MINC 2.0, like MINC 1.0, defines both a ``voxel'' and a ``world'' spatial coordinate system. Voxel coordinates are analogous to array indices: The voxel coordinate values are nonnegative integers ranging from zero to one less than the number of data points along the axis, with the origin fixed at one corner of the image.
In contrast, the ``world'' coordinate system reflects the real world
units and spatial orientation of the image. MINC world coordinates
are derived from voxel coordinates using the dimension variable
attributes direction_cosines, step, and start (see
section 4.3.5). The direction cosine vector is a unit
vector which is defined for each spatial dimension. Using the
notation to indicate the component of the axis in the
direction, the cosine vector
for the
axis can be written:
(1) |
(2) |
(3) |
Finally, any homogeneous coordinate in voxel space can be transformed to its equivalent in world space using the equation:
(4) |
While MINC 2.0 files may store data in any orientation, MINC 2.0 follows common medical imaging practice in defining the orientation of world spatial coordinates relative to patients. The convention is that the positive axis increases from patient left to right, the positive axis increases from patient posterior to anterior, and the positive axis increases from patient inferior to superior.