Endoscopes are medical devices designed to collect the smallest images from inside the human body. Endoscopes can be divided into two categories: rigid endoscopes, in which relay lenses are stacked together generally creating a high-quality image, and flexible ones-devices of lower cost where a camera is inserted near the tip of the endoscope while the image is transmitted via wires or fiber optics. In today&#;s articles, we will talk about the design parameters of a rigid endoscope. In a future blog entry, we will discuss the design of flexible endoscopes.
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There are some basic components of any endoscopic system. First, we need a light source. Traditionally, xenon sources are used (and less commonly, metal halide and tungsten halogen). In recent years, LED broadband LED sources are becoming more common. The second thing needed is a way to transfer the light inside the human body. Usually, a ring of optical fiber is used for this task.
It is important to note that the illumination light doesn&#;t go through the optical lenses used to observe the surgery. Endoscopes use the light that is tissue-scattered to create the image to avoid the final image degradation from the back light reflection in the optical system.
Traditionally, rigid endoscopes use relay lenses to relay the image from an objective lens to the camera sensor. In a past article, we talked about the fundamentals of relay lenses where we talked about a borescope which has a design very similar to a rigid endoscope. A schematic of a rigid lens can be seen in Figure 1.
It is possible to substitute some of these relay lenses by Gradient Refractive Index lenses (GRIN). The main advantage of GRIN lenses is that they simplify the manufacture of endoscopes. A drawback however is the reduction of field-of-view and a diminished brightness due to the low GRIN lenses numerical aperture.
As with any medical instrumentation, there are very strict controls on endoscope designs from their length and diameter, to the field of view and image resolution. However, some aberrations that may be important in an industrial setting are less important here. For example, distortion is well accepted due to the wide-angle view needed.
Sometimes the surgeon needs to observe a portion of the body not directly in front of the endoscope tip. In the case of knee surgery using endoscopes (arthroscopes), it is sometimes necessary to observe at a direction of 30 to 70-degrees. So it is necessary to attach prisms to the endoscope tip.
In case of physical dimensions, the diameter is a very important parameter. A typical knee arthroscope has a shaft diameter of 4-mm. Contained in that diameter we have an outer tube, a smaller tube that contains the illumination systems, and a third tube that contains the relay lenses. This limits the size of the optical components to around 2.8 mm in diameter.
The lens diameter sets the limit of almost every other parameter. The lens diameter limits the lens f-number and therefore the image brightness. Even if there is a need for higher illumination, there is a restriction on the number of optical fibers that can be added in the remaining space.
The ISO standard is used to evaluate the endoscope performance. This is an old standard but it is still useful and can help set up a basic framework to develop our designs.
There is a lot more information about the design of endoscopes and a single article won&#;t be enough to cover all possible topics. If you are interested in learning more, I&#;ll recommend D. Leiner&#;s book &#;Digital Endoscope Design&#; as a good introduction to this topic.
&#; Rigid tube endoscope: The main body of the mirror body cannot be bent or twisted, and the depth and distance of the rigid tube endoscope entering the body are much lower than those of the flexible tube endoscope.
&#; Flexible tube endoscope: the first standard of cavity mirror with soft and flexible body.
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&#; Rigid tube endoscope: According to the structure, it can be divided into straight rod non-separable tube endoscope, straight rod separable tube endoscope, curved tube non-separable tube endoscope and curved tube separable tube endoscope four categories. Various types of rigid endoscope structures generally include an outer scope tube (or sheath), a scope body, a light guide beam interface, an eye end nozzle, and an imaging interface part.
Unlike flexible tube endoscopes, some rigid tube endoscopes have no instruments and water gas channels, such as laparoscopy, thoracoscopy, mediastinoscopy, arthroscopy, etc. All kinds of operating instruments need to enter the body cavity through another incision to complete the operation under the monitoring and cooperation of rigid endoscope.
&#; Flexible tube endoscope: generally includes the following basic structures: a front end, a bending part, an insertion tube, a manipulation part, an eye receiving part and an imaging interface part. The front end part is a rigid part, and the end surface is provided with a plurality of cavities and window surfaces, which are respectively an outlet hole for water supply and air supply, an outlet hole for biopsy forceps, an objective lens and a light guide window.
The water and air supply is a common outlet. When gas is injected, the gas enters the human body cavity from this hole to expand the cavity; when water is injected, the water comes out of this hole to wash the objective lens surface and the surgical field of vision, so that the field of view is kept clear. The outlet hole of negative pressure suction and biopsy forceps is the same nozzle. When too much liquid in the cavity hinders observation, press the suction button, and the liquid can be sucked into the suction bottle through this hole. Biopsy forceps and other therapeutic instruments also enter the body cavity through this hole.
The bending part is located between the front end and the insertion tube, and is composed of many annular parts to form a coiled tube, and each pair of adjacent annular parts can move in all directions. The insertion tube is also called the mirror body or the hose part, and the inside is all kinds of pipes and wires. The operation part includes angle control knob, suction valve button, water and air supply button and biopsy tube opening. The operator can operate various buttons here to complete endoscopy and treatment.
&#; Rigid tube endoscope: It mainly enters the sterile tissues and organs of the human body or enters the sterile chamber of the human body through a surgical incision, such as laparoscope, thoracoscope, arthroscope, intervertebral disc mirror, ventriculoscope, etc.
&#; Flexible tube endoscope: It mainly completes the examination, diagnosis and treatment through the natural orifices of the human body. Such as gastroscope, colonoscopy, laryngoscope, bronchoscope, etc. mainly enter the human body through the human digestive tract, respiratory tract and urinary tract.
Hysteroscopy, cystoscopy and colonoscopy all have flexible and rigid scopes.
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