Sensor characterization in memory

Details Sensor characterization in memory Sensor characterization in memory

2001-03-19

2001-11-20

Kim, Robert H. (Department: 2882)

X-ray or gamma ray systems or devices

Electronic circuit

With display or signaling

C378S098700

Reexamination Certificate

active

06320934

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to sensor characterization in an X-ray imaging system. In particular, the invention relates to storing sensor characterization in a memory resident in an X-ray image sensor.
X-ray image sensors comprising an array of active pixels are replacing film as the preferred tool for obtaining X-ray images of teeth and other body parts. An active pixel is a semiconductor device capable of converting an optical image into an electronic signal. When incident radiation interacts with a photosite, charge carriers are liberated and can be collected for sensing. The number of carriers collected in a photosite represents the amount of incident light impinging on the site in a given time period.
There are two basic devices, i.e., photodiodes and photogates, with many variants, employed to collect and sense charge carriers in a photosite. Variants of photodiodes include Pinned, P-I-N, Metal-Semiconductor, Heterojunction, and Avalanche. Photogate structures include Charge Coupled Devices (CCD), Charge Injection Devices (CID), and variants that include virtual phase, buried channel and other variations that utilize selective dopants which are used to control charge collection and transfer underneath and between the photogate(s) and the sense node.
Complementary metal-oxide semiconductor (CMOS) image sensors now also are available. Most CMOS imagers use Active Pixel Sensor (APS) technology, which utilizes an amplifier for each pixel. Due to process variations during the manufacture of these amplifiers, the actual gain and offset of each amplifier is slightly different from those of the other amplifiers. As a result, APS imagers suffer from high fixed pattern noise (FPN) problems. A solution to the variation in gain from one amplifier to another is to implement a unity gain amplifier (UGA) for every pixel. Each UGA requires the use of at least six field effect transistors (FETs).
An active column sensor (ACS) is described in U.S. Pat. No. 6,084,229. The ACS embodies the recognition that all but one of the FETs (the input FET) of each UGA for each pixel of a column in a CMOS active pixel sensor are redundant. In the ACS, the redundant FETs are absent, and replaced with a single shared UGA amplifier at each column for all the pixels in the column. The ACS has one dual input FET per pixel, and four or so shared FETs at each column.
Electronic image sensors, such as CCD or CMOS pixel sensors, have been adapted to be X-ray sensitive elements in dental and medical applications. The digital X-ray sensor is used to detect and record X-ray images which typically are downloaded to a personal computer via a cable. Examples of use of CCD-type and other X-ray image sensors in dental and/or medical environments are described in U.S. Pat. Nos. 5,671,738 and 5,744,806, which are incorporated herein by reference. An X-ray detector that comprises a plurality of CMOS active pixel sensors is described in U.S. Pat. Nos. 5,912,942 and 6,069,935.
Virtually all imaging sensors, CMOS, CCD, CID, etc., have defects, noise, etc., inherent in their design. One of the most common defects is dead pixels that do not record information. Other common defects include offset and variable sensitivity pixels, rows and columns. Typically, manufacturers of imaging sensors define an acceptable level of defects for their application, characterize these defects in production testing and create a software map (characterization) of the noise and defects of the sensor. This software map is then used in software filters to minimize the effect of the defects, so they do not appear in the images generated by the sensor. Without the filters, the dead pixels appear as black dots in the image.
In a typical X-ray imaging system in which image signals are supplied by an imaging sensor to a computer and processed by the computer before being displayed, a sensor characterization file corresponding to the sensor is stored on, for example, a floppy disk, and downloaded to such computer from the floppy disk. When the imaging sensor captures an image and supplies a corresponding image signal to the computer, the computer then applies a software filter using the sensor characterization file to eliminate the appearance of defects in the pixels and create an acceptable looking image which is displayed on a display or another medium. However, since users often forget the download step, images of poor quality frequently are obtained under those circumstances.
SUMMARY OF THE INVENTION
The present invention provides an X-ray image sensor comprising, in accordance with one embodiment, an array of sensor elements, and a sensor characterization storage device integrated with the X-ray image sensor. The sensor characterization storage device stores sensor characterization information identifying defects in the array of sensor elements. The sensor characterization information may include a list of sensor elements corresponding to dead pixels, or otherwise non-typical response pixels, rows or columns.
The present invention also provides an X-ray imaging system comprising, in accordance with one embodiment, an X-ray image sensor and a computer. The X-ray image sensor provides to the computer a signal corresponding to image data from the array of sensor elements.
The computer may retrieve the sensor characterization information from the X-ray image sensor and process the image data from the array of sensor elements using the sensor characterization information to correct for the defects in the array of sensor elements. The computer may replace data from a sensor element identified in the sensor characterization information as corresponding to a dead pixel, or other characterized defect, by interpolating data from sensor elements corresponding to live pixels surrounding the dead pixel.
The X-ray image sensor may have an integrated interpolator. The interpolator retrieves the sensor characterization information from the sensor characterization storage device, and processes image data read out from the array of sensor elements to correct for the defects in the array of sensor elements by using the sensor characterization information. The interpolator may replace data from a sensor element identified in the sensor characterization information as corresponding to a dead pixel, or other characterized defect, by interpolating data from sensor elements corresponding to live pixels surrounding the dead pixel or other characterized defect. The array of sensor elements may have a non-destructive readout structure. The interpolator may use the sensor characterization information to identify the sensor elements corresponding to live pixels surrounding the dead pixel, and read out the data from these sensor elements.
The X-ray image sensor may further comprise a processing section including a defect determination module. The defect determination module may determine at selected times the defects in the array of sensor elements by analyzing image data read out from the array of sensor elements. For example, the defect determination module in the processing section may, under command from the computer, perform the defect determination. In another embodiment, the computer generates the new sensor characterization information by analyzing the image data read out from the array of sensor elements to determine the defects in the array of sensor elements, and communicates the new sensor characterization information to a processing section in the X-ray image sensor.
The processing section uses the defects information provided by the defect determination module or the computer to generate new sensor characterization information and replace the sensor characterization information in the sensor characterization storage device with the new sensor characterization information. The information also, or alternatively, may be used to notify a user and/or a controlling device, and/or shut down the sensor.


REFERENCES:
patent: 5182624 (1993-01-01), Tran et al.
patent: 5331166 (1994-07-01), Yamamoto et al.
patent: 5744806 (1998-04-01), Frojd
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