This might be kind of a basic question, but how would you go about calculating the slice timing for an ascending scan? I’m trying to convert to BIDS from NIfTIs acquired on a Philips (don’t have access to the DICOMS). I was under the impression that–let’s say there are 34 slices in one TR–the slice timing would just be 34 equal increments of the TR i.e. if the TR were 2, the slice timings would be [0, .058, .117 … 1.941]. But when I look at other people’s SliceTiming vectors, the numbers go up and down. I must be misunderstanding what SliceTiming is exactly…
The slice timings should be in the same order as the slices are saved to disk. So assuming an axial scan with the inferior slice saved first to disk and subsequent slices in the volume moving superior, the slice timing for ascending would be [0t, 1t, 2*t… (n-1)*t] where n is the number of slices and t is the time between slices. Assuming no temporal gap between volumes, t = TR/n where TR is the repetition time (in seconds). Note that sparse designs often insert a temporal gap after all slices. Like wise, the Philips dynamic motion correction requires a bit of time (~16ms) at the end of each scan. Historically, slices were ascending, descending, interleaved ascending and interleaved descending. However, multi-band sequence use very unusual slice acquisition orders. You can always pragmatically determine slice acquisition order by intentionally rotating your head during a volume.