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 . fig1 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 . 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 . 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 . 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 . 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 . fig2 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 fig1 , 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 . before the treatment session begins in step 202 , the patient 104 of fig1 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 . 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 . 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 . 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 . 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 fig1 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 . fig3 a is a schematic diagram 300 illustrating a limited angle acquisition approach , according to one embodiment of the disclosure . in conjunction with fig1 , 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 . 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 . fig3 b is a schematic diagram 320 illustrating another limited angle acquisition approach , according to one embodiment of the disclosure . in conjunction with fig1 , 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 . in one implementation , unlike the configuration shown in fig3 a , 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 . fig3 c 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 fig1 , 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 . fig3 d is a top view of the same limited angle acquisition approach shown in fig3 c , 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 . fig3 e 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 fig1 , 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 fig3 c ( 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 . fig4 is a flow chart illustrating a method 400 of acquiring projection radiographs , according to one embodiment of the disclosure . using the approach shown in fig3 a 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 . in conjunction with fig1 , 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 fig3 a . 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 fig5 a and 5b below . continuing with fig4 , 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 . fig5 a 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 fig2 and step 404 of fig4 . in conjunction with fig1 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 fig3 c , 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 . 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 fig4 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 . 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 . 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 . 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 . fig5 b 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 fig5 a , step 552 also corresponds to step 202 of fig2 and step 404 of fig4 . 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 fig1 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 fig2 and step 406 of fig4 . similar to fig5 a , 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 . 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 . in step 556 , the control system 116 creates an enhanced 2d image . unlike the sliding arc tomosynthesis approach of fig5 a , 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 fig1 may determine when to access the imaging data corresponding to the short arc based on optimization considerations . before describing step 558 of fig5 , fig6 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 . referring back to fig5 , 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 fig5 a , 3d information is determined in step 560 , and the average shift is calculated in step 562 . fig7 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 fig1 calculates an average shift as shown in fig5 a and fig5 b 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 . 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 . in other embodiments , in conjunction with fig1 , 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 . 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 fig1 , 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 . 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 . 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 .
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 .