referring now to fig1 , 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 fig2 , 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 fig2 , 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 fig3 , 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 fig4 , 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 fig6 , 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 fig4 , 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 fig7 , 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 fig5 , 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 .
a computed tomography machine 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 of the properties of the patient being imaged .