Optimized aperture selection imaging computed tomography system and method

Abstract

The invention provides a method and imaging system for operating imaging computed tomography using a radiation source and a plurality of detectors to generate an image of an object. The method includes: defining a desired image characteristics; and performing calculations to determine the pattern of fluence to be applied by the radiation source, to generate said desired image quality or characteristics. Then, the radiation source is modulated, to generate the intended pattern of fluence between the beam source and the object to be imaged. The desired image characteristics can comprise at least one of desired levels of contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR), and may provide at least one of: desired image quality in at least one defined region of interest; and at least one desired distribution of said image quality.

1 . A method for operating imaging computed tomography using a radiation source and a plurality of detectors to generate an image of an object, the method comprising the steps of:
 a) defining desired image characteristics;   b) performing calculations to determine the pattern of fluence to be applied by the radiation source, to generate said desired image characteristics; and   c) modulating the radiation source to generate said pattern of fluence between the beam source and the object to be imaged.   
     
     
         2 . The method of  claim 1 , wherein said desired image characteristics comprise at least one of desired levels of contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR). 
     
     
         3 . The method of  claim 2 , wherein said desired image characteristics providing at least one of: desired image quality in at least one defined region of interest; and at least one desired distribution of said image quality. 
     
     
         4 . The method of  claim 1  wherein performing said calculations comprises solving an inverse problem using an iterative solution. 
     
     
         5 . The method of  claim 4 , wherein said calculations being performed include considering at least one of:
 i) The dependence of the image quality on primary fluence transiting through the object.   ii) The dependence on scatter fluence to the detector.   iii) The dependence upon scattered dose to the object and its dependence on φ(θ,u,v).   iv) The exposure dependent DQE of the detector DQE(φ(θ,u,v)).   
     
     
         6 . The method of  claim 3 , including constraining the calculations by a dose distribution constraint. 
     
     
         7 . A method as claimed in  claim 4  including defining a region of interest from a library of population modules or at least one previously acquired image of the object. [alternatively a pre-scan of the image can be conducted] 
     
     
         8 . The method of  claim 7  comprising performing said calculations to determine the pattern of fluence by solving the inverse problem according to the equation:
     m ( u,v ) I ( u,v )= G   −1   [C ( {right arrow over (r)} )]  [7]
 
 
       where C({right arrow over (r)}) is and image metric defining the desired image characteristic, I (u, v) represents the intensity of the radiation applied to the object from the source and m(u, v) represents the modulation of the radiation by the object, and where G −1  is an operator which relates the image metric C({right arrow over (r)}) to the applied radiation intensities, and wherein the method further comprises iteratively solving the equation:
   min{∥C({right arrow over (r)})−C i ({right arrow over (r)})∥}
 
 
       where, for each step i, the image metric C i ({right arrow over (r)}) is calculated and compared to the desired quantity C({right arrow over (r)}). 
     
     
         9 . The method of  claim 8 , including lower and upper bounds on the image metric, whereby in the reconstructed image:
       C   ( {right arrow over (r)} )≦ {circumflex over (f)} ( {right arrow over (r)} )≦   C   ( {right arrow over (r)} )  [8]
   
       where  C ({right arrow over (r)}) and  C ({right arrow over (r)}) are lower and upper bounds respectively of the desired of C({right arrow over (r)}) at each point {right arrow over (r)}. 
     
     
         10 . The method of  claim 9 , including optimizing image characteristics and dose iteratively according to:
   min{∥C({right arrow over (r)})−C i ({right arrow over (r)})∥+w∥D({right arrow over (r)})−D i ({right arrow over (r)})∥}
   
       wherein ∥(C({right arrow over (r)})−C i ({right arrow over (r)})∥ represents optimal image quality, ∥D({right arrow over (r)})−D i ({right arrow over (r)})∥ represents optional patient dose, and w is weighting given to the dose. 
     
     
         11 . The method of  claim 10 , including providing different weighting to the image characteristics and the patient dose. 
     
     
         12 . The method of  claim 11 , including weighting at least one of the image quality and the patient dose across individual voxels according to:
   min{∥W C ({right arrow over (r)})(C({right arrow over (r)})−C i ({right arrow over (r)}))∥+w∥W D ({right arrow over (r)})(D({right arrow over (r)})−D i ({right arrow over (r)}))∥}
   
     
     
         13 . The method of  claim 1 , including providing temporal modulation of the radiation source. 
     
     
         14 . The method of  claim 1 , including providing spatial modulation of the radiation source. 
     
     
         15 . The method of  claim 1 , including both spatial and temporal modulation of the radiation source. 
     
     
         16 . The method of  claim 14  or  15 , comprising providing a temporal modulator comprising a plurality of individual elements adapted to absorb radiation, and moving these elements to provide desired temporal modulation 
     
     
         17 . An imaging system, the system comprising:
 a) a radiation source for directing a beam at an object to be imaged;   b) a modulator placed between said beam source and the object to be imaged; and   c) a computer for performing calculations based on the desired distribution of image quality to determine the pattern of fluence to be applied by said temporal modulator.   
     
     
         18 . An imaging system as claimed in  claim 17 , wherein the modulator comprises at least one of a spatial modulator and a temporal modulator. 
     
     
         19 . An imaging system as claimed in  claim 17 , wherein the modulator comprises a plurality of individual elements, each being substantially impervious to the radiation and being movable between an open position and a closed position, whereby open positions of the individual elements define an aperture permitting passage of the beam from the radiation source. 
     
     
         20 . An imaging system as claimed in  claim 19 , wherein the compensator is configured as a louvre compensator comprising a first set of substantially parallel louvres extending in one direction and a second set of parallel louvres extending in another direction, whereby by selected positioning of at least one louvre of the first set in the open position and at least one louvre of the second set in the open position an aperture is defined for passage of the beam of radiation. 
     
     
         21 . An imaging system as claimed in  claim 17 , wherein the compensator comprises a plurality of pairs of elements, the elements in each pair being elongate and extending end to end, and the pairs of elements being arranged in a side by side abutting relationship, wherein, with all the pairs of elements arranged with the ends of the elements abutting one another, the compensator is closed, and with at least one pair of elements open so that the ends of the elements of that pair are spaced apart, an aperture is defined for the beam of radiation.