Effects of magnetic resonance imaging on cardiac pacemakers and electrodes

In phantom studies we investigated the effects of magnetic resonance imaging (MRI) on pacemakers and electrodes. Twenty-five electrodes were exposed to MRI in a 1.5T scanner with continuous registration of the temperature at the electrode tip. Eleven pacemakers (five single chamber and six dual chamber) were exposed to MRI. Pacemaker output was monitored to detect malfunction in VOO/DOO and VVI/DDD modes. A temperature increase at the electrode tip of up to 63.1 degrees C was observed during 90 seconds of scanning. In seven electrodes the temperature increase exceeded 15 degrees C. Although no pacemaker malfunctions were observed in asynchronous pacing mode (VOO/DOO), inhibition and rapid pacing were observed during spin-echo imaging if the pacemakers were set to VVI or DDD mode. Pacemaker function was not impaired during scanning with gradient-echo sequences. Next to pacemaker dysfunction, electrode heating has to be considered a possible adverse effect when exposing patients with pacemakers to MRI.     

Dynamic fields generated by rapid crack growth

Stresses and strains near a rapidly propagating crack tip are affected by the mass density of the material. This paper starts with a brief summary of analytical results for near-tip dynamic fields as predicted by linear elastic fracture mechanics. Next, exact expressions are derived for dynamic crack-line strains, for mode-III crack propagation in a nonlinear elastic material and in an elastic perfectly-plastic material. These expressions are valid on the crack line from the moving crack tip to the moving boundary with the region of linearly elastic deformation. For steady-state crack growth, a critical strain criterion is used to compute the relation between external load and crack tip speed. The required external load increases with crack-tip speed.     

Stiction issues and actuation of RF LIGA-MEMS variable capacitors

High aspect ratio variable capacitors have been fabricated using deep X-ray lithography and electroplating. Stiction phenomena applicable to high aspect ratio devices are presented, including the conditions for stiction to occur and the critical dimensions of structures. Actuation tests at 3 GHz are also presented and show a maximum capacitance of 0.86 pF with no actuation voltage and a minimum capacitance of 0.70 pF with an actuation voltage of 20 V just before pull-in, which gives a tuning range of 1.23:1. Corresponding Q-factor values are 49.3 and 70.8 respectively. After pull-in, the measured capacitance is 0.61 pF, corresponding to a tuning range of 1.41:1, with a maximum Q-factor of 102.9.     

Characterisation of defects in very high deep-etch X-ray lithography microstructures

 In deep X-ray lithography synchrotron radiation is applied to pattern several hundred micrometer thick resist layers. This technique has been used to obtain micro structures with an aspect ratio up to 100 and dimensions in the micrometer range. The structures are characterised by straight walls and a typical sidewall roughness of approximately 50 nm. To be able to fabricate n-coherent structures with any lateral shape and to have the possibility to use these resist microstructures in an additional electroforming process the resist is usually mounted on a ceramic or metallic substrate. Due to the different thermal expansion coefficients of the resist material and the substrate a developing temperature of 37 °C produces cracks in the resist structures depending on the microstructure design. These defects are not observed if the developing temperature is reduced to 20 °C. Better structure quality is obtained using the GG-developer instead of MIBK/IPA, but the developing rate is decreased. Measurements of the developing rate of PMMA in GG-developer at different temperatures show that the contrast of the developer-resist system is increased at 20 °C compared to 37 °C. Received: 25 August 1997/Accepted: 3 September 1997     

Total and local heat transfer from a smooth circular cylinder in cross-flow at high reynolds number

The total and local heat transfer from a smooth circular cylinder to the cross flow of air has been measured over the Reynolds number range 3 × 10⁴ < Re < 4 × 10⁶. The interaction between flow and heat transfer is discussed. In particular, the boundary-layer effects on the heat transfer, such as transition from laminar to turbulent flow or boundary-layer separation, are considered in conjunction with the distributions of local static pressure and skin friction.     

Elastodynamic near-tip fields for a crack propagating along the interface of two orthotropic solids

The elastodynamic stress field near a crack tip rapidly propagating along the interface between two dissimilar orthotropic elastic solids is solved numerically, for in-plane motion. The cartesian displacements are sought in the separated forms, $\text{r}{}^{\mathrm{p}}\text{U(θ)}$ and $\text{r}{}^{\mathrm{p}}\text{V(θ)}$, $\text{r}$ and θ being polar coordinates centered at the moving tip. This reduces the mathematical statement of the problem to two complex second-order linear ordinary differential equations for complex functions $\text{U(θ)}$ and $\text{V(θ)}$. By means of the finite difference method, a matrix eigenvalue problem of the type $\text{ΣA}{}_{\mathrm{ij}}\text{(p)X}{}_{\mathrm{j}}\text{= 0}$, is obtained where $\text{A}{}_{\mathrm{ij}}\text{(p)}$ are polynomials of the complex variable $\text{p}$ and $\text{X}{}_{\mathrm{j}}$, are complex unknowns. An iterative numerical scheme for determining $\text{Im(p)}$ is developed and the roots $\text{p}$ as well as angular stress and displacement distributions are calculated and plotted for various material combinations. Comparison with exact solutions for the case of dissimilar Isotropie solids indicates good accuracy of the numerical solution. The orthotropic nature of the materials is shown to have a significant effect on stress maximums.     

Impulsivity-reflectivity and cognitive development in preschoolers: A longitudinal analysis of developmental and trait variance.

Administered the Stanford-Binet Intelligence Scale short form, J. Wright's (1973) measure of impulsivity-reflectivity (IR), and a measure hypothesis behavior to 102 preschoolers. Results show that mental age was more appropriate than IQ as an index of developmental variance in IR. (PsycINFO Database Record (c) 2010 APA, all rights reserved)