Patient Position (0018, 5100) will tell you if the patient was scanned 
head-first supine, feet-first prone, head-first prone, etc. 
Instance Number (0020, 0013), also commonly known as slice number, 
contains no information about spatial location and isn't even guaranteed to 
be unique. Slice Location (0020, 1041) is useful, if it exists, 
but you can't count on it always existing because it's a Type 3 (optional) 
attribute. To have a robust solution, you need to use 
Image Position Patient (0020, 0032) together with 
Image Orientation Patient (0020, 0037) and Patient Position (0018, 5100) 
to properly order the slices in space. 
Image Position Patient gives you a vector from the origin to the center of 
the first transmitted pixel of the image. 
Image Orientation Patient gives you vectors for the orientation of the rows 
and columns of the image in space. 
Patient Position tells you how the patient was placed on the table relative 
to the coordinate system.