Manlio G. Abele

Stereotactic surgical system controlled by computed tomography.

The three-dimensional data obtained by computed tomographic (CT) scanning offer an advantage in using this imaging technique for stereotactic surgical procedures. This requires interfacing of CT image data with a stereotactic guide. In the performance of functional procedures where the surgical target must be identified from brain landmarks, such as the anterior and posterior commissures, an image reconstruction technique that presents in an image high spatial resolution structural information must be used. The description of a fully hardware- and software-interfaced CT-directed stereotactic surgical system is presented. The logic of operation and examples of images reconstructed with a high spatial resolution algorithm are illustrated. The experimentally determined measurement of an electrode tip localization with this system is within 1 pixel or +/- 0.5 mm in any direction.

Applications of yokeless flux confinement

A recently developed generalized technique for the production of large confined magnetic fields is used to design a number of novel permanent‐magnet flux sources. The simplest of these produces a transverse field in a long cavity of a rectangular cross section. In such structures, uniform fields that are of the order of the remanence of the material used are obtained easily. Applications include: NMR imaging, general electron device biasing, and the provision of bases for various periodic magnetic structures, such as wigglers, undulators, and twisters. Slight elaboration of such configurations can produce fields of up to three times the remanence. The fields of such structures can be readily adjusted by mechanical means, with continuous variation from zero to the maximum field. The wigglers and twisters afford instantaneous conversion from wiggler to twister and conversely. Adaptations of the basic principles can be used to produce radial, conical, and other unusual field orientations.

A general method for flux confinement in permanent‐magnet structures

This paper presents a general method for the design of permanent‐magnet structures that provide uniform magnetic fields of up to several teslas in cavities of arbitrary shape. The mathematical criteria that govern confinement of flux within both yoked and yokeless magnetic structures are defined. The design procedure affords an exact determination of the distribution and magnetization of the magnetic material required for generation and confinement of the desired field within a cavity of given geometry. The method is applicable to magnets composed of ferrites and high‐energy product materials for which the analysis can be based on a linear approximation to the demagnetization curve.

MAXIMALLY EFFICIENT PERMANENT MAGNET STRUCTURES

A general theory is developed for determining the most efficient magnetic structure that generates a specified magnetic field. The structure is assumed to be composed of linear, isotropic magnetized material, occupying a given geometrical region. The efficiency is quantified by a figure of merit defined as the external field energy produced by the magnet divided by the maximum amount of energy that can be stored by the magnetized material. A set of equations is derived that determines the distribution of remanence needed to maximize this figure of merit, and several theorems concerning maximally efficient structures are given. Simple two‐dimensional examples are used to illustrate the theory.

Generation Of Uniform High Fields With Magnetized Wedges

The properties of permanent magnets composed of uniformly magnetized wedges are presented in this paper. The magnets are fully open structures capable of generating strong fields exceeding the remanence of the material.

Field computation in permanent magnets

The authors present a method of field computation for permanent magnets designed to generate a uniform field in the region of interest. The computation technique is particularly suitable for magnetic structures used in magnetic resonance imaging. The method is applied to the design of yokeless, yoked, and hybrid prismatic magnetic structures. In particular, it is applied to the correction of the field inhomogeneity of open magnets by means of a modulation of the magnetic material and by the introduction of high-permeability materials. >

Optimum design of yokeless permanent magnets

This paper discusses the selection of the design parameters and the optimum geometry of yokeless permanent magnets that generate a uniform field H0 within an arbitrary closed cavity. The optimum design minimizes the volume of magnetic material and it is determined by the geometry of the cavity, the magnitude of intensity H0, and a characteristic point F that controls the configuration of the magnetostatic potential as well as the flux of the magnetic induction. The optimization procedure is applied to a two‐dimensional magnet with a quadrilateral cross section.

Generation of highly uniform magnetic fields with magnetized wedges

A novel method of producing a highly uniform magnetic field by means of permanently magnetized wedges is presented. The method uses a pair of central wedges to generate a primary field and pairs of compensating blocks to improve the field uniformity. The geometry of the central wedges and compensating blocks are determined by an analytic procedure. Using the method, magnetic structures can be designed that allow access to the uniform field over a solid angle of approximately 2/spl pi/ steradians.

Equivalent structures of permanent magnets and electric currents designed to generate uniform fields

Generation and confinement of a magnetic field by means of permanent magnets as well as time‐independent electric currents are analyzed, and the conditions of existence of uniform field solutions are established in the general case of structures that enclose a region of arbitrary geometry. Basic properties of distributions of the electric currents are compared with the properties of yokeless structures of permanent magnets intended to generate the same uniform field within the same region.

Compensation of non-uniform magnetic properties of components of a yokeless permanent magnet

The authors present a method for compensating for nonuniformity of magnetization in a yokeless permanent magnet designed for clinical applications of nuclear magnetic resonance (NMR). The field generated by the magnet is confined within the magnetic material without the use of an external yoke. The method involves two steps: compensation in the individual magnet components and shimming of the assembled magnet. The authors concentrate on the first step, and, in particular, the compensation of the lowest harmonics of the field. >