Photon Counting X-ray Detectors: Innovations and Applications in Medical Imaging

1. Characterisation and Application ofPhoton Counting X-ray Detector SystemsDisputation seminar

2. 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

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. 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. 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 Section 2: The Medipix project Illustration from http://medipix.web.cern.ch/MEDIPIX/

11. The project is directed from the Cern microelectronics group 16 European institutes are participating Collaboration 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. 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 Medipix 1 Medipix1 system

13. 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 Medipix 2 Medipix2 mounted for dental imaging

14. 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) Medipix 2 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 26 - 30 keV 4 - 70 keV

17. Tooth image for varying energy

18. 8 - 10 keV 34 -38 keV 56 - 60 keV Colour image of the tooth • Colour X-ray image from RGB coding of three images

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. Sharing area Slit Charge sharing • Crosstalk between pixels 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. Silicon about 3 % absorption in a 300 µm detector (40 keV) CdTe almost 100 % point absorption Strong X-ray flourescence X-rays X-rays ~3 % uniform absorption 100 % point absorption Si Flourescence X-rays 43 % CdTe Charge drift

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

27. Narrow beam 10x10 µm Monochrome energy40 keV ESRF measurements The European Synchrotron Radiation Facility

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

29. 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 CdTe spectrum

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

31. Cumulative spectrum on a300 µm thick detector Silicon spectrum

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

33. Alignment becomes important 700 µm thick silicon detector

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