Structure of Endoscope

An endoscope often consists of a rigid or flexible tube, a light delivery system, an image relay system, and in many cases, an instrument/biopsy channel. To support the proper functioning of an endoscope, dedicated electronics and mechanical parts, water, and air supply are also part of a complete endoscopy system.

In a rigid tube endoscope, rod lenses, prisms, and other rigid optical components are used to relay both the illumination light and the image. It is often called a rigid endoscope. Rigid endoscopes are often used in shallow and easy-to-access body cavities such as the nose and throat. It is also used for normally closed body cavities through a small incision such as the abdominal or pelvic cavity (laparoscope) and the interior of a joint (arthroscope).

A flexible tube endoscope is called a *flexible endoscope*. Usually a flexible optical fiber bundle is used to transmit the illumination light, while another fiber bundle is used to relay the image formed by the objective lens at the distal end to the proximal end eyepiece for direct visualization or for interface with a video camera. This type of flexible endoscope is called a fiberoptic endoscope.

Images can also be captured by placing a miniaturized CCD chip (charge coupled device) at the distal end of the endoscope. In this case, the images are relayed electrically and displayed directly on a viewing monitor. This type of endoscope is called a video endoscope. Flexible endoscopes are often used in hard-to-access body cavities and lumens such as the gastrointestinal tract, the respiratory tract, and the urinary tract.


Figure below show the mechanical structures of an endoscope using a fiberoptic colonoscope as an example. A colonoscope is used to visualize the colon and rectum and has the most complicated structure among all endoscopes. The distal tip contains the illumination optics, the imaging objective, an instrument/biopsy channel, and the mechanics for providing air, water, and suction. The bending section contains pulling wires, passing back through the length of the instrument shaft to the angulation control at the proximal head and enabling the tip to deflect at large angles as high as 180° degrees and in four directions.

The proximal head of the endoscope contains the eyepiece optics, the entry into the instrument/biopsy channel, and angulation control knobs, air/water and suction valves. An umbilical cord connects the proximal head to the light source to obtain illumination light through a light guide and to transmit air, water, and suction through other tubes. The length of a colonoscope could be as long as 2 meters.

The distal tip of an endoscope is shown in Figure below, illustrating the locations of the imaging objective lens, the illumination channel, the air/water channel, and the instrument channel. Two illumination channels are located on either side of the objective lens to more uniformly illuminate the imaging area. A large instrument channel (usually 2–4 mm in diameter) allows the passage of fine flexible accessories (e.g., biopsy forceps, cytology brushes, therapeutic devices) from a port on the endoscope proximal head through the shaft and into the field of view. It also facilitates suction and the removal of tissue (thus, sometimes called a biopsy channel).

The many spectroscopy analysis catheters and microscopy imaging catheters to be discussed later in this book are also delivered through this channel. Therapeutic endoscopes sometimes have twin channels to facilitate more efficient operations.
The air/water can transmit air to insufflate and expand the organ being examined.

The air is supplied from a pump usually located inside the light source and is controlled by the air/water valve in the endoscope proximal head. The air system also applies pressure to a water bottle so that a jet of water can be squirted across the lenses to clean them. In colonoscopies there is an additional proximal opening for the air/water channel to facilitate high-pressure flushing with a syringe.

The insertion tube contents are schematically shown in Figure below. It contains the illumination fiber bundle, the imaging fiber bundle, the angulation wires, the air pipe, the water pipe, and the instrument channel. For mechanical protection, these functional components are wrapped by intertwined metal bands and then covered with metal wire mesh along the length of the tube. The outside covering tube is a durable plastic sheath capable of withstanding caustic body fluids and also disinfectants used in cleaning the endoscopes. For a new type of variable stiffness colonoscope (Innoflex, Olympus Optical Co.), an extra steel coil is built into the insertion tube that can be expanded or contracted as required through a control knob to vary the degree of flexibility and stiffness. This permits the insertion tube to be adjusted according to an individual patient’s own anatomical conditions and contours of the colon.