X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2000-06-26
2001-10-23
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S019000
Reexamination Certificate
active
06307915
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for triggering of X-ray imaging sensors. In particular, the invention relates to a method for triggering of an X-ray imaging sensor using a non-destructive readout capability of the sensor.
BACKGROUND OF THE INVENTION
Electronic image sensors, such as charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) pixel sensors, are replacing film as X-ray sensitive elements in dental and medical applications. 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.
In a typical configuration of an X-ray imaging system, the object to be imaged is placed between an X-ray generator and the image sensor. The image sensor registers the amount of radiation passing through the object. A concern in such an application of an X-ray sensor is to synchronize the image capture sequence of the image sensor with the output of the X-ray generator.
It is well known in the art that a dark signal accumulates in the sensor even when it is not exposed to X-rays. The dark current adds noise and reduces the dynamic range of the sensor. The dark signal is dependent on temperature and integration time. Since all electronic image sensors are subject to dark current, many techniques have been proposed to address the dark signal problem.
Methods of compensating for the dark signal in an X-ray image sensor are described in, for example, U.S. Pat. No. 5,519,437, which is incorporated herein by reference.
To minimize dark signal remnants, the sensor may be cleared of signal just prior to the onset of radiation and read out immediately after the end of the radiation pulse. Also, it is desirable to make the image acquisition period as short as possible.
Several approaches have been taken to address the synchronization concern. Methods known in the art for triggering an X-ray image sensor are described in, for example, U.S. Pat. No. 6,002,742, which is incorporated herein by reference.
In one known arrangement, the image sensor is electrically connected to the X-ray source and image acquisition is controlled by a timer of the X-ray source. The disadvantage of such an arrangement is that the image sensor can only be used together with a limited number of different X-ray sources since the connection between X-ray source and image sensor is not sufficiently standardized.
In a second arrangement, one or more supplementary X-ray sensing elements are located close to the imaging area to detect start and end of the X-ray pulse. A signal is sent from the supplementary sensing element to the control circuitry of the image sensor to control image acquisition. The use of supplementary sensing elements adds size and complexity to the image sensor arrangement. The supplementary sensing elements also have to cover a significant part of the image area to minimize the risk of being shaded by a dense part of the object to be imaged.
In a third arrangement, the image sensor itself is continuously read out while waiting for exposure. The signal, either from one pixel or summed from many pixels, is compared to a fixed or variable threshold to determine the onset of radiation. A disadvantage of this method, as used with, for example, CCD sensors, is that the readout is destructive and requires shifting of the image along the sensor. In arrangements with short exposure times the method causes loss of signal and image smearing. It is not possible to determine end of exposure using this arrangement in a system having an image sensor with destructive readout.
SUMMARY OF THE INVENTION
It is an objective of this invention to provide an improved method for the triggering of X-ray sensors.
Another objective of this invention is to provide a method for the triggering of X-ray sensors that requires minimal adaptation in order to be used with any of the known X-ray sources.
Yet another objective of this invention is to provide a method for the triggering of X-ray sensors that does not increase the size and complexity of the image sensor arrangement.
It is also an objective of this invention to provide a method for the triggering of X-ray sensors that is affected minimally by the risk of being shaded by a dense part of an object to be imaged.
It is yet another objective of this invention to provide a method for the triggering of X-ray sensors that does not cause loss of signal and/or image smearing.
In accordance with the present invention, a method for synchronizing an image capture sequence of an X-ray imaging system with an output from an X-ray source, the imaging system having a solid state radiation sensor with non-destructive readout of a plurality of pixels arranged in columns and rows, comprises the steps of randomly selecting reference pixels from the plurality of pixels of the detector, reading out each of the reference pixels regularly at a predetermined waiting time interval, and generating a start-of-exposure signal if the readout signal, integrated over the predetermined waiting time interval, at a predetermined number of the reference pixels exceeds a predetermined reference signal level.
Alternatively, the start-of-exposure signal may be generated if a rate of increase in the readout signal at the predetermined number of the reference pixels exceeds a predetermined reference rate.
The method may further comprise the steps of reading out the reference pixels at a second regular time interval after the start-of-exposure signal is generated, and comparing a rate of increase in the signal read out from the reference pixels with a predetermined rate value, and generating an end-of-exposure signal at least a second predetermined time interval after determining that the rate of increase in the signal read out from a second predetermined number of the reference pixels falls below a predetermined rate value. If the X-ray source is AC-coupled, the second predetermined time interval may be set according to a maximum time between pulses in the AC-coupled X-ray source.
Alternatively, the end-of-exposure signal may be generated a maximum expected exposure time interval after the start-of-exposure signal is generated.
A predetermined number of rows or columns located in at least two different areas of the sensor may be selected as the reference pixels.
These and other objectives and advantages of the invention would be apparent from the specification as a whole.
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AFP Imaging Corporation
Cooper & Dunham LLP
Kiknadze Irakli
Kim Robert H.
LandOfFree
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