OPTIMIZATION
Patient Dose in Digital
Radiography: Using
Exposure Indices to
Optimize Techniques.
What is meant by “optimum”
optimum”? Possibities:
Possibities:
Least patient ESE for specified image quality (IQ)
Least patient effective dose for specified IQ
Best IQ for specified patient dose
Etc
“Optimum”
Optimum” may vary according to preferences
Acceptance limits develop around ideal level
CR/DR Radiographic Techniques
Before CR/DR techniques can be effectively
optimized, they must first to stabilized. Issues:
Very wide latitude (dynamic range): 103 to 104 :1
obscures concept of “correct”
correct” exposure:
– Exposure level and displayed “density”
density” unrelated :
All images processed for optimal “image density”
density”
Lack of visual feedback about exposure factors:
– Technique factors do not “stabilize”
stabilize”, often inappropriate
– AEC systems may be calibrated in appropriately
Patient exposure and image quality vary widely
Exposure Indices (EI)
Examples and data used here are mostly from Fuji
and Konica computed radiography (CR). EIs are
expressed per manuf.
manuf. standards. In both cases:
– EI inversely related to plate exposure
– Calibrated at 80 kVp with no added filtration
– Yields SS-number of 200 with 1 mR at plate
Fuji: SS-Number
S = 200/Emedian (mR)
mR)
– Based an examexam-specific histogram analysis
Konica: SS-Value
S = 200/Emean (mR)
mR)
– Based an examexam-specific anatomic region selection
Illustrating the Issue
AP LL-Spine Exam
Fuji CR
Phototimed (AEC)
Excellent positioning
Excellent Collimation
S-Number: 68
Fuji range (from user’
user’s
manual): 100 - 400
How well are Existing EIs working ?
Clinical Ranges of Exposure Indices
Manual Techniques:
Average
Exp Index
Coeff of
Variation
Target
Range
185
144
152
213
139
37%
48%
78%
73%
51%
Not provided
Not provided
Not provided
Not provided
Not provided
Site 1:
AP Foot
(Hosp ED) AP Femur
(Konica CR) Lat Forearm
Port Grid Abd
Port Chest
(high frustration level)
How well are Existing EIs working ?
Clinical Ranges of Exposure Indices
Phototimed (AEC): *
Average
Coeff of
Exp Index Variation
CR versus Film: Portable AP chest
Target
Range
Site 1:
PA Chest
Hosp ED
AP Abdomen
(Konica CR) AP L-Spine
340
211
268
16%
40%
45%
Not provided
Not provided
Not provided
Site 2:
Office
(Fuji CR)
515
179
168
41%
60%
45%
200-600
100-400
100-400
*-Mostly
PA Chest
AP Abdomen
AP L-Spine
Perspective: Variability of CR/Film
CR:
Film/Screen 1:
Film/Screen 2:
Std Dev
of film OD *
Exposure
C.V.**
—
0.53 OD
0.41 OD
51%
43%
32%
*-Of Selected lung field region
**-Based on typical sensitometry for general purpose film/scre
(3.4 gamma between 1.5 and 2.5 OD, 1.8-2.0 Target OD)
Contributing Factors:
Technical Factors:
– Calibration levels of readers (all images)
– Calibration methods vs clinical use
– Calibration of AEC (AEC images)
Exposure Index Factors:
– Lack of understanding of existing EIs (TG(TG-116)
– Lack of appropriate visual feedback
– VendorVendor-specific selection of relevant image region
(volume of interest, VOI) for EI calculation
– Exam specific target ranges (if any provided)
Reader
Calibration
(matching)
Indicated plate
exposure 101020% lower than
indicated by
exposure index
Reader Calibration and Variability:
Reader
1
2
3
4
5
6
7
8
9
10
Measured S-Number*
No filter
TG116 Filter
160
105
175
115
183
121
175
118
200
126
200
124
162
108
167
113
163
114
175
120
Average:
CV:
176
8%
116.4
6%
* 80 kVp, 72" SID
Action Step 1: Calibrate CR/DR Units
For now, service engineers will need to
adhere to manufacture’
manufacture’s calibration protocol
(TG(TG-116 and IEC will eventually standardize)
Assure EI matches manufactures’
manufactures’ indicated
value for reference exposure level (eg
(eg,, 1 mR)
mR)
For CR, should try to use same generator for
all readers (especially reader of same model)
Verify proper dosimeter calibration if using
filtered beam
Unit
1
2
3
4
5
6
7
8
9
10
11
12
Type S-Num
HF
470
3P
365
HF
167
HF
139
HF
318
HF
270
1P
499
1P
439
1P
504
1P
521
1P
352
1P
552
Average 383
C.V.:
36%
Est mR
at plate
0.45
0.58
1.16
1.53
0.61
0.75
0.50
0.57
0.50
0.48
0.72
0.46
0.69
47%
AEC
CALIBRATION
Table Bucky
8” acrylic (25x25 cm)
80 kV
Fuji Carbon readers
(Similar variations
for upright receptor)
Action Step 2: Calibrate AEC Units
AEC Tests: Geometry and Phantoms
Use clinically relevant phantoms (eg
(eg,, acrylic)
Should, as closely as possible, achieve target
EI for standardized conditions (eg
(eg,, 8”
8” acrylic,
center AEC detector, 80 kVp,
kVp, etc)
L-Spine/Abdomen Phantom
AEC CALIBRATIONCALIBRATION-CAVEATS
Achieving a particular EI for an acrylic AEC phantom
DOES NOT guarantee the corresponding exposure
based on the EI calibration process was received by
the detector (for example, 1 mR for a 200 SS-number)
will need to determine receptor exposure directly
RECEPTOR EXPOSURE CAVEATS
May need “cassette frame”
frame” to circumvent PBL
Dosimeter usually will not fit on top of cassette
Also need to verify dosimeter calibration
Measuring Receptor Exposures
AEC CAVEATS – Part 2
Specifying an AEC receptor dose based on a
speed class (e.g., 1 mR for a 200 speed class)
does NOT necessarily imply that EI target
ranges should be centered on that speed class
value (e.g., may not be centered on 200)
Cu/Al calibration filters do not equal a patient
Depends on anatomic segmentation algorithm
(one selecting lung fields on a PA chest will
yield different EIs than one using full anatomy)
VOI Selection
Fuji: Full anatomic
area (exam(exam-based
histogram anal to
unexposed areas
and raw radiation
Konica:
recognition of
relevant anatomy
(exam specific)
VOI Selection
Fuji/Agfa: Based
on histogram
analysis of full
anatomic area
Konica:
recognition of
relevant anatomy
(exam specific)
VOI Selection
Fuji: Full anatomic
area (exam(exam-based
histogram anal to
unexposed areas
and raw radiation
Konica:
recognition of
relevant anatomy
(exam specific)
Anatomic Selection: AEC Sensor Areas
Suggested VOI
Selection Option
Allow user to select
VOI corresponding to
clinically used AEC
fields
Example:
Example: PA Chest
Use of VOI Corresponding to AEC Fields
Based on analysis of 54 (phototimed
(phototimed)) PA chest exams.
EI (Konica SS-value) based manually set VOI area
(approximately) corresponding to left/right AEC fields
Konica
Default ROI
Average EI:
Std Dev:
C.V.:
340
53
16%
VOI corresponding
to L/R AEC cells
222
22
10%
In addition to Final Image... show exposure based “view”
view”
In addition to Final Image... show exposure based “view”
view”
SUMMARY
EIs seem to vary more widely than did film O.D
“Good”
Good” exams often fall outside Manuf criteria
Variations not just technique selectionselection-related
EIs for CR miscalibrated -Physics action item
AECs miscalibrated for CR -Physics action item
VOI selection process perhaps largest source of
clinical EI ranges
AECAEC-field based VOIs or direct visual feedback
may allow for better technique stabilization
Overexposed (by
(by 50%)
50%)
UnderOver--exposed
Under