Choosing the Right Medical Endoscope
Choosing the Right Medical Endoscope
A rigid endoscope is often used for applications where access to the inspection area is not very sinuous, such as cranial sinuses, the ear canal, urinary devices, etc. It is coupled to a light source and sometimes a video camera is attached to the lens.
It can have a wide-angle lens, which allows for wide framing of close objects. It also offers high-definition images thanks to transmission by optical lenses. The rigid structure of the endoscope is due to the succession of lenses aligned in the tube. This allows the practitioner to obtain a clear image of the inspected area.
Lastly, rigid endoscopes can have one or more channels through which an endoscopic instrument can pass. These can be forceps to grasp or remove foreign bodies or tissue samples; scissors to cut tissue; brushes to collect cells or a lace to catch polyps for example.
Operative Instruments
Surgical endoscopes utilize high performance optical technology. The choice of endoscope requires a good deal of consideration since it is one of the most expensive pieces of surgical equipment both in terms of capital cost and maintenance. A good endoscope carries out three tasks. It must be able to transmit a good level of brightness, both in bringing light into the operative field and carrying the image to the camera. It must have large depth of field and minimal distortion of image viewed. Finally, the endoscope should also be able to withstand the mechanical and thermal constraints of the sterilization processes.
The endoscope is the instrument that has the delicate duty of extending the human eye into anatomical cavity and collecting the images to guide the surgery.
This runs along the length of the endoscope carrying light from the light post to the tip of the scope. The light is carried in a bundle of glass fibers. At each end of the channel the fibers are cut, polished, and capped to produce a light-transmitting surface.
Set at an angle to the main axis of the scope is a connection point for the cold light source cable. This connection must be of high optical and mechanical quality in order to avoid loss of light and overheating.
The last lens in the series, at the proximal end of the camera, magnifies and focuses the image for transmits to the camera. This part of the endoscope is designed to allow coupling to the camera lens.
A series of spaced glass rod lenses continually refocus the image up through the scope. Metal spacers separate each rod lens to maintain the appropriate focal distance between lenses. The number of lenses depends on the length of the endoscope. A longer scope requires more lenses but this reduces overall luminosity and increases the fragility of the system. In order to minimize the reduction in luminosity due to reflective loss all of the endoscope&#;s lenses are treated with an anti-glare system. This consists in coating each lens surface with a thin layer of magnesium fluoride, using a vacuum pack process. Thus, it is important to avoid damaging the optical surface of the endoscope.
The lens located at the distal end of the endoscope captures the image and directs it back through the endoscope. Its focal length is generally fixed. It determines the magnification of the object and the field of view of the endoscope. By changing the angle between the long axis of the lens and the axis of the endoscope the direction or &#;angle of view&#; can be changed.
So the endoscope can be divided into five distinct parts: (1) objective lens system, (2) rod-lens assembly, (3) ocular lens, (4) light post, and (5) light channel.
The rigid endoscope is constituted by a metal tube that contains two channels. One carries light to illuminate the operative field and the other carries the image of the operative field to the camera. The image channel is made up of a series of lens that are successively placed one after the other and separated by spacers. As the image passes through each lens it is inverted and the number of lenses depends on the length of the endoscope. This system is known as the Inverting Real Image Lens System (IRILS) and permits refraction of the light while maintaining overall luminosity of the endoscope. The &#;eyepiece&#; lens at the end of the series acts to magnify the image before it reaches the camera.
The desirable features of brightness and depth of field, and angle of view and minimal distortion, require opposite characteristics from the endoscope. This makes it difficult and costly to construct endoscopes with a large depth and width of field, high brightness, and minimal distortion.
The angle of view is the angle subtended by two points diametrically opposite to each other at the extremity of the field of view and the tip of the endoscope. By changing the curvature of the objective lens the angle of view can be changed. However, increasing the curvature of the lens produces more distortion in the peripheral part of the field of view (barrelling or fish-eye effect). Again higher the quality of the lens, the less the distortion.
There are two reasons why the periphery of the field of view can appear dark. The first is simply that the shape of the image transmitted to the camera by the endoscope is not exactly the same shape as the CCD. This problem can be corrected by image processing that can crop the image. The second cause is due to reduced light transmission through the periphery of the lens. The poorer the quality of the lens the more exaggerated this effect becomes.
The depth of field is the distance between near and far objects in the field, which appear focused. For a particular laparoscope this is fixed being determined by the configuration of the lenses and the caliber of the scope with a narrower scope having a deeper depth of field.
The brightness of source or of a surface is the perception of the amount of light produced by these sources and reflected from that surface. The brightness of the image diminishes further the light that has to travel and the narrows the channel along which it has to pass. Thus a long, small caliber endoscope will transmit an image of much reduced brightness compared to a short, wide caliber scope.
The focal length in an optical device is the measure of the lens focus. In particular, it coincides with the distance (in mm) between the lenses and the focal plane when the object to be focused is at infinity (so the optical beams are parallel to the axis of the system). The focal length of the endoscope is determined by the aperture of lens at the tip of the laparoscope (objective lenses). An object at the focal distance appears in its natural size within the image. However, when the object is brought closer to the lens, then it appears magnified whereas if it is moved further away then it appears smaller between the system and the object. If the endoscope is coupled to a camera fitted with a mechanical zoom the focal length of the camera can be adjusted. By moving the Charge-Coupled-Device (CCD) sensor away from the eyepiece lens the image is magnified but the field of view is reduced. However, most camera zooms are digital, which means that the image processor in the camera control unit simply magnifies each pixel. Thus although the image may appear magnified the resolution is diminished, leading to a decrease in the quality of the image, which becomes more apparent the more the digital zoom is used.
The optical quality of the endoscope is determined by the same optical laws that govern optical quality in photography.
Choice of Endoscopes
Most endoscopes used in laparoscopic and endoscopic surgery are monocular endoscopes. A wide choice is available on the market. The choice of the endoscope depends on the specific features of the instrument and the type of surgery to be performed. The characteristics of endoscopes are defined by a set of parameters to understand the type of performance that can support.
Direction (Angle) of View
Endoscopes with varying tip angulations ranging from 0° (standard forward looking scope) to 120° are available.
When laparoscopy was first introduced for digestive procedures, endoscopes with angulated tips of 25, 30, or 40° were widely used. Currently, their use is in decline as the brightness and field of view offered by angled endoscopes is generally inferior to those offered by 0° endoscopes. However, recent angled endoscopes with much improved optical properties are just being introduced. It should be noted that the view provided by an angled endoscope could impair surgical performance particularly in surgeons with limited experience.
Most laparoscopic surgical procedures can be performed with straight (0°) endoscopes. Some surgeons, however, prefer to use endoscopes with an angled line of sight, usually 30°, in order to explore areas that are difficult to visualize (gastroesophageal reflux disease surgery, urological surgery, gynecological surgery).
For the beginner who wants to approach the video-assisted breast surgery, we suggest to use the 0° endoscope, because its usage is more intuitive and immediate. In fact the lens collects images that come before the end of the endoscope. The 30° endoscope is recommended only to a more mature user, experienced in the management of the capture horizon since the correct orientation of the images varies widely with the rotation of the endoscope around its longitudinal axis. In fact, being the loggia mammary similar to a hemisphere, the use of 30° endoscope which eases the vision in the recesses and anfractuosities, it is not so essential.
We try to summarize the characteristics of the endoscopes underlining the advantages of each one.
most common type of scope, wide choice o market;
effective for most procedures;
higher brightness;
larger field of view;
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larger depth of field;
but unable to visualize some operative areas.
The advantages of forward looking endoscope (0°) are the following:
possibility to explore areas of the operative field that are difficult to access with a 0° endoscope.
The advantages of 30, 50, 70° endoscopes are the following:
lower brightness;
more difficult to control field of view;
light post must be kept vertical to avoid rotating field of view.
The disadvantages of 30, 50, 70° endoscopes are the following:
Width of View
The width of view of the endoscope usually varies from 20 to 60° although scopes with widths of views of 80° now exist.
Endoscopes are available with diameters from 3 to 12 mm. In general, the narrower the scope the less bright the image and the narrower the width of field. Certain manufacturers have developed endoscopes that are <3 mm in diameter. The principles of function of these endoscopes differ from conventional endoscopes. The image is transmitted via optical fibers, as the rod-lens system is not adapted for such small diameters. The depth of field, optical quality, and light transmitted by these endoscopes are considerably inferior to the standard 10 mm endoscopes. However, the optical quality of these micro-endoscopes is improving, although they remain difficult to use because they are so fragile.
Several manufacturers have developed endoscopes that provide a stereoscopic view of the operating field. Stereoscopic endoscopes transmit two images to a pair of video cameras. The image has to be viewed on a special projection system and a number of different systems exist. The use of stereoscopic systems is currently limited. The only regular use of the stereoscopic vision is for Trans-anal Endoscopic Microsurgery (TEM). They have also been incorporated into robotic laparoscopic systems.
With regard to the video-assisted minimally invasive breast surgery, the choice between endoscopes available is limited to those of 5 and 10 mm in diameter. In principle, the 5 mm endoscopes are efficient enough to provide a perfect vision of the operating field. This depends mainly on the fact that the virtual cavity in which the operation is performed (the mammary loggia), is sufficiently small to be well illuminated with the light produced from the 5 mm endoscope. In this way, we can limit the use of 10 mm endoscope that, although more potent in terms of brightness, are bulker and can create some limitation in the management of the instruments especially in the single port setting. Concerning the robotic platform, the choice does not arise as there is only 8 mm endoscope available. At the moment according to the present optical technology, it is not possible to have optics with two channels for the images acquisition of a smaller diameter.
Flexible and Semi-Flexible Endoscopes
Certain manufacturers have developed systems for surgical endoscopes that are comparable to flexible endoscopes. Two different types of systems exist:
The first has a standard rigid section and a flexible tip. This articulated part consists of a fiber optic bundle just like those found in flexible gastrointestinal endoscopes. The image is otherwise transmitted from the tip to the eyepiece of the endoscope as a light image.
A more innovative system uses a CCD sensor placed at the distal end of the endoscope. Thus, the shaft of the endoscope transmits an electrical signal rather than a light image.
Neither of these two systems are widely used.
All these features define the quality of an endoscope. Often, however, the manufacturers do not report the data for these characteristics and for an average user is still difficult to correctly interpret these data with the expected performance of the instrument. In this case then it is more convenient to rely on a series of small practical devices that allow us to assess the degree of reliability and quality of the endoscope.
So it is possible to check the quality of an endoscope as follows:
&#;Fish-eye&#; distortion can be checked by viewing a written text on a piece of paper through the endoscope. If the text looks flat, then the distortion is limited. However, if it looks spherical, the distortion is important.
The depth of field can be assessed by viewing a line of text and analyzing the total near-far distance over which the text appears in focus. This should be over 10 cm. In endoscopes providing a smaller depth of field the surgeon will have to repeatedly readjust the scope&#;s position.
The brightness of an endoscope is dependent not only upon its light channel and lens system but also upon the light cable and light source. Before assessing the endoscope for light transmission it must be attached to an optimized light source.
Gauge of Endoscope
The wider the endoscope the more light transmitted and thus better the final clarity of the viewed image. Narrower endoscopes have the advantage of passing down smaller trocars but generally have a narrower field of view. Thus a narrow scope may be perfectly adequate for a diagnostic or simple procedure and results in a smaller scar. However, for more complex procedures a broader scope gives a brighter and larger field of view.
An ideal endoscopic surgery kit would include a complete range of endoscopes (0&#;25, 30, 40, 50, and 70°) and a variety of gauges (12, 10, 5, and even 3 mm). At the present time, since cameras are not sterilized, it is difficult to change endoscopes during the operation while maintaining proper sterilization. Before starting the operation, the surgeon usually has to choose the endoscope that will be used throughout the procedure.
Usage Warning
Most endoscopes have no particular adjustment possibilities. Endoscopes should be used at body temperature or room temperature to prevent too much fog from forming on the end of the endoscope. Warming the scope in a flask of hot saline can be particularly effective. A variety of liquid preparations is available and can be applied to the tip to help reduce fogging.
Endoscopes are designed to be tough in order to protect the fragile systems of lenses that are glued within the scope. If there is an extreme temperature increase or an impact, the orientation of the lenses can be disturbed and the quality of the vision can be altered irreparably. This explains the often short life-span of endoscopes in the operating room.
Before the operation it is important to check the overall condition of the endoscope, search for signs of impact that could have dislodged a lens and caused a distortion of the image or a decrease in the light transmitted.
The light fiber bundle can also be damaged resulting in a decrease in the amount of light transmitted to the operating field.
It is recommended to carefully clean the endoscopes with a plastic brush and soap, especially the end inserted in the abdomen. Indeed, the accumulation of blood and protein at the end of the endoscope, combined with the heat from the light, cause a coagulation of the proteins and a progressive alteration of the quality of the light and image transmitted. For this reason, the tips of endoscopes must be frequently cleaned.
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