Direct measurement of magnetic field gradient waveforms

Abstract

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.

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

“…The entire pulse sequence is repeated with specified temporal offsets in order to increase the temporal resolution of the measurement. The time‐domain representation of the MFGM signal when low flip angle RF pulses are employed may be represented as where γ is the gyromagnetic ratio, G is the gradient, t p is the phase encode time, t eff is the effect of the RF pulse being applied in the presence of the magnetic field gradient and is constant when low flip angle pulses are employed, a is the radius of the sphere that encloses the sample, b is the sample offset from the gradient isocenter, and j is the imaginary unit. The sign of the phase of this equation is consistent with but differs from our earlier work with the MFGM method .…”

“…However, the overall impact of SNR on the phase of the MR signal is also of importance. In particular, MFGM measurements are directly affected along with those that employ MFGM such as impulse response‐based gradient waveform pre‐equalization .…”

“…Impulse response‐based pre‐equalization employing MFGM will be affected due to the inherent uncertainty in the temporal evolution of the magnetic field gradient waveform. The effect of this uncertainty on the gradient waveform ultimately experienced by the sample following the application of pre‐equalization techniques is now considered.…”

“…Noise in acquired MR data is ubiquitous and approaches such as signal averaging and filtering are commonly employed to minimize its effect on the acquired data. The impact of noise on the phase of the MR signal is of particular concern for pure‐phase encode techniques such as the magnetic field gradient waveform monitor (MFGM) where the phase evolution of the measured signal represents the temporal evolution of the gradient waveform. A general approach to the characterization of noise for MR measurements employing pure‐phase encode techniques is presented along with its impact on the MFGM technique.…”

Quantification and compensation of eddy-current-induced magnetic-field gradients

“…Alternatively, the position-dependent frequency behavior from multiple, point-like samples and an array of microcoils can be used to map the field without requiring sample repositioning [22–23]. The point-like sample approach has more recently been applied to monitor arbitrary gradient-waveform performance [24] and extended using a highly-doped, disk-like 1-D sample [25]. (Note that the 1-D method developed in this report uses a non-doped, extended, rod-like, 1-D sample.)…”