Molecular imaging is a family of techniques that visualizes biological processes in living subjects by using probes that bind to specific molecular targets. In nuclear medicine, PET (positron emission tomography) and SPECT (single-photon emission computed tomography) are workhorse modalities because they can detect tiny (trace) amounts of radiolabeled compounds and quantify target-related signals in vivo. Within PET/SPECT, targeted peptides have become a major probe class: short amino-acid sequences engineered to recognize receptors or other biomarkers (often overexpressed in tumors or diseased tissue), then “tagged” with a radionuclide so the binding event becomes imageable. 1) What Makes Peptide Targeting So Useful in PET and SPECT? Peptides sit in a sweet spot between small molecules and antibodies: High affinity and specificity (when well-designed): peptides can be tuned to fit receptor binding pockets or interaction surfaces, producing strong target-to-background contrast. Fast pharmacokinetics: many peptides clear from blood relatively quickly, which can reduce background signal and enable same-day imaging workflows (depending on isotope half-life and probe design). Chemically modular: it’s typically straightforward to add linkers, chelators, or stabilizing modifications without destroying binding—if the chemistry is placed away from the binding “hot spots.” In practice, peptide probes often target cell-surface receptors…
