Evaluation of MR signal index for the assessment of occupational manganese exposure of welders by measurement of local proton T-1 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, 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, 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. (c) 2006 Elsevier Inc. All rights reserved.