Abstract:
Systems and methods include coordinated (KV) and megaelectronvolt (MV) computerized tomography (CT) 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Non-Provisional patent application Ser. No. 11/193,160, filed Jul. 29, 2005, now U.S. Pat. No. 7,453,976, the disclosure of which is incorporated herein by reference, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/682,170, entitled “A Technique for On-Board CT Reconstruction Using Both Kilovoltage and Megavoltage Beam Projections for 3-D Treatment Verification,” filed May 17, 2005, now lapsed, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The invention is in the field of medical imaging and more specifically in the field of computerized tomography. 
     2. Related Art 
     Computerized tomography (CT) is an imaging technique wherein x-rays are used to obtain two-dimensional projection images at a variety of different angles around a target being examined. Computer techniques are then used to generate a three-dimensional representation of the target by combining the two-dimensional projection images. The three-dimensional representation can be viewed, sliced and rotated by a user. 
     CT systems can generally be characterized by the energies of the x-rays used, such as kilovoltage (kV) and megavoltage (MV) imaging. In kV imaging, x-rays with energies in the kiloelectronvolt range are generated and detected. In MV imaging, x-rays with energies in the megaelectronvolt range are generated and detected. Each of these types of imaging has advantages and disadvantages. For example, kV imaging may be subject to interference from tooth fillings and MV imaging may cause radiation damage to the DNA of living cells. MV imaging is sometimes used therapeutically as a cancer treatment. 
     In diagnostic CT imaging hundreds of two-dimensional projection images are recorded as an x-ray source and detector are rotated around the target. The quality of the final three-dimensional representation is dependent on the number of two-dimensional projection images used to generate the three-dimensional representation. The time required to record hundreds of two-dimensional projection images can be a problem when the target is a patient because the patient must stay still during the imaging process. Typically, diagnostic CT imaging is performed using kV imaging because of the danger to the patient of using MV x-rays to generate so many projection images. 
     One therapeutic use of MV x-rays is referred to as intensity-modulated radiation therapy (IMRT). IMRT enables caregivers to deliver an extremely conformal dose of high energy x-rays to a well defined treatment volume while minimizing radiation damage to nearby organs and tissues. The success of IMRT is largely dependent on the accuracy of patient positioning and target localization. Therefore, it is important to have an efficient and effective method to confirm the position of the patient and the target volume within the patient. Without confirmation of the position of the target volume, the x-ray dose may harm healthy tissue and miss the tissue requiring treatment. In many situations a volume that is larger than the volume of tissue to be treated is exposed to high energy x-rays in order to compensate for errors in patient positioning, organ motion, and target localization uncertainties. This results in an undesirable exposure of healthy tissue to these x-rays. 
     There is, therefore, a need for improved methods of imaging that provide greater speed of analysis and greater accuracy for target localization. 
     SUMMARY 
     Systems and methods including more than one x-ray source and detector combination are used to generate separate two-dimensional projection images. Each source/detector combination is moved relative to the target in order to create a series of overlapping projection images. By operating each source/detector combination in parallel in time, the time required to generate a series of two-dimensional projection images can be substantially reduced. In comparison to the time requirements and resulting resolution of the prior art, this time savings can be used to generate a three-dimensional representation in a shorter time and/or to generate a three-dimensional representation with better resolution in the same time. 
     In various embodiments of the invention, the more than one x-ray sources are configured to generate x-rays in different energy ranges. For example, in some embodiments, one source/detector combination is used to generate projection images using kV x-rays while another source/detector combination is used to generate projection images using MV x-rays. These source/detector combinations may operate in parallel. Thus, two different projection images can be obtained at the same time. The projection images generated by one source/detector combination are optionally scaled such that they can be combined with projection images generated by the other source/detector combination. The combined projection images are then used to generate three-dimensional representations of a target. 
     The three-dimensional representations may be used for target localization. For example, in some instances, therapeutic MV x-rays are used to provide medical treatment while at the same time generating MV projection images of a target area. These MV projection images are combined with kV projection images recorded in parallel with the MV projection images, in order to generate a three-dimensional representation that can be used for real-time target localization. 
     (copies of independent claims go herexxx) 
    
    
     
       BRIEF DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram of an imaging system, according to various embodiments of the invention; 
         FIG. 2  is a photograph showing of parts of the imaging system of  FIG. 1 , according to one embodiment of the invention; 
         FIG. 3  is a flowchart illustrating methods of generating a three-dimensional representation of a target using the imaging system of  FIG. 1 , according to various embodiments of the invention; 
         FIG. 4  is a graphical representation of projection angles used to generate projection images, according to various embodiments of the invention; 
         FIG. 5  is a scatter plot of kV and MV conversion parameters, according to one embodiment of the invention; 
         FIG. 6  illustrates the conversion of MV projection image data to kV projection image data, according to one embodiment of the invention; 
         FIGS. 7A and 7B  are CT images obtained using MV and kV imaging, respectively, according to one embodiment of the invention; 
         FIGS. 8A-8F  are CT images obtained using various imaging modes, according to one embodiment of the invention; and 
         FIGS. 9A-9C  include CT images generated using various imaging modes, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  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  FIG. 1  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. 
       FIG. 2  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 .  FIG. 2  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  FIG. 2  is typically replaced by a patient. 
       FIG. 3  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 . 
       FIG. 4  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  FIG. 3 , 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 . 
       FIG. 5  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  FIG. 4 . Because, in some embodiments, the kV source/detector combination is oriented at a position approximately orthogonal to the MV source/detector combination, Gantry  130  ( FIG. 1 ) 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  FIG. 5  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  FIG. 5  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  FIG. 3 . 
       FIG. 6  illustrates the conversion of MV projection image data to kV projection image data using the conversion parameter P calculated as discussed with respect to  FIG. 5 . 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  FIG. 6  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. 
       FIGS. 7A 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  FIGS. 1 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  FIGS. 7A 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. 
       FIGS. 8A-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.  FIG. 8A  illustrates a CT image generated using 48 different MV projection images obtained at Gantry  130  angles between 100 degrees and 6 degrees.  FIG. 8B  illustrates a CT image generated using 48 different kV projection images obtained at Gantry  130  angles between 94 degrees and 0 degrees.  FIG. 8C  illustrates a CT image generated using both the 48 MV projection images used to generate  FIG. 8A  and the 48 kV projection images used to generate  FIG. 8B . The kV and MV projection images were obtained and combined using, for example, the methods illustrated by  FIG. 3 . 
     The CT image of  FIG. 8C  includes more detail than either the CT images of  FIG. 8A  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. 
       FIGS. 8D and 8E  include CT images. The CT image shown in  FIG. 8D  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  FIG. 8E  was generated using 96 different kV projections acquired using Gantry  130  angles between 190 degrees and 0 degrees. For comparison,  FIG. 8F  includes a diagnostic CT image, reconstructed by using almost 1000 projections acquired with a Philips AcQsim CT simulator. 
       FIGS. 9A-9C  illustrate the use of projection images that cover different volumes within Target  210 .  FIGS. 9A 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. 
       FIG. 9C  includes a CT image generated using a combination of both the kV projection images used to generate the CT image of  FIG. 9A  and the MV projection images used to generate the CT image of  FIG. 9B . As a result of the combination, the center region of the CT image of  FIG. 9C  is enhanced relative to that shown in  FIG. 9A . 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.