Abstract:
An x-ray computed tomography system uses a broad area x-ray emitter and detector to allow both in plane and out of plane x-ray projections not restrained to spiral or helical scans.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on provisional application 60/341,560 filed Dec. 14, 2001 and entitled “Virtual Spherical Anode Computer Tomography”. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agencies: NIH HL62425. The United States has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to x-ray computed tomography (CT) and specifically to a CT system and method providing high-speed data acquisition. 
     Conventional x-ray computed tomography may employ an x-ray source collimated to produce a narrow fan-shaped beam directed along the transverse plane through a patient to be received by linear multi-element detector array. 
     The x-ray source and detector array may be mounted on a gantry to be rotated about a patient to obtain “projections” measuring x-ray attenuation at the different gantry angles along a slice plane though the patient. A “projection set” of such projections, for example, of projections obtained over a range of gantry rotation of 180 degrees plus the angle subtended by the fan beam, may be “reconstructed” into a tomographic image. The tomographic image shows a cross section of the patient along the slice plane. 
     The collection of a projection set can be viewed as an acquisition of lines of data in “k-space”. Each projection (i.e. the x-ray attenuation values measured by each element of the multi-element detector at one gantry angle) provides one line of data in a k-space plane at the same gantry angle and in the slice plane. A projection set provides a series of lines of k-space data extending like spokes of a wheel from the center of k-space. K-space is a frequency domain version of the tomographic image, with data near the center of k-space representing low-frequency image data and data at the edges of k-space providing high-frequency image data. A two-dimensional Fourier transform of k-space data collected by the projections provides the tomographic image. 
     Reconstructions using too few projections or a limited number of gantry angles may produce images with “artifacts”, typically streaks, that mar the tomographic image. 
     In a normal CT acquisition, multiple projection sets are obtained along different, sequential slice planes by movement of the patient within the gantry. The slices may be assembled to provide data for an arbitrary volume of the patient which may then be reconstructed into cross-sectional images along arbitrary planes. 
     The time required to collect projection data over a volume using conventional fan-beam tomography can be substantial and may preclude the use of tomography in situations where large volumes are to be monitored in real-time (for example, in contrast studies) or where there is unavoidable patient or organ motion during the acquisition time. 
     One method of increasing CT acquisition speed is by using a spiral or helical scan in which the gantry containing the x-ray source and detector is rotated continuously as the patient is moved. This process eliminates the need for stopping and starting the gantry and table. 
     Wider fan beams may also be used with rectangular detector arrays having, for example, 500 or 1,000 detector elements in the scan plane and sixteen detector elements perpendicular to the scan plane. This arrangement allows multiple slices to be collected at one time or allows for a steeper helical pitch. 
     While these systems provide for more rapid data acquisition, they are fundamentally limited by the rotational speed of the gantry. This latter limitation has been addressed by “electron beam” systems in which a steerable electron beam scans a hemi-cylindrical anode array providing what is in effect an x-ray source rotating about the patient within a plane but without mechanical movement. 
     SUMMARY OF THE INVENTION 
     The present invention provides improved volumetric acquisition of CT data by breaking free from the planar acquisition pattern to acquire sequential projections both aligned with and oblique to the slice plane. K-space is filled, not on a plane-by-plane basis, like spokes on wheels but in a three-dimensional starburst pattern. This out-of-plane acquisition provides a number of benefits including the ability to collect missing k-space data caused by beam divergence without patient translation, increased flexibility in selecting projection angle to minimize or measure tissue motion, and the ability to rapidly collect sparse projection sets for large volume imaging on a real time basis without being constrained to a helical or orbital acquisition sequence. 
     In this regard, the acquisitions of the present invention are analogous to three-dimensional isotropic magnetic resonance imaging (MRI) in which the x, y and z magnetic gradient fields are rapidly switched to allow for “projection” angles sampling three dimensions of k-space in a short time. When used with magnetic resonance imaging, this multi-dimensional sampling allows for time-resolved angiography and other dynamic applications in which it is desired to follow the time course of an image contrast medium. 
     Unlike magnetic resonance imaging, however, in which different projection angles may be obtained simply by changing the gradient fields, in x-ray CT, an actual x-ray beam at different three-dimensional angles must be obtained. The present invention addresses this problem with a large-area x-ray emitter that approximates a spherical source. A broad-area detector opposing the anode allows full utilization of the data obtained both in and out of slice plane. Undersampling and interleaving can be used to further decrease sampling time. 
     Specifically, the present invention provides an x-ray tomography system having a patient support and an x-ray source positionable proximate to the patient support. The x-ray source may include an areal anode and an electron gun where the electron gun is on the opposite side of the areal anode with respect to the patient support and is steerable to direct a beam of electrons at a rear side of the areal anode to cause the transmissive emission of x-rays from a front side of the areal anode. The x-rays are directed toward at least one point on the patient support over a range of latitudinal and longitudinal angles, at least one of which is no less than 180 degrees. 
     A multi-element detector opposes the x-ray source about the patient support for receiving the x-rays and a controller communicates with the x-ray source and the multi-element detector to steer the electron beam to acquire a series of projection sets each including measurements of x-ray attenuation of x-rays over the range of latitudinal and longitudinal angles. 
     Thus, it is one object of the invention to provide an x-ray CT machine that may acquire freely acquire projections outside of a single-slice plane. 
     The areal anode may be curved about the patient support. 
     It is another object of the invention to provide increased angular projection range for a given anode area. 
     The areal anode may be a hemi-cylinder. 
     Thus, it is an object of the invention to provide a compact anode shape that accommodates a patient, for example, supine on a patient table. 
     The tomography system may contain a geometric corrector receiving the projections and weighting them to simulate projection sets acquired with a hemispherical anode. 
     Thus, it is another object of the invention to provide for the use of a convenient hemi-cylindrical or other non-spherical anode surfaces while minimizing artifacts caused by varying source-to-patient distances. 
     The areal anode may be a portion of a sphere. 
     Thus, it is an object of the invention to provide a simple surface that requires no geometric correction. 
     The detector may be planar, curved about the patient in a hemi-cylinder or a section of a sphere. 
     Thus, it is another object of the invention to provide flexibility to use standard planar detector arrays, or alternatively, to use detector arrays that have minimized surface areas. 
     The detector and areal anode may be curved about the patient support and the detector may be x-ray transmissive and include a portion fitting between opposed surfaces of the areal anode. 
     Thus, it is another object of the invention to provide a method of obtaining greater than 180 degrees of x-ray beam angle necessary for artifact free tomographic reconstruction without mechanical movement. 
     The areal anode may include a collimator. 
     Thus, it is another object of the invention to provide for scatter reduction with a fixed areal anode. 
     The areal anode&#39;s surface may be dimpled to intercept the electron beam. 
     Thus, it is yet another object of the invention to allow optimization of the anode surface for multiple x-ray angles. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified perspective view of a CT machine suitable for acquiring out-of-plane projections per the present invention and having co-axial opposed hemi-cylindrical anodes and detectors about a patient table; 
         FIG. 2  is a cross-sectional view along line  2 - 2  of  FIG. 1  showing overlap of the detector and anode such as allows for greater than 180 degrees range of projection angle within a slice-plane; 
         FIG. 3  is a detailed fragmentary view of the anode of  FIG. 2  showing a dimpling of its surface to provide for an improved electron target and showing an associated collimator; 
         FIG. 4  is a diagram of k-space showing normals to acquisition planes in k-space obtained with the present invention; 
         FIG. 5  is a cross-sectional view along  5 - 5  of  FIG. 1  showing a geometric error introduced using a hemi-cylindrical rather than a hemispherical detector and anode; 
         FIG. 6  is a diagrammatic representation of successive k-space acquisitions using a averaging process used to offset the sparsity of high-frequency data; 
         FIG. 7  is a flow chart of use of the invention during acquisition of a contrast image; 
         FIG. 8  is a flow chart showing the steps of obtaining each of the images of  FIG. 7 ; 
         FIG. 9  is a view similar to that of  FIG. 1  showing an alternative embodiment of the of  FIG. 1  having hemispherical detector and anode; and 
         FIG. 10  is a figure similar to that of  FIGS. 1 and 9  showing CT machine having a planar anode and detector array. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a CT system  10  suitable for use with the present invention provides a horizontal patient table  12  extending generally along a longitudinal axis  14  on which a patient (not shown) may be supported in a supine position. 
     A detector array  16  is curved about the patient table  12  following the curved wall of a hemi-cylinder whose axis is aligned with the longitudinal axis  14 . The inner surface of the detector array  16  supports a series of detector elements  18  arranged in rows and columns facing the patient table  12 . The detector elements  18  may thus receive x-rays transmitted through the patient on the patient table  12  from a variety of different latitudinal angles  22  being generally those in a vertical plane or longitudinal angles  24  being generally those in a horizontal plane. The detector elements  18  may be conventional scintillation type x-ray detectors but may also include ionization type or CZT detectors or other such detectors well-known in the art and preferably attain at least a thirty frame-per-second readout speed. 
     Opposite to the detector array  16 , with respect to the table  12 , is an areal anode  26  also curved around the patient table  12  to follow the curved wall of a hemi-cylinder aligned along the longitudinal axis  14 . The diameter of curvature of the areal anode  26  is greater than that of the detector array  16  so that it may partially enclose the latter. The areal anode  26  can be constructed, for example, of an outer supporting material of low-Z such as beryllium, and an inner target material of high-Z material such as tungsten. The target material faces the patient table  12 . 
     Areal anode  26  is held within an airtight housing  30  that is evacuated and also encloses an electron gun  32 . Electron gun  32  is displaced horizontally so that the areal anode  26  is between the patient table  12  and the electron gun  32 . An electron beam  34  from the electron gun  32  may pass through the beryllium of the areal anode  26  to strike the tungsten to produce an x-ray beam directed toward the patient table  12 . 
     The electron gun  32  includes an electron source  36 , for example a heated filament, to produce electrons that are formed into an electron beam  34  drawn toward the areal anode  26  by an accelerating voltage maintained between the areal anode and electron gun  32 . Electrostatic or magnetic yokes  40  positioned about the electron gun  32  allow steering of the electron beam  34  to a variety of arbitrary target spots  42   a  and  42   b.    
     The yokes and electron source  36  may communicate with a gun control circuit  44  connected via a bus to a computer  46  that may thus control the electron beam  34  turning it on and off and selecting the particular focal spot  42   a  or  42   b  to which the electron beam  34  is directed. Ideally, thirty or more different focal spots  42  per second would be targeted. Various focus change coils (not shown) may also be controlled by computer  46  to accommodate the difference in distance between the electron gun  32  and the areal anode  26  for the different focal spots  42   a  and  42   b.    
     Computer  46  also communicates with table control  48  that provides for translation of the table  12  along the longitudinal axis  14  during scanning as will be described. An operator console  50  provides for input of data to the computer  46  for control of the CT system  10  and for the readout of the images and other data which may be stored on mass storage device  51  also communicating with computer  46 . 
     Referring now to  FIG. 2 , the detector array  16  may subtend an latitudinal angle  22  of substantially greater than 180 degrees about a patient on the patient table  12 , and ideally, will extend in an angular range  54  somewhat more than 180 degrees so as to be able to receive data for a complete projection set within a slice plane perpendicular to the longitudinal axis  14 . The areal anode  26  will extend in a range of latitudinal angles  22  of less than 180 degrees dictated by the requirement that the electron beam  34  from a single electron gun  32  to be able to strike its outer surface. This requirement may be relaxed if multiple vertically displaced electron guns are used. Nevertheless, the larger diameter of the areal anode  26 , with respect to the diameter of the detector array  16 , is selected so that the areal anode  26  may partially surround the detector array  16 . In this way, electron beams  34  and  34 ′ representing the highest and lowest limits of a range of latitudinal angles  22  obtainable by the electron gun  32 , when striking the areal anode  26  can produce cone beams of x-rays having central rays  52  and  52 ′ that flank a range  56  of latitudinal angles  22  of over 180 degrees. 
     Referring to  FIG. 5 , a lesser range  58  of longitudinal angles  24  of central rays  52 ″ and  52 ′″ may be obtained using electron beams  34 ″ and  34 ′″ at the limits of the longitudinal angles  24  of the electron gun  32 . Typically, this angle will be at least twenty-five degrees to ninety degrees. This angular range is sufficient to fill in the k-space data normally lost by divergence of a cone beam scanned about a single slice plane. Greater angles are limited by excessive path length through the supine patient. 
     The angular ranges  54 ,  56 , and  58  may be measured along a surface of an imaginary sphere located so that the surface of the sphere at points of intersection between the center rays and the spherical surface is normal to the central rays  52 . 
     Referring now to  FIG. 3 , the areal anode  26  may include a stepped or dimpled surface providing a series of targets  60  that the electron beam  34  may strike. X-rays  62  emitted from the inside of the areal anode  26  may be received by a collimator  64  having laminae  66  forming x-ray  62  into a cone-beam  68  of the desired size, desired central ray  52 , and causing reduced scatter. The laminae  66  may change in angle over the surface of the areal anode  26 . 
     Referring now to  FIG. 4 , the area of detection provided by the detector array  16  allows a series of different projection planes in k-space to be obtained with normals  70  having arbitrary latitudinal angles  22  and longitudinal angles  24 . Using cone beams  68  prevents direct acquisition of k-space data because of partial volumes covered by the diverging beams. K-space data can however be created from images reconstructed using cone beam algorithms such as those taught by Greaneat or Katsevich. See generally, P. Grangeat, Theoretical background of the cone beam reconstruction algorithm. Doctoral thesis, Ecole Nationale Supericure des Telecommunications, 1987; P. Grangeat, An analysis of the divergent beam X-ray transform based on the 3D Radon transform. Presented at the seminar at the Mathematicshes Forshunginstitut Oberwolfach: Theory and Applications of Radon Transforms., 1986; A. Katsevitch, Theoretically exact FBP-type inversion algorithm for spiral CT, SIAM Journal on Applied Mathematics 62(2002), 2012-2026, hereby incorporated by reference. 
     These normals  70  are generally aligned with the central rays  52  of the x-ray beams  68 . Per the central slice theorem, a set of evenly distributed planes within a spherical k-space volume may be obtained. To the extent necessary, the data of the detector elements  18  may be interpolated and rebinned to provide the proper orientation of the planes. Alternatively, this adjustment may be performed by interpolation (for example to a grid) in k-space. 
     Referring to  FIG. 5 , the hemi-cylindrical shape of the detector array  16  and an areal anode  26  will cause a geometric intensity variation among projections. For example, the first x-ray beam  68  extending approximately perpendicularly to the longitudinal axis  14  will have a smaller detector footprint  72  than a detector footprint  72 ′ of a similarly diverging beam  68 ′ canted with respect to the longitudinal axis  14 . This geometric distortion may be corrected by a set of weighting factors determined mathematically or empirically for the particular geometry of the detector array  16  and the areal anode  26 . The weighting factors are applied to the measured projections so as to produce a set of attenuation measurements at the detector array  16  that are equivalent to a detector array  16  and areal anode  26  that spherical in shape. Other standard CT processing of the data may also be performed. 
     Referring now to  FIG. 7 , the present invention is particularly well-adapted to following contrast agents injected into the body of the patient where the flexibility of positioning the projections permits a more uniform, sparse collection of k-space data. In such a study, as indicated by initial process block  76 , a mask image is first obtained of the volume of interest. This mask image may typically be acquired at a high resolution and obtained by directing the electron gun  32  to a large number of target areas  42   a  and  42   b  on the surface of the areal anode  26 . 
     At process block  78 , x-ray contrast media is injected into the patient and at process block  80  a series of contrast image acquisitions is begun. 
     Referring to  FIG. 8 , for each contrast image, a sparse projection set may be acquired per process block  84 . At process block  88 , geometric and other necessary CT corrections are performed on these projections and k-space data  90 , shown in  FIG. 6 , can be developed from the projections as described above. This k-space data set  90  desirably includes projections whose center rays are distributed in both latitudinal angle  22  and longitudinal angle  24  but provides sparse k-space data in an outer region  92  and more highly sampled k-space data in an inner region  94  as a result of a radial convergence/divergence of the planes of data. This outer region  92  corresponds to high-frequency image data and the inner region  94  responds to low-frequency image data. 
     The sparsity of high-frequency image data of the outer region  92  allows rapid acquisition of volume data but will generally produce image artifacts. Accordingly, at a process block  96 , following the geometric correction, an averaging of k-space data is performed wherein a second subsequent k-space data set  90 ′ is acquired and the data of the outer region  92  of k-space data set  90 ′ is averaged through averaging block  98  to produce an averaged outer region  100 . This second k-space data set  90  preferably uses different projections (e.g. by using focal spots  42   b  when k-space data set  90  using focal spots  42   a ). The inner region  94  of k-space data set  90  is then spliced directly into the inner region  94  of the k-space data set  90 ′ and the combined k-space inner region  94  and averaged outer region  100  is reconstructed by a three dimensional Fourier transform as indicated by process block  108  of  FIG. 8 . In this way, data artifacts are significantly reduced while providing essentially real-time imaging of low-frequency data. 
     It will be understood that these zones in k-space need not be distinct zones, but may involve a continuous weighting function, and further, that the averaging may be conducted with a weighting function over two or more successive k-space datasets  90 ,  90 ′ and  90 ″ and may also be conducted for the data of the inner region  94  using a different averaging window than that used for the outer region  92 . Variations of temporal filtering could include the use of matched filtering to weight sequential time frames in response the degree of contrast present at each point in time. 
     At process block  82 , the mask image and the contrast image (as averaged) may be subtracted to provide an image providing substantially showing the contrast media alone. The steps of process blocks  80  and  82  may be repeated so as to provide a real-time indication of the motion of the contrast medium through the body. During the acquisition of either of the mask and contrast images, motion of the table  12  may be provided, if necessary to provide for additional projection angles or to reduce the longitudinal length of the detector array  16  and areal anode  26  or to increase the possible imaging volume that may be obtained. 
     The above image processing steps, including reconstruction of an image from k-space data, may be performed by the computer  46  executing a stored program employing the above steps augmented by steps well known in the art. 
     Referring now to  FIG. 9 , geometric correction may be eliminated by using a truly hemispherical detector array  16 ′ and hemispherical areal anode  26 ′. Such a detector array  16 ′ and anode  26 ′ may be overlapped as shown in  FIG. 2  to provide proper projection set ranges, or alternatively, the detector array  16 ′ and anode  26 ′ may be rocked slightly about the patient by a motion  110 . This latter approach which may also allow separating between the detector array  16 ′ and the areal anode  26 ′ as shown for improved patient access. 
     A similar system may use planar detector array  16 ″ and planar areal anode  26 ″ such as simplifies fabrication, but requires a larger area for comparable longitudinal range as shown in  FIG. 10 . 
     It is specifically intended that the present invention not be limited to the embodiments, and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. For example, the areal anode system described may be replaced by multiple distributed x-ray sources performing a similar function, for example, to those using multiple electron guns. The invention may be used with a stationary table, a periodically incremented table or with constant table motion. An alternative to the angiographic application described above is the imaging of anatomical regions such as the lung for the purpose of tumor detection. In this case, bone subtraction using the thresholding and reprojection method may be used.