Video monitor

The description of the video monitor covers all aspects of the video monitor in the operating room. The technical key steps of the surgical procedure are presented in a step by step way: monitors, principles of function, ideal requirements/laparoscopy, available material, usage and adjustments, usage problems.

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Video   monitor

Authors
A Garcia, D Mutter
Abstract
The description of the video monitor covers all aspects of the video monitor in the operating room.
The technical key steps of the surgical procedure are presented in a step by step way: monitors, principles of function, ideal requirements/laparoscopy, available material, usage and adjustments, usage problems.
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Publication
2003-10
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E-publication
WeBSurg.com, Oct 2003;3(10).
URL: http://www.websurg.com/doi-ot02en307a.htm

Video   monitor

1. Introduction
Endoscopic surgical images are usually viewed on a video monitor that displays the images captured by the camera.
2. Monitors
• CRT monitors
Cathode ray tube (CRT) monitors are used in the majority of operating rooms.
• LCD screens
The recent introduction of flat liquid crystal display (LCD) screens has not yet replaced conventional screens. The video monitor is the final element of the video chain for the surgeon. As with the other elements of this chain, the weakest link determines the final overall quality (”a chain is only as strong as its weakest link”, as the old adage goes).
3. Principles of function
• Principles of function
• Image reproduction
An image is reproduced on a CRT video monitor by rapidly sweeping a succession of horizontal lines across the screen, from top to bottom, with electron beams generated by 3 electron guns, hitting the phosphorescent screen coating through a shadow mask (a metal screen filled with holes).
• Types of masks
Each beam is dedicated to a type of phosphor that gives off a different color of light when struck: red, green, and blue (referred to as RGB).
• Red green blue
These three primary colors can be added together to create the full visible spectrum. The electron beams “illuminate” each of these phosphor dots with a certain intensity in order to create the color of the image.
• Standards
The European standards (PAL: Phase Alternate Line, SECAM: Séquentiel Couleur A Mémoire) and American (NTSC: National Television System Committee) are different, but their principles are the same.
• Interlacement
CRT video systems using the PAL, SECAM and NTSC standards are interlaced. Each frame consists of two fields displayed in two passes.
Firstly, only one out of two horizontal lines are scanned from the top to the bottom of the screen (odd lines = odd field). Then, the other lines are scanned (even lines = even field). Thus, with the principle of interlacing, the monitor displays the first field of alternating lines over the entire screen, and then displays the second field to fill in the gaps left by the first field. In Europe, the renewing of the image occurs 25 times per second on a 50 Hertz monitor (capable of renewing the frame 50 times per second). The video frames are displayed every 1/25th of a second and contain two interlaced fields displayed 1/50th of a second each. The complete image (2 fields) is therefore refreshed every 1/25th of a second. Due to the fact that the retention of the image by the eye’s retina (approximately 1/30th of a second) is longer than the refresh rate of the frames, the brain does not perceive the individual frames, but the complete restitution of the image. In addition, following the electron impact, the phosphors have a retention effect that allows them to hold the light long enough to trick the brain into perceiving a persistent frame. The double scanning of the frames avoids flickering on the screen and improves the clearness of the image. Each field (even and odd) corresponds to one half of a 312.5 line frame, which gives a complete 625 line frame (only 576 lines are used for the image, the remaining lines are used for sound, synchronization and for other signals, such as Teletext). Interlacing is preferable to progressive scan, in which all lines in a frame are displayed in one pass from top to bottom before the next frame appears, because when the speed of the scan is insufficient, the first lines begin to disappear when the beam reaches the bottom of the screen.
With the NTSC standard, the operating system is exactly the same. The only difference involves the number of horizontal scan lines and the refresh rate of the frames (due to the oscillation frequency of the electric current).
• CRT video screen resolution
• “100 Hertz” monitor
Video monitors dedicated to surgical use are often available in 100 Hz versions. This means that the image will be refreshed 50 times/second instead of 25 times/second as is the case for a conventional screen, i.e. the frames are refreshed twice as often and twice as quick. Consequently, the image is softer, smoother and flickers less, which decreases eyestrain.
• Resolution
The horizontal resolution of surgical video monitors has increased twofold during the past 10 years and is now nearing that of high-definition monitors.
The resolution of a television screen is determined by the number of horizontal scan lines (= vertical resolution that is predefined by video standard used) multiplied by the number of vertical scan lines (= horizontal resolution). These are determined by the number of side-by-side dots (phosphors) that can be reproduced within any one scan line, and therefore by the distance that separates them. In practice, this refers to the maximum spacing of vertical lines in the horizontal plane that can be placed on a surface while distinguishing the whites from the blacks. The closer these lines are to each other, the more the number of visible lines on a given surface of the screen increases. The horizontal definition of a monitor is given by its manufacturer, who measures it with test charts.
Example:
For a video chain with a horizontal definition of 300 lines, 300 alternating dark lines and white spaces can be counted on a reference surface on the center of the screen. Most mono-CCD cameras have a horizontal resolution of 400 lines. However, today’s newest video cameras have a horizontal resolution of 800 lines or sometimes 900 lines. An optimal use of these cameras requires the use of high definition monitors, which currently offer a horizontal resolution of 800 to 900 lines. As mentioned in the specifications above, HDTV (High Definition TeleVision) monitors offer a horizontal resolution of over 1000 lines. One must keep in mind, however, the fact that the vertical resolution (= number of horizontal lines) is determined by the video standard used (PAL = 625 lines, NTSC = 525 lines, HDTV PAL = 1250 lines and HDTV NTSC = 1050 lines).
• Size of CRT screen
The size of a video monitor screen is defined by its diagonal (including the frame surrounding the screen).
The most common diagonals are 36, 42, 51 or 63 cm (14, 17, 19 or 25 inch).
Generally, a distance equivalent to 4 to 5 times the diagonal of the screen is advised for viewing surgical videos. This is why 51 cm screens are recommended in laparoscopy, as they allow the surgeon and surgical team to see well up to 2.5 meters (8 feet) from the monitor. Working with smaller screens can cause harmful eyestrain.
• Connections
Video monitors have the same analog connections as endoscopic cameras in order to be compatible.
There are composite connections (BNC) where the brightness (light intensity) and the chrominance (color) are combined into a single signal, and component connections (Y/C and RGB), where the parameters are partially separated. It is important to note that on new digital cameras there are DVI (Digital Video Interface) digital outputs that can directly transmit a digital signal without digital-to-analog conversion. This type of connection is used only with a digital monitor (e.g. LCD).

The following connections are found at the back of the monitor:
- BNC (Bayonet Neill Concelman) = Video out;
- Y/C = S-video = S-VHS: two separate cables, one for light intensity, the other for the colors, that are mixed together (Y = brightness synchronization signal, C = chrominance);
- RGB (red-green-blue): three BNC connectors of equal length, one for each primary color. The brightness is not conveyed, but calculated by adding the three colors according to the formula: Y = 1 = white = 0.3 R 0.59 V 0.11 B;
- RGB-S: fourth cable transports the image synchronization signal, used for synchronizing the different video equipment.
The quality of the transmitted image is dependant on the connections used. From an electronic point of view, the image quality depends on the quantity of information transmitted between the image captured by the camera and received by the monitor, as well as on the signal processing quality. The lowest quality is obtained by a composite BNC connection, the optimal quality is obtained by a component RGB connection, provided that the calibration of the entire video chain is perfect. This is necessary if a correct white balance and a faithful reproduction of the colors is to be achieved. However, since the resolution is generally limited by the monitor, there is no notable improvement with a RGB connection as compared to a Y/C connection. It should be noted that with a RGB connection, the colors cannot be modified on the monitor.
• Flat screen LCD
• 1
Flat screens are digital computer monitors as opposed to analog video monitors.

There are two main types of LCD screens:
- DSTN (dual-scan twisted nematic) screens, better known as passive matrix screens, in which the activation of each pixel (picture element) is controlled by sending a charge down a vertical column and a horizontal row. When they meet, they untwist the pixel.
- TFT (thin film transistor) screens, also referred to as active matrix screens, in which each pixel is controlled by its own transistor.

Only TFT screens are used in operating rooms.

The principles behind LCD technology are different than those behind CRT monitors. Instead of a scanning electron beam, there is a grid made up of liquid crystals grouped in subpixel RGB triads placed in front of a light source.
• 2
The property of these liquid crystals is their nematic structure, and the way they change directions when an electric current is applied. This modifies the polarization of the light, which then either passes or does not pass through the liquid crystal.
The resolution of these screens is not calculated by the number of scan lines, but by the maximum display surface. This corresponds to the number of pixels that are physically present on the grid (e.g. 1600 x 1200). The other intervening parameter is the pitch, i.e. the distance between the pixels, calculated in millimeters. The lower the pitch, the closer the pixels are to each other, improving the resolution. A good LCD monitor has a pitch of less than 0.28 mm.

The refresh rate does not depend on the frequency of the alternating current (AC), but on the refresh frequency defined by the video signal used (PAL = 25 images/second, NTSC = 30, HDTV PAL = 50 and HDTV NTSC = 60).

LCD screens offer real advantages over CRT screens. Due to the lack of a cathode ray tube, they are usually lighter and less bulky. Their size is measured diagonally in inches, and corresponds to the actual viewable area. A 19-inch LCD screen roughly corresponds to a 51 cm CRT screen. Another advantage is the fact that the image is not scanned as in a CRT monitor, and therefore does not cause eyestrain, even when the viewer is close to the screen. Moreover, there is no longer a direct emission of electromagnetic radiation, which is particularly perceptible close-up. Finally, the system of a fixed grid composed of liquid crystals delivers the same quality image to all points of the screen. As a result, there is no longer a problem of fuzziness or distortion on the edges of the screen.

There are disadvantages, however. To begin with, the display quality remains inferior. This is mainly due to the response time of each pixel. When the image changes rapidly, the viewer can discern an image lag. The other major drawback concerns the viewing angle. This was more flagrant with passive matrix screens, which became illegible when the viewers were not exactly opposite them. Considerable progress has been made with today’s active matrix screens that often feature a viewing angle of over 70°, both horizontally and vertically. In practice, the image quality progressively worsens as the viewer moves away from the center of the screen. Furthermore, LCDs have difficulty producing black and very dark grays. As a result they generally have lower contrast than CRTs and the color saturation for low intensity colors is also reduced. Finally, they are more fragile than CRT screens. LCDs can progressively have many weak or stuck pixels, which are permanently and irreparably off.
• Plasma screen
• 1
Plasma technology has made it possible to produce very large sized, flat screens that provide a wide screen display (aspect ratio: 16:9). A plasma display consists of a layer of gas injected between two parallel panels of glass, both of which are covered with rows of electrodes. When an electrical current is applied, the gas reacts to form plasma producing UV light, which in turn reacts with the red, green, and blue phosphors in each pixel to produce visible light. Unlike other display technologies, where the image is scanned across the screen, in plasma screens all pixels are
• 2
Plasma screens provide an image of better quality than LCD screens. However, plasma screen image quality remains inferior to CRT screen image quality, especially in terms of the color accuracy and contrast.

Plasma screens are too large to be used in standard size operating rooms, and none are conform to medical standards in 2003.
4. Ideal requirements/laparoscopy
For ideal conditions in laparoscopy, high-performance video systems are required. Originally, many laparoscopic units were equipped with monitors with a vertical resolution of 625 lines (PAL standard) and a horizontal resolution of 300 to 400 lines. Today, CCD sensors have optimized the camera image, so it is recommended to use monitors with a horizontal resolution of 800 to 900 lines. As a general rule, the resolution of a laparoscopic monitor should be as close as possible to the resolution of the camera, in order to prevent any avoidable signal degradation.
5. Available material
Monitors are characterized in terms of the diagonal measurement of the screen, the horizontal resolution and adjustment possibilities. The monitor should be adjusted according to the camera image. It is recommended for standard usage in laparoscopy to work with a monitor with a 51 cm (19 inch) screen diagonal, an image refresh rate of 100 Hz, and a horizontal resolution of 800 lines. These capacities currently provide a very good display of the image transmitted by most endoscopic cameras.
6. Usage and adjustments
• Installation
The installation of monitors in the operating room depends on the frequency of laparoscopic use. Usually, a monitor is placed on a laparoscopic unit and moves around with the rest of the equipment. It is advisable to install a second monitor in the operating room, opposite the first monitor, to provide the assistant with a good view of the operation. In addition, it allows the surgical team, as well as the anesthesiologists, to follow each surgical step from wherever they are standing and adapt their actions throughout the operation. Ideally, a third, fixed monitor can be installed in a predetermined place in the operating room.
• Life-span
Contrary to popular belief, a CRT monitor does not last forever. The alignment of the electron beam that scans the screen after it flows through the shadow mask progressively loses its precision. The electron guns progressively lose their power, and the phosphorescent layer on the screen is progressively altered, especially if a fixed image is displayed over a long period of time. One should consider that a video monitor no longer works to its full capacity after 6000 hours of use, keeping in mind that the cathode ray tubes are designed to function for about 20,000 hours. This life-span is reached by most users after 3 to 6 years, since the monitor is often lit up for much longer periods than it is actually used. For this reason, the electron beam needs to be recalibrated by the manufacturer. If it has lost too much power, or if the phosphorescent layer has become altered, the monitor must be changed.
• Color temperature
As explained in the cold light source and camera chapters, adjustments made on the video chain must be adapted to the color temperature of the cold light source. Today, surgery is usually performed with metal halide or xenon arc lamps with a color temperature of 5600 to 6000 degrees Kelvin (K). Most video monitors are set around 6500K. Once this has been verified, no particular adjustments are needed. The color temperature setting on computer screens is higher (9500K) and therefore needs to be adjusted. These adjustments can usually be done via the menu mode of the monitor (see the manufacturer’s user guide).
• Possible adjustments
Possible adjustments of a CRT monitor:
Most monitors have similar adjustments:
- Color or chroma: adjusts the amount of overall color in the picture. This adjustment is inoperative if the monitor is connected to a video source with an RGB connection.
- Phase: makes red colors appear purpler or greener (only works on non-RGB, NTSC standard).
- Contrast: modifies the difference in luminosity between the darkest and brightest part of the image. It also modifies the reproduction and the display of the colors.
- Brightness: adjusts the quantity of luminosity in the image. It can compensate for a lack of light but makes the image look blurry and pale, altering the quality of display.
- Aperture: smooths or sharpens the outlines of the image. This adjustment is particularly visible when there is a text on the screen and is inoperative with an RGB connection.
- Degauss: demagnetizes the CRT when an undesirable magnetic field distorts the image or the colors. Degaussing occurs automatically when the monitor is turned on. The screen should not be demagnetized for at least 10 minutes after the last demagnetization.
- Input selectors (Line A, B, RGB): chooses the signal corresponding to the connection used for the video source (BNC, Y/C, RGB).
7. Usage problems
Aside from the loss of the electron beam with a total image loss, the monitor is effected by few alterations. The main anomaly is caused when the monitor is placed close to an electromagnetic source or a metallic mass. This brings about a deviation of the electron beam on the screen, producing a colored halo that is most often green. The solution consists in moving the screen away from the source of the electron deviation. In case of an alteration of the display, the monitor should be sent back to the manufacturer for verification and recalibration. If the problem is due to a defective cathode ray tube, it needs to be replaced. Because of the high cost of this repair, the entire monitor is usually replaced.
8. Conclusions
As the final element of the video chain, the monitor is often the limiting factor of the chain, and therefore should not be overlooked. Its quality and its resolution should be equal to those of the camera, in order to provide optimal visual comfort for the surgeon.