X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-08-27
2002-05-07
Porter, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S008000, C378S004000, C378S005000
Reexamination Certificate
active
06385279
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for positioning an x-ray beam in a multi-slice CT imaging system.
In at least one known computed tomography (CT) 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. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during 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. 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.
In a multi-slice system, movement of an x-ray beam penumbra over detector elements having dissimilar response functions can cause signal changes resulting in image artifacts. Opening system collimation to keep detector elements in the x-ray beam umbra can prevent artifacts but increases patient dosage. Known CT imaging systems utilize a closed-loop z-axis tracking system to position the x-ray beam relative to a detector array. It would be desirable to provide a closed-loop system that operates during patient scanning to maintain the x-ray beam penumbra in the z-axis at a position relative to a detector array edge to minimize patient dosage, but far enough away from the edge to reduce artifacts.
BRIEF SUMMARY OF THE INVENTION
There is therefore provided, in one embodiment, a method for positioning an x-ray beam on a multi-slice detector array of an imaging system in which the detector array has rows of detector elements and is configured to detect x-rays in slices along a z-axis. The method includes steps of comparing data signals representative of x-ray intensity received from different rows of detector elements and positioning the x-ray beam in accordance with a result of the comparison.
The above described embodiment and systems performing this method periodically adjust the x-ray beam position to maintain the beam penumbra at a minimal distance from the detector array edge, so that patient dosage is minimized and imaging artifacts are reduced.
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Rabe Jerome D.
Toth Thomas L.
Woloschek Steven J.
Armstrong Teasdale LLP
Hobden Pamela R.
Horton Esq. Carl B.
Porter David P.
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