An FDTD model for calculation of gradient-induced eddy currents in MRI system

In most magnetic resonance imaging (MRI) systems, pulsed magnetic gradient fields induce eddy currents in the conducting structures of the superconducting magnet. The eddy currents induced in structures within the cryostat are particularly problematic as they are characterized by long time constants by virtue of the low resistivity of the conductors. This paper presents a three-dimensional (3-D) finite-difference time-domain (FDTD) scheme in cylindrical coordinates for eddy-current calculation in conductors. This model is intended to be part of a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The singularity apparent in the governing equations is removed by using a series expansion method and the conductor-air boundary condition is handled using a variant of the surface impedance concept. The numerical difficulty due to the asymmetry of Maxwell equations for low-frequency eddy-current problems is circumvented by taking advantage of the known penetration behavior of the eddy-current fields. A perfectly matched layer absorbing boundary condition in 3-D cylindrical coordinates is also incorporated. The numerical method has been verified against analytical solutions for simple cases. Finally, the algorithm is illustrated by modeling a pulsed field gradient coil system within an MRI magnet system. The results demonstrate that the proposed FDTD scheme can be used to calculate large-scale eddy-current problems in materials with high conductivity at low frequencies.

Minimal instrumentation may compromise failure diagnosis with an ecological interface

Interfaces designed according to ecological interface design (EID) display higher-order relations and properties of a work domain so that adaptive operator problem solving can be better supported under unanticipated system conditions. Previous empirical studies of EID have assumed that the raw data required to derive and communicate higher-order information would be available and reliable. The present research examines the relative advantages of an EID interface over a conventional piping-and-instrumentation diagram (PID) when instrumentation is maximally or only minimally adequate. Results show an interaction between interface and the adequacy of the instrumentation. Failure diagnosis performance with the EID interface with maximally adequate instrumentation is best overall. Performance with the EID interface drops more drastically from maximally to minimally adequate instrumentation than does performance with the PID interface, to the point where the EID interface with minimally adequate instrumentation supports nonsignificantly worse performance than does the equivalent PID interface. Actual or potential applications of this research include design of instrumentation and displays for complex industrial processes.

A prospective assessment of SRS-24 scores after endoscopic anterior instrumentation for scoliosis

Study Design. Prospective clinical case series. Objective. To evaluate the clinical outcome of anterior endoscopic instrumention for scoliosis using the SRS-24 questionnaire and to examine how these scores change over a 2-year follow-up period. Summary of Background Data. Anterior endoscopic instrumentation correction has several advantages compared with open procedures. However, the clinical results of this technique using a validated outcome measure have rarely been reported in the literature. Methods. A total of 83 consecutive patients underwent endoscopic anterior instrumentation performed at a single unit. Patients completed the SRS-24 questionnaire before surgery and at 3, 6, 12, and 24 months after surgery. The SRS-24 scores were compared between each of the follow-up intervals. Results. The pain, general self-image, and function from back condition domains improved after surgery (P < 0.05). Activity level significantly improved between 3 and 6 months, and both function domains improved between 6 and 12 months (P < 0.05). None of the domains increased significantly after 1 year. Conclusions. Endoscopic anterior instrumentation for scoliosis significantly improved pain, self-image, and function. The greatest improvement in function occurred between 6 and 12 months after surgery. The SRS-24 scores at 1 year from surgery may provide a good indicator of patient outcome in the long-term.