Basic Endoscopic Equipment - Wiley-Blackwell

1 Basic Endoscopic Equipment1 The modern era of endoscopy began with the development offibreoptic instruments in the 1960s. For most purposes these arebeing supplanted by video chip endoscopes in the 1990s. Detailsof instruments for specific purposes, accessories and precisemethods for management are outlined in other chapters. Thischapter serves to introduce the true beginner to some principleswhich are common to all endoscopes and endoscopes are complex (Fig. ). Basically, theyconsist of a control head and a flexible shaft with a manoeuvr-able tip.

2 The head is connected to a light source via an umbilical cord, through which pass other tubes transmitting air, water andsuction, etc. The suction channel is used for the passage of diag-nostic tools ( biopsy forceps) and therapeutic andsuction buttonsBiopsyportControl headLight source andair/water supplyInstrumentshaftBiopsyforcepsDeflec tabletip Fig. endoscope 12 Fig. internal reflection oflight down a glass bundle showing the packing fraction or dead spacebetween instruments and video-endoscopesFibreoptic instrumentsThese are based on optical viewing bundles, well described as a highly flexible piece of illuminated spaghetti.

3 The viewingbundle of a standard fibre-endoscope is 2 3 mm in diameter andcontains 20 000 40 000 fine glass fibres, each close to 10 m indiameter. Light focused onto the face of each fibre is transmittedby repeated internal reflections (Fig. ). Faithful transmissionof an image depends upon the spatial orientation of the individ-ual fibres being the same at both ends of the bundle (a coherent bundle). Each individual glass fibre is coated with glass of alower optical densit to prevent leakage of light from within thefibre, since the coating does not transmit light.

4 This coating andthe space between the fibres causes a dark packing fraction ,which is responsible for the fine mesh frequently apparent in thefibreoptic image (Fig. ). For this reason, the image quality of afibreoptic bundle, though excellent, can never equal that of arigid lens system. However, fibreoptic bundles are extremelyflexible, and an image can be transmitted even when tied in aknot. In most modern instruments the distal lens which focusesthe image onto the bundle is fixed, and a pin-hole aperture givesa depth of focus from 10 15 cm down to about 3 mm.

5 The imagereconstructed at the top of the bundle is transmitted to the eyevia a focusing lens, adjustable to compensate for individualdifferences in are mechanically similar to fibre-endoscopes, with acharged couple device (CCD) chip and supporting electronicsmounted at the tip, to and fro wiring replacing the opticalbundle and further electronics and switches occupying the siteof the ocular lens on the upper part of the control head. Remov-ing any need to hold the instrument close to the endoscopist seye has hygienic advantages (avoidance of splash contamina-tion) and also gives the opportunity for radical changes ofinstrument design and handling techniques in the subtleties of different CCD systems in design and perfor-mance are beyond the scope of this book.

6 However, in essence, aCCD chip is an array of 33 000 100 000 individual photo cells(known as picture elements or pixels) receiving photonsreflected back from the mucosal surface and producing electronsin proportion to the light received. In common with all othertelevision systems the individual receptors of the CCD respondonly to degrees of light and dark, and not to colour. Colour CCDs have extra pixels to allow for an overlay of multipleprimary colour filter stripes, making the pixels under a particu-lar stripe respond only to light of that particular colour (Fig.)

7 Black and white (or, more correctly, sequential system) CCDscan be made smaller, or potentially of higher resolution, by theexpedient of illuminating allthe pixels with intermittentprimary colour strobe-effect lighting produced by rotating acolour filter wheel within the light source (Fig. ). The sequen-tial primary colour images (in the gut mostly red, some greenand little blue) are stored transiently in banks of memory chipsin the processor and fed out sequentially to the red/blue/greenelectron guns of the TV monitor.

8 The large numbers of chips andsophisticated computer image-processing technology used tooptimize the underlying single CCD output account for theexcellence of the image produced by sequential CCD systems(and the high price involved), as well as the relatively Endoscopic Equipment3 Fig. red, green and blue filters in the colour filtermosaic r = redg = greenb = bluerrbbggrggPixelCCD(chargecoupleddevic e)ImageXenonlampLightsourceLight guideconnectorVideoprocessorRed, green,blue imagememoriesCCDL ensesLight guidesLesionRed, green,blue strobeilluminationat polypRotating filterwheel (red,green, bluefilters)VideoscopeRed, green, bluelight fromxenon lampImageRGBFig.

9 Colour or fibre-endoscope?The screen-image quality of present video-endoscopes equalsthat of present fibrescopes in both colour and resolution. Video-endoscopy scores greatly by the fact that everyone can view theimage simultaneously, with a clarity previously restricted to theendoscopist alone (teaching side-arms and add-on televisioncameras introduce optical interference and reduce quality).Whereas optical fibre technology is near its maximum theoreti-cal performance (since below the 6 8 m fibre diameterapproached in modern bundles there is massive loss of lighttransmission), there is no reason why the 10 m pixel size ofpresent CCDs should not be reduced to around 1 m.

10 This meansthat future CCDs can be smaller, but also that the greatlyincreased numbers of pixels will increase resolution and allowthe use of high-definition TV monitors. The objection that video-endoscopes introduce artificial colour values is untenablesince: (i) they can be shown in technical studies to give a remark-ably faithful rendering of test charts; (ii) the visual assessment oflesions depends little on absolute colour values; and (iii) there isthe inescapable fact that individual perception of colour variessignificantly the extreme example being colour blindness.