ce ▁protocol " ▁and ▁when ▁near ▁peak ▁performance , ▁could ▁be ▁inject ed ▁with ▁th al li um - 2 0 1 . ▁ ▁The ▁is ot ope ▁required ▁exercise ▁for ▁an ▁additional ▁minute ▁to ▁enh ance ▁circul ation ▁of ▁the ▁is ot ope . ▁ ▁Using ▁nuclear ▁cam eras ▁of ▁the ▁day ▁and ▁given ▁the ▁limitations ▁of ▁T l - 2 0 1 , ▁the ▁first ▁" str ess " ▁image ▁could ▁not ▁be ▁taken ▁until ▁ 1 ▁hour ▁after ▁" str ess ". ▁ ▁In ▁keeping ▁with ▁the ▁concept ▁of ▁comparison ▁images , ▁the ▁second ▁" str ess " ▁image ▁was ▁taken ▁ 4 ▁hours ▁after ▁" str ess " ▁and ▁compared ▁with ▁the ▁first . ▁ ▁The ▁movement ▁of ▁T l - 2 0 1 ▁reflected ▁differences ▁in ▁t issue ▁delivery ▁( blo od ▁flow ) ▁and ▁function ▁( m ito ch ond rial ▁activity ). ▁ ▁The ▁relatively ▁long ▁half - life ▁of ▁T l - 2 0 1 ▁( 7 3 ▁hours ) ▁forced ▁do ctors ▁to ▁use ▁relatively ▁small ▁() ▁dos es ▁of ▁T l - 2 0 1 , ▁al beit ▁with ▁relatively ▁large ▁do se ▁expos ure ▁and ▁t issue ▁effects ▁( 2 0 ▁m S v ). ▁ ▁The ▁poor ▁quality ▁images ▁resulted ▁in ▁the ▁search ▁for ▁is ot op es ▁which ▁would ▁produce ▁better ▁results . ▁ ▁The ▁introduction ▁of ▁te ch net ium - 9 9 m ▁is ot op es ▁By ▁the ▁late ▁ 1 9 8 0 s , ▁two