No abstract

We describe a scheme to generate the ground-state network for carbon clusters. Combining ' it is reported that structures for Cs2 and Cs4 always contain seven-member ring defects although we now know that a much more stable structure exists with only five-and six-membered rings. The energy to break the carbon bonds and rearrange the network is so high that it is difficult to reach the ground state unless one starts from a structure which already has the correct topology.To overcome the difficulties mentioned above, we devised a scheme for generating energetically favorable structures for large fullerenes. In this scheme, we first focus on generating the correct topological networks for energetically favorable structures. Instead of looking at the network connecting individual atoms, we focus our attention on the "face-dual" network obtained by linking the centers of each of the polygonal face of the cage structure. Since each atom of the fullerene is threefold coordinated, the face-dual network consists of a triangular mesh. For a fullerene with N atoms, the number of polygonal faces is N/2+2. We represent each of the faces by a point lying on a predetermined surface (a sphere for example) and obtain the ground-state network by simulated annealing. Initially these points are placed at random and interact with one another through a two-body repulsive potential. The face-dual network obtained can then be inverted to obtain the fullerene structure. In this scheme, whether a face is pentagonal or hexagonal is determined by the number of nearest-neighboring faces. Thus the change from pentagon to hexagon can occur easily with a small energy barrier and even for quite large number of atoms, the system does not get trapped in local metastable states. Also, this scheme distributes the faces as evenly as possible on the constrained surface and the resultant topological networks tend to separate the pentagonal faces as far apart as possible which makes the structures generated good candidates for low-energy fullerene structures. By varying the shape of the constraining surface (changing 7333

A polyacrylamide gel (PAG) containing bovine serum albumin (BSA) is introduced as a new tissue-mimicking phantom for the purpose of visualizing three-dimensional coagulation temperature distribution during radiofrequency ablation (RFA). The coagulation temperature of the phantom can be changed at the same range of biological tissue (50-60 degrees C) by adjusting the pH from 4.3 to 4.7. The phantom is transparent except in thermal coagulation regions which are ivory white. The physical properties of the phantom, such as density, electrical conductivity and specific heat capacity, are very favorable, similar to those of soft tissues. We illustrate the usefulness of the phantom in visualizing RFA lesions. This phantom has magnetic resonance properties which change drastically upon thermal coagulation, enabling its use for the characterization of RFA device, quality assurance, treatment planning and treatment verification. The PAG containing BSA, whose pH was adjusted from 4.3 to 4.7, is an attractive tissue-mimicking phantom suitable for RFA investigations.