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Charged-Coupled Device and Video Endoscopy
Modern flexible video endoscopes with photosensitive
charged-coupled device (CCD) chips were introduced in the
1990s as an extension of the development of first fiberoptic
endoscopes in 1960s. The CCD was first invented by researchers
at the Bell Labs as a new type of computer memory. Soon it
became apparent that it could be used as a light sensitive
element, which could capture images similar to a film in a
camera. The small size, low voltage, low energy consumption,
and low cost have lead to rapid proliferation of the CCD.
Many optic instruments including endoscopes and digital cameras
have been equipped with CCD for image capturing.
CCD chip is made of silicon, which is sensitive
to light of wavelengths less than 1.1 microns. (The wavelength
of visible light being 0.4-0.7microns). The surface of CCD
is divided to several hundred thousand to several million
identical light sensitive elements or pixels. Pixels are arranged
in an orderly fashion on the chip. The resolution of the images
is directly related to the number of the pixels. The light
energy packets (photons) falling on each pixel is absorbed
by the silicon and cause a reaction to take place. This reaction
generates electrical charge (electrons). The generated electrons
are hold in the pixel until the reading phase. Once it has
been read, the cycle starts again.
The ratio of generated electron to the received
energy defines the sensitivity of the CCD, which has a relatively
constant value over a wide range of absorbed energies. The
quantum efficiency of a CCD is the ratio between the number
of produced electrons to the number of incident photons at
a given wavelength. This number is non-linearly correlated
with the frequency of the light.
Image-reading is performed by the orderly
shifting of electrons to the adjacent cells. First, all rows
are shifted vertically (downward, Figure 1) and the last row
is placed in a Horizontal Shift Register (HSR). Then the data
in the HSR is serially transmitted to the A/D (Analog to Digital)
converter of a video processor. This process of row shifting
to the HSR is repeated until all the stored electrons are
transmitted to the video processing unit (Video Clip 1).
REPRODUCTION OF COLOR IMAGES
The CCD is inherently a monochromic sensor. Reproduction of
color images requires separate measurement of the intensity
of the red, green and blue components of light. In video endoscopy
the two techniques of "sequential illumination"
and "static filter" are used for this purpose.
In the "sequential illumination" technique the area
under examination is exposed consecutively to red, green and
blue lights. A Xenon lamp with a rotating wheel filter provides
primary light colors for exposure. The video processor is
synchronized with this wheel and temporarily stores the returning
signals from CCD in three memory banks depending on the color
of the light at the time of exposure. After receiving the
data for all three colors, the video processor either generates
synchronized RGB signals or combines them as a composite color
video signal for the monitor to display.
Another technique used in video endoscopy to generate images
is the "static filter." This filter, which is composed
of multiple primary color filter stripes, is mounted on the
CCD during device fabrication. Each CCD pixel responds only
to the light of the particular color of its filter. The major
benefit of this approach is the reduction of the complexity
of the system. However, this approach also reduces the effective
resolution of the image.
BLOOMING PHENOMENON AND ANTI-BLOOMING GATES
Each pixel has a limited capacity for electrical charge. The
maximum charge an individual pixel can hold before saturating
and leaking it to surrounding pixels (full well) varies among
CCDs. The maximum well capacity depends on the size of the
pixel. The larger the pixel, the more it can hold the charge,
before it leaks to the adjacent pixels.
If the CCD is subject to overexposure, it will produce image-smearing
due to a phenomenon known as "blooming," which deteriorates
the quality of the image. In order to prevent this image deterioration
some sensors offer anti-blooming gates that are manufactured
on the chip to prevent leakage of charge. However, since anti-blooming
gates are constructed into the light sensitive areas of the
chip, they reduce the size of the pixel by about 30%, diminishing
the sensitivity of the CCD. Chips with anti-blooming gates
are generally not recommended, if overexposure can be avoided
altogether.
Figure 2 below is an illustration of a typical CCD without
anti-blooming gates. Pixels are 15 microns by 15 microns with
a capacity of 165,000 electrons.
Figure 3 below is an illustration of a typical
CCD with anti-blooming gates. Pixels are 15 microns by 15
microns with capacity of 120,000 electrons.
FUTURE OF VIDEO ENDOSCOPY
The resolution and sensitivity of CCD chips are constantly
improving. This will result in a significant enhancement of
the quality of video endoscopes. Production of smaller CCDs
with higher resolutions will provide the opportunity to include
two or more sensors in an endoscope. By using two sensors
to look at a lesion from different angles, the examiner will
have a stereoscopic view of the lesion, which will facilitate
diagnostic and therapeutic procedures.
Bibliography
1. Charge-Coupled Device (CCD) Image Sensors. Kodak
CCD Primer, #KCP-001.
2. Sivak M. Gastroenterologic Endoscopy. Philadelphia:
WB Saunders, 1987.
3. Zuccaro G. Video endoscopy and the charge-coupled device.
In: Sivak M, ed. Gastrointestinal Endoscopy Clinics
of North America, Vol. 2(2). Philadelphia: WB Saunders, 1992.
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