Eddy Current Compensation Research Articles

Software and hardware integration of a microprogrammable state machine for NMR imaging

We have integrated a commercially available microprogrammable state machine (Tecmag PULSkit) for use as a magnetic resonance pulse programmer. Providing the capability for active research environment imaging protocols, it features timing resolution of 100 nsec, ten 16-bit loop counters, and individually addressable look-up tables. This integration involved hardware and software integration with a VAX 11 750 at several levels. Hardware: Each of the three gradient channels employs three digital-to-analog converters (DACs). An 8-bit, 4-quadrant, multiplying DAC generates the gradient waveform shape. A 12-bit DAC generates the multiplying DAC scaling voltage, controlling gradient amplitude and sign. A third 12-bit DAC produces a gradient offset (shim) voltage. An eddy current compensation network is present for each gradient channel. Software: The software design philosophy was to create a flexible interface (interactive window environment), while not constraining complex manipulation of the hardware (direct use of the pulse-sequence compiler primitives and microprogramming). The software levels include (a) pulse-sequence microprogramming, (b) pulse-sequence compiler, (c) interactive parameter specification, and (d) canned pulse-sequence microcode library.

Optimization of eddy-current compensation

A comprehensive method for optimizing the preemphasis applied to pulsed gradients in NMR experiments in order to compensate induced eddy-current fields is presented. First, the eddy-current effects are measured without any compensation active. The eddy-current fields with spatial symmetries different from those of the pulsed gradient are also taken into account. Next, the measured response functions are analyzed with exponentially decaying terms. An exact solution of the required compensation is provided by numerical inversion of the response function using Laplace transformation. The calculated compensation terms are implemented in hardware. This yields a highly improved suppression of the eddy-current effects, including induced fields with spatial symmetries different from those of the switched gradient. The result is independent of the waveform or timing of the pulsed gradient and can compete with shielded gradient systems.

Eddy current compensation by direct field detection and digital gradient modification

We demonstrate an empirical magnetic field gradient waveform optimization technique which compensates for eddy current distortions and nonideal gradient amplifier performance. The desired gradient response is used as a trial function, and a pickup coil in the magnet bore detects the actual pulsed gradient. The temporal behavior of the gradient is digitized and recorded with the existing NMR spectrometer hardware and is used for the calculation of a new trial function table. The method and some typical results as well as some practical considerations are described.

Exact temporal eddy current compensation in magnetic resonance imaging systems

A step-response method has been developed to extract the properties (amplitudes and decay time constants) of intrinsic-eddy-current-sourced magnetic fields generated in whole-body magnetic resonance imaging systems when pulsed field gradients are applied. Exact compensation for the eddy-current effect is achieved through a polynomial rooting procedure and matrix inversion once the 2 N properties of the N-term decay process are known. The output of the inversion procedure yields the required characteristics of the filter for spectrum magnitude and phase equalization. The method is described for the general case along with experimental results for one-, two-, and three-term inversions. The method's usefulness is demonstrated for the usually difficult case of long-term (200-1000-ms) eddy-current compensation. Field-gradient spectral flatness measurements over 30 mHz-100 Hz are given to validate the method.

Eddy current compensation in toroidal deflection yokes

A toroidal yoke for an electron-beam lithography system is compensated for deflection placement errors that normally result from eddy currents generated within the coil windings, as well as driver current settling time and inductive or capacitive coupling within the yoke. Compensation is achieved through the addition of passive conductive material along the outer periphery of the yoke. Both theoretical and experimental results are presented. Agreement is shown to be very good in a specific E-beam lithography system.

Design study of toroidal deflection yokes with eddy-current compensation

Eddy-current-compensated toroidal deflection yokes are more convenient to precisely design and fabricate than their saddle-type equivalents. A simple and practical design criterion to obtain nominal compensation, which equalizes the (coil area)(ampere-turn) products for the inner and outer toroidal yokes, has been experimentally supported.

Eddy Current Torque Compensation in a Spin Stabilized Earth Satellite

Eddy Current Torque Compensation in a Spin Stabilized Earth Satellite