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How to Make an Exposure Chart for
Computed Radiography?
Uwe Ewert, Uwe Zscherpel,
Mirko Jechow, Bernhard Redmer
t@b
d
[email protected]
April 2012,
Ewert et al.
Essential Parameters in Radiology
1
Outline
 Basics
 Comparison of optical density and grey value in Computed Radiography
 Determination of required minimum Grey Value GVmin for CR via Contrast
Sensitivity (EPS)
 Determination of required minimum Grey Value GVmin for CR via Signal-to
Noise Ratio (SNR)
(
)
 Measurement procedure for exposure diagram
 Test exposures of IPs by new Draft of ASTM E 2033 and ISO/DIS 17636-2
 EPS method
 SNR method
 Design of exposure diagram on basis of grey values
 Usage of exposure diagram
 Conclusions
April 2012,
Ewert et al.
Exposure Chart for CR
2
Exposure Chart for Films
©2007 General Electric Company.
GEIT-30158
GEIT
30158 (01/07) GE Inspection Technologies
Industrial Radiography Image forming techniques
April 2012,
Ewert et al.
Exposure Chart for CR
The final graph shows the
required mA▪minutes for
exposure at a given tube kV
under standard conditions for a
given film system
mA▪min
Exposure Chart for Films
Standard conditions are here:
 Material: steel
 Film Focus Distance: 700 mm
 Optical density: 2.0
 Given film and exposure conditions
Radiographers are trained, how to
calculate the exposure values under
modified conditions.
©2007 General Electric Company.
GEIT 30158 (01/07) GE Inspection Technologies
GEIT-30158
Industrial Radiography Image forming techniques
April 2012,
Ewert et al.
Exposure Chart for CR
mm Fe
CR Response to Dose vs. Grey Values
 Operators want to define a minimum Grey Value as equivalent to the optical
density in Film Radiography!
 But, the optical density and grey values of digital images cannot be compared
without relation to the image quality
 The grey values depend on:
 Bit depth
 Scanner gain
 Photo multiplier voltage
 Imaging plate efficiency
 Scanning LASER intensity
 Exposure
p
conditions ((kV, mA▪min, distance, shielding,
g filters).
)
 The image quality can be measured by
 IQIs,, as e.g.
g wires,, step
p holes,, plate
p
holes or byy
 Measurement of essential parameters as e.g. the normalized SNR.
grey
y
 The EPS and the normalized SNR can be converted to calibrated minimum g
values GVmin which are required for CR exposure charts.
April 2012,
Ewert et al.
Exposure Chart for CR
Determination of the minimum Grey Value GVmin by the
EPS method
See ASTM E 2033 draft and the Draft on “Practice for the Use of … Computed
Radiology for Aerospace Casting Inspections” (USA: MAI – group)
- A smooth ¾ inch (19 mm) steel plate with a set of plate holes is radiographed at 200
kV in ≥ 1 m distance (ASTM E 746)
- Other IQIs are on the plate
to increase the information
on image response.
 Th
The exposure iis performed
f
d
with different mA▪min settings
 A graph is generated, see next
pages
E 746
S
EPS= equivalent penetrameter sensitivity ((see ASTM
E 746, E 747, E 1025))
April 2012,
Ewert et al.
Essential
Exposure
Parameters
Chart in
forRadiology
CR
6
Example: EPS test with DÜRR HD CR scanner at
20 µm pixel size
Do you see the holes?
¾” (19 mm) steel plate, 200 kV
1 mm steel step for
measurementt off µeff
April 2012,
Ewert et al.
CNR  0.8
Exposure Chart for CR
7
EPS Method as Proposed by ASTM E 2033 Draft (Annex I)
Use EPS test plates (E 746) as follows:
-
Expose the EPS plate set on ¾” (19 mm) of steel at conditions, as defined
in ASTM E 746
746, about 12 times with doubling dose values (mA▪min)
-
Measure for each exposure the GV and EPS value
-
Plot a graph of EPS vs. Grey Value GV
Qualification procedure!
April 2012,
Ewert et al.
Exposure Chart for CR
Determination of Minimum Grey Value GVmin:
EPS test
EPS measurement
conditions:
see ASTM E 746
ASTM 2033 Draft
table A.2
Grey value
EPS= equivalent penetrameter sensitivity (see ASTM E 746, E 747, E 1025)
 Plot a graph with measured EPS as function of grey value and
 determine from the graph the minimum required grey value GVmin for the required EPS value of
the table A.2 of ASTM E 2033 (draft).
April 2012,
Ewert et al.
Exposure Chart for CR
9
Determination of Minimum Grey Value GVmin:
SNR Method
 The required minimum ´Grey Value’ Gvmin can also be determined by SNR
measurements with available software.
 This
Thi is
i based
b
d on the
th following
f ll i equation:
ti
Depends
p
on Hardware: effective p
pixel size
Magnification
Focal spot size, source size
New Formula
d visible
image
b
SR
 PT 
 eff  SN
SNR
} {
Material,
keV, Source type
Scattered radiation
Screens and filters
April 2012,
Ewert et al.
SNR - Signal to noise ratio
µeff – specific contrast,
effective attenuation coefficient
SRb – effective
ff ti pixel
i l size
i iin th
the iimage,
basic spatial resolution of image
p
time
Exposure
Tube current, Activity
Detector efficiency
Source-to-Detector Distance
Exposure Chart for CR
Determination of Minimum Grey Value GVmin:
SNR Method (ISO/DIS 17636-2)
Annex C: Determination of basic spatial resolution SRb
Annex D: Determination of normalized SNRN from SNRmeasured
April 2012,
Ewert et al.
Essential
Exposure
Parameters
Chart in
forRadiology
CR
11
Step Wedge Method:
April 2012,
Ewert et al.
Linear and logarithmic
g
step
p wedge
g
8 mm Cu filter
Exposure Chart for CR
How to determine the minimum gray values GVmin:
(E 2033, Annex 1)
Use sstep
ep wedge
edge method
e od o
of ASTM
S
E 2446
6o
or ISO
SO 16371-1
63
as follows:
o o s
-
Expose the step wedge
-
Measure in each step
p the g
gray
y value and SNR value
-
Plot a graph and determine the minimum gray value for the required
normalized SNR, corresponding to the required ISO testing class or ASTM
level.
level
Calibration procedure!
April 2012,
Ewert et al.
Exposure Chart for CR
Determination of Minimum Grey Value GVmin:
SNR Method
- The measurement is based on the fixed correlation of SNR and Grey
Values of a CR system (IP and scanner) with fixed scanner settings.
ISO/DIS 17636-2 requires for
Class A: SNRN ≥ 70 and for
Class B: SNRN ≥ 100
ASTM E 2033 (Draft) requires for
Level I: SNR ≥ 80 (@SRb = 70 µm)
Level II: SNR ≥ 56 (@SRb = 70 µm)
Scanner parameters as
gain scan speed
gain,
speed, laser
intensity, scan pixel
resolution and others
shall not be modified for
qualification and usage
in the field.
GV
GV
Measure the SNR and mean
grey value in all steps
Grey value
Grey value
April 2012,
Ewert et al.
Exposure Chart for CR
SNRnorm = SNR * 88,6µm/SR
µ
b
How to Plot an Exposure Graph on the basis of
minimum gray values GVmin:

Determine the minimum grey value GVmin for the specific SR system and
its parameter setting

D t
Determine
i diff
differentt GVmin values
l
ffor diff
differentt IP types
t
and
d scanner settings
tti

Determine different GVmin values for testing class A and testing class B of
ISO/DIS
SO/ S 17636-2
636 or for ASTM
S
E 2033
033 ((Draft)) level I and level II

Define a reference value GVreference for the exposure chart.

Plot the required
q
mA▪minutes as function of the material thickness, which
is required to achieve the GVreference under standard conditions.

The operator may define the required GVmin value depending on the testing
requirements.
requirements
April 2012,
Ewert et al.
Exposure Chart for CR
Exposure Chart for Computed-Radiography
B (mA xmin)
min)
B (mA▪m
100
•
•
•
•
•
•
•
10
•
1
FDDref = 1000 mm
GVref = 26000
SNRN = 100
Material = Fe
IP = ST VI
DÜRR CR35 NDT
BAM
BAM-certified
tifi d scanparameter
No lead screens
600 kV
450 kV
Bw 
300 kV
B  GVmin  FDD 2
2
GVref  FDDref
01
0,1
GVmin_B = 26000
w
Fe(mm)
(mm)
GVmin_A = 10000
w Fe
Exposure chart,
chart based on a qualified minimum grey value GVreferenz
30
40
50
60
70
80
90
100
- Measurement of the exposure value (mA min) for different tube voltages,
which are required to obtain the qualified GVreferenz .
- Half logarithmic presentation with system parameters.
April 2012,
Ewert et al.
Exposure Chart for CR
Conclusions

Exposure charts for CR can be designed in analogy to film radiography
on the basis of grey values in the digital images

It is required to determine for each Imaging Plate type and scanner and
its parameter setting a required minimum Grey Value GVmin.

Two equivalent methods were developed:

The EPS method is based on the visual determination of EPS
values
l
under
d standardized
t d di d conditions
diti
b
by visual
i
l evaluation
l ti off di
digital
it l
images and measured Grey Values

The SNR method is based on measured normalized SNR values
and measured Grey Values

Different GVmin values apply for testing ISO class A and B or ASTM level
I and
d II

The relative exposure times between CR and film radiography are
different for class A and B or level I and II.
April 2012,
Ewert et al.
Exposure Chart for CR
End
[email protected]
April 2012,
Ewert et al.
Essential Parameters in Radiology
18