The production technology of silica-based image guide had been established by Japanese engineer Atsushi Utsumi who had been working with Mitsubishi Cable, Co. Ltd [2]. A cross-sectional schematic structure of his waveguide is illustrated in Fig. 1.3, comparing it to conventional optical fiber bundle used in a fiber endoscope. The optical fiber bundle is basically a human-work assembling fiber, which contains several thousands of independent pixel fibers. Because the optical fiber bundle was expensive and its pixel size is limited to 20,000–40,000 pixels, a CCD endoscope had been replaced. In contrast, the silica-based image guide is a unified fiber, which contains several thousands of independent core in a common clad region. Since this silica-based image guide can be fabricated by a wiredrawing process corresponding to fit large-lot production, it should be very thin and inexpensive. Despite pixel size being limited to approximately 10 k, this silica-based image guide has been used in an angioscope due to its thinness and inexpensiveness. Technical challenges in the fabrication of this image guide are core diameter stability/distribution and optimization to suppress optical leakage between each core. When the core diameter distribution is scattered, some core has a specific color due to the mode difference in small amount of the transmission mode (see 1.1.2). Moreover, because the clad region is common for all cores, the optical leakage exists as close-set pixel to pixel. This leakage reduces image quality to be a bleary image. A few Japanese optical fiber fabricators only can produce a high-quality silica-based image guide so far.

The specification of the silica-based image guides for the coronary angioscope is summarized in Table 1.1 with one particular new image guide made of a plastic material. The information of this table is based on specification catalogs from each fabricator [3–5]. The author also tries to predict and indicate undisclosed information in Table 1.1. Three thousand or six thousand pixels are employed in general. The minimum bending radius should be less than 15 mm for the coronary angioscope. The outer diameter of the image guide should be less than around 0.4 mm because of both stiffness and size. The glass fibers need a coating layer to avoid a scratch of the fiber outside to keep their bending capability because of the brittle fracture characteristics of glass materials. The angioscope image taken in the 1990s was dark and not sharp because of low pixel density (equal to low pixel size in the image) using a low-NA image guide in the early years [6, 7]. The standard pixel size in an angioscope image in the 1990s was 3000, but a 6000 pixel image is generally used in recent years. The recent advance of high-NA silica-based image guide can make bright and sharp angioscopic image by increasing pixel density as well as suppressing cross leakage. Another advantage of the high-NA image guide is a wide view angle of the image. Since edge distortion is observed in the wide view angle image, in general, an observer feels spatial effect even from a two-dimensional image. One particular waveguide made of plastic material in Table 1.1 has the obvious capability to be applied in the coronary angioscope but has not been applied yet. The cost of plastic fiber might be less than tenth part of the silica-based image guide. Moreover, plastic material has lower stiffness than silica glass. These characteristics are helpful to make ideal coronary angioscope.