## 3 Results
### Imaging of Biological Samples  
This study was performed under a research protocol approved by the Animal Research Committee of Yamagata University. Two male Wistar rats were anesthetized initially using diethylether and subsequently by intraperitoneal injections of pentobarbital sodium salt (30 mg/kg). The weights of Rat 1 and Rat 2 were 430 g and 480 g, respectively. After \(\sim\)15 min, the head was secured in a stereotaxic apparatus. We cut windows with diameters of \(\sim\)6 mm on the skull using a dental drill and removed the dura (Figure 5a). We measured the displacement of skull surfaces and cortexes using a laser displacement sensor (Keyence, LC-2450, wavelength, 670 nm; precision, 0.5 \(\upmu\)m; spot diameter, 45 \(\upmu\)m \(\times\) 20 \(\upmu\)m; sampling frequency, 1 kHz). A typical result for Rat 1 is shown in Figure 5b. The displacements on the skull were slow and approximately a few micrometers, and those of the cortex were \(\sim\)20 \(\upmu\)m synchronizing with a breath rate of 50 min\({}^{-1}\). During measurements, electrocardiograms (EKGs) were recorded using a bioelectric amplifier ([PERSON], MEG-1200) and a head amplifier ([PERSON], JH-110). The typical result of Figure 5c for Rat 1 shows that the pulse of the EKG was 350-370 beats/min and stable through the measurements. The stereotaxic apparatus containing the rat was placed perpendicularly on the 3D micrometer slide stage. The cortex was brought near the SMMF using a micrometer slide stage, which controlled the insertion length (IL) of the SMMF into the cortex. The SMMF was fixed in front of the objective lens using a stainless-steel jig (Figure 6a,b). The photographs of the SMMF and a cortex surface were recorded using another CCD camera from above (Figure 6c). During the measurements, a physiological saline solution was poured with a syringe to prevent the cortex from drying out. After the SMMF was inserted into the cortex using the micrometer slide stage, 100 _en face_ OCM images were obtained by scanning the reference mirror in steps of 2 \(\upmu\)m. The size of the OCM image was 200 \(\times\) 200 pixels. The total scan depth was 147 \(\upmu\)m, considering the refractive index of tissues of 1.3526 [34]. 50 _en face_ images were averaged at each scan depth. The measurement time was 5 min owing to the depth-scanning speed with a slow stepping motor. The ILs of the SMMF ranged from zero in contact, to a maximum depth of 4 mm with steps of 0.5 mm each. These ILs correspond to  
Figure 5: (**a**) Photograph of windows on the skull of Rat 1; R, rostral; L, lateral; (**b**) variations of displacements of cortex surface and skull with time; (**c**) EKG of Rat 1, pulse 350–370 beats/min. displacements of the micrometer slide stage and approximate depths of the tip of the SMMF below the tissue surface. The measured position of Rat 1(Figure 6d) was at \(-2.3\) mm rostral (R) and 3 mm lateral (L) to the Bregma in the left primary somatosensory cortex. We also cut an 11 mm \(\times\) 9 mm window on the skull of Rat 2 and removed the dura. The measured positions were P1: 2.17 mm R, 1.0 mm L; P2: 1.98 mm R, 1.0 mm L; P3: 1.70 mm R, 1.0 mm L; P4: 1.98 mm R, 1.25 mm L relative to the Bregma in the left secondary motor cortex. The cortex thicknesses were 1.4-1.9 mm [41; 48]. These measured positions were practically selected to avoid blood vessels. The measurement points for Rat 1 were distributed in the depth direction across the somatosensory cortex to the hippocampus. After measurements, the brain of Rat 1 was excised and stained with the standard H&E stain process. The tissue was cut by the coronal plane and near the wound created by the SMMF. The inserted path in the atlas in Figure 6d and photograph in Figure 7a shows that the tip of the SMMF reached the dentate gyrus (DG) in the hippocampus. The wound at the cortex surface can be observed. According to Reference [49], the lined-up oligodendrocytes and the pyramidal cell layer can also be observed in Figure 7b,c.