WO2009115982A1

WO2009115982A1 - Computed tomography scanner apparatus and method for ct-based image acquisition based on spatially distributed x-ray microsources of the cone-beam type

Info

Publication number: WO2009115982A1
Application number: PCT/IB2009/051083
Authority: WO
Inventors: Gereon Vogtmeier; Joerg Bredno; Rainer Pietig; Michael Grass; Michael Overdick
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2008-03-21
Filing date: 2009-03-17
Publication date: 2009-09-24

Abstract

The present invention refers to distributed X-ray source computed tomography (CT) based imaging systems and, more particularly, to a CT scanner apparatus and method for a CT-based image acquisition and 3D reconstruction using stationary, partly stationary or non-stationary source-detector setup geometries with at least one detector ring fixedly attached to a gantry (101) and a number of contiguous or partly contiguous radiator arrays (RA₁, RA₂, …, RA_N ),each comprising a number of spatially distributed X-ray microsources (R, MS) of the cone-beam type, which may preferably be fabricated in carbon nanotube (CNT) technology, thus allowing higher sampling rates for an improved temporal resolution of acquired CT images as needed for an exact reconstruction of fast moving objects (such as e.g. the myocard) from a set of acquired projection data. Different arrangements of CNT-type radiator or source arrays (RA₁, RA₂, …, RA_N ) and detector arrays (DA₁, DA₂, …, DA_N ) as described with reference to exemplary embodiments of the present invention are proposed which, within improved sampling constraints as mentioned above, serve to enhance the spatial resolution of the reconstructed images by reducing artifacts that may result from missing or partial available image acquisition data.

Description

Computed Tomography Scanner Apparatus and Method for CT-Based Image Acquisition Based on Spatially Distributed X-ray Microsources of the Cone-Beam Type

FIELD OF THE INVENTION

The present invention refers to distributed X-ray source computed tomography (CT) based imaging systems and, more particularly, to a CT scanner apparatus and method for a CT -based image acquisition and 3D reconstruction with stationary, partly stationary or non-stationary source-detector setup geometries with at least one detector ring fixedly attached to a gantry and a number of contiguous or partly contiguous radiator arrays, each comprising a number of spatially distributed X-ray microsources of the fan- or cone-beam type, which may preferably be fabricated in carbon nanotube (CNT) technology, thus allowing higher sampling rates for an improved temporal resolution of acquired CT images as needed for an exact reconstruction of fast moving objects (such as e.g. the myocard) from a set of acquired projection data. Different arrangements of CNT -type radiator arrays (source arrays) and detector arrays as described with reference to exemplary embodiments of the present invention are proposed which, within improved sampling constraints as mentioned above, serve to enhance the spatial resolution of the reconstructed images by reducing artifacts that may result from missing or partial available image acquisition data.

BACKGROUND OF THE INVENTION

Today, computed tomography has found wide acceptance in different fields such as clinical diagnosis, industrial inspection, and security screening. In the clinical field, dedicated high-resolution CT imaging systems have recently emerged as important new tools for cancer research. From the relevant literature, various CT systems are known which make use of a 3D rotational X-ray examination device having one or more X-ray tubes and the same number of X-ray detectors placed on a gantry that is rotating around the patient (third generation). Modern fourth generation CT systems use 3D rotational X-ray examination devices which may be equipped with a stationary detector ring with multiple pixels that is mounted on the stator of the gantry, and only the X-ray tube is rotating with the rotor of the gantry around the patient. In any case, however, CT scanning is based on a rectilinear propagation and attenuation of X-rays. A tomographic imaging system thereby acquires a series of X-ray projections from a range of angles around the subject. Each projection represents the value (or collection of values in a multi-element X-ray detector) of the X-ray attenuation line integral through the object along the line from an X-ray source to an X-ray detector. Imaging an object to be graphically reconstructed at equiangular-spaced views over 180° forms a complete set of projection data. Tomographic image reconstruction creates a 2D image (or 3D volume) from the measured projection data. The angular distance between successive projections and the X-ray detector pitch are two of the primary factors controlling the resolution of the reconstructed image.

In a tomosynthesis system, an X-ray source is moved to a few discrete locations for acquiring voxel data for a set of 2D projection images of an object to be graphically visualized, wherein said projection images are taken from distinct projection angles. The voxel data of the obtained set of projection images are then used as input data for a 3D reconstruction algorithm which calculates a three-dimensional view of the object to be displayed on a screen by an image rendering application. Tomosynthesis images thereby provide a limited depth resolution - an improvement on projection radiography - without the expense and increased dose of conventional CT imaging.

Aside therefrom, X-ray microcomputed tomography (micro-CT) has proved as a cost-effective means for detecting and characterizing soft-tissue structures, skeletal abnormalities and tumors in live humans and animals. Micro-CT systems provide high- resolution images (typically 50 μm or less), rapid data acquisition (typically 5 to 30 minutes), excellent sensitivity to skeletal tissue and good sensitivity to soft tissue, particularly when contrast-enhancing media are employed.

Since the introduction of computer-based tomographic imaging about 30 years ago, advanced diagnostic imaging technologies have revolutionized the practice of medicine. X-ray computed tomography and magnetic resonance imaging studies are routinely performed to examine a patient's anatomy whereas single photon emission tomography and positron emission tomography provide functional maps of metabolic processes. These new technologies have become common tools of the clinical arsenal and have touched virtually every aspect of modern medicine.

In the recent decade, high-resolution microtomographic imaging has emerged as a powerful in-vivo imaging tool for preclinical cancer studies, especially for small animal studies in the basic biomedical sciences. Micro-CT systems have been used primarily for bone imaging studies due to the high contrast between calcified and soft tissue, but they have also been shown to be effective for soft-tissue imaging, particularly when a contrast medium is employed. These new imaging systems permit researchers to non-invasively screen small animals (such as e.g. mice and rats) for mutations or pathologies and to monitor disease progression and response to therapy. Aside therefrom, micro-positron emission tomography and high-resolution single photon emission tomography systems with pinhole and parallel- hole collimators have found use in functional brain imaging and gene expression studies, and magnetic resonance microscopy (MRM) has emerged as an important technology for in- vivo anatomic studies of soft tissue structure. Due to the relatively low costs of X-ray imaging, however, micro -CT has emerged as a cost-effective, attractive alternative to magnetic resonance microscopy (MRM) for a number of research applications.

In a typical configuration of early small animal CT systems, an animal (e.g. a mouse) is placed on a rotating stage between an X-ray source and an X-ray detector. The image spatial resolution is primarily determined by the X-ray source focal spot size, the detector element size and the system geometry whereas the contrast resolution is principally set by the X-ray flux and detector element size. In the early 1980s, the available X-ray detector elements were too large to provide the resolution required to image rodents so some early investigators replaced the detector block with a translating X-ray film cassette. The X- ray film was subsequently processed and digitized, providing data sets with sufficient resolution (-150 μm) to reconstruct useful images of small animal organs. By 1984 high- resolution X-ray detector technology had improved, and Burstein et al. reported a ~50-μm resolution image of a mouse thorax obtained using a conventional X-ray source operated at an electron accelerating potential of 90 kVp and a 512-element linear array of X-ray detectors (cf. Burstein, P. J.; Bjorkholm, R. C; Chase, R. C; Seguin, F. H.: ,,The largest and smallest X-ray computed tomography systems", in: Nucl. Instrum. Methods Phys. Res. 1984; Vol. 221, pp. 207-212). During this period, conventional CT systems were also employed to simultaneously image multiple mice with relatively low resolution (> 800 μm) but very high throughput (eight mice at a time, 9.6 seconds per image).

Nowadays, novel X-ray source technologies which are based on arrays of spatially distributed, microfabricated multi-pixel X-ray sources using field emission from carbon nanotubes (CNT) are opening up new approaches to X-ray imaging. In recently developed imaging applications, multi-pixel carbon nanotube field emission X-ray sources are used that are capable of generating spatially and temporally modulated radiations. X-ray microsources of the CNT type are small X-ray radiation emitting devices that are made from layers of carbon having a size of a few nanometers.

With the possibility of creating dense arrays comprising discrete and individually controlled X-ray microsources which are spatially distributed, new imaging setup geometries for micro-CT and CT systems as well as also for X-ray systems, such as e.g. tomosynthesis systems, become technically feasible.

In 1987, Flannery et al. brought X-ray microtomography into a new era with the introduction of a three-dimensional imaging system using a two-dimensional detector comprising a phosphor plate optically coupled to a charge-coupled detector (CCD) array (cf. Flannery, B. P.; Deckman, H. W.; Roberge, W. G.; D'Amico, K. L.: ,,Three-dimensional X- ray microtomography"; in: Science. 1987, Vol. 237, pp. 1439-1444 and Flannery, B. P.; Roberge, W. G.: ,,Observational strategies for three-dimensional synchrotron microtomography"; in: J. Appl. Phys. 1987; Vol. 62, pp. 4668-4674). To acquire a large number of X-ray photons in each micro-pixel (-2.5 μva x 2.5 μm) these investigators employed a synchrotron X-ray source beam line in place of the conventional X-ray tube. During this time the Ford Motor Company Research Laboratories also developed a three- dimensional microtomography system for industrial applications using a laboratory X-ray tube for the source and an image intensifier screen coupled to a video readout. As a part of this effort the scanner was used to study the subchondral bone architecture in guinea pigs with osteoarthritis, human cancellous bone, and trabecular bone structure. A fundamental contribution of the Ford group was the development of a new three-dimensional cone-beam image reconstruction algorithm (commonly referred to as Feldkamp algorithm), which remains one of the most widely used volumetric reconstruction algorithms (cf. Feldkamp, L. A.; Davis, L. C; Kress, J. W.: ,,Practical cone-beam algorithm"; in: J Opt. Soc. Am. 1984; Vol. 1, pp. 612-619).

In the 1990s a number of groups have developed microtomography systems for high-resolution specimen analysis. Most of these systems employ CCD-based detector arrays, micro-focus X-ray tubes, and have reconstructed image resolutions between 20 μm and 100 μm. The majority of the studies performed using these instruments focused on high- density tissue such as bone or teeth for which magnetic resonance imaging is less successful.

For in-vivo small animal studies, particularly for large population studies, rotating CT scanner configurations can be cumbersome because the object must be confined in a rotating carrier designed to prevent soft-tissue organ motion. Recently, dedicated small animal micro-CT scanners have been developed in which the X-ray source and X-ray detector both rotate about a fixed ,,patient bed", which is such like clinical CT systems. Both scanners employ conventional X-ray generators but use different configurations. One of the systems acquires relatively high-speed sequential single-slice images using an array of twelve semiconductor detectors (cf. ,,Stratec Medizintechnik XCT Research M"; published by: Stratec Medizintechnik Gmbh, Durlacher Str. 35, D-75172 Pforzheim, Germany), whereas the other acquires volumetric images using a 1024x1024 element CCD detector array (cf. Paulus, M. J. et al: ,,A new X-ray computed tomography system for laboratory mouse imaging"; in: IEEE Trans. Nucl. ScL 1999; Vol. 46, pp. 558-564). For an exact 3D reconstruction of an object to be graphically visualized by means of computed tomography imaging, a large number of 2D projection images from a wide viewing angle range are required. A serious limitation is the incompleteness of projection data acquired by a conventional short-scan circular source trajectory. Cone artifacts, which may result from that incompleteness, occur as a smearing and shading artifact and may thus superpose severely important low-contrast details.

Numerous investigations on source trajectories that satisfy Tuy's completeness condition (see Tuy, H. K.: ,,An inversion formula for cone-beam reconstruction"; in: SIAMJ. Appl. Math, 1983) can be found in the literature: Among many others, saddle trajectories (Pack, J.; Noo, F.; Kudo, H.: investigation of saddle trajectories for cardiac CT imaging in cone-beam geometry"; in: Phys. Med. Biol. 49, pp. 2317-2336, 2004), non-planar, non-closed trajectories optimized for C-arm devices (Schomberg, H.: ,,Complete source trajectories for C-arm systems and a method for coping with truncated cone-beam projections"; in Proc. Meeting on Fully 3-D Image Reconstruction in Radiology and Nucl. Med., 2001), circle-and- arc trajectories optimized for CT gantries (Ning, R.; Tang, X.; Conover, D.; Yu, R.: ,,Flat panel detector-based cone beam computed tomography with a circle-plus-two-arcs data acquisition orbit: Preliminary phantom study"; in: Med. Phys. 30, pp. 1694-1705, 2003), circle-and-line trajectories (Zeng, G. L.; Gullberg, G. T.: ,,A cone -beam tomography algorithm for orthogonal circle-and-line orbit"; in: Phys. Med. Biol. 37, pp. 563-577, 1992 as well as Johnson, R.; Hu, H.; Haworth, S.; Cho, P.; Dawson, C; Linehan, J.: ,,Feldkamp and circle-and-line cone -beam reconstruction for 3D micro-CT of vascular networks"; in: Phys. Med. Biol. 43, pp. 929-940, 1998 and Kudo, H.; Saito, T.: ,,Fast and stable cone-beam filtered back-projection method for non-planar orbits"; in: Phys. Med. Biol. 43, pp. 747-760, 1998) have been investigated.

Aside from a good spatial resolution, fast scanning speed is essential to image objects in rapid motion such as e.g. for diagnosis of cardiovascular diseases, CT fluoroscopy or airport luggage inspection. Scanning speed and temporal resolution, however, is critical for tomographic imaging of objects in rapid motion. Current CT scanners record projection images sequentially in the time domain by employing a step-and-shoot process using a conventional single-pixel X-ray source. This inefficient serial data acquisition scheme demands an increasingly high X-ray peak power and gantry rotation speed, both of which have approached the physical limits of conventional CT -based imaging systems.

SUMMARY OF THE INVENTION It may thus be an object of the present invention to provide a CT -based imaging system with an enhanced source-detector arrangement for improving spatial and temporal resolution when imaging and reconstructing fast moving objects, such as e.g. small animals or a patient's myocard, by means of radiographic imaging techniques based on X-ray imaging. Today, the development of spatially distributed X-ray sources allows new geometries for micro-CT and CT systems. For the concept of a stationary X-ray scanner without moving parts, a fast sequential switching of the X-ray sources is necessary, and the placement of the X-ray sources in combination with the X-ray detectors should allow a good sampling to achieve proper image reconstruction. In view of this object, the present invention uses the fact that microfabricated

X-ray sources of the CNT type, aside from their small size, are characterized by another attractive feature which consists in that they can be switched on and off very fast. According to the present invention, this switching capability is utilized to replace the moving of a CT or micro-CT gantry with a sequential switching of spatially distributed X-ray microsources. A first exemplary embodiment of the present invention refers to a computed tomography scanner apparatus with a source-detector system which comprises at least one stationarily mounted multi-source radiator array comprising a plurality of spatially distributed, sequentially switchable X-ray microsources addressed by a programmable switching sequence in a contiguous or at least partly contiguous sequential placement fixedly attached to a gantry having a substantial cylindrical gantry opening, said X-ray microsources being configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X- ray imaging. In addition to that, said source-detector system comprises at least one stationary detector ring carrying a detector array which comprises a number of two or more than two spatially distributed X-ray detectors fixedly attached to said gantry. According to the present invention, it is further foreseen that the total radiator surface size of the at least one multi- source radiator array and the total detector surface size of the at least one detector array, the density of arrangement of said X-ray microsources on each contiguous partial radiator surface of said radiator array and spatial distribution of all contiguous partial radiator surfaces over the interior surface of the substantial cylindrical gantry opening are chosen such that a complete set of 2D projection images is acquired which allows an exact 3D reconstruction of an image volume of interest without blurring or spatial aliasing artifacts.

For example, the spatially distributed X-ray microsources of said at least one multi-source radiator array and the spatially distributed X-ray detectors of said at least one detector array may be consecutively placed at adjacent angular positions along the perimetral edge of a polygon or circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening such that adjacent X-ray microsources as well as adjacent X-ray detectors do not mutually overlap. As an alternative, the spatially distributed X-ray microsources of said at least one multi-source radiator array and the spatially distributed X-ray detectors of said at least one detector array may be alternately placed at different angular positions or elsehow regularly interleaved along the perimetral edge of a polygon or circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening such that adjacent pairs of X-ray microsources and X-ray detectors do not mutually overlap.

The spatially distributed X-ray microsources of said at least one multi-source radiator array may be arranged in multiple transversal rows perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening. These rows may have a flat surface aligned with at least one edge segment or edge of a polygon lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening or, alternatively, a curved surface aligned with at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of said gantry opening.

As an alternative aspect of this first embodiment, the spatially distributed X- ray detectors of said at least one detector array may be arranged on at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, and the spatially distributed X-ray microsources of said at least one multi-source radiator array may be arranged on at least one arc segment of a helical trajectory winding around the longitudinal symmetry axis of the substantial cylindrical gantry opening, wherein the radius of the helical trajectory may be smaller than, equal to or greater than the radius of the circle. It may further be provided that each of said arc segments extends over an angular range of more than 180° such that the arc segment of said radiator array and the arc segment of said detector array mutually overlap in an azimuthal direction perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening.

According to a further alternative aspect of this first embodiment, the spatially distributed X-ray detectors of said at least one detector array are arranged on at least one arc segment of a first circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, and the spatially distributed X- ray microsources of said at least one multi-source radiator array are arranged on at least one arc segment of a second circle interlocking into the at least one arc segment of said first circle and having the same center coordinates as said first circle, wherein the radius of the second circle may be smaller than, equal to or greater than the radius of the first circle. Thereby, said first circle and said second circle may be arranged such that said second circle lies in a plane which is inclined by an acute angle with respect to the cross-sectional plane of said first circle.

In any case, each X-ray microsource may be aligned with a corresponding X- ray detector of said at least one detector array lying diametrically opposite to the respective radiation element, thus being irradiated by the radiation beam emitted from this radiation element.

The number of the spatially distributed X-ray microsources contained in the at least one stationarily mounted multi-source radiator array and the number of the spatially distributed X-ray detectors contained in the at least one detector array of the stationary detector ring as well as the switching frequency for sequentially switching said X-ray microsources on or off, respectively, may be chosen such that Ny quist- Shannon's sampling theorem is fulfilled when imaging a moving object so as to exactly reconstruct said object without any temporal aliasing artifacts. In a setup geometry where the number of spatially distributed X-ray microsources is too small to fulfill Nyquist-Shannon's sampling theorem, the computed tomography scanner apparatus may be adapted to perform a correction method which provides a sufficient image quality.

Preferably, it may be provided that the spatially distributed X-ray microsources of said at least one multi-source radiator array are given by a number of individually addressable single-pixel X-ray microsources of the cone -beam type using field emission cathodes in the form of carbon nanotubes. Using spatially distributed micro fabricated X-ray sources allows a very flexible placement of the respective radiator elements. The above-described placement of multiple CNT -type X-ray microsources in combination with an array of X-ray detectors further allows a good sampling to achieve a proper reconstruction of an object to be graphically visualized. It can be seen that the stationary placement of both X-ray sources and X-ray detectors is different compared to the known solutions of the third or fourth generation CT systems as described in the relevant literature. Moreover, the proposed solution based on spatially distributed X-ray microsources uses much more small radiation elements (in the range of hundreds to thousands), and the number of X-ray detectors may also be higher (e.g. three or more).

As an alternative, it may be provided that the spatially distributed X-ray microsources of said at least one multi-source radiator array are given by a number of cold emission X-ray microsources or a number of thermal X-ray microsources with a biased cathode for providing a grid switching of the emitted X-ray beams.

A second exemplary embodiment of the present invention refers to a computed tomography scanner apparatus with a source-detector system which comprises at least one first multi-source radiator array comprising a plurality of spatially distributed, sequentially switchable X-ray microsources addressed by a programmable switching sequence in a contiguous or at least partly contiguous sequential placement on a gantry having a substantial cylindrical gantry opening, said X-ray microsources being sequentially arranged in at least one transversal row having a curved surface aligned with at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, wherein said X-ray microsources are configured to emit multiple rays of cone -beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. In addition to that, said source-detector system comprises at least one further multi-source radiator array comprising a plurality of spatially distributed X-ray microsources in a contiguous or at least partly contiguous sequential placement, said X-ray microsources being consecutively arranged in any direction orthogonal to the azimuthal direction of the above- mentioned transversal row, wherein said X-ray microsources are configured to emit multiple rays of cone-beam radiation from distinct longitudinal positions parallel to the longitudinal symmetry axis of the substantial cylindrical gantry opening. Moreover, a detector ring carrying a detector array which comprises a number of two or more than two spatially distributed X-ray detectors arranged on said gantry is provided. According to the present invention, it is further foreseen that size, length and spatial orientation of the at least two multi-source radiator arrays and the total detector surface size of the at least one detector array, the density of arrangement of said X-ray microsources on each contiguous partial radiator surface of said radiator arrays and spatial distribution of all contiguous partial radiator surfaces over the interior surface of the substantial cylindrical gantry opening are chosen such that a complete set of 2D projection images is acquired which allows an exact 3D reconstruction of an image volume of interest without blurring or spatial aliasing artifacts. The present invention thereby proposes different setup geometries arranged for performing a circle-and-line cone -beam 3D reconstruction algorithm.

Preferably, said X-ray microsources may be consecutively arranged in at least one rectilinear, longitudinal column perpendicular to the transversal row of X-ray microsources contained in said first multi-source radiator array. As an alternative, said X-ray microsources may be consecutively arranged in at least one curvilinear column orthogonal to the above-mentioned transversal row. This line scan can be regarded as an add-on to the conventional short-scan circular path. A 3D reconstruction algorithm which is performed based on such a circle-and-line setup geometry has been proven as theoretically exact, possesses an efficient, shift- invariant filtered back-projection (FBP) structure and solves the long object problem.

According to this embodiment, it may be foreseen that said gantry is non- rotationally mounted to a stationary mounting unit and wherein the spatially distributed X-ray microsources of said at least one further multi-source radiator array are fixedly attached to this stationary mounting unit (configuration #1).

Alternatively, it may be foreseen that said gantry is rotationally mounted to a stationary mounting unit for journaling said gantry. In the latter case, the spatially distributed X-ray microsources of said at least one further multi-source radiator array may either be fixedly attached to the stationary mounting unit (configuration #2), which requires that the at least one further multi-source radiator array is used for detection irradiation when the corresponding detectors of the at least one detector array are in the diametrically opposed position, or fixedly attached to the rotational gantry and thus being moved together with said at least one first multi-source radiator array and said at least one detector array around the longitudinal symmetry axis of the substantial cylindrical gantry opening in case of rotational movements of the rotational gantry about this axis (configuration #3).

As a further refinement of configuration #3, it may be foreseen that the detector array, the at least one first multi-source radiator array and/or the at least one further multi-source radiator array are rotationally movable in an azimuthal direction about the aforementioned longitudinal symmetry axis of the substantial cylindrical gantry opening relative to said gantry.

In this context, it should be mentioned that all the above described systems have a limited capability to employ scatter grids for scatter reduction. While the source- detector system in the latter two configurations may be equipped with a linear scatter grid in column direction, this has to be omitted for the first configuration. All X-ray microsources have to be switchable to program the sequence of the line acquisition with respect to the circular arc acquisition.

Compared to setup geometries of conventional CT scanners according to the rotational gantry type having a single non-stationary X-ray source, this X-ray source would have to be moved in two orthogonal directions to achieve the same result, or the patient table would have to be moved. Both solutions are not optimal since components of the examination system would have to be moved (the X-ray tube or the patient table), which may result in inconsistent data acquisition as given by movement artefacts or speed limitations. The computed tomography scanner apparatus according to said second exemplary embodiment may thereby be adapted for performing an image acquisition in an azimuthal direction along the circular arc segment of the transversal row of X-ray microsources contained in said at least one first multi-source radiator array (arc acquisition), said arc acquisition covering an azimuthal range of 180° at maximum, and a further image acquisition in a direction covering the entire length of a rectilinear (line acquisition) or curvilinear column of X-ray microsources (further arc acquisition) orthogonal to this circular arc segment as contained in said at least one further multi-source radiator array.

A third exemplary embodiment of the present invention is directed to an image processing, visualization and archiving system comprising a computed tomography scanner apparatus as described above.

A fourth exemplary embodiment of the present invention is dedicated to a method for operating a computed tomography scanner apparatus as described above with reference to said first exemplary embodiment. Said method thereby comprises the steps of sequentially switching on individual consecutive columns of X-ray microsources contained in said at least one multi-source radiator array, said columns being arranged parallel to the longitudinal symmetry axis of the substantial cylindrical gantry opening, for a predefined image acquisition time, thereby using a constant switching frequency which is chosen such that Nyquist-Shannon's sampling theorem is fulfilled when imaging a moving object so as to reconstruct said object without any temporal aliasing artifacts. After said image acquisition time has expired, they are sequentially switched off.

According to this embodiment, the steps of sequentially switching individual consecutive columns of X-ray microsources on or off, respectively, may be cyclically repeated at a constant repetition rate. Said repetition rate may be chosen such that each column of X-ray microsources is switched on and off once per switching cycle. It may be foreseen, however, that some of these X-ray microsources may be switched on and off several times in one scanning sequence so as to enhance the temporal resolution of the scanned image sequence and/or improve the obtained 2D intensity profiles in terms of their signal-to-noise ratios.

A fifth exemplary embodiment of the present invention is dedicated to a method for operating a computed tomography scanner apparatus as described above with reference to said second exemplary embodiment. Such as described above referring to the method according to said fourth embodiment of the present invention, this method thereby comprises the steps of sequentially switching on individual consecutive columns of X-ray microsources contained in said at least two multi-source radiator arrays, said columns being arranged parallel to the longitudinal symmetry axis of the substantial cylindrical gantry opening, for a predefined image acquisition time, thereby using a constant switching frequency which is chosen such that Nyquist-Shannon's sampling theorem is fulfilled when imaging a moving object so as to reconstruct said object without any temporal aliasing artifacts. After said image acquisition time has expired, they are sequentially switched off.

The steps of sequentially switching individual consecutive columns of X-ray microsources on or off, respectively, may also be cyclically repeated at a constant repetition rate, and said repetition rate may also be chosen such that each column of X-ray microsources is switched on and off once per switching cycle.

According to this method, image data may be acquired in azimuthal direction along the circular arc segment of the transversal row of X-ray microsources contained in said at least one first multi-source radiator array (arc acquisition), said arc acquisition covering an azimuthal range of 180° at maximum, and in a further direction, thereby covering the entire length of a rectilinear column of X-ray microsources orthogonal to this circular arc segment as contained in said at least one further multi-source radiator array (line acquisition). Alternatively, image data may be acquired in azimuthal direction along the circular arc segment of the transversal row of X-ray microsources contained in said at least one first multi-source radiator array, said arc acquisition covering an azimuthal range of 180° at maximum, and in a further direction, thereby covering the entire length of a curvilinear column of X-ray microsources orthogonal to this circular arc segment as contained in said at least one further multi-source radiator array.

The image data is reconstructed with the aid of a circle-and-line cone -beam reconstruction algorithm, thereby acquiring a number of 2D projection images from different angular positions of a circular trajectory, augmented with further 2D projection images obtained by a linear scan along a longitudinal path segment perpendicular to the circular trajectory. While the acquisition of cone-beam projection data along a circle may lead to cone-beam artifacts, the proposed circle-and-line geometry setup allows a complete data acquisition resulting in exact object recovery. Especially for plate-like objects perpendicular to the longitudinal symmetry axis of the gantry, the circle-and-line algorithm produces superior 3D reconstructions compared to the Feldkamp algorithm.

Finally, according to a sixth exemplary embodiment of the present invention, a computer program product is provided for executing a method as described above with reference to said fourth or fifth exemplary embodiment, respectively, when being implemented and running on a control unit for controlling said computed tomography scanner apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS These and other advantageous aspects of the invention will be elucidated by way of example with respect to the embodiments described hereinafter and with respect to the accompanying drawings. Therein,

Fig. Ia shows a configuration of a conventional CT scanner apparatus as known from the prior art, Fig. Ib shows a schematic block diagram of the CT scanner apparatus illustrated in Fig. Ia,

Fig. 2 is a schematic view of a multi-source, multi-detector X-ray imaging system utilizing three X-ray tubes and detectors as in a configuration according to the prior art, Fig. 3 shows a schematic diagram of a fourth-generation 3D-rotational CT system according to the prior art (see left part of the figure) as well as a conventional multiplexing imaging system using a nine-pixel X-ray source and a digital area detector (see right part of this figure), wherein a single-pixel X-ray tube is mechanically rotated around a patient to be non-invasively examined by means of radiographic imaging, so as to acquire a set of 2D projection images from an anatomical or pathological object of interest in the interior of said patient's body,

Fig. 4 shows a detailed view of one X-ray pixel of a multi-pixel X-ray source as known from the prior art, wherein each individual X-ray pixel of the depicted multi-pixel X-ray source is comprised of a CNT -based field emission cathode, a dielectric spacer with a thickness of e.g. 150 μm, an extraction gate and a focusing electrode, wherein the cathode is realized as a thin CNT composite film deposited on a metal substrate by electrophoresis and wherein pulsing of the X-ray radiation is realized by applying a pulsed voltage signal with desired pulse width and repetition rate to the gate of the corresponding MOSFET circuit, and Figs. 5a-h show eight distinct source-detector setup geometries for the computed tomography scanner apparatus according to the present invention, and

Fig. 6 shows a flowchart illustrating the process of the proposed method according to said fourth or fifth exemplary embodiment of the present invention, respectively.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following, the configuration of an X-ray system with a spatially distributed multi -pixel X-ray source according to an exemplary embodiment of the present invention will be explained in more detail with respect to special refinements and referring to the accompanying drawings.

In Fig. Ia, a conventional CT imaging system 100 as known from the prior art (such as disclosed in US 6,937,689 B2) is shown which comprises a rotational gantry 101 rotating about the longitudinal axis 108 of a patient's body 107 or any other object to be examined, said rotational gantry 101 having at least one X-ray source or tube 102 that projects a beam of X-rays 106 towards an X-ray detector array 103 placed at the opposite side of said gantry. The X-ray detector array 103 is equipped with a plurality of detector elements 103a which together sense the projected X-rays passing through the patient's body 107 to be examined between X-ray detector array 103 and X-ray source 102. Each detector element 103a generates an electrical signal that represents the intensity of an impinging X- ray beam and can hence be used to estimate the attenuation of the beam as it passes through the object.

In a rotational CT scanner such as depicted in Fig. Ia, a 3D volume is calculated by reconstructing and stacking individual 2D slices. Most recently developed micro-CT systems employ two-dimensional detector arrays, allowing the acquisition of a truly three-dimensional data set. Since the initial work by Feldkamp et al.(cf. Feldkamp, L. A.; Davis, L. C; Kress, J. W.: ,,Practical cone-beam algorithm"; in: J Opt. Soc. Am. 1984; Vol. 1, pp. 612-619), 3D cone -beam reconstruction algorithms have become an active area of research. For example, Dusaussoy developed a Radon inversion, exact cone-beam reconstruction method that is effective independent of cone angle size (cf. Dusaussoy, N. J. : ,,VOIR, a volumetric image reconstruction algorithm based on Fourier techniques for inversion of the 3-D radon transform"; in: IEEE Trans Image Process. 1996; Vol. 5, pp. 121- 131). Schaller et al. have developed a fast, approximate cone-beam reconstruction algorithm suitable for small cone angles and an efficient Radon-inversion method for exact cone-beam reconstruction (cf. Schaller, S.; Flohr, T.; Steffen, P.: ,,An efficient Fourier method for 3-D radon inversion in exact cone -beam CT reconstruction"; in: IEEE Trans Med Imaging. 1998; Vol. 17, pp. 244-250).

Despite these recent advances, the Feldkamp algorithm remains the most commonly employed cone -beam reconstruction method because of its straightforward implementation and its applicability to practical tomography systems. Feldkamp' s algorithm is a 3D backprojection method that can be derived directly from the fan-beam equations for filtered backprojection. It is an approximate solution, but has tremendous practical utility for most cone -beam tomography applications.

In the schematic block diagram of Fig. Ib, only a single row of detector elements 103a is shown (i.e., a detector row). However, a multi-slice detector array such as denoted by reference number 103 comprises a plurality of parallel rows of detector elements 103a such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan. Alternatively, an area detector may be utilized to acquire cone -beam data. Moreover, the detector elements 103 a may completely encircle the patient 107. Fig. Ib also shows a single X-ray source 102; however, many such X-ray sources may be positioned around gantry 101. Operation of X-ray source 102 is governed by a control mechanism 109 of CT system 100. Control mechanism 109 includes an X-ray controller 110 that provides power and timing signals to one or more X-ray sources 102. A data acquisition system 111 (DAS) belonging to said control mechanism 109 samples analog data from detector elements 103 a and converts the data to digital signals for subsequent processing. An image reconstructor 112 receives sampled and digitized X-ray data from DAS 111 and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer 113, which stores the image in a mass storage device 114. The image reconstructor 112 may be specialized hardware residing in computer 113 or a software program executed by this computer.

The computer 113 also receives signals via a user interface or graphical user interface (GUI). Specifically, said computer receives commands and scanning parameters from an operator console 115 which in some configurations may include a keyboard and mouse (not shown). An associated display 116 (e.g., a cathode ray tube display) allows the operator to observe the reconstructed image and other data from computer 113. The operator- supplied commands and parameters are used by computer 113 to provide control signals and information to X-ray controller 110, DAS 111, and a table motor controller 117 (also referred to as ,,movement controller") in communication with a patient table 104, which controls a motorized patient table 104 so as to position patient 107 in gantry 101. Particularly, patient table 104 moves said patient 107 through gantry opening 105.

In some configurations, computer 113 comprises a storage device 118 (also referred to as ,,media reader"), for example, a floppy disk drive, CD-ROM drive, DVD drive, magnetic optical disk (MOD) device or any other digital device including a network connecting device such as an Ethernet device for reading instructions and/or data from a computer-readable medium, such as a floppy disk 119, a CD-ROM, a DVD or another digital source such as a network or the Internet, as well as yet to be developed digital means. In other configurations, computer 113 executes instructions stored in firmware (not shown). The computer may be programmed to perform functions described herein, and as used herein, the term ,,computer" is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits and other programmable circuits, and these terms are used interchangeably herein. Referring now to Fig. 2, an interior view of a gantry 101 of a computed tomography (CT) imaging system that does not require a rotating X-ray source is illustrated. One such system utilizing multiple X-ray sources is disclosed, for example, in U.S. Pat. No. 6,385,292 Bl, issued May 7, 2002. Other multi-source type X-ray CT apparatus are disclosed in U.S. patent application Ser. No. US 2003/0072407 Al, published April 17, 2003, and U.S. Pat. No. 4,592,079, issued May 27, 1986. A continuously formed tube could also be used. In Fig. 2, a first, second and third X-ray source 201a, 201b and 201c are used to generate respective X-rays 203a, 203b and 203c. These sources are representative of many such sources surrounding gantry 101. Each of X-rays 203a, 203b and 203c impinge upon a corresponding detector segment 202a, 202b and 202c of detector 103 shown in Figs. Ia and Ib. By using a device without a rotating gantry, each of the X-ray sources, X-rays, and detectors are fixed relative to one another. A few representative examples are shown as X-ray sources 201a, 201b and 201c, X-rays 203a, 203b and 203c and detectors 202a, 202b and 202c. One or multiple sources may be activated during a certain scanning interval. Such systems can be substantially faster in the generation of an image than systems using a rotating gantry. Faster scan times are achievable because the location of the source of the X-ray beam used for scanning can be electronically switched, thereby making fast cardiac imaging possible. Power levels can be reduced relative to rotating gantry configurations because imaging system configurations of the present invention can be positioned closer to the patient, thereby reducing the emitted X-ray intensities needed for imaging.

To improve temporal resolution of a CT imaging apparatus 100 having multiple or distributed X-ray sources, an optimal time-sequential sampling pattern is provided. Projections are collected in a range from 0° to 360°, subject to a time-sequential constraint that there is only one projection acquired at any time instant. Thus, the sampling is considered time-sequential. The sampling pattern specifies the angular position of which projection data set, i.e. the angular orientation of the view, to collect at any given time. It is optimal in the sense that it maximizes the temporal inter-projection interval, while providing a method for eliminating motion artifacts. This method can also be extended to configurations in which multiple sources are turned on simultaneously.

By applying computed tomography systems equipped with spatially distributed X-ray sources of the CNT type as will now briefly be described with reference to Fig. 3, which shows a schematic diagram of a conventional CT scanner 300a (see left part of the figure) in comparison with a conventional multi-beam X-ray source 300b (see right part of the figure) as described in the study ,,Multiplexing radiography using a carbon nanotube based X-ray source" (in: Appl. Phys. Lett. 89, 064106 (2006); DOL 10.1063/1.2234744; published: August 9, 2006) by J. Zhang, G. Yang and Y. Z. Lee, it can be demonstrated that multiplexing radiography is a feasible approach that enables parallel imaging (which means a simultaneous collection of multiple projection images) by means of commonly known frequency multiplexing techniques. With such a computed tomography system, both a drastic increase of the image acquisition speed and a significant reduction of the emitted X-ray peak power can be achieved without compromising the obtained imaging quality. The general term of multiplexing means a process to combine multiple data channels to form one composite data stream for transmission on a common medium. Demultiplexing, on the other hand, is the recovering of the original separate channels from a multiplexed signal.

Up to now, multiplexing techniques have not been applied to X-ray radiography, partly due to limitations of the X-ray source technology. Conventional X-ray tubes, such as referred to by reference number 301 in the left part of Fig. 3, are single-pixel devices that generate radiation from one focal point (herein also referred to as ,,pixel") on the X-ray anode. As a result, mechanical motion of either the X-ray source 301 and the irradiated X-ray detector array 302 placed diametrically opposite to the X-ray source or the object O to be graphically visualized is required to obtain a set of 2D projection images that are needed for a 3D reconstruction of the object. The radiation wave form can not be readily programmed, which makes coding and decoding difficult.

These limitations can be mitigated by multi-pixel field emission X-ray technology as known from the relevant literature. A multi-pixel X-ray source, which uses carbon nanotubes (CNTs) as the field emission cathode, is able to generate a plurality of spatially distributed X-ray beams (pixels) with a programmable intensity, pulse width, and repetition rate. The X-ray pixels are individually addressable through simple electronics. Spatially distributed X-ray source technology thus opens the door for system configurations such as stationary CT scanners that record the multiple views without mechanical motion of the gantry and multiplexing tomographic imaging, both of which have the potential to significantly increase the CT imaging speed.

The principle of multiplexing imaging can be explained by a conventional multi-beam X-ray source configuration 300b comprising a multi-beam X-ray source 303 and a flat panel X-ray detector 304 according to the prior art as shown in the right part of Fig. 3 which performs an image acquisition technique based on the orthogonal frequency division multiplexing (OFDM) algorithm. The employed multi-beam X-ray source 303 in the depicted configuration comprises a linear array of nine CNT electron field emission cathodes, a shared common gate, electrostatic focusing optics and a molybdenum target housed in 10^"8 Torr vacuum.

A detailed schematic view showing one pixel of a multi-pixel X-ray source as known from the prior art is depicted in Fig. 4. To ensure uniform emission across the pixels a rheostat R_D may be put in series with each CNT cathode 404, which also functioned as a ballast resistor to minimize the current fluctuation. Gate voltage U_g and rheostat R_D of the depicted configuration are calibrated to achieve the desired current and thus X-ray flux from each pixel. Voltage U/ of a electrostatic focusing electrode 402 is adjusted for each pixel to obtain a uniform 200 μm focal spot size for all nine pixels. During operation, voltage U_a at the anode 401 is stationary at a voltage level of e.g. 40 kV, and voltage U_g applied to the gate electrode 403 is adjusted to obtain, for example, up to 1 mA tube current per pixel. Activation and modulation of X-ray radiation are achieved by programming the input voltage pulse train which is applied to the gate electrode G of the metal-oxide semiconductor field effect transistor (MOSFET) control circuit 405 connected to the CNT cathode 404 (see Fig. 4), which serves as a toggle switch. The radiation from a particular pixel is switched on when the input voltage at the corresponding MOSFET gate is equal to 5 V and is switched off when it is equal to 0 V. Although performing a sampling at twice the Nyquist frequency (fd = 2f_max) can ensure a correct reconstruction of the acquired image data, detector frame rate/_? needs to be sufficiently faster than the highest pulsing frequency f_max to obtain a set of image data which is free of under-sampling aliasing artifacts. For example, a 10-fold oversampling may be applied (fd = Nf_max with N= 10). This is because overlapping intensity contributions from the primary and higher harmonics of the input signals and their ripples in the Fourier spectrum introduce imaging artifacts due to inter-channel interferences.

In the following sections, eight setup geometries for the computed tomography scanner apparatus according to the present invention as depicted in Figs. 5a-h shall briefly be described.

Fig. 5a shows a first setup geometry of a computed tomography scanner apparatus according to a first configuration of the present invention's first exemplary embodiment which comprises a stationarily mounted multi-source radiator array RA comprising a plurality of spatially distributed X-ray microsources MS (herein also referred to as radiator elements indicated by reference sign ,,R") in a contiguous sequential placement fixedly attached to a gantry having a substantial cylindrical gantry opening, said X-ray microsources being configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. Furthermore, said setup geometry comprises at least one stationary detector ring which carries a detector array DA comprising three pairwise adjacent rectangular X-ray detector plates D (flat panel detectors) fixedly attached to said gantry. According to the present invention, the X-ray microsources MS are given by a number of switchable radiation elements R that are addressed by a programmable switching sequence for being sequentially switched on or off, respectively.

In the herein depicted configuration, the spatially distributed X-ray microsources MS (radiator elements R) of said multi-source radiator array RA are consecutively placed at adjacent angular positions along the perimeter of a half circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening. They are arranged in three transversal rows RAi, RA₂ and RA3 perpendicular to the longitudinal symmetry axis (not shown) of the cylindrical gantry, which is assumed as pointing in the z-direction of a 3D Cartesian coordinate system spanned by the three orthogonal coordinate axes x, y and z (not shown). In contrast thereto, the spatially distributed X-ray detectors D of said detector array DA are consecutively placed at adjacent angular positions along the perimetral edges of a regular 2N-sided polygon (with N> 2) lying in this cross-sectional plane (here exemplarily depicted for N= 3, thus yielding a hexagon). This arrangement is such that adjacent X-ray microsources MS as well as adjacent X-ray detectors D do not mutually overlap. As can be taken from Fig. 5a, said transversal rows have a curved surface aligned with an 180° arc segment of a circle lying in a cross- sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening. As an alternative to this first setup geometry, it may be foreseen that the spatially distributed X-ray microsources MS (radiator elements R) of said multi-source radiator array RA, such as the spatially distributed X-ray detectors D of said detector array DA, are arranged on Nplanar radiator fields which are consecutively placed at adjacent angular positions along the perimetral edges of the aforementioned regular 2N-sided polygon, each radiator field comprising K-L X-ray microsources (with K, L » 100) consecutively arranged in the longitudinal and transversal direction, respectively.

Fig. 5b shows a second setup geometry of a computed tomography scanner apparatus according to a second configuration of the present invention's first exemplary embodiment which also comprises a stationarily mounted multi-source radiator array RA comprising a plurality of spatially distributed X-ray microsources MS (radiator elements R) fixedly attached to a gantry having a substantial cylindrical gantry opening. This time, however, they are arranged in a partly contiguous sequential placement interleaved with three spatially distributed rectangular X-ray detector plates D (flat panel detectors) of a detector array DA, wherein the latter is also fixedly attached to said gantry (in the following referred to as ,,first interleaved arrangement"). Said X-ray microsources are again configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. As already described above with reference to said first configuration depicted in Fig. 5a, said X-ray microsources MS are given by a number of switchable radiation elements R that are addressed by a programmable switching sequence for being sequentially switched on or off, respectively.

According to the herein depicted setup geometry, areas containing said spatially distributed X-ray microsources MS and areas containing said detector plates D regularly interleave along the perimeter of a regular 2N-sided polygon (with N > 2) lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening (exemplarily depicted for N= 3, thus yielding a hexagon). In each second perimetral edge of said hexagon, the X-ray microsources MS of said radiator array RA are arranged in three transversal rows RAi, RA₂ and RA₃ perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, which is again assumed as pointing in the z-direction of a 3D Cartesian coordinate system spanned by the three orthogonal coordinate axes x, y and z. The spatially distributed detector plates D of said detector array DA are placed at the remaining angular positions along the perimetral edges of said hexagon. Again, this arrangement is such that adjacent X-ray microsources MS as well as adjacent X-ray detectors D do not mutually overlap. As can be taken from Fig. 5b, said transversal rows have a flat surface, each aligned with an perimetral edge segment of the hexagon lying in the respective cross-sectional plane.

According to four slightly modified design variants of this embodiment, setup geometries with smaller source arrays and larger detectors than depicted in Fig. 5b, setup geometries with larger source arrays and smaller detectors than depicted in Fig. 5b or setup geometries where the X-ray microsources MS of the radiator array RA are closer to or more far away from the hexagon center than the X-ray detectors D of the detector array DA may be foreseen.

In Fig. 5c a third setup geometry of a computed tomography scanner apparatus according to a third configuration of the present invention's first exemplary embodiment is shown that also comprises a stationarily mounted multi-source radiator array RA comprising a plurality of spatially distributed X-ray microsources MS (radiator elements R) fixedly attached to a gantry having a substantial cylindrical gantry opening. Similarly to as depicted in Fig. 5 a, both the X-ray microsources MS of said radiator array RA as well as the detector plates D of said detector array DA are consecutively arranged. Said X-ray microsources are again configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X- ray imaging. As described above with reference to said first configuration depicted in Fig. 5a, said X-ray microsources MS are given by a number of switchable radiation elements R that are addressed by a programmable switching sequence for being sequentially switched on or off, respectively.

According to the herein depicted setup geometry, the spatially distributed X- ray detectors D of said detector array DA are arranged on at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, and the spatially distributed X-ray microsources MS of said stationary multi-source radiator array RA are arranged on at least one arc segment of a helical trajectory winding around the longitudinal symmetry axis of the substantial cylindrical gantry opening, wherein each of said arc segments may extend over an angular range of more than 180° and thus may mutually overlap in an azimuthal direction perpendicular to said longitudinal symmetry axis. As shown in Fig. 5c, it may be provided that the radius of the helical trajectory is smaller than, equal to or greater than the radius of the circle.

According to an alternative of this configuration, the spatially distributed X- ray detectors D of said detector array DA may be arranged on at least one arc segment of a first circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, and the spatially distributed X-ray microsources MS of said stationary multi-source radiator array RA may be arranged on at least one arc segment of a second circle interlocking into the at least one arc segment of said first circle and having the same center coordinates as said first circle, said first circle and said second circle being arranged such that said second circle lies in a plane which is inclined by an acute angle with respect to the cross-sectional plane of said first circle. The radius of said second circle may thereby be smaller than, equal to or greater than the radius of said first circle.

In this connection, it should be noted that other setup geometries with spatially distributed X-ray microsources MS of a stationary multi-source radiator array RA being arranged on arc segments of two or more circular or helical trajectories winding around the longitudinal symmetry axis of the substantial cylindrical gantry opening are also conceivable (see Figs. 5g and 5h).

Fig. 5d shows a fourth setup geometry of a computed tomography scanner apparatus according to a fourth configuration of the present invention's first exemplary embodiment which also comprises a stationarily mounted multi-source radiator array RA comprising a plurality of spatially distributed X-ray microsources MS (radiator elements R) fixedly attached to a gantry having a substantial cylindrical gantry opening. This time, N radiator columns of a spatially distributed multi-source radiator array (with N > 2; here exemplarily depicted for N= 11), each column comprising five longitudinally arranged X-ray sources, are arranged in an alternate placement interleaved with N rectangular detector plates D (flat panel detectors) of an X-ray detector array DA, wherein both the X-ray microsources MS and the detector plates D are fixedly attached to said gantry (in the following also referred to as ,,second interleaved arrangement"). Said X-ray microsources are again configured to emit multiple rays of cone -beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. As already described above with reference to said first configuration depicted in Fig. 5a, said X-ray microsources MS are given by a number of switchable radiation elements R that are addressed by a programmable switching sequence for being sequentially switched on or off, respectively.

According to the herein depicted setup geometry, areas containing the longitudinal columns of spatially distributed X-ray microsources (R or MS) and areas containing said detector plates D regularly interleave along the perimetral edges of a regular 2N-sided polygon (which for N= 11 yields an icosikaidigon) lying in a cross -sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening. In each second perimetral edge of said polygon, the X-ray microsources MS of said radiator array RA are arranged in K transversal rows perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening (with K » 100; here: K = 5 rows exemplarily sketched), which is again assumed as pointing in the z-direction of a 3D Cartesian coordinate system spanned by the three orthogonal coordinate axes x, y and z. The spatially distributed detector plates D of said detector array DA are placed at the remaining angular positions along the perimetral edges of the polygon. Preferably, this arrangement is such that adjacent X-ray microsources MS as well as adjacent X-ray detectors D do not mutually overlap. As can be taken from Fig. 5b, said columns have a flat surface, each aligned with an perimetral edge segment of the polygon lying in the respective cross-sectional plane.

A fifth setup geometry of a computed tomography scanner apparatus according to a fifth configuration of the present invention's first exemplary embodiment is depicted in Fig. 5e which also shows a stationarily mounted multi-source radiator array RA comprising a plurality of spatially distributed X-ray microsources MS (radiator elements R) fixedly attached to a gantry having a substantial cylindrical gantry opening. This time, in contrast to the configuration depicted in Fig. 5d, Nradiator fields of a spatially distributed multi-source radiator array (with N > 2; here exemplarily depicted for N= 5), each radiator field comprising KL X-ray sources (with K, L » 100) consecutively arranged in the longitudinal and transversal direction, respectively, are arranged in an alternate placement interleaved with N rectangular detector plates D (flat panel detectors) of an X-ray detector array DA, wherein both the X-ray microsources MS and the detector plates D are fixedly attached to said gantry (in the following also referred to as ,,third interleaved arrangement"). Said X-ray microsources are again configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. As already described above with reference to said first configuration depicted in Fig. 5a, said X-ray microsources MS are given by a number of switchable radiation elements R that are addressed by a programmable switching sequence for being sequentially switched on or off, respectively.

According to the herein depicted setup geometry, areas containing said fields of spatially distributed X-ray microsources MS and areas containing said detector plates D regularly interleave along the perimetral edges of a regular 2N-sided polygon (which for N= 5 yields a decagon) lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening. In each second perimetral edge of said polygon, the X-ray microsources MS of said radiator array RA may be arranged in K transversal rows perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, which is again assumed as pointing in the z-direction of a 3D Cartesian coordinate system spanned by the three orthogonal coordinate axes x, y and z. The spatially distributed detector plates D of said detector array DA are placed at the remaining angular positions along the perimetral edges of the polygon. Preferably, this arrangement is such that adjacent fields of X-ray microsources MS and said detectors plates D do not mutually overlap. As can be taken from Fig. 5e, said radiator fields have a flat surface, each aligned with an perimetral edge segment of the polygon lying in the respective cross- sectional plane.

A sixth setup geometry of a computed tomography scanner apparatus according to anyone of the three configurations of the present invention's second exemplary embodiment is depicted in Fig. 5f. It comprises a first multi-source radiator array RAi comprising a plurality of spatially distributed, sequentially switchable X-ray microsources MS (radiator elements R) addressed by a programmable switching sequence in a contiguous or at least partly contiguous sequential placement fixedly attached to a gantry 101 having a substantial cylindrical gantry opening 105, said X-ray microsources being sequentially arranged in at least one transversal row having a curved surface aligned with at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening 105, wherein said X-ray microsources MS are configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging. The herein depicted setup geometry additionally comprises a further multi-source radiator array RA₂ comprising a plurality of spatially distributed X-ray microsources MS in a contiguous or at least partly contiguous sequential placement, said X- ray microsources MS being consecutively arranged in any direction orthogonal to the azimuthal direction of the above-mentioned transversal row, wherein said X-ray microsources MS are configured to emit multiple rays of cone -beam radiation from distinct longitudinal positions parallel to the longitudinal symmetry axis of the substantial cylindrical gantry opening 105. Furthermore, a detector ring carrying a detector array DA which comprises a number of two or more than two spatially distributed X-ray detectors D (such as e.g. flat panel detectors) fixedly attached to said gantry may be foreseen.

In the herein depicted configuration, the spatially distributed X-ray microsources MS contained in said first multi-source radiator array RAi are consecutively placed, to be more precisely, at adjacent angular positions along the perimeter of a 180° arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis of the substantial cylindrical gantry opening, said 180° arc segment constituting the aforementioned transversal row, wherein the symmetry axis is again assumed as pointing in the z-direction of a 3D Cartesian coordinate system spanned by the three orthogonal coordinate axes x, y and z (not shown). In contrast thereto, the spatially distributed X-ray detectors D of said detector array DA are consecutively placed at adjacent angular positions along the perimetral edges of a regular 2N-sided polygon (N > 2) lying in this cross- sectional plane (here exemplarily depicted for N= 3, thus yielding a hexagon). This arrangement is such that adjacent X-ray microsources MS as well as adjacent X-ray detectors D do not mutually overlap. As can be taken from Fig. 5f, said transversal row has a curved surface aligned with the 180° arc segment of said circle. The spatially distributed X-ray microsources MS contained in said further multi-source radiator array RA₂ are consecutively placed along the entire length of a longitudinal column parallel to the aforementioned longitudinal symmetry axis z of the substantial cylindrical gantry opening 105. Due to the programmable switching option of the sequentially switchable X-ray microsources MS, the projections from all source positions can be acquired very fast. This is the necessary precondition for a reconstruction without motion artefacts but with high temporal resolution. As an alternative to this setup geometry, it may be foreseen that the employed flat panel detectors depicted in Fig. 5f may be replaced with at least one detector array DA comprising a number of spatially distributed X-ray detectors D or small detector modules consecutively arranged at adjacent angular positions along at least one perimetral edge of the 2N-sided polygon or along at least one arc segment of said circle such as in a standard rotational CT scanner setup.

A flow chart for illustrating the proposed method for operating a computed tomography scanner apparatus as described above with reference to said fourth or fifth exemplary embodiment, respectively, is shown in Fig. 6. After having received (Sl) a switching command (power-on signal) for initiating a new CT imaging session, a first column (or row) of X-ray microsources MS contained in a multi-source radiator array or a group of adjacent columns (or rows) of X-ray microsources MS, said columns being arranged parallel to the longitudinal symmetry axis z of the substantial cylindrical gantry opening, is switched on (S2). Otherwise, the procedure is continued in a loop with step Sl after a predefined delay time At until such a switching command has been received. In the first case, it is continuously determined (S3) whether a predefined image acquisition time T has expired. According to the present invention, this acquisition time is given by the reciprocal value of a switching frequency s = HT which is chosen such that Νyquist-Shannon's sampling theorem is fulfilled when imaging a moving object O so as to reconstruct said object without any motion or temporal aliasing artifacts. If this is the case, said column (or row) of X-ray microsources MS or said group of adjacent columns (or rows) is switched off again, and a neighboring column (row) of X-ray microsources MS or a neighboring group of adjacent columns (rows) contained in said multi-source radiator array is switched on (S4). When receiving (S5) a switching command (power-off signal) for terminating the running CT imaging session, the procedure is finished. Otherwise, it is continued with step S2 to be cyclically repeated at a constant repetition rate given by switching frequency^.

As described above, the advantageous effect of the present invention is to enhance the both temporal and spatial resolution for a 3D image reconstruction, the latter effect being achieved by optimizing the alignment of the radiator and detector arrays. To be more precise, the angular positioning of the individual X-ray detectors is optimized. The use of smaller X-ray detectors is beneficial as the costs for these smaller chips are lower, which is due to a better yield and due to the possibility of a mass production. The flexibility of the placement and the reduced distance for the X-ray beam from source to detector is also beneficial for this application compared to large detector setups.

APPLICATIONS OF THE PRESENT INVENTION The present invention and the above exemplary embodiments can be used in the scope of stationary CT or microtomography systems that are equipped with spatially distributed X-ray sources of the cone-beam type. The invention can especially be applied in small animal imaging or other high-speed medical image acquisition and image processing scenarios where it is beneficial to generate two- or three-dimensionally reconstructed high- resolution images of fast moving objects (such as e.g. the myocard) at an image acquisition rate which is high enough to fulfill Shannon-Nyquist's sampling theorem so as to avoid disturbing motion or aliasing artifacts and thus to improve the obtained image quality.

Although the specific configurations described herein refer to CT imaging systems having a stationary X-ray detector and a plurality of stationary X-ray microsources, each capable of projecting a directed X-ray beam (not necessarily all at once), it is contemplated that the benefits of the invention described herein may accrue to imaging modalities other than CT. Additional, although the herein described method and apparatus are described as belonging to a medical setting, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning system for an airport or any other kind of transportation center.

While the present invention has been illustrated and described in detail in the drawings and in the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, which means that the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word ,,comprising" does not exclude other elements or steps, and the indefinite article ,,a" or ,,an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. A computer program may be stored/distributed on a suitable medium, such as e.g. an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as e.g. via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

CLAIMS:

1. A computed tomography scanner apparatus with a source-detector system comprising at least one stationarily mounted multi-source radiator array (RA) comprising a plurality of spatially distributed, sequentially switchable X-ray microsources (R or MS) addressed by a programmable switching sequence in a contiguous or at least partly contiguous sequential placement fixedly attached to a gantry (101) having a substantial cylindrical gantry opening (105), said X-ray microsources (R or MS) being configured to emit multiple rays of cone -beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging, and further comprising at least one stationary detector ring carrying a detector array (DA) which comprises a number of two or more than two spatially distributed X-ray detectors (D) fixedly attached to said gantry (101), wherein the total radiator surface size of the at least one multi-source radiator array (RA) and the total detector surface size of the at least one detector array (DA), the density of arrangement of said X-ray microsources (R or MS) on each contiguous partial radiator surface (RAi, RA₂ or RA₃) of said radiator array (RA) and spatial distribution of all contiguous partial radiator surfaces (RAi, RA2 and RA3) over the interior surface of the substantial cylindrical gantry opening (105) are chosen such that a complete set of 2D projection images is acquired which allows an exact 3D reconstruction of an image volume of interest without blurring or spatial aliasing artifacts.

2. The computed tomography scanner apparatus according to claim 1, wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi- source radiator array (RA) and the spatially distributed X-ray detectors (D) of said at least one detector array (DA) are consecutively placed at adjacent angular positions along the perimetral edge of a polygon or circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) such that adjacent X-ray microsources (R or MS) as well as adjacent X-ray detectors (D) do not mutually overlap.

3. The computed tomography scanner apparatus according to claim 1, wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi- source radiator array (RA) and the spatially distributed X-ray detectors (D) of said at least one detector array (DA) are alternately placed at different angular positions or elsehow regularly interleaved along the perimetral edge of a polygon or circle lying in a cross- sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) such that adjacent pairs of said X-ray microsources (R or MS) and X-ray detectors (D) do not mutually overlap.

4. The computed tomography scanner apparatus according to anyone of claims 2 or 3, wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi- source radiator array (RA) are arranged in multiple transversal rows (RAi, RA₂, RA₃) perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105).

5. The computed tomography scanner apparatus according to claim 4, wherein said transversal rows (RAi, RA₂, RA₃) have a flat surface aligned with at least one edge segment or edge of a polygon lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105).

6. The computed tomography scanner apparatus according to claim 4, wherein said transversal rows (RAi, RA₂, RA₃) have a curved surface aligned with at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105).

7. The computed tomography scanner apparatus according to claim 2, wherein the spatially distributed X-ray detectors (D) of said at least one detector array (DA) are arranged on at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) and wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi-source radiator array (RA) are arranged on at least one arc segment of a helical trajectory winding around the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105).

8. The computed tomography scanner apparatus according to claim 7, wherein each of said arc segments extends over an angular range of more than 180° such that the arc segment of said at least one radiator array (RA) and the arc segment of said at least one detector array (DA) mutually overlap in an azimuthal direction perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105).

9. The computed tomography scanner apparatus according to claim 2, wherein the spatially distributed X-ray detectors (D) of said at least one detector array (DA) are arranged on at least one arc segment of a first circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) and wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi-source radiator array (RA) are arranged on at least one arc segment of a second circle interlocking into the at least one arc segment of said first circle and having the same center coordinates as said first circle, said first circle and said second circle being arranged such that said second circle lies in a plane which is inclined by an acute angle with respect to the cross-sectional plane of said first circle.

10. The computed tomography scanner apparatus according to anyone of the preceding claims, wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi- source radiator array (RA) are given by a number of individually addressable single-pixel X- ray microsources of the cone-beam type using field emission cathodes in the form of carbon nanotubes.

11. The computed tomography scanner apparatus according to anyone of claims 1 to 9, wherein the spatially distributed X-ray microsources (R or MS) of said at least one multi- source radiator array (RA) are given by a number of cold emission X-ray microsources or a number of thermal X-ray microsources with a biased cathode for providing a grid switching of the emitted X-ray beams.

12. A computed tomography scanner apparatus with a source-detector system comprising at least one first multi-source radiator array (RAi) comprising a plurality of spatially distributed, sequentially switchable X-ray microsources (R or MS) addressed by a programmable switching sequence in a contiguous or at least partly contiguous sequential placement on a gantry (101) having a substantial cylindrical gantry opening (105), said X-ray microsources (R or MS) being sequentially arranged in at least one transversal row having a curved surface aligned with at least one arc segment of a circle lying in a cross-sectional plane perpendicular to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105), wherein said X-ray microsources (R or MS) are configured to emit multiple rays of cone-beam radiation from distinct azimuthal positions around an imaging volume of interest to be graphically visualized by means of X-ray imaging, at least one further multi-source radiator array (RA₂) comprising a plurality of spatially distributed X-ray microsources (R or MS) in a contiguous or at least partly contiguous sequential placement, said X-ray microsources (R or MS) being consecutively arranged in any direction orthogonal to the azimuthal direction of the above-mentioned transversal row, wherein said X-ray microsources (R or MS) are configured to emit multiple rays of cone-beam radiation from distinct longitudinal positions parallel to the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105), and further comprising at least one detector ring carrying a detector array (DA) which comprises a number of two or more than two spatially distributed X-ray detectors (D) arranged on said gantry (101), wherein size, length and spatial orientation of the at least two multi-source radiator arrays (RAi, RA₂) and the total detector surface size of the at least one detector array (DA), the density of arrangement of said X-ray microsources (R or MS) on each contiguous partial radiator surface of said radiator arrays (RAi, RA₂) and spatial distribution of all contiguous partial radiator surfaces over the interior surface of the substantial cylindrical gantry opening (105) are chosen such that a complete set of 2D projection images is acquired which allows an exact 3D reconstruction of an image volume of interest without blurring or spatial aliasing artifacts.

13. The computed tomography scanner apparatus according to claim 12, wherein said X-ray microsources (R or MS) are consecutively arranged in at least one rectilinear, longitudinal column perpendicular to the transversal row of X-ray microsources (R or MS) contained in said first multi-source radiator array (RAi).

14. The computed tomography scanner apparatus according to claim 12, wherein said X-ray microsources (R or MS) are consecutively arranged in at least one curvilinear column orthogonal to the transversal row of X-ray microsources (R or MS) contained in said first multi-source radiator array (RAi).

15. The computed tomography scanner apparatus according to anyone of claims 12 to 14, wherein the gantry (101) is non-rotationally mounted to a stationary mounting unit and wherein the spatially distributed X-ray microsources (R or MS) of said at least one further multi-source radiator array (RA₂) are fixedly attached to this stationary mounting unit.

16. The computed tomography scanner apparatus according to anyone of claims

12 to 14, wherein the gantry (101) is rotationally mounted to a stationary mounting unit for journaling said gantry.

17. The computed tomography scanner apparatus according to claim 16, wherein said detector array (DA), the at least one first multi-source radiator array (RAi) and/or the at least one further multi-source radiator array (RA₂) are rotationally movable in an azimuthal direction about the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) relative to said gantry (101).

18. The computed tomography scanner apparatus according to claim 16, wherein the spatially distributed X-ray microsources (R or MS) of said at least one further multi-source radiator array (RA₂) are fixedly attached to the stationary mounting unit.

19. The computed tomography scanner apparatus according to claim 16, wherein the spatially distributed X-ray microsources (R or MS) of said at least one further multi-source radiator array (RA₂) are fixedly attached to the rotational gantry (101), thus being moved together with said at least one first multi-source radiator array (RAi) and said at least one detector array (DA) around the longitudinal symmetry axis (z) of the substantial cylindrical gantry opening (105) in case of rotational movements of the rotational gantry (101) about this axis.

20. The computed tomography scanner according to anyone of claims 12 to 19, adapted for performing an image acquisition in an azimuthal direction along the circular arc segment of the transversal row of X-ray microsources (R or MS) contained in said at least one first multi-source radiator array (RAi), said image acquisition covering an azimuthal range of 180° at maximum, and a further image acquisition in a direction covering the entire length of a rectilinear or curvilinear column of X-ray microsources (R or MS) orthogonal to this circular arc segment as contained in said at least one further multi-source radiator array (RA₂).