Traditional body composition evaluation used manual tools, so most metrics are based on manually segmenting a few slices. If you want to reproduce these established metrics then you can do the same - manually segment slices and work with cross-sectional areas (computed by Cross-sectional area module).
If you are ready to challenge the established methods and work with more accurate 3D metrics then you can segment entire body regions and work with segment volumes (computed by Segment Statistics module).
Segment Statistics module computes metrics for each segment independently.
Number of voxels multiplied by volume of one voxel.
“Surface area” is the surface area of the segment you see in the 3D view. For a single slice it is approximately 2x larger than the area of the cross-section, because the segment is flat and its two sides each has approximately the same area as the cross-sectional area.
You can segment the structures that might interfere with your analysis and do not include them in your measurements.