X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
2000-11-15
2002-10-01
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S004000
Reexamination Certificate
active
06457861
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS (IF APPLICABLE)
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT (if applicable)
Not Applicable.
BACKGROUND OF THE INVENTION
The preferred embodiments of the present invention generally relate to medical diagnostic imaging systems, and in particular relates to a method and apparatus for correcting electronic offset and gain variations in medical imaging systems employing solid state detectors.
X-ray imaging has long been an accepted medical diagnostic tool. Xray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial, and abdominal images that often include information necessary for a doctor to make an accurate diagnosis. X-ray imaging systems typically include an xray source and an x-ray sensor. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against the x-ray sensor as an x-ray technologist positions the x-ray sensor and the x-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the x-ray sensor then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected x-ray energy from the x-ray sensor and prepares a corresponding diagnostic image on a display.
The x-ray sensor may be a conventional screen/film configuration, in which the screen converts the x-rays to light that exposes the film. The x-ray sensor may also be a solid state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations.
One embodiment of a solid state digital x-ray detector may be comprised of a panel of semiconductor FETs and photodiodes. The FETs and photodiodes in the panel are typically arranged in rows (scan lines) and columns (data lines). A FET controller controls the order in which the FETs are turned on and off. The FETs are typically turned on, or activated, in rows. When the FETs are turned on, charge to establish the FET channel is drawn into the FET from both the source and the drain of the transistor. Due to the imperfect nature of the amorphous silicon FETs, the charge is retained temporarily when the FET is turned off and bleeds out, decaying, over time. which corrupts desired the signal in the form of an offset. The source of each FET is connected to a photodiode. The drain of each FET is connected to readout electronics via data lines. Each photodiode integrates the light signal and discharges energy in proportion to the x-rays absorbed by the detector. The gates of the FETs are connected to the FET controller. The FET controller allows signals discharged from the panel of photodiodes to be read in an orderly fashion. The readout electronics convert the signals discharged from photodiodes. The energy discharged by the photodiodes in the detector and converted by the readout electronics is used by an acquisition system to activate pixels in the displayed digital diagnostic image. The panel of FETs and photodiodes is typically scanned by row. The corresponding pixels in the digital diagnostic image are typically activated in rows.
The FETs in the x-ray detector act as switches to control the charging of the photodiodes. When a FET is open, an associated photodiode is isolated from the readout electronics. The associated photodiode is discharged during an x-ray exposure. When the FET is closed, the photodiode is recharged to an initial charge by the readout electronics. Light is emitted by a scintillator in response to x-rays absorbed from the source. The photodiodes sense the emitted light and are partially discharged. Thus, while the FETs are open, the photodiodes retain a charge representative of the x-ray dose. When a FET is closed, the voltage across the photodiode is restored to re-establish a desired voltage across the photodiode. The measured charge amount to re-establish the desired voltage becomes a measure of the x-ray dose integrated by the photodiode during the length of the x-ray exposure.
Readout electronics read the output signal from the x-ray detector panel. When the readout electronics are activated to read out the output signal from the x-ray detector panel, an electronic offset may be added to the resulting image. For example, some excess charge may “leak” from the readout electronics and add to the output signal. The charge leakage from the readout electronics may induce structured artifacts (including ghost images or distortions) in the x-ray image. The offset, such as charge leakage, from the readout electronics can be measured initially by acquiring a “dark” image. A “dark” image is a reading done without x-ray exposure. A “dark” image simply activates the FETs on the x-ray detector panel and reads the output signal through the readout electronics. Thus, a “dark” image may determine the offset, such as charge leakage, from the FET controller readout electronics. By subtracting the “dark” image pixel value from the actual “expose” x-ray image pixel value of a desired object, the offset (i.e., charge leakage) effects from sources such as the readout electronics may theoretically be eliminated.
Gain calibration is performed on the detector and electronics in order to provide gain correction coefficients for the x-ray image on a pixel by pixel basis. Gain calibration includes the sensitivity of the detector and the gain of the readout electronics. A flat field uniform x-ray exposure, with only an x-ray calibration phantom that uniformly attenuates the exposure, is used for gain calibration. Thus, it is desirable to perform gain calibration infrequently. After exposure, pixels in the gain calibration image are examined. Pixels that have a small response (less than the mean) are multiplied by a factor greater than one. Pixels that have a large response (greater than mean) are multiplied by a factor less than one. Pixels that exhibit a response below a given threshold are mapped out as “dead” pixels. Pixels above a second given threshold are also mapped out. Pixels above the second threshold will probably saturate too easily. Pixels that saturate too easily will probably not return any additional signal, exhibiting limited dynamic range.
X-ray images may be used for many purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from x-ray imaging has applications in many fields, including medicine and manufacturing.
In any imaging system, x-ray or otherwise, image quality is of primary importance. In this regard, x-ray imaging systems that use digital or solid state image detectors (“digital x-ray systems”) face certain unique difficulties. Difficulties in a digital x-ray image could include image artifacts, “ghost images,” or distortions in the digital x-ray image. One source of difficulty faced by digital x-ray systems is offset (i.e., electronic leakage) and gain variation of readout electronics used in digital x-ray systems.
In an ideal image adjustment, offset correction may be performed as described above by subtracting the value of a “dark” image pixel from the value of a corresponding pixel in an exposed x-ray image. The result may be multiplied by a gain calibration coefficient described above. However, variation in gain and offset in readout electronics may affect offset correction and gain calibration.
Changes in temperature may have an effect on readout electronics. The output signals from the x-ray detector panel are very small. Since the output signals are very small, readout electronics are very sensitive. The sensitive readout electronics are susceptible to changes in temperature. Differences in temperature at different times will affect the signal read out by the readout electronics at the different times. Differences in temperature between gain calibration and at the time ex
Narasimhan Swami
Petrick Scott
Vafi Habib
Dellapenna Michael A.
Dunn Drew A.
GE Medical Systems Global Technology Company LLC
McAndrews Held & Malloy Ltd.
Vogel Peter J.
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