Spatial transformations of diffusion tensor magnetic resonance images

The authors address the problem of applying spatial transformations (or “image warps”) to diffusion tensor magnetic resonance images. The orientational information that these images contain must be handled appropriately when they are transformed spatially during image registration. The authors present solutions for global transformations of three-dimensional images up to 12-parameter affine complexity and indicate how their methods can be extended for higher order transformations. Several approaches are presented and tested using synthetic data. One method, the preservation of principal direction algorithm, which takes into account shearing, stretching and rigid rotation, is shown to be the most effective. Additional registration experiments are performed on human brain data obtained from a single subject, whose head was imaged in three different orientations within the scanner. All of the authors' methods improve the consistency between registered and target images over naive warping algorithms

Erosion-Corrosion in Pipe Flows of Particle-Laden Liquids

The transmission of particle-bearing liquids in pipes has motivated continuing research into erosion mechanisms and the distribution of erosion rates over wetted surfaces. This chapter covers these initiatives with particular reference to erosion-corrosion modelling within bends and straight sections of cylindrical pipes manufactured in a variety of materials and transporting a variety of liquids. Erosion-corrosion modelling techniques such as submerged slurry jets and rotating cylinder electrodes have been used to study factors influencing material degradation. Improvements in computational fluid dynamics (CFD), such as the development of a moving deforming mesh (MDM) have improved the accuracy of CFD models in predicting pipe wall erosion rates. Combined discrete phase tracking approaches such as the CFD-DPM-DEM (discrete phase-discrete element model) have helped improve computational efficiency. Wall impact erosion models are calibrated using laboratory scale tests. Validation of CFD models using full-scale test data is rare, meaning their accuracy is still largely unreported. Material testing has helped to identify the resilience of prospective pipeline materials to erosion-corrosion, while modifications to internal geometry and pipe section have shown potential to improve erosion-corrosion resistance

SWI-informed diffusion tensor tractography

Introduction In diffusion tensor tractography (DTT), white matter structure is inferred in vivo by reconstructing fiber tracts from diffusion weighted images (DWI). Recently [1], white matter structure has also been shown at 7T using susceptibility weighted imaging (SWI) [2]. Most notably, SWI shows excellent contrast between the highly myelinated optic radiation (OR) and the surrounding white matter [3]. Because DTT attempts to reconstruct tracts from voxels orders of magnitude larger than the underlying substrate, it suffers from partial volume effects in voxels that contain multiple or incoherently oriented tracts, resulting in false positive and false negative tracts. Tractography might therefore benefit from the combination of the diffusion tensor with the white matter contrasts in SWI which can be obtained at a much higher resolution. We have adapted a DTT algorithm to include the structure tensor [4] of the SWI magnitude in order to improve tractography in locations where DWI and SWI provide complementary information. Methods DWI (3T; 32-ch coil; twice-refocused spin echo EPI; 61+7 gradient directions; b=1000 s/mm2; TR=8300 ms; TE=95 ms; matrix size=110x110; FOV=220x220 mm; slice thickness=2.0 mm; number of slices=64) and SWI (7T; 8-ch coil; sagittal orientation; TR=36 ms; TE=25 ms; flip angle=15°; matrix size=448x336; FOV=224x168 mm; slice thickness=0.5 mm; number of slices=208; BW=120 Hz/px; acquisition time=20 min) were recorded from a healthy volunteer. The DWI mean b0 image and SWI were bias field corrected and then coregistered with FSL using the normalized mutual information algorithm and weighting volumes. The diffusion tensor and structure tensor fields were reconstructed from the DWI and SWI volumes, respectively. The structure tensor was calculated as the partial derivatives [dxx,dxy,dxz; dyx,dyy,dyz; dzx,dzy,dzz] of the SWI magnitude and every structure tensor component was smoothed (FWHM = 2.5 mm). Tractography was performed using Camino (PICo; 5000 iterations; curvature threshold = 80º; FA threshold = 0.10; step size = 0.50 mm). The structure tensor information was incorporated by requiring that the tracking direction be in the plane orthogonal to the first eigenvector of the structure tensor (ST ϵ1). This plane is assumed to be aligned to the direction of the tract that causes the intensity variation. The tracking direction within this plane is determined as the projection of the diffusion tensor onto the plane. To avoid adapted tracking directions where the structure tensor was non-informative, it was used only if the first eigenvalue of the structure tensor (ST λ1) > 100. For evaluating the performance, seeds were placed in the OR posterior to the point where it merged with the splenium of the corpus callosum (SCC). Waypoints were created anterior to the split in both the OR and SCC. Fractions of streamlines crossing these waypoints were extracted for both DTT and SWI-informed DTT. Results and Discussion Although the main tracts were similar for both DTT and SWI-informed DTT, the algorithms often showed very different branching patterns and more subtle differences in the course of the tracts. Examples are provided in Fig 1&2. In Fig 1 a putatively more accurate tracking of the OR using SWI-informed DTT compared to DTT is shown after seeding in the posterior OR. The seed at the merging of the OR and the SCC resulted in markedly different results for DTT and SWI-informed DTT (Fig 2). Frontal branches emerged for SWI-informed DTT, but not for DTT. A presumably non-veridical split is seen in the SCC for SWI-informed DTT (black arrow), where the structure tensor seems to cause a bias towards the borders of the tract. Fractions of streamlines entering OR/SCC were 0.026 for DTT vs. 0.336 for SWI-informed DTT. Conclusions A modification of a method was proposed to overcome some limitations of diffusion tensor tractography. It was shown that the contrast within the white matter in susceptibility weighted images can provide additional information for tractography algorithms, leading to increased sensitivity at specific locations. To have an unambiguous validation of the findings of SWI-informed DTT, an ex vivo validation of white matter connectivity has to be performed. We have shown that SWI-informed DTT reveals white matter fiber tracts that were not found using standard DTT

Role of the metal cation in the dehydration of the microporous metal–organic frameworks CPO-27-M

The dehydration of the CPO-27-M (M-MOF-74, M = Zn, Co, Ni, Mg, Mn, Cu) metal-organic framework series has been investigated comprehensively using in situ variable temperature powder X-ray diffraction (VT-PXRD) and thermal analysis (TG) coupled with mass spectrometry (MS). Significant differences in the order of water desorption from different adsorption sites on heating are found with varying metal cation in the otherwise isostructural material. For all CPO-27-M (except M = Cu), water is bonded significantly more strongly to the accessible open metal sites, and these water molecules are only desorbed at higher temperatures than the other water molecules. CPO-27-Cu is an exception, where all water molecules desorb simultaneously and at much lower temperatures (below 340 K). MS and TG data show that all CPO-27-M start to release traces of CO2 already at 300–350 K, and thus long before bulk thermal decomposition is observed. Only for CPO-27-Co, the CO2 release is essentially constant on its baseline between 450 and 700 K, and it is the only CPO-27-M member that shows a stable plateau in the TG in this region. Additional rehydration studies on CPO-27-Co show that the MOF incorporates any water molecules present until the pores are fully loaded. CPO-27-Co consequently behaves as an efficient trap for any water present

Mapping complex cell morphology in the grey matter with double diffusion encoding MR: A simulation study

This paper investigates the impact of cell body (namely soma) size and branching of cellular projections on diffusion MR imaging (dMRI) and spectroscopy (dMRS) signals for both standard single diffusion encoding (SDE) and more advanced double diffusion encoding (DDE) measurements using numerical simulations. The aim is to investigate the ability of dMRI/dMRS to characterize the complex morphology of brain cells focusing on these two distinctive features of brain grey matter. To this end, we employ a recently developed computational framework to create three dimensional meshes of neuron-like structures for Monte Carlo simulations, using diffusion coefficients typical of water and brain metabolites. Modelling the cellular structure as realistically connected spherical soma and cylindrical cellular projections, we cover a wide range of combinations of sphere radii and branching order of cellular projections, characteristic of various grey matter cells. We assess the impact of spherical soma size and branching order on the b-value dependence of the SDE signal as well as the time dependence of the mean diffusivity (MD) and mean kurtosis (MK). Moreover, we also assess the impact of spherical soma size and branching order on the angular modulation of DDE signal at different mixing times, together with the mixing time dependence of the apparent microscopic anisotropy (μA), a promising contrast derived from DDE measurements. The SDE results show that spherical soma size has a measurable impact on both the b-value dependence of the SDE signal and the MD and MK diffusion time dependence for both water and metabolites. On the other hand, we show that branching order has little impact on either, especially for water. In contrast, the DDE results show that spherical soma size has a measurable impact on the DDE signal's angular modulation at short mixing times and the branching order of cellular projections significantly impacts the mixing time dependence of the DDE signal's angular modulation as well as of the derived μA, for both water and metabolites. Our results confirm that SDE based techniques may be sensitive to spherical soma size, and most importantly, show for the first time that DDE measurements may be more sensitive to the dendritic tree complexity (as parametrized by the branching order of cellular projections), paving the way for new ways of characterizing grey matter morphology, non-invasively using dMRS and potentially dMRI

Anderson-Yuval approach to the multichannel Kondo problem

We analyze the structure of the perturbation expansion of the general multichannel Kondo model with channel anisotropic exchange couplings and in the presence of an external magnetic field, generalizing to this case the Anderson-Yuval technique. For two channels, we are able to map the Kondo model onto a generalized resonant level model. Limiting cases in which the equivalent resonant level model is solvable are identified. The solution correctly captures the properties of the two channel Kondo model, and also allows an analytic description of the cross-over from the non Fermi liquid to the Fermi liquid behavior caused by the channel anisotropy.Comment: 23 pages, ReVTeX, 4 figures av. on reques

Interfaces with a single growth inhomogeneity and anchored boundaries

The dynamics of a one dimensional growth model involving attachment and detachment of particles is studied in the presence of a localized growth inhomogeneity along with anchored boundary conditions. At large times, the latter enforce an equilibrium stationary regime which allows for an exact calculation of roughening exponents. The stochastic evolution is related to a spin Hamiltonian whose spectrum gap embodies the dynamic scaling exponent of late stages. For vanishing gaps the interface can exhibit a slow morphological transition followed by a change of scaling regimes which are studied numerically. Instead, a faceting dynamics arises for gapful situations.Comment: REVTeX, 11 pages, 9 Postscript figure

Interacting one dimensional electron gas with open boundaries

We discuss the properties of interacting electrons on a finite chain with open boundary conditions. We extend the Haldane Luttinger liquid description to these systems and study how the presence of the boundaries modifies various correlation functions. In view of possible experimental applications to quantum wires, we analyse how tunneling measurements can reveal the underlying Luttinger liquid properties. The two terminal conductance is calculated. We also point out possible applications to quasi one dimensional materials and study the effects of magnetic impurities.Comment: 38 pages, ReVTeX, 7 figures (available upon request

Scaling Regimes, Crossovers, and Lattice Corrections in 2D Heisenberg Antiferromagnets

We study scaling behavior in 2D, S=1/2 and S=1 Heisenberg antiferromagnets using the data on full q-dependences of the equal time structure factor and the static susceptibility, calculated through high temperature expansions. We also carry out comparisons with a model of two coupled S=1/2 planes with the interlayer coupling tuned to the T=0 critical point. We separately determine the spin-wave velocity c and mass $m=c/\xi$, in addition to the correlation length, $\xi$, and find that c is temperature dependent; only for T\alt JS, it approaches its known T=0 value $c_0$. Despite this temperature dependent spin-wave velocity, full q- and $\omega$-dependences of the dynamical susceptibility $\chi(\bf q,\omega)$ agree with the universal scaling functions computable for the $\sigma$-model, for temperatures upto $T_0 \sim 0.6c_0/a$. Detailed comparisons show that below $T_0$ the S=1 model is in the renormalized classical (RC) regime, the two plane model is in the quantum critical (QC) regime, and the S=1/2 model exhibits a RC-QC crossover, centered at T=0.55J. In particular, for the S=1/2 model above this crossover and for the two-plane model at all T, the spin-wave mass is in excellent agreement with the universal QC prediction, $m\simeq 1.04\,T$. In contrast, for the S=1/2 model below the RC-QC crossover, and for the S=1 model at all T, the behavior agrees with the known RC expression. For all models nonuniversal behavior occurs above $T\sim 0.6c_0/a$. Our results strongly support the conjecture of Chubukov and Sachdev that the S=1/2 model is close to the T=0 critical point to exhibit QC behavior.Comment: 13 pages, REVTeX with attached PostScript (see file for addl info

Los métodos de diagnóstico de la sarna sarcóptica en cerdos

Poster apresentado no II Congreso Ibérico de Epidemiologia Veterinária, que decorreu em Barcelona, na FVUAB de 2 a 5 de Fevereiro de 2010.El ácaro astigmatídeo Sarcoptes scabiei (Figura 1), que causa la sarna, es una especie adaptada a diferentes hospedadores y con especial importancia en el cerdo. La sarna es una enfermedad parasitaria de la piel comunes en los animales estabulados o explotados en virtud de las malas condiciones de higiene y por lo general se produce a finales de invierno o principios de primavera. La importancia económica de la enfermedad se asocia con disminución en la producción, con la devaluación de los canales en el matadero y el uso continuo de acaricidas en los animales infectados (Damriyasa et al., 2004)