Abstract
Time-of-flight measurement is an important advancement in PET scanners to improve image reconstruction with a lower delivered radiation dose. This article describes the monolithic ASIC for the TT-PET project, a novel idea for a high-precision PET scanner for small animals. The chip uses a SiGe Bi-CMOS process for timing measurements, integrating a fully-depleted pixel matrix with a low-power BJT-based front-end per channel, integrated on the same 100 μm thick die. The target timing resolution of the scanner is 30 ps RMS for electrons from the conversion of 511 keV photons. The system will include 1.6 million channels across almost 2000 different chips. A full-featured demonstrator chip with a 3×10 matrix of 500×500 μm2 pixels was fabricated to validate each block. Its design and experimental results are presented here.
Highlights
: Time-of-flight measurement is an important advancement in PET scanners to improve image reconstruction with a lower delivered radiation dose
A novel synchronization scheme using a patent-pending TDC is used to allow the synchronization of 1.6 million channels across almost 2000 different chips at picosecond-level
: Analogue electronic circuits, Digital electronic circuits, Front-end electronics for detector readout, Timing detectors, Pixelated detectors and associated VLSI electronics layer is composed by two 100 μm thick monolithic pixel silicon detectors placed side by side, a 50 μm lead converter and dielectric glue layers, as shown in figure 2
Summary
After some small-scale test structures, a 3×10 matrix of fully-featured pixels (shown in figure 3) was submitted in a MPW run in Spring 2017. Given an accurate enough TDC (TDCs with precision of a few ps can be found in literature[4]), the uncertainty is dominated by the effect of the analog front-end This includes different factors, such as the pixel-to-pixel threshold variation, the intrinsic electronic noise of the preamplifier and the distribution of charge collection time in the substrate. Due to the much larger peaking time compared to the target time resolution, time walk must be taken into account and compensated when calculating the time of arrival because different input charges can change the time stamp by hundreds of ps, as shown in figure 8 This is possible by estimating the charge performing a time-over-threshold measurement and correcting the time-walk error off-line. Stage of the discriminator uses very small NMOS transistors, leading to a significant pixel-to-pixel threshold mismatch (simulations showed a 3σ value of 100 mV) To compensate for this effect, an 8-bit calibration DAC is included in each front-end. The total current produced by the DAC can be tuned to change the calibration range
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