Analysis of eddy currents in a gradient coil

To model the z-coil of an MRI-scanner, a set of circular loops of strips turned out to be a good approximation. The result of this ring model is a current distribution, which only depends on the axial direction. In reality, a z-coil shows a variation in the tangential direction as well, due to eddy currents induced by other coils. In order to take the dependence of the tangential direction into account, we introduce rectangular pieces of copper (called islands) in between the rings. In this paper the current distribution in a set of rings and islands, driven by an external applied source current is investigated. The source, and all excited fields, are time harmonic, and the frequency is low enough to allow for a quasi-static approximation. Due to induction eddy currents occur which form the so-called edge-effect. The edge-effect depends on the applied frequency and the distances between the strips, and causes higher impedances. From the Maxwell equations, an integral equation for the current distribution in the strips is derived. The Galerkin method is applied, using global basis functions to solve this integral equation. Using Legendre polynomials for the axial direction turns out to be an appropriate choice. It provides a fast convergence, so only a very restricted number of Legendre polynomials is needed.

Wave propagation in thin-walled aortic analogues

Research on wave propagation in liquid-filled vessels is often motivated by the need to understand arterial blood flow. Theoretical and experimental investigation of the propagation of waves in flexible tubes has been studied by many researchers. The analytical one-dimensional frequency domain wave theory has a great advantage of providing accurate results without the additional computational cost related to the modern time domain simulation models. For assessing the validity of analytical and numerical models well defined in-vitro experiments are of great importance. The objective of this paper is to present a frequency domain analytical model based on one-dimensional wave propagation theory and validate it against experimental data obtained for aortic analogues. The elastic and viscoelastic properties of the wall are included in the analytical model. The pressure, volumetric flow rate and wall distention obtained from the analytical model are compared with experimental data in two straight tubes with aortic relevance. The analytical results and the experimental measurements were found to be in good agreement when the viscoelastic properties of the wall are taken into account