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xyzt_units
nifti1.h header documentation
/---------------------------------------------------------------------------/ /* UNITS OF SPATIAL AND TEMPORAL DIMENSIONS: ---------------------------------------- The codes below can be used in xyzt_units to indicate the units of pixdim. As noted earlier, dimensions 1,2,3 are for x,y,z; dimension 4 is for time (t). - If dim[4]=1 or dim[0] < 4, there is no time axis. - A single time series (no space) would be specified with - dim[0] = 4 (for scalar data) or dim[0] = 5 (for vector data) - dim[1] = dim[2] = dim[3] = 1 - dim[4] = number of time points - pixdim[4] = time step - xyzt_units indicates units of pixdim[4] - dim[5] = number of values stored at each time point
Bits 0..2 of xyzt_units specify the units of pixdim[1..3] (e.g., spatial units are values 1..7). Bits 3..5 of xyzt_units specify the units of pixdim[4] (e.g., temporal units are multiples of 8).
This compression of 2 distinct concepts into 1 byte is due to the limited space available in the 348 byte ANALYZE 7.5 header. The macros XYZT_TO_SPACE and XYZT_TO_TIME can be used to mask off the undesired bits from the xyzt_units fields, leaving "pure" space and time codes. Inversely, the macro SPACE_TIME_TO_XYZT can be used to assemble a space code (0,1,2,...,7) with a time code (0,8,16,32,...,56) into the combined value for xyzt_units.
Note that codes are provided to indicate the "time" axis units are actually frequency in Hertz (_HZ), in part-per-million (_PPM) or in radians-per-second (_RADS).
The toffset field can be used to indicate a nonzero start point for the time axis. That is, time point #m is at t=toffset+m*pixdim[4] for m=0..dim[4]-1.
DEFINED CODES
#define NIFTI_UNITS_UNKNOWN 0 /! NIFTI code for unspecified units. // Space codes are multiples of 1. / #define NIFTI_UNITS_METER 1 /! NIFTI code for meters. / #define NIFTI_UNITS_MM 2 /! NIFTI code for millimeters. / #define NIFTI_UNITS_MICRON 3 /! NIFTI code for micrometers. /
/ Time codes are multiples of 8. / #define NIFTI_UNITS_SEC 8 /! NIFTI code for seconds. / #define NIFTI_UNITS_MSEC 16 /! NIFTI code for milliseconds. / #define NIFTI_UNITS_USEC 24 /! NIFTI code for microseconds. /
/* These units are for spectral data: / #define NIFTI_UNITS_HZ 32 /! NIFTI code for Hertz. / #define NIFTI_UNITS_PPM 40 /! NIFTI code for ppm. / #define NIFTI_UNITS_RADS 48 /! NIFTI code for radians per second. */
MISCELLANEOUS C MACROS
#define XYZT_TO_SPACE(xyzt) ( (xyzt) & 0x07 )
#define XYZT_TO_TIME(xyzt) ( (xyzt) & 0x38 )
#define SPACE_TIME_TO_XYZT(ss,tt) ( (((char)(ss)) & 0x07) \
| (((char)(tt)) & 0x38) )
