Modelling the gradient system as linear and time-invariant: Do the assumptions break down at higher field strength?

The gradient system transfer function (GSTF) is a broadly used model to characterize the gradient chain and correct for imperfections e.g. in non-Cartestian sampling trajectories. The model assumes linearity and time-invariance (LTI) of the system. In this work, we indirectly examine the LTI compliance across two different field strengths by comparing different GSTFs based on varying subsets of input gradient shapes for the model computation. Overall, only small changes in the GSTF were observed upon adding different input gradients for the system characterization. However, the differences were more pronounced at 7T compared to 3T.