Unoccupied states of individual silver clusters and chains on Ag(111)

Size-selected silver clusters on Ag(111) were fabricated with the tip of a scanning tunneling microscope. Unoccupied electron resonances give rise to image contrast and spectral features which shift toward the Fermi level with increasing cluster size. Linear assemblies exhibit higher resonance energies than equally sized compact assemblies. Density functional theory calculations reproduce the observed energies and enable an assignment of the resonances to hybridized atomic 5s and 5p orbitals with silver substrate states.Comment: 9 pages, 8 figure

Mixed biexcitons in single quantum wells

Biexcitonic complexes in a ZnSe single quantum well are investigated by spectrally resolved four-wave mixing (FWM). The formation of heavy-heavy-hole XXh and of mixed heavy-light-hole XXm biexcitons showing binding energies of Δh=4.8meV and Δm=2.8meV is identified by polarization selection rules. The coherent dynamics of the FWM response and the observed FWM intensity ratio between the XXh and XXm biexciton-induced nonlinear signals are in agreement with the solution of an extended optical Bloch equation

Understanding Hatch-Dependent Part Properties in SLS

Selective laser sintering of polymers (SLS) is on the verge from pure prototyping to producing individualized complex parts for series application. As the parts are generated layer-wise and the influence of process-parameters as well as part orientations are well-known, the aim of the paper is to point out the influence of the layer-wise manufacturing in dependence of the hatching strategy on the resulting part properties as these are constant process-steps. Therefore, tensile bars with different number of layers but constant layer-thickness were produced using different hatching strategies and investigated depending density, surface roughness and mechanical properties. The results showed a strong increase of the mechanical properties, ductile breaking behavior and part density as well as decreasing surface roughness with higher layer numbers as well as the hatching strategies. Therefore, the results point out significant interaction between constant process steps and resulting part properties.Mechanical Engineerin

Understanding the Influence of Energy-Density of the Layer Dependent Part Properties in Laser-Sintering of PA12

As the demand for individualization and complex parts is continuously growing, laser-sintering of polymers is on the edge from a pure prototyping technology to manufacturing parts for applications in series production. The influences on resulting parts and layer depending part properties are well known in the literature but the understanding of the interaction between process parameters and layer dependent properties is missing and limiting the dimensioning. Within this study, tensile bars with different amounts of layers and energy densities were produced and investigated for the resulting mechanical properties, roughness, density and the degree of particle melt. The results showed a strong interaction between the energy density and amount of layers, which results in differences in the fracture behavior as well as the mechanical properties. Therefore, the presented results enable the prediction of necessary part thickness for dimensioning thin parts with laser-sintering.Mechanical Engineerin

Towards patient specific catheter selection: Computation of aortic geometry based on fused MRI data

In coronary angiography, a catheter's tip has to be directed through the aorta towards the ostium - the region where the coronary arteries arise. Due to the anatomical variation in different humans, there is no common catheter which can be used for all patients. Thus, in a trial and error procedure cardiologists find a catheter that fits to the patient's anatomy. To replace this time consuming approach by providing a computer aided planning tool to be used prior to the intervention is the focus of our work. First of all, it is necessary for such a system to derive geometrical parameters for the patient's aorta as well as for the different available catheters. Based thereon, the best fitting catheter can be selected. In this paper, we discuss the first step: the computation of geometrical parameters from the patient's image data. Due to the setting defined by our clinical partner, two MRI data sets are acquired and should be used for the computation. This requires a specific image processing pipeline which we present here and which has to our knowledge not been proposed so far. Furthermore, we show first results obtained for real clinical data sets and discuss the subsequent steps for the development of the catheter selection tool

Aortic morphometry at endograft position as assessed by 3D image analysis affects risk of type I endoleak formation after TEVAR

PURPOSE The purpose of this study was to identify morphologic factors affecting type I endoleak formation and bird-beak configuration after thoracic endovascular aortic repair (TEVAR). METHODS Computed tomography (CT) data of 57 patients (40 males; median age, 66 years) undergoing TEVAR for thoracic aortic aneurysm (34 TAA, 19 TAAA) or penetrating aortic ulcer (n = 4) between 2001 and 2010 were retrospectively reviewed. In 28 patients, the Gore TAG® stent-graft was used, followed by the Medtronic Valiant® in 16 cases, the Medtronic Talent® in 8, and the Cook Zenith® in 5 cases. Proximal landing zone (PLZ) was in zone 1 in 13, zone 2 in 13, zone 3 in 23, and zone 4 in 8 patients. In 14 patients (25%), the procedure was urgent or emergent. In each case, pre- and postoperative CT angiography was analyzed using a dedicated image processing workstation and complimentary in-house developed software based on a 3D cylindrical intensity model to calculate aortic arch angulation and conicity of the landing zones (LZ). RESULTS Primary type Ia endoleak rate was 12% (7/57) and subsequent re-intervention rate was 86% (6/7). Left subclavian artery (LSA) coverage (p = 0.036) and conicity of the PLZ (5.9 vs. 2.6 mm; p = 0.016) were significantly associated with an increased type Ia endoleak rate. Bird-beak configuration was observed in 16 patients (28%) and was associated with a smaller radius of the aortic arch curvature (42 vs. 65 mm; p = 0.049). Type Ia endoleak was not associated with a bird-beak configuration (p = 0.388). Primary type Ib endoleak rate was 7% (4/57) and subsequent re-intervention rate was 100%. Conicity of the distal LZ was associated with an increased type Ib endoleak rate (8.3 vs. 2.6 mm; p = 0.038). CONCLUSIONS CT-based 3D aortic morphometry helps to identify risk factors of type I endoleak formation and bird-beak configuration during TEVAR. These factors were LSA coverage and conicity within the landing zones for type I endoleak formation and steep aortic angulation for bird-beak configuration