By Danny Scheffer
CMOS image sensors open up the design of cost-efficient and robust endoscopes for one-time use.
Endoscopes are used to examine the inside of live organisms, as well as cavities in technical structures, by means of image acquisition and presentation. Originally developed for medical diagnostics in humans, today they generally are used as advanced tools in the visual examination of difficult-to-enter cavities.
The broadest application of endoscopes is in medical practice, where they are used for mirroring the inside of the body through natural orifices or for minimally invasive surgery. Medical endoscopy has had a large impact on the handling, flexibility of use, and manufacturing cost of endoscopes. The strict hygiene conditions required in a surgical theater or treatment room necessitate the highest possible sterility of all applied treatments and equipment.
A rigid endoscope is composed of a direct-sight endoscope with glass lenses and adjustable ocular, a lateral connection for a light conductor, an adaptor that allows focusing, and a CCD camera head. This configuration is also called a video endoscope; it has been in use for more than a quarter century. But it exhibits three significant disadvantages: lack of image quality, the need for sterilization, and high manufacturing cost.
Research at the Millennium Research Group in April 2006 ("US Markets for Laparoscopic Devices 2006") indicates reduced image quality in comparison to electronic endoscopes. This is because the lenses used are made of glass, which causes image losses of at least 1% at each lens surface. Before the image reaches the camera, 25% to 30% of image quality is irrevocably lost. Above all, this pertains to the best case--when the endoscope is new.
Furthermore, all the video endoscopes in use today are laid out for repetitive use. They must be cleaned and sterilized after each application. Such a cycle of reconditioning can take up to 40 minutes. It exposes the equipment, as well as the staff, to toxic and burning chemicals, which in time may lead to failure of the system. To a large degree the lifespan of endoscopes is therefore determined by their cleaning, sterilizing, and transport conditions.
Trend to one-time-use solutions
Medical applications carry highest demands in hygiene. For a long time, they have been met by using one-way products. A strong trend in today's medical technology is to transfer this one-way concept to the equipment, as well. This pertains especially to equipment parts that come into contact with patients. In endoscopy this means instead of using endoscopes reconditioned for each new application using one-time-use components delivered and stored sterilized.
A typical example of the one-way endoscope concept is the approach taken by Micro-Imaging Solutions (Englewood, CO, USA; www.micro-imaging.us). Its new, ergonomically shaped endoscope design has the light conductor connected through the handle, as are the connections for air insufflator, irrigator, and exhaust pump, and a needle for using flexible tools. At the top of the handle there is a four-way mouse activator to navigate the menu screen. The mirror for selecting the direction, the rigid lens, and the CMOS image converter chip are all housed in the tip of the one-way endoscope.
Another trend in medical electronics is minimizing the diameter to be able to undertake more detailed examinations. Consequently, the CMOS technology used must be scalable. Currently produced sensors are 1 x 1 mm. Soon there will be ones measuring 0.5 x 0.5 mm.
Advantages of CMOS technology
In contrast to CCD image converters, sensor chips based on energy-saving CMOS technology offer several advantages for video endoscopes. CMOS circuits are highly immune to magnetic fields generated by medical RF equipment and therefore do not need shielding. A single supply voltage of 1.8 to 2.5 V for light loads is sufficient. All this simplifies endoscope design.
In addition, CMOS image sensors can be manufactured cheaply on existing CMOS lines. In high-volume production such a component should be available at less than US$ 10. This, in turn, pushes the cost of an endoscope for one-time use to less than US$ 200.
Another advantage of CMOS technology is the feasibility of integrating additional standard logic circuitry on the image sensor chip--placing drivers, converters, and evaluation logic next to image capture. Due to the high density of such chips, this yields compact one-chip sensors. The integration of further system functionality, leading to autonomous optoelectronic sensor systems, depends, in principle, on only the economic targets and conditions such as component size, production volume, and development cost.
The extremely small form factor of endoscopic image converters necessitates careful consideration of how much additional functionality can be realized, since this determines the number of connections. A minimum sensor configuration needs just four connections (pads): ground, supply voltage, and two counterphase outputs (pixel data and *pixel data). A separate clock line is not necessary because pixel frequency can be reconstructed from the signals.
The concept of an endoscope for one-time use can be optimally adapted to its examination task through a specifically designed sensor chip. Cypress Technology offers resolutions from 100 x 100 pixels to 1150 x 1150 pixels, equaling 1.3 Mpixels. Pixel sizes can go from 2.5 to 6 µm, and image repetition can be in the realm of 30 to 60 frames/s. Power consumption is dependent on various factors, covering 20 to 99 mW. A measure of sensitivity is the noise level in read mode. It is at a low count of 18 electrons.
Miniaturizing CMOS image sensors to allow the practical application of ever finer examination methods has its own hazards: smaller image points receive less light than larger ones. At the required resolution and diameter, pixel size is more or less fixed. In any actual design, it is important to utilize pixel size as good as possible and increase each pixel's sensitivity by additional measures. Among these is designing for the lowest possible noise level for use at body temperature to increase signal/noise dynamics.
Microlenses are usable for this application. They collect light over the entire pixel surface and concentrate it on the light-sensitive barrier layer. The disadvantages of microlenses, their small usable angle, are not relevant here: rigid lenses can be calculated and built for outputting the light practically in parallel. This is already used today for coupling glass fiber cables via rigid lenses to traditional flexible endoscopes with separate image sensors.