A Novel Time-Based Readout Scheme for a Combined PET-CT Detector Using APDs

Abstract

Abstract-This paper summarizes CERN R&D work done in the framework of the European Commission's FP6 BioCare Project. The objective was to develop a novel "time-based" signal processing technique to read out LSO-APD photodetectors for medical imaging. An important aspect was to employ the technique in a combined scenario for both Computer Tomography (CT) and Positron Emission Tomography (PET) with effectively no tradeoffs in efficiency and resolution compared to traditional single mode machines. This made the use of low noise and yet very high-speed monolithic front-end electronics essential so as to assure the required timing characteristics together with a high signal-to-noise ratio. Using APDs for photon detection, two chips, traditionally employed for particle physics, could be identified to meet the above criteria. Although both were not optimized for their intended new medical application, excellent performance in conjunction with LSO-APD sensors could be derived. Whereas a measured energy resolution of 16% (FWHM) at the 511 keV photo peak competes favorably with that of 'classical' PMTs, the coincidence time resolution of 1.6 ns FWHM with dual APD readout is typically lower. This is attributed to the stochastic photon production mechanism in LSO and the photon conversion characteristic of the photo diode, as well as to the fluctuations in photon conversion, albeit the APD's superior quantum efficiency. Also in terms of CT counting speed, the chosen readout principle is limited by the intrinsic light decay in LSO (40 ns) for each impinging X-ray.Index Terms-Avalanche photodiodes (APD), lutetium-oxyorthosilicate (LSO), time-based readout, time-over-threshold discrimination.

A Systematic Study to Optimize SiPM Photo-Detectors for Highest Time Resolution in PET

“…It should be noted that, owing to the time-over-threshold method, signal discrimination in NINO is subject to time walk, predominantly for low input charges [11]. However, as one can see from Fig.…”

“…These were originally developed for the LHC program at CERN [7], [9] and are now being adapted to the readout of photo-detectors. Also, a newly developed "time-based" readout scheme was employed, allowing the simultaneous record of the pulse width encoding the photon energy and the mutual pulse delay containing the time information of the event [10], [11].…”

The dynamic time-over-threshold method for multi-channel APD based gamma-ray detectors

“…Time-over-threshold (ToT) [1][2][3] is a pulse processing method in which an input pulse signal is compared with a preset threshold level in order to convert an input charge into a digital pulse with a corresponding time width. As it can be implemented without an independent analog-to-digital converter, The ToT method has advantages over conventional pulse height systems in terms of circuit integration and power consumption.…”

Design of an adjustable bias circuit using a single-sided CMOS supply for avalanche photodiodes

“…Avalanche photodiodes are used in a wide range of lowlight sensing applications including astronomy [1], DNA sequencing [2], light detection and ranging (LIDAR) [3] and medical sensing [4]. Planar avalanche photodiodes (APDs) have been developed over a number of years that typically require bias voltages above 25 V [5], [6].…”