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How Endoscopes Work
Endoscopes are flexible instruments combining fiber-optics (for illumination) and charge-coupled devices (for imaging) that are used in medicine to visualize the interior of otherwise inaccessible sites, such as the lumen of hollow organs. Endoscopes are used to examine the gastrointestinal (GI) tract, the bronchial system, the ureters and other body cavities. They have numerous uses in non-medical settings as well and are central to the modern management of many GI disorders.
Endoscope Design
The endoscope consists of three basic parts: the tip, the insertion tube and the control section ( Fig. 3.1 ).
The Control Section
The control section is the most versatile part of the endoscope. It is typically held in the left hand of the endoscopist and is used to maneuver the endoscope and introduce accessories. The tip can be maneuvered using the twin dials located on the control section. The larger dial deflects the tip up and down, whereas the smaller dial is responsible for lateral control (i.e., left or right) ( Fig. 3.2 ). These dials can be locked into place to hold a particular position of the tip. Some ultra-thin specialized endoscopes may have only a single dial for up/down angulation and require application of torque for sideways maneuverability. Duodenoscopes used for endoscopic retrograde cholangiopancreatography (ERCP) and linear echoendoscopes have an additional dial for controlling the elevator.
There are two openings in the front of the control section that accommodate specially designed buttons. One button is used for applying suction at the tip of the endoscope. The second button has the dual functions of air insufflation and washing the camera lens. There are separate buttons that can be used to freeze and record images, selectively change the wavelength of the light (see later section on Image Processing ), and alter the focus of the camera. Some buttons are also programmable for specific functions.
The control section is attached to the insertion tube. It also contains an entry port (“instrument channel”) that allows passage of many different endoscopic accessories, such as biopsy forceps, electrocautery probes, and snares, through the length of the endoscope. Some colonoscopes incorporate a variable-stiffness function that is controlled by a rotatable dial at the level of the control head.
Insertion Tube
The insertion tube is the part of the endoscope that enters the body. It contains a “working channel” of variable diameters, which permits passage of endoscopic accessories. It is also involved in the application of suction. Some endoscopes have a “power wash” function to direct a jet of water towards a target in the lumen. This can be very useful for cleaning debris that may coat the mucosa or obscure the lens. Angulation wires that are connected to the up-down and right-left control wheels run the length of the insertion tube. These are used to flex or even retroflex the tip of the endoscope.
The insertion tube is made from multiple layers of polymers that provide durability as well as flexibility. Spiral metal bands wound in opposite directions run through the length of the insertion tube: they transmit force and torque (twist) from the end of the tube to its tip ( Fig. 3.3 ). Certain specialty endoscopes have devices to create variable rigidity of the insertion tube. This requires adjusting a cable that increases or decreases tension within the insertion tube ( Fig. 3.4 ). The changes in stiffness do not extend to the distal 15 to 20 centimeters of the insertion tube, where separate inputs control the use of the so-called bending section.
Tip of the Endoscope
The objective lens is among the many components of the endoscope mounted on the distal end or the tip of the insertion tube. This lens may be forward-facing, oblique, or side-viewing. In close proximity of the lens lies a nozzle that directs a jet of water to clean debris off the lens. A charge-coupled device (CCD) unit connected to objective lens is mounted inside the tip ( Fig. 3.5A ). The CCD and the objective lens form an integrated system that allows seamless transmission of images from the tip of the endoscope through the insertion tube directly into the processing unit through an “umbilical cord” (connector).
The illumination system (“light source”) transmits light into the field of view. The distal tip is also a source of air and water insufflation. The largest opening in the endoscope is a port for passage of a biopsy forceps or other endoscopic accessories. The spatial orientation of the various ports and components of the endoscopes is important (see Fig. 3.5B ), especially when planning complex endoscopic interventions, such as large polypectomies. Specialized endoscopes have accessories geared towards specific purposes, such as ultrasonographic (EUS) transducers in echoendoscopes (see later).
The endoscope is integrated with an image processor through a connector (the umbilicus mentioned earlier). Electronic images are sent from the processor to a color monitor (TV screen). The processor also houses a powerful (usually quartz-halogen) light source, an air pump and a water bottle ( Fig. 3.6 ). The modern endoscope is a precision tool for performing a wide range of diagnostic and therapeutic procedures.
Image Processing
In the earliest iteration of the endoscope, fiber-optic imaging was used to transmit the light from the illuminated end of the endoscope to the eye piece. The fiber-optics were encased in bundles that ran the length of the endoscope and used the principle of total internal reflection of light. These fiber-optic cables were susceptible to damage and the optical arrangement required the endoscopist to hold the endoscope close to the eye.
As technology progressed, CCDs came into being and heralded the era of video endoscopy, which displays images on a screen. This freed the endoscopist from the tyranny of the eye piece, enabling an ergonomically friendly procedure with a substantially lesser risk of body fluid splashes. The CCD is an integrated circuit with photocells that generate electrical charge when hit by light (photons). The amount of charge produced is proportional to the brightness of the light. The system requires the use of a bright xenon lamp to produce white light, which is then transmitted through a filter to provide illumination. The reflected light is then processed by the CCD to an image on the monitor. Using multiple filters, a mosaic of colors can be produced, resulting in a video image. Hence, the CCD essentially converts an image into a sequence of electronic signals which, after appropriate processing, are transformed into an image on the monitor screen.
Before current technology allowed the etching of filters directly on to a chip, a rotating wheel with primary color filters was used to synthesize the fine-color image. This system suffered from a pronounced “flicker effect”. On modern CCDs, each pixel is given one of the three primary color filters (red, blue, green) by metal oxide etching. One pixel will measure the intensity of light through a red filter; adjacent pixels do the same through blue and green filters. When the values of the colors and brightness levels from the three adjacent pixels are combined in a process repeated throughout the whole CCD, a full-color video image is the result ( Fig. 3.7 ).
Advances in CCD design and, more recently, complementary metal oxide semiconductor technology, have resulted in smaller chips with an increased number of pixels and increased resolution.
Along with advances in image resolution, progress has also been made in magnifying the endoscopic images. High-magnification endoscopes are defined by the capacity to perform optical zoom by using a movable lens in the tip of the endoscope. Optical zoom obtains a magnified image of the target while maintaining image display quality. This is distinguished from electronic or digital magnification, which simply enlarges the image on the display, with a consequent decreased pixel density and decreased image quality. Certain endoscopes even employ a movable, motor-driven lens in the tip of the scope that can change the focal distance and provide a magnified view of the mucosal surface ( ).
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