[Brant goose colonies near snowy owls: internest distances in relation to lemming and arctic fox abundance].

Brant goose colonies around snowy owl nests have been studied near Meduza Bay (73 degrees 21' N, 80 degrees 32' E) and in the lower reaches of the Uboinaya River (73 degrees 37' N, 82 degrees 10' E), the northwestern Taimyr Peninsula, from 1999 to 2006. All brant nests within 680 m from an owl nest have been regarded as an individual colony. The results show that the area of the colony is always larger than the guarded area around the owl nest. In years of high abundance of lemmings, brant geese nest generally closer to the owl nest than in years of high abundance. When arctic foxes are abundant, however, brant geese nest significantly closer to owls than when the foxes are scarce, irrespective of lemming abundance. The mechanism of brant colony formation around owl nests is based on a number of stimuli.

Local-scale fluxes of carbon dioxide in urban environments: methodological challenges and results from Chicago.

Much attention is being directed to the measurement and modeling of surface-atmosphere exchanges of CO2 for different surface types. However, as yet, few measurements have been conducted in cities, even though these environments are widely acknowledged to be major sources of anthropogenic CO2. This paper highlights some of the challenges facing micrometeorologists attempting to use eddy covariance techniques to directly monitor CO2 fluxes in urban environments, focusing on the inherent variability within and between urban areas, and the importance of scale and the appropriate height of measurements. Results from a very short-term study of CO2 fluxes, undertaken in Chicago, Illinois in the summer of 1995, are presented. Mid-afternoon minimum CO2 concentrations and negative fluxes are attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources in a deep mixed layer. Poor night-time atmospheric mixing, lower mixed layer depths, biospheric respiration, and continued missions from mobile and fixed anthropogenic sources, account for the night-time maxima in CO2 concentrations. The need for more, longer-term, continuous eddy covariance measurements is stressed.

Potential effect of Anoplophora glabripennis (Coleoptera: Cerambycidae) on urban trees in the United States.

Anoplophora glabripennis Motschulsky, a wood borer native to Asia, was recently found in New York City and Chicago. In an attempt to eradicate these beetle populations, thousands of infested city trees have been removed. Field data from nine U.S. cities and national tree cover data were used to estimate the potential effects of A. glabripennis on urban resources through time. For the cities analyzed, the potential tree resources at risk to A. glabripennis attack based on host preferences, ranges from 12 to 61% of the city tree population, with an estimated value of $72 million-$2.3 billion per city. The corresponding canopy cover loss that would occur if all preferred host trees were killed ranges from 13-68%. The estimated maximum potential national urban impact of A. glabripennis is a loss of 34.9% of total canopy cover, 30.3% tree mortality (1.2 billion trees) and value loss of $669 billion.

Attenuation artifacts in SPECT: effect of wrap-around lung in 180 degrees cardiac studies.

UNLABELLED: We estimated that in 75%-90% of PET 82Rb patients the left lung appeared to wrap around the anterior aspect of the left ventricle. We used clinical PET 82Rb myocardial perfusion studies as the input to a SPECT computer simulation model to determine if patients with left lung wrap-around displayed consistent artifacts in reconstructed SPECT images. In particular, we sought an explanation for the hot lateral wall seen in SPECT images from normal female and male patients. METHODS: Attenuated SPECT 201Tl emission data were simulated from a mid-ventricular slice in 10 randomly selected clinical PET 82Rb studies with left lung wrap-around. In these same cases, the influence of left lung wrap-around was removed by assigning the left lung an attenuation coefficient which matched that of the heart. Five randomly selected clinical PET 82Rb studies without left lung wrap-around were also processed with our model. RESULTS: In all 10 cases with left lung wrap-around, reconstructed SPECT images showed the hot lateral wall artifact with a mean septal-to-lateral wall count ratio of 0.86. With left lung wrap-around removed in the same 10 patients, reconstructed images did not show hot lateral wall (mean septal-to-lateral wall count ratio = 1.07). The 5 cases without left lung wrap-around did not show hot lateral wall (mean septal-to-lateral wall count ratio = 1.04) and the ratios changed little with the filling of the left lung (mean septal-to-lateral wall count ratio = 1.05). CONCLUSION: Results of our PET-to-SPECT computer simulation model showed that the hot lateral wall artifact found in SPECT myocardial perfusion images was related to the orientation and positions of the left ventricle and the left lung.

Fundamentals of 180 degree acquisition and reconstruction in SPECT imaging.

The accuracy of the reconstructed images obtained from a 180 degrees SPECT acquisition is directly related to the effects of resolution and attenuation in the acquired projection data. Computer simulation studies show that the tomographic point spread functions and the quality of 201Tl myocardial perfusion transaxial images depend upon the specific 180 degrees arc used for reconstruction. Significant distortions are predicted in 201Tl myocardial images reconstructed from both 180 degrees and 360 degree scans; with signal to noise being significantly better for 180 degree scans. An anterior 180 degrees scan with a starting angle between right lateral and 45 degrees RAO in 201Tl myocardial imaging is recommended. Reconstructed images acquired from 180 degrees and 360 degree elliptical orbits are predicted to show more distortion than those obtained from circular acquisitions.

Distance-weighted backprojection: a SPECT reconstruction technique.

A method of reconstruction of single-photon emission computed tomographic (SPECT) images--distance-weighted filtered backprojection--is described. The quality of SPECT images using the standard filtered backprojection algorithm for data obtained from 360 degrees acquisitions is limited by the loss of spatial resolution with distance, and the attenuation and scatter inherent in the planar images used for reconstruction. In contrast to the standard techniques in which a reconstructed pixel receives equal contributions from 180 degrees opposed views, distance--weighted filtered backprojection applies a variable weighting factor to the data such that the pixel receives greater weight from the closest planar views. Data derived from hot and cold spot phantoms as well as from various clinical studies show better spatial and contrast resolution with this new technique compared with the conventional 360 degrees algorithm.

Regional distribution of myocardial blood flow measured by single-photon emission tomography: comparison with in vitro counting.

An emission computed tomography system (SPECT), which uses a single large-field-of-view gamma camera, was evaluated for its ability to measure the relative distribution of myocardial blood flow and to assess the effect of attenuation, scatter, and cardiac motion on the tomographic images. Normalized regional myocardial counts from the SPECT images of the living dogs correlated closely with those from the anatomic slices and the samples counted at necropsy except for an over-estimate of tracer in the perfusion defect (SPECT) 57.7 compared to tissue count 32.1; p less than 0.05. The differences were less for the other imaging conditions. Heart and thorax motion, attenuation, and scatter contributed less than 25% to the over-estimate of defect counts. We conclude that the SPECT system accurately reflects regional distribution of myocardial blood flow except for overestimation of flow in regions of perfusion defects. Small perfusion defects might therefore be missed, but no artifactual defects are created.