typical embodiments of the invention include two or more x - ray source / detector combinations . each of these source / detector combinations includes an x - ray source and an x - ray detector . the x - ray source is configured to generate x - rays and direct them toward the associated x - ray detector . the x - ray detector is configured to detect received x - rays in a spatially resolved manner and to generate resulting projection image data . typically , a target is placed between the x - ray source and x - ray detector for analysis . the detected x - rays are , therefore , representative of a projection of the target onto the x - ray detector . in medical applications , the target is often part of a patient . each of the source / detector combinations is configured to image an overlapping volume within the target . for example , in some embodiments , the path of x - rays from a center of an x - ray source to a center of an x - ray detector can be represented by a beam axis , and each source / detector combination is configured such that their respective beam axes intersect in a target region . each source / detector combination is optionally operated in parallel to generate projection image data . this parallel operation can be used to reduce the time required to generate a three - dimensional representation of a target and / or increase the resolution of the resulting three - dimensional representation . in some embodiments , parallel operation includes simultaneous generation of x - rays . in some embodiments , parallel operation includes generation of x - rays by one x - ray source while the detector of another source / detector combination is involved in a data transfer process . in these embodiments , the generation of x - rays and data transfer processes may alternate between source / detector combinations . in alternative embodiments , each source / detector combination is operated in series . in some embodiments , a first source / detector combination is configured to generate x - rays in one energy range while a second source / detector combination is configured to generate x - rays in another energy range . for example , the first source / detector combination can be configured to generate projection image data using kv x - rays while the second source / detector combination can be configured to generate projection image data using mv x - rays . as described further herein , projection image data generated using one energy range is optionally scaled such that it can be combined with projection image data generated using another energy range in order to generate a three - dimensional representation . in some embodiments , the use of more than one energy range to generate a three - dimensional representation allows for reduction of disadvantages associated with a particular energy range . for example , the use of mv x - rays may reduce the generation of artifacts seen in kv only data , and the use of kv x - rays may reduce injury to tissue surrounding the target volume that would be caused by mv x - rays . fig1 is a block diagram of an imaging system 100 configured to generate a three - dimensional representation of a target positioned in a target region 105 , and optionally configured to provide a therapeutic dose of x - rays to a treatment region within target region 105 . the instance of imaging system 100 illustrated in fig1 includes two source / detector combinations . a first combination , referred to as the mv detector / source combination , is configured to generate projection image data using mv x - rays . the mv detector / source combination includes a mv x - ray source 110 and a mv x - ray detector 115 disposed such that their associated beam axis passes through target region 105 . a second combination , referred to as the kv detector / source combination , is configured to generate projection image data using kv x - rays . the kv detector / source combination includes a kv x - ray source 120 and a kv x - ray detector 125 disposed such that their associated beam axis intersects the beam axis of the mv detector / source combination within target region 105 . in typical embodiments , mv x - ray source 110 , mv x - ray detector 115 , kv x - ray source 120 and kv x - ray detector 125 are coupled to a movable gantry 130 . gantry 130 is configured to rotate around target region 105 under the control of motor 135 . in alternative embodiments , motor 135 is configured to rotate a target within target region 105 . in these embodiments , gantry 130 is optionally stationary . the kv detector / source combination is optionally rotated around target region 105 independent of the mv detector / source combination . imaging system 100 further includes a control logic 140 configured for operating motor 135 , mv x - ray source 110 , mv x - ray detector 115 , kv x - ray source 120 , and kv x - ray detector 125 . control logic 140 typically includes a processor and memory configured for storing projection image data . control logic 140 is further configured to control an optional conversion logic 145 , a ct construction logic 150 and an optional display 155 . conversion logic 145 is configured to scale projection image data received from mv x - ray detector 115 such that the scaled data can be combined with projection image data received from kv x - ray detector 125 . in alternative embodiments , conversion logic 145 is configured to scale projection image data received from kv x - ray detector 125 such that the scaled projection image data can be combined with projection image data received from mv x - ray detector 115 . the scaling can include logarithmic functions known in the art . the operation of conversion logic 145 is described further elsewhere herein . ct construction logic 150 is configured to construct a three - dimensional representation of a target using two - dimensional projection images of the target generated using both mv x - ray detector 115 and kv x - ray detector 125 . the construction process can be performed using several alternative construction techniques known in the art . for example , in various embodiments , the filtered back - projection technique or multi - level scheme algebraic reconstruction technique ( mls - art ) are used for construction of a three - dimensional representation . optional display 155 is configured for viewing various data generated using mv x - ray detector 115 and kv x - ray detector 125 , and for viewing three - dimensional representations of a target constructed using ct construction logic 150 . typically , three - dimensional representations are viewed as cross - sections of the three - dimensional representation . these cross - sections are referred to as ct images . fig2 is a photograph showing of parts of imaging system 110 , according to one embodiment of the invention . the mv source / detector combination and kv source / detector combination are positioned such that their respective beam lines intersect at approximately a right angle in target region 105 . fig2 shows an experimental instance of a target 210 positioned within target region 105 . as is discussed further herein , this experimental instance of target 210 includes a simulation of part of a human head and a device used to study resolution in x - ray imaging . in practice , the instance of target 210 shown in fig2 is typically replaced by a patient . fig3 is a flowchart illustrating methods of generating a three - dimensional representation of target 210 using imaging system 100 , according to various embodiments of the invention . in this method the kv source / detector combination and the mv source / detector combination are used to generate separate projection image data . optionally , a first set of projection image data generated using one of these source / detector combinations is scaled such that it can be combined with a second set of projection image data generated using an other of these source / detector combinations . for example , the projection image data generated using the mv source / detector combination may be scaled such that it can be combined with projection image data generated using the kv source / detector combination . following the scaling process , the scaled first set of projection image data and the second set of projection image data are used to construct a three - dimensional representation of target 210 . in a generate kv x - ray step 310 , kv x - ray source 120 is used to generate x - rays in the kv energy range . in a direct kv x - ray step 320 , these kv x - rays are directed through target region 105 in order to image target 210 . target 210 blocks passage of these kv x - rays as a function of the adsorption cross - section of target 210 . in a detect kv x - ray step 330 , those kv x - rays that pass through target 210 are detected using kv x - ray detector 125 . the detection of these x - rays includes generation , and optionally storage , of projection image data representative of a projection of target 210 on kv x - ray detector 125 . in a generate mv x - ray step 340 , mv x - ray source 110 is used to generate x - rays in the mv energy range . in a direct mv x - ray step 350 , these mv x - rays are directed through target region 105 . the mv x - rays can be used or imaging and / or treatment . in some embodiments , an aperture is used to reduce the volume within target region 105 that is exposed to the mv x - rays . while the volume within target region 105 that is exposed to kv x - rays and the volume within target region 105 that is exposed to mv x - rays are not necessarily the same , these volumes will typically overlap . some of the mv x - rays are attenuated as they pass through an instance of target 210 within target region 105 . in a detect mv x - ray step 360 , those mv x - rays that pass through target 210 are detected using mv x - ray detector 115 . the detected x - rays are used to generate projection image data representative of a projection of target 210 on mv x - ray detector 115 . detect kv x - ray step 330 and detect mv x - ray step 360 optionally both include a phase in which projection image data is transferred from kv x - ray detector 125 and mv x - ray detector 115 , respectively , to memory associated with control logic 140 . in an optional move gantry step 370 , motor 135 is used to move gantry 130 . this movement rotates the kv source / detector combination and / or the mv source / detector combination relative to target region 105 . in alternative embodiments , motor 135 is used to move target 210 while gantry 130 remains stationary . following move gantry step 370 , if further projection image data is required for the construction of a desired three - dimensional representation of target 210 , then the method returns to generate kv x - ray step 310 . if sufficient data has been generated for the construction of a desired three - dimensional representation of target 210 then the method proceeds to a scale mv x - ray data step 380 . in some embodiments , steps 310 through 370 are repeated numerous times in order to generate projection image data at a sufficient number of different projection angles to generate a desired three - dimensional representation . a projection angle is the angular position of an x - ray source around target region 105 relative to a fixed reference angle . for example , an angular position directly above target region 105 may be assigned 0 degrees while an angular position directly below target region 105 is assigned 180 degrees . the larger the number of different projection angles the greater the resolution of the three - dimensional representation , and the longer the imaging process takes . in some embodiments , gantry 130 is rotated such that both mv and kv projection images are generated at overlapping projection angles . typically , some or all of steps 310 - 330 are performed in parallel in time ( e . g ., at times that are at least partially overlapping ) with steps 340 - 360 . for example , any of steps 310 - 330 may be performed parallel in time with generate mv x - ray step 340 . thus , two different projection images , optionally using two different x - ray energies , can be generated at the same time . thus , in some embodiments , kv x - ray source 120 and mv x - ray source 110 are used to generate x - rays simultaneously . in these embodiments , scatter correction is optionally used to reduce cross - talk between each source / detector combination . for example , in one embodiment , scatter correction is used to reduce the generation of noise at kv x - ray detector 125 resulting from x - rays generated using mv x - ray source 110 and scattered to kv x - ray detector 125 by target 210 . fig4 is a graphical representation of projection angles used to generate projection images , in various embodiments of the invention . relative to a projection angle arbitrarily labeled 0 degrees , kv x - ray source 120 is rotated to various projection angles between 270 degrees and 10 degrees by moving gantry 130 . at the same time , mv x - ray source 110 , which is fixed at a position on gantry 130 at a position approximately 90 degrees from kv x - ray source , is rotated to various projection angles between 0 degrees and 100 degrees . using this rotation scheme , projection images are recorded between projection angles of 0 degrees and 10 degrees using both the kv source / detector combination and the mv source / detector combination . as is discussed further herein , the projection image data generated at these overlapping projection angles may be used to determine scaling factors for converting projection image data obtained using x - rays of one energy for combination with projection image data obtained using x - rays of the other energy . referring again to fig3 , in a scale mv x - ray data step 380 projection image data generated using mv x - ray detector 115 in detect mv x - ray step 360 is scaled such that it can be combined with projection image data generated using kv x - ray detector 125 in detect kv x - ray step 330 . this scaling is performed using conversion logic 145 . typically , the scaling process involves multiplication of the projection image data by a scaling factor or application of a non - linear scaling function . in alternative embodiments , the kv projection image data is scaled for combination with the mv projection image data . in a construct image step 390 , ct construction logic 150 is used to generate a three - dimensional representation of target 210 , or cross - section thereof , using both the scaled mv projection image data and the kv projection image data . the construction of the three - dimensional representation can be performed using any of the known algorithms for generating three - dimensional representations from two - dimensional projections known in the art of computerized tomography . cross - sections of the three - dimensional representation are optionally displayed to a user using display 155 . fig5 is a scatter plot of kv and mv conversion parameters , according to one embodiment of the invention . these data are generated by comparing projection images obtained at the same projection angles using x - rays of two different energies , such as kv and mv energies . these same projection angles include , for example , the angles between 0 degrees and 10 degrees as shown in fig4 . because , in some embodiments , the kv source / detector combination is oriented at a position approximately orthogonal to the mv source / detector combination , gantry 130 ( fig1 ) is rotated 90 degrees in order for the mv source / detector combination to generate projection images at the same projection angles as the kv source / detector combination . the scatter plot shown in fig5 is generated by examining pixels in the mv projection images and noting the intensity of detected mv x - rays at those pixels . the corresponding pixels ( e . g ., the pixels representing the same positions ) in the kv projection images are then examined and the intensity of the detected kv x - rays at those corresponding pixels are noted . in some instances , the noted values are normalized by dividing data obtained with an instance of target 210 placed in target region 105 by data obtained without any target placed in target regions 105 ( e . g ., background data ). the kv and mv intensity values are then plotted . a conversion parameter p for both kv and mv projection images is calculated using the formula p = log [( i b − i 0 )/( i b − i )]. where i 0 was the pixel value from the open field and i was the pixel value from the original projection image . i b was the background pixel value . this formula is based on an assumption that the radiation beam was attenuated exponentially through the imaging object . in fig5 the plotted intensities have been fitted to a line 510 . line 510 is optionally a linear function , in which case the slope and intercept can be used to convert mv projection image data to kv projection image data in scale mv x - ray data step 380 of fig3 . fig6 illustrates the conversion of mv projection image data to kv projection image data using the conversion parameter p calculated as discussed with respect to fig5 . the data shown represents a cross - section across a projection image . mv projection image data is represented by mv line 610 , kv projection image data is represented by kv line 620 , and converted mv projection image data is represented by converted data line 630 . discrepancies between the kv profile and the converted mv profile in fig6 are partially attributable to the use of a linear fit used to determine proportionality constant p . in alternative embodiments , a non - linear fit is used . in various embodiments , different approaches are used to determine the conversion parameter p . for example , in some embodiments , projection images are acquired using a ct phantom having regions of different density . because the regions within the ct phantom are well characterized , data taken using kv x - rays and mv x - rays can be compared typically at the same projection angle . once the conversion parameter p is determined then a target 210 of interest ( e . g ., a patient ) is place in target region 110 and the previously determined conversion parameter p is used to convert the projection image data of the target 210 of interest . in some embodiments , the conversion parameter p is determined using a less precisely characterized target 210 of interest . in these embodiments , data obtained at overlapping projection angles are used to determine the conversion parameter p . in some embodiments , both a ct phantom and overlapping projection angles are used to determine conversion parameter p . in some embodiments , the projection images generated using kv x - rays and mv x - rays are of different dimensions . for example , the volume covered by mv x - rays may be truncated such that it includes only a subset of the volume covered by kv x - rays . this arrangement may be desirable when the mv x - rays are used therapeutically and there is a wish to limit the exposure of healthy tissue to mv x - rays . thus , in some embodiments of the invention , a larger target volume is covered by kv x - rays for the purpose of imaging target 210 while a smaller target volume is covered by mv x - rays for the purpose of treatment . further , those mv x - rays used for treatment are optionally also used to enhance the three - dimensional representation by combining the mv projection image data with the kv projection image data as described herein . resolution of the three - dimensional representation is enhanced in the volume of target 210 receiving therapeutic x - rays . fig7 a and 7b are ct images obtained using mv and kv imaging , respectively , according to one embodiment of the invention . a ct image is a cross - section of a three - dimensional object generated using computerized tomography . in this embodiment , projection images were acquired using a varian clinac 21ex accelerator ( varian medical systems ) as mv x - ray source 110 to generate mv x - rays , and amorphous silicon electronic portal imager ( a si500 detector ) as mv x - ray detector 115 to detect the generated mv x - rays . a varian medical system &# 39 ; s on - board imager ™ ( obi ), including both kv x - ray source 130 and kv x - ray detector 125 , was used to generate and detect kv x - rays . these systems were mounted orthogonally on gantry 130 as shown in fig1 and 2 . three robotically controlled exact ™ ( varian medical systems ) supportive arms were used to position mv x - ray detector 115 , kv x - ray detector 125 and kv x - ray source 120 such that the beam lines of the mv source / detector combination and kv source / detector combination intersected orthogonally near the center of target region 105 . the active imaging area for both kv and mv source / detector combinations was 397 × 298 mm . the matrix size for mv x - ray detector 115 was 1024 × 768 pixels with 2 bytes depth , and the matrix size for kv x - ray detector 125 could be either 2048 × 1536 pixels ( high resolution ) or 1024 × 768 ( low resolution ) pixels with 2 bytes depth . a high - performance scatter rejection grid was mounted in the front of kv x - ray detector 125 . the x - ray tube of kv x - ray source 120 had a target angle of 14 degrees and two focal spots : a nominal small spot of 0 . 4 mm and a nominal large spot of 0 . 8 mm , per iec ( international electrotechnical commission ) 60336 . the heat load capacity of this x - ray tube is 600 k heat units , while the heat loading of the x - ray tube housing is 2m heat units . the x - ray generator , within kv x - ray source 120 , had a maximum output of 32 kw . projection images acquired using the kv source / detector combination could be achieved with two different modes : digital radiography , both in high or low resolution , and digital fluoroscopic imaging with a frame rate of 7 or 15 frames per second . the kv source / detector combination also had a cone - beam ct acquisition mode which could acquire over 650 projections within less than 70 seconds . this mode is used for clinical applications . in order to acquire the ct images shown in fig7 a and 7b , a target 210 was placed within target region 105 . target 210 included a head phantom and a contrast phantom taped together . the head phantom ( rando ® anthropomorphic phantom by phantom laboratories , salem , n . y .) and contrast phantom ( mini ct qc phantom model 76 - 430 , nuclear associates , ny ) are experimental tools configured to simulate a clinical instance of target 210 , such as a patient . the dimensions of the contrast phantom were six inches in diameter and one inch in thickness . the contrast phantom included insertions of different densities in six 1 . 125 - inch circular holes . projection images using the mv x - ray source 110 were acquired using gantry angles starting from 100 degrees to 270 degrees ( iec convention ), with an interval of 2 degrees . the projection images using the kv source were acquired using gantry angles starting from 190 degrees to 0 degrees . a total of 96 projections were acquired at each x - ray energy . because of the orthogonal relationship between of the mv source / detector combination and the kv source / detector combination , when mv x - ray source 110 was at 0 degrees , kv x - ray source 120 was at 270 degrees . fig8 a - 8f include several cross - sections of three - dimensional representations ( e . g ., ct images ) of one instance of target 210 generated using the techniques described herein . fig8 a illustrates a ct image generated using 48 different mv projection images obtained at gantry 130 angles between 100 degrees and 6 degrees . fig8 b illustrates a ct image generated using 48 different kv projection images obtained at gantry 130 angles between 94 degrees and 0 degrees . fig8 c illustrates a ct image generated using both the 48 mv projection images used to generate fig8 a and the 48 kv projection images used to generate fig8 b . the kv and mv projection images were obtained and combined using , for example , the methods illustrated by fig3 . the ct image of fig8 c includes more detail than either the ct images of fig8 a or 8 b . this improved detail is due to the greater number of projections used and possibly the different sensitivities of the kv and mv x - rays . for example , the kv projection images show more contrast resolution for soft tissues while the mv images are less susceptible to some types of interferences . fig8 d and 8e include ct images . the ct image shown in fig8 d was generated using 96 different mv projection images acquired using gantry 130 angles between 100 degrees and 270 degrees , and the ct image shown in fig8 e was generated using 96 different kv projections acquired using gantry 130 angles between 190 degrees and 0 degrees . for comparison , fig8 f includes a diagnostic ct image , reconstructed by using almost 1000 projections acquired with a philips acqsim ct simulator . fig9 a - 9c illustrate the use of projection images that cover different volumes within target 210 . fig9 a and 9b include ct images generated using only 12 full kv projection images , and only 12 truncated mv projection images , respectively . the mv projection images were generated using an x - ray beam truncated by an aperture to reduce the volume exposed to mv x - rays . in some embodiments , the mv x - rays are truncated to minimize the exposure of tissues outside a treatment volume to mv x - rays . fig9 c includes a ct image generated using a combination of both the kv projection images used to generate the ct image of fig9 a and the mv projection images used to generate the ct image of fig9 b . as a result of the combination , the center region of the ct image of fig9 c is enhanced relative to that shown in fig9 a . thus , mv treatment x - rays are combined with kv imaging x - rays to achieve greater ct image quality during treatment . this improvement may be used to refine the treatment volume and reduce the exposure of healthy tissue to harmful x - rays . several embodiments are specifically illustrated and / or described herein . however , it will be appreciated that modifications and variations are covered by the above teachings and within the scope of the appended claims without departing from the spirit and intended scope thereof . for example , while mv x - ray source 110 and kv x - ray source 120 are used herein as an example , either may be replaced by alternative radiation sources . in some embodiments both sources are configured to generate the same types and / or energies of radiation . in some embodiments , the methods discussed herein are used to generate four - dimensional ct data that includes a time dependent three - dimensional representation of target 210 . in some alternative embodiments , projection images generated using a kv source / detector combination are used to generate a first three - dimensional representation , and projection images generated using a mv source / detector combination are used to generate a second three - dimensional representation . the first and second three - dimensional representations are then combined using ct construction logic 150 to form one or more ct images . in these embodiments , scale mv x - ray data step 380 and conversion logic 145 are optional . in some embodiments , more than two source / detector combinations are used to generate projection images parallel in time . control logic 140 , conversion logic 145 and / or ct construction logic 150 are each optionally embodied in hardware , firmware , or software stored in memory . in some embodiments , the projection images generated using one or more of the source / detector combinations are each one pixel line , e . g ., one - dimensional . these projection images are each representative of the attenuation of x - rays along a line through target region 105 . in these embodiments , a plurality of one - dimensional projection images may be used to generate a two - dimensional representation of target 210 , using the systems and method of the invention . the adaptation of the systems and method of the invention to the generation of two - dimensional representations from one - dimensional projection images would be apparent to a person of ordinary skill in the art . in some embodiments , mv projections are generated using x - rays configured for x - ray treatment of a patient . in these embodiments , a computerized tomography image constructed using kv projection image data and mv projection image data may be used for identifying and / or viewing the treatment volume . in these embodiments , mv x - rays are used for both treatment and imaging . the embodiments discussed herein are illustrative of the present invention . as these embodiments of the present invention are described with reference to illustrations , various modifications or adaptations of the methods and or specific structures described may become apparent to those skilled in the art . all such modifications , adaptations , or variations that rely upon the teachings of the present invention , and through which these teachings have advanced the art , are considered to be within the spirit and scope of the present invention . hence , these descriptions and drawings should not be considered in a limiting sense , as it is understood that the present invention is in no way limited to only the embodiments illustrated . exact ™ and on - board imager ™ are registered trademarks of varian medical systems , inc .
systems and methods include coordinated and megaelectronvolt computerized tomography imaging . kv and mv data are combined using a normalization process in order to generate ct images . the resulting ct images can include an improved signal to noise ratio in comparison to ct images generated using either kv or mv imaging alone . the coordinated kv and mv imaging process may be accomplished in significantly less time than using kv or mv imaging alone . this time savings has advantages in treatment verification . the mv projections are optionally generated using mv x - rays configured for x - ray treatment . in these cases the combined projections will reflect the treatment volume .