Abstract:
A contrast-enhanced digital tomosynthesis system with a source configured to emit penetrating particles toward an object, a detector configured to acquire a series of projection images of the object in response to the penetrating particles from the source, a positioning apparatus configured to position the source relative to the object and the detector, and an imaging system coupled to the source, the detector, and the positioning apparatus. The imaging system is configured to control the positioning apparatus to position the source and detector relative to the object, control the source and the detector to acquire the series of projection images, and construct a tomographic volume capable of exhibiting super-resolution morphology and contrast-enhancement arising from injection of an exogenous contrast agent from data representing the acquired series of projection images or a subset thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/911,761, titled “DYNAMIC FOUR-DIMENSIONAL CONTRAST ENHANCED TOMOSYNTHESIS”, filed on Dec. 4, 2013, the entirety of which is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Aspects of this invention were made with government support under one or more of: contract number W81XWH-11-1-0229 titled “Dynamic Contrast-Enhanced Digital Breast Tomosynthesis” awarded by the U.S. Army Medical Research and Material Command, located at Fort Detrick, Md., 21702-5102. The government has rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The need for a medical imaging modality that overcomes the obscuration of overlapping bodily features has given rise to digital tomosynthesis, a three-dimensional (3D) x-ray imaging technique used for cancer screening and diagnosis. However, 3D tomosynthesis is limited in the detection of structures that do not demonstrate readily discernible radiographic features, such as tumor bloodflow. Modalities like magnetic resonance imaging (MRI) are sensitive to the abnormal metabolic activity of tumor tissue but do not have the anatomical resolution of tomosynthesis. 
       SUMMARY OF THE INVENTION 
       [0004]    Aspects of the invention include a contrast-enhanced digital tomosynthesis system. The system may include a source configured to emit penetrating particles toward an object, a detector configured to acquire a series of projection images of the object in response to the penetrating particles from the source, a positioning apparatus configured to position the source relative to the object and the detector, and an imaging system coupled to the source, the detector, and the positioning apparatus. The imaging system may be configured to control the positioning apparatus to position the source and detector relative to the object, control the source and the detector to acquire the series of projection images, and construct a tomographic volume capable of exhibiting super-resolution morphology and contrast-enhancement arising from injection of an exogenous contrast agent from data representing the acquired series of projection images or a subset thereof. 
         [0005]    Further aspects of the invention include a contrast-enhanced digital tomosynthesis method for imaging an object. The method may include positioning at least one source of penetrating particles relative to at least one detector and the object, the at least one detector having an array of pixels, and acquiring a series of images. The method may also include constructing at least one tomographic volume capable of exhibiting super-resolution morphology and contrast-enhancement arising from injection of an exogenous contrast agent from data representing the acquired series of projection images or a subset thereof. 
         [0006]    Additional aspects of the invention include a contrast-enhanced digital tomosynthesis method for obtaining four dimensional image data. The method includes injecting a contrast agent into the blood stream of a patient, acquiring a series of images of the patient over a plurality of different angles, where one image is acquired at each angle, and constructing four dimensional image data for the patient from the acquired series of images. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings, with like elements having the same reference numerals. This emphasizes that according to common practice, the various features of the drawings are not drawn to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures: 
           [0008]      FIG. 1  is a graph depicting movement of contrast agents through normal and cancerous tissue; 
           [0009]      FIGS. 2A, 2B, 2C and 2D  are block diagrams depicting systems for acquiring 4D image data in accordance with aspects of the invention; 
           [0010]      FIG. 3  is an illustration depicting an imaging system according to aspects of the invention; 
           [0011]      FIGS. 4A, 4B and 5  are illustrations depicting imaging of a patient in accordance with aspects of the invention; 
           [0012]      FIGS. 6A, 6B, 7A, 7B, 7C, and 7D  are illustrations depicting the reconstructions of 4D image data in accordance with aspects of the invention; 
           [0013]      FIGS. 8A, 8B, and 8C  are flowcharts showing steps for reconstruction processes according to aspects of the invention. 
           [0014]      FIG. 9  is a perspective view of a phantom for use in testing the 4D image acquisition system in accordance with aspects of the invention; 
           [0015]      FIG. 10  is a graph comparing observed concentration over time to actual concentration over time within the phantom in accordance with aspects of the invention; 
           [0016]      FIG. 11  is a graph characterizing spatial resolution of the 4D image acquisition system in accordance with aspects of the invention; 
           [0017]      FIG. 12  is a graph characterizing MTF over spatial frequency for various angles in accordance with aspects of the invention; 
           [0018]      FIG. 13  is a graph characterizing spatial frequency at 50% MTF versus central angle according to aspects of the invention; and 
           [0019]      FIG. 14  is a graph characterizing MTF versus spatial frequency for various numbers of projections in accordance with aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Aspects of the invention enable dose-reduced four-dimensional contrast enhanced tomosynthesis. Embodiments combine the merits of tomosynthesis and MRI (anatomical data, such as breast imaging, chest imaging, musculo-skeletal imaging, etc., and perfusion data) to achieve dynamic four-dimensional (4D) contrast-enhanced tomosynthesis. 4D imaging involves acquisition of 3D images over time. By looking at the movement of the contrast agent, blood flow through a 3D volume can be visualized. 
         [0021]    Typical PET/CT, MR/CT, x-ray/MR, etc., imaging processes require registration between modalities upon which a patient is being imaged. The systems, methods, and devices disclosed herein provide contrast-enhanced tomosynthesis imaging without requiring registration between modalities. Generally, data for tomosynthesis imaging utilized for producing images and/or rendering of an object/patient being imaged is acquired simultaneously according to aspects of the invention, such that registration between modalities is not required. 
         [0022]    Perfusion data can help draw conclusions about abnormal angiogenesis, which is a well-characterized precursor to tumor development as shown in  FIG. 1 .  FIG. 1  is a graph  10  that depicts the change in concentration of contrast of an image over time in contrast-enhanced tomosynthesis imaging (CETI). Perfusion data indicative of the presence of a tumor results in a different trend of the contrast concentration over time, as shown by line  100 , versus perfusion data indicative of normal results (e.g., no presence of a tumor), as shown by line  102 . 
         [0023]    Obtaining 4D data simply by acquiring a series of 3D tomosynthesis reconstructions at different time-points, however, would greatly increase the radiation dosage to the patient. As used herein, those of skill in the art will understand reference to the object being imaged may include any object, patient, particular anatomy of a patient, etc., which may be imaged by the CETI systems, methods, and devices disclosed herein. In accordance with the invention, instead of acquiring a full set of projections at each time-point, fewer images are acquired (e.g., one single-energy or dual-energy projection (e.g., a pair of projections made at two different energies or a single projection made with a detector capable of resolving the projection data into two or more energy bins) per time-point, with each projection acquired at a different projection angle). A sliding window (e.g., a window function) may be applied over the projections to generate a series of reconstructions. Windowing produces a moving average of the original data along two dimensions—time and angle, and results in a 4D reconstruction in which the 3 spatial dimensions and the 4 th  time dimension can be fully interrogated. 
         [0024]      FIGS. 2A-2D  depict tomosynthesis systems  200 ,  202 ,  204 ,  206  in accordance with aspects of the invention. The system  200  at  FIG. 2A  includes a tube generator  20 , a source  21  for emitting a beam  23  of particles (e.g., x-ray particles, neutrons, protons, particles of gamma, ultraviolet, visible, and/or infrared wavelengths, etc.), a filter  22  (e.g., a filter wheel adapted to select one of many filters), a stage  24  with object  26  being imaged, a detector  28 , and a computer  29  that coordinates the other components of the system  200 . Tomographic resolution may be achieved by motion of the object  26  being imaged, motion of the source  21 , and/or a combination of motion of the object  26  and motion of the source  21 . The tube generator  20 , source  21  and the detector  28  may be generally configured for x-ray particle generation and x-ray projection detection as is described herein. It will be understood by one of skill in the art that the tube generator  20 , source  21  and the detector  28  may be alternatively or additionally configured for generation and detection of additional or other particles that may be utilized for CETI. (e.g., neutrons, protons, etc.). The system  200  may be automated. The object  26  may be breast tissue or other tissue of an anatomy of a patient being imaged. In an embodiment, the object  26  is coupled to a contrast injection apparatus (not shown, but depicted as contrast injection  27  in  FIGS. 2B-2D ). The computer  29  may be positioned locally or remote to the other components. 
         [0025]      FIGS. 2B-2D  depict systems  202 ,  204 ,  206  with different configurations in accordance with aspects of the invention. In the system  202  depicted  FIG. 2B , the object  36  is coupled to (e.g., mounted to, attached to, etc.) a rotary stage  25 . The rotary stage  25  is configured to rotate the object  26  along predetermined arcs such that the source  21  emits radiation (x-rays  23 ) to the object  26  at various angles, providing multiple projections that are detected by the detector  28  for 4D CETI. At  FIG. 2C , the system  204  includes a source  31  that is made up of multiple emitters  32 ,  33 ,  34 ,  35 ,  36 . The emitters  32 - 36  are fixed at various positions with respect to the object  26 , such that multiple projections of the object  26  are produced at various angles that are detected by the detector  28  for 4D CETI. The system  206  shown at  FIG. 2D  has a source  41  that includes multiple emitters  42 ,  43 ,  44 ,  45 ,  46 , and the detector includes a first detector  47  and a second detector  48 . The first detector  47  is offset at a distance and an angle with respect to the second detector  48 , such that each detector  47 ,  48  detects projections of the object  26  at various angles. In each of the systems  200 ,  202 ,  204 ,  206 , the detectors  28 ,  47 ,  48 , the sources  21 ,  31 ,  41 , emitters  32 - 36 ,  42 - 46 , tubes  20 , and/or objects  26  may be fixed, configured to move, or both. It will be understood to those of skill in the art that multiple sources and multiple detectors may refer to multiple unique sources and detector, or equally can refer to multiple source positions and multiple detector positions achieved through moving the source or detector to these positions. Although the configurations of systems  200 ,  202 ,  204  and  206  are depicted independent of each other, one of skill in the art will understand from the disclosure herein that any combination of the features of systems  200 ,  202 ,  204  and  206  may be utilized for CETI in accordance with the invention. 
         [0026]    A contrast agent is injected into the blood stream of a patient in accordance with aspects of the invention. The contrast agent may be an exogenous contrast agent such as an Iodine-based contrast agent (e.g., a commercial iodinated contrast agent, which typically have a large molecular structure that encompasses 1 to 6 iodine atoms). 
         [0027]      FIG. 3  depicts an imaging system according to aspects of the invention. The system  30  includes an X-ray detector  300 , an x-ray tube  302  (e.g., a device to emit penetrating particles such as x-ray radiation), and a patient  304  positioned such that the x-ray detector  300  may detect the emissions transmitted from the tube  302  to the patient  304 .  FIG. 3  depicts the tube  302  in various positions relative to the patient  304 . At each position, a unique image of the patient  304  may be captured and processed via the x-ray detector  300 . In an embodiment, the x-ray tube  302  is adapted to move between the various positions depicted in  FIG. 3 . In one embodiment, the system  30  includes a plurality of x-ray tubes fixed at various locations relative to the patient  304 . In addition, multiple x-ray tubes may be fixed in position, while the object being imaged is rotated with respect to the x-ray tubes and detector(s). For example, one or more x-ray sources may be combined with one or more stationary detectors, one or more stationary x-ray sources may be combined with one or more moving detectors, one or more moving x-ray sources may be combined with one or more moving detectors, multiple fixed electronically switched x-ray sources may be combined with one or more electronically switched detectors, one or more fixed x-ray sources may be combined with one or more fixed detectors combined with a translation and/or rotation stage to move the target, etc. It is contemplated that a combination of x-ray tubes/sources adapted for movement relative to the patient  304  and x-ray tubes/sources fixed at positions relative to the patient  304 , as well as additional detectors, fixed and/or adapted to move, positioned accordingly for multiple tubes/sources may be utilized for tomosynthesis imaging of a patient. 
         [0028]    Referring next to  FIGS. 4A, 4B and 5 , a tomosynthesis system  40  according to aspects of the invention is depicted. The system  40  includes an x-ray detector  400  and an x-ray  402  tube configured for tomosynthesis imaging of a patient  404 .  FIG. 4A  depicts a single x-ray tube  402  that is adapted to move in a semi-circular direction (shown as an arc  406 ) with respect to the patient  404  and/or the detector  402 . One of skill in the art will understand that the x-ray tube  402  may be adapted to move in both the clockwise and counterclockwise directions, and may also be configured to move in a full 360 degree rotation with respect to the patient  404 . Additionally, the tube(s)  402  may also be configured for movement/placement across various families of motion (e.g., linear motion, motion on a curved path, random motion, and motion; see, for example, U.S. Pat. No. 8,233,690) coordinate systems and various lengths/dimensions for CETI of a patient (e.g., axes along a Cartesian coordinate system; radial, theta and phi axes along a spherical coordinate system; radial and theta axes along a polar coordinate system; radial, phi, and azimuthal axes along a cylindrical coordinate system, etc.). 
         [0029]      FIG. 4B  depicts various positions ( 410 - 417 ) at which the x-ray tube  402  may be configured to emit x-ray radiation for detection and image capture via the detector  400 . Although eight positions ( 410 - 417 ) are depicted, it will be understood to one of skill in the art that less or more positions may be used from the disclosure herein. In an embodiment, the positions ( 410 - 417 ) at which the x-ray tube  402  is adapted to emit x-ray radiation are dependent upon a predetermined time interval (e.g., every 5 seconds). The time intervals may be determined based upon the timing of the motion under consideration (e.g., the time course over which the tumor fills with the contrast agent), the window needed to achieve minimally sufficient 3D image quality (e.g., when five projections are used at 5 seconds per projection, the minimum window width would be 25 seconds (five times 5), which is acceptable for a phenomenon with a one minute peak), etc. 
         [0030]    At  FIG. 5 , positions  413 - 417  are depicted in greater detail. At each position, a projection is captured via emission of x-ray radiation from the tube  402  and detection by the detector  400 . At position  413 , the projection  513  is captured by the detector  400  and is associated with the time at which the tube  402  is located at position  413 . 
         [0031]    The process is repeated for position  414  and projection  514 , position  415  and projection  515 , position  416  and projection  516 , and position  417  and projection  517 . Each projection  513 - 517  detected by the x-ray detector  400  is rendered into an image that is unique relative to one another, and each image is associated with a position of the x-ray tube  402  and/or the time associated with the position of the x-ray tube  402 , and the data is interpolated for reconstruction.  FIGS. 4A-5  illustrate that projections (e.g., projections  513 - 517 ) are acquired at different angles within a limited angular range around the object  26  ( FIG. 2 ) (e.g., patient  304 / 404 ), but that each projection is acquired at a different time point. 
         [0032]      FIGS. 6A-7D  depict examples of the reconstruction process according to aspects of the invention. At  FIGS. 6A and 6B , subsets of these projections (indicated by “image  1 ,” “image  2 ,” “image  3 ,” “image  4 ,” “image  5 ,” and “image  6 ”) can be used to generate reconstructions.  FIG. 6B  illustrates eight projections with reconstructions (“image  1 ”—“image  6 ”) formed from three of the projections. It is contemplated that more or fewer projections/reconstructions may be employed. 
         [0033]    In another example as illustrated in  FIGS. 7A-7D , it is not necessary for projections forming a reconstruction to be from the same cycle of projections.  FIGS. 7A-7D  depict a first cycle of projections  700 , a second cycle of projections  702  and a third cycle of projections  704 . Each cycle  700 ,  702 , and  704  employs an x-ray tube configured to emit x-ray radiation at various positions relative to the patient according to a predetermined time interval. Although each cycle  700 ,  702  and  704  is shown to have eight positions, more or fewer positions may be utilized based on differing time intervals, angles/positions of the tube with respect to the patient, etc. 
         [0034]    In  FIG. 7A , a first reconstruction  708  is made from the eight projections in the first cycle  700 . In  FIG. 7B , a second reconstruction  710  is made from the last seven projections of the first cycle  700  and the first projection  712  of the second cycle  702 . In  FIG. 7C , a third reconstruction  714  is made from the last six projections of the first cycle  700  and the first two projections  712  and  713  of the second cycle  702 . This reconstruction process may be repeated such that a final reconstruction  716  is made from the eight projections of the third cycle  704 , as is shown in  FIG. 7D . 
         [0035]    Those of skill in the art will understand from the disclosure herein that various positions, angles, time intervals, projection combinations, cycles, etc., may be utilized for tomosynthesis reconstruction and imaging. For example, the time intervals between the positions of the x-ray tube(s) may be constant, or may not be constant. Various time intervals (e.g., a shorter time interval between a first position and a second position, and a longer time interval between a second position and a third position, etc.) may be utilized, and any number of positions may be selected for the CETI of the patient. Furthermore, in embodiments where cycles are utilized for CETI of the patient, each cycle may include the same number of projection or a different number of projections (e.g., a first cycle being constructed of five projections and a second cycle being constructed of seven projections, etc.). Each reconstruction may not necessarily be constructed in chronological order. Those of skill in the art will recognize from the disclosure herein that various positions, cycles, time intervals, chronological orders, cycles sizes, etc., may be utilized to conduct the CETI of the patient according to aspects of the invention. Additionally, it is not mandatory that projections be angularly adjacent to one another. It is also feasible to use multiple cycles to produce images with enhanced super-resolution. Enhanced super-resolution is achieved by offsetting the angles used in each cycle by small increments so that the range of angles is more finely sampled. 
         [0036]    The capability to compute oblique reconstructions permits the usage of sets of projections that are not centered around the same angle to obtain reconstructions from the same viewing angle. Reconstruction software (such as RTT&#39;s (Real-Time Tomography, LLC of Villanova, Pa.) Briona reconstruction software) provides dynamic real-time reconstructions that allow on-demand planar and oblique angle reconstructions, filtering updates and super-resolution magnified views. See, for example, U.S. Pat. No. 8,233,690. 
         [0037]    Additional improvements and features are added to the reconstruction software to provide reconstruction of 4D tomosynthesis datasets in accordance with aspects of the invention. Referring to  FIGS. 8A-8C , flowcharts  80 ,  82 , and  84  of steps for reconstruction of 4D tomosynthesis datasets are depicted. Each of the captured projection images  802  may be pre-processed  803  and buffered in on-board GPU memory or a combination of system and GPU memory (e.g., memory buffered projections  804 ). The memory buffered projections  804  are then indexed  805  for retrieval. According to the desired projection subset size  806  and time point  808 , the software pulls the appropriate projections/projection subsets  810  at particular timepoints from the indexed projections  805  into the reconstruction calculation. The reconstruction computation  812  then takes place, followed by post-processing  814  to generate the reconstructed image  816 . Since the reconstruction process is dynamic, both the projection subset size  806  and timepoint  808  can be interactively changed by the user along with all the other reconstruction and filtering parameters. The subset size  806  can be varied from 1 to number of projections in a single acquisition cycle. 
         [0038]    To improve computation efficiency as depicted in flowcharts  82  and  84 , the reconstruction can take advantage of the fact that some projections remain constant between nearby timepoints. Backprojection computations from individual projections may be buffered. This allows different timepoints to be reconstructed by reusing the buffered backprojections from the projections which remain constant and only performing the backprojection for the updated projections at the new timepoint. As shown in flowchart  82 , the first reconstruction timepoint processes the projection subset at timepoint t  821  and sends the projection subset to be reconstructed  823 . Each of the projections in the subset  821  is computed as a backprojection  824  and stored in the backprojection buffer memory  826 . Then, as depicted in flowchart  84 , the backprojection  824  stored in the backprojection buffer  826  is reused for reconstruction at the next time point. Backprojections  2  through M can be pulled from the backprojection buffer  826 , and only the new backprojection M+1 needs to be computed in  824  for the updated timepoint. Such features may be conducted in real-time. In such embodiments, reconstruction in 3D is started once sufficient data is accumulated to make the first reconstruction. After the image is acquired, the image is moved to a buffer and the 3D image is updated immediately. Thus, the 3D data is periodically (e.g., once per image acquisition) and automatically updated. These real-time modes may be utilized in real-time biopsy needle guidance implementations. 
         [0039]    The software processes described above may also be implemented with other tomographic reconstruction methods such as filtered backprojection, backprojection filtering, and iterative methods. 
         [0040]    The reconstruction flexibility opens up several options. If a higher spatial resolution image is desired, the user may elect to trade-off some temporal resolution and reconstruct using a larger subset size (e.g., larger window). Conversely, if a higher temporal resolution is desired, the user can trade off some spatial resolution by using a smaller projection subset size (e.g., smaller window). In an embodiment, the CETI reconstruction software permits the user to select which projections are used at each time point to create each individual 3D image. The number of projections used for each reconstruction may be increased or decreased. Increasing the number of projections for a reconstruction increases tomographic resolution while decreasing temporal resolution. As such, a user may select a larger number of projections to increase the tomographic resolution with the trade-off that the temporal resolution would decrease when such an increase in tomographic resolution is desired. Accordingly, the user may decrease the number of projections to improve the temporal resolution when improved temporal resolution is desired over improved tomographic resolution. Advantageously, by varying the selection of the number of projections that are used to create each image at each time point, the user may dynamically alter the degree of tomographic and temporal resolution as desired to achieve each 4D reconstruction. Filtering can also be dynamically adjusted to changes in the reconstruction. For example, the noise filtering can be dynamically adjusted to compensate for magnification or projection subset size changes. 
         [0041]    Additionally, each projection is shown such that the plane of interest is portrayed at the plane of a display (e.g., a video monitor) configured to output the image and adapted to operate with the CETI reconstruction software. This advantageously provides a system in which the object of interest and the plane of the object of interest is held fixed on the display while other objects in different planes are permitted to move, providing a rendering with a 3D appearance on a 2D projection of images. 
         [0042]      FIG. 9  depicts a phantom  90 . The phantom  90  mimics the flow of contrast agent through human tissue and is used to generate the data depicted in  FIGS. 3A-7D  in place of object  26  (e.g., human tissue, a patient, etc.). In an embodiment, the phantom  90  is constructed of Lucite (and/or other tissue mimicking the structure and appearance of plastic) and may be of a size and thickness of the object being imaged (e.g., about 200 square centimeters and about 5 centimeters in thickness for breast imaging). The phantom  90  may include a single cylindrical chamber that spans the width of the phantom  90  to simulate a vessel. In one embodiment, the phantom includes attachments at each end to connect to a pump and a reservoir to simulate the action of a heart of a patient (e.g., to pump fluid through the cylindrical chamber). A contrast agent (or other background fluid) may be injected into the tubing of the phantom  90  to simulate the inflow and outflow of the contrast agent such that the phantom  90  is able to provide dynamic flow. The illustrated phantom  90  enables pumping of fluid (e.g., contrast agent) with a time-varying concentration through a central chamber. Along the top edge of the phantom  90  are closed cylindrical chamber of various diameters utilized for calibration standards. 
         [0043]    Experiments were conducted in which a time-varying concentration of contrast agent was continuously pumped through the phantom  90  and a series of projection images were acquired over a 32 degree angular range with one projection every 2 degrees. Reconstructions were performed using a sliding window of 17 projections. 
         [0044]    Reconstruction data was analyzed to determine the observed contrast concentration in the phantom  90 .  FIG. 10  is a graph  1000  illustrating a comparison between observed concentration over time to the actual concentration over time. The results illustrate close agreement between the expected and observed data. 
         [0045]    The spatial resolution of the system was characterized by analyzing the modulation transfer function (MTF). Sample data of the MTF at different distances between the reconstruction plane and an object of interest are shown in the graph  1100  depicted at  FIG. 11 . 
         [0046]    At  FIG. 12 , a graph  1200  depicting MTF versus frequency is shown.  FIG. 12  shows the distribution of MTFs across a 64° range of reconstruction slice angles (−32° to 32° with respect to the plane parallel with the detector, in 2° increments). There exists a distinct trend in the data showing that the MTF degrades 360 as the obliquity of the reconstruction plane increases, shown in  FIG. 12  by the transition from superior to inferior curves. The variation in the MTF with angular range is very small (compare with  FIG. 11 ). This supports the assertion that the time point and window size can be varied in the reconstruction. 
         [0047]    At  FIG. 13 , a chart  1300  depicting central angle versus spatial frequency is shown. The frequency at which the MTF drops below 50% of its maximum value is plotted for each of the 33 different reconstruction slice angles in  FIG. 13 . The trend is clearly visible in this figure. A second order polynomial fit was plotted through this data as shown in  FIG. 13 . In an ideal system, the peak of the hyperbola would be located at 0°, but some outliers and noise exist in the data.  FIG. 12  shows that the MTF drops by 4.1% when comparing the peak MTF to the MTF at a 32° obliquity from the peak, based on the trendline data. 
         [0048]      FIG. 14  is a graph  1400  showing the variation of spatial frequency versus MTF.  FIG. 14  shows the MTF in the plane of the test tool for a varying number of projections used to create the reconstruction. There is little change in the MTF as the number of projections changes, and there is no distinguishable trend so long as the angular range remains the same. This appears to be true regardless of which n projections are used to reconstruct a subset of size n; that is, given a set of 17 projections, a reconstruction using any 5 of them will produce the same MTF as if all 17 projections were used in the reconstruction, provided the outer-most projection angles are included in the five. The variation in the MTF with number of projections is very small (compare with  FIG. 11 ). This supports the assertion that the time point and window size can be varied in the reconstruction. 
         [0049]    The experiments and analysis demonstrate that it is feasible to perform reconstructions from oblique projection sets without significant degradation of the in-plane resolution. Further, these data explicitly demonstrate that the reconstructed images support super-resolution in combination with functional (flow) information. 
         [0050]    In addition, the imaging systems, methods, and devices disclosed above may be utilized in additional applications. For example, a surgical instrument (e.g., a biopsy needle) may be inserted into a patient, a series of images of the tissue of the patient are acquired at various angles, and 4D images are constructed for the surgical instrument relative to the tissue of the patient. When such images are combined with real-time reconstruction, the image data may provide a physician (or a robotic positioning system) with real-time guidance information to direct the surgical procedure (e.g., a tissue biopsy), and to monitor the removal of the tissue of interest. 
         [0051]    Another example includes injecting, optionally, a contrast agent into an object of interest, acquiring a series of images of the object over various angles, with one image acquired for each angle. 4D image data for the object may be constructed from the acquired series of images. Such applications may be utilized in monitoring of flow in pipes, tubes, pipelines, etc., where the material under examination may have sufficient intrinsic radiographic contrast so that an exogenous contrast material is not required. Additionally, the object may include material that, under examination, lacks sufficient intrinsic radiographic contrast, so that an exogenous contrast material is required. 
         [0052]    Another example includes bolus tracking. Bolus tracking is a method in which low dose images are made with reduced frequency. These images are analyzed for the presence of the contrast agent. Once contrast agent is sensed in the image, images with higher dose and higher frequency are acquired for clinical use. In this way, the patient radiation dose is minimized and the timing of the image relative to the appearance of the contrast agent is improved. 
         [0053]    Furthermore, the systems, methods, and devices may also be utilized to observer objects under motion in 3D as a function of time. As an example, the motion of airplane control surfaces may be observed to search for faults in the structure of the wing as a function of the control surface position. 
         [0054]    Although the invention has been described above with reference to imaging tissue, especially breast tissue, in a human body, the invention has broader applicability. For example, the inventive concepts described herein may be used for nondestructive testing, e.g., it could be used for pipelines, etc. to assess flow in real time—oil pipelines, etc. may use technology to check for inclusions in the pipes that could cause problems. Suitable modification will be understood by one of skill in the art from the description herein. 
         [0055]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.