Abstract:
A computed tomography machine ( 1 ) provides for improved dose efficiency by calculating an optimized set of beam intensities to produce the desired image quality. Determination of the beam weights is based on an a priori modeling ( 53 ) of the properties of the patient being imaged.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a PCT application and claims priority to Provisional Patent Application Ser. No. 60/760,896, filed on Jan. 20, 2006, titled Partial Volume Imaging Using Portal Images, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to computed tomography machines that are free-standing or that are part of a radiation therapy machine, and in particular to a computed tomography machine providing a reduced dose to the patient. 
     External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope or a megavoltage energy x-ray source, such as a linear accelerator. The external source produces a collimated radiation beam directed along an axis of radiation toward a tumor site. The adverse effect of irradiating healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the radiation beam into the patient along a variety of radiation axes with the beams converging on the tumor site. 
     Intensity modulated radiation therapy (IMRT) provides an external beam of radiation composed of individually intensity-modulated “beamlets”. The intensities or weights of the beamlets are calculated to provide a desired dose pattern of radiation for an arbitrary shaped tumor within the patient while minimizing radiation in other areas. One such system, commercially available from TomoTherapy, Inc. of Madison, Wis., provides a radiation source that rotates about the patient on a helical line producing a fan beam of multiple beamlets, each beamlet controlled by a movable shutter. 
     With the increased accuracy possible with IMRT, the problem arises of obtaining precise knowledge of the position, shape, and size of the target which often changes between the time that the treatment plan was developed and the actual treatment time. In image guided radiation therapy (IGRT), the target geometry is monitored at the time of radiation treatment so that the treatment plan may be adjusted accordingly. Ideally, the imaging system used for this purpose is computer tomography (CT) which can provide precise three-dimensional imaging of a target needed for maximizing the benefit of IMRT. 
     One method of providing IGRT with CT is to provide a rail system to transfer the patient between an adjacent CT and the IMRT machines. Alternatively, a CT machine may be incorporated into the IMRT machine itself, either by adding a kilovoltage x-ray source and detector, or by using the radiation therapy megavoltage source and adding megavoltage detector. 
     Generally, this latter approach of megavoltage CT (MVCT) requires high imaging doses because of the penetrating nature of megavoltage beams, low detector efficiency and the need to repeat imaging on a daily basis for patient positioning. A common strategy to reduce the exposure to the patient in MVCT is to reduce the area of the imaging, for example, by limiting radiation principally to the treatment area, this latter option making dual use of the radiation for both treatment and imaging. Unfortunately, reduction of the exposure area in CT creates “partial volume” artifacts in the reconstructed image. These artifacts are caused by structure outside of the image area which affects the measurements of the image region to the extent that radiation passes through this outside structure to reach the image area. Because the outside structure is not fully characterized by imaging, its attenuating influence is not fully cancelled, and the result is streaks in the image. 
     U.S. Pat. No. 6,618,467 issued Sep. 9, 2003, assigned to the assignee of the present invention, and hereby incorporated by reference, describes an MVCT system that addresses this problem of partial volume artifacts by performing a pre-scan of an entire patient slice with low flux radiation to obtain a low resolution measurement of the outside structure. This low flux scan is combined with the radiation used during the treatment to provide images with reduced partial volume artifacts. 
     Alternative methods of dealing with the partial volume artifacts are described for example, in U.S. Pat. No. 4,878,169 issued Oct. 31, 1989, and U.S. Pat. No. 6,810,102 issued Oct. 26, 2004, which also augment CT projection data taken of a limited region of interest with full slice projection data obtained at a different time. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that minimizing the region of exposure does not necessarily optimize the imaging dose for a given quality of image. The contribution of each measuring radiation beamlet to the quality of the resultant tomographic image varies significantly as a complex function of the internal structure of the patient being imaged. 
     Generally, the structure of the patient being imaged is unknown, however, the present inventors have determined that in many cases there will be sufficient a priori knowledge about the patient, for example, when the images are part of a sequence of images, when there is a planning image, or when the internal structure of the patient conforms to standard patterns. In these cases, the a priori knowledge about the patient may be used to intelligently select beamlets according to their contribution to image quality and thereby effect an arbitrary trade-off between dose and image quality. Generally, for a given image quality, the dose may be reduced from what would be required if uniform exposures across beamlets are used. 
     Specifically then, the present invention provides a computed tomography imaging machine that includes a radiation source providing a radiation beam divisible into beamlets each being individually controllable in intensity. A radiation detector receives and measures these beamlets after they have passed through a patient held on a patient support between the radiation source and radiation detector. A controller holds a stored model of the patient and using the stored model, determines and controls the intensity of the radiation beam in the beamlets based on a calculated contribution by the beamlet to a quality of a tomographic image. 
     Thus it is one aspect of at least one embodiment of the invention to provide a computed tomography machine providing sophisticated control of the intensity of individual beamlets to precisely tailor the radiation dose to a desired image quality. 
     The determination of the intensity of the radiation beams may be performed by an inverse calculation in which intensities are iteratively modified based on a comparison between a calculated image using the stored model and the stored model itself. 
     It is thus another aspect of at least one embodiment of the invention to provide a beamlet analysis technique that is applicable to a wide variety of different images. 
     The intensity of the radiation beam in the regions may controlled to reduce dose to the patient for a given image quality. 
     It is thus another aspect of at least one embodiment of the invention to significantly decrease dose to a patient for CT imaging. 
     The stored model may be a tomographic image of the patient. 
     It is thus another aspect of at least one embodiment of the invention to provide a system that may create a stored model from automatically acquired data without the need for manual construction of a model of the patient. 
     The tomographic image may be a previous image of the patient provided by the computed tomography imaging machine. 
     It is another aspect of at least one embodiment of the invention to provide a system that may significantly decrease dose in CT cinematography. 
     The previous image may be obtained with the controller controlling the intensity of the regions of the radiation beam. 
     It is thus another aspect of at least one embodiment of the invention to allow the system to make use of its own optimized images for the purpose of optimizing future images. 
     The radiation source may be kilovoltage x-ray sources and/or megavoltage x-ray sources. 
     It is another aspect of at least one embodiment of the invention that it may work with either radiation therapy systems, combined radiation therapy machines and CT machines, or stand-alone CT systems where it is desired to control the dose. 
     The radiation source may be a multi-leaf collimator providing a plurality of leaves moving into and out of the radiation beam to control the intensity within the regions wherein one leaf is associated with each region. 
     It is thus another aspect of at least one embodiment of the invention to provide a simple mechanism for independently controlling the beamlets of a CT system. 
     Alternatively, the radiation source may include opposing shutter blades moving across multiple regions to control the intensity within the regions. 
     It is another aspect of at least one embodiment of the invention to provide a system that may work broadly with other forms of beam intensity control. 
     The controller may control the intensity of the radiation beams assuming a precondition of a subset of regions of given intensity. 
     It is another aspect of at least one embodiment of the invention to provide a system that may work with equipment providing limited control of beam intensities. 
     The subset of regions of given intensity may be obtained from a radiation therapy treatment plan. 
     It is another aspect of at least one embodiment of the invention to provide a system that can work with a predefined radiation plan, and hence a set of predefined beamlets, to reduce additional exposure to the patient. 
     The intensity of the beamlets may be controlled to on or off states or among a range of intensities. 
     It is an aspect of the present invention that it may work with a range of different types of beamlet modulation systems. 
     The computed tomography imaging machine may include multiple radiation sources and wherein the radiation beam is provided by no less than one radiation source. 
     It is another aspect of at least one embodiment of the invention that it may work with computed tomography machines having multiple radiation sources so long as one of the radiation sources or the combination of the multiple radiation sources provides for control of the beamlet weights. 
     The controller further may reconstruct a tomographic image from the attenuation signals from the controlled intensity radiation beams after augmenting the attenuation signals with information from the model. 
     It is another aspect of at least one embodiment of the invention that it may be used in combination with other methods of reducing partial volume artifacts. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary perspective view of a simplified computed tomography machine having multiple radiation sources for rotation about a patient support; 
         FIG. 2  is a schematic representation of one of the sources of  FIG. 1  showing the opposed radiation source and detector together with a duty-cycle modulating shutter system such as controls the intensity of individual beamlets of a radiation beam; 
         FIG. 3  is a figure similar to that of  FIG. 2  showing an alternative shutter system with continuously positionable collimator blades; 
         FIG. 4  is a data flow chart showing use of an acquired model of the patient and a radiation plan to provide optimized augmenting beams for enhancing image quality; 
         FIG. 5  is a figure similar to that of  FIG. 4  showing an alternative embodiment not using a radiation plan and suitable for stand-alone CT systems; 
         FIG. 6  is a program flow chart corresponding to the data flows of  FIG. 4  or  5 ; and 
         FIG. 7  is a detailed program flow chart of the imaging step of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , in one embodiment, a tomographic imaging system  10  may provide a rotating gantry  12  rotating about an axis  14  as shown by arrow  15 . A first and second radiation source  16  and  18  may be attached to the gantry  12  to rotate within a plane perpendicular to the axis  14 . 
     The gantry may include an open bore  20  receiving a patient supporting table  22  for positioning the patient within the plane  21  at an arbitrary location for scanning. The table  22  may be moved during rotation of the gantry  12  for so-called helical scans and the like. 
     Each of the radiation sources  16  and  18  may generate a radiation beam  26  directed along the plane  21  and received by a corresponding detector  28  also mounted on the gantry  12 . The beam  26  may be a fan beam providing for a single plane of detection or a cone beam providing for multiple planes of detection as is understood in the art. 
     One of the sources  16  may be a kilovoltage source for acquiring projection data at the corresponding detector  28  for the reconstruction of a tomographic image. Generally, as will be understood in the art, a projection set provides attenuation data through the patient along a number of ray lines in the radiation beam  26  at different gantry angles over a range equal to at least the angle subtended by the ray lines plus 180 degrees. 
     In this case, the other radiation source  18  may be a megavoltage source for radiation treatment. In this case, the radiation source  18  may provide for a component of an IMRT machine such as is well known in the art. 
     It is also contemplated that both radiation sources  16  and  18  may be kilovoltage sources for dual source computed tomography or, in yet another embodiment, that there be only one source  18  used for both radiation treatment and megavoltage tomography. 
     Referring now to  FIG. 2 , the radiation beam  26  used for tomography may be divided into a series of beamlets  30  along different ray-lines. Each beamlet is defined by a shutter system  32 , positioned between the radiation source  16  or  18  and detector  28 , which may individually control the intensity or weight of the different beamlets  30 . In the embodiment of  FIG. 2 , the beamlet weights are controlled by moving a radio opaque shutter, having a width in plane equal to the width of the beamlet  30 , into and out of the radiation beam  26  perpendicularly to the plane  21 . Changing the proportion of time during which the shutters block the beamlet  30 , “duty cycle” modulates the beamlets. A shutter system suitable for this purpose is described generally in U.S. Pat. No. 5,317,616, issued May 31, 1994 and entitled: Method And Apparatus For Radiation Therapy, assigned to the assignee of the present invention and hereby incorporated by reference. In contrast to the radiation therapy application described in this patent, however, the shutter system  32  is intended to modulate imaging radiation. A similar shutter system may form part of an IMRT system that may also be part of the present invention. 
     Referring to  FIG. 3 , in an alternative embodiment the shutter system  32  may employ two or more continuously positionable collimator blades  34  movable along the plane  21  whose motion again serves to provide individual control of the weights of the different beamlets  30  according to the relative time that the beamlets  30  are un-occluded. 
     Referring now to  FIG. 4 , in a first embodiment of the invention, a controller  40  executing a stored program  42 , may control the above elements of the tomographic imaging system  10  to obtain a full tomographic projection set  44  of a patient that is sufficient projection data to fully reconstruct a tomographic image of the patient. This full tomographic image provides a “forward” model  53  of the attenuating properties of the tissue of the patient along a slice through plane  21 . Alternatively the forward model  53  may be derived from a tomographic scan taken on another machine or an approximation of the patient tissue properties, for example, based on standard patient anatomy. 
     In this embodiment, the controller  40  also receives a radiation treatment plan  46  which describes a set of weights to be used for IMRT beamlets  30  such as will be used to provide treatment to a patient. The radiation treatment plan  46  may be based on the full tomographic projection set  44 , and as such, the full tomographic projection set represents a so-called planning image. 
     Before the radiation treatment, the controller  40  reviews the forward model  53  from the tomographic projection set  44  and the radiation treatment plan  46  to calculate an augmenting radiation plan  50 . During the radiation treatment, the patient is exposed to the radiation prescribed by the radiation treatment plan (shown as beamlets  30 ′) detected by a detector  28  and the radiation of the augmenting radiation plan  50  (shown as beamlets  30 ″) also detected by a detector  28  to produce a combined exposure of the patient that provides improved projections data (obtained from detectors  28 ) for the reconstruction of a tomographic image  52  that may be displayed on output device  54 . This image  52  may be used as part of an IGRT system to confirm correct dose placement. The augmenting radiation plan  50  may be implemented using a kilovoltage radiation source or megavoltage radiation source used for the IMRT. Importantly, and as will be described, the augmenting radiation plan  50  provides different beam weights for adjacent beamlets  30 , specially selected to increase the image quality of the image  52  with reduced dose burden. 
     Referring now generally to  FIG. 6 , the program  42  produces a set of beamlet weights  43  for the augmenting radiation plan  50  by an “inverse” iterative process using a forward model  53  describing the internal structure of the patient, for example, as taken from a previous full tomographic projection set  44 . The goal of the iterative process to achieve uniform imaging dose to the imaging region of interest, which is usually the area being treated plus an appropriate margin, while minimize dose outside of it. Generally the forward model  53  will not be a perfect representation of the slice or slices of the patient to be imaged, or else there would be no reason to perform the imaging, however, the process accommodates errors in the forward model  53  and, as additional images are acquired, works to reduce those errors. 
     The beamlet weights  43  by the “inverse” iterative process for the augmenting radiation plan  50  are selected by a weight adjuster  56 , which will be described further, and which receives a set of constraints  58  being, in the embodiment of  FIG. 4 , the weight values of the beamlets  30  used in the radiation treatment plan  46 . These weights should not be decreased, and thus constrain the weight adjuster  56  in its selection of the beamlet weights  43 . Other constraints related to the limitations of the tomographic imaging system  10  may also be included. 
     An initial set of beamlet weights  43 , within the constraints  58 , is selected by the weight adjuster  56  to be an arbitrarily low set of beamlet weights  43  consistent with low dose to the patient. The beamlet weights  43  are provided to the forward model  53  and a simulated image  59  is generated. This simulated image  59  is produced by integrating the attenuation indicated by the forward model  53  along each of the paths of the beamlets  30  according to the weights or intensity of the beamlets  30 . In this calculation, the beamlet weights  43  will include the weight required by the radiation treatment plan  46  plus any amount of augmenting radiation to be determined by this process. 
     The simulated image  59  is then compared to the forward model  53  itself at comparison block  57  and a difference value  60  is produced indicating image quality. Generally, the lower the difference value  60 , the higher the quality of the image and the higher the difference value  60  the lower the quality of the image. The difference value  60  may be a straight summation of the magnitude of the differences over each pixel of the simulated image  59  and/or may be weighted according to the structure of particular interest or the absolute amount of the differences. 
     If the difference value  60  is greater than desired (based on a predetermined target image quality) then the program  42  returns to the weight adjuster  56  and new beam weights are selected within the constraints  58 . The modification of the beamlet weights  43  may be according to any number of well-known algorithms including simulated annealing or genetic algorithms that incrementally move the beam weights upward until the desired dose is achieved. 
     This process of adjusting beamlet weights  43 , modeling them and checking the difference may be repeated for a number of iterations until the difference value  60  drops to an acceptable level. At this time the program  42  proceeds to an actual scan  62  and the beamlet weights  43  are used to control a shutter system  32  on the megavoltage source or the megavoltage source and a separate kilovoltage source to obtain scan projection data  68 . 
     The resulting scan projection data  68  (possibly being a combination of multiple sources  16  and  18 ) is provided to an image reconstructor  66  which produces an image  52 . This image  52  may be displayed and may serve as a model for determining additional beamlet weights  43  for a next scan. 
     Referring now to  FIG. 7 , in a further embodiment, the image reconstructor  66  may take the scan projection data  68  from the scan  62 , such as generally represents an attenuation sinogram, and may further process it to supplement those projections (e.g., attenuation data from particular beamlets  30  at a particular gantry angles) which were associated with low or zero beam weights. This may be done, for example, by a combiner  70  that combines the scan projection data  68  obtained from the scan  62  with data from the forward model  53 , reprojected by reprojector  67 , to form model projection data  63 . Generally the combiner  70  splices in projections from the model projection data  63  according to what data is missing from scan projection data  68 . 
     When scan projection data  68  is obtained with a binary switching of the shutters for example, with beamlet weights  43  that are either at zero or 100 percent, the combiner  70  simply takes the projections from the model projection data  63  to fill in the missing projections of scan projection data  68 . 
     Where scan projection data  68  includes continuously varying beam weights, then a weighting function is adopted to combine the scan projection data  68  with the model projection data  63 . Thus for example if a beamlet  30  at a given projection is operating at 20% of maximum intensity, then 20% of the value of that projection in the scan projection data  68  is combined with 80% of the value of the corresponding projection in the model projection data  63 . 
     The combined data set  72  from the combiner  70  is provided to reconstructor  74 , for example, one that uses filtered back projection, to produce an interim image  76 . 
     This interim image  76  is then reprojected by reprojector  78  to turn it back into sinogram data which replaces the model projection data  63 . This process of combining and reprojecting is repeated iteratively. After a number of iterations that may be determined empirically, an output image  52  is provided. 
     Referring to  FIG. 5 , this invention may also be used outside of the radiation therapy context for a CT machine that provides for a reduced dose to the patient. In this case, the tomographic projection set  44 , for example, a first frame of a cinematographic sequence, provides for the model  53  which is used to optimize the beamlet weights  43  used for subsequent images to provide reduced dose at the desired image quality. As each frame is acquired the data of the acquired image may become the new model  53  as indicated by arrow  47 . 
     When the above invention is used on a kilovoltage CT system, the shutter system may be much lighter and more easily constructed than the shutter used in radiotherapy. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.