The neurofilament light chain protein (NfL) is a suggested general biomarker for neuronal loss.1 Its release from neurons into cerebral spinal fluid, and detection in blood has seen it established as a blood-based marker in the presymptomatic diagnosis and assessment of neurodegenerative disorders. However, due to low concentrations and indirect detection via antibodies, the pathophysiological interpretation of these results is still being developed. Here, we describe a strategy for exploiting positron emission tomography (PET) imaging using isosteric protein mimics following the installation of a fluorine-18 label, providing sensitive, real-time information on the dynamics and trafficking of NfL.2 We build on our previous radical mediated protein modification methods3 to generate [18F]fluoroalkyl radicals that can be rapidly reacted at pre-defined sites on the target protein. We show this 18F label circumvents the limits of current methods that integrate 18F into proteins through the bio-conjugation of bulky, unnatural groups, that may perturb NfL’s assembly and functional properties from those in the natural state. These prosthetic-free, protein radiotracers can be generated in excellent radiochemical yield (up to 67%) via a semiautomated protocol in just 15 mins and allowed for high sensitivity dynamic observations in blood, brain and cerebrospinal fluid, enabling observations of spinal flow kinetics using proteins. This methodology demonstrates the broad potential of a near-zero-size labelling technology for the functional study of proteins in whole organisms without interfering with their biological activity and native assembly.