Evaluation of MR signal index for the assessment of occupational manganese exposure of welders by measurement of local proton T1 relaxation time

Abstract

Magnetic resonance (MR) imaging has become a useful tool for the estimation of occupational exposure to manganese (Mn). However, validity of signal indices on T 1-weighted imaging, a proxy measurement of T 1 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 T 1-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 T 1 relaxation time measured at the GP. Both PI and T 1 relaxation time were significantly increased in the welders compared to the non-welders. T 1 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 T 1 relaxation time and PI. However, blood Mn level correlated only with T 1 relaxation time, and not with PI. T 1 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 T 1 relaxation time and PI, especially at the lower level of signal intensity. Although both T 1 and PI are good biomarkers for the environmental Mn exposure, non-linearity of the relationship between T 1 and PI must be considered in the assessment of Mn exposure using MRI.