How It Works

The Third Eye Retroscope (TER; Avantis Medical Sys­tems, Inc.) is a self-contained endoscope that can be advanced through the instrument channel of a standard colonoscope and was developed specifically because of the inability of the standard forward-viewing colonoscope to detect all polyps located behind folds and flexures in the colon during screening procedures.

When the TER is positioned beyond the tip of the colonoscope, it permits a retrograde view of the colon. Whereas the colonoscope reveals an image of the colon in a forward direction, the TER looks backward, toward and beyond the tip of the colonoscope, providing a view of the proximal aspect of the folds as the colonoscope simultaneously views their distal surfaces. The disposable TER has a 2.5-mm external diameter, and the tip enclos­ing the camera is 3.5 mm wide. To prevent triggering the automatic iris (a brightness reduction feature) built into the colonoscope, the TER has a polarized light emitting diode (LED) matched to a polarized filter affixed to the colonoscope tip before the procedure begins. As a result, the colonoscope light is as bright as usual and is not dimin­ished in intensity by the built-in light sensor, leading to full illumination for both forward and retrograde viewing.

The working end of the TER is composed of 3 con­nected units with a straightforward-viewing camera at the end. When the instrument emerges from the colo- noscope, the tip flexes so that the camera segment points backward at 180 degrees. This brings it face to face with the lens of the colonoscope and parallel with the colo- noscope’s long axis (Figure 1). The short section carrying the light assumes its position parallel to the faceplate of the colonoscope; thus, its illumination is directed toward the colonoscope tip and reveals the proximal aspect of folds while the colonoscope views their distal sides. As the TER is advanced from the colonoscope’s instrumentation channel, the shaft of the TER is then adjusted to place the camera several centimeters from the tip of the colono­scope. The TER lens is rotated to place it in line with the water jet from the colonoscope. This affords the ability to clean the lens if it becomes soiled. The TER, containing an integrated light source and microchip for visualization, depends upon a separate image processor. The images aretransmitted by a tiny complementary metal oxide semi­conductor chip to the processor and then to the monitor, where side-by-side live video is projected with the colono- scopic image on the left and the TER image on the right (Figure 2).
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Figure 1. The Third Eye Retroscope

Figure 1. The Third Eye Retroscope automatically turns 180 degrees when extended from the instrument channel of the standard colonoscope. A miniature video camera is housed in its tip and provides a continuous retrograde image during withdrawal through the colon. The light-emitting diode in the bent portion of the instrument provides illumination for the camera. A polarized hood on the tip of the colonoscope prevents the bright light emitted by the retroscope from interfering with the automatic iris of the colonoscope.

When the TER is advanced several centimeters beyond the tip of the colonoscope, the area behind the folds and the valleys between the folds can be readily observed, providing a complete evaluation of the areas hidden from the forward-viewing colonoscope. Because of the self-contained LED illumination and the 135-degree angle of view, the previously dark area behind the colo- noscope tip can be visualized for several centimeters. The transmitted images from the microchip on the TER and the colonoscope are viewed simultaneously by a trained endoscopist. A CTC simulation model has shown that a retrograde 135-degree angle of view device, coupled with a standard forward-viewing colonoscope (140-degree angle of view), will permit almost full optical visualization of the entire distal and proximal colonic surfaces and the crevices between colonic folds.

In the CTC simulation study, the addition of a sim­ulated retrograde-viewing auxiliary imaging device to a standard forward-viewing 140-degree angle of vision led to nearly complete surface visualization, with a 10-fold decrease in the area that would have been missed with a simulation model having a 170-degree angle of view.
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Figure 2. In the forward view

Figure 2. In the forward view of the colonoscope, the shaft of the Third Eye Retroscope is visible, as is light from the light-emitting diode (A). The retrograde view from the retroscope reveals the area behind folds and flexures (B). The retroscope looks back at the colonoscope, and the proximal edges and valleys between folds are brightly illuminated by its integrated light source.

The marked additional mucosal visualization seen in simulation models combining 140 degrees of forward view and 135 degrees of reverse view (such as with the TER) may be preferred to recently developed optics providing a 360-degree view (Aer-O-Scope, GI View), which has a substantial fish-eye effect. In this simula­tion model, there does not appear to be any additional benefit to using a 170-degree angle-of-view colonoscope instead of a 140-degree instrument when associated with the additional advantage of the TER.