Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-12-28
2003-09-09
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S425000, C600S473000, C600S476000, C378S062000, C382S128000, C250S208100, C348S308000
Reexamination Certificate
active
06618604
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 present invention generally relates to medical diagnostic imaging systems, and in particular relates to a method and apparatus for correcting the digital image offset induced by Field Effect Transistor (FET) photo-conductive effects in medical imaging systems employing solid state detectors.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray 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 x-ray 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. The source of each FET is connected to a photodiode. Each photodiode integrates the light signal emitted by the scintillator above it in response to the absorption of x-rays and discharges energy in proportion to the x-rays absorbed. The gates of the FETs are connected to the FET controller. The Image Acquisition Module reads signals discharged from the panel of FETs and photodiodes. The Image Acquisition Module converts the signals discharged from the panel of FETs and photodiodes. The converted energy discharged by the photodiodes in the detector is used by the Image Acquisition Module 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 and discharging of the photodiodes. When a FET closes, an associated photodiode is recharged to an initial charge. While the FETs are open, the photodiodes are bombarded with x-rays. The number of x-rays experienced by each photodiode corresponds to the x-ray dose. The x-rays are absorbed by the scintillator above the photodiode, which emits light and discharges the photodiodes in contact therewith. Thus, after the conclusion of the exposure, while the FETs are open, the photodiodes retain a charge representative of the x-ray dose. When a FET is closed, a certain amount of charge is applied thereto in order to re-establish a desired charge across the photodiode. When a FET is closed, the amount of charge required to restore the initial charge on each photodiode is measured. The measured charge amount becomes a measure of the x-ray dose integrated by the scintillator, with the resulting light integrated by the photodiode during the length of the x-ray exposure.
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. In particular, digital x-ray systems must meet stringent demands on Critical to Quality (CTQ) measurements in order to provide a usable image. CTQ measurements include image resolution, image resolution consistency (e.g., comparing an image from one system to another system), and image noise (artifacts, “ghosts,” or distortions in the image). In the past, however, digital x-ray systems were often unable to meet CTQ requirements or provide consistent image quality. This deficiency in part may be due to process variations in the semiconductor fabrication techniques used to manufacture solid state digital image detectors. Additionally, the decrease in image quality may be due to the inherent charge retention properties of semiconductor materials used in imaging technology.
Many semiconductor devices exhibit photo-conductive characteristics. Photo-conductivity is an increase in electron conductivity of a material through optical (light) excitation of electrons in the material. Photo-conductive characteristics are exhibited by the FETs used as switches in solid state x-ray detectors. Ideally, FET switches isolate the photodiode from the electronics which measure the charge restored to the photodiode. FETs exhibiting photo-conductive characteristics do not isolate the photodiode from the system, when the FETs are open. Consequently, charge from multiple photodiodes is restored simultaneously by the Image Acquisition Module. The Image Acquisition Module can not distinguish to which photodiodes the charge is restored, which corrupts the image acquisition process. The unintended charge leakage through the FETs may produce artifacts or ghost images or may add a charge offset to component values in the digital x-ray image.
FETs and other materials made of amorphous silicon also exhibit a characteristic referred to as charge retention. Charge retention is a structured phenomenon and may be controlled to a certain extent. Charge retention corresponds to the phenomenon whereby not all of the charge drawn into the FET to establish a conducting channel is forced out when the FET is turned off. The retained charge leaks out of the FET over time, even after the FET is turned off, and the leaked charge from the FET adds an offset to the signal read out of the photodiodes by the x-ray control system.
The FETs in the x-ray detector exhibit charge retention characteristics when voltage is applied to the FETs to read the rows of the x-ray detector. The detector rows are generally read in a predetermined manner, sequence, and time interval. The time interval may vary between read operations for complete frames of the x-ray image. When a FET is closed, the charge on an associated photodiode is restored by a charge measurement unit but the FET retains a portion of the charge. When the FETs are opened, between read operations, a portion of the charge retained by the FETs leaks from the FETs to a charge measurement unit. The amount of charge that leaks from the FETs exponentially decays over time. The next read operation occurs before the entire retained charge leaks from the FETs. Consequently, the charge measurement unit measures during each read operation an amount of charge that was retained by the FETs during the previous read operation.
The charge remaining on the FETs when a new read operation is initiated is referred to as the initial charge retention. The initial charge retention sto
Boudry John Moore
Cronce Richard Gordon
Perry Douglas I.
Petrick Scott William
Dellapenna Michael A.
GE Medical Systems Global Technology Company LLC.
Lateef Marvin M.
Lin Jeoyuh
McAndrews Held & Malloy Ltd.
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