VIPPIX: A readout ASIC for the next generation of human brain PET scanners

Abstract

Positron emission tomography (PET) is a molecular imaging technique used for several decades in nuclear medicine that provides precise physiological information of the human body, what is crucial in oncology, cardiology, and neuropsychiatry. Due to physical, the best spatial resolution of PET is approximately 1 mm for human brain scanners. Unfortunately, the minimum resolution of the best commercial PET scanners for humans is 4-5 mm due to technological limitations. In this thesis, an application specific integrated circuit (ASIC) to readout the energy and the time stamp of a high-density and highly-granulated Cadmium Telluride detector for a novel PET scanner design is presented. The research presented here was realized within the framework of Voxel Imaging PET pathfinder ERC project to develop detector modules for positron emission tomography applications and reach their actual physical limits. The VIP PET scanner is based on the stacking of 2-mm thickness pixelated hybrid detectors. Every CdTe detector is pixelated into an array of 10 x 10 voxels of 1 mm x 1 mm x 2 mm size and is connected to a pixelated ASIC to readout independently the energy and the time stamp of every photon interaction. The VIP PET is based on more than 6 million voxels with independent energy and timing readout to reach the physical limits of PET. The simulated performance based on the properties of CdTe detectors shows a scatter fraction of approximately 4 % due to an excellent energy resolution of 1.6 % FWHM of the CdTe detectors. By simulation, the VIP PET is able to distinguish 1 mm size radioactive point-like source. The characterization of 2-mm thickness CdTe detectors using commercially available single-channel readout electronics is reported. Using a Sodium-22 radioactive source, the 511 keV photopeak resolution and the coincidence time resolution of back-to-back photons were measured with – 1000 V/mm detector bias voltage at -8 Celsius degree. An energy resolution of 1.6 % FWHM and a time coincidence resolution of 6 ns FWHM were obtained for photoelectric interactions. The architecture of the VIPPIX ASIC, i.e. the ASIC developed for VIP project, is based on an array of 10 x 10 independent pixel electronics controlled by a global controller and a common time to digital converter (TDC). Additional voltage and current references are generated in the ASIC’s back-end with a temperature sensor and a chip-ID cell. Every pixel electronics composes of a programmable gain preamplifier with detector’s leakage dynamic compensation, a tuneable peak-time pulse shaper connected to a peak-and-detect circuit, a 10-bit analog-to- digital converter (ADC), a pulse discriminator with adjustable offset, and a local pixel digital controller. The measured equivalent noise charge (ENC) of the pixel is 150 e- RMS and the trigger time jitter is approximately 1 ns for energy depositions larger than 200 keV. The time resolution of the integrated TDC is 600 ps FWHM. Twelve wafers of VIPPIX ASIC has been fabricated and characterized. Best quality ASICs have been mounted on 720 CdTe detectors and stacked in 18 detector modules with 40 hybrid detectors each to build the VIP PET prototype. Five modules have been characterized with a Sodium-22 radioactive source. The performance of approximately 18000 pixels shows a resolution of 2.2 % FWHM and a coincidence time resolution of 60 ns FWHM for 511 keV photopeak at -250 V/mm detector bias. Therefore, the main goal of the research has been accomplished. A new PET design based on pixelated Cadmium Telluride detectors using dedicated readout ASICs has been successfully fabricated, partly characterized, and is ready for image acquisition.