Transcript PowerPoint-presentation

Characterisation and Application of
Photon Counting X-ray Detector Systems
Disputation seminar
2015-07-07
Disposition
• Introduction
– Motivation for research and development of X-ray imaging
• Short description of the Medipix project
• Applications
– Dose reduction in medical imaging
– Material recognition
• Characterisation of the Medipix system
– Charge sharing
• Conclusions
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Section 1: Introduction
• Basics on X-ray detectors
– X-ray detectors are available on the market, why do any research?
– What is photon counting?
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X-rays
• Discovered in 1895 by
W. K. Röntgen
• Generated by radioactive decay
• Medical images for surgery
• Cancer therapy
– High doses
X-ray image from Siemens
• Today the entire population is affected by X-ray imaging
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Negative effects of radiation
• Ionizing radiation induces cancer
• No lower limit found
• Reduction of the X-ray dose
– Reduction of the cancer frequency
– Reduction of the costs for society
• For the individual
– The risk is small compared to other cancer inducing factors
– Attend X-ray examinations recommended by the medical expertise
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Example: Mammography
• Examination on regular basis for all females
• New tumours are small and easy to treat
– Argument for short interval between examinations
• Each examination increases the lifetime dose
and the statistical risk for cancer development
– Argument for long interval between examinations
Mammography device from
Sectra AB
• A compromise between risk and benefit has to be made
– With improved detectors the dose at each examination can be
reduced
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Detector improvement
• With improved detectors the dose at each examination
can be reduced
• The examination interval can be decreased with remained
lifetime dose
• More cancer tumours will be discovered at an early stage
• More cancers will be successfully treated
Lives will be saved!
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Readout principles
• Photons generates a charge cloud in the semiconductor
• Charge integrating
– Intensity equals a sum of charge
• Photon counting
– The intensity equals the number of photons
– The lowest energies must be discriminated, otherwise thermal
noise is counted as photons
– The energy or ”colour” of each photon can be measured
• Photon counting makes colour X-ray imaging possible
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Illustration of photon counting
Commercials of MicroDose from
Mamea imaging AB and Spectra Imtec AB
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Section 2: The Medipix project
• A pixellated photon counting
readout chip
– One readout circuit per pixel
• Requires deep submicron
CMOS processes
• Detector matrix bump bonded
to the readout chip
• Detectors of silicon, CdTe and
GaAs
Illustration from http://medipix.web.cern.ch/MEDIPIX/
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Collaboration
• The project is directed from the
Cern microelectronics group
• 16 European institutes are
participating
Institut de Física d'Altes Energies IFAE Barcelona
University of Cagliari
Commissariat à l'Energie
Atomique CEA
European Organization for Nuclear Research CERN
Czech Academy of Sciences
Czech Technical University in Prague (CTU)
Friedrich-Alexander- Universität Erlangen-Nürnberg (FAU)
European Synchrotron Radiation Facility ESRF
Albert-Ludwigs- Universität Freiburg-i.B.
University of Glasgow
Medical Research Council MRC Mittuniversitetet
Mid-Sweden University (Mitthögskolan) MSU
Università di Napoli Federico II
National Institute for Nuclear and High-Energy Physics NIKHEF
Università di Pisa
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Map with collaborators logotypes
Medipix 1
• 1 µm SACMOS technology
–
–
–
–
–
170 µm square pixels
64x64 pixels
15 bit counters
Low energy threshold
3 bits individual threshold
adjustment
• Operated by standard PC
connected to an interface circuit
Medipix1 system
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Medipix 2
• Smallest pixel size for now
– 55 µm square pixels
– 256x256 pixels (1,4x1,6 cm)
• Dead area minimized on three
sides
– Chipboards with 2x4 chips
exists
• Operated by a standard PC
Medipix2 mounted for dental imaging
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Medipix 2
• 0.25 µm CMOS technology
– 13 bit counters
– Upper and lower threshold
• Each with 3 bits threshold
adjustment
• Individual leakage current
compensation (GaAs)
• Positive and negative charge
signal (CdTe)
Description of the Medipix2 readout circuit for each pixel
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Section 3: Applications
• Dose reduction in dental imaging
• Material recognition
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Inverval or full spectra
• The relative contrast can be improved by applying an
energy interval in dental imaging
Relative contrasts
0.70
0.59
26 - 30 keV
4 - 70 keV
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Tooth image for varying energy
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Colour image of the tooth
• Colour X-ray image
from RGB coding of
three images
8 - 10 keV
34 -38 keV
56 - 60 keV
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Material recognition
– Possible to distinguish between Si
and Al although the full spectrum
absorption
is equal
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Section 4: Characterisation
•
•
•
•
Description of charge sharing
Simulation of charge sharing
Measurements with narrow monochrome source
Slit measurements
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Charge sharing
Column profile
100
Intensity (%)
• Crosstalk between pixels
120
80
60
40
20
0
-200
-100
0
100
200
Relative distance from center of pixle
Sharing area
Slit
Colour X-ray image of a slit achieved with a Medipix2 Si-detector.
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Charge sharing
• Physical components of charge sharing
–
–
–
–
–
–
Beam geometry and scattering
Quantisation error
Absorption width
X-ray fluorescence
Charge drift
Back scattering
Colour X-ray image of a slit achieved with a Medipix2 Si-detector.
• High energy photons can be divided into several low
energy counts (Red colour in image)
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Charge drift
• Silicon
• CdTe
– about 3 % absorption in a
300 µm detector (40 keV)
– almost 100 % point absorption
– Strong X-ray flourescence
X-rays
X-rays
~3 % uniform absorption
Si
Flourescence X-rays
43 %
CdTe
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100 % point absorption
Flourescence
• Flourescence is a
problem for CdTe
detectors
– Low energies has to
be discriminated, to
achieve reasonable
spatial resolution
Colour X-ray image of a slit achieved with a Medipix2 CdTe-detector.
Colour X-ray image of a slit achieved with a Medipix2 Si-detector.
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Simulation of charge sharing
• Charge sharing highly distorts
the measured spectrum (Si)
Medipix2 25 V bias
Medipix2 100 V bias
Sim. (50 x 50 micron2)
Spectrum of X-ray source
0.025
Relative photon flux [a.u.]
– Overdepletion supresses
charge sharing slightly
0.03
0.02
0.015
0.01
0.005
0
0
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20
40
Energy [keV]
60
80
ESRF measurements
• Narrow beam 10x10 µm
• Monochrome energy
40 keV
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The European Synchrotron Radiation Facility
CdTe point spread function
• The 10 µm wide beam is
centered on a pixel
• For low energies signal is
measured 165 µm away
Point spread function for variyng low threshold setting
10000
1000
– Flourescence
36 keV
Counts
30 keV
100
26 keV
22 keV
16 keV
10
1
-200
-150
-100
-50
0
50
100
Distance from centre of beam (µm)
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150
200
CdTe spectrum
• Spectrum from the pixel
where the 10 µm wide
beam is centered
40 keV synchrotron spectrum
35000
30000
– Threshold window 2 keV
– Some photons deposits a
fraction of their energy
outside the pixel
Counts
• Low energy tail
25000
20000
Illuminated pixel
Neigbour pixel
15000
10000
5000
0
15
20
25
30
35
40
45
Center of window energy (keV)
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50
CdTe neighbour pixels spectra
• Neighbour pixels
6000
– Charge sharing behaviour
5000
• Far neighbour
– Tenfold exposure time
– Distrurbances at 24 keV
and 28 keV
4000
left 110 µm
right 110 µm
below 110 µm
3000
above 110 µm
right 275 µm
2000
1000
0
15
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20
25
30
35
40
45
50
Silicon spectrum
• Cumulative spectrum on a
300 µm thick detector
300 µm Si 40 keV synchrotron spectrum
2500
2000
D epth prof ile
1
Absorption (a.u.)
1
Absorption (a.u.)
1500
Surf ace prof ile
0.8
0.6
0.4
0.2
0
60
120 180 240
Depth (µm)
300
counts
300 µm Si
0.8
0.6
0.4
Cumulative function
1000
Spectrum
500
0.2
0
10
20
30
40
Distance (µm)
50
0
5
10
15
20
25
30
35
40
-500
Lower threshold energy (keV)
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45
300 micron, Si, 40keV, 170 e- noise, 10 micron std in absorption
profile
Simulation versus measurements
300 µm Si 40 keV cumulative functions for different
distance from the center of the pixel
3000
2500
0 µm cum
counts
2000
5 µm cum
10 µm cum
1500
15 µm cum
20 µm cum
1000
25 µm cum
500
0
5
10
15
20
25
30
Energy (keV)
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35
40
45
700 µm thick silicon detector
• Alignment becomes important
40 keV prof ile in 700 µm Si
D epth prof ile
1
0.8
0.6
0.4
0.42
0.36
1
Absorption (a.u.)
0.8
0.6
0.4
0.2
20
40 60
80
Distance (µm)
Pix el size 55 µm
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0 100 200 300 400 500 600 700
Depth (µm)
Surf ace prof ile
Surf ace prof ile
Absorption (a.u.)
0.48
0.3
1
700 µm
0.54
0 100 200 300 400 500 600 700
Depth (µm)
0
D epth prof ile
0 µm0.6
Absorption (a.u.)
0 µm
Absorption (a.u.)
40 keV prof ile in 700 µm Si
0.8
0.6
0.4
0.2
0
100
700 µm
Pix el size 55 µm
20
40 60
80
Distance (µm)
100
Conclusions
• Photon counting X-ray systems can lead to significant
dose reduction (paper IV)
– With the next version of Medipix the technology is probably mature
enough to be transfered to product developement
– Colour imaging can be used to discern different materials in an
object (paper III)
– Energy dependence in image correction methods needs to be
considered (paper II)
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Conclusions
• Charge sharing degenerates the spectral information
– Charge sharing corrections can be implemented into the readout
electronics
– The 3D detector structure supresses charge sharing (paper I)
• CdTe and GaAs detectors are less mature than Silicon
– Flourescence becomes a problem
– For 1 mm thickness the charge cloud is in the same size as the
55 µm pixel
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Acknowledgements
• Thanks to:
– My supervisors doc. Christer Fröjdh and prof. Hans-Erik Nilsson
– My colleagues at the electronics design department
– My colleagues in the Medipix collaboration
• The Mid-Sweden University, the KK-foundation and the
European Commission are greately acknowledged for
their financial support
• Thanks to my family Monica, Johan and William
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