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R Lecomte

Publications and source records attributed to R Lecomte.

At least 19 recordsLinked to original sources
PubMed1999

Energy dependence of nonstationary scatter subtraction--restoration in high resolution PET.

In previous works, the determination of object and detector scatter kernels from line source measurements was described and their application in scatter correction was investigated. It was also shown that low energy data contains a large fraction of useful events (true and detector scatter events). In the present work, data acquired in multispectral mode was summed in single broad energy windows of lower energy thresholds varying from 129 to 516 keV in steps of 43 keV and a constant upper energy threshold of 645 keV. Line-source projections were fitted by extracting the object and detector scatter kernels as a function of energy threshold. These kernels were then used to process scatter by the nonstationary convolution subtraction-restoration method in phantom images. After scatter correction, the sensitivity is found to increase by up to 64% at the lower threshold of 129 keV, relative to the conventional photopeak energy window (344-645 keV). Whereas contrast and spatial resolution are degraded as the energy discriminator is lowered, such degradation is fully recovered by the scatter correction. As a result of scatter correction, the noise increases insignificantly in hot regions but substantially in cold regions, in proportion of the amount of scatter.

Image Processing, Computer-Assisted↗
PubMed1999

High-resolution PET imaging for in vivo monitoring of tumor response after photodynamic therapy in mice.

UNLABELLED: The aim of this study was to investigate the use of [18F]fluoro-2-deoxy-D-glucose (FDG) and a small-animal PET scanner to assess early tumor response after photodynamic therapy (PDT) in mice. PDT consists of intravenous administration of a photosensitizer that accumulates preferentially in tumor tissue, followed by local illumination of the tumor with red light. Two different photosensitizers were used: Photofrin (PII), which has been approved for clinical use, and disulfonated aluminum phthalocyanine (AIPcS), which is a second-generation drug. These drugs have been shown to induce tumor necrosis through different action mechanisms, i.e., mainly initial vascular stasis (PII) or direct tumor cell kill (AIPcS). FDG PET was used to follow both perfusion and metabolic activity in the tumor tissue. METHODS: The study was performed using a mouse model implanted with two contralateral murine mammary tumors (5 mm diameter x 2.5 mm thickness) on the back. Only one tumor was subjected to PDT, whereas the other tumor served as a control. A total of 13 mice were studied, 1 without illumination, 3 at 30 min and 3 at 2 h after PDT with both PII-PDT and AIPcS-PDT. Dynamic PET imaging of the mice, which were placed in pairs in a prostate position parallel to the transaxial planes of the Sherbrooke animal PET scanner, was performed after a bolus injection of 11 MBq (300 microCi) FDG. Blood samples were collected concurrently from 1 mouse during each study using an automated microvolumetric blood sampler. RESULTS: Analysis of the tumor time-activity curves showed that (a) scans during the first 3 min provided an estimate of tumor perfusion, as confirmed by the blood samples; (b) the tumor FDG uptake after 15 min was a direct measurement of tumor metabolism clearly demonstrating the relative efficacy of the two PDT drugs; and (c) the tumor tracer concentration in the interval 3-15 min after FDG injection is an appropriate indicator of the different mechanisms of tumor necrosis through indirect vascular stasis (PII) or direct cell kill (AIPcS). CONCLUSION: This pilot study confirmed the feasibility of using dynamic in vivo PET imaging for assessing early tumor response to PDT in mice.

Aluminum↗
PubMed1997

Pre-processing variance reducing techniques in multispectral positron emission tomography.

Stochastic fluctuations and systematic errors severely restrict the potential of multispectral acquisition to improve scatter correction by energy-dependent processing in high-resolution positron emission tomography (PET). To overcome this limitation, three pre-processing approaches which reduce stochastic fluctuations and systematic errors without degrading spatial resolution were investigated: statistical variance was reduced by smoothing acquired data in energy space, systematic errors due to nonuniform detector efficiency were minimized by normalizing the data in the spatial domain and the overall variance was further reduced by selecting an optimal pre-processing sequence. Selection of the best protocol to reduce stochastic fluctuations entailed comparisons between four smoothing algorithms (prior constrained (PC) smoothing, weighted smoothing (WS), ideal low-pass filtering (ILF) and mean median (MM) smoothing) and permutations of three pre-processing procedures (smoothing, normalization and subtraction of random events). Results demonstrated that spectral smoothing by WS, ILF and MM efficiently reduces the statistical variance in both the energy and spatial domains without observable spatial resolution loss. The ILF algorithm was found to be the most convenient in terms of simplicity and efficiency. Regardless of the position of subtraction of randoms in the sequence, reduction of the systematic errors by normalization followed by spectral smoothing to suppress statistical noise produced the best results. However, subtraction of random events first in the sequence reduces computation load by half since the need to pre-process this distribution before subtraction is removed. In summary, normalizing data in the spatial domain and smoothing data in energy space are essential steps required to reduce systematic errors and statistical variance independently without degrading spatial resolution of multispectral PET data.

Biophysical Phenomena↗
PubMed1996

Nonstationary scatter subtraction-restoration in high-resolution PET.

UNLABELLED: Although removal of object scatter has been shown to improve both contrast and quantitation accuracy, subtraction of detector scatter leads to marginal contrast enhancement and negligible resolution recovery at the expense of reduced sensitivity and increased statistical noise. Since detector scatter has correct information about radioactivity but slightly erroneous information about source location, we suggest that this component should be restored to preserve sensitivity and improve resolution. METHODS: A scatter correction model that consecutively removes object scatter and restores detector scatter is proposed. The scatter components are processed in the spatial domain using nonstationary scatter kernels. The detector scatter restoration kernel is obtained by piecewise inversion in the Fourier space. The model was tested using line source and hot spot phantom measurements. RESULTS: Object scatter subtraction increased contrast substantively with no effect on resolution. Detector scatter restoration recovered resolution almost completely with modest contrast enhancement in small lesions. Spillover effects were reduced to less than 5% for hot spots > or = 3 x FWHM, at the expense of moderate noise amplification. CONCLUSION: While subtraction of object scatter is necessary for contrast enhancement and quantitation accuracy, restoration of detector scatter preserves sensitivity and improves quantitation accuracy by reducing spillover effects in high-resolution PET.

Image Enhancement↗
PubMed1995

Assessment of scatter components in high-resolution PET: correction by nonstationary convolution subtraction.

UNLABELLED: This paper describes a new approach to determine individual scatter kernels and to use them for scatter correction by integral transformation of the projections. METHODS: Individual scatter components are fitted on the projections of a line source by monoexponentials. The position-dependent scatter parameters of each scatter components are then used to design non-stationary scatter correction kernels for each point in the projection. These kernels are used in a convolution-subtraction method which consecutively removes object, collimator and detector scatter from projections. This method is based on a model which assumes that image degradation results exclusively from Compton interactions of annihilation photons, thus neglecting further Compton interactions of object scatters with collimator and detector. RESULTS: Subtraction of the object scatter component improved contrast typical of what is obtained with standard convolution-subtraction methods. The collimator scatter component is so weak that it can be safely combined with object scatter for correction. Subtraction of detector scatter from images did not improve contrast because statistical accuracy is degraded by removing counts from hot regions while cold regions (background) remain unchanged. CONCLUSION: Subtraction of object and collimator scatter improves contrast only. The slight gain in image sharpness resulting from the subtraction of detector scatter does not justify removal of this component at the expense of sensitivity.

Humans↗
PubMed1994

High-resolution PET imaging and quantitation of pharmaceutical biodistributions in a small animal using avalanche photodiode detectors.

UNLABELLED: The feasibility of high-resolution PET using BGO-avalanche photodiode detectors for in vivo imaging and quantitation of the biodistribution of radiopharmaceuticals in small animals is demonstrated. A prototype PET camera consisting of two scanning arrays of eight EG&G C30994 solid-state scintillation detectors was used to simulate a 310-mm diameter dual-ring animal tomograph having a 130-mm port and three imaging slices, each about 3.5 mm thick. The spatial resolution (FWHM) is 3 mm or less, isotropic and uniform throughout the 120-mm diameter field of view. METHODS: Female Fischer 344/CRBL rats implanted with subcutaneous mammary adenocarcinoma tumors were injected with copper-tetrasulfophthalocyanine (CuPcS4), a potential sensitizer for the photodynamic therapy of cancer, labeled with 64Cu (T1/2 = 12.7 hr, beta +:19%). RESULTS: In spite of the low specific radioactivity of 64Cu and other inherent limitations, organs such as the liver, kidneys and the tumor could be resolved with sufficient detail for their separation and quantitation. Apart from the tumor, agreement was obtained between the biodistributions measured by PET and by scintillation counting. The discrepancy for the tumor measurement results from averaging the radioactivity over the entire tumor volume when, in fact, CuPcS4 does not completely penetrate the tumor. This incomplete penetration is noted on the PET images. CONCLUSIONS: PET based on avalanche photodiode detectors provides an accurate measurement of target organ and tumor tissue concentrations. These preliminary results demonstrate the potential of very high resolution PET for biodistribution studies in small animals.

Animals↗
PubMed1992

Analytical study of performance in a 3D PET scanner.

The use of arrays of small discrete detectors and thin slices to achieve a high isotropic spatial resolution in positron emission tomography (PET) results in systems with a low ring sensitivity. In a multi-ring system, the overall sensitivity can be considerably improved by removing the interslice collimators to make full use of all cross slices coincidences, but this is achieved at the expense of increased scatter and accidental rates in the image. For imaging of small laboratory animals (diameter of 10 cm or less), the relieved burden of scatters, and to some extent accidentals, suggests that volumetric imaging may be of particular value. In order to evaluate the performance to be expected from a small animal PET scanner (10 cm diameter field) with and without the interplane collimators, the incident event rates for singles (unscattered and single scattered) and true, scatter and accidental coincidences were evaluated analytically. The performance was evaluated for various source sizes and activities in terms of five criteria: true/single ratio, scatter fraction, accidental fraction, image contrast and noise effective sensitivity. As a result of a 3-fold increase in scatter fraction and of a significant increase in accidental fraction for larger sources, the image contrast (true/(scatter+accidental)) is observed to always be inferior with the collimators removed. However, a significant improvement in noise effective sensitivity is obtained with the volumetric configuration, especially for small size sources placed at the centre of the field of view. It is concluded that the volumetric configuration is more advantageous than the multislice configuration to image small animals because the gain in sensitivity overcomes the loss of accuracy due to higher scatter and accidental rates.

Animals↗
PubMed1986

Dependence of the coincidence aperture function of narrow BGO crystals on crystal shape and light encoding schemes.

Although the spatial resolution in positron emission tomography is improved by reducing the crystal width, the resolution becomes increasingly non-uniform towards the edges of the field of view. This is mainly due to penetration of the gamma rays through one or more crystals before detection and the inability of current systems to resolve the depth of interaction within the crystal. This paper compares crystals with straight and pointed tips with and without tungsten septa to determine the effects of pointed tips and septa on spatial resolution and efficiency. Monte Carlo calculations are compared with extensive experimental results obtained from 16 coincidence lines between two four-BGO crystal/dual PMT detector assemblies. The spectral efficiencies for the different combinations are compared. A modest improvement in signal-to-noise ratio at the maximum resolvable frequency is demonstrated when using tungsten septa and pointed crystals. The effects of the optical coding scheme used are discussed and it is shown that most coding schemes cause diminishment of spatial resolution even at the centre of the field of view, resulting from Compton scattering to adjacent crystals.

Electrons↗
PubMed1986

Wedge-shaped scintillation crystals for positron emission tomography.

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.

Crystallization↗
PubMed1979

Trace elements and acute phase reactants in gold treated rheumatoid arthritis patients.

Abnormal concentrations of trace elements, sulfhydryl groups, amino-acids and serum proteins were found in patients with rheumatoid arthritis similar to other chronic inflammatory conditions. These changes have been called the phase reaction. In this study, a systematic profile of the phase reactants was established in rheumatoid arthritis at the onset and during its modulation by chrysotherapy (gold sodium thiomalate 25 mg i.m. weekly). It was shown that this treatment, if successful, is capable of reverting to normal measures of the phase reaction. It was also found that the pattern of the profile varied in individuhe eventual beneficial or toxic effects. In these preliminary studies, no parameters or group of parameters of the phase reaction were predictive of the therapeutic outcome. Extended observations (over 6 months) and more individual profiles are being analyzed to gain insight into these features of inflammation and their modulation by chrysotherapy.

Adult↗
PubMed1979

Trace element determination in serum by proton-induced x-ray emission.

We report the use of proton-induced x-ray emission (PIXE) as an analytical method in clinical research. The experimental set-up and target preparation procedures are briefly discussed together with the methods of automatic data acquisition and analysis. Results from a clinical project involving rheumatoid patients receiving chrysotherapy are presented.

Arthritis, Rheumatoid↗