X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-10-15
2002-06-11
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000, C378S015000
Reexamination Certificate
active
06404842
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to tomographic imaging, and more particularly to methods and apparatus for reducing aliasing artifacts in computerized tomographic (CT) imaging in a twin beam imaging system.
In at least one known 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 an “imaging plane”. The x-ray beam passes through an 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 an 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.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms which weight the collected data as a function of view angle and detector channel index. Specifically, prior to filtered back projection, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The helical weighting algorithms also scale the data according to a scaling factor, which is a function of the distance between the x-ray source and the object. The weighted and scaled data is then processed to generate CT numbers and to construct an image that corresponds to a two dimensional slice taken through the object.
In some known clinical scans, projection weighting prior to image reconstruction is performed. For example, halfscan weighting has been implemented to shorten scan time by approximately 40%. Underscan weighting is used to suppress patient motion in axial scan protocols. On the other hand, helical weightings (“High Quality” or HQ mode, and “High Speed” or HS mode, for example) have been used to avoid artifacts resulting from constant table translation during a scan. However, because of a lack of quarter-detector offset, halfscan sampling without focal spot deflection typically does not produce aliasing-free images for high-resolution kernels.
BRIEF SUMMARY OF THE INVENTION
In one aspect, a method for weighting projection data is provided. The method includes helically twin beam scanning an object at a selected pitch to acquire a first data sample from a first detector row and a second data sample from a second detector row. The first data sample includes first conjugate data and first two pi data. The second data sample includes second conjugate data and second two pi data. The first conjugate data and the second conjugate data are conjugate each other, and the first two pi data and the second two pi data are two &pgr; projection angle apart. The method also includes identifying a center view of the first conjugate data and the second conjugate data, and weighting at least one of the first conjugate data, the second conjugate data, the first two pi data, and the second two pi data inverse to the identified center view.
In another aspect, a method for weighting projection data is provided. The method includes helically twin beam scanning an object at a selected pitch to acquire a first data sample from a first detector row and a second data sample from a second detector row. The first data sample includes first conjugate data and first two pi data. The second data sample includes second conjugate data and second two pi data. The first conjugate data and the second conjugate data are conjugate each other, and the first two pi data and the second two pi data are two &pgr; projection angle apart. The method also includes identifying a center view of the first conjugate data and the second conjugate data, wherein the center view is &pgr;/p from &bgr;
1
and &pgr;/p from &bgr;
2
. Wherein p is a helical pitch, &bgr;
1
is the projection angle at which the first detector row intersects a plane of reconstruction, and &bgr;
2
is the projection angle at which the second detector row intersects the plane of reconstruction. The method also includes weighting the first data sample according to
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where &bgr; is a projection angle, &bgr;
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is the identified center view, &ggr; is a fan angle, and &agr; is a relative strength parameter.
In a further aspect, a twin beam computerized tomographic (CT) imaging system for imaging an object is provided. The imaging system includes a detector array, at least one radiation source, and a computer coupled to the detector array and radiation source. The computer is configured to helically twin beam scan an object at a selected pitch to acquire a first data sample from a first detector row and a second data sample from a second detec
Armstrong Teasdale LLP
Bruce David V.
GE Medical Systems Global Technology Company LLC
Hobden Pamela R.
Horton Esq. Carl B.
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