▁re ceptor . ▁When ▁the ▁radio active ▁is ot ope ▁in ▁the ▁lig and ▁dec ays ▁it ▁can ▁be ▁measured ▁by ▁posit ron ▁emission ▁tom ography ▁( P ET ) ▁or ▁single ▁phot on ▁emission ▁computed ▁tom ography ▁( S PE CT ). ▁In ▁in ▁v ivo ▁systems ▁it ▁is ▁often ▁used ▁to ▁quant ify ▁the ▁binding ▁of ▁a ▁test ▁mole c ule ▁to ▁the ▁binding ▁site ▁of ▁radi ol ig and . ▁The ▁higher ▁the ▁aff inity ▁of ▁the ▁mole c ule ▁the ▁more ▁radi ol ig and ▁is ▁disp la ced ▁from ▁the ▁binding ▁site ▁and ▁the ▁increasing ▁radio active ▁decay ▁can ▁be ▁measured ▁by ▁sc int il log raph y . ▁This ▁ass ay ▁is ▁commonly ▁used ▁to ▁calculate ▁binding ▁constant ▁of ▁mole cules ▁to ▁re cept ors . ▁ ▁The ▁transport ▁of ▁the ▁radi ol ig and ▁is ▁described ▁by ▁re ceptor ▁k inet ics . ▁ ▁History ▁ ▁Rad i ol ig ands ▁are ▁cred ited ▁with ▁making ▁possible ▁the ▁study ▁of ▁bi om ole cular ▁behaviour , ▁a ▁previously ▁myster ious ▁area ▁of ▁research ▁that ▁had ▁ev aded ▁research ers . ▁With ▁this ▁capacity ▁radi ol ig and ▁techniques ▁enabled ▁research ers ▁to ▁identify ▁re ceptor ▁devices ▁within ▁cells . ▁ ▁Radio active ▁is ot op es ▁commonly ▁used ▁▁▁ ▁T rit ium , ▁ 3 H ▁ ▁Car bon - 1 4 , ▁ 1 4 C ▁ ▁Sul fur - 3 5 , ▁ 3 5 S ▁ ▁I od ine - 1 3 1