A technique for producing geometrically accurate magnetic resonance images (MRIs) with undistorted intensity in the face of high levels of static field inhomogeneity arising from either source is presented. The technique requires the acquisition of two images of the same object with altered gradients. On the basis of a knowledge of these gradients it employs an automatic postprocessing step that exploits some invariant characteristics of the distortions to produce a rectified image from the two acquired images. No phantom imaging is involved and no operator interaction is required. The technique is theoretically justified and compared to other techniques, and experimental results that show that the technique works are presented. The improved accuracy in geometry and intensity may improve reliability of stereotactic surgery, may enhance the feasibility of both clinical and industrial imaging via external fields, and may increase the resolution of microscopic imaging.

A general and straightforward model was developed for the design of passively -switched lasers. With the secondthreshold criterion and using a numerically fitting procedure, the output pulse energy was expressed as an analytical function of the initial transmission of the saturable absorber and the reflectivity of the output coupler. An analytical expression for the optimal output reflectivity was also obtained for maximizing the output pulse energy of a passively -switched laser with a given initial transmission of the saturable absorber. Excellent agreement was studied between the present results and detailed theoretical computations. A Nd:YAG laser with Cr 4+ :YAG as a saturable absorber was performed to illustrate the use of the present model. Index Terms-Passively -switched laser, saturable absorber, solid-state laser, Cr 4+ :YAG.

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We studied the finite-size efFect on the first-order metal-insulator phase transition and the accompanying tetragonal-to-monoclinic structural transition of VO2 films. The VO2 films were epitaxially grown by a metal-organic chemical-vapor-deposition technique on the (101) growth plane of a 125-A-thick Ti02 buffer layer which was also epitaxially predeposited on polished sapphire (1120) substrates. The thickness of the VO2 films in this study ranges from 60 to 310 A. We find that VO2 films grow isomorphically on the Ti02 bufFer layer resulting in a high degree of epitaxial VO2 films.We determined structural correlation lengths of the VO2 films parallel and normal to the growth plane from the x-ray-diÃraction widths of VO2 reflections at room temperature.The structural order parameter associated with the monoclinic distortion and the change in resistivity associated with the metal-insulator phase transition were simultaneously measured using x-ray-diKraction and resistivity measurements. It was found that the transition temperature, width of the transition, and the estimated electronic gap are dependent on the structural correlation length normal to the growth plane. These dependences are discussed in terms of finite-size and substrate efFects on the first-order phase transition.

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An efficient Q-switched dual-wavelength laser with self-frequency Raman conversion in composite Nd:YVO(4) and intracavity sum-frequency generation in BBO is reported. With an input pump power of 17.5 W, average power of 0.53 W at the first-Stokes 1176 nm and average power of 1.67 W at the sum-frequency mixed 559 nm are simultaneously generated at a pulse repetition rate of 100 kHz, corresponding to a total conversion efficiency of 12.5%.

A framework to estimate, with an intelligent and systematic approach, the applicability of a controller derived for a low-order model to a higher-order model is presented. This framework is applied to the test case of nonlinear feedback control of the two-dimensional unsteady separated flow past a plate perpendicular to the free-stream velocity. A hierarchy of models of increasing complexity is generated by using vortex methods. A low-order point vortex model and a high-order vortex blob model are used to simulate the roll-up of the separated shear layer. A nonlinear controller able to manipulate the wake by means of a suction point located on the downstream wall of the plate is given in closed form for the point vortex model. This controller is applied to the vortex blob model by defining a center of circulation. The topological equivalence of the phase spaces of the two models is verified. Finally, the two models are used to simulate the same flows using the same controller and the results are discussed.