Direct Measurement of Magnetic Field Gradient Waveforms

As eddy currents increase with gradient amplitude and faster slew rate, they have become a greater problem with the advent of higher-performance gradients in modern MRI scanners. Success in eddy current reduction techniques such as active gradient shielding and waveform pre-emphasis, however, require that the residual eddy currents must be measured with high accuracy for image improvement. Traditional MR based gradient calibration techniques, whether based on an entire FID or a gradient echo, measure the integral of gradient waveforms. We have however previously proposed a different category of methods, which employ pure phase encode FIDs and have proven advantageous in directly measuring the gradient waveforms. In this article, we review the basis of these pure phase encode methods. In keeping with the instructional nature of CMRA, we undertake this review by describing specific experiments and the line of the thought behind the experiments. The pure phase encode approach is sensitive to low amplitude gradients (0.001-1 G/cm), and also permits measurement of high amplitude gradients (10-300 G/cm). The inverse Fourier transform permits ready understanding of these pure phase encode methods. The accuracy of pure phase encode gradient measurement is significantly improved by a multiple FID point acquisition, which permits high temporal resolution of the gradient waveform. The accuracy of gradient measurements is also analyzed and improved through elimination of potential artifacts. As one example of the capability of these methods, waveform measurements were undertaken to reduce the repetition time TR for centric scan SPRITE experiments. (C) 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 349-360, 2010.