
8/2/2011
30 Exposures – No Changes
Bone and
Mostly Bone Bone Only
Soft Tissue
Mean
398.1
277.9
231.2
Std
2.164
0.712
0.774
17
Chest Exposures
All Image
Anatomy
Mean
466.00
462.17
Std
2.280
2.483
18
9
8/2/2011
ROI Effects on DI
Bone and
Mostly Bone Bone Only
Soft Tissue
4 mAs
-1.0
-1.0
-1.0
6 mAs
+1.1
+1.1
+1.1
19
DICOM Changes
• CP1024 written to support new Exposure Indices
• Became “Final Text” June 21, 2010
• Use is not mandatory
20
10
8/2/2011
DICOM Changes
• Legacy Exposure Indices
Attribute Name
Relative X-Ray Exposure
Tag
(0018,1405)
Sensitivity
(0018,6000)
Type
Attribute Description
3
Relative x-ray exposure on the plate.
Meaning of values is implementation
specific. May be used to adjust the
dynamic range of the plate digitizer
(scanner).
Note: This value is manufacturerspecific. DICOM specifies
standard Attributes in Table
10.x-1 ‘Exposure Index Macro’
which are recommended.
3
Read out sensitivity.
Note: This value is manufacturerspecific. DICOM specifies
standard Attributes in Table
10.x-1 ‘Exposure Index Macro’
which are recommended.
21
DICOM Changes
• New Exposure Indices
Attribute Name
Exposure Index
Tag
(0018,1411)
Target Exposure Index
(0018,1412)
Deviation Index
(0018,1413)
Type
Attribute Description
3
Measure of the detector response to
radiation in the relevant image region of an
image acquired with a digital x-ray imaging
system as defined in IEC 62494-1.
Notes: 1. A string rather than binary
Value Representation is used for
this Attribute, in order to allow the
sender to control the precision of
the value as suggested in the
report of AAPM Task Group 116.
2. This index value is scaled as
defined by IEC 62494
62494-1.
1
3
The target value used to calculate the
Deviation Index (0018,1413) as defined in
IEC 62494-1.
3
A scaled representation of the difference of
the Exposure Index compared to the Target
Exposure Index as defined in IEC 62494-1
and the report of AAPM TG 116.
22
11
8/2/2011
Recommendations
• Insist that all new CR and DR equipment support
IEC 62494-1
–
Both EI and DI
• Push your PACS vendor to support and display DI
on all workstations (CP1024)
• Attempt to upgrade older CR and DR systems to
add support for IEC 62494-1
• Develop a set of Target Exposure Values
• Setup a training program on the use of EI, DI
and action limits
23
Learning Objectives
 1. Understand AAPM recommendations,
 2. Understand the IEC standard,
 3. Understand factors that affect Exposure and
Deviation Index values.
 4. Understand DICOM support for Exposure and
Deviation Index values.
24
12
8/2/2011
Final Comment
“It is
i essential
ti l th
thatt all
ll aspects
t off good
d
clinical technique be adhered to and an
appropriate DI value should not be
interpreted as proof of good work.”
TG116 Report Section X
25
Thank You
UCD School of Medicine
& Medical Science
Scoil an Leighis agus
Eolaíocht An Leighis UCD
13