We describe the Sherbrooke positron emission tomograph, a very high resolution device dedicated to dynamic imaging of small laboratory animals. Its distinctive features are: small discrete scintillation detectors based on avalanche photodiodes (APD) to achieve uniform, isotropic, very high spatial resolution; parallel processing for low deadtime and high count rate capability; multispectral data acquisition hardware to improve sensitivity and scatter correction; modularity to allow design flexibility and upgradability. The system implements the "clam-shell" sampling scheme and a rotating rod transmission source. All acquisition parameters can be adjusted under computer control. Temperature stability at the detector site is ensured by the use of thermoelectric modules. The initial system consists of one layer of 256 modules (two rings of detectors) defining 3 image slices in a 118 mm diameter by 10.5 mm thick field. The axial field can be extended to 50 mm using 4 layers of modules (8 rings of detectors). The design constraints and engineenng aspects of an APD-based PET scanner are reviewed and preliminary results are reported. * This work was supported in part by the Medical Research Council of Canada under Grant MA-8549. The first author is a senior scholar of Le Fonds de la Recherche en S a d did Qrribec.
Grassland heterogeneity, defined by its components of spatial pattern, vertical structure, and species composition, is one of the most important indicators of prairie habitat. Maintaining grassland under conservation without disturbance may result in homogeneity at multiple spatial scales that could reduce wildlife diversity as a consequence. Therefore, monitoring grassland conditions that contribute to diversity can be critical for wildlife habitat and ecological integrity. Remote sensing, with multi-spatial, multi-spectral, and multi-temporal resolutions plus newly developed analytical techniques, provides a potential tool for measuring grassland heterogeneity under different management regimes quickly, efficiently, and at low cost. The objectives of this study were 1) to evaluate the heterogeneity of grassland under grazing and conservation management practices spatially and vertically, and 2) to investigate the feasibility of using remotely sensed data to measure grassland heterogeneity. The study area was Grasslands National Park of Canada and its surrounding pastures. Field data were collected in the 1999 growing season by measuring the grassland vertical profile in a fixed spatial array. A Landsat Enhanced Thematic Mapper Plus (ETM+) image was acquired for the same year. A grey level co-occurrence matrix (GLCM) texture analysis was applied to the Landsat ETM+ imagery to compare the grasslands under grazing and those under the conservation practice. The results derived from field measurement show that the variation of vertical structures of grasslands differ significantly under grazing and conservation management regimes. Optical remote sensing data could detect the spatial variation of grasslands under these two management practices. Texture analysis is effective at 15 m resolution, which confirmed other studies that grassland heterogeneity is at about 15 meter.
We describe the Sherbrooke positron emission tomograph, a very high resolution device dedicated to dynamic imaging of small laboratory animals. Its distinctive features are: small discrete scintillation detectors based on avalanche photodiodes (APD) to achieve uniform, isotropic, very high spatial resolution; parallel processing for low deadtime and high count rate capability; multispectral data acquisition hardware to improve sensitivity and scatter correction; modularity to allow design flexibility and upgradability. The system implements the "clam-shell" sampling scheme and a rotating rod transmission source. All acquisition parameters can be adjusted under computer control. Temperature stability at the detector site is ensured by the use of thermoelectric modules. The initial system consists of one layer of 256 modules (two rings of detectors) defining 3 image slices in a 118 mm diameter by 10.5 mm thick field. The axial Celd can be extended to 50 mm using 4 layers of modules (8 rings of detectors). The design constraints and engineering aspects of an APD-based PET scanner are reviewed and preliminary results are reported.
A novel integrated semiconductor photonic switch, based on carrier-induced refractive index changes, has been designed and fabricated for use at near infrared wavelengths (890-920 nm, 750-780 nm and 745-775 nm). These switches are intended for use in quantum sensors which rely on the spectroscopy of caesium, rubidium or potassium atoms respectively. The beam-steering properties of the 890-920 nm device are presented and its extinction ratio measured to be 13.4 dB. This measurement was limited by coupling efficiency. Subsequent changes made to the testing equipment include the implementation of an automated testing routine. This new experimental setup will facilitate the full characterisation of the 890-920 nm device and the newly fabricated optical switches, designed for operation in the wavelength ranges 750-780 nm and 745-775 nm respectively.
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