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ITK orientation codes are used. The orientation code maps the voxel coordinate system I (column) J (row) K (slice) to right-handed coordinate system X Y Z. Minor rank notation is used, characters R (right), L (left), A (anterior), P (posterior), I (inferior), S (superior) identify from direction.

RAI means:
X: patient Right to left
Y: patient Anterior to  posterior
Z: patient Inferior to superior

Note: sometimes major rank notation (towards) is used, the same coordinate system may be called DICOM LPS or Nrrd left-posterior-superior (right to Left, anterior to Posterior, inferior to Superior).