Suitably designed optical fibers are shown to offer a powerful tool for functional brain mapping, facilitating the detection of Cherenkov radiation generated by relativistic positrons from positron-emitting radionuclides. Fiber probes support a low-loss delivery of the high-intensity short-wavelength part of the Cherenkov spectrum, which is strongly absorbed by biotissues, and lower the threshold positron energy ε required for Cherenkov radiation relative to ε values typical of biotissues, thus enhancing the conversion of positron energy into Cherenkov radiation.