## 2 Materials and Methods
### Statistical Analysis  
The correlations of VBM-derived regional brain volumes with blood dioxin levels were evaluated using a multiple regression model in SPM12 using the data of the 32 men.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Dioxin Congeners** & \multicolumn{4}{c}{**Below LOD**} & \\ \cline{2-7}
**PCDD Congeners (pg/g Lipid)** & **LOD (ppt)** & **No.** & **\%** & **GM** & **GSD** & **Min** & **Max** \\ \hline Nonortho-PCB (pg/g lipid) & & & & & & & \\ \hline TCB \#77 & 0.05 & 0 & 0 & 56.8 & 1.6 & 17.8 & 186 \\ TCB \#81 & 0.05 & 31 & 96.9 & ND & ND & ND & ND \\ PeCB \#126 & 0.28 & 30 & 93.8 & ND & ND & ND & ND \\ HxCB \#169 & 0.05 & 2 & 6.3 & 43.6 & 1.7 & 16.2 & 145 \\ \hline TEQs (pg-TEQ/g lipid) & & & & & & & \\ \hline TEQ-PCDDs & & & 21.8 & 1.5 & 9.1 & 69.2 \\ TEQ-PCDFs & & & 8.6 & 1.3 & 5.0 & 14.1 \\ TEQ-PCDDs/Fs & & & 30.8 & 1.4 & 14.5 & 74.1 \\ TEQ-nonortho PCBs & & & 1.1 & 3.8 & 0.01 & 5.89 \\ TEQ-PCDDs/Fs/nonorthoPCBs & & 32.4 & 1.4 & 14.5 & 79.4 \\ \hline \hline \end{tabular} \(n\): number of subjects; GM: geometrical mean; GSD: geometrical standard deviation; Min: minimum; Max: maximum. LOD (ppt): limit of detection (ppt: parts per trillion): it was defined as a signal-to-noise (S/N) ratio of peak height of chromato-gram = 3. Average of LODs in measurements of each dioxin congener is shown. ND: not detected.  
\end{table}
Table 1: _Cont._The normalization process of the DARTEL algorithm normalized the global brain volume of each subject to the same value while conserving regional differences in brain matter volume. Blood dioxin congener levels were treated as a covariate of interest after base-10 logarithm transformation to improve normality. The total intracranial brain volume and age of the subjects were treated as confounding covariates. Two linear contrasts (positive or negative correlations) were used to estimate associations between blood dioxin levels and brain regions.  
Two-sample \(t\)-tests in SPM12 were used to compare regional brain volumes between subjects with and without perinatal dioxin exposure indicated by maternal residency in Bien Hoa during pregnancy. These tests were performed after adjusting for total intracranial brain volume and age.  
The Montreal Neurological Institute (MNI) coordinates of the voxel of maximal statistical significance in each cluster (connected voxels defined by a prespecified statistical threshold) were extracted. For all VBM analyses, the cluster-based false-discovery rate (FDR) for multiple comparisons combined with the peak detection threshold was used for testing statistical significance [18]. The Anatomy toolbox in SPM12 [19] was used to identify anatomical regions showing significant results in the VBM analysis. \(p<0.05\) was considered statistically significant in all tests.  
SPSS version 22.0 (IBM, Armonk, NY, USA) was used for statistical analyses of global brain volume. General linear models were used to compare global brain volume parameters (independent variables) between subjects with and without perinatal dioxin exposure (a fixed factor), after adjusting for confounding factors correlated with global brain volume (covariates), including age (years) and height (cm).