The effect of concomitant gradient fields on MRI with long readout radial-based trajectories.

Abstract

Purpose: To demonstrate in theory and practice that concomitant gradient fields (CCGF) can produce substantial imaging artifacts in scans utilizing radial-based trajectories and to provide strategies to mitigate these effects.

Theory and methods: A framework was developed to relate concomitant gradient phase to local point-spread-function distortion, which was used to evaluate the effects of trajectory choice and imaging parameters on imaging artifacts. Gradient waveforms for realistic imaging scenarios were simulated and used to determine the effect of CCGF. Phantom and in vivo experiments were performed at 3 T to validate theoretical predictions.

Results: CCGF-induced artifacts are shown to be produced in part by increased variation in concomitant gradient phase across view angles. This is shown to increase with increasing gradient strength and contrast index in radial-based trajectories. Phase variation across view angles and the associated artifacts are shown to be effectively diminished via azimuthal rotation down the echo train in the helical EPI and helical stack-of-stars trajectories introduced in this work.

Conclusions: Concomitant gradient fields are found to produce non-negligible imaging artifacts in long readout radial-based trajectories due to variations in the associated phase accrual across view angles. Azimuthal rotation of the readout direction down the echo train, as implemented in the helical EPI and helical stack-of-stars trajectories, is shown in simulations, phantoms, and in vivo to mitigate these effects. Substantial improvement is seen in cases of nonaxial imaging with multiple contrasts, quickly varying B₀ inhomogeneity, and/or high gradient amplitudes.