@mgxd Thanks for the response. That would make sense, but I’ll quote the BIDS spec (my emphasis):
PhaseEncodingDirection : Possible values: “i”, “j”, “k”, “i-”, “j-”, “k-”. The letters “i”, “j”, “k” correspond to the first, second and third axis of the data in the NIFTI file. The polarity of the phase encoding is assumed to go from zero index to maximum index unless ‘-’ sign is present (then the order is reversed - starting from the highest index instead of zero). PhaseEncodingDirection is defined as the direction along which phase is was modulated which may result in visible distortions. Note that this is not the same as the DICOM term InPlanePhaseEncodingDirection which can have “ROW” or “COL” values. This parameter is required if a corresponding fieldmap data is present or when using multiple runs with different phase encoding directions (which can be later used for field inhomogeneity correction).