X-ray or gamma ray systems or devices – Accessory – Alignment
Reexamination Certificate
1998-12-15
2001-03-27
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Accessory
Alignment
C378S062000
Reexamination Certificate
active
06206566
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray apparatus of the type having an X-ray examination system which with an X-radiation source and an X-ray detector which can be displaced relative to an examination subject for the pickup of 2D projections of a region of the subject, with subsequent reconstruction of 3D images of the region of the subject.
2. Description of the Prior Art
X-ray apparatuses of the above type commonly have a C-arm for mounting the X-ray source and the X-ray detector, the C-arm being mounted in a holding device such that it can be displaced in motorized fashion along its perimeter in a defined angle range (orbital motion). To obtain 2D projections from various projection angles for the reconstruction of 3D images—of a body region of a living organism, for example—in the pickup of the 2D projections of the body region of the organism, the C-arm is displaced along its perimeter subsequent to corresponding placement relative to the living organism to be examined. 3D images of the body region of the organism are subsequently reconstructed from the 2D projections captured with the X-ray examination system during the displacing motion. The reconstruction of 3D images is preconditioned by the precise knowledge of the projection geometries, i.e. the knowledge of the positions and orientations of the X-ray source and of the X-ray detector with respect to a stationary coordinate system during each of the individual 2D projections.
It has proven problematic that known stationary C-arm X-ray apparatuses, and quite particularly mobile C-arm X-ray devices, exhibit mechanical instabilities, particularly with respect to the displacement of the C-arm along its perimeter, so that the actual displacing motion of the X-ray examination system deviates from the ideal displacing motion due to deformations of the C-arm. Thus, the precision in the reproducibility of the projection geometries which is necessary for a reconstruction of 3D images cannot be achieved, particularly with the known mobile C-arm X-ray devices, for which reason additional position detection systems are necessary in order to be able to determine the projection geometries in every 2D projection. The following two methods are known for determining the projection geometries:
a) German OS 195 12 819 (corresponding to U.S. Pat. No. 5,706,324) teaches the utilization of a marker ring, usually made of plexiglass with inserted metal structures, which is arranged around the body region of the examined organism. The metal structures of the marker ring are visible in the 2D projections of the examined body region, so that the respective projection geometries of the 2D projections can be calculated from their position. This method has the disadvantage that the marker ring has a relatively large diameter, so that the distance between the X-ray source and the marker ring is very small (a few centimeters), particularly given mobile C-arm X-ray devices having a relatively small C-arm. The metal structures are thus imaged with significant enlargement in the 2D projections, so that large parts of the 2D projections are covered by the metal structures. Furthermore, only a small region of the metal structures of the marker ring is imaged in the 2D projections, so that the determination of the projection angle with the aid of the low number of imaged metal structures is difficult.
b) Gauging measurements are performed prior to the actual patient measurement, under the assumption that the system behavior, i.e. essentially the displacement of the C-arm, is largely reproducible. This method is very time-consuming and can be used only given mechanically reinforced stationary C-arm X-ray devices. Application in mobile X-ray devices is impossible, due to the previously mentioned mechanical instabilities of such X-ray devices, mechanical stabilzation being out of the question for mobile X-ray devices due to the large weight increase, which restricts mobility.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an X-ray apparatus of the abovementioned type wherein the determination of the projection geometries is simplified and is suitable not only for a stationary X-ray apparatus but also for a mobile apparatus.
This object is inventively achieved in an X-ray apparatus with an X-ray examination system including an X-ray source and an X-ray detector which can be displaced relative to a subject for the pickup of 2D projections, with means for determining extrinsic and intrinsic imaging parameters, i.e. for determining the projection geometries of the X-ray system in each 2D projection, and with control and computing means for reconstructing 3D images from the 2D projections with the aid of the extrinsic and intrinsic imaging parameters, wherein the means for determining the intrinsic imaging parameters include X-ray-positive marks which are allocated to the X-ray source and which are arranged, substantially in one plane, in the path of an X-ray beam emanating from the X-ray source, the geometric positions of the marks relative to each other and to the X-ray source being known. For determining the projection geometries in each 2D projection, means for determining extrinsic imaging parameters and means for determining intrinsic imaging parameters are thus provided. The extrinsic imaging parameters describe the position and orientation of the focus of the X-ray source as a reference point, or the position and orientation of an arbitrarily selected zero point of the detector surface of the X-ray receiver as a reference point, for example, in a first stationary coordinate system. The intrinsic imaging parameters specify the geometric relation between the X-ray source and the X-ray detector—i.e., the distance of the X-ray source and the X-ray detector from one another, the orientation of the X-ray source and of the X-ray detector relative to one another, and the displacement of the X-ray detector perpendicular to the axis of the center beam of an X-ray beam emanating from the X-ray source, for example—in a second coordinate system, whose origin is preferably located at the reference point, i.e. at the focus of the X-ray source or at the zero point of the detector surface, for example. The position of the origin and the orientation of the second coordinate system—whose origin is located at the focus of the X-ray source, for example, and which, like the marks, is displaced relative to a subject together with the X-ray source in various 2D projections—is specified, for every 2D projection, by the extrinsic imaging parameters, as already noted.
In the examination of a subject, for each 2D projection of the subject, a matrix I of the intrinsic imaging parameters and a matrix E, which contains the extrinsic imaging parameters, are determined, whereby, according to P=I*E, a projection matrix P results for each 2D projection, each projection matrix P comprising the projection geometries of the corresponding 2D projection which are necessary for the reconstruction of 3D images. The projection matrices, which the control and computing means calculate from the extrinsic and intrinsic imaging parameters, are used for the reconstruction of 3D images from the 2D projections.
The means for determining the extrinsic imaging parameters are operable independent of the means of the intrinsic imaging parameters, so that the determination of the extrinsic and intrinsic imaging parameters is possible separately and thus is simplified in relation to the evaluated signals. The intrinsic imaging parameters are obtained using the X-radiation, with X-ray-positive marks which are arranged in one plane being allocated to the X-ray source such that they are imaged in the 2D projections. Since the geometric positions of the marks relative to each other and to the X-ray source are known in the second coordinate system, for example, whose origin is situated at the focus of the X-ray source, the intrinsic imaging parameters—i.e., the distance between the X-ray source and the X-ray detector, the
Dunn Drew A.
Porta David P.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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