Since it was first introduced in the field of medical imaging in the early 1980s, MRI has become essential for the diagnosis and treatment of musculoskeletal conditions. Most imaging in the United States is performed on high-field (Ͼ1.0T), whole-body scanners. However, for reasons discussed below, imaging at low (Ͻ0.5T) and medium (0.5-1.0T) field strengths using small, low-cost, easily installed scanners in imaging centers and physicians' offices is gaining increasing popularity. Such scanners can be very useful for imaging the upper and lower extremities, from the shoulder to the fingers and the hips to the toes. In this review we provide an overview of the different available extremity scanners and their advantages and disadvantages, briefly review the literature regarding their use, and discuss our experience in using low-field extremity scanners to evaluate joints.

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In this paper, a constrained boundary integral equation is presented for the analysis of three-dimensional magnetostatic problems. This formulation uses a simple layer of charge on the interface surface to represent the permeable object. A constraining condition that sets the total integrated surface charge to zero is used to make this formulation unique. In addition, a method of generating exact analytical solutions for use in validating the boundary element code is presented. A test comparison using this method will be shown.

A single finite element formulation using the magnetic (H) field vector directly is proposed for analysis of electromagnetic fields throughout the frequency spectrum. Results for waveguide and cavity analysis, as well as recent solutions to benchmark low-frequency eddy current examples such as the ‘‘Bath cube,’’ demonstrate the flexibility of the formulation. Applying earlier finite element methods to vector Helmholtz or diffusion equation problems, various workers have obtained incorrect solutions because the eigenmode spectra of the discrete (finite element) operators for such problems may contain eigenvalues and eigenmodes which do not correspond to modes of the underlying continuum (physical) problem. Such ‘‘spurious’’ modes have long been documented in high-frequency modal analysis. They are clearly identified as the cause for error in deterministic problems. Error is avoided by employing finite element operators whose spectra contain no spurious components. Application of the formulation may be limited by computer round-off at matrix assembly which affects solenoidality of magnetic fields in the solutions. Furthermore, the extreme values encountered in low-frequency eddy current analyses lead to ill conditioning and information loss and subsequent unreliability of the solution. These numerical instabilities may be overcome by parametric adjustment of permittivities, or by increased computer word length.