Radial imaging with multipolar magnetic encoding fields

We present reconstruction methods for radial magnetic resonance imaging (MRI) data which were spatially encoded using a pair of orthogonal multipolar magnetic fields for in-plane encoding and parallel imaging. It is shown that a direct method exists in addition to iterative reconstruction. Standard direct projection reconstruction algorithms can be combined with a previously developed direct reconstruction for multipolar encoding fields acquired with Cartesian trajectories. The algorithm is simplified by recasting the reconstruction problem into polar coordinates. In this formulation distortion and aliasing become separate effects. Distortion occurs only along the radial direction and aliasing along the azimuthal direction. Moreover, aliased points are equidistantly distributed in this representation, and, consequently, Cartesian SENSE is directly applicable with highly effective unfolding properties for radio-frequency coils arranged with a radial symmetry. The direct and iterative methods are applied to simulated data to analyze basic properties of the algorithms and for the first time also measured in vivo data are presented. The results are compared to linear spatial encoding using a radial trajectory and quadrupolar encoding using a Cartesian trajectory. The direct reconstruction gives good results for fully sampled datasets. Undersampled datasets, however, show star-shaped artifacts, which are significantly reduced with the iterative reconstruction

Settlement Scaling in the Northern Maya Lowlands: Human-Scale Implications

Settlement scaling theory predicts that higher site densities lead to increased social interactions that, in turn, boost productivity. The scaling relationship between population and land area holds for several ancient societies, but as demonstrated by the sample of 48 sites in this study, it does not hold for the Northern Maya Lowlands. Removing smaller sites from the sample brings the results closer to scaling expectations. We argue that applications of scaling theory benefit by considering social interaction as a product not only of proximity but also of daily life and spatial layouts. Investigadores de relaciones de escala en asentamientos predicen que densidades altas resultan en el aumento de interacciones social, lo cual estimula productividad. Relaciones de escala entre población y área de asentamiento se manifiestan para varias sociedades antiguas pero, como se ve en nuestra muestra de 48 sitios, no se manifiestan para el norte de la Península de Yucatán. Quitando sitios pequeños produce resultados más semejantes a las expectativas de escala. Aplicaciones de relaciones de escala tienen que considerar interacciones sociales como producto no solamente de proximidad sino de la vida cotidiana y patrones de espacio.</p