Abstract:
One embodiment of the present disclosure sets forth a method for determining a movement of a target region using tomosynthesis. The method includes the steps of accessing a first set of projection radiographs of the target region over a first processing window defined by a first range of projection angles, accessing a second set of projection radiographs of the target region over a second processing window defined by a second range of projection angles, wherein the first processing window moves to the second processing window, and comparing a first positional information derived from the first set of the projection radiographs and a second positional information derived from the second set of the projection radiographs with the first positional information to determine the movement of the target region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/354,800, filed Jan. 16, 2009, having Attorney Docket No. VAR-0001-US-REG. 
     
    
     BACKGROUND 
       [0002]    Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
         [0003]    Various systems and methods exist to provide radiation therapy treatment of tumorous tissue with high-energy radiation. Many forms of radiation treatment benefit from the ability to accurately control the amount, location, and distribution of radiation within a patient&#39;s body. Such control often includes using a multi-leaf collimator to shape a radiation beam to approximate that of the tumorous region. 
         [0004]    Many existing radiation treatment procedures require a location of a target region to be determined in order to accurately register the target region relative to a radiation source before radiation is applied to the target region. Computed tomography (“CT”) is an imaging technique that has been widely used in the medical field. In a procedure for CT, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination. The x-ray source generates and directs an x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process. The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer. By taking a plurality of readings from multiple angles around the patient, relatively massive amounts of data are thus accumulated. The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of a volume of the bodily region being examined. Cone-bream computed tomography imaging (CBCT) which uses a flat panel detector is typically used in radiation therapy systems. 
         [0005]    CT has found its principal application in examination of bodily structures or the like which are in a relatively stationary condition. In some cases, it may be desirable to continuously monitor a position of a target region while a treatment procedure is being performed. However, currently available apparatus that supports CT may not be able to generate tomographic images with sufficient quality or accuracy in part due to intra-fraction motion caused by inadvertent patient shifts or natural physiological processes. For example, breathing or expelling gas through the rectum has each been shown to cause degradation of quality in CT images. In such cases, it would be desirable to track a movement of the target region to ensure that a treatment radiation beam is accurately aimed towards the target region. In existing radiation treatment systems, tracking of the target region does not use a CT imaging technique. This is because collecting a sufficient quantity of CT image data for image reconstruction may take a long time, and therefore may not be performed at a fast enough rate to provide sufficiently current information to adjust the treatment radiation beam. 
         [0006]    Another approach to 3D localization is “3D point tracking” which relies on taking individual projection radiographs and localizing high density implanted fiducial markers in each projection, for example by using the pixel coordinates of the markers&#39; centroids. Then triangulation is performed to find the 3D position of a marker by using different radiographs taken at different projection angles. However, finding the pixel coordinates of a high density marker in a single X-ray projection can be difficult. Overlaying anatomy and external structures are an important source of failure of these techniques. Very often, the X-ray quantum noise and scattered radiation result in the failure to detect or localize a marker using automatic image analysis algorithms. 
         [0007]    Conventional portal imaging techniques use treatment “beam&#39;s-eye view” (“BEV”) imaging to track both inter- and intra-fraction motion. One drawback is that most BEV imaging occurs at MV energies, which is less dose-efficient than imaging at kilo-volt (kV) energies. Another drawback is that, if high density fiducial markers are used, the markers may not be exposed to BEV at all times, thus causing treatment to be interrupted for purposes of repositioning the multi-leaf collimator blades. Interruption of treatment is particularly undesirable for arc-therapies. 
         [0008]    Some radiation therapy treatment systems are equipped with kV imaging systems mounted to the gantry whose projection angle is orthogonal to the treatment beam. The imaging techniques used with such an orthogonal system also can include CT imaging and 3D point tracking. An advantage of the kV system is its higher dose efficiency. Moreover, the imaging target can be exposed at all times during treatment since the kV source is only used for imaging. Nevertheless, the motion-related problems with full CT acquisitions still exist as can SNR and other limitations associated with acquiring a single projection radiograph for 3D point tracking. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the above recited features of the disclosure can be understood in detail, a more particular description of the disclosure may be had by reference to embodiments, some of which are illustrated in the drawings. It is to be noted, however, that the drawings illustrate only typical embodiments and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
           [0010]      FIG. 1  is a schematic diagram illustrating a treatment radiation system, according to one embodiment of the disclosure; 
           [0011]      FIG. 2  is a flow chart illustrating a method of performing real-time tracking using tomosynthesis, according to one embodiment of the disclosure; 
           [0012]      FIG. 3A  is a schematic diagram illustrating a limited angle acquisition approach, according to one embodiment of the disclosure; 
           [0013]      FIG. 3B  is a schematic diagram illustrating another limited angle acquisition approach, according to one embodiment of the disclosure; 
           [0014]      FIG. 3C  is a side view schematic diagram illustrating yet another limited angle acquisition approach, according to one embodiment of the disclosure; 
           [0015]      FIG. 3D  is a top view schematic diagram illustrating the limited angle acquisition approach of  FIG. 3C , according to one embodiment of the disclosure; 
           [0016]      FIG. 3E  is a side view schematic diagram illustrating another limited angle acquisition approach, according to one embodiment of the disclosure; 
           [0017]      FIG. 4  is a flow chart illustrating a method of acquiring projection radiographs, according to one embodiment of the disclosure; 
           [0018]      FIG. 5A  is a flow chart illustrating a method of processing projection radiographs using sliding arc tomosynthesis, according to one embodiment of the disclosure; 
           [0019]      FIG. 5B  is a flow chart illustrating a method of processing projection radiographs using short arc tomosynthesis, according to one embodiment of the disclosure; 
           [0020]      FIG. 6  is a schematic diagram illustrating an axial view of the voxel row, according to one embodiment of the disclosure; and 
           [0021]      FIG. 7  is a flow chart illustrating a method of adjusting treatment real-time, according to one embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Various embodiments are described hereinafter with reference to the figures. It should be noted that the figures are not drawn to scale. It should also be noted that the figures are only intended to facilitate the description of embodiments. They are not intended as an exhaustive description of the disclosure or as a limitation on the scope of the disclosure. In addition, an aspect described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments. 
         [0023]      FIG. 1  is a schematic diagram illustrating a treatment radiation system  100 , according to one embodiment of the disclosure. The treatment radiation system  100  includes a first radiation source  102 , an electronic portal imaging device (EPID)  106 , a second radiation source  108  mounted on a gantry  110 , a flat panel detector  114 , and a control system  116 . The first radiation source  102  is aimed towards a patient  104  and to the EPID  106 . The patient  104  has markers, which may be high density objects that can be localized using x-ray projection radiographs. Some examples of a marker include, without limitation, a bone, a surgical clip, or other high contrast object. In one scenario, the patient  104  may have a prostate gland implanted with gold marker beads. 
         [0024]    In one implementation, the second radiation source  108  is situated at a right angle to the first radiation source  102 . A radial direction (r)  112  here is defined as the direction from the second radiation source  108  through the isocenter to the flat panel detector  114 . The information acquired by the treatment radiation system  100  is analyzed by the control system  116 , which adjusts the first radiation source  102  and the rotation of the gantry  110  accordingly. 
         [0025]    In the illustrated embodiments, the first radiation source  102  is a treatment radiation source for providing treatment energy with a collimator system for controlling a delivery of the treatment beam, and the second radiation source  108  is an imaging radiation source. In other embodiments, in addition to being a treatment radiation source, the first radiation source  102  can also provide imaging data. In other embodiments, the first radiation source  102  can provide imaging data without providing treatment energy. The treatment energy generally refers to those energies of 160 kilo-electron-volts (keV) or greater, and more typically 1 mega-electron-volts (MeV) or greater. The imaging energy can include treatment energies and also energies below the high energy range, more typically below 160 keV. 
         [0026]    In the illustrated embodiments, the control system  116  includes a processor for executing instructions, a monitor for displaying data, and an input device, such as a keyboard or a mouse, for inputting data. In the illustrated embodiments, the gantry  110  is rotatable, and during a treatment session, the gantry  110  rotates about the patient  104 , as in an arc-therapy. Here, “treatment session” generally refers to the session in which the patient  104  is imaged and/or treated. The operations of the first radiation source  102 , the collimator system, and the gantry  110  are controlled by the control system  116 , which provides power, timing, rotation, and position control based on received signals. Although the control system  116  is shown as a separate component from the gantry  110 , in alternative embodiments, the control system  116  can be a part of the gantry  110 . 
         [0027]    It should be noted that the treatment radiation system  100  should not be limited to the configuration described above, and that the system can also have other configurations. For example, instead of the shown ring-configuration, the system can include a C-arm or other types of an arm to which the first radiation source  102  or the second radiation source  108  is secured. It should also be noted that the treatment radiation system  100  can have one or more radiation sources. Other configurations may include a single radiation source with multiple detectors, or multiple sources with a single detector. 
         [0028]      FIG. 2  is a flow chart illustrating a method  200  of performing real-time motion tracking using tomosynthesis, according to one embodiment of the disclosure. In conjunction with  FIG. 1 , in step  202 , the control system  116  accesses imaging data during a treatment session. Here, “imaging data” generally refers to projection radiographs, which as discussed above, can come from the first radiation source  102 , the second radiation source  108 , or a combination of the two sources. In step  204 , the control system  116  processes the imaging data to determine 3D information from the markers during the treatment session. In step  206 , the control system  116  adjusts the radiation source(s) based on information associated with 3D positions. Here, “real-time motion tracking” broadly refers to the motion tracking that occurs while the treatment session is ongoing. Similarly, “real-time adjustment” of the first radiation source  102  also broadly refers to the adjustment that occurs while the treatment session is ongoing. Steps  202 ,  204 , and  206  are performed concurrently with treatment and repeated until the session ends. Each of the above steps is performed independently from the other steps. They may also be performed simultaneously. 
         [0029]    Before the treatment session begins in step  202 , the patient  104  of  FIG. 1  is set-up and positioned on the treatment radiation system  100 . Set-up may involve acquiring projection radiographs to localize the markers for comparison with digitally reconstructed radiographs from a reference scan. Alternatively, Cone Beam CT (CBCT) may be used to localize both soft tissue and markers. The position of the patient  104  is adjusted according to the localization information. 
         [0030]    Independent from step  202 , in one implementation, the gantry rotates continuously, and the second radiation source  108  and the flat panel detector  114  are used to acquire projection radiographs at regularly spaced intervals. In another implementation, the second radiation source  108  and the flat panel detector  114  are used to acquire imaging data with gaps in a certain angular range. In yet another implementation, the second radiation source  108  and the flat panel detector  114  are used to acquire imaging data at predetermined gantry angles. In still another implementation, the control system  116  determines when the second radiation source  108  and the flat panel detector pair  114  are used to acquire imaging data based on optimization considerations. In one implementation, the second radiation source  108  and the flat panel detector  114  refer to the On-Board Imaging (OBI) system from Varian Medical Systems, Inc. 
         [0031]    Alternatively, the first radiation source  102  and the EPID  106  can together also generate imaging data. For example, projection radiographs, acquired by the EPID  106  while using the first radiation source  102  at high energies (e.g., for treatment), may be used. In another example, projection radiographs, acquired by the EPID  106  while using the first radiation source  102  at low energies (e.g., for imaging), may be used. 
         [0032]    In one implementation, during a treatment session, the first radiation source  102  is configured to alternate between delivering beams for treating a target region and delivering beams for generating imaging data for tomosynthesis. In another implementation, a combination of imaging data from utilizing the first radiation source  102  and the second radiation source  108  can be used for tomosynthesis. In implementations with multiple radiation sources, a combination of imaging data from the radiation sources can be used for tomosynthesis. 
         [0033]    The radiation source adjustment in step  206  can be done in a number of ways. For example, the collimator blades of the first radiation source  102 , the second radiation source  108 , or the combination of the two radiation sources of  FIG. 1  can be modulated so that the markers are irradiated to minimize the extra dose delivered to the patient  104 . In addition, the position of the collimator blades may be adjusted during treatment whenever the markers are determined not to be in the field-of-view to maintain illumination of the markers. In another example, the dose may vary according to the projection angle. To illustrate, the dose for lateral views of the pelvis may be higher than the does for anterior-posterior views. This can be achieved either by adjusting the mAs per projection or by adjusting the projection density or sampling rate as a function of the projection angle. 
         [0034]      FIG. 3A  is a schematic diagram  300  illustrating a limited angle acquisition approach, according to one embodiment of the disclosure. In conjunction with  FIG. 1 , here, the second radiation source  108  is aimed towards the patient  104  through to the flat panel detector  114 . There are N projection radiographs in the sequence spanning an angular range Δθ  302  (also referred to as a “tomographic angle Δθ  302 ”.) A first projection angle  304  is denoted as θ i−N+1 . A last angle  306  is denoted as θ i , corresponding to current time point t i , which is the most current time that imaging data is being acquired. In other implementations, the limited angle acquisition approach can be used by the first radiation source  102  or other radiation sources. 
         [0035]    As mentioned above, in one implementation, the second radiation source  108  and the flat panel detector  114  pair can rotate 360° around the gantry  110  and about the patient  104  in an arcuate manner (e.g., in a counter clockwise or clockwise direction) and can generate an image every 1°. The second radiation source  108  and the flat panel detector  114  pair may also move in tandem. In other implementations, the second radiation source  108  and the flat panel detector  114  pair can be configured to rotate through a set of different rotational angles, generate a different number of images, or have gaps in the angular range over which the images are acquired. 
         [0036]      FIG. 3B  is a schematic diagram  320  illustrating another limited angle acquisition approach, according to one embodiment of the disclosure. In conjunction with  FIG. 1 , the second radiation source  108  is aimed towards the patient  104  through to the flat panel detector  114 . There are N projection radiographs in the sequence spanning an angular range Δθ  322  (also referred to as a “tomographic angle Δθ  322 ”.) A first projection angle  324  is denoted as θ i−N+1. A last angle 326 is denoted as θ   i , corresponding to current time point t i , which is the most current time that imaging data is being acquired. 
         [0037]    In one implementation, unlike the configuration shown in  FIG. 3A , the second radiation source  108  and the flat panel detector  114  may move in different directions. For example, suppose the patient  104  lays on a patient patent in an X-Y plane. The second radiation source  108  may move in a positive Y direction, while the flat panel detector  114  may move in a negative Y direction. In addition, rather than rotating 360° around the gantry  110  and the patient  104 , the second radiation source  108  and the flat panel detector  108  pair may generate an image every 1° or another rotational angle from a side of the patient  104 . 
         [0038]      FIG. 3C  is a side view of a schematic diagram  340  illustrating yet another limited angle acquisition approach, according to one embodiment of the disclosure. In conjunction with  FIG. 1 , the second radiation source  108  and optionally a third radiation source  350  are aimed towards the patient  104  through to the flat panel detector  114 . 
         [0039]      FIG. 3D  is a top view of the same limited angle acquisition approach shown in  FIG. 3C , according to one embodiment of the disclosure. The second radiation source  108  and the optional third radiation source  350  may rotate in a circle and in a first plane above the patient  104  and the flat panel detector  114 . The flat panel detector  114  is in a second plane, which is in parallel to the first plane. There are M projection radiographs in the sequence spanning an angular range Δθ  342  (also referred to as a “tomographic angle Δθ  342 ”) as the second radiation source  108  rotates clockwise. Similarly, there are also N projection radiographs in the sequence spanning an angular range Δθ  344  (also referred to as a “tomographic angle Δθ  344 ”) as the third radiation source  350  rotates clockwise. In one implementation, M may not equal to N. Also, the rotation of the one or more radiation sources may occur independently from the rotation of a gantry, such as the gantry  110 . 
         [0040]      FIG. 3E  is a side view of a schematic diagram  360  illustrating another limited angle acquisition approach, according to one embodiment of the disclosure. In conjunction with  FIG. 1 , the second radiation source  108  and optionally a third radiation source  360  are aimed towards the patient  104  through to the flat panel detector  114  and optionally a second flat panel detector  362 . Unlike the approach shown in  FIG. 3C  (i.e., the flat panel detector  114  does not rotate), the second radiation source  108  and the third radiation source  360  may rotate in a circle and in a first plane, and the flat panel detector  114  and the second flat panel detector  362  may also rotate in a circle and in a second plane. The first plane is in parallel to the second plane. As a result, additional projection radiographs from different angular ranges may be acquired. 
         [0041]      FIG. 4  is a flow chart illustrating a method  400  of acquiring projection radiographs, according to one embodiment of the disclosure. Using the approach shown in  FIG. 3A  as an example, in step  402 , a processing window is defined by the tomographic angle Δθ  302 . In one implementation, the processing window can range from 3° to 40°. The method  400  can be carried out by the first radiation source  102 , the second radiation source  108 , or other radiation sources in other implementations. 
         [0042]    In conjunction with  FIG. 1 , in step  404 , the control system  116  accesses the imaging data within the processing window taken from time t i−N+1  to t i , corresponding to the N projection radiographs taken between projection angles θ i−N+1    304  to θ i    306  in  FIG. 3A . As an example, for a sliding arc window, the control system  116  accesses  21  projection radiographs taken between projection angles 20° and 40° (1 projection radiograph taken at each degree interval). In step  406 , at time t i , a window of N projection radiographs starting at time t i−N+1 , is processed. Two methods of processing the windows of projection radiographs, sliding arc tomosynthesis and short arc tomosynthesis, are further described in conjunction with  FIGS. 5A and 5B  below. 
         [0043]    Continuing with  FIG. 4 , in step  408 , the control system  116  moves on to the next processing window (i=i+1). The moving of the processing window  302  may occur while treatment is still ongoing so that at the next time increment t i+1  the projection radiograph at time t i−N+1  may be dropped. If the coordinates of the backprojection matrix do not rotate in place with the acquisition, then processing may continue by subtracting the projection radiograph at t i−N+1  and adding the projection radiograph at time t i+1 . In the above sliding arc example, at the next time increment, the projection radiograph at the projection angle 20° is dropped, and the new projection radiograph taken at this time increment, corresponding to projection angle 41°, is added. In other words, the processing window  302  slides to encompass projection angles 21° through 41°. Steps  404 ,  406 , and  408  are repeated throughout the treatment period. In the example above, 21 projection radiographs from 21° to 41° are first processed. Then 21 projection radiographs from 22° to 42° are subsequently processed, and so on, with the 20° processing window sliding over 1° at each time increment. In an optional step  410 , the movement of the processing windows ends when treatment ends. 
         [0044]      FIG. 5A  is a flow chart illustrating a method  500  of processing projection radiographs using sliding arc tomosynthesis, according to one embodiment of the disclosure. The method  500  can be carried out by the first radiation source  102 , the second radiation source  108 , or other radiation sources in other implementations. Step  502  corresponds to step  202  of  FIG. 2  and step  404  of  FIG. 4 . In conjunction with  FIG. 1  and the example above for sliding arc tomosynthesis, the control system  116  accesses  21  projection radiographs for each 20° processing window. Steps  504 ,  506 , and  508  further illustrate steps  204  and  406 . At time t i , a window of N projection radiographs starting at time t i−N+1 , is processed. In step  504 , reconstruction using techniques such as, without limitation, backprojection or filtering. Step  504  can also include preprocessing techniques, such as, without limitation, performing logarithmic transforms. When more than one radiation source and/or more than one flat panel detector are utilized to acquire the radiograph projections (e.g., the approaches illustrated in  FIGS. 3C ,  3 D, and  3 E and discussed above), in one implementation, the different radiograph projections generated from distinct radiation source and flat panel detector pairs are processed using, for example, filtering and backprojection operations, to reconstruct an image. Other reconstruction methods may involve iterative techniques. 
         [0045]    Backprojection is a common algorithm used in the tomographic reconstruction of clinical data. When an n-dimensional object is projected, each projection radiograph is an n−1 dimensional sum of its density along the projection axis. The reverse function is called back projection and regenerates the original object. In some implementations, the orientation of the backprojection matrix may rotate with the image acquisition system, which is a more “natural” coordinate system for tomosynthesis reconstruction. Here, the radial direction is defined as being parallel to the central projection angle θ p =(θ i−N+1 +θ i )/2 of the imaging system and a lateral direction as being orthogonal to the radial direction. For tomosynthesis reconstruction, two axes of the backprojection (or reconstruction) matrix are in lateral directions while the third axis is directed in the radial direction which in general, has lower spatial resolution than the lateral axes. In alternative implementations, the backprojection matrix can be fixed in the normal Cartesian coordinate system (e.g., left-right, anterior-posterior, superior-inferior) used for imaging and radiotherapy. One common method of backprojection is known and “shift and add tomosynthesis.” In particular, the projection radiographs acquired using the approach described in conjunction with  FIG. 4  above are shifted and added in the plane of interest to bring the markers in focus, while structures in other planes are distributed and thus appear blurred. 
         [0046]    Before backprojection is performed, the data may be processed to enhance certain spatial frequencies and depress others. An example is the frequency-domain ramp filter that is used for the Feldkamp, Davis, and Kress (FDK) reconstruction algorithm. Compensation may be made for 1/r 2  effects as prescribed by the FDK algorithm. After backprojection, the data may be filtered to deblur the image. Deblurring techniques can include spatial frequency filtering selective plan removal, iterative restoration, matrix inversion, or other techniques known in the art. 
         [0047]    In some implementations, fully iterative reconstruction methods are used where the data are first backprojected and then forward projected for comparison with the original projection radiographs. Examples of iterative techniques include, but are not limited to, ART, EM MLEM, and OSEM. 
         [0048]    In step  506 , the positions of the markers are determined. In one implementation, a different method is employed to identify the axis along the radial direction for the markers than the axes along the lateral directions. Some techniques for detecting the markers include, without limitation, calculating the 3D center-of-mass of each marker, curve fitting, or peak finding. In step  508 , an average shift is determined based on how the markers have moved relative to their position just before treatment starts. This original or starting position may be determined using multiple techniques including but not limited to CBCT or tomosynthesis involving the first and/or second source-detector pair. 
         [0049]      FIG. 5B  is a flow chart illustrating a method  550  processing projection radiographs using short arc tomosynthesis, according to one embodiment of the disclosure. The method  550  can be carried out by the first radiation source  102 , the second radiation source  108 , or other radiation sources in other implementations. Like  FIG. 5A , Step  552  also corresponds to step  202  of  FIG. 2  and step  404  of  FIG. 4 . However, as a variant of the sliding arc tomosynthesis approach described above, the arc length can be smaller for marker tracking using short arc tomosynthesis. As an example, the control system  116  of  FIG. 1  here generally accesses  6  projection radiographs for each 3° processing window. The angular range of a processing window for short arc tomosynthesis can be approximately 3 degrees or may correspond to a compromised larger arc length to include approximately minimum  6  projection radiographs. Steps  554 ,  556 ,  558 ,  560  and  562  further illustrate step  204  of  FIG. 2  and step  406  of  FIG. 4 . Similar to  FIG. 5A , the reconstruction performed in step  554  also may include techniques such as, without limitation, backprojection or filtering, and the preprocessing techniques possibly utilized in step  554  include techniques such as, without limitation, performing logarithmic transforms. 
         [0050]    In step  554 , in one implementation, when short arc tomosynthesis reconstructs small volumes, a number of slices are generated by back projecting the input projection radiographs without putting them through 2D spatial filtering. The depth covered by these slices cover several times the depth of the volume of interest. In such an implementation, a 3D filter can be applied to the generated slices to remove the blurred out-of-slice structures for the slices of interest. This method is described in the United States Patent Application No. US 2007/0237290 A1 of Varian Medical Systems, Inc. 
         [0051]    In step  556 , the control system  116  creates an enhanced 2D image. Unlike the sliding arc tomosynthesis approach of  FIG. 5A , which generates 3D information, this short arc tomosynthesis approach combines projection radiographs acquired over a small arc of gantry rotation to generate a 2D image with significant enhancement of the target region. This 2D image is used instead of the original projection radiographs for marker tracking. Steps  552 ,  554 , and  556  are repeated at least once, using a different short arc window, in order to generate multiple 2D images. In one implementation, this short arc can “slide” as in sliding arc tomosynthesis, and the imaging data corresponding to the short arc may be acquired continuously. In another implementation, the imaging data corresponding to the short arc may be acquired with gaps in a certain angular range. In yet another implementation, the imaging data corresponding to the short arc may be acquired at predetermined gantry angles. In still another implementation, the control system  116  of  FIG. 1  may determine when to access the imaging data corresponding to the short arc based on optimization considerations. 
         [0052]    Before describing step  558  of  FIG. 5 ,  FIG. 6  is a schematic diagram  600  illustrating an axial view of a voxel row, according to one embodiment of the disclosure. For the purpose of 3D tracking, short arc DTS voxels  602  can be visualized in the planes that are parallel to the imager rotation axis (also referred to as “slice planes” or “lateral planes”) and normal to the imaging axis at a center projection  604  of an acquisition arc  606 . The voxel dimensions are small in the slice plane and long in the direction parallel to the imaging axis at the arc center (i.e. the radial direction). The long voxel dimension, or large “voxel depth,” corresponds to low depth resolution, and is due to the short arc used for tomosynthesis. Depth resolution is determined by triangulation, and is not an objective of individual images. Short arc tomosynthesis enhances the image of markers in the presence of noise and reduces the effect of overlaying objects outside the volume occupied by the markers. For example, in the case of multiple markers in prostate, the voxel depth can be about 2 to 3 cm, thus minimizing the effect of any overlaying bony structures or external objects. This voxel depth is achievable with short arcs of 2 to 3 degrees. 
         [0053]    Referring back to  FIG. 5 , in step  558 , the control system  116  performs triangulation using multiple 2D images generated from step  556 . Three dimensional tracking by triangulation uses two or more images that are reconstructed from short arcs centered at different gantry angles. Geometrically, the 2D images of the markers can be viewed as projection radiographs of the markers onto a new image plane that is defined by the slice plane  602 . In one implementation, triangulation using two of these images can be achieved by intersecting the two lines that go through the 2D position of the target, found in each image, and where each line also connects to the radiation source position for the corresponding arc center. This intersection of the two lines is also referred to as a triangulated position. In such an implementation, the geometric calibration parameters of each 2D image can vary with the gantry angle corresponding to the short arc center; the source-to-image plane distance is the normal distance of the radiation source position corresponding to arc center, to the slice plane; this is different from source-to-flat panel (physical image sensor) distance and can vary with the gantry angle corresponding to short arc center. Similar to  FIG. 5A , 3D information is determined in step  560 , and the average shift is calculated in step  562 . 
         [0054]      FIG. 7  is a flow chart illustrating a method  700  of performing real-time treatment adjustment, according to one embodiment of the disclosure. In one implementation, after the control system  116  of  FIG. 1  calculates an average shift as shown in  FIG. 5A  and  FIG. 5B  and discussed above, drift data for tracking or repositioning based on the average shifts from two processing windows are also calculated. Specifically, in one implementation, the control system  116  determines whether low frequency motion has occurred in step  702 . If low frequency motion is indeed detected, the control system  116  is configured to use linear prediction methods to compensate for the temporal delay in step  704 . The average temporal delay in seconds (T r ) in the radial direction is given by the equation T r =NΔt/2, where N is the number of projection radiographs processed, and Δt is the sampling period of the projection radiographs. In step  706 , the drift data is fed back to the treatment system. In step  708 , the multi-leaf collimator will be adjusted so that the first radiation source  102  is directed to compensate for the drift. 
         [0055]    If in step  702  the control system  116  instead determines that a transient motion has occurred, then in step  710  treatment is stopped until the transient motion dissipates. In step  712 , the control system  116  determines whether the transient motion has returned to the initial position. If so, the drift data is fed back to the treatment system and the multi-leaf collimator will be adjusted, in steps  706  and  708 , respectively. If not, in step  714 , the control system  116  can acquire a beam&#39;s eye projection from the first radiation source  102  to reset marker positions in the radial direction. After the marker positions are reset, the drift data is again fed back to the treatment system in step  706  and the multi-leaf collimator is adjusted to compensate the drift data in step  708 . 
         [0056]    In other embodiments, in conjunction with  FIG. 1 , the control system  116  is configured to not only adjust treatment beams from the first radiation source  102 , but also interleave imaging beams from the second radiation source  108 , imaging beams from the first radiation source  102 , treatment beams from the first radiation source  102 , and other data signals. 
         [0057]    In the illustrated embodiment, the methods  400 ,  500 , and  550  can be performed while treatment occurs. In an alternative implementation, the methods  400 ,  500 , and  550  can be performed using the projection radiographs acquired prior to a current treatment session. In some implementations, in conjunction with  FIG. 1 , the 3D information of the markers determined using the methods  500  and  550  can also be used to verify a location of a target region, to track a movement of the target region, and/or to control an operation of the first radiation source  102 , and/or the collimator. In the illustrated embodiment, the method  700  adjusts the multi-leaf collimator in real-time during treatment. In other implementations, the method  700  can adjust gantry speed, delivery dose, or other treatment parameters. 
         [0058]    One embodiment of the disclosure may be implemented as a program product for use with a computing device. The programming instructions of the program product define functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, DVD disks readable by a DVD driver, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive, hard-disk drive, CD-RW, DVD-RW, solid-state drive, flash memory, or any type of random-access memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the present disclosure, are embodiments of the present disclosure. 
         [0059]    While the foregoing is directed to embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. Therefore, the above examples, embodiments, and drawings should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the disclosure as defined by the following claims.