Francois Cayouette

DETECT2000: an improved Monte-Carlo simulator for the computer aided design of photon sensing devices

We introduce a new version of DETECT. DETECT is a Monte-Carlo simulator developed for the Computer Aided Design (CAD) of optical photon sensing devices. The simulator generates individual emission photons in specified locations of a photon-emitting device and tracks their passage and interactions in active and passive components of the system. Extensive options are available in the simulator to model the geometry of the photon sensing device, to account for the time and wavelength distribution of emission photons, to track their interactions with surfaces, to account for their possible absorption and re-emission by a wave-shifting components and to model their detection by pixelated photomultipliers or photodiodes. DETECT2000 is a very significant upgrade of DETECT97, which has long been established in the nuclear medicine instrumentation community for its accuracy to model the performances of high resolution energy and position sensitive gamma-ray detectors. The 2000 version of DETECT offers an accelerated version of the simulator which has been redesigned in the object-oriented C++ language. New features such as the tracking of the time and wavelength history of individual optical photons have been added.

Anode position and last dynode timing circuits for dual-layer BGO scintillator with PS-PMT based modular PET detectors

New high spatial resolution positron emission tomography (PET) detectors for small animal and breast imaging have been developed. In these detectors, position-sensitive photomultipliers (R7600-C12) and dual-layer pixelated bismuth germanate (BGO) crystals are employed to detect and localize gamma rays. Modified high-voltage dividers with last dynode readout circuits, front-end anode position, and last dynode timing schemes have been investigated and developed for these detectors. Methods for combining four position-sensitive photomultipliers (PS-PMTs) with simple four (X/sup -/, X/sup +/, Y/sup -/, and Y/sup +/) outputs have been developed to further simplify the position processing. The front-end circuits are small so they can be fitted into the detectors structure. A prototype of two detector modules, each having two PS-PMTs with corresponding electronic circuits, has been built for evaluation. The crystal elements of 10/spl times/10 in the proximal layer and 9/spl times/9 in the distal can be clearly identified.

A prototype modular detector design for high resolution positron emission mammography imaging

Current challenges facing us in developing dedicated position emission tomography (PET) system for metabolic breast mammography (PEM) and small animal (ANIPET) are to achieve high spatial resolution (less than 2 mm) and high efficiency. It is also crucial to extend the sensitive areas of PEM detectors to their periphery in order to overcome the difficulty in imaging near a patients chest wall. This limitation of the periphery dead region was revealed in the clinical trials of our previously developed PEM-I systems. In the new study, we developed prototype detectors by using position-sensitive photomultiplier tubes (PS-PMTs) and pixelated bismuth germanate (BGO) crystals with depth encoding scheme to detect and localize gamma rays. The procedures in crystal processing include cutting, polishing, encapsulating, separating, and re-gluing. We also developed front-end electronic circuits including high-voltage dividers, anode resister chains, position readout circuits, and last dynode timing circuits. Methods for combining four PS-PMTs with simple X+, X-, Y+, Y- outputs have been developed to further simplify the position recording. The detectors were constructed in the structure of array (two in the system) - module (four in each array) - unit (four in each module). The basic unit of one crystal and one PS-PMT can be as field replaceable. Our new prototype detectors show that the proposed PEM-II system has a spatial resolution of 1.8 mm (vs. 2.8 mm in PEM-1), a timing resolution of 10.3 ns (vs. 12 ns in PEM-I), and a field-of-view of 88 mm /spl times/ 88 mm (vs. 64 mm x 56 mm in PEM-1). Compared with our previous PEM-I system, it demonstrates that the design improves the spatial resolution, enhances the detector field-of-view, and significantly reduces the peripheral dead regions.

Monte-Carlo modeling of scintillator crystal performance for stratified PET detectors with DETECT2000

In order to determine the theoretical performance of a multi-layer scintillator crystal used in small PET scanners, we have used the DETECT2000 Monte-Carlo simulation of the light transport in scintillation crystal software. The results given by this software demonstrate that some of the individual crystals in the block could not be distinguished. Also we have found out that the layer connecting the different crystals degrades the performance of the scintillation block. Simulation of thinner interconnecting layers suggest that much better crystal identification could be obtained.

Optimizing position readout circuits in positron emission tomography front-end electronics

Most of the front-end position readout circuits in PET detectors originate from the Anger logic design. to order to simplify the PMT readout circuits, many different schemes of combining multiple PMTs with simplified (X/sup -/, X/sup +/, Y/sup -/, Y/sup +/) outputs have been investigated. Crystal irradiation image produced from eight-bit digitized (X/sup -/, X/sup +/, Y/sup -/, Y/sup +/) often shows noticeable artifacts. These artifacts are mainly caused by truncation and round-off errors, which occur in the irradiation image processes. To examine these errors, we applied the root-sum-square (RSS) analysis and the uniform distribution analysis methods. In addition, we studied the correspondent function of the event energy with image regions. It is demonstrated that energy non-uniformity could be introduced if signal gains in the front-end analog circuits were not adjusted properly. We also discussed how to judge balanced gains in the position circuits. In general, we aim at optimizing the front-end position readout circuits.

Monte Carlo simulation using DETECT2000 of a multi-layered scintillation block and fit to experimental data

The authors have used DETECT2000, a Monte Carlo simulation program of the light photon transport inside a scintillation block, to improve the performance of their next positron emission mammography system. The scintillation blocks of the system are composed of two arrays of crystals that are held together by an interconnecting layer. First, they used previously acquired experimental data on the block to find good simulated surface parameters for the scintillation block. They have found that the surface of the crystals are smooth and have good reflectivity. After a set of parameters was found to replicate the behavior of the block, the model geometry was modified in order to increase the blocks performance. The DETECT2000 simulations determined that the removal of the interconnecting layer would yield the best results. The experimental results on a modified block yielded excellent results, showing the power of Monte Carlo simulations.

A new surface parameterization for modeling thin layers of reflector material in the DETECT2000 optical modeling program

A new surface parameterization has been implemented in DETECT2000 that allows for the efficient representation of a thin film of reflector material with non-negligible transmission. Material is usually modeled in DETECT by specifying the optical attenuation and scatter lengths. For a material such as Teflon, commonly used as a reflector material for nuclear medicine scintillators, the scatter length can be very short. Modeling the propagation of photons in a material with a very short scattering length can be computationally intensive as there can be many scatter interactions before the photon either exits the material or is absorbed. In a detector design that makes generous use of Teflon, most of the computations can be spent simply transporting photons in the Teflon. To address this issue, a parameterized surface has been implemented which is specified not by the bulk optical scatter and attenuation lengths, but rather by a reflection and transmission coefficient. This allows computationally efficient modeling of thin film reflectors where surface properties and transmission are relevant.

Generic Real-Time Tracking Method on Semi-Dynamic Scenes

This paper describes a tracking method capable of following multiple targets for a generic scene in real-time. The tracker has no a priori knowledge of the objects and does not know how many objects will be present in the scene. The tracker uses straight-forward techniques to extract, identify and track the targets within the image sequence