Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-01-18
2002-09-10
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C250S208100
Reexamination Certificate
active
06448545
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to precision imaging systems. More particularly, the invention relates to precision endoscopic (and other, e.g. mammographic) imaging systems that operate at low levels of radiation to form a high-resolution image.
BACKGROUND OF THE INVENTION
In endoscopic imaging systems, high image resolution and high sensitivity (or low radiation). is an important system characteristic. This is particularly true in medical imaging through where the clarity and contrasts within an image directly affect the diagnostic capabilities of a physician. That is, the higher the resolution and the higher the sensitivity, the earlier and easier the detection of abnormalities is. Likewise, industrial uses such as quality control of product components operate in much the same manner and lack of detection of abnormalities can have similarly disastrous results.
Higher resolution images at low radiation levels can also help distinguish aspects of the image thus presenting additional valuable information. For example, if the image shows certain fractal duct structures then a physician may be able to deduce that a tumor is benign. Further, accurate representation of objects in the image, as to image size, for example, assists in diagnosis. That is, the observation of a stable tumor size over time alleviates the fear of malignancies without intrusive and invasive operations.
Applications of fluorescent endoscopy compare tissue regions based upon different image signals for at least two light wavelengths, e.g. red and green images for the same tissue. However, the process is typically limited by detector noise and the rate at which the different color images are provided. The noise is typically at the level of 100 electrons, while the rate of acquiring a red and green image pair is as much as a few hundred milliseconds or more, requiring patient immobilization and mechanically fixed, i.e. tripod-mounted endoscope probes which seriously inhibit the usability of present day fluorescent endoscopes. The latter could be solved by using a pair of photo detectors assemblies for measuring the intensity of light emitted from the tissue at two different wavelengths simultaneously. However, two detectors are clumsy and requires expensive cross-correlation which is costly in time and expense.
A proximity electron bombardment charge-coupled device (EBCCD) has no focussing. To have acceptable resolution, the distance between the EBCCD chip and photocathode must be kept small (0.7-0.8 mm). At this distance, it is impossible to apply high voltage to the elements of the EBCCD. The maximum gain is typically limited to a few hundred. Moreover, positive ions are produced by energetic electrons which after hitting the anode, strike back at the cathode to produce spurious electrons, which: hit the anode again at different positions from the original electron. Such spurious electrons produce noise in the image signal, destroys the image resolution and shorten the life of the EBCCD. Therefore, intensified image CCDs have heretofore included micro-channel-plate (MCP) elements to reduce these noise source. However, the MCP itself is noisy with only about 60% acceptance.
Prior intensified CCD devices all use the (light-spreading) phosphor screen as the transmitting medium and the (noisy and with small acceptance) MCP as the amplification means. All such intensified devices have poor modulate transfer functions (MTF) and poor image quality.
Medical diagnostic imaging systems utilizing x-ray image intensifier tubes are well known in the art. The image intensifier tube has as a component a scintillator that converts an x-ray image, representing the absorption of x-rays by the :structure to be depicted, into visible light. Devices such as this are widely used for medical observation. The visible light can then be made to impinge upon a photographic film or a photosensitive detector that electronically records the image. The film can then be developed for direct review, at the expense of time, or the electronic signals from the detector can be processed and transmitted to a cathode-ray tube (‘CRT’) or photographic recording system.
FIG. 1
shows a prior art scintillator
10
, which is generally formed by depositing cesium iodide by vacuum evaporation onto a substrate
14
. The thickness of the cesium iodide, or structured cesium, deposited generally ranges from 150-500 microns. The cesium iodide is deposited in the form of needles
12
each with a diameter of 5-10 microns. Since the refractive index of cesium iodide is 1.8, a fiber optic effect is obtained. This effect minimizes the lateral diffusion of the light within the scintillating material. A scintillator of this type, for example, is described in U.S. Pat. No. 4,803,366 dated Feb. 7, 1989.
The resolution of the image intensifier tube depends on the capacity of the cesium iodide needles
12
to properly channel the light. Non-uniformity (i.e. positions dependent light yield across the needles) and cross talks between the needles can result in large non-Gaussian tails which degrade the spatial image resolution. The cesium iodide as well as another popular material, sodium iodide, used as x-ray converters all have relatively low densities and thus low detective quantum efficiency (“DQE) if a thin layer of scintillator is used, and/or poor spatial resolution if a thick layer of scintillator is used.
These factors can be seen with more particularity in
FIG. 2
which shows the blooming of a single pixel imaged using these conventional scintillators. The vertical axis represents intensity of the pixel and the horizontal axis represents position relative to the center of the pixel with respect to light. One skilled in the art will understand that the broader a particular function of light for a pixel appears on this graph, the lower potential resolution on a photosensitive medium, such as film or a CRT, since this will represent a blooming and a potential for cross-talk between individual pixels. Each line represents different prior art systems. Line
20
represents a Lanex fast screen; line
22
represents a non-structured cesium iodide crystal layer of 220 Micron thickness; line
24
represents a structured cesium iodide layer of 220 micron thickness; line
26
represents a Lanex fine screen; and line
28
represents a structured cesium iodide layer of 75 micron thickness.
Often, as is the case with x-rays, the radiation used to create the image has potentially harmful effects on the subject of the examination. Devices with higher DQE reduce the required radiation doses per viewing and allow more frequent viewings for the observation of the growth rate of abnormalities. The density of cesium iodide and sodium iodide crystals is low, thus, prior art scintillators have a low DQE when the scintillator is thin. DQE can be raised by increasing the thickness, but this is done at the expense of spatial resolution.
Conventional methods of fabrication of scintillators, such as vacuum deposition or chemical vapor deposition, have difficulty making films of single crystals of more than a few microns thick. This, in turn, detrimentally affects the light conversion efficiency of the scintillator.
Once the scintillator converts the x-ray image into visible light, there is often still the problem of inadequate light to adequately resolve objects clearly in the image by a detector in the image intensifier tube. The problem is common in various other applications such as endoscopic or laparoscopic imaging, and non-medical imaging such as night-vision photography, for example. Commercially available systems of the aforementioned types generally use as a detector a room temperature charge-coupled device (“CCD”) to electronically capture the image-bearing light. Such a CCD has no gain and, therefore, low signal-to-noise ratio, thus requiring intense light illumination. Each pixel in the CCD converts incoming photons into electron-hole pairs. This conversion is made with an efficiency about 30%. Mainly due to the thermal noise of the readout elect
Allen Stephone
Matzuk Stephen G.
Syncrotronics Corp.
LandOfFree
Precision endoscopic imaging system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Precision endoscopic imaging system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Precision endoscopic imaging system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2870518