Image analysis – Applications – Biomedical applications
Reexamination Certificate
1993-06-22
2001-04-17
Chang, Jon (Department: 2623)
Image analysis
Applications
Biomedical applications
C378S004000, C378S901000
Reexamination Certificate
active
06219441
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to reconstruction of images from three-dimensional data acquired with x-ray CT or SPECT scanners.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection 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 3D scan the x-ray beam diverges to form a cone beam that passes through the object and impinges on a two-dimensional array of detector elements. Each view is thus a 2D array of x-ray attenuation measurements and the complete scan produces a 3D array of attenuation measurements. Either of two methods are commonly used to reconstruct a set of images from the acquired 3D array of cone beam attenuation measurements. The first method described by L.A. Feldkamp et al in “Practical Cone-Beam Algorithm”,
J. Opt. Soc. Am
., A/Vol. 1, No. 6/June 1984 is a convolution backprojection method which operates directly on the line integrals of the actual attenuation measurements. The method can be implemented easily and accurately with current hardware and it is a good reconstruction for images at the center or “midplane”, of the cone beam. The Feldkamp method employs the conventional convolution—back projection form, but this is an approximation that becomes less accurate at larger cone beam angles. The second method proposed by Pierre Grangeat in “Mathematical Framework of Cone Beam 3D Reconstruction Via the First Derivative of the Radon Transform”,
Mathematical Methods In Tomography
, Herman, Louis, Natterer (eds.), Lecture notes in Mathematics, No. 1497, pp. 66-97, Spring Verlag, 1991, provides an accurate solution to the image reconstruction task based on a fundamental relationship between the derivative of the cone beam plane integral to the derivative of the parallel beam plane integral. While this method is theoretically accurate, it requires mathematical operations that can only be solved using finite numerical calculations that are approximations. The errors introduced by the implementation of the Grangeat method can be greater than Feldkamp and these errors are not correlated with cone beam angle.
SUMMARY OF THE INVENTION
The present invention relates to a computer tomography system which produces a three-dimensional array of data from which a set of 2D image slices can be reconstructed. More specifically, the system includes a 2D array of detector elements for receiving photons in a cone beam produced by a source while the two are rotated about a central axis to acquire data at a series of views, an image reconstructor which employs filtered back projection of the acquired cone beam data to produce image data; means for identifying data missing from the acquired cone beam data and estimating values for the missing cone beam data values; means for calculating correction image data from the estimated cone beam data values; and combining the correction image data with the back projection image data to produce an image slice.
A general object of the invention is to accurately reconstruct image slices from 3D cone beam data. A filtered back projection method is employed to accurately and efficiently produce the main part of the reconstruction. A second set of image data is also produced by estimating values not supported by the projection. Correction image data is produced from these estimated values and the resulting correction image is combined with the back projection image to produce the corrected image slice.
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Herman. “Image Reconstruction from Projections: The Fundamentals of Computerized Tomography.” Academic Press, pp. 108-117, 1980.*
Mathematical Framework of Cone Beam 3D Reconstruction Via the First Derivative of the Radon Transform, LETI/DSYS/SETLA/90-4433/PG, Nov. 1990, G. T. Herman, et al.
Practical Cone-Beam Algorithm,J. Opt. Soc. Am.A/vol. 1 No. 6/pp. 613-619/Jun. 1984, L. A. Feldkamp, et al.
Cabou Christan G.
Chang Jon
General Electric Company
Quarles & Brady LLP
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