A novel target-field method for finite-length magnetic resonance shim coils: I. Zonal shims

This paper presents a new approach for the design of genuinely finite-length shim and gradient coils, intended for use in magnetic resonance imaging equipment. A cylindrical target region is located asymmetrically, at an arbitrary position within a coil of finite length. A desired target field is specified on the surface of that region, and a method is given that enables winding patterns on the surface of the coil to be designed, to produce the desired field at the inner target region. The method uses a minimization technique combined with regularization, to find the current density on the surface of the coil. The method is illustrated for linear, quadratic and cubic magnetic target fields located asymmetrically within a finite-length coil.

An inverse design of an open, head/neck RF coil for MRI

Radio-frequency (RF) coils are a necessary component of magnetic resonance imaging (MRI) systems. When used in transmit operation, they act to generate a homogeneous RF magnetic field within a volume of interest and when in receive operation, they act to receive the nuclear magnetic resonance signal from the RF-excited specimen. This paper outlines a procedure for the design of open RF coils using the time-harmonic inverse method. This method entails the calculation of an ideal current density on a multipaned planar surface that would generate a specified magnetic field within the volume of interest. Because of the averaging effect of the regularization technique in the matrix solution, the specified magnetic field is shaped within an iterative procedure until the generated magnetic field matches the desired magnetic field. The stream-function technique is used to ascertain conductor positions and a method of moments package is then used to finalize the design. An open head/neck coil was designed to operate in a clinical 2T MRI system and the presented results prove the efficacy of this design methodology.

Dental and skeletal pathology in lungfish jaws and tooth plates

Many lungfish of the tooth plated lineage, both fossil and living, are affected by dental and skeletal pathologies including dental caries, abscesses and cysts within the bone or tooth plate, osteopenia, bone hypertrophy, and malocclusion. These conditions, while influenced in part by structural relationships of soft and hard tissues in the tooth plates, jaw bones and surrounding oral tissues, can also be used as indicators of the kind of environment inhabited by the fish. The disease processes have specific structural consequences, related either to the pathology or to attempts to heal the damage, and usually alter the form and function of the tooth plate or bone. Consequently they can be distinguished from postmortem diagenetic or taphonomic effects, which alter the structure in less specific ways and show no sign of healing. Dental caries, the most common pathological condition in dipnoan dentitions, is recognisable in lungfish from the Devonian of Western Australia, the Tertiary of South Australia and the Northern Territory and from living lungfish in south east Queensland. Other pathologies have a more sporadic occurrence.

Ultrastructure of developing tooth plates in the Australian lungfish, Neoceratodus forsteri (Osteichthyes : Dipnoi)

While the lungfish dentition is partially understood as far as morphology and light microscopic structure is concerned, the ultrastructure is not. Each tooth plate is associated with a dental lamina that develops from the inner layer of endodermal cells that form the oral epithelium. Dentines, bone and cartilage of the jaws differentiate from mesenchyme cells aggregating beneath the oral endothelium. Enamel, in the developing and in the mature form, has similarities to that of other early vertebrates, but unusual characters appear as development proceeds. Ameloblasts are capable of secreting enamel, and, with mononuclear osteoclasts, of remodelling the bone below the tooth plate. The forms of dentine, all based largely on an extracellular matrix of collagen and mineralised with biological apatite, differ from each other and from the underlying bone in the ultrastructure of associated cells and in the mineralised extracellular matrices produced. Cell processes emerging from the odontoblasts and from the osteoblasts vary in length, degree of branching and of anastomoses between the processes, although all of the cell types have large amounts of rough endoplasmic reticulum. Mineralisation of the extracellular matrices varies among the enamel, dentines and bone in the tooth plate. In addition, the development of the hard tissues of the tooth plates indicates that many of the similarities in fine structure of the dentition in lungfish, to tissues in other fish and amphibia, apparent early in development, disappear as the dentition matures. (C) 2003 Elsevier Ltd. All rights reserved.

Developmental anomalies in the tooth plates and jaw bones of lungfish

Lungfish of the tooth-plated lineage, both fossil and living, may be affected by alterations in the permanent tooth plates and associated jaw bones as they grow. In a few taxa, the unusual structures may be so common that they must be considered as normal for those species, or as a variation of the normal condition. In others the condition is rare, affecting only a few individuals. Variations, or anomalies, may appear in the growing tissues of the lungfish tooth plate at any time in the life cycle, although they usually appear early in development. Once the changes appear, they persist in the dentition. The altered structures include divided or intercalated ridges, short ridge anomaly, changes in the shape, number and position of cusps, pattern loss, and fused ridges or cusps. Criteria used to distinguish alteration from normal conditions are the incidence of the character in the population, the associated changes in the jaw bone, and the position of the altered structure in the tooth plate. The occurrence of similar changes across a wide range of different species suggests that they may have a genetic cause, especially when they are a rare occurrence in most taxa, but common enough to be a part of the normal variation in others. Prevalence of related anomalies throughout the history of the group suggests that dipnoans of the tooth-plated lineage are closely related, despite significant differences in morphology, microstructure, and function of the denfitions.