Endoscopic vision technology
In the past endoscopic procedures were done without the aid of monitors. The operator visualized 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 visualizing with only one eye.
As new methods of communication developed, the introduction of television brought about a significant impact. 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 visualize 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.
Laparoscopic Video monitor
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.
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 color and memory). The broadcasting standards for each are summarized below:
|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 is, 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 color 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 color 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 color image. This is done by super- imposing the data for color on the existing black and white picture. The black and white signal is monochromatic and combines with the composite color signal. This gives the final color signal. Luminance (brightness) is delivered by the black and white signal. Chrominance (color) is delivered by the color signal. It is called composite as it contains the three primary color information's (red, green and blue). A system that combines luminance and chrominance into one signal is called a 'compound system'.
Color 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 `color burst' can prevent this. The color on a monitor can be calibrated. This can be done manually by using the standard color 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 visualized on the monitor unless they are wired. Monitor cables are of three types. The RGB cable has 3 wires one for each primary color. 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 realize 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 pattern centric. He applies these to minimal access surgery and finds a dissociation of pattern centric 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 visualize 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 polarized 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 normalizing 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 inter-pupillary 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 visualize 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 patien'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 color.
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.||Mark||Otto||@mdo|
|Visible lines (92%)||1125||1250||1050|
|Pixels per line||1035||1150||966|
|Total number of pixels||1895085||2340250||1650894|
|Field frequency(cps)||60||50||59, 94|
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 is 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.
Laparoscopic camera is one of the very important instruments and should be of good quality. Laparoscopic camera today's available is either of single chip or three chip. We all know that there are three primary colour (Red, Blue and Green). All the colours are mixture of these three primary colour in different proportion. In single chip camera all these 3 primary colour is sensed by single chip. In three chip camera there are 3 CCD- Chips for separate capture and processing of 3 primary colour.
These 3 chip camera has unprecedented colour reproduction and highest degree of fidelity. Three chip camera has high horizontal image resolution of more than 750 lines.
Focusing of laparoscopic camera
Laparoscopic camera need to be focused before inserting inside the abdominal cavity. At the time of focusing it should be placed at a distance of approximately 5 cm away from the target. 5 cm distance is optimum for focusing because at the time of laparoscopic surgery most of the time we keep the telescope at this distance.
White balancing of camera
White balancing should be performed before inserting camera inside the abdominal cavity. White balancing is necessary everytime before start of surgery because every time there is some addred impurities of colour due to following variables
Difference in voltage
Different cleaning material used to clean the tip of telescope which can stain the tip
Scratches wear and tear of the telescopes eye piece, Object piece and CCD of camera
When we do white balancing by keeping any white object infront of telescope attached with camera camera senses that white object as reference and adjust its all primary colour (Red, Blue and Green) to make a pure natural white colour.