Nigrosome 1 (NG1), a small cluster of dopaminergic cells in the substantia nigra and visible in the susceptibility map-weighted magnetic resonance image (SMwI), is severely affected in Parkinson's disease (PD). However, the degree of nigrostriatal degeneration according to the visibility of NG1 has not yet been well elucidated. We consecutively recruited 138 PD and 78 non-neurodegenerative disease (non-ND) patients, who underwent both 18 F-FP-CIT positron emission tomography (PET) and SMwI. Three neurologists and one radiologist evaluated the visibility of NG1 in SMwI. The participants were thereby grouped into visible, intermediate, and non-visible groups. Nigrostriatal dopaminergic input was calculated using the specific binding ratio (SBR) of the 18 F-FP-CIT PET. We determined the threshold of regional SBR for discriminating NG1 visibility and the probability for NG1 visibility according to regional SBR. Visual rating of NG1 showed excellent interobserver agreements as well as high sensitivity and specificity to differentiate the PD group from the non-ND group. NG1 was visible in seven patients (5.1%) in the PD group, who had relatively short disease duration or less severe loss of striatal dopamine. The threshold of putaminal SBR reduction on the more affected side for the disappearance of NG1 was 45.5%, and the probability for NG1 visibility dropped to 50% after the reduction of putaminal SBR to 41% from the normal mean. Almost half loss of nigrostriatal dopaminergic input is required to dissipate the hyperintensity of NG1 on SMwI, suggesting its utility in diagnosing PD only after the onset of the motor symptoms.