Image analysis – Applications – Biomedical applications
Reexamination Certificate
1999-04-30
2004-03-02
Patel, Jayanti K. (Department: 2625)
Image analysis
Applications
Biomedical applications
C378S004000, C378S008000, C378S012000, C378S062000
Reexamination Certificate
active
06701000
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an imaging system, and more particularly, to reducing artifacts caused by detector signal lag.
In at least one known imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location.
In at least one known type of imaging system, commonly known as a computed tomography (CT) system, a group of x-ray attenuation measurements, i.e., projection data, from the detector array is referred to as a “view”. A “scan” of the object comprises a set of views made at different projection angles, or view angles, during at least one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. Typically, each slice represents less than approximately 2 cm of coverage of the patient in the patient or z-axis and is generated from data collected from 984 views during a rotation of the gantry. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT system collects data utilizing a large flat panel digital x-ray device, or detector, having a plurality of pixels arranged in rows and columns. However, such flat panels suffer from detector lag. The detector lag causes a significant portion of the signals from previous samples to incorrectly bias subsequent samples. A significant cause of the lag is related to the electron de-trapping resulting from the high density electronic defects in an energy gap. De-trapping times range from a few milliseconds to as long as 100 seconds. As a result of the non-uniformity of the lag, artifacts, such as rings and bands, occur in the reconstructed images.
It is desirable to provide an imaging system which utilizes a solid-state detector to generate substantially “artifact free” volumetric images. It would also be desirable to provide such a system which reduces a detector lag artifacts without significantly increasing the time required to generate the images.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained in a digital x-ray imaging system which, in one embodiment, collects projection data from a plurality of views and reduces detector residual signals between the collection of adjacent views. More specifically and in one embodiment, the imaging system includes an x-ray source and at least one solid-state x-ray detector. To generate volumetric images, at least one of the x-ray source and the x-ray detector are rotated around the object of interest. For each identified view, x-rays are emitted from the x-ray source toward the x-ray detector and projection data is collected for the view. During an inactive period between the collection of projection data for adjacent views, the emission of x-rays is stopped and each pixel of the detector is simultaneously energized at least once.
More specifically, during the inactive period, each scan line of the detector is simultaneously energized. The simultaneous energizing of all of the scan lines reduces a residual signal of each pixel. As a result, when projection data is collected for a subsequent view, the signal level of each pixel more accurately reflects the attenuation of the object of interest for the current view. Using the projection data collected for the plurality of views, cross-sectional images of the object of interest are generated.
Using the above described imaging system, detector lag artifacts are reduced to generate substantially “artifact free” volumetric images. In addition, the system does not significantly increase the amount of time required to generate the images.
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Horton Esq. Carl B.
Patel Jayanti K.
Tabatabai Abolfazl
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