how to make an exposure chart for how to make an exposure
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BerlinBAM
How to Make an Exposure Chart forHow to Make an Exposure Chart for Computed Radiography?
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Uwe Ewert, Uwe Zscherpel, Mirko Jechow, Bernhard Redmer
April 2012,Ewert et al. Essential Parameters in Radiology
1
Outline Basics Basics
Comparison of optical density and grey value in Computed Radiography Determination of required minimum Grey Value GVmin for CR via Contrast min
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 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
Conclusions
2
Exposure Chart for Films
©2007 General Electric Company. GEIT-30158 (01/07) GE Inspection Technologies
April 2012,Ewert et al. Exposure Chart for CR
GEIT 30158 (01/07) GE Inspection TechnologiesIndustrial Radiography Image forming techniques
Exposure Chart for Films
mA▪m
inThe final graph shows the required mA▪minutes for exposure at a given tube kVexposure at a given tube kV under standard conditions for a given film system
Standard conditions are here: Material: steel Film Focus Distance: 700 mm Optical density: 2.0 Given film and exposure conditions
mm Fe
Given film and exposure conditions
Radiographers are trained, how to calculate the exposure values under
©2007 General Electric Company. GEIT 30158 (01/07) GE Inspection Technologies
calculate the exposure values under modified conditions.
April 2012,Ewert et al. Exposure Chart for CR
GEIT-30158 (01/07) GE Inspection TechnologiesIndustrial Radiography Image forming techniques
CR Response to Dose vs. Grey Values Operators want to define a minimum Grey Value as equivalent to the optical 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 qualitywithout relation to the image quality The grey values depend on:
Bit depth Scanner gain Photo multiplier voltage Imaging plate efficiency Imaging plate efficiency Scanning LASER intensity Exposure conditions (kV, mA▪min, distance, shielding, filters).p ( g )
The image quality can be measured by IQIs, as e.g. wires, step holes, plate holes or by, g , p , p y Measurement of essential parameters as e.g. the normalized SNR.
The EPS and the normalized SNR can be converted to calibrated minimum grey
April 2012,Ewert et al. Exposure Chart for CR
g yvalues GVmin which are required for CR exposure charts.
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- Other IQIs are on the plate to increase the information on image response.
Th i f d The exposure is performed with different mA▪min settings
A graph is generated, see next pagespages
E 746
EPS ( S )
April 2012,Ewert et al. Exposure Chart for CR 6Essential Parameters in Radiology
EPS= equivalent penetrameter sensitivity (see ASTM E 746, E 747, E 1025)
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
CNR 0.81 mm steel step for
t fmeasurement of µeff
April 2012,Ewert et al. 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 about 12 times with doubling dose values (mA▪min)in ASTM E 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 GVPlot 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 Drafttable 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 GV for the required EPS value of
April 2012,Ewert et al. Exposure Chart for CR 9
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).
Determination of Minimum Grey Value GVmin: SNR Method
The required minimum ´Grey Value’ Gvmin can also be determined by SNR measurements with available software.
Thi i b d th f ll i ti This is based on the following equation:
Depends on Hardware: effective pixel sizeNew Formula
p pMagnificationFocal spot size, source size
SNR Signal to noise ratio
SNRSRPTdeff
imageb
visible
SNR - Signal to noise ratio µeff – specific contrast,
effective attenuation coefficientSR ff ti i l i i th iSNeff
{}Material, Exposure time
SRb – effective pixel size in the image,basic spatial resolution of image
{}keV, Source typeScattered radiationScreens and filters
pTube current, ActivityDetector efficiencySource-to-Detector Distance
April 2012,Ewert et al. Exposure Chart for CR
Determination of Minimum Grey Value GVmin: SNR Method (ISO/DIS 17636-2)
Annex C: Determination of basic spatial resolution SRbAnnex D: Determination of normalized SNR from SNRAnnex D: Determination of normalized SNRN from SNRmeasured
April 2012,Ewert et al. Exposure Chart for CR 11Essential Parameters in Radiology
Step Wedge Method: Linear and logarithmic step wedgeStep Wedge Method: g p g8 mm Cu filter
April 2012,Ewert et al. Exposure Chart for CR
How to determine the minimum gray values GV :How to determine the minimum gray values GVmin:(E 2033, Annex 1)
Use step wedge method of ASTM E 2446 or ISO 16371-1 as follows:Use s ep edge e od o S 6 o SO 63 as o o s- Expose the step wedge- Measure in each step the gray value and SNR valuep g y- Plot a graph and determine the minimum gray value for the required
normalized SNR, corresponding to the required ISO testing class or ASTM levellevel.
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.
Scanner parameters as gain scan speed laser
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)
gain, scan speed, laser intensity, scan pixel resolution and others shall not be modified forshall not be modified for qualification and usage in the field.
GV
Measure the SNR and mean grey value in all stepsGV
Grey value
SNRnorm = SNR * 88,6µm/SRbGrey value
Grey value
April 2012,Ewert et al. Exposure Chart for CR
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 i diff t GV l f diff t IP t d tti Determine different GVmin values for different IP types and scanner settings Determine different GVmin values for testing class A and testing class B of
ISO/DIS 17636-2 or for ASTM E 2033 (Draft) level I and level II SO/ S 636 S 033 ( ) Define a reference value GVreference for the exposure chart. Plot the required mA▪minutes as function of the material thickness, which q
is required to achieve the GVreference under standard conditions. The operator may define the required GVmin value depending on the testing
requirementsrequirements.
April 2012,Ewert et al. Exposure Chart for CR
Exposure Chart for Computed-Radiography100
• FDDref = 1000 mm• GVref = 26000• SNRN = 100
10
x m
in)
SNRN 100• Material = Fe• IP = ST VI• DÜRR CR35 NDT
BAM tifi dmin
)
1
B (m
A x
600 kV
• BAM-certified scan-parameter
• No lead screens
B (m
A▪m
0 1
450 kV
300 kV 2
2min
refrefw FDDGV
FDDGVBB
Exposure chart based on a qualified minimum grey value GV
0,130 40 50 60 70 80 90 100
w Fe (mm)w Fe (mm)GVmin_B = 26000GVmin_A = 10000
Exposure chart, based on a qualified minimum grey value GVreferenz
- Measurement of the exposure value (mA min) for different tube voltages, which are required to obtain the qualified GVreferenz .
April 2012,Ewert et al. Exposure Chart for CR
- Half logarithmic presentation with system parameters.
Conclusions
Exposure charts for CR can be designed in analogy to film radiography on the basis of grey values in the digital imageson 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
l d t d di d diti b i l l ti f di it lvalues under standardized conditions by visual evaluation of digital 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 d III and 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
different for class A and B or level I and II.
April 2012,Ewert et al. Essential Parameters in Radiology 18Essential Parameters in Radiology