Introduction to Endoscopes

Endoscopes have traditionally been considered either rigid or flexible, depending on the geometry required by the physician in accessing different areas of the body. Rigid endoscopes were the high-quality devices with stacks of lenses arranged to relay images from the tip of the scopes to eyepieces or large video cameras outside the body. These rigid endoscopes are surgical devices and are inserted through temporary access ports created by the physician. Flexible endoscopes were lower-quality devices, typically fiber-optic or with a miniature video camera at the tip directly coupled with an equally small camera lens.

Due largely to cell phone camera improvements, the state-of-the-art for miniature video cameras has undertaken a renaissance in image quality. The endoscope industry has been a fortunate beneficiary of this progress. Improvements in miniature video cameras initially advanced flexible endoscopes, allowing them to be made with a smaller diameter and higher resolution. This was obviously a boon to patients, providing access to additional areas in the body with less trauma and with faster recovery times.

Rigid endoscopes are also benefitting from image sensor improvements. With Olympus Corporation as the forerunner, we are now seeing “chip-on-tip” endoscopes with quality that rivals their relay lens predecessors. We are even seeing the newest three-dimensional (3-D) endoscopes, previously using relay lenses, now using pairs of miniature cameras positioned side-by-side.

Components of an Endoscope System

There are common components to all endoscopes, whether rigid or flexible. For example, Figure below shows a standard 10-mm diameter laparoscope. All visible light scopes have a light source because it is always dark inside the body. (The notable exceptions are infrared endoscopes which rely on the internal blackbody emission of the patient and do not require a light source.)

Xenon sources are the most common type of light although metal halide and tungsten halogen are sometimes used. Light-emitting diode (LED) sources are increasingly seen, with the best models being as bright as xenon and having much longer bulb life.

With typical light source configurations, a requirement exists for transferring the illumination from the light source to the endoscope. The geometry consists of a cable comprising a bundle of optical fibers with the ends bonded or fused and then polished. In flexible endoscopes, the light cable is often built into the endoscope itself.

Once illumination passes through the light cable to the proximal end of the endoscope, it must then be transmitted down the shaft and into the body.
Another bundle of optical fibers is usually used for this function, with the fibers surrounding the optical system like a ring light. The illumination thus reaches the operative site. A small portion of this light scatters off the area of interest and is imaged by the optical system where it can viewed by the physician with a video monitor. The various optical systems used in endoscopes are described in the next section.

By the way, it should be noted that illumination light never goes through the same optical system as the lenses (see Figure below). The reason for this is practical: an extremely small fraction of the light emitted from the light source contributes to forming the image. Glare from back-reflections off the inner surfaces of the lenses will drastically reduce the image contrast even if multilayer antireflection coatings are used.

Of course there are more parts to a rigid scope than just the optics and illumination system. As an example consider the typical components used in a knee arthroscope system (Figure below). The arthroscope itself is typically 4 mm in diameter including the lens system and the fiber-optic illumination system. Surrounding the
optical shaft is another tube called a cannula. There is clearance between the scope shaft and the cannula to allow irrigation to be applied to the knee in order to distend the joint cavity. Irrigation aids in visualization by helping to clear blood and debris during surgery.

To start the arthroscopic procedure, the surgeon inserts a sharp pointed rod called a trocar into the cannula and pushes both instruments at the same time through the skin and tissue outside of the joint cavity. The trocar is then replaced by a blunt-shaped rod called an obturator, which is used to open up access to the joint cavity, minimizing possible damage caused by the sharp trocar. Once the cannula is in position, the obturator is removed and the arthroscope is inserted to start the procedure.

It is interesting to note that when the tip of the arthroscope is immersed in water during surgery (typical in arthroscopy), the field of view visible to the surgeon is cut by about 1/3 because of refraction in water versus air.