Methods and systems for establishing a kinetic model input function (IF) in positron emission tomography and single-photon emission computed tomography are provided. A position of interaction along a scintillating fiber coil is determined by: detecting a first plurality and second plurality of photons at first and second ends of the scintillating fiber coil; associating the first plurality of photons and the second plurality of photons with the interaction event based on a timing parameter; and determining a position of interaction for the interaction event based on a comparison between a first parameter of the first plurality of photons and a second parameter of the photons in the second plurality of photons.