<Li> Tc labelled Tetrofosmin and Sestamibi for myocardial perfusion imaging with SPECT </Li> <Li> Xe - gas for absolute quantification of brain perfusion (rCBF) with SPECT </Li> <Li> O - labeled water for brain perfusion (rCBF) with PET (absolute quantification is possible when measuring arterial radioactivity concentration) </Li> <Li> Rb - chloride for measuring myocardial perfusion with PET (absolute quantification is possible) </Li>

Which of the following is the best measure of the adequacy of cellular perfusion