A hybrid, inverse approach to the design of magnetic resonance imaging magnets

This paper describes a hybrid numerical method of an inverse approach to the design of compact magnetic resonance imaging magnets. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first, kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. The emphasis of this work is on the optimal design of short MRI magnets. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric MRI magnets as well as asymmetric magnets. The results highlight that the method can be used to obtain a compact MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1 m in length, significantly shorter than current designs. Viable asymmetric magnet designs, in which the edge of the homogeneous region is very close to one end of the magnet system are also presented. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)00303-5].

Current density mapping approach for design of clinical magnetic resonance imaging magnets

Novel current density mapping (CDM) schemes are developed for the design of new actively shielded, clinical magnetic resonance imaging (MRI) magnets. This is an extended inverse method in which the entire potential solution space for the superconductors has been considered, rather than single current density layers. The solution provides an insight into the required superconducting coil pattern for a desired magnet configuration. This information is then used as an initial set of parameters for the magnet structure, and a previously developed hybrid numerical optimization technique is used to obtain the final geometry of the magnet. The CDM scheme is applied to the design of compact symmetric, asymmetric, and open architecture 1.0-1.5 T MRI magnet systems of novel geometry and utility. A new symmetric 1.0-T system that is just I m in length with a full 50-cm diameter of the active, or sensitive, volume (DSV) is detailed, as well as an asymmetric system in which a 50-cm DSV begins just 14 cm from the end of the coil structure. Finally a 1.0-T open magnet system with a full 50-cm DSV is presented. These new designs provide clinically useful homogeneous regions and have appropriately restricted stray fields but, in some of the designs, the DSV is much closer to the end of the magnet system than in conventional designs. These new designs have the potential to reduce patient claustrophobia and improve physician access to patients undergoing scans. (C) 2002 Wiley Periodicals, Inc.

New Symmetric (61,16,4) Designs Invariant Under the Dihedral Group of Order 10

∗ This work has been partially supported by the Bulgarian NSF under Contract No. I-506/1995.

Konstruktion symmetrischer Designs

In dieser Arbeit werden zehn neue symmetrische (176,50,14) Designs und ein neues symmetrisches (144,66,30) Design durch Vorgabe von nichtauflösbaren Automorphismengruppen konstruiert. Im Jahre 1969 entdeckte G. Higman ein symmetrisches (176,50,14) Design, dessen volle Automorphismengruppe die sporadische einfache Gruppe HS der Ordnung 44.352.000 ist. Hier wurden nun Designs gesucht, die eine Untergruppe von HS zulassen. Folgende Untergruppen wurden betrachtet: die transitive und die intransitive Erweiterung einer elementarabelschen Gruppe der Ordnung 16 durch Alt(5), AGL(3,2), das direkte Produkt einer zyklischen Gruppe der Ordnung 5 mit Alt(5) und PSL(2,11). Die transitive Erweiterung von E(16) durch Alt(5) lieferte zwei neue Designs mit Automorphismengruppen der Ordnungen 960 bzw. 11.520; letzteres konnte auch mit der transitiven Erweiterung erhalten werden. Die Gruppe PSL(2,11) operiert auf den Punkten des Higman-Designs in drei Bahnen; sucht man nach symmetrischen (176,50,14) Designs, auf denen diese Gruppe in zwei Bahnen operiert, so erhält man acht neue Designs. Die übrigen Gruppen lieferten keine neuen Designs. Schließlich konnte ein neues symmetrisches (144,66,30) Design unter Verwendung der sporadischen Mathieu-Gruppe M(12) konstruiert werden. Dies war zu diesem Zeitpunkt außer dem Higman-Design das einzige bekannte symmetrische Design, dessen volle Automorphismengruppe im Wesentlichen eine sporadische einfache Gruppe ist.

Integrating tracking in concentrating photovoltaics using laterally moving optics

In this work the concept of integrating tracking in concentrating photovoltaics is briefly summarized and possible fields of application are classified. A previously proposed system setup relies on the use of two rotational symmetric laterally moving plano-convex lenses to achieve 500× concentration over an angular range of ±24 ◦ . However, the circular lens apertures are less suitable for application in lens array structures. A new design algorithm based on the Simultaneous Multiple Surface algorithm in three dimensions (SMS3D) demonstrates the ability to address this problem. Performance simulations show that the resulting non-rotational symmetric design outperforms its conventional rotational symmetric counterpart

Design of spherical symmetric gradient index lenses

Spherical symmetric refractive index distributions also known as Gradient Index lenses such as the Maxwell-Fish-Eye (MFE), the Luneburg or the Eaton lenses have always played an important role in Optics. The recent development of the technique called Transformation Optics has renewed the interest in these gradient index lenses. For instance, Perfect Imaging within the Wave Optics framework has recently been proved using the MFE distribution. We review here the design problem of these lenses, classify them in two groups (Luneburg moveable-limits and fixed-limits type), and establish a new design techniques for each type of problem.

Preliminary design of symmetric arch-gravity dams

In this paper, a set of design parameters, such as the slopes of upstream and downstream faces of the dam, radius of the upper arch, width of the dam at the top level and height of the vertical upper part of the dam, are given as function of the valley characteristics when the dam is situated, such as its geometry and its geotechnical properties. These tables have been obtained using a regression of the design parameters of an arch-gravity dam with a minimum concrete volume, placed in a large number of valleys with different characteristics and properties. Elasticites for these design parameters are also discussed.

Topology Optimized Design, Microfabrication and Characterization of Electro-Thermally Driven Microgripper

This article presents a systematic and logical study of the topology optimized design, microfabrication, and static/dynamic performance characterization of an electro-thermo-mechanical microgripper. The microgripper is designed using a topology optimization algorithm based on a spatial filtering technique and considering different penalization coefficients for different material properties during the optimization cycle. The microgripper design has a symmetric monolithic 2D structure which consists of a complex combination of rigid links integrating both the actuating and gripping mechanisms. The numerical simulation is performed by studying the effects of convective heat transfer, thermal boundary conditions at the fixed anchors, and microgripper performance considering temperature-dependent and independent material properties. The microgripper is fabricated from a 25 mm thick nickel foil using laser microfabrication technology and its static/dynamic performance is experimentally evaluated. The static and dynamic electro-mechanical characteristics are analyzed as step response functions with respect to tweezing/actuating displacements, applied current/power, and actual electric resistance. A microgripper prototype having overall dimensions of 1mm (L) X 2.5mm (W) is able to deliver the maximum tweezing and actuating displacements of 25.5 mm and 33.2 mm along X and Y axes, respectively, under an applied power of 2.32 W. Experimental performance is compared with finite element modeling simulation results.

Reconstruction of large cranial defects in nonimmunosuppressed experimental design with human dental pulp stem cells

The main aim of this study is to evaluate the capacity of human dental pulp stem cells (hDPSC), isolated from deciduous teeth, to reconstruct large-sized cranial bone defects in nonimmunosuppressed (NIS) rats. To our knowledge, these cells were not used before in similar experiments. We performed two symmetric full-thickness cranial defects (5 x 8 mm) on each parietal region of eight NIS rats. In six of them, the left side was supplied with collagen membrane only and the right side (RS) with collagen membrane and hDPSC. In two rats, the RS had collagen membrane only and nothing was added at the left side (controls). Cells were used after in vitro characterization as mesenchymal cells. Animals were euthanized at 7, 20, 30, 60, and 120 days postoperatively and cranial tissue samples were taken from the defects for histologic analysis. Analysis of the presence of human cells in the new bone was confirmed by molecular analysis. The hDPSC lineage was positive for the four mesenchymal cell markers tested and showed osteogenic, adipogenic, and myogenic in vitro differentiation. We observed bone formation 1 month after surgery in both sides, but a more mature bone was present in the RS. Human DNA was polymerase chain reaction-amplified only at the RS, indicating that this new bone had human cells. The us e of hDPSC in NIS rats did not cause any graft. rejection. Our findings suggest that hDPSC is an additional cell resource for correcting large cranial defects in rats and constitutes a promising model for reconstruction of human large cranial defects in craniofacial surgery.

An inverse design method for elliptical MRI magnets

An inverse, current density mapping (CDM) method has been developed for the design of elliptical cross-section MRI magnets. The method provides a rapid prototyping system for unusual magnet designs, as it generates a 3D current density in response to a set of target field and geometric constraints. The emphasis of this work is on the investigation of new elliptical coil structures for clinical MRI magnets. The effect of the elliptical aspect ratio on magnet performance is investigated. Viable designs are generated for symmetric, asymmetric and open architecture elliptical magnets using the new method. Clinically relevant attributes such as reduced stray field and large homogeneous regions relative to total magnet length are included in the design process and investigated in detail. The preliminary magnet designs have several novel features.

Compact clinical MRI magnet design using a multilayer current density approach

In this work, a new method of optimization is successfully applied to the theoretical design of compact, actively shielded, clinical MRI magnets. The problem is formulated as a two-step process in which the desired current densities on multiple, cc-axial surface layers are first calculated by solving Fredholm equations of the first kind. Non-linear optimization methods with inequality constraints are then invoked to fit practical magnet coils to the desired current densities. The current density approach allows rapid prototyping of unusual magnet designs. The emphasis of this work is on the optimal design of short, actively-shielded MRI magnets for whole-body imaging. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric, and asymmetric MRI magnets. Magnet designs are presented for actively-shielded, symmetric magnets of coil length 1.0 m, which is considerably shorter than currently available designs of comparable dsv size. Novel, actively-shielded, asymmetric magnet designs are also presented in which the beginning of a 50-cm dsv is positioned just 11 cm from the end of the coil structure, allowing much improved access to the patient and reduced patient claustrophobia. Magn Reson Med 45:331540, 2001. (C) 2001 Wiley-Liss, Inc.

A design method for superconducting MRI magnets with ferromagnetic material

The emphasis of this work is on the optimal design of MRI magnets with both superconducting coils and ferromagnetic rings. The work is directed to the automated design of MRI magnet systems containing superconducting wire and both `cold' and `warm' iron. Details of the optimization procedure are given and the results show combined superconducting and iron material MRI magnets with excellent field characteristics. Strong, homogeneous central magnetic fields are produced with little stray or external field leakage. The field calculations are performed using a semi-analytical method for both current coil and iron material sources. Design examples for symmetric, open and asymmetric clinical MRI magnets containing both superconducting coils and ferromagnetic material are presented.

Elicited bid functions in (a)symmetric first-price auctions

We report on a series of experiments that examine bidding behavior in first-price sealed bid auctions with symmetric and asymmetric bidders. To study the extent of strategic behavior, we use an experimental design that elicits bidders' complete bid functions in each round (auction) of the experiment. In the aggregate, behavior is consistent with the basic equilibrium predictions for risk neutral or homogenous risk averse bidders (extent of bid shading, average seller's revenues and deviations from equilibrium). However, when we look at the extent of best reply behavior and the shape of bid functions, we find that individual behavior is not in line with the received equilibrium models, although it exhibits strategic sophistication.