X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-06-05
2003-12-02
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06658081
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for computed tomography (CT), of the type wherein, for scanning a subject with a conical ray beam emanating from a focus and with a matrix-like detector array for detecting the ray beam, the focus is moved on a spiral path around a system axis relative to the subject, with the detector array supplying output data corresponding to the received radiation, and wherein images having an inclined image plane relative to the system axis are reconstructed from output data supplied during the motion of the focus on a spiral segment. The invention also is directed to a computed tomography apparatus of the type having a radiation source having a focus from which a conical ray beam emanates, a matrix-like detector array for detecting the ray beam, the detector array supplying output data corresponding to the received radiation, an arrangement for generating a relative motion between radiation source and detector array, and a subject, and an image computer to which the output data are supplied, the means for generating a relative motion for scanning the subject with the ray beam and the two-dimensional detector array causing a relative motion of the focus with respect to the system, such that the focus moves on a helical spiral path relative to the system, axis having a central axis corresponding to the system axis, and whereby the image computer reconstructs images with an image plane inclined relative to the system axis from output data supplied during the motion of the focus on a spiral segment.
2. Description of the Prior Art
Various CT methods using conical x-ray beams are known particularly in conjunction with detector arrays having a number of lines of detector elements. The cone angle that thereby occurs as a consequence of the conical shape of the x-ray beam is taken into consideration in various ways.
In the simplest case (see, for example, K. Taguchi, H. Aradate, “Algorithm for image reconstruction in multi-slice helical CT”, Med. Phys. 25, pp. 550-561, 1998; H. Hu, “Multi-slice helical CT: Scan and reconstruction”, Med. Phys. 26, pp. 5-18, 1999), the cone angle is left out of consideration, with the disadvantage that artifacts occur in a large number of lines, and thus a large cone angle.
Further, an algorithm referred to as the MFR Algorithm (S. Schaller, T. Flohr, P. Steffen, “New, efficient Fourier-reconstruction method for approximate image reconstruction in spiral cone-beam CT at small cone-angles”, SPIE Medical Imaging Conf., Proc. Vol. 3032, pp. 213-224, 1997) is known, the disadvantage thereof being that a complicated Fourier reconstruction was necessary and the image quality leaves much to be desired.
Exact algorithms (for example, S. Schaller, F. Noo, F. Sauer, K. C. Tam, G. Lauritsch, T. Flohr, “Exact Radon rebinning algorithm for the long object problem in helical cone-beam CT, in Proc. of the 1999 Int. Meeting on Fully 3D Image Reconstruction, pp. 11-14, 1999 or H. Kudo, F. Noo and M. Defrise,:Cone-beam filtered back-projection algorithm for truncated helical data”, in Phs. Med. Biol., 43, pp. 2885-2909, 1998) have also been described, which have the common disadvantage of extremely complicated reconstruction.
A method and CT apparatus of the type initially described are disclosed in U.S. Pat. No. 5,802,134. In accord therewith, in contrast, images are reconstructed for image planes that are inclined by an inclination angle &ggr; around the x-axis relative to the system axis z. As a result, the (at least theoretical) advantage is achieved that the images contain fewer artifacts when the inclination angle &ggr; is selected such that a good and optimum adaptation of the image plane to the spiral path is established, insofar as possible according to a suitable error criterion, for example minimum square average of the distance measured in z-direction of all points of the spiral segment from the image plane.
In U.S. Pat. No. 5,802,134, fan data, i.e. data registered in the known fan geometry are employed for the reconstruction, the data having been acquired with the motion of the focus along a spiral segment having the length 180° plus the fan angle, for example 240°. The optimum inclination angle &ggr; is dependent on the slope of the spiral, and thus on the pitch p.
Fundamentally, the method disclosed in U.S. Pat. No. 5,802,134 can be employed for arbitrary values of the pitch p. However, an optimum utilization of the detector area available, and thus of the radiation dose applied to the patient for image acquisition (dose utilization) is not possible below the maximum pitch P
max
. This is because even though a given transverse slice, i.e. a slice of the subject residing at a right angle relative to the system axis z, is scanned via a spiral segment that is longer then 180° plus fan angle, only a spiral segment having the length 180° plus the cone angle can be utilized for values of the pitch p below the maximum pitch P
max
since the utilization of a longer spiral segment would make it impossible to adapt the image plane to the spiral path well enough.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and a CT apparatus of the type initially described wherein the cone angle is taken into consideration and wherein the preconditions for an optimum detector utilization and thus an optimum dose utilization are also established for values of the pitch p below the maximum pitch P
max
.
This object is achieved in accordance with the invention in a method for producing a computed tomography image wherein a subject is scanned with a conical x-ray beam emanating from a focus which is detected, after attenuation by the subject, using a matrix-like detector array while the focus moves along a spiral path around the subject relative to a system axis. The detector array generates output data dependent on the radiation from the x-ray beam that is incident thereon, and the output data, for a segment of the spiral path having a length that is adequate for reconstructing a CT image, are divided into a number of datasets respectively for a number of sub-segments of the aforementioned segment. Each of these sub-segments has a length that is shorter than the length that is adequate for reconstructing a CT image. For each of the sub-segments, a number of segment images is reconstructed, the segment images being in respective planes that are inclined relative to the system axis. For each sub-segment, the segment images associated therewith are combined to form a partial image with respect to a target image plane. These partial images that arise for the respective sub-segments are then combined to form a resulting CT image with respect to the target plane.
Since in the inventive method, the spiral segment whose length suffices for the reconstruction of a CT image is divided into sub-segments whose lengths are each less then the length required for the reconstruction of a CT image, the deviations of the image planes of the segment images reconstructed with respect to the sub-segments from the spiral path along the sub-segments are very small. The segment images thus contain only very slight errors caused by deviations of the image planes of the segment images from the spiral path along the sub-segments, so that the image quality in the generation of the resulting CT image is high.
The maximum inclination of the image planes of the segment images is defined from the condition that rays for the image plane of the respective segment image must be present at both ends of the sub-segment within the measurement field.
The segment images that are not useable by themselves because the length of the sub-segments is shorter then the length required for the reconstruction of a CT image are calculated in a known way, i.e. the rays most beneficial for the image plane of the respective segment image are selected from the projections for the respective sub-segment present in parallel or fan geometry according to a suitable error criterion, and are filtered and back-project
Bruder Herbert
Flohr Thomas
Stierstorfer Karl
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
LandOfFree
Computed tomography method and apparatus for optimized... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Computed tomography method and apparatus for optimized..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Computed tomography method and apparatus for optimized... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3108074