Patent Publication Number: US-2005133708-A1

Title: Method and system for three dimensional tomosynthesis imaging

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is related to and a continuation-in-part (CIP) application of application U.S. Ser. No. 10/739,541, Attorney Docket No. 130986-1, filed Dec. 18, 2003 in the U.S. Patent and Trademark Office. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates generally to the field of imaging, and more specifically to the field of tomosynthesis. In particular, the invention relates to tomosynthesis systems and methods employing new scanning trajectories for an x-ray source and a detector to yield an improved image of an object.  
      Tomography is well known for both industrial and medical applications. Conventional tomography is based on a relative motion of the x-ray source, the detector and the object. Typically, the x-ray source and the detector are either moved synchronously on circles or are simply translated in opposite directions. Due to that correlated motion, the location of the projected images of points within the object moves also. Only points from a particular slice, typically called a focal slice, will be projected always at the same location onto the detector and therefore imaged sharply. Object structures above and below the focal slice will be permanently projected at different locations. Because of that, they are not imaged sharply and will be superimposed as a background intensity to the focal slice. This principle of creating a 3D image with one slice in focus (focal slice) using a discrete number of projections is called tomosynthesis.  
      Tomosynthesis systems for medical applications, typically use an x-ray source for producing a fan or cone-shaped x-ray beam that is collimated and passes through the patient to then be detected by a set of detector elements. The detector elements produce a signal based on the attenuation of the x-ray beams. The signals may be processed to produce a radiographic projection. The source, the patient, or the detector are then moved relative to one another for the next exposure, typically by moving the x-ray source, so that each projection is acquired at a different angle.  
      By using reconstruction techniques, such as filtered backprojection, the set of acquired projections may then be reconstructed to produce diagnostically useful three dimensional images. Because the three dimensional information is obtained digitally during tomosynthesis, the image can be reconstructed in whatever viewing plane the operator selects. Typically, a set of slices representative of some volume of interest of the imaged object is reconstructed, where each slice is a reconstructed image representative of structures in a plane that is parallel to the detector plane, and each slice corresponds to a different distance of the plane from the detector plane.  
      In addition, because tomosynthesis reconstructs three dimensional data from projections, it provides a fast and cost-effective technique for removing superimposed anatomic structures and for enhancing contrast in in-focus planes as compared to the use of a single x-ray radiograph. Further, because the tomosynthesis data consists of relatively few projection radiographs that are acquired quickly, often in a single sweep of the x-ray source over the patient, the total x-ray dose received by the patient is comparable to the dose of a single conventional x-ray exposure and is typically significantly less than the dose received from a computed tomography (CT) examination. In addition, the resolution of the detector employed in tomosynthesis is typically greater than the resolution of detectors used in CT examinations. These qualities make tomosynthesis useful for such radiological tasks as detecting pulmonary nodules or other difficult to image pathologies.  
      Though tomosynthesis provides these considerable benefits, the techniques associated with tomosynthesis also have disadvantages.  
      Reconstructed data sets in tomosynthesis often exhibit a blurring of structures in the direction of the projections that were used to acquire the tomosynthesis data. This is expressed in a poor depth resolution of the 3D reconstruction or depth blurring. The degree of depth blurring depends on the scanning parameters, the distance of the object from the plane of interest, and on the size and orientation of the object relative to the scan paths. For example, the traditionally used linear scanning trajectory may lead to a limited z-resolution, and a contrast that may depend on the orientation of the anatomy to be imaged, while the circular trajectory may lead to circular artifacts, caused by out-of-plane structures, that may be mistaken for pathology. The blurring of structures may create undesirable image artifacts and inhibit the separation of structures located at different heights in the reconstruction of the imaged volume. Some existing tomosynthesis reconstruction algorithms address streaking artifacts due to acquisition at discrete focal spot locations, but do not address depth blurring.  
      Also generally, the solid angular range and complexity of the acquisition projection geometries trade off against the physical limitations of an exam. For example, projection geometries which result from simple linear focal spot trajectories using a flat fixed detector over a small angular range are faster and less demanding of the focal spot positioning apparatus. The smaller exam time mitigates patient motion artifacts for medical imaging applications. However, because the angular range of these projection geometries is small, depth blurring will be more severe in volume reconstructions of objects imaged using such an acquisition configuration. Projection geometries which result from more complex three dimensional focal spot trajectories over a larger solid angular range where a custom geometry multiple surface detector may be repositioned during the scan require a longer exam time, demand more of the focal spot positioning apparatus, demand more of the detector positioning apparatus, and may require additional design considerations for the detector shape. The longer exam time exacerbates patient motion artifacts for medical imaging applications. However, because the solid angular range of these projection geometries is larger, the depth blurring artifacts in the reconstructed volume will be reduced relative to the simpler scan.  
      Therefore there exists a need to adapt the current tomosynthesis systems to provide for new scanning trajectories to address the depth blurring of the imaged object by using more general projection geometries that may be more suited to reconstructing the region of interest and the anatomy to be imaged.  
     BRIEF DESCRIPTION OF THE INVENTION  
      Briefly, in accordance with one aspect of the present invention, an imaging system for scanning a volume of interest in an object is provided. The system includes a radiation source configured to traverse in a plurality of focal spot positions, and a detector configured to acquire a plurality of acquisition images of the volume of interest, where the detector is configured to translate beyond an edge of the detector for each focal spot position.  
      In accordance with another aspect of the invention, a method is provided for reducing missing data while scanning a volume of interest in an object using an imaging system. The method includes traversing a radiation source in a plurality of focal spot positions, and translating a detector beyond an edge of the detector, each of the plurality of detector positions corresponding to a respective focal spot position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other advantages and features of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
       FIG. 1  is a diagrammatical view of an exemplary imaging system in the form of a tomosynthesis system for scanning an object in accordance with aspects of the present technique;  
       FIG. 2  is a diagrammatical view of a physical implementation of the tomosynthesis system of  FIG. 1 ;  
       FIG. 3  is a top view of an embodiment of the present technique illustrating multiple source positions, a source trajectory and a parallel beam projection of the trajectory in an image acquisition plane;  
       FIG. 4 - FIG. 14  are a collection of top views illustrating different source trajectories and their projections in an image acquisition plane;  
       FIG. 15  is a diagrammatic view illustrating detector movement with respect to an X-ray source;  
       FIG. 16  is a top view illustrating the source and detector trajectories and their respective projections in an image acquisition plane; and  
       FIG. 17  is a diagrammatic view illustrating translation of a detector to eliminate motion blurring and capturing missing data during an image acquisition. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS  
       FIG. 1  illustrates diagrammatically an imaging system  10  which may be used for acquiring and processing image data. In the illustrated embodiment, the system  10  is a tomosynthesis system designed both to acquire original image data, and to process the image data for display and analysis in accordance with the present technique. In the embodiment illustrated in  FIG. 1 , the imaging system  10  includes a source  12  of radiation which is typically x-ray radiation in tomosynthesis, the source  12  is freely movable in three dimensions with respect to the object  18  and a detector  22 . In this exemplary embodiment, the x-ray radiation source  12  typically includes an x-ray tube and associated support and filtering components.  
      A stream of radiation  16  is emitted by the source  12  and impinges an object  18 , for example, a patient in medical applications. A portion of the radiation  20  passes through or around the object and impacts a detector array, represented generally at reference numeral  22 . Detector elements of the array produce electrical signals that represent the intensity of the incident x-ray beam. These signals are acquired and processed to reconstruct an image of the features within the object. A collimator  14  may define the size and shape of the x-ray beam  16  that emerges from the x-ray source  12 .  
      Source  12  is controlled by a system controller  24  which furnishes both power and control signals for tomosynthesis examination sequences, including positioning of the source  12  relative to the object  18  and the detector  22 . Moreover, detector  22  is coupled to the system controller  24 , which commands acquisition of the signals generated in the detector  22 . The system controller  24  may also execute various signal processing and filtration functions, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. In general, system controller  24  commands operation of the imaging system to execute examination protocols and to process acquired data. In the present context, system controller  24  also includes signal processing circuitry, typically based upon a general purpose or application-specific digital computer, associated memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth.  
      In the embodiment illustrated in  FIG. 1 , system controller  24  is coupled to a positional subsystem  26  which positions the x-ray source  12  relative to the object  18  and the detector  22 . In alternative embodiments the positional subsystem  26  may move the detector  22  or even the object  18  instead of the source  12  or together with the source  12 . In yet another embodiment, more than one component may be movable, controlled by positional subsystem  26 . Thus, radiographic projections may be obtained at various angles through the object  18  by changing the relative positions of the source  12 , the object  18 , and the detector  22  via the positional subsystem  26  according to various embodiments illustrated herein below in detail.  
      Additionally, as will be appreciated by those skilled in the art, the source of radiation may be controlled by an x-ray controller  30  disposed within the system controller  24 . Particularly, the x-ray controller  30  is configured to provide power and timing signals to the x-ray source  12 . A motor controller  32  may be utilized to control the movement of the positional subsystem  26 .  
      Further, the system controller  24  is also illustrated comprising a data acquisition system  34 . The detector  22  is typically coupled to the system controller  24 , and more particularly to the data acquisition system  34 . The data acquisition system  34  receives data collected by readout electronics of the detector  22 . The data acquisition system  34  typically receives sampled analog signals from the detector  22  and converts the data to digital signals for subsequent processing by a processor  36 .  
      The processor  36  is typically coupled to the system controller  24 . The data collected by the data acquisition system  34  may be transmitted to the processor  36  and moreover, to a memory  38 . It should be understood that any type of memory adapted to store a large amount of data may be utilized by such an exemplary system  10 . Also the processor  36  is configured to receive commands and scanning parameters from an operator via an operator workstation  40 , typically equipped with a keyboard and other input devices. An operator may control the system  10  via the input devices. Thus, the operator may observe the reconstructed image and other data relevant to the system from processor  36 , initiate imaging, and so forth.  
      A display  42  coupled to the operator workstation  40  may be utilized to observe the reconstructed image and to control imaging. Additionally, the image may also be printed on to a printer  44  which may be coupled to the processor  36  and the operator workstation  40 . Further, the operator workstation  40  may also be coupled to a picture archiving and communications system (PACS)  46 . It should be noted that PACS  46  may be coupled to a remote system  48 , radiology department information system (RIS), hospital information system (HIS) or to an internal or external network, so that others at different locations may gain access to the image and to the image data.  
      It should be further noted that the processor  36  and operator workstation  46  may be coupled to other output devices which may include standard or special purpose computer monitors and associated processing circuitry. One or more operator workstations  40  may be further linked in the system for outputting system parameters, requesting examinations, viewing images, and so forth. In general, displays, printers, workstations, and similar devices supplied within the system may be local to the data acquisition components, or may be remote from these components, such as elsewhere within an institution or hospital, or in an entirely different location, linked to the image acquisition system via one or more configurable networks, such as the Internet, virtual private networks, and so forth.  
      Referring generally to  FIG. 2 , an exemplary imaging system utilized in a present embodiment may be a tomosynthesis imaging system  50 . In an arrangement similar to that described above, the tomosynthesis imaging system  50  is illustrated with a source  12  and a detector  22  between which an object, illustrated as a patient  18  may be disposed. The source of radiation  12  typically includes an x-ray tube which emits x-ray radiation from a focal point  52 . The stream of radiation is directed towards a particular region  54  of the patient  18 . It should be noted that the particular region  54  of the patient  18  is typically chosen by an operator so that the most useful scan of a region may be made.  
      In a typical operation, x-ray source  12  is positioned at a predetermined distance above the patient  18  and projects an x-ray beam from the focal point  52  and toward detector array  22 . The x ray source is configured to move in a plurality of focal spot positions. In specific embodiments, the source is configured to move in first and second dimensions generally parallel to the plane  56  of the detector  22  and in a third dimension generally perpendicular to the plane  56  of the detector  22 . The detector  22  is disposed in a spaced apart relationship with respect to the source  12  and at a predetermined distance from the patient  18 . The detector in one embodiment is kept stationary. Another embodiment includes detector movement and is described in detail herein below. The region of interest  54  or anatomy to be imaged is placed between the x-ray detector  22  and a plurality of x-ray focal spot positions  52  (described in greater detail in the description of  FIG. 3 ), and the focal spot is scanned to other positions consistent with image acquisition time, dose, signal to noise ratio, and mechanical complexity requirements. The detector  22  is generally formed by a plurality of detector elements, generally corresponding to pixels, which sense the x-rays that pass through and around a object of interest  54 , such as particular body parts, for instance the chest, lungs and so on. In one embodiment, the detector  22  consists of a 2,048×2,048 rectangular array of elements which correspond to a pixel size of 200 μm×200 μm, though other configurations and sizes of both detector  22  and pixel are of course possible. Each detector element produces an electrical signal that represents the intensity of the x-ray beam at the position of the element at the time the beam strikes the detector. The movement of the x-ray source is described in detail with reference to the discussion of  FIG. 3 - FIG. 14  herein below. In one embodiment the distance between the source  12  and the detector  22  is approximately 180 cm and the total range of motion of the source  12  is between 31.5 cm and 131 cm, which translates to ±5° to ±20° where 0° is a centered position. In this embodiment, typically at least 11 projections are acquired, covering the full angular range.  
      The processor  36  is typically used to control the entire tomosynthesis system  50 . The main processor that controls the operation of the system may be adapted to control features enabled by the system controller  24 . Further, the operator workstation  40  is coupled to the processor  36  as well as to a display, so that the reconstructed image may be viewed.  
      As the x-ray source  12  is moved in three-dimensions in reference to the plane  56  of detector, in accordance with the different embodiments described herein, the detector  22  collects data of the attenuated x-ray beams. Data collected from the detector  22  then typically undergo pre-processing and calibration to condition the data to represent the line integrals of the attenuation coefficients of the scanned objects. The processed data, commonly called projections, are then typically backprojected to formulate an image of the scanned area. In tomosynthesis, a limited number of projections are acquired, typically thirty or less, each at a different angle relative to the object and detector. Reconstruction algorithms are typically employed to perform the reconstruction on this data to reproduce the initial images.  
      Once reconstructed, the image produced by the system of  FIGS. 1 and 2  reveals the three dimensional relationship of internal features of the patient  18 . The image may be displayed to show these features and their three dimensional relationships. Though the reconstructed image may comprise a single reconstructed slice representative of structures at the corresponding location within the imaged volume, more than one slice is typical.  
       FIG. 3  illustrates an example of the source movement wherein the radiation source  12  is configured to traverse in a plurality of focal spot positions  52  yielding a plurality of focal spot trajectories  58 . The focal spot trajectory  52  is defined as a curve traced out by joining the focal spot positions  52  from the start of the exam until the end of the exam (called total exam time). Further, each focal spot trajectory  58  defines a two dimensional focal spot projection  60  in an image acquisition plane  62 . Thus the image acquisition plane  62  is defined as a plane which receives two dimensional projections of the three dimensional focal spot trajectories. The focal spot positions  52  comprise at least two positions at unequal distances from the image acquisition plane  62 , thus defining a three dimensional movement of the source  12 .  FIG. 3  illustrates a specific embodiment using a parallel projection geometry. The two-dimensional curve  60  identifies the projection of the three dimensional trajectory  58 , in one direction, onto an acquisition identification plane  62 . In  FIG. 3 , the trajectory  58  is projected in the z dimension, but any direction can be used. Thus, for a given choice of acquisition identification plane  62 , the focal spot trajectory  58  is identified as a two dimensional curve  60 . This association between three dimensional scan trajectories and two dimensional curves in an image acquisition plane is independent of the detector. It would be appreciated by those skilled in the art that perspective projection geometries will be obtained if the image acquisition is from a perspective of a point within the object  18 .  
       FIG. 4  through  FIG. 14  illustrate a collection of exemplary top views depicted generally by reference numerals  66 ,  68 ,  70 ,  72 ,  74   76 ,  78 ,  80 ,  82 ,  84 , and  86  showing different source trajectories  58  and their projections  60  in an image acquisition plane  62  in reference with the plane  56  of the detector  22 . Non-limiting examples of focal spot positions  52  include focal spot positions spaced equally on the focal spot trajectory, the focal spot positions having equal angular increment along the focal spot trajectory. Similarly, the non-limiting examples for the focal spot projections include at least one of a circle, an ellipse, a square, a rectangle, a composition of at least two closed curves intersecting at a point, a composition of a set of non-intersecting closed curves, and a spiral in the image acquisition plane. More examples of the focal spot projections include a shape including at least one of a saw tooth curve, a sine wave, a square wave, or an arbitrary curve, wherein the amplitude of the shape is defined by the volume of interest being imaged.  
      Thus it would be appreciated by those skilled in the art that various configurations of source movements are possible yielding a number of three dimensional focal spot location trajectories which are associated with respective two dimensional projections.  FIG. 4  through  FIG. 14  are just illustrative example for few of these possibilities.  
      Non-limiting examples of source movement include a path aligned to match the volume of interest being imaged and a path aligned to match a selection of features of the volume of interest being imaged. For example, for an anatomy that is long in one direction, it is beneficial to scan in the perpendicular direction. Also, the angular range is chosen such as to achieve the required depth sharpness for the imaged anatomy. A larger angular range leads to a better depth sharpness. Furthermore, it may be useful to align the scanning trajectory with certain features of the anatomy. For example, in chest imaging, it may be advantageous to scan along multiple lines that are aligned with the long axes of the lungs. Furthermore, it may be beneficial to scale the scanning trajectory according to the size of the patient, or the size of the anatomy. In one embodiment, additional parameters that are needed for this optimization of the scanning trajectory can be derived from a single pre-shot of the anatomy. This trajectory may be derived in conjunction with a pre-existing model of the anatomy.  
      Also the source in one example is configured to traverse in at least one of a step and shoot mode and in a continuous mode. In step and shoot mode, the positional subsystem  26  moves the focal spot to the desired location  52  and then nominally stops moving. The electron beam then excites the anode at that focal spot location any number of times. Then the positional subsystem  26  moves the focal spot to a new position and acquires at the new position similarly, etc. In step and shoot mode, focal spot locations  52  are nominally fixed on the trajectory  58 . In continuous mode, the positional subsystem  26  moves the focal spot through the nominal focal spot location  52  without stopping completely. In continuous mode, there may be some acceleration and/or deceleration of the focal spot near the nominal focal spot location. If the continuous mode is used, the detector  22  can be simultaneously moved in an effectively opposite direction to reduce motion blurring. In yet another example, the source is a distributed anode and the electron beam is steered toward a plurality of focal spot locations  52  on the distributed anode. In yet another example, the source is a distributed field emitter and the x-rays are directed at the detector from each source location. In a specific example, at least one radiation source  12  is configured to direct radiation toward a detector  22  through the subject of interest  18  from a plurality of focal spot positions  52  defining a desired two dimensional projection  60  in an image acquisition plane  62  as described hereinabove. The focal spot positions  52  are displaced from one another in first and second dimensions generally parallel with the image acquisition plane  62  at a distance from the image acquisition plane  62  in a third dimension. Similarly the focal spot positions  52  can be selected in a variety of ways to suit a particular exam or anatomy being imaged.  
      Furthermore, in accordance with another embodiment, as illustrated in  FIG. 15  and  16 , the detector  22  is traversed in a plurality of detector positions  53 , that is, it is translated, tilted, and/or rotated, each of the detector positions  53  corresponds to a respective focal spot position  52 . Referring to  FIG. 16 , repositioning the detector serves to keep the region of interest  54  centered on the detector  22 . In another example the detector  22  is repositioned to limit the missing data from the region of interest  54 . Referring to  FIG. 16 , the detector positions yield a detector trajectory  59  defining a two dimensional detector projection  61  in the image acquisition plane  62 . The detector  22  in a specific embodiment is configured to move during imaging sequences in first and second dimensions in the plane  56  of the detector, and in a third dimension to vary the distance between the source  12  and the detector  22 . The dynamic detector location, orientation or configuration in these embodiments is associated with a two dimensional projection  61 . Thus a number of projection geometry trajectories, are defined as the coupled set of focal spot projection and detector projection,  60  and  61  respectively, for a given plane  62 . As will be evident to those skilled in the art in the embodiments described hereinabove, the motion of the object  18  can be removed, that is, if the object  18  moves during the total exam time, the focal spot trajectory and the detector trajectory can be adjusted to effectively remove the relative motion of the object. This is possible because in the embodiments described hereinabove, the projection of a locus of points in the object on the detector remains stationary during the scan by adjusting the focal spot trajectory and the detector trajectory. The locus of points include but are not limited to a plane and a curved surface.  
       FIG. 17  is a diagrammatic view of a specific embodiment where the detector  22  is traversed beyond an edge of the detector in order to reduce the effects due to motion blurring in continuous Tomosynthesis application and capture missing data. In a specific embodiment, a radiation source  12 , (not shown) is configured to traverse in a plurality of focal spot positions  52  and a detector  22  configured to acquire a plurality of acquisition images, of the volume of interest in the object  54 , the detector  22  is translated beyond an edge  90  of the detector  22  for each focal spot position  52 . As it would be well understood by those skilled in the art, the divergent nature of the x-ray beam leads to missing data even at a radiation source position of 0 degrees. Thus the fraction of the object volume not imaged by the detector is approximately 10% in the x-direction. Since the imaging system is symmetric in x and y planes, an additional 10% is lost in the orthogonal view (y-direction). In one example it has been observed that as the source angle is increased, the fraction of missing data stays constant up to an angle of approximately 6.5 degrees (arctan (20.5/180)), where the source is positioned over an edge of the detector. Up to this point, the missing data is shifted from one side of the object to the other, with no net increase of missing data. However, after the source moves over the edge of the detector, additional loss of data at the far edge is not compensated by increasing coverage at the near edge, and the fraction of missing data increases. Hence the missing data increases with source angle for larger sources angles. It will be well understood by those skilled in the art that the missing data is more severe for a smaller source to imager distance (SID), since the divergence angle of the source is greater, and hence the missing data regions at the edges of the object are larger. The effect of missing data as described hereinabove can be largely compensated for by translating the detector to a new position for each source position beyond the edge of the detector as referenced hereinabove. It has been observed that even for a large source angle of 30 degrees, the missing data is reduced significantly [from 45% greater than the theoretical minimum to only 10% greater than the theoretical minimum], by translating the detector appropriately for each source position.  
      In another exemplary embodiment, the detector  22  is translated in an opposite direction  92  with respect to a radiation source movement depicted generally by the reference numeral  94  in  FIG. 17 . This has an additional advantage of reducing motion-blurring artifacts during scanning. As the source moves during the image acquisition process, the projection of a point in the object on the detector moves as well. This motion of the projection of a point in the object during the firing of the x-ray source results in motion blurring in the image. By moving the detector in the opposite direction from the tube, this relative motion and the associated motion blurring can be eliminated at a single plane in the image and reduced elsewhere. By choosing, for example, to move the detector to eliminate motion blurring on a plane at a height half through the object, the worst case motion blurring can be significantly reduced compared to the nominal case where the detector is not moved.  
      It will be well appreciated by those skilled in the art that there may be other criteria besides reduction of missing data and reducing motion blurring which may be used to determine the appropriate detector motion. For example, other image quality factors such as minimizing the variation in number of projections contributing to a voxel in the 3D image, minimizing edge effects, or other criteria can be used.  
      As would be also appreciated by those skilled in the art the technique also includes the methods for scanning and acquiring images using the various embodiments of the invention described hereinabove.  
      While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.