## 2 Subjects and Methods
### Human Neck Measurements  
The subject is a healthy woman who was informed of the aim and procedures of this study. This study was approved by the ethical committee of Hamamatsu University School of Medicine (No. 15-082). To confirm the tracheal and thyroid positions, a cervical MRI scan was performed (Discovery MR750, GE Healthcare, Waukesha, WI, USA). The MR images were also used to make an optical fiber holder in our laboratory. Since the same neck position must be reproduced in the DOT measurements, a vacuum cushion for radiotherapy was used for the MRI scan to fix the neck position. Figure 1: (**a**) Measurement setting; (**b**) optical fiber holder (front); (**c**) optical fiber holder (back); (**d**) schematic of the figure, which shows positions of the holder, fibers, and thyroid. Source fibers are marked in red, and detection fibers are shown in blue. The holder (Figure 1b,c) was made based on the 3D MR neck images by using 3D-computer-aided design (CAD) software (Fusion360, Autodesk, San Rafael, CA, USA) and a 3D printer (Agilista, Keyence, Osaka, Japan). Since the thyroid is a superficial organ and the minimum distance between the skin and the thyroid was about 1 cm in this subject, 1.5 cm was used as the shortest source-detector distance. A total of 15 holes in the holder were arranged at an interval of 1.5 cm and 7 source and 8 detector fibers were inserted into the holes alternately so that the whole thyroid was illuminated (Figure 1d). As a result, 56 datasets (7 \(\times\) 8) were obtained. Each fiber was placed perpendicular to the skin surface. Optical fibers were fixed firmly to the holder by the screw cap on each hole of the holder. Moreover, the empty holes were filled with black polyoxymethylene (POM) cylinders. This fiber arrangement enables us to detect light passing through the thyroid and reduce influences of any strong absorption occurring in the internal jugular vein and the common carotid artery, which are far from the optical fibers. The black natural rubber (KSNR-10054T, HIKARI CO., LTD., Osaka, Japan) inside the holder (Figure 1c) is used to absorb light at the interface between air and the skin so that the Robin boundary condition is applied to numerical modeling of light propagation. In addition, it plays the role of a cushion between the rigid holder and the neck. The holder with the optical fibers was attached to the neck of the subject using the triumphal-arch shaped support. The subject was lying down on the vacuum cushion to be able to reproduce the neck position for the MRI scan. Prior to the neck measurements, we measured the IRFs (Figure 2b-d) in all combinations of the source and detection fibers by using an 8 ch IRF module, which enabled us to obtain eight IRFs for one source fiber simultaneously. The 8 ch IRF module (7.6 (D) \(\times\) 10 (W) \(\times\) 10 (H) cm) has one source fiber port on one side and eight detection fiber ports on the other side (Figure 2a). A diffuser is placed inside each port. The detection fiber ports are arranged at regular intervals on the circumference of a circle (a diameter of 30 mm) of which center is opposite to the source fiber port. The space between the source fiber and the detection fibers is filled with air, and the distance between every source-detection pair is 7.5 cm. Figure 2: Instrumental response function (IRF) and time-of-flight (TOF) distribution in the neck. (**a**) 8 ch IRF module; (**b**) IRF and TOF distribution at 763 nm of S2D2; (**c**) IRF and TOF distribution at 801 nm of S2D2; (**d**) IRF and TOF distribution at 836 nm of S2D2. Light was emitted from each source fiber about every 11 s, and light transmitted through the neck was detected at all the eight detection fibers simultaneously. The TRS-80 automatically adjusted the optical attenuators placed in front of each PMT. A total measurement time was about 120 s, including the switching time of the optical switch and the time for adjusting the attenuators.