Magnetic resonance (MR) imaging has become a useful tool for the estimation of occupational exposure to manganese (Mn). However, validity of signal indices on T1-weighted imaging, a proxy measurement of T1 relaxation time shortening due to Mn deposition, needs to be evaluated in an occupationally exposed population. We performed MR imaging on 20 male welders and 10 age- and gender-matched, non-office workers in a shipyard. We measured the relative signal intensity ratio of the globus pallidus (GP) to the frontal white matter on T1-weighted MR images (pallidal index, PI). We also assessed the visual scale for the relative signal intensity of the GP, which was graded as 0-2 by a radiologist. We evaluated both signal index and visual scale in relation to the T1 relaxation time measured at the GP. Both PI and T1 relaxation time were significantly increased in the welders compared to the non-welders. T1 relaxation time began to decrease from visual scale 2, while PI began to increase from visual scale 1. Environmental Mn exposure indices correlated well with both T1 relaxation time and PI. However, blood Mn level correlated only with T1 relaxation time, and not with PI. T1 relaxation time correlated with PI only at the higher level of PI. These results indicate the possible inaccuracy of visual scoring and the discrepancy between T1 relaxation time and PI, especially at the lower level of signal intensity. Although both T1 and PI are good biomarkers for the environmental Mn exposure, non-linearity of the relationship between T1 and PI must be considered in the assessment of Mn exposure using MRI.