Semiconductor Detectors and Applications on X-ray imaging Natalie Diekmann Particle Physics 1 NIKHEF

Outline Detector requirements in medical imaging Integrating and counting detectors SNR and Flat Field Correction Map The Medipix2 detector Energy weighting Dependence between energy weighting and SNR

Detector requirements in medical Imaging As in high energy physics: high 2 dimensional spatial resolution high sensitivity. But also: Low dose to patient <3mGy: this means low information loss, high charge collection efficiency, little noise Compactness, stable operation, simple handling Room temperature operation Low costs

Integrating and counting detectors integrating detector: counting detector: if

SNR and Flat Field Correction Map Noise: –Photonic noise: statistical fluctuations in incident photon flux. –Electronic noise: dark currents, amplifier and discriminator noise –Fixed pattern: sensitivity fluctuations

Flat Field Correction Can be applied because we have fixed pattern noise Macroscopic: Detectors have wave pattern of high and low counting regions. Microscopic: fixed pattern noise due to variations in pixel geometry or electric field components When beam passes object the spectrum hardens. This affects the flat field correction map gain constant because it is energy dependent. SNR can be increased from 20 to 160

Medipix2 Hybrid semiconductor detector Active area of about 2 cm². Pixel have side length of 55 um. 256 x 256 pixels form the pixel matrix. Counting detector with two energy thresholds in each pixel

Energy weighting Integrating detectors: w prop. E (CCD or photographic film) Counting detectors: w=1(Medipix1) How can we achieve a better energy weighting ? Multiple DAC readout chips, simulation by threshold scan with Medipix 2

Best energy region: W=1W=E works well

counting integrating weighting SNR enhancement Water/ Breast Adipose/ Breast Integrating1.0 Counting Weighting2.01.8

Bibliography The Influence of Energy Weighting on X-ray Imaging Quality Juergen Giersch, University of Erlangen, Germany Design of analog-to-digital converters for energy sensitive hybrid pixel detectors David San Segundo Bello, NIM A466 (2001) X-ray imaging Using Single Photon Processing with Semiconductor Pixel Detectors Bettina Mikulek And many many thanks to Prof. Jan Visschers