X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-09-30
2001-09-04
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C378S901000
Reexamination Certificate
active
06285733
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a computed tomography method which includes the following steps: generating, while using a radiation source, a conical radiation beam which traverses an examination zone or an object present therein, generating a circular relative motion, including a rotation about an axis of rotation, between the radiation source on the one side and the examination zone or the object on the other side, acquiring, while using a detector unit, measuring data which is dependent on the intensity in the radiation beam to the other side of the examination zone during the relative motion, reconstructing the spatial distribution of the absorption within the examination zone from the acquired measuring data. The invention also relates to a computed tomography apparatus for carrying out the above method.
2. Description of Related Art
A method of the kind set forth is known from a publication by L. A. Feldkamp et al. “Practical cone-beam algorithm”, Journal of Optical Soc. Am. A/Vol. 1, No. 6, 1984, pp. 612-619. The known method in principle consists of the following steps:
a) multiplying, for all radiation source positions, all measuring values by a weighting factor which corresponds to the cosine of the angle enclosed by the ray along which the measuring value has been acquired with respect to the central ray;
b) subjecting the measured values thus weighted to a high-pass filtering operation.
c) backprojecting the measured values into the examination zone along the rays along which they have been measured. The contribution of a measuring value to the absorption value of a voxel must then be weighted by a factor which is dependent on the distance between the relevant voxel and the radiation source position.
Because this last step must be carried out for all voxels of the volume to be reconstructed and for all radiation source positions, it requires a long calculation time. It is a further drawback of the known method that it only enables the reconstruction of the absorption in voxels which have been continuously exposed to X-rays during the examination. Such voxels are situated in a discus-shaped region which is concentrically situated relative to the axis of rotation. However, it is desirable to perform a reconstruction in a flat, cylindrical slab-like region. If the reconstruction, however, is limited to the slab-like region in the conical beam which is exposed to X-rays in all radiation source positions, only a very narrow reconstruction region will be obtained.
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 propose a computed tomography method of the kind set forth in which the amount of calculation work is reduced and reconstruction of the absorption distribution is possible in a thicker, flat slice of the examination zone. On the basis of a method of the kind set forth this object is achieved according to the invention by taking the following steps:
a) rebinning the measuring data so as to form a number of groups, each group containing a plurality of planes which extend parallel to one another and parallel to the axis of rotation and in each of which a respective fan beam is situated,
b) one-dimensional filtering of the data produced by the rebinning in each group in a direction perpendicular to the direction of the planes,
c) reconstructing the spatial distribution of the absorption by backprojection of the filtered data of a plurality of groups.
Whereas according to the known method the measuring values acquired in the individual radiation source positions are always directly subjected to a filtering operation, in the method according to the invention there is first performed a rebinning operation during which fan beams picked up in different radiation source positions are group-wise combined, after which filtering takes place. The advantage of the additional rebinning operation resides in the fact that the contributions of the individual measured values to a voxel need not be multiplied by a distance-dependent factor during the subsequent reconstruction of the absorption distribution. The reconstruction is thus significantly simplified. The image quality is also enhanced. Moreover, an attractive ratio of the irradiated volume to the volume in which the absorption distribution can be reconstructed is obtained.
The invention is also based on the recognition of the fact that the absorption can be reconstructed in each voxel if the voxel has been irradiated from an angular range of at least 180°. It can be demonstrated that all voxels which satisfy this requirement are situated in a flat slab which extends perpendicularly to the axis of rotation and whose thickness is greater than the dimensions at the edges of the discus-shaped region in which the absorption can be reconstructed by means of the known method.
The rebinning on a flat, rectangular virtual detector, which extends perpendicularly to the planes of each group and contains the axis of rotation, significantly simplifies the subsequent one-dimensional filtering operation.
The slab in which the absorption distribution can be reconstructed contains voxels which have been irradiated from an angular range of more than 180°. However, because the reconstruction requires only an irradiation angle range of 180° (being the angular range covered by the (parallel) projection of the rays from the radiation source to a voxel onto a plane perpendicular to the axis of rotation, or covered by the components of the vectors from the radiation source to the voxel in the plane of rotation of the radiation source) measuring values from some radiation source positions may be ignored. A simple possibility of determining the radiation source positions whose measuring data are to be taken into account for the reconstruction of the absorption in individual voxels includes reconstructing the absorption of voxels with an irradiation angle range of at least 180° while taking into account the filtered measuring data from exclusively the radiation source positions which are separated from the voxel by a plane which contains the axis of rotation and extends perpendicularly to the plane defined by the relevant voxel and the axis of rotation.
The absorption distribution in said slab can in principle be reconstructed by taking into account only as many radiation source positions for the reconstruction of all voxels as are required to obtain an irradiation angle range of exactly 180° is obtained. However, this slab also contains voxels with an irradiation angle range of 360°. If such voxels were also reconstructed with an irradiation angle range of only 180°, the signal-to-noise ratio obtained for these voxels would not be as good as possible. Therefore, in a hybrid reconstruction method, the volume of the slab that can be reconstructed is subdivided into two sub-volumes: a first sub-volume which contains exclusively voxels which have been irradiated from all radiation source positions (i.e., with an irradiation angle range of 360°), and a second sub-volume in which the irradiation angle range is less than 360° (i.e., with an irradiation angle range of at least 180° but less than 360°). During the reconstruction of the absorption in the voxels of the first sub-volume, all measuring values are taken into account whereas only the measuring values from an irradiation angle range of 180° are taken into account for the reconstruction of the voxels in the second sub-volume.
A computed tomography apparatus for carrying out the method according to the invention includes a radiation source, a detector unit which is coupled thereto, a drive device for making an object present in the examination zone and the detector device perform a rotation about the axis of rotation relative to one another, and a reconstruction unit for reconstructing the spatial distribution of the absorption w
Grass Michael
Proksa Roland
Bruce David V.
U.S. Philips Corporation
Vodopia John F.
LandOfFree
Computed tomography method utilizing a conical radiation beam 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 utilizing a conical radiation beam, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Computed tomography method utilizing a conical radiation beam will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2524632