How Endoscopes Work

Published on 13/02/2015 by admin

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Chapter 3 How Endoscopes Work

Insertion Tube

The insertion tube of the endoscope is a major differentiating feature among endoscopes designed for gastroenterology. Although obvious differences exist depending on the application of the endoscope (e.g., the extra long length of the enteroscope, the thinness of a transnasal esophagoscope), the subtler differences between endoscope models are just as important. This is especially true for colonoscopes; although endoscopists may prefer using a particular colonoscope model for various reasons, the instrument’s insertion tube characteristics more than anything else likely cause an endoscopist to pick a particular colonoscope as the instrument of choice. If any single specification of the instrument can determine the speed and ease with which the endoscopist can insert the instrument, it is the mechanical characteristics of the insertion tube.

Endoscope manufacturers have put significant effort into refining the construction of the insertion tube and selecting ideal materials. Fig. 3.2 shows the internal components of a typical colonoscope. The insertion tube usually contains (1) tubes for suction (biopsy), air, and water feeding; (2) often an additional tube for a forward water jet; (3) four angulation control wires; (4) fine electrical wires connecting the charge-coupled device (CCD) image sensor at the distal tip of the endoscope to the videoprocessor; and (5) delicate glass fibers for bringing light from the light source to the distal end of the endoscope. Colonoscopes with adjustable insertion tube flexibility have an additional component—a tensioning wire to control insertion tube stiffness. Duodenoscopes also have an additional wire/coil sheath running the length of the insertion tube for controlling the up-and-down position of the forceps elevator (see later discussion). It is the task of the endoscope designer to pack all of these individual components into the smallest space possible while still providing freedom for the components to move about without damaging the more fragile elements (CCD wires, fiberoptic strands) as the instrument is torqued and flexed during use. A dry powdered lubricant is applied to all internal components to reduce the stress that they place on each other during manipulation of the insertion tube.

Insertion Tube Flexibility

As previously mentioned, the handling characteristics of the insertion tube are extremely important, particularly for colonoscopes. For easy insertion, the instrument must be capable of accurately transmitting all of the subtle movements and torque applied by the endoscopist. Any rotation that the endoscopist applies to the proximal portion of the shaft (torque) must be transferred to the distal tip of the instrument in a 1 : 1 ratio, although this capability is lost when the instrument is looped. The torquing ability of the instrument is facilitated by flat, spiral metal bands that run just under the skin of the insertion tube (see Fig. 3.2). Because these bands are wound in opposite directions, they lock against one another as the tube is torqued, accurately transmitting rotation of one end of the tube to the other. At the same time, gaps between these spiral bands allow the shaft to flex freely. The bands also give the insertion tube its round shape. Their stiffness prevents the internal components of the insertion tube from being crushed by external forces. These spiral bands are covered by fine strands of stainless steel wire, braided into a tubular mesh. A plastic polymer layer, typically black (or dark green on colonoscopes), is extruded over this wire mesh to create the smooth outer surface of the insertion tube. The polymer layer provides an atraumatic, biocompatible, and watertight surface for the insertion tube. It is usually marked with numbers to gauge the depth of insertion.

Experience has shown that a more rigid insertion tube is optimal for examining the fixed anatomy of the upper GI tract. The colon, with its tortuosity and freely moving loops, is best examined by a more flexible instrument. The instrument should be sufficiently floppy (nonrigid) to conform easily to the tortuous anatomy of the patient and to exert minimum force on the colon wall and attached mesentery. The instrument also should have sufficient column strength to prevent buckling when the proximal end of the instrument is pushed. In addition to its flexibility, the colonoscope should have sufficient elasticity to pop back into a straightened condition when it is pulled back; this aids in removing loops.

Obtaining the best combination of flexibility, elasticity, column strength, and torquing ability is the art and science of insertion tube design. Improvements in one of these characteristics often negatively affect one or more of the others. The final design is usually a compromise between these ideal characteristics, confirmed by months of clinical testing. To improve insertion further, the flexibility of both gastroscope and colonoscope insertion tubes typically varies from end to end. As Fig. 3.3 illustrates, the distal 40 cm of a colonoscope insertion tube is significantly more flexible than the proximal portion. This variation in flexibility is achieved by changing the formulation of the tube’s outer polymer layer as it is extruded over the wire mesh during manufacturing. As Fig. 3.4 illustrates, the extruder contains two types of resins, one significantly harder than the other. Initially, as the distal end of the insertion tube passes through the machine, a layer of soft resin is applied to the distal 40 cm of the wire mesh. This soft resin is gradually replaced by the hard resin within a transition zone near the middle of the tube. The proximal portion of the insertion tube (50 to 160 cm) is constructed of only the hard resin.1 The end result is an insertion tube that has a soft distal portion for atraumatically snaking through a tortuous colon, with a stiffer proximal portion that is effective at preventing loop reformation in the portions of the colon that have already been straightened by the colonoscope.

Adjustable Flexibility

Clinical experience has shown that endoscopists often disagree over what constitutes the ideal insertion tube. This disagreement may be due to differences in training, insertion technique, or past experience. In addition, some endoscopists have expressed a desire to change the characteristics of the insertion tube during the procedure, based either on insertion depth or on the patient’s anatomy, which has led to the development of an insertion tube with adjustable stiffness.2

Colonoscopes with adjustable stiffness have a tensioning wire that runs the length of the insertion tube (see Fig. 3.2). The amount of tension in this wire is controlled by rotating a ring at the proximal end of the insertion tube, just below the control section (Fig. 3.5). When the pull wire in this stiffening system is in the “soft” position, the stiffening system provides no additional stiffness to the insertion tube beyond that provided by the wire mesh and polymer coat. As Fig. 3.5 illustrates, when the control ring is rotated to one of the “hard” positions, an angled slot in the control ring pulls on the slide pin at the end of the pull wire, stretching the pull wire and placing it under heavy tension. This tension stiffens the coil wire that surrounds the pull wire and adds significant rigidity to the insertion tube. As Fig. 3.3 illustrates, although the base stiffness of the insertion tube is established by varying the mixture of hard and soft resins in the polymer base layer, the insertion tube can be stiffened further at will during the procedure by rotating the stiffness control ring. The variable stiffness mechanism does not run the entire length of the insertion tube so that the distal portion of the endoscope is not affected.

Distal Tip

Fig. 3.6 illustrates the components found in the distal tip of a typical end-viewing endoscope, such as a gastroscope or colonoscope. The larger of the circular glass lenses on the distal tip is the objective lens. This lens focuses a miniature image of the GI mucosa on the surface of a solid-state CCD image sensor. The image sensor sends a continuous stream of images back to the videoprocessor via a collection of very fine electrical wires. The objective lens and CCD unit are tightly sealed to prevent condensation from fogging the image and to protect the imaging system from damage if fluid were to enter the instrument accidentally. Light to illuminate the interior of the body travels through the instrument via fiberoptic illumination fibers. This light is evenly dispersed across the endoscope’s field of view via a light guide lens system.

Some endoscopes have a single illumination system (as shown in Fig. 3.6). Other endoscope models have two fiberoptic bundles and two light guide lenses to improve illumination on both sides of the biopsy forceps (e.g., snare) and to facilitate the packing of components within the insertion tube. The channel used for biopsy and suction exits close to the objective lens on the distal tip. The position of the biopsy channel relative to the objective lens determines how accessories appear in the image as they enter the visual field. On some instruments, the snare or biopsy forceps appears to emanate from the lower right corner of the image; on other instruments, these accessories enter the visual field from the lower left corner, and so forth. When planning difficult procedures, such as piecemeal polypectomy or hemostasis, it is crucial that the endoscopist know where the accessories will enter into his or her field of view.

The insertion tube also contains small tubes that carry air and water through the instrument (see Fig. 3.2). These tubes typically merge into a single tube a few inches from the distal tip (see Fig. 3.9 further on). This combined air/water tube connects to the air/water nozzle on the tip of the instrument. Under control of the endoscopist, water can be fed across the objective lens to clean it, or air can be fed from the nozzle to insufflate the GI tract. Some gastroscopes and colonoscopes have an additional water tube and a water-jet nozzle on the distal tip for washing debris from the mucosa (see Fig. 3.6).

Fig. 3.7 illustrates the components found in the distal end of a typical duodenoscope. Fig. 3.7A is a schematic cross section through the optical and illumination systems found in the distal tip of the duodenoscope. The objective lens for viewing the tissue is now located on the side of the distal end rather than on the very tip of the instrument. A prism is used to deflect the angle of view 90 to 105 degrees and to convert the instrument into a side-viewing endoscope. The illumination fibers are likewise steeply bent at the tip of the instrument, directing the light to emanate from the side. As in end-viewing instruments, an air/water nozzle positioned near the objective lens directs water across the lens to clean it, followed by air to blow away any remaining water droplets. Air from this nozzle is also used to insufflate the patient. All duodenoscopes have a forceps elevator to deflect actively the tip of any accessory passed through the channel. The elevator mechanism is shown in Fig. 3.7B. This elevator normally lies in a recess within the tip of the endoscope (lowered position). When the endoscopist wishes to raise the accessory up into the field of view, he or she operates a thumb control on the control section of the instrument (not shown). This thumb control pulls on the elevator wire, lifting the elevator out of its recess into a raised position, deflecting the tip of the accessory up into the field of view. In some instruments, such as the V-scope (Olympus Corp, Melville, NY), this elevator has a small groove that entraps a 0.035-inch guidewire to aid in wire stabilization during accessory exchanges.

Bending Section and Tip Angulation

The distal tip of the insertion tube of the endoscope can also be manipulated by the endoscopist. The deflectable portion, referred to as the bending section, is constructed differently from the rest of the insertion tube. As Fig. 3.8 illustrates, the bending section is composed of a series of oddly shaped metal rings, each one connected to the ring on either side of it via a freely moving joint. These joints are constructed using a series of pivot pins, each one displaced from its neighbors by 90 degrees. One set of pivots allows the bending section to curl in the up-and-down direction. A second set allows the bending section to curl in the right-and-left direction. Together, they enable the bending section to curl in any direction. The direction of the curl is controlled by four angulation wires that run the length of the insertion tube (see Fig. 3.2). These four wires are firmly attached to the tip of the bending section at the 3 o’clock, 6 o’clock, 9 o’clock, and 12 o’clock positions. Pulling on the wire attached at the 12 o’clock position causes the bending section to curl in the up direction and achieves what endoscopists refer to as “up tip deflection.” Pulling on the wire attached at the 3 o’clock position causes right tip deflection. Pulling the other two wires causes down and left deflection. The endoscopist is able to pull on each of these wires in turn by rotating either the up-and-down or right-and-left angulation knobs. (For simplicity, Fig. 3.8 illustrates only the up-and-down angulation system.) Rotating the up-and-down and right-and-left knobs together produces a combined tip movement (e.g., upward and to the right) and allows the endoscopist to sweep the tip of the endoscope in any direction.

Air, Water, and Suction Systems

A schematic of a typical endoscopic air, water, and suction system is shown in Fig. 3.9. An air pump in the light source provides air under mild pressure to a pipe protruding from the light source connector of the endoscope. This air is carried by an air channel (tube) to the air/water valve on the control section. If this valve is not covered, the air simply exits from a vent hole in the top of the valve (see Fig. 3.1). This vent hole allows the air pump to pump freely when air is not needed, reducing wear and tear on the pump. If the endoscopist wants to insufflate the patient, he or she covers the vent hole with a fingertip; this closes off the vent and forces air down the air channel, exiting the instrument through the nozzle on the distal tip. A one-way valve is incorporated into the shaft of the air/water valve (see Fig. 3.1) to hold air in the patient during examination. During endoscopy, the GI tract is typically insufflated to a pressure slightly above atmospheric pressure. If it were not for this one-way valve in the system, air from the organ under examination would flow back into the nozzle on the distal tip, up the air channel in the insertion tube, and out the hole in the air/water valve whenever the operator removed his or her finger from the valve. The antireflux valve is required to keep the patient insufflated.

Water, used to clean the objective lens during the procedure, is stored in a water bottle attached to the light source or cart (see Fig. 3.9). In addition to feeding air for insufflation, the air pump also pressurizes this water container, forcing water out of the bottle and into the endoscope. This water is carried via a tube on the water bottle cap to the light source connector of the endoscope and then by a water channel up the universal cord to the air/water valve. When the endoscopist depresses the air/water valve, water continues down the water channel in the insertion tube and flows out of the nozzle at the distal tip. The nozzle directs this water across the surface of the objective lens, cleaning it.

Suction is also controlled by a valve on the control section of the endoscope. A suction source, either the hospital’s wall suction system or a portable suction pump, is connected to the light source connector of the endoscope. When the endoscopist depresses the suction valve, suction is applied to the suction/biopsy channel within the insertion tube. Any fluid (or air) present at the distal tip of the endoscope is drawn into the suction collection system. A channel-opening valve (also called a biopsy valve) closes off the proximal opening of the biopsy channel and prevents room air from being drawn into the suction collection system.

There are several inherent safety features in the design of the air, water, and suction system shown in Fig. 3.9. The air supply system has no moving parts and no mechanical valves that could stick in a continuously “on” position, resulting in accidental overinsufflation of the patient. Instead, the air simply exits the vent hole in the valve, unless the physician has his or her finger over this opening, and in the event that the suction system becomes obstructed and the endoscopist has difficulty with possible overinsufflation, he or she simply can quickly remove all valves from the endoscope. This action stops all feeding of air and water and allows the patient’s GI tract to depressurize through the open valve cylinders.

Illumination System

Endoscopes use an incoherent fiberoptic bundle to carry light from the external light source to the distal tip of the endoscope. This fiber bundle is composed of thousands of hairlike glass fibers (30 µm in diameter) that are optically coated to trap light within the fiber and to transmit light from end to end via a phenomenon known as total internal reflection. Light rays entering one end of such a fiber reflect off of the walls of the fiber many thousands of times before exiting the opposite end of the fiber. The types of glass used to make the core and cladding of the fiber and the thickness of the core and cladding all are carefully chosen to enable the fiber bundle to carry as much light as possible (see Kawahara and Ichikawa3 for a more complete discussion of fiberoptics).

Endoscopic light sources typically use 300-W xenon arc lamps to produce the intense, white light needed for videoendoscopy. These lamps also produce considerable heat. Heat sinks, infrared filters, and forced-air cooling systems within the light source prevent the fiber bundle of the endoscope from overheating and burning. A close inspection of the tip of the endoscope’s light guide reveals a burn-resistant quartz lens that serves to collect light from the light source lamp and to direct it into the endoscope (see Fig. 3.1). At the other end of the endoscope, the light guide lens at the distal tip of the instrument spreads this light uniformly over the visual field (see Fig. 3.6). An automatically controlled aperture (iris) in the light source controls the intensity of the light emitted from the endoscope tip.

When the endoscope is in a large cavity such as the stomach and significant light is required, the aperture in the light source opens up, allowing the endoscope to transmit maximum light. When the endoscope tip is very close to the mucosa and illumination is bright, the aperture in the light source automatically closes down to reduce the amount of light exiting the light source. If the illumination is too low, the video image on the monitor is dark and grainy. If the illumination is too strong, the image on the monitor is washed out (i.e., “bloom”). The videoprocessor automatically keeps the brightness of the illumination within a range that is acceptable for the CCD image sensor by carefully controlling the amount of light produced by the light source.

Solid-State Image Capture

The image sensors used in videoendoscopes are typically referred to as CCDs. These sensors are solid-state imaging devices constructed of silicon semiconductor material. The silicon on the surface of the sensor is responsive to light. When a photon of light strikes the photosensitive surface of the CCD, it displaces an electron from a silicon atom at the surface. A free, negatively charged electron is produced in the silicon material along with a corresponding positively charged “hole” in the crystalline structure of the silicon where the electron was previously bound. This action is referred to as the photoelectric effect and is illustrated in Fig. 3.10. As additional photons hit the surface of the sensor, additional free electrons and additional corresponding holes are created. The charges built up in the sensor are directly proportional to the amount of light falling on the CCD. Also, these charges are created regardless of the color of the light falling on the sensor.

Although a single photosensitive element is useful for measuring the brightness of light falling on a surface (as in a light meter), it cannot reproduce an image. To reproduce an image, the photosensitive surface must be divided up into a matrix of thousands of small, independent photosites. When an image is focused on the surface of such a sensor, the brightness of the image is automatically measured at each individual photosite within the matrix. Knowing the brightness of every point in the image allows a vision system to reproduce the image accurately. The CCD is a common component of such a solid-state vision system. The surface of a CCD image sensor is divided into a rectangular array of discrete photosites, individually referred to as picture elements, or pixels. Fig. 3.11 illustrates a CCD sensor with such an array. In a video image endoscope, the CCD is located in the distal tip of the instrument directly behind the objective lens (as shown in Fig. 3.6). The objective lens focuses a miniature image of the observed mucosa directly on the surface of this sensor. The pattern of light falling on the CCD (i.e., the image) is instantly converted into an array of stored electrical charges because of the photoelectric effect previously described. Because the charges stored in each of the individual pixels are isolated from neighboring pixels, the sensor faithfully transforms the optical image into an electrical replica of the image.