Endoscopes

TTTS. Endoscopes

Although some endoscopes were specifically designed for fetoscopy in the 1970s, all endoscopes currently used are either custom-made or adaptations from other surgical specialties. Familiarity with technological aspects of the endoscopes is important in order to understand their capabilities and limitations. 

The endoscope has two fundamental systems: a light bundle that carries light to the tip of the endoscope, and the optic pathway which carries back the image under observation. The light bundle can be located at the periphery of the scope, but several other arrangements are also possible. Transmission of the image from the object to the eyepiece can be performed in three basic ways: fiberoptics, solid rod lens, or multiple lens. 

Fiberoptics 

Fiberoptic endoscopes use the internal reflection properties of certain materials, such as glass, to propagate light along a fiber. This property is based on the angle of incidence of light on the glass. By adjusting the angle, the light ray can be made to bounce within the fiber until it emerges at the other end. The endoscope is made using a tightly packed bundle of such fibers (collimation). Some of the fibers are used to transmit the light from the light source, while the rest are optical fibers that transmit the image back to the operator. Fiberoptic endoscopes are typically flexible, and are commonly used in the gastrointestinal and pulmonary tract, as well as in cardiovascular surgery. The larger the number of fibers, the clearer the image is. Advances in the collimation process allow more fibers to be packed together, with less intervening collimation material, therefore providing an improved image without increasing the outer diameter of the instrument. Flexible endoscopes can be particularly useful in the treatment of patients with anterior placentas.9 The endoscope must be both flexible and steerable, and have a working channel. It is important for the surgeon to know the angle of flexion of the endoscope (with and without a laser fiber) within the operating channel, to understand the capabilities of the instrument (Figure 9.5). Although visualization is not nearly as good as with rigid endoscopes, flexible endoscopes may play an important role in selected patients with anterior placentas. 

Self-focusing rod lens 

This technology also uses the reflective properties of glass or other materials to transmit light along a fiber of such material. However, in contrast to the fiberoptic endoscope in which the rays emerge at different points of the end surface of the fiber, the solid rod lens has the ability to focus the incident rays on a plane to provide an image. Solid rod lenses, to our knowledge, are not available with a diameter of less than 1.5 mm. 

Multilens endoscopes 

Ringleb and von Rohr in 1908 first developed endoscopes that included multiple lenses within the shaft of the endoscope. The advantage of this system over the single lens optics of prior endoscopes was an improvement in the brightness, higher magnification, and an erect image. The space between the lenses was air. The lenses were spaced throughout the shaft of the endoscope according to their focal length. In 1959, the English physicist Hopkins designed an endoscope that replaced the air spaces with glass rods, and the lenses were replaced by annular spacers. 

The optical effect of the glass rod is to shorten the length of the optical path, which results in a higher angle of aperture and brighter relay-lens systems (Figure 9.6a). The lenses could be eliminated altogether by making glass rods with optical surfaces (Panoview endoscope). These endoscopes provide the best image available, and should be preferentially used. In addition to their different optical properties, endoscopes also vary in their length, diameter, and whether they are purely diagnostic or operative. 

After testing different diameter endoscopes, we have settled on diagnostic and operative endoscopes 3.3 mm in diameter. Because operating endoscopes must sacrifice optical space for the operating or irrigation channel, they will typically have less resolution than diagnostic endoscopes of similar caliber. Therefore, we often combine diagnostic and operating endoscopes during surgery. The length of the endoscope is also critical. 

Because patients with TTTS may have different degrees of polyhydramnios, and because access to the amniotic cavity may be hindered by placental location, the ideal endoscope should span the entire length of the uterine cavity. Figure 9.6b shows our current set of diagnostic and operating endoscopes.