X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-09-21
2001-08-28
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06282256
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a computed tomography method which includes the following steps:
generating a conical radiation beam which traverses an examination zone or an object present therein,
generating a relative motion, including a rotation about an axis of rotation, between the radiation beam and the examination zone or the object,
acquiring, during the relative motion, measuring data which is dependent on the intensity in the radiation beam to the other side of the examination zone,
reconstructing the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit.
The invention also relates to a computed tomography apparatus for carrying out the above method.
2. Description of Related Art
A “conical” beam is to be understood to mean a beam of finite dimensions in two mutually perpendicular directions and is detected by a detector unit which is suitable for spatially resolved measurement in these two directions of the intensity of the beam which has been attenuated in the examination zone. A method of this kind is known from a publication by L. A. Feldkamp et al. “Practical Cone Beam Algorithms”, Journal of Optical Soc. Am. A, Vol. 1, No. 6/pp. 612-619, 1984.
It is a fundamental drawback of CT methods (CT=computed tomography) utilizing conical radiation beams that some voxels (volume elements) in the examination zone are only temporarily exposed to the radiation during the relative motion between the radiation source and the examination zone and that the absorption in these voxels cannot be reconstructed from the measuring data acquired by the detector unit. The part of the examination zone in which the spatial absorption distribution can be reconstructed, therefore, is always smaller than the part exposed to the radiation.
The known method utilizes a reconstruction algorithm for reconstructing the absorption within the rotationally symmetrical zone which is exposed to radiation during the entire relative motion and is shaped like a disc, the reconstruction in practice being limited to a plane slice within this zone. The known method is based on a circular relative motion.
However, there are also CT methods which involve a helical relative motion. In the case of such methods the absorption is not reconstructed in the voxels which are present in the radiation beam at the beginning or at the end of the relative motion.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the present invention to enlarge the zone in which the absorption distribution is reconstructed. This object is achieved according to the invention in that the method also includes the following steps for the reconstruction of the spatial distribution of the absorption:
a) defining at least one first and one second sub-volume within the overall volume traversed by the radiation beam,
b) reconstructing the spatial distribution of the absorption within the first sub-volume by means of a first reconstruction algorithm,
c) reconstructing the spatial distribution of the absorption within the second sub-volume by means of a second reconstruction algorithm which deviates from the first reconstruction algorithm.
The invention is based on the consideration that the known methods always utilize only one reconstruction algorithm for the reconstruction. For each voxel the absorption is then reconstructed by means of the same number of calculation steps of the same kind. Each reconstruction algorithm is subject to given secondary conditions (for example, the condition that all voxels in the volume to be reconstructed have been exposed to radiation during the entire relative motion) which are satisfied only in a part of the overall volume (the first sub-volume). These secondary conditions are adequate but not necessary for the reconstruction, i.e. there are additional voxels which do not satisfy this secondary condition but do satisfy less severe secondary conditions which are also adequate for a reconstruction, utilizing a different reconstruction algorithm, even if the signal-to-noise ratio could then be less attractive. These voxels are situated in another part of the overall volume (the second sub-volume).
The reconstruction zone can thus be enlarged by utilizing a hybrid reconstruction method involving a first reconstruction algorithm in a first sub-volume and a second reconstruction algorithm in a second sub-volume (other than the first sub-volume). The second reconstruction algorithm may include calculation steps of the same kind as the first algorithm, but a different number thereof. The term “different reconstruction algorithm” is to be broadly interpreted in this sense.
This invention includes an embodiment which can be used in the case of a circular trajectory (i.e. a trajectory where the relative motion between the radiation source and the detector unit on the one side and the examination zone on the other side is shaped as a circle). The criterion for assignment to the two sub-volumes is then given by the irradiation angle range (being the angular range covered in the relevant plane by the (parallel) projection of the rays from the radiation source to a voxel in a plane which is perpendicular to the axis of rotation, or is covered by the components of the vectors from the radiation source to the voxel in the plane of rotation of the radiation source). Voxels with an irradiation angle range of 360° (which voxels are exposed to radiation during the entire relative motion) are assigned to the first sub-volume and voxels with an irradiation angle range of at least 180° (but less than 360°) are assigned to the second sub-volume which bounds the first sub-volume to both sides and has sides extending perpendicularly to the axis of rotation.
The reconstruction of the absorption of the voxels in the first sub-volume is then performed by means of a first reconstruction algorithm which utilizes a reconstruction angle range of 360° (the reconstruction angle range is to be understood to mean the angular range covered by the (parallel) projections of the rays, used for the reconstruction, from the radiation source to a voxel in a plane which is perpendicular to the axis of rotation). For example, the algorithm described in the previously mentioned publication can be used as the reconstruction algorithm. For the second sub-volume use can be made of a reconstruction algorithm for which a reconstruction angle range of 180° suffices; as is known, CT methods involving a plane fan-shaped radiation beam also utilize reconstruction algorithms which operate with a reconstruction angle range of only 180°.
This invention includes two alternatives for the reconstruction of the absorption in these voxels. In conformity with a first alternative, measuring data is taken into account only from an irradiation angle range of exactly 180°. In conformity with a second alternative, all measuring data is taken into account which has been determined for rays through the relevant voxel, but the contributions by rays whose projection passes through the voxel from 180° offset directions are weighted in such a manner that their overall weight equals that of a single ray (i.e. a ray for one direction for which no ray occurs in the opposite direction). In this case the reconstruction is equivalent to a reconstruction with a reconstruction angle range of 180°, but a more attractive signal-to-noise ratio is obtained.
When the part of the examination zone that can be reconstructed by means of a circular trajectory does not suffice, the examination zone can be scanned along two adjacently situated circular scanning paths. This invention also includes an embodiment which is suitable for such a case. A (disc-shaped) intermediate zone which constitutes a third sub-volume then exists symmetrically with respect to the two circles along which the re
Grass Michael
Proksa Roland
Bruce David V.
U.S. Philips Corporation
Vodopia John F.
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