Measurements of the Solid-body Rotation of Anisotropic Particles in 3D Turbulence

We introduce a new method to measure Lagrangian vorticity and the rotational dynamics of anisotropic particles in a turbulent fluid flow. We use 3D printing technology to fabricate crosses (two perpendicular rods) and jacks (three mutually perpendicular rods). Time-resolved measurements of their orientation and solid-body rotation rate are obtained from stereoscopic video images of their motion in a turbulent flow between oscillating grids with $R_\lambda$=$91$. The advected particles have a largest dimension of 6 times the Kolmogorov length, making them a good approximation to anisotropic tracer particles. Crosses rotate like disks and jacks rotate like spheres, so these measurements, combined with previous measurements of tracer rods, allow experimental study of ellipsoids across the full range of aspect ratios. The measured mean square tumbling rate, $\langle \dot{p}_i \dot{p}_i \rangle$, confirms previous direct numerical simulations that indicate that disks tumble much more rapidly than rods. Measurements of the alignment of crosses with the direction of the solid-body rotation rate vector provide the first direct observation of the alignment of anisotropic particles by the velocity gradients of the flow.Comment: 15 pages, 7 figure

Nulling interferometry: impact of exozodiacal clouds on the performance of future life-finding space missions

Earth-sized planets around nearby stars are being detected for the first time by ground-based radial velocity and space-based transit surveys. This milestone is opening the path towards the definition of missions able to directly detect the light from these planets, with the identification of bio-signatures as one of the main objectives. In that respect, both ESA and NASA have identified nulling interferometry as one of the most promising techniques. The ability to study distant planets will however depend on exozodiacal dust clouds surrounding the target stars. In this paper, we assess the impact of exozodiacal dust clouds on the performance of an infrared nulling interferometer in the Emma X-array configuration. For the nominal mission architecture with 2-m aperture telescopes, we found that point-symmetric exozodiacal dust discs about 100 times denser than the solar zodiacal cloud can be tolerated in order to survey at least 150 targets during the mission lifetime. Considering modeled resonant structures created by an Earth-like planet orbiting at 1 AU around a Sun-like star, we show that the tolerable dust density for planet detection goes down to about 15 times the solar zodiacal density for face-on systems and decreases with the disc inclination. The upper limits on the tolerable exozodiacal dust density derived in this study must be considered as rather pessimistic, but still give a realistic estimation of the typical sensitivity that we will need to reach on exozodiacal discs in order to prepare the scientific programme of future Earth-like planet characterisation missions.Comment: 17 pages, accepted for publication in A&

Structure and Magnetization of Two-Dimensional Vortex Arrays in the Presence of Periodic Pinning

Ground-state properties of a two-dimensional system of superconducting vortices in the presence of a periodic array of strong pinning centers are studied analytically and numerically. The ground states of the vortex system at different filling ratios are found using a simple geometric argument under the assumption that the penetration depth is much smaller than the spacing of the pin lattice. The results of this calculation are confirmed by numerical studies in which simulated annealing is used to locate the ground states of the vortex system. The zero-temperature equilibrium magnetization as a function of the applied field is obtained by numerically calculating the energy of the ground state for a large number of closely spaced filling ratios. The results show interesting commensurability effects such as plateaus in the B-H diagram at simple fractional filling ratios.Comment: 12 pages, 19 figures, submitted for publicatio

Detecting Lesion Bounding Ellipses With Gaussian Proposal Networks

Lesions characterized by computed tomography (CT) scans, are arguably often elliptical objects. However, current lesion detection systems are predominantly adopted from the popular Region Proposal Networks (RPNs) that only propose bounding boxes without fully leveraging the elliptical geometry of lesions. In this paper, we present Gaussian Proposal Networks (GPNs), a novel extension to RPNs, to detect lesion bounding ellipses. Instead of directly regressing the rotation angle of the ellipse as the common practice, GPN represents bounding ellipses as 2D Gaussian distributions on the image plain and minimizes the Kullback-Leibler (KL) divergence between the proposed Gaussian and the ground truth Gaussian for object localization. We show the KL divergence loss approximately incarnates the regression loss in the RPN framework when the rotation angle is 0. Experiments on the DeepLesion dataset show that GPN significantly outperforms RPN for lesion bounding ellipse detection thanks to lower localization error. GPN is open sourced at https://github.com/baidu-research/GP

Common mistakes and pitfalls in magnetic resonance imaging of the knee

This pictorial review presents an overview of common interpretation errors and pitfalls in magnetic resonance imaging (MRI) of the knee. Instead of being exhaustive, we will emphasize those pitfalls that are most commonly encountered by young residents or less experienced radiologists

Prickle1 is required for EMT and migration of zebrafish cranial neural crest

The neural crest—a key innovation of the vertebrates—gives rise to diverse cell types including melanocytes, neurons and glia of the peripheral nervous system, and chondrocytes of the jaw and skull. Proper development of the cephalic region is dependent on the tightly-regulated specification and migration of cranial neural crest cells (NCCs). The core PCP proteins Frizzled and Disheveled have previously been implicated in NCC migration. Here we investigate the functions of the core PCP proteins Prickle1a and Prickle1b in zebrafish cranial NCC development. Using analysis of pk1a and pk1b mutant embryos, we uncover similar roles for both genes in facilitating cranial NCC migration. Disruption of either gene causes pre-migratory NCCs to cluster together at the dorsal aspect of the neural tube, where they adopt aberrant polarity and movement. Critically, in investigating Pk1-deficient cells that fail to migrate ventrolaterally, we have also uncovered roles for pk1a and pk1b in the epithelial-to-mesenchymal transition (EMT) of pre-migratory NCCs that precedes their collective migration to the periphery. Normally, during EMT, pre-migratory NCCs transition from a neuroepithelial to a bleb-based and subsequently, mesenchymal morphology capable of directed migration. When either Pk1a or Pk1b is disrupted, NCCs continue to perform blebbing behaviors characteristic of pre-migratory cells over extended time periods, indicating a block in a key transition during EMT. Although some Pk1-deficient NCCs transition successfully to mesenchymal, migratory morphologies, they fail to separate from neighboring NCCs. Additionally, Pk1b-deficient NCCs show elevated levels of E-Cadherin and reduced levels of N-Cadherin, suggesting that Prickle1 molecules regulate Cadherin levels to ensure the completion of EMT and the commencement of cranial NCC migration. We conclude that Pk1 plays crucial roles in cranial NCCs both during EMT and migration. These roles are dependent on the regulation of E-Cad and N-Cad