## 3 Results
### Imaging of Biological Samples
However, it was difficult to discuss the absolute depth, as the tissue shrank owing to the staining process. Figure 6: (**a**) Photograph of the objective lens, SMMF with jig, rat brain on 3D slide stage, and the objective lens from the upper part; (**b**) extended photograph of (**a**); (**c**) photograph of the cortex into which the SMMF was inserted; (**d**) positions of the SMMF in Rat 1. Figure 7: (**a**) Photograph of tissue near the wound with the SMMF of Rat 1; (**b**) Photograph of the area indicated by B1 in (**a**). The arrow indicates oligodendrocytes lined up; (**c**) Photograph of the area indicated by B2 in (**a**). The arrow indicates the pyramidal cell layer. _En face_ (x-y) and resliced (x-z) images with an IL of 2.0 mm are shown in Figure 8a,b, respectively (see film of Figure 8a,b). These images include many shapes such as grains and short fibers, and are localized at depths from approximately 30 \(\upmu\)m to 60 \(\upmu\)m. Typical _en face_ images at depths of 2.055 mm and 2.536 mm, showing the gray and white matters, respectively, are shown in Figure 8c,d. The dotted line circles denote a field of view (FOV) with a diameter of 47 \(\upmu\)m. The influence of the mode pattern as shown in Figure 4b is not remarkable but is superimposed on the OCM images. The figure insets show the intensity profiles as indicated by yellow bars. The diameters of the short fibers and grains are \(\sim\)2 \(\upmu\)m, whereas some short fibers are longer, i.e., from 5 \(\upmu\)m to 10 \(\upmu\)m. In nerve tissues, myelinated axons manifest themselves as a high-backscattering region [35], with their diameters ranging from 0.5-4.0 \(\upmu\)m [50]. The measurement points for Rat 2 were also distributed in the depth direction from the motor cortex to the forceps minor of the corpus callosum. At positions P1 to P3 of Rat 2, images were obtained by inserting the SMMF to a depth of 3.5 mm with steps of 0.5 mm each. At the position P4, images were measured to a depth of 4.0 mm, and _en face_ (x-y) and resliced (x-z) images are shown in Figure 8e,f (see movies of Figure 8e,f), respectively. Typical _en face_ images at depths of 2.038 mm and 2.547 mm are shown in Figure 8g,h, respectively. From the figure insets, the diameters of the short fibers and grains are similar to those in Figure 8c,d. In Figure 8_t, en face_ images are more localized in a narrower depth region and will be discussed later. As mentioned before, in vivo OCM images of rat and mouse cortexes have been previously obtained using a sealed cranial window with a glass coverslip to reduce the tissue movement. The imaging of single myelinated fibers has been demonstrated using scanning-type spectral domain OCM with an axial resolution of 4.7 \(\upmu\)m and a lateral resolution of 0.9 \(\upmu\)m [37], with which myelin fibers perpendicular to the optical axis were visualized [35]. The imaging of myelin fibers was also demonstrated using an OCM based on a Linnik interferometer with an axial resolution of 1.2 \(\upmu\)m and  
Figure 8: (**a**) _En face_ (x–y) OCM image with an insertion length (IL) of 2.0 mm in Rat 1 (See Video S1); (**b**) Resliced (x–z) images of (**a**) (Video S2); (**c**) Typical _en face_ image at a depth of 2.055 mm of (**a**); (**d**) Typical _en face_ image at a depth of 2.536 mm with an IL of 2.5 mm in Rat 1; (**e**) _En face_ (x–y) OCM image with an IL of 4.0 mm at P4 in Rat 2 (Video S3); (**f**) Resliced (x–z) images of (**e**) (Video S4); (**g**) Typical _en face_ image at a depth of 2.038 mm with an IL of 2.0 mm at P4 in Rat 2; (**h**) Typical _en face_ image at a depth of 2.547 mm with an IL of 2.5 mm at P4 in Rat 2.  
a lateral resolution of 0.5 \(\upmu\)m [36]. There are three reasons why nerve fibers appear discontinuous in the images obtained: (i) Structural disruptions of the fiber; (ii) speckling in the images; and (iii) interference fringes along the fiber's length owing to interference [36].