Initially, {HEAD ~ a set} of {TAIL ~ focus points} are identified that were used in the scan of a virtual slide image 
of
Initially , { HEAD ~ a set } of { TAIL ~ focus points } are identified that were used in the scan of a virtual slide image
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH