Fig. 2.

(A) Mean values of the frequency evolution ($\Delta f=\gamma \Delta B)$ over the whole brain for each of the 4 repeats (only two repeats were available for Case 2, which is not presented here). The dashed horizontal line corresponds to the Δf value that accumulates phase of π with echo spacing of 6.6 ms ($\frac{\pi }{2\pi \times 6.6\times {10}^{-3}}=75.76\text{rad}/\mathrm{s}$), which leads to aliasing when Δf is above this value (indicated by the light green dots). This was observed in Cases 1, 5 and 12. (B) Blue circles: Translations along the phase encoding direction needed to align echo six of repeats 2-4 to echo two of repeat 1. Red squares: Increments of mean Δf over the whole brain between the labelled repeat and the preceding repeat. The correspondence of patterns between blue and red squares is consistent with our hypothesis that both effects are driven by a common source, hypothesised to be due to B₀ drift induced by temperature change of the passive shim material.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)