## 4 Discussion  
The practical applications of an SMMF present some problems. First, the signal intensities in _en face_ images are influenced by the LP mode. We reduced the reflection of the SMMF using polarization to avoid the saturations of CCD, but a better approach would be to use optical index-matching with non-polarization optics to avoid influences of mode patterns in the images. A second challenge is the contact between the SMMF facet and the tissue. Ideally, the SMMF should be smoothly inserted into the tissue and the facet should adhere to it completely. At present, as the facet is as cut, good contact with the tissue could not be confirmed. We are investigating a process to modify the tip of the SMMF to remedy this issue.  
Rat cortexes consist primarily of neurons, glial cells, and nerve fibers. The densities of neurons and glial cells such as astrocytes, microglia, and oligodendrocytes have been reported. Assuming that these cells are uniformly distributed in tissues, the mean interval distances between cells can be estimated. Such densities, mean distances, and typical cell sizes are summarized in Table 1.  
As the FOV in this measurement of an SMMF is 47 \(\upmu\)m in diameter, and the measured depths span are approximately 20-90 \(\upmu\)m, statistically, one or two cells would be observed in the measured volume considering the distances listed in Table 1. We occasionally measured low signal-intensity regions in  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **Density 1/mm\({}^{3}\)** & **Distance \(\upmu\)m** & **Size \(\upmu\)m** & **Reference** \\ \hline Neuron [1] & & 40,000 & 29.2 & \(\sim\)20 & [37] \\ Glial cell & Astrocyte [1] & 20,000 & 36.8 & 4.9–5.4 & [37, 51] \\ Microglia [1] & 1060 & 98.1 & 2.2–3.1 & [51, 52] \\ Oligodendrocyte [2] & 7713 & 50.6 & \(\sim\)4.3 & [51, 53] \\ \hline \hline \end{tabular}  
* Rat, somatosensory cortex; 2) Rat, spinal cord.  
\end{table}
Table 1: Typical distances and sizes of various cells.  
Figure 10: (a) Dependence of sums of signal intensity in each _en face_ image on the scan depth for Rat 1 for each IL; (b) dependence of sums of signal intensity in each _en face_ image on the scan depth at P4 of Rat 2 for each IL; (c) dependence of integrations of signal intensities on IL for Rat 1 and Rat 2-P1 to P4, where each arrow indicates the position of maximum integration of signal intensities for Rat 2 from P1 to P4.  
these volumes. These regions might correspond to neurons, but this was not confirmed. Through in vivo and ex vivo imaging of rat brains, we irregularly obtained bright images with diameters ranging from 5-7 um in _en face_ images. The possibility that other glia--in addition to neurons--might possess backscattering properties has been reported [37]. This will be investigated in more detail.  
For applications of OCT for the imaging of cytoarchitecture and myeloarchitecture in brains, connectivity, neuropathic progression, and developmental appearance have been discussed [30]. By inserting an SMMF up to approximately 6 mm, FF-OCM has the potential to image deeply to obtain useful information. Further, with the use of intrinsic optical signals, new insights into morphology and its functions might be obtained. However, for practical applications, the overall performance in terms of measurement speed, sensitivity, and system size must be improved.