A collection of slices with the same orientation. Multi-stack acquisitions are possible when the stack parameter is greater than one, for example in lumbar spine CT exams with individual angulation (oblique) for every disc.

The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:

From these basic units many other units are derived and named.