1. Characterisation and Application of Photon Counting X-ray Detector Systems Disputation seminar

2. Disposition Introduction Short description of the Medipix project
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

3. Section 1: Introduction
Basics on X-ray detectors
X-ray detectors are available on the market, why do any research?
What is photon counting?

4. X-rays Discovered in 1895 by W. K. Röntgen
Generated by radioactive decay
Medical images for surgery
Cancer therapy
High doses
Today the entire population is affected by X-ray imaging
X-ray image from Siemens

5. 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

6. 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
A compromise between risk and benefit has to be made
With improved detectors the dose at each examination can be reduced
Mammography device from Sectra AB

7. Lives will be saved! 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!

8. 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

9. Illustration of photon counting
Commercials of MicroDose from Mamea imaging AB and Spectra Imtec AB

10. 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

11. 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
Mid-Sweden University (Mitthögskolan) MSU
Università di Napoli Federico II
National Institute for Nuclear and High-Energy Physics NIKHEF
Università di Pisa
Mittuniversitetet
Map with collaborators logotypes

12. 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

13. 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

14. 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

15. Section 3: Applications
Dose reduction in dental imaging
Material recognition

16. Inverval or full spectra
The relative contrast can be improved by applying an energy interval in dental imaging
Relative contrasts
0.70
0.59
keV
keV

17. Tooth image for varying energy

18. Colour image of the tooth
Colour X-ray image from RGB coding of three images
keV
keV
keV

20. Material recognition
Possible to distinguish between Si and Al although the full spectrum absorption is equal

21. Section 4: Characterisation
Description of charge sharing
Simulation of charge sharing
Measurements with narrow monochrome source
Slit measurements

22. Charge sharing Crosstalk between pixels Sharing area Slit
Colour X-ray image of a slit achieved with a Medipix2 Si-detector.

23. Charge sharing Physical components of charge sharing
Beam geometry and scattering
Quantisation error
Absorption width
X-ray fluorescence
Charge drift
Back scattering
High energy photons can be divided into several low energy counts (Red colour in image)
Colour X-ray image of a slit achieved with a Medipix2 Si-detector.

24. Charge drift Si CdTe Silicon CdTe X-rays X-rays
about 3 % absorption in a 300 µm detector (40 keV)
CdTe
almost 100 % point absorption
Strong X-ray flourescence
X-rays
~3 % uniform absorption Si
X-rays
100 % point absorption
CdTe
Flourescence X-rays 43 %

25. 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.

26. Simulation of charge sharing
Charge sharing highly distorts the measured spectrum (Si)
Overdepletion supresses charge sharing slightly

27. ESRF measurements The European Synchrotron Radiation Facility
Narrow beam 10x10 µm
Monochrome energy 40 keV

28. CdTe point spread function
The 10 µm wide beam is centered on a pixel
For low energies signal is measured 165 µm away
Flourescence

29. CdTe spectrum
Spectrum from the pixel where the 10 µm wide beam is centered
Threshold window 2 keV
Low energy tail
Some photons deposits a fraction of their energy outside the pixel

30. CdTe neighbour pixels spectra
Charge sharing behaviour
Far neighbour
Tenfold exposure time
Distrurbances at 24 keV and 28 keV

31. Silicon spectrum Cumulative spectrum on a 300 µm thick detector

32. Simulation versus measurements
300 micron, Si, 40keV, 170 e- noise, 10 micron std in absorption profile
Simulation versus measurements

33. 700 µm thick silicon detector
Alignment becomes important

34. 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)

35. 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

36. 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