Abstract:
The origin of a cone or fan beam of B, γ, or x radiation is moved in a continuous path such as a circle or is stepped among a two-dimensional grid of preselected points. The cone beam of radiation passes through a patient ( 14 ) in an imaging region ( 12 ) and is detected by a detector array ( 16 ). A data collection circuit ( 20 ) samples the detector array to generate radiation intensity sub-images. A circuit ( 56 ) monitors the shifting of the focal spot and controls an image shifting circuit ( 58 ) to shift physical coordinates of the sampled sub-image analogously. A sub-imaging combining circuit ( 60 ) interleaves or otherwise combines spatially shifted sub-images. In one embodiment, the combined sub-images forms a higher resolution composite image representation. In another embodiment, a plurality of combined, spatially shifted sub-images are collected at angularly offset orientations around the subject and are reconstructed into a higher resolution composite image representation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with digital x-ray imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention also has application in conjunction with computed tomography (CT) scanning and nuclear cameras and is not limited to the aforementioned application. 
     The most prominent difference between digital x-ray scanning and classical plane film x-ray is the x-ray detection system. An array of digital detectors replaces the sheet of photosensitive film. Each individual detector within the array detects the intensity of the radiation incident upon its face during the scan and converts that information into an electrical signal. The combined signals are processed and ordered by a computer and converted into a visible display. 
     Each individual detector reports only one intensity value, the average intensity along a path with about the same cross-section as the individual detector. However, the data is typically treated as if intensity variations are due solely to the x-ray from the source to the center of the detector. In this manner, the detectors can be thought of as individual pixels of a larger image. The smaller the pixels are, the finer the resolution and the more precise the resulting image. Conversely, if the individual detectors are too small, only a small amount of radiation strikes each detector during a sampling interval. Low amounts of received radiation are sensitive to statistical fluctuation and sampling errors. 
     In a more complex solution, the paths of the central rays are shifted half a pixel and the detectors sampled again. In this manner, the effective number of pixels is doubled. However, mechanically shifting the detector array rapidly and precisely is difficult. Often, vibration occurs providing uncertainty and inaccuracy in the true path of the central rays, which blurs the resultant image. 
     U.S. Pat. No. 4,637,040 to Sohval and Freundlich alternates between the two x-ray sources. The patent also suggests physically shifting a single x-ray rouse during rotation of a CT scanner, such as with a pair of x-ray tubes or a single tube with two distinct focal spots. 
     The present invention contemplates a new method and apparatus for increasing the spatial resolution in two dimensions of a digital x-ray system that overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a diagnostic imaging apparatus emits a beam of radiation into an imaging region where it penetrates a subject and is detected on the other side by an array of detectors, and processed into a visual representation of the interior of the subject. 
     In accordance with another aspect of the present invention, the imaging apparatus emits multiple beams of radiation, each time from a different source point, and combines the detected information into a higher resolution representation of the interior of the subject. 
     According to a more limited aspect of the present invention, the imaging apparatus uses an x-ray that generates electrons with a cathode and accelerates them towards an anode where they are converted into x-rays, steering the electrons along the way with a plurality of deflectors. 
     According to a more limited aspect of the present invention, the electrons are moved continuously or incrementally around a closed path on the anode. 
     According to another aspect of the present invention, a method is provided wherein the resolution of a digital x-ray apparatus is improved by using a plurality of point sources of x-rays, detecting the x-rays after passage through a subject, and combining multiple images into one, higher resolution image. 
     One advantage of the present invention is that it increases the spatial resolution of a digital x-ray scanning system. 
     Another advantage of the present invention is that it requires no moving parts to be added to the system. 
     Another advantage of the present invention is that it increases digital sampling density. 
     Yet another advantage of the present invention is that it is cost-effective. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a digital x-ray scanner in accordance with the present invention; 
     FIG. 2 is an x-ray trajectory plot that illustrates the path of a plurality of x-rays from an oscillating source, through a plane of imaging, to an array of detectors in accordance with the present invention; 
     FIG. 3 is a diagrammatic illustration of the reconstruction process in accordance with the present invention; and, 
     FIG. 4 is a cross-sectional view of an x-ray emitter looking in along the z-axis in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a digital x-ray system is provided. A radiation source  10  emits a cone or fan of B, γ, or other penetrating radiation which passes through an imaging region  12  and through a subject  14  to a two-dimensional radiation detector array  16 . In a CT embodiment, a fan or cone of radiation is projected onto a one or two-dimensional array. The x-ray source is mounted for rotation about the subject. The detectors either rotate with the x-ray source or extend in a circumferential arc around the subject. In nuclear medicine, the radiation source is a radioisotope. 
     The x-ray or other radiation is differentially attenuated by the tissue along each ray between the source and individual detectors. Optionally, a collimator  18  collimates the radiation beam into individual beams focused on the central portion of each detector. Physical filters for beam hardness correction are optionally disposed between the source and the subject  14 . Each individual detector in the detector array  16  senses the intensity of the x-rays incident upon its face. This intensity value is turned into a gray scale value which is read by a data collection circuit  20 . The gray scale values range from white to black, black corresponding to all of the x-rays reaching the detector, and white corresponding to none of the x-rays reaching the detector. Typically, the detection devices are capable of resolving approximately 2 32  gray scale values, i.e., resolve the gray scale with 32 bit accuracy. 
     In the simplest case, the radiation source  10  emits x-rays from a stationary point or focal spot, which provides the detector array  16  with only one view of the subject  14 . Subsequently, each individual detector reports one intensity value to a data collection circuit  20 . The single set of projection data from the data collection circuit  20  is the electronic image representation. In the CT embodiment, each single set of projection data is one of the views for collective reconstruction into an image. 
     In accordance with the present invention, the x-ray source  10  has a mechanism to vary the position of the focal spot or other source of the x-rays in two dimensions, for the purpose of providing the detector array  16  with multiple different views of the subject  14 . This concept is illustrated in FIG.  2 . With reference to FIG. 2, the source moves about a source path  40 , a circle in the illustrated embodiment. 
     As the source moves along its path, the ray seen by any individual one of the detectors traverses a corresponding path  42  through an imaging plane  44 . In the illustrated embodiment, the source is positioned at each of four locations along the source path  40 . Of course, the source can be positioned at a larger or smaller number of positions, or can move continuously around the path  40 . 
     With reference again to FIG. 1, a user input device  50  enables the user to instruct the system regarding a desired resolution improvement. A resolution control circuit  52  issues control signals to an x-ray source driver circuit  54  which causes the x-ray source to move continuously or intermittently around the source path  40 . The resolution control circuit also sends a sampling signal to the data collection circuit  20  to control sampling of the detector array in coordination with movement of the x-ray source. After the data is collected at one of the positions of the x-ray source, the collected data is moved to a sub-image memory  55 . A source shift monitoring circuit  56  monitors movement of the x-ray source and sends a corresponding signal to an image shifting circuit  58 . More specifically, as seen in FIG. 2, the data sensed by any given pixel of the array shifts along the path  42  on the image plane. The image shift circuit  58  creates a corresponding shift in the data. An image combining or reconstruction device  60  combines the shifted data. In the digital x-ray embodiment, the combined, shifted data is loaded into an image memory  62 . In the CT embodiment, each of the combined, shifted data sets is one view. The one or two-dimensional views generated at different angular orientations around the subject are reconstructed by a reconstruction processor  64  using a convolution-backprojection or other conventional reconstruction algorithm into a two or three-dimensional image representation that is loaded in the image memory  62 . In nuclear cameras, volume images are generated analogously from the γ radiation emitted by radioisotopes. A video processor  66  converts images from the image memory into appropriate format for display on a human-readable monitor  68 , such as a video monitor, LCD display, active matrix display, or the like. 
     A four sampling point embodiment of this concept is illustrated in FIG.  3 . Although circular paths are illustrated, it is to be appreciated that other trajectories are also contemplated. In this example, there are four detectors in the array  16  and four sub-images each containing four pixels are generated. Pixel values  101 ,  105 ,  109 , and  113  are sampled by the upper left detector. Pixel values  104 ,  108 ,  112 , and  116  are sampled by the lower right detector, and so forth. As the x-ray source rotates clockwise around the path  40 , data is collected at four points. The size of the source path  40  is selected relative to the geometry of the image plane and the detector array such that the second sampling position  102 ,  106 ,  110 ,  114  is shifted by a half pixel to the right. In the third position around the source circle, each ray passes through the image plane a half pixel to the right and a half pixel down and is sampled as sample values  104 ,  108 ,  112 ,  116 . In the fourth position of the source around the path  40 , the ray passes through the image plane a half pixel below the first sampling position. The shifting circuit  58  shifts each of the four sub-images by a half pixel in the appropriate lateral and vertical direction and the combining processor  60  loads the shifted images into the image memory  62 . Of course, this four element array is for simplicity of illustration. In practice, arrays are more commonly 256×256, 512×512, 1024×1024, or the like. 
     With reference to FIG. 4, the x-ray source is an x-ray tube, in the preferred embodiment. The x-ray tube  10  includes a cathode  70  having a filament and a rotating anode  72 . Optionally, the anode can be stationary. Beams of electrons emanating from the filament are accelerated toward the anode and deflected around or to selected points along the source path  40  by the four deflection plates  74 ,  76 ,  78 ,  80 ,( 80  behind  78 ). Two of the deflection plates  74 ,  76  control the movement of the beam in the y-direction, and two of the plates  78 ,  80  control the movement of the beam in the x-direction. 
     In the preferred embodiment, the voltage applied to the deflection plates varies sinusoidally. Plates  78  and  80  are 90° out of phase with respect to plates  74  and  76 . As a result, the electron beam traces a circle that processes about the source path  40  continuously. Alternately, the voltages can be applied to the deflection plates in steps to step the electron beam around the path  40  in steps. 
     Alternate deflection techniques are also contemplated. For example, magnetic coils can be utilized to deflect the electron beam. As yet another alternative, the x-ray tube can be mechanically moved. As yet another example, the cathode or the anode can be moved within the x-ray tube. As yet another option, the x-ray tube can have multiple cathodes, each focused on an incrementally shifted portion of the anode so that the focal spot is shifted by switching from cathode to cathode. As yet another option, a single cathode can be provided with multiple, offset filaments. 
     It is to be appreciated that the focal spot can be moved in other than circular trajectories. For example, the detector array can be a one-dimensional array of detectors. The focal spot is then swept back and forth either in steps or continuously in a direction parallel to the one-dimensional array. After sampling the detector array with the x-ray spot in each of a plurality of positions, e.g., four, the subject is indexed relative to the x-ray source and detector array in a direction perpendicular to the detector array. In the new position, the focal spot is again swept and another series of one-dimensional images collected. The sub-images of each line are interleaved and the lines are stacked to form a two-dimensional image. Alternately, the x-ray source and detector can be rotated around the subject and the interleaved data lines reconstructed to form a slice image representation. This same principal can be extended to volumetric images using either two-dimensional arrays or physically stepped one-dimensional arrays. 
     With two-dimensional detector arrays, various patterns for moving the focal spot are contemplated. For example, the focal spot can be stepped among the four corners of the square. For a finer resolution, the focal spot can be stepped among an nxn array of linear positions arranged in a grid, where m,n are plural integers. As another option, a large number of shifted images are generated and stacked. The pixels of the resultant image are projected through the stacked images and weightedly averaged. This technique is particularly advantageous when the focal spot is not sampled at a periodically changing position and where the resolution of the final image does not match the resolution of the interleaved shifted images. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.