Gliomas are characterized by intratumoral histological heterogeneity, coexisting foci of low and high grade. First, in low-grade gliomas, neoangiogenesis has not yet developed and cellularity is low, so alterations on perfusion MRI may not be present. Second, a non-negligible number of high-grade gliomas show none, patchy, or weak contrast enhancement on MRI, so they can be misdiagnosed as low-grade glioma, preventing their correct management. We present 4 cases of patients in which F-fluorocholine PET defined the anaplastic tumor component and therefore the tumor aggressiveness, solving the limitations of MRI.