An X-ray-based method for the determination of the contrast agent propagation in 3-D vessel structures

A method for the determination of the contrast-agent propagation in vessel trees is presented. A standard three-dimensional (3-D) rotational angiography procedure is performed to reconstruct the morphology of the contrast-filled vessel tree in a 3-D volume. An additional fluoroscopy projection series acquired with a fixed projection angle delivers the temporal information of the bolus propagating. The mapping of the propagation information from the two-dimensional projections to the 3-D image data set is the topic of this paper. A symbolic tree structure is built up that represents the vessel tree including bifurcations. Neighborhood relations between vessel pieces are given in three dimensions. This facilitates filtering procedures and plausibility controls of the resulting time dependent 3-D data set. The presented method proved to be accurate with phantom data and gives novel insight in the feeding structure of arterio-venous malformations and aneurysms.     

Implementation of an optical method for the real-time determination of uniaxial strain and vessel mechanics

The determination of the mechanical properties of vascular growth substrates has seen increasing interest in the bioengineering field. Mechanical features such as rupture strength, compliance characteristics, and viscoelastic properties of vascular grafts are important for their design and are indicative of their success in vivo. Thus a simple inexpensive measurement technique for these parameters would be useful. In this report we describe the implementation of an optical method for the measurement of vessel distention under a transluminal pressure gradient. It is based on the concept of laser light occlusion and allows for real time noncontact diameter measurements of hollow vessels < or = 2 cm. We demonstrate precise and reproducible measurements of diameter changes of less than 10 microm and, further, with the simultaneous determination of both strain and luminal pressure, were able to determine the elastic modulus of commercially available polymeric vessels. Comparison of the manufacturer specifications and our own measurement of the elastic modulus of these vessels, validate the effectiveness of our system. The advantages of this technique are its relative low cost, ease of implementation, high resolution, and flexibility stemming from its modular setup.     

Application of the method of lines to cylindrical inhomogeneous propagation structures

The method of lines is applied to the Helmholtz equation in cylindrical co-ordinates to analyse cylindrical inhomogeneous propagation structures such as finlines in circular waveguide housings. The numerical results for the frequency dependent propagation characteristics and the dominant mode characteristic impedance for the finline structures are presented.<>     

Dynamics of a contrast agent microbubble attached to an elastic wall

A modified Rayleigh-Plesset equation is derived to model the oscillation of a contrast agent microbubble attached to an elastic wall. The obtained equation shows that contact with the wall affects the bubble oscillation as if the bubble oscillated in a liquid with a changed (effective) density. Depending on the wall properties, the effective density can be either higher or lower than the real liquid density and hence the natural frequency of the attached bubble can be either lower or higher than the natural frequency of the same bubble in an unbounded liquid. Numerical simulations are made for a contrast bubble with shell properties similar to those used in the Marmottant shell model. The cases of a rigid wall and a plastic wall are compared. The properties of the plastic wall are set to correspond to walls of OptiCell chambers commonly used in experiments. It is shown that contact with the rigid wall decreases the natural frequency of the bubble as compared to its natural frequency in an unbounded liquid, whereas contact with the OptiCell wall increases the natural frequency of the bubble. Bubble resonance curves for three cases are compared: the bubble in an unbounded liquid; the bubble at a distance from an OptiCell wall; the bubble in contact with an OptiCell wall. Results obtained for a 2- μm -radius bubble insonified with a 10-cycle, 40 kPa, 2.1 MHz Gaussian pulse show that contact with the OptiCell wall leads to the following effects. The amplitude of the radial oscillation of the attached bubble is decreased by about 70% as compared to that of the same bubble in an unbounded liquid. The fundamental component in the spectrum of the scattered pressure of the attached bubble is decreased by 12 dB. A strong second harmonic appears in the spectrum of the scattered pressure of the attached bubble; its magnitude is about 11.5 dB higher than the level corresponding to the case of an unbounded liquid.     

A versatile method for analyzing paraxial optical propagation in dielectric structures

This paper presents a fast and accurate quasi-analytic model for studying optical field propagation in weakly guiding dielectric structures. The proposed efficient and versatile computational scheme is obtained by merging the Hermite-Gauss (HG) total field expansion with the numerical collocation method and is particularly suited for longitudinally nonuniform structures. By means of a quasilinearization scheme, the same procedure has also been successfully applied to the analysis of field propagation in Kerr-nonlinear media. The latter achievement gives an indication of the great potentialities offered by this straight forward method. Several examples are discussed in the paper and in all cases the results computed by the proposed method favorably compare with those from alternative methods.     

The determination of the propagation constant for the travelingwave in an infinite ground plane

The propagation constant for the travelling wave in a trough in an infinite ground plane is examined. The null-field integral is used to determine the electromagnetic field in the trough structure, and pulse basis functions give the distribution of the aperture fields. From this, the propagation constant is solved for, using the Newton-Raphson iterative scheme. Various sizes of geometries are examined. The far-field patterns are calculated and compared with other solutions, thereby validating the integral formulation which subsequently provided the propagation constant. Measurements of two trough geometries are performed to validate the theoretical results