## 3 Results  
Figure 4 shows horizontal sections of DOT images of \(\mu_{a}\) at 801 nm, which is an isosbestic point for oxy-Hb and deoxy-Hb, at the 15 th iteration by the trachea-tissue boundary grid (Figure 3b) at different slices. The DOT images are shown on the T2-weighted MR images. Figure 4b shows the section at the middle of the isthmus of the thyroid. Figure 4a,c are sections 3 mm above and below the middle of the isthmus of the thyroid. The trachea-tissue boundary grid region is shown in Figure 4c (dashed white line). The color bar shows the reconstructed \(\mu_{a}\) values. In Figure 4a-c, the outline of the thyroid is denoted by white lines. The shape of the thyroid, where the \(\mu_{a}\) values are about 0.075/mm, is reconstructed in the DOT images, while the position of the reconstructed thyroid shifts slightly towards the skin. The \(\mu_{a}\) values of the background tissue (muscles) were about 0.02/mm. Although the right internal jugular vein (IJV) was included in the grid, it was not reconstructed in the image because the optical fibers were arranged far from the IJV. In fact, the \(\mu_{a}\) values in the IJV were not updated and the initial ones remained (0.027/mm) during the iteration process.  
Figure 5 shows tomographic images of \(\text{StO}_{2}\) calculated from the \(\mu_{a}\) values at the three wavelengths. High \(\text{StO}_{2}\) (around 90%) regions were observed, which corresponded approximately to the reconstructed thyroid regions in Figure 4. The \(\text{StO}_{2}\) in the background tissues, mainly muscles, was around 65%. Although the \(\mu_{a}\) values at the isthmus of the thyroid were much smaller than those in the bilateral lobes, the \(\text{StO}_{2}\) distribution was relatively homogeneous, and \(\text{StO}_{2}\) of the thyroid was 82% to 92%. As a result, the isthmus of the thyroid was more clearly imaged compared to that in Figure 4.  
Figure 6 shows horizontal sections of DOT images of \(\mu_{a}\) at 801 nm at the 35 th iteration by the homogeneous grid (Figure 3c). Although the existence of the thyroid may be read from these reconstructed images, the shape of the thyroid is not correctly reconstructed.  
Figure 4: Tomographic images of the absorption coefficient at 801 nm by the trachea–tissue boundary grid on horizontal sections at (**a**) 3 mm above the middle of the thyroid isthmus, (**b**) the middle of the thyroid isthmus, (**c**) 3 mm below the middle of the thyroid isthmus. The T2-weighted MR images are shown in the background. The shape of the thyroid is outlined by the white line. In Figure 4a, positions of the muscle (MU), internal jugular vein (JV), common carotid artery (CCA), trachea (TR), thyroid (TH), and skin (SK) are shown. In Figure 4c, the grid region is denoted by the dashed white line.  
Figure 5: Tomographic images of \(\text{StO}_{2}\) by the trachea–tissue boundary grid of horizontal sections at (**a**) 3 mm above the middle of the thyroid isthmus, (**b**) the middle of the thyroid isthmus, and (**c**) 3 mm below the middle of the thyroid isthmus. The T2-weighted MR images are shown in the background. The shape of the thyroid is outlined by the white line.  
Figure 6: Same as Figure 4 but for the homogeneous grid.