The 2D grayscale X-ray image data (i.e. the projection data) usually undergoes a filtering step using conventional mathematical signal processing methods. The mathematical filtering is a convolution with a suitable function, said function indicating how different frequencies are to be weighted. Mathematical filtering is followed by actual 3D back projection. The entire process is also known as filtered back projection as described, for example, in the book by A. C. Kak and M. Slaney, “Principles of Computerized Tomographic Imaging”, IEEE Press, 1988.
It has now been recognized that, to reconstruct a region of interest of an object, it suffices to map only half of said region of interest in the grayscale X-ray images, provided that 2D grayscale X-ray images are taken at angular positions of X-ray source and flat-panel X-ray detector over a full circle (i.e. through 360°).
This is based on the recognition that each ray which passes from the X-ray source of a particular angular position to a detector element on the flat-panel X-ray detector has a correspondence at another angular position where the radiation passes in the opposite direction, but on the same path.
This fact is utilized e.g. in DE 10 2008 051 157 A1 to virtually enlarge a detector, cf. also the article by Holger Kunze and Frank Dennerlein: “Cone beam reconstruction with displaced flat panel detector.” Proc. of the Fully 3D, 2009. In the case of the “differentiated back projection” method as described in Seungryong Cho, Dan Xia, Erika Pearson, Charles A. Pelizzari, and Xiaochuan Pan: “Half-fan-based Region-of-interest Imaging in Circular Cone-beam CT for Radiation Therapy”, Proc. of the Fully 3D, 2009, there does not even have to be an overlapping region, for example.
It is now known that a 3D reconstruction does not necessarily provide all the information necessary for an assessment. This problem is frequently overcome by obtaining a second 3D reconstruction. For example, a first 3D image dataset is acquired using X-rays of a particular frequency or energy, and a second X-ray image dataset is obtained using X-rays of a different frequency or energy. With this so-called dual-energy imaging, different materials can then be separated, as one material attenuates the X-ray radiation of one frequency slightly more than that of another frequency, and in the case of another material precisely the opposite may apply. Using suitable image fusion methods, this enables in particular different materials to be separated and image quality improvements to be achieved.
DE 10 2008 056 891 A1 describes a computed tomography (CT) scanner in which an X-ray filter is used to produce an unfiltered and a filtered radiation component of a fan beam, said radiation components having different X-ray spectra. In order to operate the scanner in a dual-energy mode, the processing unit analyzes a measurement signal of the unfiltered radiation component separately from a measurement signal of the filtered radiation component. An X-ray detector is of curved design to detect the entire radiation.
U.S. Pat. No. 7,620,141 B2 discloses a computer tomograph in which a flat-panel X-ray detector is used and the tomograph's X-ray source is operated at different voltages to generate two different kinds of X-ray fan beams alternately one after the other.
U.S. Pat. No. 7,476,023 B1 discloses an X-ray emitter for an X-ray apparatus, comprising two X-ray sources. The X-ray sources are interconnected such that their beams always have the same focal spot. In conjunction with a dual-energy mode, it is proposed, because of the common focus, to generate beams with different energy levels consecutively.
However, acquiring a plurality of 3D image datasets is very time-consuming.