D. Schmitt

Université de Sherbrooke, Sherbrooke, Quebec, Canada

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Publications (9)19.82 Total impact

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    ABSTRACT: The problem of extracting point spread functions from detector aperture functions in high-resolution PET is addressed. In the limit of very small size detectors relative to the ring dimensions, assumptions are made that lead to a fast and simple computation model yielding point spread functions with negligible errors due to the reconstruction algorithm. The methods allows one to assess accurately the intrinsic performance of a PET tomograph, and it appears to be adequate to relate the imaging capabilities in every point of the camera reconstruction field to the geometric and physical characteristics of the detection system. The method was developed as an investigation tool to help design the next generation of very-high-resolution PET tomographs
    IEEE Transactions on Medical Imaging 04/1988; · 4.03 Impact Factor
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    ABSTRACT: A novel type of positron camera achieving nearly isotropic spatial resolution and uniform sensitivity throughout the useful field of view is proposed. The design is based on a quad scintillator dual-channel detector module using two avalanche photodiodes as photodetectors. Original features include bilayered scintillators to measure the depth of interaction, tilted detectors relative to the radial direction to ensure proper sampling in a stationary mode, multiring capability, and an independent acquisition channel for every discrete detector. Significantly better resolution is obtained toward the edges of the camera field, and the sampling uniformity near the center is improved. Multiring systems with a slice thickness of 5 mm can be implemented to achieve true volume imaging with symmetrical 3-D spatial resolution. The performance of the proposed design is analyzed in terms of resolution, sensitivity, contrast, and field uniformity.
    IEEE Transactions on Nuclear Science 03/1988; · 1.22 Impact Factor
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    ABSTRACT: The need for compact, fast, low-noise front-end electronics in high resolution positron emission tomography (PET) has prompted this effort to design a preamplifier suitable for avalanche photodiode-based scintillation detectors. Due to the small signals from the detectors (< .03 pC/MeV), a preamplifier with ultra-low noise performance in the 5 to 20 MHz range is essential to achieve the timing resolution required by the PET application. Out of many available technologies, a new third generation MOSFET wqas selected and implemented as input transistor in an original charge sensitive (CSP) design. Performance among the best reported to date are obtained. The new design was implemented as a dual-channel preamplifier in high density hybrid (thick film) technology.
    IEEE Transactions on Nuclear Science 03/1987; · 1.22 Impact Factor
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    ABSTRACT: 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.
    Physics in Medicine and Biology 06/1986; 31(5):491-506. · 2.70 Impact Factor
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    ABSTRACT: A light-tight, hermetically sealed module for use in high resolution positron emission tomography systems is described. The module has external dimensions 3.8×13.2×33 mm and contains two 3×5×20 Bismuth Germanate (BGO) scintillators, each with its own 3×3 mm silicon avalanche photodiode. When stacked, the vertical packing fraction is 80%. As measured with a 137Cs (662 keV) source, the typical energy resolution is 20% at 22°C, reducing to 16% at 0°C. The single detector time resolution for the 22Na gamma at 511 keV is typically less 20 ns at 22°C, reducing to less than 15 ns at 0°C. Further cooling does not improve the performance since the emission time of light from BGO increases at lower temperature. Preliminary results with Gadolinium Orthosilicate show similar energy resolution, better timing resolution (under 10 ns), but as is known, a slightly poorer photofraction and stopping power.
    IEEE Transactions on Nuclear Science 03/1986; · 1.22 Impact Factor
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    D Schmitt, R Lecomte, E LeBel
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    ABSTRACT: 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.
    Journal of Nuclear Medicine 02/1986; 27(1):99-104. · 5.77 Impact Factor
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    ABSTRACT: Spectroscopic and timing characteristics of a gamma-ray detector based on silicon avalanche photodiodes (APD) and bismuth germanate (BGO) scintillators are reported. The APD used in this work is a large area (25 mm2) device of the "reach through" type having a quantum efficiency of 70% at 480 nm. When coupled to a 5×5×3 mm3 BGO crystal, its spectroscopic performance is found to be superior to that of a PMT coupled to the same crystal, owing largely to its higher quantum efficiency. At room temperature, the APD achieves better energy resolution than any other (unity gain) solid state photodetector currently available; at -15 °C, its spectroscopic performance is comnparable to that of large area p-i-n photodiodes at -150 °C. Preliminary measurements show that the timing performance of the new detector allows it to be used for both energy and coincidence discrimination in positron emission tomography (PET). Results obtained with a realistic BGO crystal shape and size for PET are also presented. Limitations and possible improvements are discussed.
    IEEE Transactions on Nuclear Science 03/1985; · 1.22 Impact Factor
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    ABSTRACT: Silicon avalanche photodiodes of the "reach-through" type have been used as scintillation detectors with device performance closely approaching that of PMT's. The device consists of a 6 × 6mm diode with a 5 × 5mm photosensitive area in a 6.7 × 6.7 × 2.0mm package which is suitable for mounting in an array. Typical characteristics at room temperature are: quantum efficiency ¿50% at ¿=415nm; surface dark current <200nA; bulk dark-current (before gain) <1nA; noise current (at optimum gain of 20 to 50) <1pA/Hz½; response time <5ns; capacitance ¿21pF; biasing voltage ¿200-400 Volts. When coupled to a NaI(Tl) scintillator (9 × 9 × 38mm) photo-peak resolutions (FWHM) for 662keV radiation of 10.4% at 21°C and 9.8% at 0°C were measured. The measured noise equivalent primary charge of one detector at a gain of 50 was 121 electrons (51 electrons rms) at room temperature. The significance of this new detector to high resolution imaging systems and detector arrays is discussed.
    IEEE Transactions on Nuclear Science 03/1984; · 1.22 Impact Factor
  • R. Lecomte, D. Schmitt, G. Lamoureux
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    ABSTRACT: The current trend in positron emission tomography (PET) towards very high spatial resolution tomographs raises the problem of detecting high energy gamma-rays with a high spatial accuracy while preserving the overall sensitivity of the camera. It is the purpose of this paper to investigate the effects of some geometrical parameters on the detection efficiency and intrinsic spatial resolution of a circular array of narrow closely packed detectors. Two approaches were employed: the first one uses the Monte-Carlo technique to simulate the interactions of gamma-rays in the detectors; the second one is based on the linear attenuation on a beam of gamma-rays impinging on a detector array. Aperture functions and modulation transfer functions were obtained for various configurations of the detectors and septas. Expressions for the resolution and the spectral signal-to-noise ratio were derived. It is shown that minor modifications to the geometry can improve both the efficiency and intrinsic resolution of the detector array.
    IEEE Transactions on Nuclear Science 03/1984; · 1.22 Impact Factor