Roger Lecomte

Université de Sherbrooke, Nuclear Medicine and Radiobiology, Faculty Member

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Ph.D., Professor at Université de Sherbrooke, QC, Canada

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Papers by Roger Lecomte

Novel detector technology for clinical PET

European Journal of Nuclear Medicine and Molecular Imaging, Dec 24, 2008

Introduction Positron emission tomography (PET) is the most sensitive of all medical imaging moda... more Introduction Positron emission tomography (PET) is the most sensitive of all medical imaging modalities for quantitatively probing biologic processes at the molecular level. However, spatial resolution in PET is significantly inferior to that of other imaging modalities that can provide exquisite images of the anatomy, such as X-ray computed tomography (CT) or magnetic resonance (MR) imaging. Objective It has been one of the outstanding challenges of the last decade to combine PET with these complementary imaging modalities in order to synergistically exploit the benefits of each modality and to enhance the role of PET in pre-clinical research as well as in clinical routine and research. Discussion The simple juxtaposition of tomographs around a common axial bed, such as with current PET/CT technology, is very successful in allowing sequential acquisition of PET and anatomical data. However, novel imaging combinations are being considered that would enable simultaneous, or at least concurrent, dual-modality imaging through combined PET/MR or PET/CT. The development of these new integrated instruments creates new bewildering challenges for PET detection systems, which, in addition to the ability to measure annihilation radiation in PET, must satisfy several other critical requirements.

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Wedge-shaped scintillation crystals for positron emission tomography

PubMed, 1986

The use of wedge-shaped scintillation crystals for positron emission tomography is reevaluated wi... more The use of wedge-shaped scintillation crystals for positron emission tomography is reevaluated with the aim of delimiting its range of benefit. A linear attenuation simulation model is used to generate the detector geometric aperture functions, and the investigation is carried out in terms of a newly introduced shape-independent definition of the spatial resolution, the "spectral resolution." Contrary to previous expectations, it is concluded that shaped crystals do not improve the performance of high resolution detection systems, either with or without intercrystal septa. However, wedges are found to be useful for lower resolution systems and the boundaries of applicability are estimated for NaI, BaF2, GSO, and BGO scintillators.

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Analytical study of the effect of collimation on the performance of PET cameras in 3-D imaging

IEEE Transactions on Nuclear Science, Apr 1, 1990

An analytical model was developed to evaluate the 3-D imaging performance of cylindrical PET (pos... more An analytical model was developed to evaluate the 3-D imaging performance of cylindrical PET (positron emission tomography) systems with the multislice and the volumetric configurations. Point-source event rates for singles and for true and scattered coincidences were obtained by numerical integration of the equations for probability of incidence on the detectors. Data were subsequently combined to extract the sensitivity and background (scatters and accidentals) count rates for various source dimensions and activity densities. Event rates before detection were evaluated, and an ideal tomograph geometry was assumed in order to investigate the fundamental properties inherent to the two tomograph designs. For small sources placed in a scattering cylinder, results indicate higher scatter and accidental fractions in the volumetric system. If the source activity is adjusted to produce the same true event rate (same sensitivity) in each tomograph, a comparable image contrast can be expected in the volumetric system for small size source and for high activity concentrations. >

Bioreactors for Tissue Engineering

Elsevier eBooks, 2011

ABSTRACT This article reviews reports on three-dimensional mammalian tissue growth in bioreactors... more ABSTRACT This article reviews reports on three-dimensional mammalian tissue growth in bioreactors and the corresponding requirements. The needs for nutrient supply and waste removal of some tissues are reviewed. Many studies conclude that oxygen supply appears to be one of the most important factors limiting tissue development. Various correlations to describe oxygen mass transfer are presented and discussed here with the aim to provide some guidance to design, build, and test reactors for tissue mass culture. To obtain thick tissue substitutes clinically valuable, dimensionless and other types of analysis tend to point out that diffusive transport has to be matched with an important convection to bring sufficient oxygen molecular flux to the growing cells located within a tissue mass. An update of our previous review paper related to the use of bioreactors in tissue engineering is presented. Finally, a section reviews noninvasive imaging techniques, such as positron emission tomography and magnetic resonance imaging, which can be used to monitor bioreactor cultures nondestructively.

Technology challenges in small animal PET imaging

Nuclear Instruments and Methods in Physics Research, Jul 1, 2004

Positron Emission Tomography (PET) is a non-invasive nuclear imaging modality allowing biochemica... more Positron Emission Tomography (PET) is a non-invasive nuclear imaging modality allowing biochemical processes to be investigated in vivo with sensitivity in the picomolar range. For this reason, PET has the potential to play a major role in the emerging field of molecular imaging by enabling the study of molecular pathways and genetic processes in living animals non-invasively. The challenge is to obtain a spatial resolution that is appropriate for rat and mouse imaging, the preferred animal models for research in biology, while achieving a sensitivity adequate for real-time measurement of rapid dynamic processes in vivo without violating tracer kinetic principles. An overview of the current state of development of dedicated small animal PET scanners is given, and selected applications are reported and discussed with respect to performance and significance to research in biology.

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Biomedical Imaging: SPECT and PET

Nucleation and Atmospheric Aerosols, 2007

Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are non... more Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are non‐invasive nuclear imaging techniques relying on the use of tomographic reconstruction methods to provide 3D representations of the distribution of radiolabeled molecules in vivo. Differences in the underlying physical principles determine the achievable spatial resolution, sensitivity, specificity and observation time span of these two imaging modalities. Their specific characteristics are described and the current technology developments and design tradeoffs are reviewed.

Study of the resolution performance of an array of discrete detectors with independent readouts for positron emission tomography

IEEE Transactions on Medical Imaging, 1991

The intrinsic resolution characteristics of an original detection system for very high resolution... more The intrinsic resolution characteristics of an original detection system for very high resolution positron emission tomography (PET) based on arrays of small discrete crystals individually coupled to solid state photodetectors are evaluated. The results obtained demonstrate the excellent three-dimensional (3-D) resolving capability of a detection system based on the EG&amp;G/RCA C30994 detector module. The analysis of the various components contributing to the resolution has shown that the geometry of the detector and the intercrystal scattering both contribute to limit the achievable intrinsic resolution. The geometrical component is dominant off the central region of the camera field as a result of intercrystal penetration when the array is irradiated at an angle. Intercrystal scattering must be properly rejected to achieve the intrinsic resolution determined by the detector geometry.

Revisiting motion compensation models in PET image reconstruction

Positron emission tomography (PET) image reconstruction in the presence of periodic motion, such ... more Positron emission tomography (PET) image reconstruction in the presence of periodic motion, such as heartbeat and breathing, has been actively investigated since the introduction of gated data acquisition. While each gated sinogram can be reconstructed independently, better images can be obtained by incorporating all gated sinograms, which are considered statistically independent, into an iterative reconstruction scheme. However, this gain in image quality is achieved at the expense of substantial extra computational resources to handle all gated sinograms simultaneously. We provide a comparative study with a model that is built upon the interdependence of the parameters defining the stochastic processes behind each gated sinogram to yield a single merged sinogram. We show, using simulated and experimental datasets, that images of similar quality than with the classical model can be produced, while requiring much less computing resources.

A microvolumetric blood counter/sampler for metabolic PET studies in small animals

IEEE Transactions on Nuclear Science, Aug 1, 1998

Quantitative metabolic imaging in small animals with positron emission tomography (PET) requires ... more Quantitative metabolic imaging in small animals with positron emission tomography (PET) requires the determination of the tracer concentration in whole blood, arterial plasma and metabolites as a function of time. A blood counting and sampling system was designed to simultaneously measure the time-activity curve as microvolumes of blood are collected. The system consists of a flow-through counter made of a plastic scintillator to detect positrons and of a computer-controlled blood sampler based on the concept of bubble segmentation. The number and size of samples, the withdrawal speed and the sampling time are programmable and can lbe modified on-line. Samples as small as 10 p1 can be repetitively obtained from an implanted arterial catheter in the femoral vein or artery of small rats (150 g) or the jugular vein of mice (20 g). For medium sampling speed (1 00 yll/min) at a constant rate, the standard deviation of the sample activity is typically less than 1%. By cutting the tubing at the bubbles at the end of the experiment, samples are made akailable for further processing and biochemical analysis. This apparatus has become an essential tool for quantitative amimal PET studies, allowing easy, reliable sampling at a low cost.

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Parameterizing the Angular Distribution of Emission: A model for TOF-PET low counts reconstruction

arXiv (Cornell University), May 5, 2021

Low-count reconstruction remains a challenge for Positron Emission Tomography (PET) even with the... more Low-count reconstruction remains a challenge for Positron Emission Tomography (PET) even with the recent progress in time-of-flight (TOF) resolution. In this context, the bias between the acquired histogram, consisting of low values or zeros, and the expected histogram, obtained from the forward projector, is propagated to the image, resulting in a biased reconstruction. This situation could be exacerbated with finer resolution of the TOF information, which further sparsifies the acquired histogram. We propose a new approach to circumvent this limitation of the classical reconstruction model. It consists of extending the description of the reconstruction scheme to also explicitly include the projection domain by Parameterizing the Angular Distribution of Emission (PADE). This parametrization has greater degrees of freedom than the log-likelihood model, which can not be harnessed in classical circumstances. We hypothesize that with ultra-fast TOF this new approach would not only be viable for low-count reconstruction, but also more adequate than the classical reconstruction model. As a proof of concept, an implementation of this approach is compared to the log-likelihood model by investigating two-dimensional simulations of a hot spots phantom. The proposed model achieves similar contrast recovery coefficients as MLEM except for the smallest structures where the low-count nature of the simulations makes it difficult to draw conclusions. However, this new model seems to converge toward less noisy solutions than MLEM. These results suggest that the PADE approach has potential for low-count reconstruction with ultra-fast TOF.

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Measurement of basal cerebral perfusion and metabolic activity in LDLr −/− ;hApoB +/+ mice

The FASEB Journal, Apr 1, 2011

The aim of this study was to investigate the feasibility of using positron emission tomography (P... more

CT acquisition using PET detectors and electronics

2003 IEEE Nuclear Science Symposium. Conference Record (IEEE Cat. No.03CH37515), 2003

The emergence of PET/CT multi-modality imaging has provided the ability to sequentially obtain an... more The emergence of PET/CT multi-modality imaging has provided the ability to sequentially obtain anatomic and functional information using adjacent PET and CT scanners without having to move the patient. To avoid the need for successive PET and CT scans, we have investigated the possibility of acquiring both the anatomic and functional images using the same detection system, based on conventional

Fast point spread function computation from aperture functions in high-resolution positron emission tomography

IEEE Transactions on Medical Imaging, Mar 1, 1988

In developing the new generation of positron cameras with spatial resolution approaching the phys... more In developing the new generation of positron cameras with spatial resolution approaching the physical limit, a very careful investigation of the system performance as a function of detector characteristics becomes essential. The greater probability for intercrystal spillage between small detectors is largely responsible for the increased dependence of the tomograph performance on the detection system. To facilitate the system design, fast point source simulation and reconstruction methods incorporating a full description of the in-plane detector aperture functions were developed. The result is the point spread function (PSF) of a linear axial source transposing accurately at the image level the physical and geometrical characteristics of the detection system. From the PSF can be extracted the system sensitivity and precise spatial resolution values. The method is applied to the study of a high-resolution tomograph based on small discrete detectors.

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Cardiac Metabolic Pathway Remodelling in Response to Sunitinib Malate: Potential Role for Positron Emission Tomography in the Cardio-Oncology Setting

Interscapular brown adipose tissue denervation does not promote the oxidative activity of inguinal white adipose tissue in male mice

American Journal of Physiology-Endocrinology and Metabolism, 2018

Brown adipose tissue (BAT) thermogenesis is a key controller of energy metabolism. In response to... more Brown adipose tissue (BAT) thermogenesis is a key controller of energy metabolism. In response to cold or other adrenergic stimuli, brown adipocytes increase their substrate uptake and oxidative activity while uncoupling ATP synthesis from the mitochondrial respiratory chain activity. Brown adipocytes are found in classic depots such as in the interscapular BAT (iBAT). They can also develop in white adipose tissue (WAT), such as in the inguinal WAT (iWAT), where their presence has been associated with metabolic improvements. We previously reported that the induction of oxidative metabolism in iWAT is low compared with that of iBAT, even after sustained adrenergic stimulation. One explanation to this apparent lack of thermogenic ability of iWAT is the presence of an active iBAT, which may prevent the full activation of iWAT. In this study, we evaluated whether iBAT denervation-induced browning of white fat enhanced the thermogenic activity of iWAT following cold acclimation, under be...

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Asbestos pollution levels in river water measured by proton-induced X-ray emission (PIXE) techniques

Environmental Pollution Series B, Chemical and Physical, 1983

ABSTRACT

$Research paper thumbnail of High spin states and band structure in ^{97}Tc$

High spin states and band structure in ^{97}Tc

Physical Review C, 1982

ABSTRACT

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Coulomb-excitation studies ofGe70,Ge72,Ge74, andGe76

Physical Review C, 1980

The characteristics of the low-lying levels of ' Ge, "Ge, "Ge, and "Ge have been investigated via... more The characteristics of the low-lying levels of ' Ge, "Ge, "Ge, and "Ge have been investigated via multiple Coulomb excitation effected by bombarding isotopically enriched thick targets with 36 to 42 MeV "0 ions. These measurements allowed the excitation of the 2+, 2+', 0+', and 4+ levels in all nuclei. The E2 transition values of the y rays deexciting the 2+' and 4+ levels to the 2+ states are compared with those recently measured in "Ge and "Ge, A close inspection of this set of data clearly reveals a change of nuclear properties between N = 38 and N = 40. This is in agreement with the possible existence of a shape transition around N = 40. NUCLEAR REACTIONS 7 '7 ' 4' ~Ge(' 0 f60'y), E =36 to 42 MeV; measured I" Eyp deduced B(E X}f 8{M1) f cog Tf / 2 Enriched targets. The low-ener gy spectra of the doubly even Se, Ge, and Zn nuclei have been extensively investigated in the past years. ' Cl.early, these nuclei belong to a region of weak deformation and have a large instability character emphasized by the pos- sible' existence of a structural change around X =40. However, even though the hypothesis of a shape transition is very pertinent, different shapes '9L.

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Measurement of the static quadrupole moments of the first 2+ states in 76Se, 78Se, 80Se and 82Se

Nuclear Physics A, 1977

The reorientation effect in Coulomb excitation was employed to measure the static quadrupole mome... more The reorientation effect in Coulomb excitation was employed to measure the static quadrupole moments of the first 2+ excited states in 76Se, 78Se, 80Se, and 82Se. Beams of 4He and 16O were used on thin highly enriched targets and the resulting spectra of the well resolved backward elastic and inelastic scattered particles were detected by four surface-barrier detectors. For constructive interference via the second 2+ excited states the determined quadrupole moments are: -0.34+/-0.07 e.b for76Se; -0.26+/-0.09 e.b for78Se; -0.31+/-0.07 e.b for80Se; -0.22+/-0.07 e.b for82Se.

Trapping of fluorescent light in cylindrical scintillators

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1989

Abstract A theoretical model of the light transport in ideal cylindrical scintillators is propose... more

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