X-ray system for the iterative determination of an optimal coordinate transformation between overlapping volumes that have been reconstructed from volume data sets of discretely scanned object areas
This disclosure relates to an X-ray system for obtaining referencing information in the form of a coordinate transformation between a first volume and a second volume, with the volumes partially overlapping. In this case two partially overlapping subareas of an extended object having a continuous distribution of a physical size are discretely sampled with a measuring apparatus; the volumes are reconstructed and the overlapping volumes are iteratively changed, wherein a bandpass filter is applied to the overlapping volumes, where said bandpass filter changes the Fourier domain representation of the overlapping volumes. The filtered overlapping volumes, in which the artifacts, induced by the discrete sampling, are matched, are compared by means of a measure of similarity, which is iteratively maximized by means of mathematical optimization. The coordinate transformation with the maximum result for the measure of similarity is used as the optimal coordinate transformation for combining the two volumes in the correct position. One application is the combination of 3D X-ray volumes that have been reconstructed by means of a cone beam computed tomography scanner, preferably a 3D C-arm.
1. An X-ray system suitable for iterative determination of an optimal coordinate transformation between a first N-dimensional partial volume V 1 and a second N-dimensional partial volume V 2 of an extended object, wherein the partial volume V 1 and the partial volume V 2 partially overlap in overlapping volumes OV 1 and UOV 2 , said X-ray system comprising an imaging device and one or more processors configured to perform operations comprising at least:
a) recording, using the imaging device, the partial volume V 1 and the partial volume V 2 by discrete sampling of the extended object;
b) applying a bandpass filter to the overlapping volumes OV 1 and UOV 2 within each of a plurality of iteration steps i to obtain filtered overlapping volumes FOV 1 ( i ) and FOV 2 ( i ), wherein applying said bandpass filter comprises:
removing, from the overlapping volume OV 1 ( i ), frequencies that are not contained in the overlapping volume UOV 2 ( i ); and
removing, from the overlapping volume UOV 2 ( i ), frequencies that are not contained in the overlapping volume OV 1 ( i ); and
c) determining the optimal coordinate transformation by iterative optimization of a measure of similarity between the filtered overlapping volumes FOV 1 ( i ) and FUOV 2 ( i ).
2. The X-ray system as claimed in claim 1 , wherein the bandpass filter determines the frequencies which are to be removed based on information about a scanning mode or a scanning geometry.
3. The X-ray system as claimed in claim 1 , wherein the bandpass filter determines the frequencies to be removed in a Fourier domain of the overlapping volumes OV 1 ( i ) and UOV 2 ( i ) based on an absolute amount present therein falling below a predetermined threshold value.
4. The X-ray system as claimed in claim 1 , wherein the frequencies to be removed are removed temporarily from the two overlapping volumes by the bandpass filter in a spatial domain and separately for each recording of the overlapping volumes OV 1 ( i ) and UOV 2 ( i ) before calculation of the measure of similarity.
5. The X-ray system as claimed in claim 1 , wherein the frequencies to be removed are removed temporarily from a Fourier domain of the two overlapping volumes OV 1 ( i ) and UOV 2 ( i ) by the bandpass filter in the Fourier domain and separately for each iteration step i before calculation of the measure of similarity, under a current assumption of a coordinate transformation.
6. The X-ray system as claimed in claim 1 , further comprising a cone beam computed tomography scanner configured to perform the recording of the partial volume V 1 and the partial volume V 2 .
7. The X-ray system as claimed in claim 6 , wherein the cone beam computed tomography scanner is a mobile C-arm.
8. The X-ray system as claimed in claim 6 , wherein the overlap of the two partial volumes V 1 and V 2 is achieved by adjusting a support structure of the extended object, and wherein the cone beam computed tomography scanner is configured to prompt an operator to perform a manual adjustment of the support structure before the start of the recording and to receive a confirmation of the manual adjustment from the operator.
9. The X-ray system as claimed in claim 8 , wherein the support structure has a position sensor, and wherein the cone beam computed tomography scanner ensures a correct adjustment by monitoring the position sensor.
10. The X-ray system as claimed in claim 6 , wherein the cone beam computed tomography scanner is moved in space with respect to the extended object to achieve the overlap.
11. The X-ray system as claimed in claim 10 , wherein the cone beam computed tomography scanner is moved by a motor.
12. The X-ray system as claimed in claim 10 , wherein the cone beam computed tomography scanner has a position sensor and ensures a correct overlap of the partial volumes V 1 and V 2 by evaluating the position sensor.
13. The X-ray system as claimed in claim 6 , wherein the overlap of the two partial volumes V 1 and V 2 is achieved by adjusting a support structure of the extended object, and wherein a determined coordinate transformation between the cone beam computed tomography scanner and the object support structure is stored.
14. The X-ray system as claimed in claim 13 , wherein the stored coordinate transformation is used to combine subsequently recorded partial volumes, directly without re-optimization.
15. The X-ray system as claimed in claim 1 , wherein the overlap of the two partial volumes V 1 and V 2 is achieved by adjusting a support structure of the extended object.
16. The X-ray system as claimed in claim 15 , wherein the support structure is adjustable by a motor.
17. The X-ray system as claimed in claim 16 , further comprising a cone beam computed tomography scanner configured to perform the recording of the partial volume V 1 and the partial volume V 2 , wherein the motor adjusts the support structure by means of the cone beam computed tomography scanner to achieve the overlap.
18. The X-ray system as claimed in claim 1 , wherein the X-ray system is further configured to combine the partial volumes V 1 and V 2 into a combined volume using the determined optimal coordinate transformation.
19. The X-ray system as claimed in claim 18 , wherein the combining the partial volumes V 1 and V 2 comprises, after applying the coordinate transformation at each coordinate, generating the combined volume in the overlapping area of the partial volumes V 1 and V 2 by weighted averaging of the measured values present in the partial volumes V 1 and V 2 at each coordinate.
20. The X-ray system as claimed in claim 19 , wherein a weight of the weighted averaging is a function of a cone beam angle present at a coordinate.
21. The X-ray system as claimed in claim 18 , wherein the combined volume is generated in the overlapping area of the partial volumes in such a way that the frequencies contained in a Fourier domain of one of the overlapping volumes are first transmitted to a Fourier domain of the other overlapping volume at such points where the other overlapping volume has no frequencies.
22. The X-ray system as claimed in claim 1 , wherein, after the combination of the partial volumes V 1 and V 2 , further partial volumes are added to the resulting combined volume.
23. A tangible, non-transitory computer-readable storage medium having stored thereon a computer program which can be loaded directly into a storage unit of a control unit of a cone beam computed tomography scanner, in particular, a C-arm, with program sections that causes an X-ray system comprising the cone beam computed tomography scanner to carry out steps a-c of claim 1 when the computer program is executed in the control unit of the cone beam computed tomography scanner.
24. A tangible, non-transitory computer-readable storage medium having stored thereon program sections which, when read and executed by a computer unit, cause an X-ray system comprising the computer unit to carry out steps a-c of claim 1 .