Ronnie Wirestam

PURPOSE: Deleterious effects from concomitant gradients are amplified at lower field strengths and stronger gradients. We aimed to show the signal errors caused by concomitant gradients across a wide range of hardware configurations and propose corrections that can be applied during experimental design and analysis.

THEORY AND METHODS: We derived a compact but accurate expression for concomitant gradients and simulated the associated signal bias across field strengths (0.03-7 T) and gradient amplitudes (20-600 mT/m) for asymmetric Maxwell-compensated diffusion encoding gradient waveforms. Bias was assigned to "concomitant dephasing" or "concomitant diffusion weighting." Concomitant dephasing was reduced by waveform correction that was deployed in the design stage. Concomitant diffusion weighting could be accounted for by using the actual gradient waveform during analysis.

RESULTS: Concomitant dephasing was caused by loss of Maxwell compensation during waveform resampling, with signal errors up to 100%, especially at low B0 and high gmax. Our waveform correction yielded a categorical improvement to signal accuracy, reducing bias to < 1% for most systems. However, bias at extreme B0-gmax-combinations remained > 1% (e.g., B0 = 3 T and gmax = 600 mT/m). Concomitant diffusion weighting caused biases above 1% at across all simulated MRI systems.

CONCLUSION: Concomitant gradients have a relevant impact on signal accuracy, especially at low fields and ultra-strong gradients. Concomitant dephasing must be considered for asymmetric waveforms, but can be largely suppressed by our correction method. Concomitant diffusion weighting is always present, regardless of waveform symmetry, but has a smaller impact and can be accounted for in the analysis.