X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-10-27
2002-10-01
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06459754
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to methods and apparatus for reconstruction of imaging data, and more particularly to cone beam correction for reconstruction of three-dimensional computed tomography imaging data.
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 at least one known multi-slice CT system, a “cone angle,” or volumetric content of measured data, is very small. Therefore, this system currently processes 3-dimensional data using a 2-dimensional algorithm. By using the Feldkamp (FDK) algorithm, which is a simple perturbation of a 2-dimensional filtered backprojection (FBP) algorithm for image reconstruction, excellent image quality is obtained for these relatively small cone angles. The FDK algorithm is not exact, however, and as the number of slices increases (for fixed slice thickness) cone angle and cone beam artifacts increase.
It would be desirable to extend 2-dimensional CT fan-beam reconstruction algorithms to the cone-beam geometry of a third-generation multi-slice CT imaging system. Such a reconstruction could be based upon a modified FDK algorithm, but the modifications would have to compensate for both a cylindrical (rather than planar) area detector, and a helical (rather than circular) source trajectory. Use of a curved detector array requires data interpolation along curved lines on the detector, the application of a new data filter, and of pre- and post-convolution weights. However, the helical source trajectory complicates voxel-driven backprojection in that variable data (projection ray) redundancy conditions are encountered across a reconstructed image. For each voxel in a reconstruction volume, it is necessary to compute, for each source position, a ray passing from the x-ray source through the voxel. Thus, approaches must be found to address the handling of data redundancy and handling the variable number of rays that contribute to voxels.
Two approaches to addressing the problems of data redundancy and the variable number of rays contributing to voxels are known. In one approach, the helical pitch is limited so that at least two (interpolated) samples are obtained for each voxel. Extra data is thrown away, retaining only 2&pgr; worth of projections. The z-resolution available from conjugate rays is ignored to keep the approach simple. However, in patient scanning, this approach is impractical, because it severely limits patient coverage and results in increases radiation dose. A second approach improves on the first by providing better image quality (IQ) and reduced patient dose, while handling any practical pitch. However, a method implementing this approach to handling the above-mentioned problems requires that all data passing through a given voxel (for given source and fan angles) are simultaneously available. As a result, methods based on the second approach are impractical to implement.
It would therefore be desirable to provide methods and apparatus that provide acceptable compromises between improved image quality and practicality in third generation CT imaging systems.
BRIEF SUMMARY OF THE INVENTION
There is therefore provided, in one embodiment, a method for multi-slice, computed tomography imaging. The method includes steps of moving an x-ray source through a trajectory; projecting an x-ray cone beam from the moving x-ray source through an object towards a curved detector; and determining contributions of segments along the trajectory to voxels in a reconstruction volume of the object, including compensating the determined contributions for curvature of the detector and x-ray cone beam geometry.
The above described embodiment and others described in detail herein provide improved image quality in third generation CT imaging systems, even for cylindrical detectors and helical source trajectories. In addition, these embodiments can be implemented within the practicality constraints imposed by CT imaging hardware and patient dosage limitations.
REFERENCES:
patent: 5377250 (1994-12-01), Hu
patent: 5400255 (1995-03-01), Hu
patent: 5430783 (1995-07-01), Hu et al.
patent: 5960056 (1999-09-01), Lai
patent: 6075836 (2000-06-01), Ning
patent: 6078638 (2000-06-01), Sauer et al.
L.A. Feldkamp, L.C. Davis, and J.W. Kress, Practical cone-beam algorithm, J. Opt. Soc. Am., vol. 1(A) 612-619, 1984.
K. Taguchi and H. Aradate, Algorithm for image reconstruction in multi-slice helical CT, Med Phys. 550-561 Apr. 1998.
H. Hu, Multi-slice helical CT: Scan and reconstruction, Med.Phys. 26(1) Jan. 1999 pp 5-18.
G. Wang et al, A General Cone-Beam Reconstruction Algorithm, IEEE Trans. on Medical Imaging, vol. 12, No. 3, 486-496 Sep. 1993.
S. Schaller, T. Flohr, and P. Steffen, A New Approximate Algorithm for Image Reconstruction in Cone-Beam Spiral CT at Small Cone-Angles, IEEE Nuclear Science Symp. Record Conf. (Anaheim, CA 1996) vol. 3 ed. A. DelGuerra 1703-9.
D.L. Parker, Optimal short scan convolutional reconstruction for fanbeam CT, Med. Phys. 9(2), Mar. 1982 pp 254-257.
D.L. Parker, Optimization of Short Scan Convolutional Reconstruction in Fan Beam CT, IEEE pp. 199-202.
G. Besson, New classes of helical weighting algorithms with applications to fast CT reconstruction, Med. Phys. 25(8), Aug. 1998 pp. 1521-1532.
G. Besson, CT fan-beam parameterizations leading to shift-invariant filtering, Inverse Probl. 12, 815-833, 1996.
Besson Guy M.
Patch Sarah K.
Armstrong Teasdale LLP
Bruce David V.
Horton Esq. Carl B.
LandOfFree
Methods and apparatus for cone beam multislice CT correction does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods and apparatus for cone beam multislice CT correction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods and apparatus for cone beam multislice CT correction will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2951569