Figure 1. Central pathways and imaging targets for apoptosis.
Both extrinsic pathway (cell-surface receptors) and intrinsic pathway (mitochondria) activate caspase-3 to finally result in apoptosis. The externalized phospholipid (i.e., PS and PE), activated caspase-3, and dissipated Δψm can be targeted by various radiotracers for noninvasive imaging of apoptosis. PS: phosphatidylserine; PE: phosphatidylethanolamine; Δψm: mitochondrial transmembrane potential.