X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-06-22
2001-06-12
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06246742
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to methods and apparatus for reconstructing image data, and more particularly to methods and apparatus for image reconstruction in a computerized tomography (CT) imaging system providing reduced X-ray exposure as compared to conventional CT imaging systems.
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.
Exposure to x-rays in CT imaging systems may present a hazard to a patient. At least in the long term, it may also present a hazard to a physician performing a procedure in the vicinity of a CT imaging system. Current CT systems provide tomographic cross-sections of a patient with a field of view that is normally around 50 cm, and a gantry opening of 70 cm. For applications in imaging a small organ such as a heart, exposing a patient with X-ray photons across the whole cross-section of the patient where the region of interest is a small organ may not be justifiable.
Several techniques have been proposed to reduce the total exposure risk. For example, the x-ray source may be turned on only when the source is on its lower trajectory, where both primary and scatter are more likely to be attenuated by the patient table. Often, after initial localization and insertion of a biopsy needle, a physician is interested in a specific and targeted anatomy region. Although turning on the x-ray source on its lower trajectory limits radiation exposure of both the patient and the physician, it still exposes more of the patient to x-rays than is desirable and does not fully shield the physician from exposure. Exposure to x-ray radiation could be reduced through the use of an x-ray source limited in fan-angle coverage to the region of interest (ROI) of the patient. The data resulting from the limited X-ray source would then be limited in terms of fan-angle coverage. However, no previously known method or apparatus provides reconstruction, from such limited data, with the image quality typical of a state-of-the art CT scanner. When direct reconstruction is attempted from such limited data, a very large object-dependent shading is introduced over the ROI, rendering the image data useless.
It would therefore be desirable to provide a reconstruction method and apparatus that provided image reconstruction from limited projection data obtained from a CT scanner. In particular, it would be desirable to obtain such high-quality reconstruction of a region of interest from data obtained from a beam of limited fan-angle extent, or from limited exposure to a wider, collimated beam. In addition, because the ROI may not be directly centered in a beam from an X-ray source of a CT scanner, it would be desirable to provide a method and apparatus for transposing the ROI into the center of the beam without shifting the patient relative to the scanner bed.
BRIEF SUMMARY OF THE INVENTION
In one embodiment, the present invention is thus a method for reconstructing an image of an object utilizing an imaging system, in which a limited width beam of radiation is emitted towards the object, the limited width beam of radiation having a fan beam angle extent selected to encompass a perimeter of a region of interest (ROI) within the object and to encompass less than a perimeter of the object itself; a set of truncated projection data of the object, including projection data of the ROI, is obtained by detecting the radiation from the limited width beam of radiation passing through the object; low frequency components of the set of truncated projection data are estimated; and an image of the ROI within the object is reconstructed utilizing the set of truncated projection data and the estimated low frequency components. In this embodiment, high frequency components of the projection over the ROI are directly measured. A corresponding apparatus embodiment in accordance with the invention is also disclosed herein.
Information from a limited set of complete projection data can be used to estimate the low frequency components of the set of truncated projection data. However, additional information from complete projections is not required for reconstructing the image of the ROI, because low frequency components can also be estimated directly from the set of truncated projection data. Because the estimated low frequency components together with the set of truncated projection data are sufficient to reconstruct a high quality image, there is no need to obtain a full set of complete projection data with a broad beam of radiation, whether or not a partial set of complete projection data is utilized to obtain the low frequency component estimates.
The above described methods and apparatus achieve high quality image reconstruction from partial, or truncated projection data, thereby providing a reduced exposure to X-ray radiation for a patient and for medical personnel attending to the patient.
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“Reduction of Truncation Artifacts in Fan Beam Transmission by using Parallel Beam Emission Data,” by Tin-Su Pan et al., IEEE Transactions on Nuclear Science, vol. 42, No. 4, Aug. 1995, pp. 1310-1320.
“Choice of Initial Conditions in the ML Reconstruction of Fan-Beam Transmission with Truncated Projection Data,” by Tin-Su Pan et al., IEEE Transactions on Medical Imaging, vol. 16, No. 4, Aug. 1997, pp. 426-438.
Besson Guy M.
Pan Tin-Su
Armstrong Teasdale LLP
Bruce David V.
Cabou Christian G.
General Electric Company
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