X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1999-03-19
2001-05-15
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S004000, C378S008000, C378S019000, C378S901000
Reexamination Certificate
active
06233308
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an imaging system, and more particularly, to artifact compensation with variable angular sampling for generating volumetric images of an object.
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 slow read-out times thereby increasing the time required to generate the image. For at least one known CT system the large flat panel is capable of collecting only 33 views per second. Utilizing a 10 second scan, only 330 views are collected, resulting in a significant reduction in the number of views collected per rotation of the gantry versus other known imaging systems. As is known in the art, such view under-sampling causes aliasing artifacts. The aliasing artifacts are the most severe in the upper peripheral of the patient as largely caused by the sharp structures of the high density elements, such as a spine. In addition, as a result of the spine orientation, the worst streaking artifact is parallel to the patient y-axis, indicating the most severe under-sampling occurs when the x-ray source is substantially aligned with the patient Y-axis.
It is desirable to provide an imaging system which generates a volumetric image of a complete object within a patient utilizing data collected from a single rotation of the gantry. It would also be desirable to provide such a system which alters the view collection rate as a function of projection angle so that aliasing artifacts are reduced and the scan is completed in a reasonable period of time.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by a digital imaging system which, in one embodiment, alters an angular spacing between samples of projection data so that image aliasing artifacts are reduced. Particularly and in one embodiment, the imaging system includes a rotatable gantry having an x-ray source that projects an x-ray beam toward a digital detector array. The digital detector array is fabricated in a panel configuration having a plurality of pixels arranged in rows and columns. The digital detector array is configured so that a volumetric image is generated for an entire object within a patient by rotating the gantry up to one complete rotation.
In operation, prior to performing a scan on a patient, at least two projection angle regions are identified with respect to the object. As the gantry is rotated through a plurality of projection angles, the angular spacing at which the projection data is collected from each region is altered. More specifically, a view sampling rate for each region is altered so that the time required to complete the scan is minimized and aliasing artifacts are reduced. Alternatively, the rotational speed of the x-ray source is decreased in areas containing sharp structures of high density objects so that the view sampling rate is increased to reduce aliasing artifacts. For those areas not containing such sharp structures, the rotational speed of the gantry is increased so that the view sampling rate is decreased.
After collecting the data utilizing the different view sampling rates, the projection data representing the collected views is interpolated to generate an additional number of views. The views are then reconstructed with known reconstruction algorithms to generate the volumetric image.
The above described system generates a volumetric tomographic image of a complete object within a patient utilizing data collected from a single rotation of the gantry. In addition, aliasing artifacts are reduced by altering the view sampling rate as a function of the projection angle. Particularly, by collecting a higher rate of views in certain regions and a lower rate of views in other regions, scan time is reduced and aliasing artifacts are reduced.
REFERENCES:
patent: 5228070 (1993-07-01), Mattson
patent: 5668845 (1997-09-01), Miguta
patent: 5828718 (1998-10-01), Ruth et al.
patent: 5848117 (1998-12-01), Urchuk et al.
patent: 0 119 664 (1984-09-01), None
Armstrong Teasdale LLP
Bruce David V.
Cabou Christian G.
General Electric Company
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