Title: Novel Detector Module for Nuclear Medicine Imaging 
Speaker: Mokhtar Chmeissani 
MOderator: Prof. Lou Xinchou
Time: 15:00, April 3
Place: Room C305, IHEP Main Building

Abstract:
Current Positron Emission Tomography (PET) scanner have limited image resolution which is in part due to the intrinsic limits set by the PET physics and in part due to the design and the material used in the PET scanner. We present a novel detector module, based on stack of pixel CdTe sensor, to reduce significantly the limitation coming from spatial and energy resolution. Each module detector has 4000 voxel (independent channels). Each voxel is 1mm x 1mm x 2mm of CdTe bonded to a dedicate pixel channel. Each ASIC (100 channels) has one TDC with time resolution of 1 ns. Each channel has its own pre-amp, shaper, discriminator, and 10 bits ADC. Each triggered voxel send its energy digitized appended with time stamp. This convert the novel detector into a true 3D gamma detector as it can precisely indicate the exact coordinated in space ( x,y, and z) of the impact point of the gamma photon. 
18 detector modules have been assembled (72000 channels) as small PET ring with solid angle/4π ≈ 0.13. Using Na22 source and bias voltage of -250V/mm, 85% of the channels are operational. The other 15% are with defects due to electronics or detector leakage current. Under such working point, the Energy resolution obtained for photo-absorption events is 2.0% (FWHM) at 511 keV. The energy resolution for 511 keV photon undergone a Compton scatter (reconstructed from two different sensors) is 3.0% (FWHM). With such energy resolution, one can reduce the scatter fraction from ~35% to ~4%. With excellent energy and spatial resolutions for the Compton gamma, one can foresee that in the near future, the Compton camera will become a viable nuclear medicine imaging modality. 

About the speaker: 
Mokhtar Chmeissani did his undergraduate studies at the faculty of Science of the Lebanese University, where he obtained his B.S.in Physics in 1983. Then he moved to University of Michigan where he did his Ph.D thesis on particle physics, Quantum Chromo Dynamics(QCD), with MKII detector at SLAC. He defended his thesis successfully in early 1991. After that he went to the European Organization for Nuclear Research (CERN) and participated on various experiments such as, L3, ALEPH, RD3, and ATLAS. His research was focused on the data analysis related to QCD and detector instrumentations. He joined the Medipix2 collaboration at CERN in 1998 and in 2001 he coordinated "Detection of Early Markers in Mammography” project, funded by the European 5th Framework Programme (FP5), to develop X-ray digital imaging detectors for mammography and general radiology based on a Photon Counting sensor.). In 2005 he developed a radiation monitor for the LISA Pathfinder, a European Space Mission for detecting gravitational waves. In 2008 he received the Silver Medal at the 36th International Salon of Inventions, Geneva, Switzerland for the concept of a 3D real time breast biopsy system. After the conclusion of the work in the breast biopsy machine, Mokhtar shifted his research towards nuclear medicine. He proposed to construct a new type of PET scanner based on pixel CdTe solid state detectors. This novel detector will yield good quality images by reducing the systematic errors due to scatter fraction and parallax. It will also open the way to build Compton Gamma Camera to operate with multi-traces simultaneously and for real-time PET imaging. The idea of this novel detector was very well received by the European Research Council (ERC) which awarded him a grant in 2010 to build a small prototype for evaluation. Beside his work on the novel PET scanner, he has lead the development of pixel sensor for spectral X-ray scanners in the field of security and quality control. The sensor has 6 thresholds making it possible to detect minute change in material density. For this project, he has invented a new trigger scheme to sum up charges in neighboring pixels to reconstruct the original spectral energy of the X-ray tube. Currently he is participating on the development of the pixel electrodes for retinal implant. 
Mokhtar has more than 25 years of expertise in the instrumentation of radiation detectors in general and the pixel detectors in particular. He holds 3 patents and one of the founder of a startup in the field of pixel radiation detector.