Fig. 5.

The main effect of time-variable sampling is to move aliased components to benign locations on the temporal frequency axis, creating regions (shown shaded here) where the overlap problem has been alleviated. Accordingly, a lower acceleration factor can be used near DC as compared to the rest of the bandwidth. Because imaged objects tend to have their full spatial extent at the DC temporal frequency, unintended spatial overlaps (see red arrow) can be considered more likely at low temporal frequencies. In contrast, quickly varying signals tend to appear mostly near the outline of the heart, and unintended spatial overlaps are thus not likely at high temporal frequencies. For these reasons we use GRAPPA, well-known for its robustness in the presence of such unintended overlaps, to reconstruct the near DC signal at lower acceleration factors, while the SPACERIP method is used to reconstruct higher frequency signals given its better regularization flexibility.