19 research outputs found
Positron kinetics in an idealized PET environment
The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the 'gas-phase' assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations
Dry and Humid Periods Reconstructed from Tree Rings in the Former Territory of Sogdiana (Central Asia) and Their Socio-economic Consequences over the Last Millennium
One of the richest societies along the Silk Road developed in Sogdiana,
located in present-day Tajikistan, Uzbekistan, and Kyrgyzstan. This urban civilisation
reached its greatest prosperity during the golden age of the Silk Road (sixth to
ninth century ce). Rapid political and economic changes, accelerated by climatic
variations, were observed during last millennium in this region. The newly developed
tree-ring-based reconstruction of precipitation for the pastmillennium revealed
a series of dry and wet stages. During the Medieval Climate Anomaly (MCA), two
dry periods occurred (900–1000 and 1200–1250), interrupted by a phase of wetter
conditions. Distinct dry periods occurred around 1510–1650, 1750–1850, and
1920–1970, respectively. The juniper tree-ring record of moisture changes revealed
that major dry and pluvial episodes were consistent with those indicated by hydroclimatic
proxy data from adjacent areas. These climate fluctuations have had longand
short term consequences for human history in the territory of former Sogdiana
Aberrant B-lymphocyte responses in lupus: inherent or induced and potential therapeutic targets
Isenberg and R. A. Mageed are supported by ArthritisResearch-UK