The endoscopy tower: what is on the cart and how the image is made

Name every unit of a flexible and a rigid endoscopy tower, explain how light reaches the tissue and how the image travels back to the monitor, and use that chain to decide where a complaint most likely starts.

Two families: flexible and rigid

Walk into any endoscopy unit and you will find two very different kinds of instrument that share the same idea: put light inside the body and bring a picture out. Flexible endoscopes are long, steerable tubes that follow the natural passages of the body: gastroscopes, colonoscopes, duodenoscopes, bronchoscopes, flexible cystoscopes, ENT and other specialist scopes. They are used mostly in the endoscopy department, bronchoscopy suite and urology. Rigid endoscopes are straight optical telescopes pushed through a small incision or a natural opening: laparoscopes, arthroscopes, rigid cystoscopes, hysteroscopes, sinus scopes. They live mostly in the operating theatre.

The two families age and fail differently. A flexible scope is a sealed, flexible pressure vessel full of wires, fibres, tubes and electronics; it bends thousands of times, goes through a reprocessing machine after every patient and is easily bitten, crushed or kinked. A rigid telescope is a precision optical tube; it fails by bending, by losing its seal and fogging, by chipped windows and by burnt light fibres. Lessons 2 and 4 deal with each in turn. In this lesson we look at the whole cart, because most complaints arrive as 'the endoscope does not work', and your first job is to know which box on the cart is really involved.

One more distinction matters: fibrescope versus videoscope. Older flexible scopes, and some thin specialist ones still made today, carry the image back through a coherent bundle of glass fibres to an eyepiece; a camera may be clipped onto that eyepiece. A videoscope has an image sensor at the tip, so the image leaves the tip as an electrical signal. Most gastroscopes and colonoscopes you will service are videoscopes.

What stands on the cart

A typical flexible endoscopy tower is a cart with a medical-grade monitor on top and, below it, a video processor and a light source. On many modern systems the light source is built into the processor; the Fujifilm EP-6000 service manual, for example, describes a processor with its own LED illumination cooled by forced air. On others it is a separate box connected by a control cable. Further down you will find an image recorder or capture unit, sometimes a printer, a keyboard for patient data, a water bottle for lens washing and, nearby, a suction source: a wall vacuum regulator or a portable suction pump with a collection canister. Many units also add a CO2 regulator for insufflation and an electrosurgical generator on the same cart or next to it.

A rigid (laparoscopic) tower looks similar but the parts are different: a camera control unit and camera head instead of a scope processor, a light source with a separate light cable, a CO2 insufflator connected to a gas cylinder or pipeline, a suction-irrigation pump, often an electrosurgical generator and, in arthroscopy or hysteroscopy, a fluid pump or a fluid-management system. Lesson 4 covers these.

Treat the cart as one system. The units share mains through a multiple socket outlet or an isolating transformer, share earth paths through video cables and the cart frame, and share a patient. In electrical safety terms this is a medical electrical system, and the electrical safety course explains why an extra box plugged into the cart's socket strip can change the leakage currents of the whole tower. The endoscope itself is an applied part: the Fujifilm EP-6000 documentation, for instance, classes the processor as Class I with a type BF applied part, under the particular standard for endoscopic equipment, IEC 60601-2-18.

How light gets in

Light starts in the light source. Three technologies are in the field. Halogen lamps are found on old or basic units. Xenon short-arc lamps were the standard for years: a quartz bulb filled with xenon under pressure, started by a high-voltage ignition pulse and then kept running by a low-voltage, high-current supply. LED light sources are now standard on new systems; some combine several LEDs of different colours so the processor can shape the spectrum for special imaging modes. Lesson 3 covers lamp life, LED units and their faults.

From the light source, the light goes into a bundle of glass fibres. On a flexible scope that bundle runs through the light guide connector (the plug at the end of the universal cord), along the universal cord, through the control body and down the insertion tube to one or two light guide lenses at the tip. On a rigid system a separate light cable carries the light to the light post of the telescope, and fibres inside the telescope carry it to the tip. These illumination bundles are non-coherent: the fibres do not need to keep their order, they only need to carry light. Every broken fibre, dirty end face, burnt adhesive or loose adapter takes a share of the light away, which is why a dim image is so often a light path problem and not a lamp problem.

The light source usually also does the brightness control. A mechanical iris in front of a lamp, or the drive current of an LED, is adjusted continuously by the processor so the image stays correctly exposed as the tip moves closer to or further from the tissue.

How the image gets out

In a videoscope the objective lens at the tip forms an image on a small CCD or CMOS sensor. Thin wires run the full length of the scope to the electrical connector, where the processor supplies power and timing to the sensor and receives the image signal. Some designs place a small amount of electronics in the connector; some use contact pins, others transfer signals and power without metal contact. The scope also carries a memory chip that tells the processor which model is connected and stores identification and correction data; this is why a processor can refuse a scope it does not support.

Colour can be made in two ways. In simultaneous systems the sensor has a colour filter over its pixels and white light illuminates the tissue; the EP-6000 manual lists its pickup method as simultaneous. In sequential systems a monochrome sensor is lit in quick succession by red, green and blue light and the processor assembles the colour image. You do not need to know which a given processor uses to start fault-finding, but it explains certain symptoms: colour fringes on fast movement point to sequential imaging, not to a fault.

In a fibrescope or a rigid telescope the optics bring the image out to an eyepiece. A rigid telescope uses a train of glass rods and lenses; a fibrescope uses a coherent image bundle in which every fibre keeps its exact position, so each broken fibre shows as a fixed black dot. On a video tower, a camera head with its own sensor sits on the eyepiece through an optical coupler, and the camera control unit does the job that the processor does for a videoscope.

The processor then corrects the colour (white balance), adjusts gain and contrast, adds any enhancement or special light mode, overlays patient data and sends the image to the monitor. Outputs vary by generation: composite and S-video on older units, analogue RGB, DVI, SDI, HDMI or similar digital outputs on newer ones. The weakest connection in the chain sets the quality of the whole picture: a high-definition processor connected to a monitor through an S-video cable will give a soft, smeared image however good the scope is.

Air, water and suction

A flexible scope is not only an optical device. Through its channels the endoscopist insufflates air or CO2 to open the lumen, washes the lens with water and sucks out fluid and gas. The air usually comes from a small pump in the processor or light source; the same air pressurises the water bottle so that water is pushed to the tip when the air/water valve is pressed. The EP-6000 manual, for example, gives a maximum supply pressure for air and for water. Suction comes from an external source through the suction connector on the light guide plug and is controlled by the suction valve on the control body. The biopsy or instrument channel usually shares its distal part with the suction channel.

This matters for fault-finding: 'no water' can be an empty or badly seated water bottle, a missing seal on its cap, a worn air/water valve, a blocked nozzle at the tip or a weak air pump, and only the last one is inside the processor. Lessons 2 and 6 come back to this.

Using the chain to start fault-finding

Before you open anything, place the complaint on the chain: light source, light path, tissue, optics, sensor, scope cable and connector, processor, output cable, monitor. Ask three questions. Is it every scope or one scope? Is it every room or one tower? Did it start after something happened: a drop, a reprocessing cycle, a new cable, a move of the cart? One scope on every tower points at the scope. Every scope on one tower points at that tower. A symptom that appears only while the electrosurgical unit is active points at interference and earthing, not at the camera.

Carry a known-good scope or camera head when you can, and learn how to swap units safely with the staff's agreement. A swap test that takes five minutes often saves a day of guessing and, more importantly, stops a healthy scope being sent away for an expensive repair.

• One scope, any tower: suspect the scope (connector, fluid inside, sensor cable, optics, light guide).
• Any scope, one tower: suspect the processor, light source, output cable or monitor.
• Only during electrosurgery: suspect interference, earthing and cable routing.
• Only after reprocessing: suspect fluid invasion, a missing cap, or chemical damage.
• Gradually worse over months: suspect lamp ageing, dirty light path or a deteriorating light cable.

On-site checklist

• List every unit on the tower with model and serial number, including the monitor, recorder and socket strip.
• Note which output of the processor feeds which input of the monitor and recorder.
• Check that only approved units are plugged into the cart's socket strip or isolating transformer.
• Check the water bottle, its cap seal and tubing; check the suction canister, filter and tubing.
• Ask staff whether the complaint affects one scope or all scopes, one tower or all towers.
• Keep a known-good scope or camera head available for swap tests when the department allows it.

Check yourself