X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2003-02-20
2004-11-16
Bruce, David V (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S017000, C378S901000
Reexamination Certificate
active
06819736
ABSTRACT:
REFERENCE TO RELATED APPLICATION
The present patent document claims priority to German Application Ser. No. DE 10207623.5, filed Feb. 22, 2002, which is hereby incorporated by reference.
1. Field
The invention relates to a method for performing computer tomography, having the following method steps: for scanning an object by means of a cone-shaped beam exiting from a focal point and by means of a matrix-like detector array for detecting the beam, the focal point is moved in relation to the object on a spiral path about a system axis, and the detector array provides output data corresponding to the received radiation; and from output data furnished during the motion of the focal point on a spiral segment, images of an object region executing a periodic motion are reconstructed, taking into account a signal, obtained in the course of the periodic motion, which reproduces the course over time of the periodic signal. The invention furthermore relates to a computed tomography (CT) apparatus, having a radiation source, from whose focal point a cone-shaped beam is emitted; a matrix-like detector array for detecting the beam, the detector array providing output data corresponding to the received radiation; a device for generating a relative motion between the radiation source and the detector array on the one hand and an object on the other; and an image computer to which the output data is supplied, and a device for creating a relative motion for scanning the object by the beam and the two-dimensional detector array caused by a relative motion of the focal point in respect to a system axis in such a way that the focal point moves on a helical spiral path in relation to the system axis, whose center axis corresponds to the system axis; and where the image computer reconstructs images of an object region executing a periodic motion from output data furnished during the motion of the focal point on a spiral segment while taking a signal into account, which reproduces the course over time of the periodic motion and which was obtained during the scanning process with the aid of an appropriate device.
2. Background
A method and a CT apparatus of this kind are known from DE 198 42 238 Al. This method is disadvantageously suitable only for detector arrays which extend a relatively short distance in the direction of the system axis.
Various CT methods using cone-shaped X-ray beams have become known, in particular in connection with detector arrays having several rows of detector elements. In them, the cone angle that is due to the cone-shaped form of the X-ray beam is taken into account in various ways.
In the simplest case (see, for example, K. Taguchi, H. Aradate in “Algorithm for Image Reconstruction in Multi-Slice Helical CTs”, Med. Phys. 25, pp. 550 to 561, 1988, or H. Hu in “Multi-Slice Helical CT: Scan and Reconstruction”, Med. Phys. 26, pp. 5 to 18, 1999), the cone angle is ignored, which has the disadvantage that with a large number of rows, and therefore a large cone angle, artifacts appear.
Moreover, the so-called MFR algorithm (S. Schaller, T. Flohr, P. Steffen in “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 to 224, 1997) has become known, but it is disadvantageous because an elaborate Fourier reconstruction is required, and the image quality leaves something to be desired.
Exact algorithms have furthermore been described (for example by S. Schaller, F. Noo, F. Sauer, K. C. Tam, G. Lauritsch, T. Flohr in “Exact Radon Rebinning Algorithm for the Long Object Problem in Helical Cone-Beam CT”, Proc. of the 1999 Int. Meeting on Fully 3D Image Reconstruction, pp. 11 to 14, 1999, or H. Kudo, F. Noo and M. Defrise in “Cone-Beam Filtered Backprojection Algorithm for Truncated Helical Data”, Phys. Med. Biol. 43, pp. 2885 to 2909, 1998), which have the disadvantage of an extremely elaborate reconstruction in common.
Another such method and CT apparatus are known from U.S. Pat. No. 5,802,134. In this reference, however, images are reconstructed for image planes which are inclined by an inclination angle &tgr; about the x-axis relative to the system axis z. By this device, the at least theoretical advantage is achieved that the images contain fewer artifacts if the angle of inclination &tgr; has been selected to be such that a good adaptation, if possible in accordance with a suitable error criterion, such as the minimum mean square value of the distance of all points of the spiral segment from the image plane, measured in the z-direction, and even an optimal adaptation of the image plane to the spiral path is provided.
In this case, the spiral path of the focal point F illustrated in
FIG. 1
is described by the following equations:
x
f
=
-
R
f
cos
α
(
1
)
y
f
=
-
R
f
sin
α
z
f
=
S
·
p
·
α
2
π
or
x
_
f
=
(
-
R
f
cos
α
-
R
f
sin
α
Sp
α
2
π
)
In the case where the detector elements of the detector array are arranged in rows extending transversely to the system axis Z and in columns extending parallel to the system axis Z, S stands for the length of one detector row in the direction of the system axis, and p stands for the pitch, where p=h/S, and h stands for the slope of the spiral path per revolution of the focal point F. &agr; is the projection angle, and an image plane will now be addressed that belongs to data that was acquired over a projection angle range of ±&agr;; the reference projection associated with this image plane is at &agr;
r
=0, and thus represents the center of the projection angle range ±&agr;. Below, &agr;
r
will be called the reference projection angle.
In the conventional spiral CT, so-called transverse section images are reconstructed, that is, images for image planes that are perpendicular to the system axis marked z and that thus include both the x-axis and the y-axis; the x- and y-axes are perpendicular to one another and to the system axis z.
In the case of U.S. Pat. No. 5,802,134, conversely, images are reconstructed for image planes that are inclined by an angle of inclination &ggr; about the x-axis to the system axis z, as shown in FIG.
2
. As a result, the at least theoretical advantage is attained that the images contain fewer artifacts if the angle of inclination &ggr; is selected such that there is a good optimal adaptation of the image plane to the spiral path, if at all possible in accordance with a suitable error criterion, such as a minimum mean square value of the distance, measured in the z-direction, of all points in the spiral segment from the image plane.
In the case of U.S. Pat. No. 5,802,134, fan beam data, that is, data acquired using fan beam geometry, which is known per se, and obtained in the motion of the focal point over a spiral segment whose length was 180° plus the fan or cone angle, such as 240°, are used for the reconstruction. Referred to the reference projection angle, &agr;
r
=0, the applicable equation for the normal vector of the image plane is
n
_
ijs
(
γ
)
=
(
0
-
sin
γ
cos
γ
)
.
The optimal angle of inclination &ggr; is evidently dependent on the slope of the spiral and thus on the pitch p.
In principle, the method known from U.S. Pat. No. 5,802,134 can be employed for arbitrary values of the pitch p. However, below the maximum pitch p
max
, optimal utilization of the available detector area and thus of the radiation dose delivered to the patient to obtain images (detector and hence dose utilization) is not possible, because even though a given transverse slice, that is, a slice of the object that is perpendicular to the system axis a, is scanned over a spiral segment that is longer than 180° plus the fan or cone angle, still in the method known from U.S. Pat. No. 5,802,134, for values of the pitch p be
Bruder Herbert
Flohr Thomas
Ohnesorge Bernd
Schaller Stefan
Stierstorfer Karl
Brinks Hofer Gilson & Lione
Bruce David V
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