Patent Publication Number: US-6219441-B1

Title: Reconstruction of images from three-dimensional cone beam data

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to reconstruction of images from three-dimensional data acquired with x-ray CT or SPECT scanners. 
     In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile. 
     The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. 
     In a 3D scan the x-ray beam diverges to form a cone beam that passes through the object and impinges on a two-dimensional array of detector elements. Each view is thus a 2D array of x-ray attenuation measurements and the complete scan produces a 3D array of attenuation measurements. Either of two methods are commonly used to reconstruct a set of images from the acquired 3D array of cone beam attenuation measurements. The first method described by L.A. Feldkamp et al in “Practical Cone-Beam Algorithm”,  J. Opt. Soc. Am ., A/Vol. 1, No. 6/June 1984 is a convolution backprojection method which operates directly on the line integrals of the actual attenuation measurements. The method can be implemented easily and accurately with current hardware and it is a good reconstruction for images at the center or “midplane”, of the cone beam. The Feldkamp method employs the conventional convolution—back projection form, but this is an approximation that becomes less accurate at larger cone beam angles. The second method proposed by Pierre Grangeat in “Mathematical Framework of Cone Beam 3D Reconstruction Via the First Derivative of the Radon Transform”,  Mathematical Methods In Tomography , Herman, Louis, Natterer (eds.), Lecture notes in Mathematics, No. 1497, pp. 66-97, Spring Verlag, 1991, provides an accurate solution to the image reconstruction task based on a fundamental relationship between the derivative of the cone beam plane integral to the derivative of the parallel beam plane integral. While this method is theoretically accurate, it requires mathematical operations that can only be solved using finite numerical calculations that are approximations. The errors introduced by the implementation of the Grangeat method can be greater than Feldkamp and these errors are not correlated with cone beam angle. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a computer tomography system which produces a three-dimensional array of data from which a set of 2D image slices can be reconstructed. More specifically, the system includes a 2D array of detector elements for receiving photons in a cone beam produced by a source while the two are rotated about a central axis to acquire data at a series of views, an image reconstructor which employs filtered back projection of the acquired cone beam data to produce image data; means for identifying data missing from the acquired cone beam data and estimating values for the missing cone beam data values; means for calculating correction image data from the estimated cone beam data values; and combining the correction image data with the back projection image data to produce an image slice. 
     A general object of the invention is to accurately reconstruct image slices from 3D cone beam data. A filtered back projection method is employed to accurately and efficiently produce the main part of the reconstruction. A second set of image data is also produced by estimating values not supported by the projection. Correction image data is produced from these estimated values and the resulting correction image is combined with the back projection image to produce the corrected image slice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system in which the present invention may be employed; 
     FIG. 2 is a block schematic diagram of the CT imaging system; 
     FIGS. 3 a  and  3   b  are pictorial views of the cone beam produced by the CT imaging system; 
     FIGS. 4 a - 4   c  are vector diagrams used to explain the image reconstructor which forms part of the CT imaging system; and 
     FIG. 5 is a block diagram of the image reconstructor which forms part of the CT imaging system of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIGS. 1 and 2, a computed tomography (CT) imaging system  10  includes a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  13  that projects a cone beam of x-rays  14  toward a detector array  16  on the opposite side of the gantry. The detector array  16  is formed by a number of detector elements  18  which together sense the projected x-rays that pass through a medical patient  15 . Each detector element  18  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through the patient. During a scan to acquire x-ray projection data, the gantry  12  and the components mounted thereon rotate about a center of rotation  19  located within the patient  15 . 
     The rotation of the gantry and the operation of the x-ray source  13  are governed by a control mechanism  20  of the CT system. The control mechanism  20  includes an x-ray controller  22  that provides power and timing signals to the x-ray source  13  and a gantry motor controller  23  that controls the rotational speed and position of the gantry  12 . A data acquisition system (DAS)  24  in the control mechanism  20  samples analog data from detector elements  18  and converts the data to digital signals for subsequent processing. An image reconstructor  25 , receives sampled and digitized x-ray data from the DAS  24  and performs high speed image reconstruction according to the method of the present invention. The reconstructed image is applied as an input to a computer  26  which stores the image in a mass storage device  29 . 
     The computer  26  also receives commands and scanning parameters from an operator via console  30  that has a keyboard. An associated cathode ray tube display  32  allows the operator to observe the reconstructed image and other data from the computer  26 . The operator supplied commands and parameters are used by the computer  26  to provide control signals and information to the DAS  24 , the x-ray controller  22  and the gantry motor controller  23 . In addition, computer  26  operates a table motor controller  34  which controls a motorized table  36  to position the patient  15  in the gantry  12 . 
     As shown best in FIG. 3 a , in the preferred embodiment of the present invention the detector array  16  is a flat array of detector elements  18 , having N r  (e.g. 1000) elements  18  disposed along the in-plane (x,y) direction, and N. (e.g. 16) elements  18  disposed along the z axis. The x-ray beam emanates from the x-ray source  13  and fans out as it passes through the patient  15  and intercepts the detection array  16 . Each acquired view is a N r  by N z  array of attenuation measurements as seen when the gantry is oriented in one of its positions during the scan. As shown in FIG. 3B, the object of the present invention is to reconstruct as set of 2D image slices  35  from the 3D array of acquired data produced by the x-ray cone beam during the scan. It can be seen that because the cone beam diverges as it passes through the patient  15 , the reconstruction of the parallel image slices  35  is not possible with a straight forward fan beam filtering and backprojection process. The present invention enables an accurate reconstruction of the image slices  35  from this acquired cone beam data. 
     Referring particularly to FIG. 4 a , let f({right arrow over (r)}) be the function of the image to be reconstructed, where {right arrow over (r)} is the position vector. In a cylindrical coordinate system one has 
     
       
         {right arrow over (r)}=(r cos φ, r sin φ, z)  (1) 
       
     
     Let Rf (, {right arrow over (n)}) be the Radon transform of f({right arrow over (r)}). As shown in FIG. 4 b , each point in the Radon space is characterized by its algebraic distance, , to the origin O and the unit vector, {right arrow over (n)}, which is along the line connecting the origin O to the point. In the object space, (,{right arrow over (n)}) defines a plane P with its normal of {right arrow over (n)} and its algebraic distance to the origin of , as shown in FIG. 4 c . As shown in FIG. 4 b , {right arrow over (n)} can be further characterized by the co-latitude angle, θ, and the longitude angle, φ, as follows: 
     
       
         {right arrow over (n)}=(sin θ cos φ, sin θ sinφ, cos θ)  (2) 
       
     
     Note that (,θ,φ) supports the entire Radon space and the entire Radon space may be denoted by S. By definition, Radon transform Rf(,{right arrow over (n)}), contains the value of integration of f({right arrow over (r)}) over the plane that is characterized by (,{right arrow over (n)}). This relation can be mathematically expressed as follows: 
     
       
         Rf(,{right arrow over (n)})=∫f({right arrow over (r)})δ({right arrow over (r)}·{right arrow over (n)}−)d{right arrow over (r)}  (3) 
       
     
     The inverse formula of the 3D Radon transform is given as follows:                f        (     r   →     )       =       -     1     8        π   2                  ∫     Q   2                  ∂     R   2          f       ∂     ϱ   2                         (         r   →     ·     n   →       ,     n   →       )               n   →                     (   4   )                         
     where the integration is over a half unit sphere in Radon space. This can be rewritten as follows:                      f        (     r   →     )       =                  -     1     8        π   2                ∫     S        ∫           ∂     R   2          f       ∂     ϱ   2              (     ϱ   ,     n   →       )          δ        (         r   →     ·     n   →       -   ϱ     )               ϱ               n   →                             =                  1     8        π   2                ∫     φ   =   0       2      x              ∫     θ   =   0     π            ∫     ϱ   =     -   ∞       ∞                ∂     R   2          f       ∂     ϱ   2                         (     ϱ   ,     n   →       )          δ        (         r   →     ·     n   →       -   ϱ     )                 sin                 θ                  ϱ             θ             φ                             (   5   )                         
     where the integration is over the entire Radon space S. 
     The Radon space S can be divided into two subspaces, subspace D which is supported by projection data, and subspace C that is not supported by projection data.                f        (     r   →     )       =         f   D          (     r   →     )       +       f   c          (     r   →     )                 (   6   )                         
     where:                  f   D          (     r   →     )       =       -     1     8        π   2                ∫     D        ∫           ∂     R   2          f       ∂     ϱ   2              (     ϱ   ,     n   →       )          δ        (         r   →     ·     n   →       -   ϱ     )               ϱ               n   →                         (   7   )                         
     and                  f   c          (     r   →     )       =       -     1     8        π   2                ∫     C        ∫           ∂     R   2          f       ∂     ϱ   2              (     ϱ   ,     n   →       )          δ        (         r   →     ·     n   →       -   ϱ     )               ϱ               n   →                         (   8   )                         
     Points in subspace C usually have small co-latitude angle, θ. Since the contribution from each point of Radon space is weighted by sin θ, points in subspace C tend to have small weighting. As a result, it is safe to conclude that f c ({right arrow over (r)}) need not be calculated with high precision in order to reconstruct accurate slice images. On the other hand, since f({right arrow over (r)}) is primarily determined by f D ({right arrow over (r)}), an accurate reconstruction of f D ({right arrow over (r)}) is crucial. 
     To accurately reconstruct f D ({right arrow over (r)}) we transform equation (7) from (,θ,φ) to the detector coordinates system (1,Θ,{right arrow over (OS)}). By doing so, the intermediate calculation of          ∂   Rf       ∂   ϱ                     
     as taught by Grangeat can be eliminated and f D ({right arrow over (r)}) can be reconstructed directly from the projection data. In making this transformation, the following relation, derived from the Central Slice Theorem, is used: 
      Σ {right arrow over (OS)} (1Θ)=∫FP {right arrow over (OS)} (ω,Θ) j2πω1 dω  (9) 
     where FP {right arrow over (OS)} (ω,Θ) is the 2D Fourier transform of P {right arrow over (OS)} (Y,Z), expressed in the polar coordinate system. From equation (9) one has:                  ∂       ∑     o                   s   →              (     l   ,   Θ     )           ∂   l       =     ∫         FP     o                   s   →              (     ω   ,   Θ     )          j                 2        πω     j      2      πωl               ω                 (   10   )                         
     The specific form of the transformation is determined by the particular machine geometry, and for the circular scanning geometry of the preferred embodiment, the image f D ({right arrow over (r)}) can be calculated as follows:                  f   D          (     r   →     )       =         1   2          ∮              Φd   2           (            -     r   →         ·   s     )     2            ∫     ∫         FP     o                   s   →              (     ω   ,   Θ     )            ω   2        sin                   Θ         j      2      π          (         Y   0        sin                 Θ     +       Z   0        cos                 Θ       )          ω               ω             Θ                 +     R        (     1     d   0       )                 (   11   )                         
     where:          Y   0     =              r   →       ·     u   ^                -     r   →         ·     s   ^                   Z   0     =          z              -     r   →         ·     s   ^                         
     The first term in equation (11) is precisely the filtering and back projection method described by Feldkamp et al in the above-cited article which is incorporated herein by reference. The second term R(l/d 0 ) may be ignored without significantly affecting image quality, however, if higher accuracy is required it may be calculated as well. In other words, the most important calculation f D ({right arrow over (r)}) in the cone beam image reconstruction can be accurately and efficiently performed using the Feldkamp et al method. 
     The remaining term f c ({right arrow over (r)}) in the final image f({right arrow over (r)}) may be calculated using the methodology disclosed in the above-cited Grangeat publication which is incorporated herein by reference. More specifically,          ∂   Rf       ∂   ϱ                     
     on the boundary of subspace D is calculated from the cone beam projection data using the following equation disclosed by Grangeat that relates the derivative of cone beam plane integral to the derivative of parallel beam plane integral:                    ∂   Rf       ∂   ϱ            (       O                     S   →     ·     n   →         ,     n   →       )       =                O                   S   →            2              O                   S   →     ×     n   →            2                         ∂       ∑     O                   S   →              (     l   ,   Θ     )           ∂   l                 (   12   )                         
     where, the line denoted by (1,Θ) is the intersection of the plane P characterized by (={right arrow over (OS)}·{right arrow over (n)}, {right arrow over (n)}) and the detector plane denoted by {right arrow over (OS)}. Next,          ∂   Rf       ∂   ϱ                     
     is estimated in subspace C. Note that since subspace C is not supported by cone beam projection data, we assume that          ∂   Rf       ∂   ϱ                     
     is continuous at the boundary of subspace D and C, and interpolate therebetween. Having calculated these values, f c ({right arrow over (n)}) is calculated using the above equation (8) and this reconstructed image data is added to that produced from equation (11) to yield the final image slices. 
     This reconstruction method is implemented in the image reconstructor  25 . Referring particularly to FIG. 5, the cone beam projection data is received from the DAS  24  as a two-dimensional array of values which are preprocessed in the standard manner at process block  40 . Such preprocessing includes correcting for known errors and offsets and calculating the minus log of the data to convert it to x-ray attenuation values. 
     The preprocessed cone beam attenuation profiles are used to separately calculate the two image terms f D ({right arrow over (r)}) and f c ({right arrow over (r)}). The main image term f D ({right arrow over (r)}) is calculated in a sequence of steps indicated by process blocks  41 - 44  which is essentially the method described by Feldkamp et al. It includes multiplying the cone beam projection data by weighting factors, as indicated at process block  41 : 
     
       
         P′ φ (Y,Z)=P φ (Y,Z)d{square root over (d 2 +L +y 2 +L +Z 2 +L )}  (13) 
       
     
     where d=distance from x-ray source to detector element. 
     The resulting projection data is then filtered by convolving it with a filter kernal as indicated at process block  42 .                    P   φ     _          (     Y   ,   Z     )       =       ∫     -   ∞     ∞                 Y   ′              ∫     -   ∞     ∞                 Z   ′              g   y          (     Y   -     Y   ′       )              g   z          (     Z   -     Z   ′       )              P   φ   ′          (       Y   ′     ,     Z   ′       )                       (   14   )                         
     where the kernals are:            g   y          (   Y   )       =     Re          ∫   0     ω   yo            ω             ω                      ω                 y                           g   z          (   Z   )       =     sin                   ω   z0          Z   /   π                   Z                     
     The filtered attenuation data is then back projected from each detector element position back along the ray extending from the point source of the x-ray cone beam. This results in a 3D image array f D ({right arrow over (r)}).                  f   D          (     r   →     )       =       1     4        π   2              ∮          φ            d   2         (     d   +     r   ·       x   ^     ′         )     2                P   φ     _          [       Y        (     r   →     )       ,     Z        (     r   →     )         ]                     (   15   )                         
     where Y({right arrow over (r)})={right arrow over (r)}·ŷ′ d/(d+{right arrow over (r)}·{circumflex over (x)}′) 
     Z ({right arrow over (r)})={right arrow over (r)}·{circumflex over (z)}d/(d+{right arrow over (r)}·{circumflex over (x)}′) 
     As is well known in the art, the image reconstructed in this manner through the midplane of the cone beam is very accurate. However, as the images move away from this midplane image, their quality decreases due to incomplete data. Corrections for this deterioration in image quality is provided by the f c ({right arrow over (r)}) image term described above and calculated as will now be described. 
     Referring still to FIG. 5, the acquired and preprocessed cone beam attenuation data is also applied to a process block  46  in which the boundaries between the C and D subspaces are calculated as set forth above in equation ( 12 ). As indicated at process block  47 , the data values in subspace C are then calculated by interpolating between values at the boundaries with subspace D, and these estimated values are applied to process block  48  which calculates the correction images f c ({right arrow over (r)}) using the above equation (8). The corresponding slices f D ({right arrow over (r)}) and f c ({right arrow over (r)}) are added together at summing point  49  to produce the final image slices  50  for the computer  26 .