A list containing xyzt.units field.
Bits 0..2 of xyzt.units specify the units of pixdim[2..4]
(e.g., spatial units are values 0,1,2,...,7).
Bits 3..5 of xyzt.units specify the units of pixdim[5]
(e.g., temporal units are multiples of 8: 0,8,16,24,32,40,48,56).
This compression of 2 distinct concepts into 1 byte is due to the
limited space available in the 348 byte ANALYZE 7.5 header.
Some NIFTI codes: 0 (unspecified units), 1 (meters), 2 (millimeters),
3 (micrometers), 8 (seconds), 16 (milliseconds), 24 (microseconds),
32 (Hertz), 40 (ppm, part per million) and 48 (radians per second).