Patent Number: 
Section: description

The present invention is directed to improved image reconstruction methods for cone beam ROI imaging of long objects with an area detector using a series of circular scans, wherein essentially the entire detector area is utilized for all source positions. More specifically, preferred embodiments of the invention comprise extensions of the data acquisition and image reconstruction techniques disclosed in the above-incorporated U.S. Pat. No. 5,383,119 to improve utilization of the detector. Rather than filter out (via a mask) the portion of the cone beam projection data (which is captured by the circular scans on the ROI boundaries) that lies outside the ROI, the cone beam data outside the ROI is preferably processed using the methods disclosed in the reference by Kudo, et al., entitled xe2x80x9cAn Extended Completeness Condition for Exact Cone-Beam Reconstruction and its Applicationxe2x80x9d, IEEE Nucl. Sci Symp. Conf. Record, 1994, Norfolk, Va., pp 1710-1714 (referred to herein as xe2x80x9cKudoxe2x80x9d), which is incorporated herein by reference. Further, the cone beam data within the ROI is acquired and processed using the methods disclosed in the above-incorporated U.S. Pat. No. 5,383,119. Image reconstruction methods according to the invention provide reconstruction of an extended ROI, i.e., a ROI that includes a portion of the object that lies beyond the boundary extents of the circular scans. Advantageously, image reconstruction methods according to the invention enable virtually full utilization of the detector, while providing reconstruction of a longer (extended) ROI with the same projection data acquired by the scanning trajectories for the method of U.S. Pat. No. 5,383,119, within the same scan time. In other words, by optimally utilizing the detector, a longer volume can be reconstructed for the same data acquisition scan time. Details of image reconstruction methods according to the invention will now be discussed with reference to FIGS. 5-9. FIG. 5 generally illustrates a cone beam data acquisition process for an image reconstruction method according to an embodiment of the present invention. More specifically, FIG. 5(a) illustrates a given source position on the upper circular scan 21, wherein an xe2x80x9cextendedxe2x80x9d portion of the long object 20 (which is located above the desired ROI xcexa9o in the figure) is reconstructed using cone beam projection data captured on the top half (T) of the detector D. FIG. 5(b) illustrates the utilization of the detector D for the upper circular scan depicted in FIG. 5(a). As shown, the cone beam projection data on the upper half (T) of the detector D is not masked (filtered) out but rather fully utilized. The cone beam projection data that is captured on the detector portion T (i.e., v greater than 0) is processed using the methods described in the Kudo reference, whereas the cone beam projection data that is captured on the detector portion (B) (i.e., for vxe2x89xa60)is processed for data combination with the bottom circular scan path data using the methods disclosed in the above-incorporated U.S. Pat. No. 5,383,119. This is to be contrasted with the method depicted in FIGS. 3 and 4b, for example, wherein the cone beam projection data for v greater than 0 is filtered out via mask M such that the cone beam projection data on the top half (T) of detector D is not utilized for image reconstruction. A similar cone beam data acquisition process is applied for the lower circular scan 22 as depicted in FIG. 6. In particular, FIG. 6(a) illustrates a given source position on the lower circular scan 22, wherein an xe2x80x9cextendedxe2x80x9d portion of the long object 20 (which is located below the desired ROI xcexa9o in the figure) is reconstructed using cone beam projection data captured on the bottom half (B) of the detector D. FIG. 6(b) illustrates the utilization of the detector D for the lower circular scan 22 depicted in FIG. 6(a). As shown, the cone beam projection data on the lower half (B) of the detector D is not masked (filtered) out, but rather fully utilized. The cone beam projection data that is captured on the bottom half (B) of the detector (i.e., v less than 0) is processed using the methods described in the Kudo reference, whereas the cone beam projection data that is captured on the upper half (T) of the detector D (i.e., for vxe2x89xa70)is processed for data combination with the upper circular scan path data using the methods disclosed in the above-incorporated U.S. Pat. No. 5,383,119. This is to be contrasted with the method depicted in FIGS. 3 and 4c, for example, wherein the cone beam projection data for v less than 0 is filtered out via mask M such that the cone beam projection data on the bottom half (B) of detector D is not utilized for image reconstruction. FIG. 7 illustrates an extended ROI, comprising a ROI xcexa9o, and extended portions xcexa9u and xcexa9d, which is reconstructed using cone beam data that is acquired from two circular scan paths 21 and 22 (FIGS. 5 and 6), and a connecting line scan L, using an image reconstruction method according to an embodiment of the invention. When the entire top circular scan data is processed using the method depicted in FIG. 5, the portion of the cone beam image with v greater than 0 yields reconstruction of the volume xcexa9u shown in FIG. 7. Similarly, when the entire bottom circular scan data is processed using the method depicted in FIG. 6, the portion of the cone beam image with v less than 0 yields reconstruction of the volume xcexa9d shown in FIG. 7. The total reconstructed volume with the two circular scan paths is extended from xcexa9o to xcexa9u ∪xcexa9o ∪xcexa9d. Further details of the data acquisition and processing method for reconstructing the extended ROI depicted in FIG. 7, will now be explained in further detail. As discussed above, the method disclosed in the above-incorporated U.S. Pat. No. 5,383,119 is extended to perform a data acquisition and image reconstruction process that improves utilization of the detector and provides reconstruction of an extended ROI, i.e., a reconstructed ROI which is larger that the reconstructed ROI obtained using the method U.S. Pat. No. 5,383,119, but with the same scanning paths and scanning time as the conventional method. To improve the utilization of the detector, the cone beam data in the v greater than 0 portion (see FIG. 5(b)), that is obtained from the upper circular scan, together with the cone beam data from a line scan, are used to reconstruct the portion xcexa9u of the long object that extends from the end of xcexa9o using the Kudo method. Similarly, using the Kudo method, the cone beam data in the v less than 0 portion (see FIG. 6(b)), that is obtained from the lower circular scan, together with the cone beam data from the line scan, are used to reconstruct the portion xcexa9d of the long object that extends from the end of xcexa9o. Details of a preferred method for processing cone beam data for the extended portions, xcexa9u and xcexa9d, using the Kudo method will now be provided. First, the portion of the cone beam image data that corresponds to the extended portions, i.e., the cone beam data for v greater than 0 for the upper circular scan, and the cone beam data for v less than 0 for the lower circular scan, is processed using the well-known xe2x80x9cFeldkampxe2x80x9d algorithm, which is disclosed for example in the article by Feldkamp, et al., entitled xe2x80x9cPractical Cone-Beam Algorithmxe2x80x9d, Journal of the Optical Society of America, Vol. 1, pp. 612-619, 1984, which is incorporated herein by reference. Essentially, with the Feldkamp method, a ramp filtering process is applied to the relevant projection data of the circular scans. Next, the line scan data are processed to fill in the data missing in the circle scan data, i.e. those integration planes which do not intersect the circle scan. Essentially, with this process, space-variant filtering is applied to the linear orbit (line scan), where a redundancy function is employed to extract the 3-D Radon Data that is inaccessible from the circular orbits and discard other multiply measures 3-D Radon data. More specifically, the line scan data are processed as follows: For each angle xcex8: (1) compute all line integrals at angle xcex8; (2) compute the derivative of the line integrals; and (3) compute a 2D backprojection of the line integral derivatives. Steps (1), (2) and (3) are performed on each line such that the integration plane defined by the line and the source position does not intersect the circle scan. The last step (4) is to compute the derivative of the resultant 2D backprojection image in the horizontal direction. The above process results in reconstruction of the extended regions xcexa9u and xcexa9d as shown in FIG. 7. Further, as noted above, the method disclosed in the above-incorporated U.S. Pat. No. 5,383, 119, is used to reconstruct the region xcexa9o. The combination of the different reconstructed regions yield the extended ROT shown in FIG. 7. Advantageously, since the data acquisition process according to one embodiment of the invention utilizes the same scan paths as disclosed in U.S. Pat. No. 5,383,119 (i.e., two circular scans and one connecting line scan), one can obtain reconstruction of an extended ROT, while maintaining the same scan time and fully utilizing the detector area. In other embodiments of the invention, the data acquisition and image reconstruction may be performed using more than two circle scans. For example, FIG. 8 illustrates an extended ROT that is reconstructed from three circular scan paths, an upper scan 21, lower scan 22 and middle scan 23, and one connecting line scan 24. The volume xcexa9obetween the upper and lower scans 21, 22 is reconstructed with the data combination method disclosed in the incorporated U.S. Pat. No. 5,383, 119. Further, the portion of the cone beam image with v greater than 0 for source positions on the upper circular scan path 21, as well as the portion of the cone beam image with v less than 0 for source positions on the lower circular scan 22, is processed using the Kudo method as discussed above. FIGS. 9a, 9b and 9c illustrate detector utilization of upper, middle and lower circular scans, respectively, for data acquisition and image reconstruction of the extended ROT shown in FIG. 8. Based on the teachings herein, an extension of the current method for 4 or more circular scans is readily apparent to one of ordinary skill in the art. Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.