内镜视频技术

Endoscopic Video technology

Prof. Dr. R. K. Mishra

                               
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Introduction

When it comes to minimal access surgery, the endoscopic monitor should be best. It should be backed by technical support. Every surgeon should make the right buying decisions for their laparoscopic operating room, surgery centre, or even your office. Whether you need a state-of-the-art colour video camera system, minimally invasive surgical instrumentation or a few new accessories, you should always give your option tailored to meet your surgical needs. Your goal should be quality, every time. This article will help you to understand the technology behind the video monitors and camera.

Historical background

In the past endoscopic procedures were done without the aid of monitors. The operator visualised the interiors of the patient directly through the eye- piece of the scope. This method was associated with many difficulties. He was the only person who could observe the procedure leading to poor co-ordination with other members of the team. As a result extensive and difficult procedures could not be performed. The magnification was very poor. Surgeons had to face problems with posture leading to discomfort and strain as his eye was always glued to the eye – piece. He had difficulties in orientation due to visualising with only one eye.

As new methods of communication developed, the introduction of television brought about a significant impact. Not only in other fields but also in Medicine. What did we gain through the introduction of television? We could get rid of most of the previous handicaps if not all. A good magnification of the image was reproduced. All members of the team could visualise the procedure. Surgeons could operate more comfortably. Complex procedures began to be undertaken and were even recorded.

Soulas in France first used television for endoscopic procedures in 1956. He demonstrated the first televised bronchoscopy. A rigid bronchoscope was attached to a black and white camera that weighed about 100 lbs.

In 1959 a laparoscopic procedure was demonstrated using a closed circuit television program using the “Fourestier method ”. This method was developed by transmitting an intense beam of light along a quartz rod from the proximal to distal ends of the laparoscope.

The first miniature endoscopic black and white television camera was developed in Australia in 1960. It weighed 350 grams, was 45mm wide and 120mm long. Because of its small dimensions it could be attached to the eyepiece.

Practical Implications

One has to think about what he wants in the further development of monitors. Would they help the surgeon in carrying out his tasks with more ease, comfort and safety?

The video monitors of today appear to satisfy most of these demands. We have progressed a long way to achieve the technology that we are using today. It is essential to have knowledge about how monitors work. This will help us to think about the advantages and disadvantages and what could be done to improve them.

Surgical monitors are no different from the T.V. We watch at home. The basic principle of image reproduction is horizontal beam scanning on the face of the picture tube. This plate is coated internally with a fluorescent substance containing phosphor. This generates electrons when struck by beams from the electron gun. As the beam sweeps horizontally and back it covers all the picture elements before reaching its original position. This occurs repetitively and rapidly. This method is called ‘horizontal linear scanning’. Each picture frame consists of several such lines depending on the type of system used.

Technology behind

The existing television systems in use differ according to the country. The U.S.A uses the NTSC (National Television System Committee) system. In European countries the PAL (Phase Alternation by Line) system is in use. There is also a French system called SECAM (Sequential colour and memory). The broadcasting standards for each are summarized below:

SYSTEM	PAL	SECAM	NTSC
Number of lines	625	625	525
Visible lines (max.)	575	575	486
Field frequency (cps) cycles per second	50	50	60
Frames per second	25	25	30

The final image depends upon the number of lines of resolution, scanning lines, pixels and dot pitch. How many black and white lines a system can differentiate gives the lines of resolution. These can be horizontal and vertical. Horizontal resolution is the number of vertical lines that can be seen and vice-versa. Pixels denote the picture elements and they are responsible for picture detail. The more number of pixels, the better the detail. They are represented on the camera chip by an individual photodiode. The restricting factor of information on a scan line is the ‘dot pitch' that represents phosphor element size.

The NTSC system has certain drawbacks. Not all the lines of resolution are used. The maximum number of lines visible are reduced by 40.Improving the resolution of the camera will not improve the monitor resolution. This is due to a fixed vertical resolution. In addition to these problems, if the phase angle is disturbed even a little it produces unwanted hues.

The PAL system is superior in certain aspects. It can overcome this problem by producing alternations over the axis of modulation of the colour signed by line. This system also deals with problems of flickering. It involves a process called ‘inter-lacing’ where odd and even lines in a field are scanned alternatively. Sequential colour and memory systems are similar to PAL in these aspects except that the signals are transmitted in sequence.

Another important aspect one has to keep in mind is the formation of the colour image. This is done by super- imposing the data for colour on the existing black and white picture. The black and white signal is monochromatic and combines with the composite colour signal. This gives the final colour signal. Luminance (brightness) is delivered by the black and white signal. Chrominance (colour) is delivered by the colour signal. It is called composite as it contains the three primary colour information’s (red, green and blue). A system that combines luminance and chrominance into one signal is called a `compound system’.

Colour values can be problematic as they can go out of phase. This is due to their high sensitivity. Applying a reference mark for the signal on the scanning line called as `colour burst’ can prevent this. The colour on a monitor can be calibrated. This can be done manually by using the standard colour bars of NTSC or by using other methods like `blue gun’. New monitors do not require this as calibration can be done automatically.

Images cannot be visualised on the monitor unless they are wired. Monitor cables are of three types. The RGB cable has 3 wires one for each primary colour. The Y/C cable has two wires one for the luminance (Y) and one for the chrominance (C) component. The composite cable consists of one pair of wires. An important factor to realise is that no matter what type of cable is used, whether it has better band -width or other advantages the final resolution depends upon the monitor used.

We face many problems with monitors in regard to minimal access surgery. But before dealing with them, a mention of the frames of reference in vision would be apt. It is beyond the scope of this essay to go through them in detail. N.J.Wade’s paper on `Frames of reference in vision’ mentions various frames namely retinocentric, egocentric, geocentric and patterncentric. He applies these to minimal access surgery and finds a dissociation of patterncentric motion (seen on the monitor) and the area of manipulation. Any visual- motor task requires a match between the co-ordinate systems operating in both vision and motor control. Knowledge of these frames can alter our perspective of the way things happen in minimal access surgery with respect to vision.

After routine use we encounter many drawbacks with the monitor. Only a 2D picture can be seen on present day monitors. The operative field is represented only by monocular depth cues. Monitor positioning is such that the visual- motor axis is disrupted. The monitor distance from the surgeon is also quite far. As a result the efficiency of the surgeon decreases. Apart from pictorial depth cues the picture can be further disturbed by anti- cues. These may originate from the monitor. Glaring effect due to reflection is one of these important anti- cues.

The endoscopes transmit resolution and contrast to the monitor. The efficacy by which this occurs determines the more delicate aspects of the image. Resolution and contrast can be measured on a specially designed optical bench and expressed as Modulation Transfer Function (MTF). If there is excessive glare in the picture then contrast and resolution decrease. Distortions of the image can occur and if these lines seem to curve outwards they are called `barrel distortion’. Field curvature occurs when there is improper focus of the Centre from other parts. Astigmatism can occur when some lines of different orientation are present in focus and others are not.

The list of drawbacks does not end here. The others encountered are mentioned below. When a moving object is shown on a monitor, unless the speed with which it is moving is similar to the refresh rate, then jerky movements will occur. This is called `temporal alaising’. This can be prevented by the use of filters, or by performing slow movements. Fatigue and headache can occur due to disturbance of saccadic eye movements. These are rapid eye movements used to visualise the borders of a field.

When a surgeon has to constantly look in a different direction and operate in another his efficiency to perform declines. The job becomes even more difficult if the monitor is positioned at a further distance-giving rise to spatial disorientation. A surgeon can perform optimally if he can look and operate in the same direction as in open surgery. This can also be called the` gaze- down position’

To overcome the problem of 2D viewing, various experiments are being done with stereoscopic systems. But 3D systems also have many disadvantages. A mention of some of them is made here. The visual cues are not similar to normal vision, they are unbalanced and can produce altered sense of depth, and they have fixed horizontal disparity.

To get a 3D picture the surgeon has to wear a liquid crystal glass with shutter technology. When the image from one eye is produced the shutter of the opposite side is closed and vice- versa. The two images are then super- imposed in the brain to get a 3D image. This is harmful to the surgeon on prolonged use, gives incorrect depth perception and results in headache and eyestrain. Another way in which 3D images can be obtained is a mechanism by which the surgeon wears a polarised glass. The shutter mechanism is present in the monitor. The final image however occurs by the fusion of the two images in the brain. Moreover there is no documented improvement with 3D over 2D systems. The current 3D systems can only be operated from a very close distance and if placed further will not produce the desired 3D effect.

With the increasing demands for technological development many new techniques are currently under trial. These seem to eliminate some of the problems encountered, but only time and repeated use will tell.

Head mounted display (HMD) is an interesting technique that aims at normalising the visual – motor axis. It consists of a monitor and the necessary connections mounted to the surgeon’s head with the power supply pack attached to the back of the surgeon’s shirt. It is not very heavy and also allows the surgeon to view peripherally. The optical characteristics are;

Lines of resolution- 420 x 320 lines.

Contrast ratio- 100: 1.

Horizontal field of vision- 220.

Diagonal field- 27.50.

Vertical field- 190.

The surgeon using the display will have to make adjustments to the interpupillary distance, focus and the distance from his eyes each time. Studies have shown the HMD to have certain advantages. It is light weight, comfortable to position, reduces mental stress, is cheaper than monitor systems, decreases eye strain, and it allows the surgeon to visualise the operative field directly (the abdomen and ports). The problems however are that the picture is granular, definition is not very good and nausea can occur.

As mentioned before the gaze- down position is said to improve the performance of the surgeon. As it brings the alignment between his hands and eyes to normal. This principle has been used in a project called `View- site’. This mechanism is used to project the operative field image onto a sterile screen placed on the patient’s abdomen close to the original area of surgery. However it cannot be used for extensive procedures as the image field is small, resolution is not unto the mark and separation and identification of tissue planes becomes difficult if bleeding were to occur.

Another system uses the same principle but instead of using a sterile screen the image is suspended in space. This is called the suspended image system’ (SIS). It basically consists of two components: a high precision retro- reflector and a beam splitter. With the help of these the system can produce images with good resolution and can suspend them on top of the patient in close vicinity to the operative site.  The advantages of this method are that there is no distortion, object can be placed anywhere, focal length is not specific and the image is similar to the original in size. This system is also said to improve the sense of depth, as there are no anti- cues. And as is obvious the visual- motor axis is correctly aligned for optimal performance.

VISTRAL is a system currently under trial. The advantage of this system is that does not allow flatness cues to occur in 2D pictures. This improves the sense of depth and it does not require binocular depth cues. It is also said to reduce fatigue and eye strain.  However this system does not bring about any changes to resolution, brightness and colour.

Another remarkable advancement in technology is the `High Definition Television ‘(HDTV). It uses component signals, the resolution of the picture is much better, and there are no distortions. They use about 1,100 lines of resolution.

Some systems currently under evaluation and their requirements are given below:

SYSTEM	JAPAN (NHK/SONY)	EUROPE (EUREKA 95)	USA
Number of lines.	1125	1250	1050
Visible lines(92%)	1035	1150	966
Pixels per line	1831	2035	1709
Total number of pixels	1895085	2340250	1650894
Field frequency(cps)	60	50	59, 94
Luminance	20	20	20
Chrominance	7	7	7

These systems however require large amounts of space, can cause problems during transmission and due to the increased definition small unwanted movements can be magnified and visual stress can be increased. More work needs to be done before the HDTV can be put forward for regular use. The alternatives to HDTV are;

The PALPLUS which is an advanced modification of PAL and D2-MAC, HD- MAC which are used for satellite transmissions.

What does the future have in store? Can it bring about technological marvels that can replicate the human eye? Future developments have to mainly concern themselves with improving resolution. This will ultimately result in excellent visual acuity and image resolution.

Prof. Dr. R. K. Mishra.