principles of industrial digital radiography2011.radioactivewastemanagement.org/download/10a -...
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Principles of industrial Principles of industrial digital radiographydigital radiography
XDRS handsXDRS hands--on sessionon session
JRCJRC--Ispra, July 6, 2011Ispra, July 6, 2011
Operational Issues in Radioactive Waste Operational Issues in Radioactive Waste Management and Nuclear DecommissioningManagement and Nuclear Decommissioning
3rd3rd International Summer SchoolInternational Summer School
JRC Ispra6 July 2011
1. X1. X--RAY BASICSRAY BASICSa.a. XX--ray physics & equipmentray physics & equipmentb.b. Image formation (contrast & signal)Image formation (contrast & signal)c.c. Imaging geometryImaging geometry
2. DIGITAL RADIOGRAPHY2. DIGITAL RADIOGRAPHYa.a. Digital imagesDigital imagesb.b. XDRS digital imagesXDRS digital imagesc.c. CalibrationCalibrationd.d. Scatter reductionScatter reduction
3. PRINCIPLES OF TOMOGRAPHIC RECONSTRUCTION3. PRINCIPLES OF TOMOGRAPHIC RECONSTRUCTIONa.a. Analytical methods (iterative methods not treated)Analytical methods (iterative methods not treated)
SummarySummary
JRC Ispra6 July 2011
XX--ray nature and generationray nature and generation
XX--rays are photons (electromagnetic radiation)rays are photons (electromagnetic radiation)They are generated by:They are generated by:•• Fluorescence (characteristics XFluorescence (characteristics X--rays)rays)•• Braking radiation (Braking radiation (BremsstrahlungBremsstrahlung))
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++++
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-
-
-
-E
JRC Ispra6 July 2011
XX--ray nature and generationray nature and generation
XX--rays tubes are used to generate Xrays tubes are used to generate X--rays in a controlled rays in a controlled manner. The user can select:manner. The user can select:•• The voltageThe voltage•• The currentThe current•• FilteringFiltering
Voltage
Cu
rren
t
XDRS max parameters:XDRS max parameters:•• 450 kV450 kV•• 20 20 mAmA
http://labspace.open.ac.uk/mod/resource/view.php?id=361885
JRC Ispra6 July 2011
XX--ray attenuation (I)ray attenuation (I)XX--rays are attenuated in any material depending on:rays are attenuated in any material depending on:•• The The energyenergy of the X raysof the X rays•• TheThe
oo thicknessthicknessoo density density oo compositioncomposition
of the of the materialmaterial
( )0
7.10
cm10 cm
g1 g
cm17.0
0tE
0 I 18.0eIeIeII3
2
==== −−
ρμ−
t
1E-02
1E-01
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1 10 100 1000
keV
Atte
nuat
ion
coef
ficie
nt (1
/cm
)
Water
Iron
Cement
I0
ρ
μ(E)ρ
JRC Ispra6 July 2011
XX--ray detectorsray detectors
XDRS is based on an axial line of 384 CdWOXDRS is based on an axial line of 384 CdWO44 crystalscrystalsCrystals emit light when hit by XCrystals emit light when hit by X--raysraysCdWOCdWO44 dimensions are 9 x 3 x 6 mm (width, height, depth)dimensions are 9 x 3 x 6 mm (width, height, depth)Each crystal is associated to (read out by) a Si photodiodeEach crystal is associated to (read out by) a Si photodiodePhotodiode converts crystal light into an electric signalPhotodiode converts crystal light into an electric signal
38
4 d
etec
tors
6 mm
9 mm
3 mm
JRC Ispra6 July 2011
XX--ray image formationray image formation
Detectors count more or fewer XDetectors count more or fewer X--rays depending on the rays depending on the attenuation met on the path from tube to detectorattenuation met on the path from tube to detectorCounts are associated to a color and displayedCounts are associated to a color and displayed
Detector array
1E-02
1E-01
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1 10 100 1000
keV
Atte
nuat
ion
coef
ficie
nt (1
/cm
)
Water
Iron
Cement
JRC Ispra6 July 2011
Imaging geometry: Imaging geometry: planar radiographyplanar radiography
The drum is scanned sideways to obtain a planar radiographyThe drum is scanned sideways to obtain a planar radiographyXDRS images have 384 x 200 pixels (200 is adjustable).XDRS images have 384 x 200 pixels (200 is adjustable).
Det
ecto
r ar
ray
Det
ecto
r ar
ray
Det
ecto
r ar
ray
JRC Ispra6 July 2011
1E-02
1E-01
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1 10 100 1000
keV
Atte
nuat
ion
coef
ficie
nt (1
/cm
)
Water
Iron
Cement
XX--ray settings: voltageray settings: voltage
Iron 0.7310
Transmission
6 10-4
240.1410Cement350.1110Water
@ 400 keV
2 10-132.910Iron40.3210Cement180.1710Water
@ 100 keV%cm-1cm
μρThicknessMaterial
400
( ) tE
0
eII ρμ−=
Changing the voltage:Changing the voltage:•• Changes penetration (higher voltage, more penetration)Changes penetration (higher voltage, more penetration)•• Changes contrast between materialsChanges contrast between materials
JRC Ispra6 July 2011
Cu
rren
tXX--ray settings: currentray settings: current
Increasing the current of the XIncreasing the current of the X--ray tube increases the number of ray tube increases the number of photons collected and thus decreases image noise.photons collected and thus decreases image noise.
Currenthttp://www.mghradrounds.org/index.php?src=gendocs&link=2008_february
JRC Ispra6 July 2011
XX--ray imaging: resolutionray imaging: resolution
To improve To improve transaxialtransaxial resolution, the detectors can be resolution, the detectors can be collimated with lead platescollimated with lead platesImproved resolution comes at a cost in noise because fewer Improved resolution comes at a cost in noise because fewer photons are collected (e.g. increase current to compensate)photons are collected (e.g. increase current to compensate)
JRC Ispra6 July 2011
XX--ray imaging: scatter reductionray imaging: scatter reduction
Scattered photons contribute an unwanted background to Scattered photons contribute an unwanted background to the image and cause a loss of contrastthe image and cause a loss of contrastScatter can be reduced by collimating Scatter can be reduced by collimating the beam, the detectors or boththe beam, the detectors or both
JRC Ispra6 July 2011
XX--ray imaging: calibrationray imaging: calibration
Not all detectors have the same response:Not all detectors have the same response:•• Intrinsically different efficiencyIntrinsically different efficiency•• Different geometric efficiencyDifferent geometric efficiency
For this reason detectors are calibrated for:For this reason detectors are calibrated for:•• Dark current: Dark current: ddii
•• Efficiency: Efficiency: eeii
i
iii e
d - data Rawtmeasuremen Corrected =
Detector array2 4 3 3 2 1 2 4
85 92 98 99 97 97 91 84
di
ei
JRC Ispra6 July 2011
Imaging geometry: Imaging geometry: tomographytomography
Data for Data for tomographictomographic reconstruction are obtained by reconstruction are obtained by rotatingrotatingthe object and repeating the lateral the object and repeating the lateral scanscan..Rotation: 360Rotation: 360°° with 10 arc seconds repeatabilitywith 10 arc seconds repeatabilityLateral scan: 762 mm span with 10Lateral scan: 762 mm span with 10--um repeatabilityum repeatability
Det
ecto
r ar
ray
JRC Ispra6 July 2011
Tomography:Tomography:image reconstruction (I)image reconstruction (I)
Projection data are Projection data are backprojectedbackprojected
4 views
8 views
projection 0°
proj
ectio
n 90°
180 views
2 views
projection 0°
proj
ectio
n 90°
All images courtesy of S. Matej, UPenn
JRC Ispra6 July 2011
( )[ ] =•ℑℑ− 1
Ramp filter
Tomography:Tomography:image reconstruction (II)image reconstruction (II)
Projection data are filtered (multiplication Projection data are filtered (multiplication with ramp filter and inversewith ramp filter and inverse--Fourier Fourier transformed) and then transformed) and then backprojectedbackprojected
4 views
8 views
projection 0°
proj
ectio
n 90°
180 views
Filtered projection 0°
Filte
red
proj
ectio
n 90°2 views
All images courtesy of S. Matej, UPenn
JRC Ispra6 July 2011
Ramp filter
Tomography:Tomography:image reconstruction (III)image reconstruction (III)
A ramp filter works well in absence or with low noiseA ramp filter works well in absence or with low noiseIn real data noise is always presentIn real data noise is always present
•• A cutA cut--off frequency is introduced to reduce noiseoff frequency is introduced to reduce noise
A sharp cutA sharp cut--off causes ringing artifactsoff causes ringing artifactsApodizationApodization functions are used to reduce ringingfunctions are used to reduce ringing
•• HammingHamming•• HannHann•• CosineCosine•• …… and many, many othersand many, many others
Ramp filter with cut-off
Ramp filter apodized
JRC Ispra6 July 2011
Summary: 3 takeSummary: 3 take--away ideasaway ideas
XX--ray imaging essentially measures electron densityray imaging essentially measures electron density•• It does not measure emitted radiation nor identify isotopesIt does not measure emitted radiation nor identify isotopes
Current changes image noiseCurrent changes image noiseVoltage changes contrast and noise:Voltage changes contrast and noise:•• High voltage: good penetration, poor contrast (material High voltage: good penetration, poor contrast (material
discrimination) in light materials discrimination) in light materials •• Low voltage: poor penetration, better contrast in light materialLow voltage: poor penetration, better contrast in light materials s