Qingguo Xie

Potential advantages of digitally sampling scintillation pulses in timing determination in PET

We investigate the use of digital signal processing (DSP) techniques for event processing in positron emission tomography (PET). In this work, we focus on the determination of the event time. We are building a digital library of pulse waveforms generated by using different photo-detectors, scintillator materials, crystal sizes, and read-out electronics of interest in PET. We employ a dataset from our library and investigate two methods for obtaining digital samples of an event pulse and several DSP algorithms for estimating the event time. Our first results, by using non-optimized DSP methods, are encouraging.

A simple all-digital PET system

Positron emission tomography~(PET) systems employ mixed-signal front-end to carry out relatively simple, and ad hoc, processing of the charge pulses generated upon event detection. To obtain, and maintain over time, proper calibrations of the mixed-signal circuitry for generating accurate event information is a challenging task due to the simplicity of the event processing, and the huge number of channels and multiplexing of the input signals found in modern PET systems. It is also difficult to modify or extend the event-processing technologies when needs arise because it would involve making changes to the circuitry. These limitations can be circumvented by applying digital signal-processing technologies for analyzing event pulses generated in PET. With digital technologies, optimized event-processing algorithms can be implemented and they can be modified or extended with ease when needed. The resulting PET data-acquisition (DAQ) system is easier to calibrate and maintain, can generate more accurate event information, and has better extendibility. In this paper, we present our work toward developing a scalable all-digital DAQ system for PET, built upon a personal-computer platform for reducing cost. We will present the overall architecture of this digital DAQ system, and describe our implementations of several components of the system.

Characterization of Silicon Photomultipliers for PET Imaging

The Silicon Photomultiplier (SiPM) is a novel photodetector being developed for high energy physics applications. SiPM is attractive for PET imaging because it is compact; provides high gain at low voltage; is insensitive to magnetic fields; has a fast timing response; and is potentially inexpensive and CMOS-technology compatible. Researchers have characterized SiPM performance mostly in areas relevant to high-energy physics and astrophysics applications, with some findings demonstrating the potential usefulness of SiPM in positron emission tomography (PET) imaging. Using our photodetector characterization test-stand equipped with a 20 GSps sample rate and 6 GHz bandwidth oscilloscope, we have measured the gains in the range of 1-3 times 105 room temperature for SiPM samples obtained from different producers. We have measured the rising times and their standard deviations. Based on the measured dark-count rates at room temperature, we have also estimated the achievable energy resolution of SiPM in PET imaging when used with LSO and BGO scintillators.

Performance evaluation of multi-pixel photon counters for PET imaging

The multi-pixel photon counter (MPPC), also known as the silicon photo-multiplier (SiPM), is a novel, solid-state photodetector that contains an array of Geiger-mode photodiodes (called microcells below) to a gain in range of 105 -106 when operating at a low voltage of 40-70 V. The device also has relatively high photon detection efficiency (PDE) and fast timing response. It is also compact and insensitive to magnetic fields. These properties of the MPPC has recently created substantial interest in using the device for PET imaging. In this paper, we evaluate and compare the performance properties of three designs of 1times1 mm2 MPPC offered by Hamamatsu for use in PET. We examine the gains of devices, and also their energy and timing resolutions when coupled to LYSO.

Performance characterization of a high-sensitivity small-animal PET scanner

In this work, we will report solid evidences supporting the proposed compact, dual-head PET scanner design for achieving an unprecedented sensitivity. We will also report our initial imaging results generated by our prototypical scanner with phantoms and rats.

A new approach for pulse processing in PET

We propose a new electronic design to overcome design limitations in PET that arise from the need to use high-cost fast analog-to-digital converters (ADCs). Our design may completely remove the use of ADCs in PET, and possibly the constant fraction discriminator (CFD) as well. In our approach, we model the electric pulse generated upon an event detection by a fast linearly rising edge followed by a slower exponential decay. Based on this modeling, basic properties of the pulse that are relevant for PET event detection, including the decay constant, the peak value and the peak time, can be determined from simple time interval measurements of the pulse. In this work, we will present methods for determining these pulse properties and propose electronic implementations of these methods. We will employ computer simulations to conduct sensitivity analysis of the proposed new event processing methods to modeling errors, noise and clock rate.

Semi-automatic position calibration for a dual-head small animal PET scanner

A dedicated small-animal positron emission tomography (PET) scanner that can provide ~30% detection sensitivity is under development at The University of Chicago. This scanner employs two HRRT (high resolution research tomography) detector heads in a compact configuration to achieve a high system sensitivity. The HRRT detector employs a quadrant-sharing configuration to substantially reduce the number of photomultipliers needed, but also creates particular challenges in calibrating the crystal position. To address these challenges, we develop a watershed segmentation method that also incorporates a curve-fitting forecast algorithm. Our results indicate that the watershed algorithm alone can correctly detect 92.77% of the positions for the 72x104 crystals of the HRRT detector head, when including the curve-fitting forecast algorithm the accuracy can be further improved to 97%.

An investigation of digital signal processing for shaped pulses for all-digital PET

We present our work toward implementing all-digital signal processing for Positron Emission Tomography (PET) event detection. In the conventional PET system, proper calibration and extending event processing are challenging tasks due to the huge number of channels and multiplexing of input signals in the mixed-signal front-end. To alleviate such limitations, we have proposed a simple all-digital PET system utilizing digital signal processing (DSP) technologies for analyzing event pulses generated in PET. In this work, we implement a Gaussian shaper circuit for scintillation pulses, which followed by a moderate sampling rate Analog-to-Digital Converter (ADC). We also evaluate two DSP algorithms for extracting time information from the digitized pulse samples, and the two algorithms examined could generate a coincidence timing resolution of ~ 2.4ns FWHM, by using a 125MSps sampling rate ADC.

The effects of respiration motion in PET/CT studies

In recent years, the clinical status of positron emission tomography(PET)/computed tomography(CT) in achieving more accurate staging of lung cancer has been established and the technology has been enthusiastically accepted by the medical community. However, its capability in chest imaging is still limited by several physical factors. As a result of typical PET/CT imaging protocol, respiration-averaged PET data and free of respiration-averaged CT data are collected in a PET/CT scanning. In this work, we investigate the effects of respiration motion. We employ mathematical and Monte-Carlo simulations for generating PET/CT data. We scale a Zubal phantom to generate 30 phantoms having various sizes in order to represent different torso anatomic states during respiration. Images reconstructed from selected scaling PET data using the respective scaling PET attenuation maps serve as baseline results. PET/CT imaging protocol is simulated by reconstruction from respiration-averaged PET data with the selected PET attenuation maps. We also reconstruct PET images from respiratory-averaged PET data with respiration-averaged PET attenuation maps, which simulates conventional PET imaging protocol. We will compare the resulting images reconstructed from the above-mentioned approaches to evaluate the effects of respiration motion in PET/CT.