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An introduction to high-resolution endoscopy and narrowband imaging

Kazuhiro Gono

Introduction

The development of narrowband imaging (NBI) started in May 1999. To confirm the idea of NBI, a study using a multi-spectrum camera capable of producing spectroscopic images and high-power light source was conducted, with this author volunteering as a test subject. The study revealed that the use of 415-nm narrowband light can improve the contrast of capillary images, which are difficult to observe under conventional white light. The first image of living tissue ever produced on NBI is shown in Figure 2.1. The development of an NBI endoscopy system proceeded in cooperation with Dr Sano of the National Cancer Center Hospital East. On December 14, 1999, based on a study using the NBI prototype, we confirmed that the technology was promising for endoscopies of colon, stomach, and esophagus. Since that time, we have developed products in cooperation with not only Japanese endoscopists, but also endoscopists from around the world in an effort to expand the capacity of the prototype. EXERA II, the next generation system equipped with both high-definition TV (HDTV) and NBI, was introduced in December 2005.

Figure 2.1 The 415-nm narrowband image of human tongue mucosa. The 415-nm narrowband image reflects the fine capillary pattern on the mucosa, which is hard to visualize under conventional white light. (Copyright K. Gono.)

At present, Olympus has two types of video endoscopy systems in use worldwide. The difference between these two systems is based on how a color image is produced. One is based on a color charge-coupled device (CCD) chip which has several tiny color filters in each pixel. This system is the 100 series and is branded as EVIS EXERA II. The second system is based on a black and white CCD, in which color separation is achieved through the use of an RGB color filter wheel within the light source unit. This system is the 200 series and is branded as EVIS LUCERA SPECTRUM. Both systems possess NBI technology. Research and development for NBI was first attempted with the EVIS LUCERA SPECTRUM system, the system predominant in Japan, the UK, and Asian countries. Once success was achieved with that system, research and development was focused on the use of NBI with the color CCD system or EVIS EXERA II. Both systems possess the same optical filter in the light source, which enables the illumination of two narrowbands within the visible spectrum of light for NBI. As such, both systems are "optically" identical. However, since both technologies are fundamentally different, there are actually some minor differences in image reproduction. For NBI, both systems are the same, as they both provide improved image contrast when viewing microvessel patterns within the superficial mucosa. If images from both systems are compared simultaneously without magnification, some observers may notice slight differences. However, these differences are quite minor and have not been shown to be clinically significant.

Apart from these optical features, the two systems do differ in the method of magnification incorporated into the endoscopes and the resulting ability to magnify the images observed. In the EXERA II system, the endoscope currently has digital zoom at 1.2× and 1.5× magnification. The HDTV format also possesses "physical zoom" properties that allow the scope tip to be advanced up to 2 mm from the mucosal surface without losing resolution. This combined feature results in a capacity for at least a 50-fold magnification. In contrast, the LUCERA system utilizes an optical zoom system, similar to that used in previous non-high-resolution zoom magnification endoscopes, that allows for magnification of the image up to 80 times. However, these numbers for mucosal magnification have somewhat limited reliability, as there are a number of variables that may affect the actual magnification of tissue that is observed, such as the size of the monitor that is used.

HDTV is a video format that provides clear and high-resolution images while NBI offers high-contrast images of blood vessels. In theory, combining these technologies will give the best performance in close observation of the mucosa. Knowledge of the design concept underlying the functions of EXERA II, its technical limitations and how to read NBI images should be helpful in learning the practical use of HDTV and NBI.

There are many "high definition" TV formats. At the time of product development, 1080i and 720p were the most popular, as they still are today. Olympus had to consider which format would provide the highest level of resolution for motion and still imaging, as well as maintain its popularity within the market so that current and future peripheral devices such as monitors, printers and digital recorders would remain compatible. As the result, 1080i was selected and has proven to be the most popular high-definition broadcast format to this day.

Unlike conventional image processing, NBI is a technology in which an image is emphasized by light. Designing such light requires an in-depth understanding of the optical characteristics of living tissue. As an introduction, this chapter first discusses the characteristics of light, including wavelength and color, as well as the interaction between living tissue and light, such as absorption and scattering. Next, it describes the value offered by HDTV and NBI in terms of image quality and the method for designing chromatic images on NBI. Finally, the chapter explains how typical endoscopic findings such as fine mucosal pattern and blood vessel images look on NBI and why they look that way, using illustrations.

Light and bio-optics

Light and wavelength

Light is an electromagnetic radiation having the characteristics of both wave and particle. When light is considered as a wave, the distance from peak to peak in each wave is called "wavelength" (Figure 2.2). Visible wavelength ranges from 400 to 700 nm. A different wavelength is perceived as a different color. Although colors look different depending on the psychological state of each individual, 400 nm, on average, is perceived as blue, 550 nm as green, and 600 nm as red. Generally, saturation decreases when light contains more wavelengths. In other words, blue light of narrow bandwidth looks more vivid compared with that of broad bandwidth. Light of broad bandwidth within the range 400-700 nm looks white.

Figure 2.2 Wavelength and related color. Wavelength is defined as the distance from peak to peak in each wave. Longer wavelengths have a reddish appearance while shorter wavelengths have a bluish appearance. (Copyright K. Gono.)

Color, absorption, and reflection

When white light illuminates the surface of an apple, the pigment in apple skin absorbs light at wavelengths of 400-550 nm. The absorbed light is converted to heat. In other words, energy in the blue-green range of the white light is converted to heat. Unabsorbed light at 550-700 nm is reflected. The reflected light reaches our eyes and the apple is perceived to be red. How would an apple look if cyan-colored light, instead of white, illuminated it? Cyan-colored light mainly consists of blue and green light, and because such light is absorbed by pigment and almost no light is reflected, the apple looks black. That is to say, white light is needed to perceive the natural color of an object. Contrarily, the light does not need to be white if it is not intended to reproduce the appearance of an object in natural color. NBI is based on this idea and has been developed for the purpose of highlighting blood vessels, not reproducing natural colors. Therefore, light other than white is used for NBI.

Light scattering

In relation to light propagation in an optically turbid medium such as diluted milk, light scattering needs to be taken into consideration in addition to reflection and transmission. Milk contains a number of fat globules of various sizes (1-100 µm). When light strikes such small particles, it diffuses three- dimensionally. This is called light scattering. When there are a multitude of particles, multiple scattering occurs as scattered light is scattered again by striking another particle. Light propagates diffusively due to this light scattering even when a flux of light such as a laser beam is injected into milk.

Absorption and scattering in tissue

A schematic diagram of the interaction between light and living tissue is shown in Figure 2.3. When light enters biological tissue, some reflects from the surface and some diffuses within the body. Multiple scattering occurs among light and small particles such as cell nuclei, cell organelles and nucleoli in the tissue. As a result, light propagates diffusively through the tissue. The propagation of light is determined by its wavelength. Red light, having a long wavelength, diffuses widely and deeply, while blue light, having a short wavelength, diffuses over a smaller range. This is shown in Figure 2.4.

Figure 2.3 Interaction between light and biological tissue. (Copyright K. Gono.)

Figure 2.4 Diffusive light propagation in a turbid medium. Red light diffuses widely and deeply in the turbid medium, while blue light does not propagate...