Rotational sandpile models: A finite size scaling study
Abstract Nontrivial critical behaviour is observed in equilibrium lattice statistical models under rotational constraint. Effect of rotational constraint on lattice statistical models at out of equilibrium situation is important to study. Critical properties of non-equilibrium steady state of sandpile models are studied under rotational constraint via finite size scaling (FSS) analysis. Not only FSS is found to be valid for a wide range of rotational sandpile models but also they are found to belong to the same universality class.
Breakdown of universality in directed spiral percolation
Abstract.Directed spiral percolation (DSP), percolation under both directional and rotational constraints, is studied on the triangular lattice in two dimensions (2D). The results are compared with that of the 2D square lattice. Clusters generated in this model are generally rarefied and have chiral dangling ends on both the square and triangular lattices. It is found that the clusters are more compact and less anisotropic on the triangular lattice than on the square lattice. The elongation of the clusters is in a different direction than the imposed directional constraint on both the lattices. The values of some of the critical exponents and fractal dimension are found considerably different on the two lattices. The DSP model then exhibits a breakdown of universality in 2D between the square and triangular lattices. The values of the critical exponents obtained for the triangular lattice are not only different from that of the square lattice but also different form other percolation models.
The effect of collimation on image quality and radiation safety in digital radiography of small animals
Collimating the primary x-ray beam to the specific area of anatomic interest has traditionally been strongly recommended to produce high quality radiographic images, even though some small animal patients can fit their entire torso on a single radiographic casette or detector. However, modern digital radiography may allow for whole-body projections for these patients at similar quality to specific collimated projections. Lateral and ventrodorsal projections of 31 small dogs were obtained to evaluate the effect of collimation on the quality of the resulting digital radiograph. Each projection was performed using both a collimated and whole-body technique. Scatter radiation was detected during each exposure as well to determine the effect of collimation on radiation safety for veterinary personnel. The resulting images were cropped to portray only the thorax, and were evaluated by five radiologists blinded to the procedure used for each image. Visual grading characteristics analysis was performed. An overall trend towards higher image quality was found in the collimated approach compared to the whole-body, but in most cases this change was not significant. Overall image quality for the ventrodorsal images was found to be significantly better in the collimated approach compared to the whole-body. Evaluation of image histograms suggests that the application of digital processing and the look-up table differs between them, and this may account for a large portion of any quality difference. Over twice as much scatter radiation was detected during the whole-body images as during the collimated images. Overall image quality was rated as adequate to high for almost all images with both approaches. Adequate collimation remains recommended to provide the highest quality images with the lowest exposure to veterinary personnel.