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The CCD, or charge couple device, was developed in 1969 by Williard Boyle and George Smith at AT&T Bells Lab in New Jersey. It was originally developed as a memory storage device but has gained widespread notion for its function in digital imaging. CCDs are commonly used in most camera or video camera devices. Therefore, they are a crucial component to colonoscopes and many other medical imaging devices.
When Smith and Boyle developed the CCD their idea was to focus light on the surface of the CCD so that the light values could be stored electrically. These values, stored in the form of charge, could then be passed from capacitor to adjacent capacitor until they were read at the end of the line. This first CCD contained 8 pixels so the imaging was rather crude. During the initial stages of the project Smith and Boyle noticed the device was extremely sensitive to alpha particles of incoming light. This made it difficult to function as a memory device, but also made it ideal for imaging.
In 1974 Fairchild Electronics had improved the CCD enough to produce them commercially. Their unit was comprised of a 100 X 100 array of pixels, which compared to today's standards isn't significant, but at the time was unheard of. Their CCD, however, did not gain attention until the following year when it was adopted for use in video cameras for television networks. By 1980 a CCD with a 320 X 512 pixel array had been incorporated into a telescope at the Kitt National Observatory in Arizona. NASA soon adopted this practice and began using CCDs in all their telescopes. Thus began the widespread use of the charge coupled device.
Retrieved from Latent Imager
The CCD we're currently investigating can be summarized in 2 layers, the first being an organized layer of silicon which acts as the photoactive layer, and the second being the transmission region. When an image is projected through the lens it becomes incident on the capacitor array. This allows the capacitors to develop a charge across them that is proportional to the light intensity at that point. Next, the charge in one capacitor is transferred to its neighboring capacitor. This continues down the line of capacitors until the end is reached. At the final capacitor in the array, where the overall charge has accumulated, the net charge is emptied into a charge amplifier which converts this charge into a voltage. This process is repeated until all of the charge present across the capacitors is converted into a corresponding voltage. This voltage can then be digitized and stored in memory or fed continuously into an analog device for transmission or recording.
As mentioned before the surface of the CCD is composed of a microscopic layer of silicon. This superficial layer of silicon is usually doped with Boron yielding a positive increase in the number of free charge carriers. Underneath this layer is a substrate layer with twice the number of free charger carriers. When light strikes the superficial layer of silicon atoms a free electron will be produced. This creates a temporary "hole" in the silicon crystalline lattice. The free electron will then be collected in a "well" known as the depletion layer which is located deep within the silicon. At the same time, the "hole" is moved away from the well, where the electron is currently present, until it is displaced elsewhere within the silicon. Movement of this "hole" is how the charge is carried through the crystalline lattice.
Retrieved From Molecular Expressions
Retrieved From Molecular Expressions
Charge Coupled Devices have been a standard in colonoscopes since their development. These CCDs provide the means for actual viewing of the inside of the colon without using a physical camera. As explained above, the CCD acts like a camera and saves the image from the lens as a compilation of voltages emitted by an array of photodiodes. This image is then transported back to the base unit where the voltage is converted back into a form viewable by us. As you can see, the CCD is an essential component to the operation of the colonoscope. Below is the datasheet from the type of CCD that would be found in a colonoscope. Retrieved from Texas Instruments
The CCD is a function of the number of pixels it contains. A pixel is the smallest element viewable on a screen. Therefore, a CCD with a higher number of pixels will produce a more detailed image. This is accomplished by decreasing the size of each individual photodiode so that more of them can be placed in the same area. Producing a more detailed image can benefit the physician in a number of ways. When speaking to Dr. Silas he specifically mentioned that only recently had the quality of the images from inside the colonoscope become high definition. Dr. Silas Interview He explained that Pentax, the company that designs the scopes he works with, made a point to mention the HD capability of their scopes. Ideally, this ability should translate to an increase in polyp detection but hasn't been proven yet.
CCDs are rather expensive to make. Each one must be covered in silicon, photoresist, and special masks multiple times during the course of the its creation. They must then be individually inspected by a technician and each pixel checked for proper function. These costs begin to add up. Moreover, the CCDs used in colonoscopes are of the highest pixel count and thus quality so their cost is even higher. A typical CCD can run between 15 and a couple hundred dollars. Thus mass production and consumption isn't cheap. To combat this rising cost the idea of using CMOS, complementary metal oxide semiconductors, instead of using CCDs has been discussed. CMOS devices are much cheaper to manufacture and can be produced faster than CCDs. This would make them ideal for "one and done" devices like the old disposable cameras. However, the tradeoff lies in the quality of the images. CMOS devices tend to have a lower pixel count and since physicians don't like to sacrifice quality for convenience, many have chosen to shy away from scopes with CMOS devices. More information on the difference between CCD and CMOS can be found at the Dalsa Wesbite.
Another area for CCD advancement is in the use of filters. Dr. Silas Interview Dr. Silas mentioned that scopes are currently equipped with filters capable of processing the image in 15 to 20 ways. These include improving the ability to see flat polyps which are thought to be more cancerous, using color processing to differentiate between discolored areas and normal tissue, removing blood so the intestinal wall can be seen more clearly, and filtering that makes blood vessels stand out so they can be studied individually. Some of these ideas are still in the developmental stage but others are well on their way to becoming a staple in colonoscopes across the globe. All however, can be related to the charge coupled device and its function in the scope.
The images below are from the colonoscope display we were able to secure courtesy of Pentax. The caption on the board reads, CCD Unit Assembly.
(1) Davidson, Michael W. Anatomy of a Charge Coupled Device Molecular Expressions: Images from the Microscope. Florida State University. [Online]. Available <http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html>.
(2) Williamson, Mark. The Latent Imager. Engineering and Technology. [Online]. Available <http://web.ebscohost.com.proxy2.library.uiuc.edu/ehost/pdfviewer/pdfviewer?hid=11&sid=a2f52d2e-4b36-4aa6-b4a2-e01669ddaad1%40sessionmgr4&vid=3>.
(3) (2010). CCD vs. CMOS. DALSA Corporation. [Online]. Available <http://www.dalsa.com/corp/markets/ccd_vs_cmos.aspx>.