X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-07-09
2003-12-16
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S008000, C378S094000
Reexamination Certificate
active
06665370
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and apparatus for computed tomography.
2. Description of the Prior Art
A computed tomography method is known wherein, a subject is scanned with a conical ray beam emanating from a focus and with a matrix-like detector array for detecting the ray beam, with the focus being moved on a spiral path around a system axis relative to the subject, and the detector array supplies output data corresponding to the received radiation, and wherein images of an object region executing a periodic motion are reconstructed from output data respectively supplied during the motion of the focus on a spiral segment, the images being reconstructed dependent on the time curve of a signal that is acquired during the scanning that reflects the time curve of the periodic motion.
A computed tomography (CT) apparatus also is known having a radiation source with a focus from which a conical ray beam emanates, a matrix-like detector array for detecting the ray beam, which supplies output data corresponding to the received radiation, means for generating a relative motion between the radiation source/detector array and a subject, on the other hand, and an image computer to which the output data are supplied, wherein the means for producing a relative motion for scanning the subject produce a relative motion of the focus with respect to a system axis such that the focus moves on a helical spiral path relative to the system axis, the middle axis of the spiral path corresponding to the system axis, a device for obtaining a signal during the scanning which represents the time curve of the periodic motion, and an image computer that reconstructs images of an object region executing the periodic motion from the detector output data respectively and the periodic motion signal.
German OS 198 42 238 discloses such a method and apparatus. A disadvantage of this method is that it is suited only for detector arrays having a relatively small extent in the direction of the system axis.
Various CT methods making use of a conical X-ray beam are known, particularly in conjunction with detectors having several lines of detector elements. The cone angle that occurs as a consequence of the conical shape of the X-ray beam is thereby taken into account in different ways.
In the simplest case (see, for example, B. 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 given a large number of detector 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. A disadvantage of this method is that a complicated Fourier reconstruction is needed and the image quality leaves something to be desired.
Exact algorithms also have been disclosed (see, 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 backprojection algorithm for truncated helical data”, in Phys. Med. Biol., 43, pp. 2885-2909, 1998), these having the common disadvantage of an extremely complicated reconstruction.
A method and CT apparatus of the type initially described also are disclosed in U.S. Pat. No. 5,802,134. As disclosed therein, images are reconstructed for image planes that are inclined relative to the system axis z by an inclination angle &ggr; around the x-axis. As a result, the (at least theoretical) advantage is achieved of the images containing fewer artifacts when the inclination angle &ggr; is selected such that a good, optimum adaptation of the image plane to the spiral path is established, insofar as possible according to a suitable error criterion, for example the minimum quadratic average of the distance of all points of the spiral segment from the image plane as measured in the z-direction.
In the case of U.S. Pat. No. 5,802,134, fan data—i.e. data registered using known fan geometry—that were acquired given the motion of the focus over a spiral segment having the length 180° plus fan angle, for example 240°, are thereby employed for the reconstruction. The optimum inclination angle &ggr; is dependent on the slope of the spiral, and on the pitch p.
The method disclosed in U.S. Pat. No. 5,802,134 can be employed for arbitrary values of the pitch p. An optimum utilization of the available detector area and thus of the radiation dose applied to the patient for image acquisition (detector and, thus, dose utilization), however, is not possible below a 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, that is longer than 180° plus fan angle is scanned over a spiral segment, only a spiral segment of the length 180° plus cone angle can be used for values of the pitch p below the maximum pitch p
max
, since the use of a longer spiral segment would make it impossible to adapt the image plane adequately well to the spiral path.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and a CT apparatus of the type initially described which are also suited, i.e. enable high-quality images, for detector arrays having a large extent in the direction of the system axis.
The above object is achieved in accordance with the invention in a computed tomography method and apparatus wherein a subject having a subject with a conical ray beam emanating from a focus and with a matrix-like detector array for detecting the ray beam, while the focus is moved on a spiral path around a system axis relative to the subject, and the detector array supplies output data corresponding to the received radiation. The output data respectively supplied during the motion of the focus on a spiral segment and having a length adequate for the reconstruction of a CT image are divided into output data with respect to sub-segments, with the length of each sub-segments being shorter than the length required for the reconstruction of a CT image. Segment images having an inclined image plane relative to the system axis are reconstructed for the sub-segments. A signal reproducing the time curve of the periodic motion is acquired during the scanning. A z-position on the system axis and a time position with respect to the periodic motion are allocated to the segment images. Segment images belonging to a desired range of z-positions and a desired range of time positions are selected such that the corresponding sub-segments have an overall length adequate for the reconstruction of a CT image. The selected segment images are at least indirectly combined into a resulting CT image with respect to a target image plane.
In the invention, the cone angle is taken into consideration since sub-segments are first formed and segment images are reconstructed with respect to the sub-segments, the deviations of the image areas from the spiral path along the sub-segments being very small for these segment images since the length of each sub-segment is shorter than the length required for the reconstruction of a CT image. The segment images thus contain only very slight deviations of the image areas 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 even given a large number of detector lines.
Since the segment images have a z-position on the system axis and a time position with respect to the time curve of the periodic motion allocated to t
Bruder Herbert
Flohr Thomas
Mayer Robert
Ohnesorge Bernd
Schaller Stefan
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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