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

Abstract

The magnetic field gradient waveform monitor (MFGM) technique permits characterization of the temporal evolution of magnetic field gradients in magnetic resonance (MR) instruments (MRIs). Knowledge of the gradient waveform performance permits the development of further techniques, such as gradient waveform pre‐equalization, that correct and optimize gradient waveform distortions due to eddy currents induced during the application of switched magnetic fields and other system limitations. The accuracy of the MFGM technique is important since the overall uncertainty of the gradient waveform measurement will propagate into an uncertainty in corrected gradient waveforms impacting the precision of the resulting MR/MRI measurements. The accuracy of MFGM is investigated through a treatment of the noise present in a MRI. A noisy receiver model provides the basis for characterization of the noise and permits examination of the overall impact of noise on the phase accumulated in a pure‐phase encoded MR signal. Ultimately, a relationship between the signal‐to‐noise ratio of a measurement and the corresponding MFGM uncertainty is developed. The theoretical development is supported through simulation in conjunction with experimental results. The propagation of uncertainties to gradient waveform pre‐equalization is also discussed.

Gradient waveform pre‐emphasis based on the gradient system transfer function

“…Such a predistortion technique can be described as “pre‐equalization” or “pre‐emphasis,” and is also used in many signal processing applications apart from MRI to precompensate distortions in signals. In MRI, the possibility of a predistortion technique, which aims to calculate the input gradient waveform needed to produce the ideal waveform, was presented in Ref , and the accuracy was investigated . However, the technique has not yet been applied to MRI sequences.…”

“…In MRI, the possibility of a predistortion technique, which aims to calculate the input gradient waveform needed to produce the ideal waveform, was presented in Ref 20, and the accuracy was investigated. 21 However, the technique has not yet been applied to MRI sequences.…”

A rapid and robust gradient measurement technique using dynamic single‐point imaging

“…The proposed SPI‐based gradient measurement technique does not require any special hardware, unlike other reported methods for PGM and MFM that need specialized equipment such as NMR field probes . Although these methods have been shown to be viable techniques to measure gradient waveforms, the use of external hardware adds complication and may be cost‐prohibitive.…”

Systematic image alteration due to phase accumulation during RF pulse excitation in pure phase encode magnetic resonance imaging

“…The effect of the phase accumulated during the RF pulse can be interpreted by adding a time evolution phase term associated with the RF pulse application to the nominal phase accumulated during the phase encoding time t p . Simulations by Goora et al indicated that this additional phase term has the same sign as that of the phase — . This additional phase can be defined with an effective pulse duration as shown in Equations and .…”

“…These factors suggest that additional phase accumulation alters the spatial information in phase encoding experiments and could lead to distortion of the reconstructed images. Recent work by Goora et al with related measurements of magnetic field gradient waveforms has motivated a reconsideration of this effect. Prior experiments conducted neglecting the phase accumulation during the RF pulse have been uniformly successful, suggesting that any effect on images obtained is minimal.…”