Abstract:
A single-slice data acquisition system (DAS) for a CT imaging system is time-multiplexed to measure more than one signal over the DAS standard sampling time interval. In one embodiment, a detector element includes a scintillator and two photodiodes aligned with respective portions of the scintillator. Each photodiode generates a signal based on the scintillator output. The signal from one of the photodiodes is subjected to a time-dependent gain during consecutive sampling intervals. The gain-adjusted signal and the signal from the other photodiode are combined, and the combined signal is processed to obtain an estimate of the z-derivative of the signal. The estimated z-derivative is then used to generate a high quality image.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for time-multiplexing data acquisition. 
     In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. 
     In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection 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 growing number of computer tomography applications it is desirable to increase the speed of data acquisition with corresponding increased patient coverage in a given amount of time. Demands increasingly are put upon the data acquisition system (DAS), particularly in multi-slice applications. To include a single-slice DAS row for each row of detectors is cost-prohibitive. It is known to use a single-slice DAS to process the signal of several combined detector rows. Although this approach may serve to limit the number of DAS systems needed, it negates the potential benefit of higher z-resolution through multiple rows in z. 
     BRIEF SUMMARY OF THE INVENTION 
     In one exemplary embodiment of the present invention, a single-slice imaging system DAS is time-multiplexed for measuring more than one signal during a DAS standard sampling time interval. More specifically, and in the one embodiment, each detector element includes a scintillator and two photodiodes aligned with respective portions of the scintillator. Each photodiode generates a signal based on the scintillator output. The signal from one of the photodiodes is subjected to a time-dependent gain during consecutive sampling intervals. The gain-adjusted signal and the signal from the other photodiode are combined, and the combined signal is processed to obtain an estimate of the z-derivative of the signal. The estimated z-derivative is then used to generate a high quality image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system. 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG.  1 . 
     FIG. 3 is a block schematic diagram of a data acquisition system applying gain according to one embodiment of the present invention. 
     FIG. 4 is a block schematic diagram of a detector element according to one embodiment of the present invention. 
     FIG. 5 is a graph showing time-dependent gain applied to intensity signals according to one embodiment of the present invention. 
     FIG. 6 is a block schematic diagram of a detector element pair according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through a medical patient  22 . Detector array  18  is fabricated in a single slice configuration. Each detector element  20  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 patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . 
     Referring to FIG.  3  and according to one embodiment of the invention, a DAS standard single slice sampling time is divided into N equal sub-intervals. N 2  time-dependent gains or gain offsets dg k  (t j ), k=1, . . . ,N; j=1, . . . N are selected to be driven by DAS  32 . Gains dg k  (t j ) are supplied to multiplier  52 . Signals i k  are gain-adjusted and summed at adder  54 . Each gain dg k  (t j ) assumes up to N different values over each of the N sub-intervals. By sampling the DAS output N times over the standard single-slice sampling time, N signals I j  are obtained which are described by the equation:          I   j     =       ∑     k   =   1       k   =   N                         i   k          x        [     1   +     d                     g   k          (     t   j     )           ]                                  
     This equation is rewritten in the form of an N-by-N linear system:            [     I   j     ]       N   ×   1       =               [   G   ]       N   ×   N            [     i   k     ]         N   ×   1                     with        :                     G     j   ,   k         =     [     1   +     d                     g   k          (     t   j     )           ]                              
     Dependent upon the selection of gains 1+dg k  (t j ), the matrix G is invertible and the inverse is stable, thus leading to:            [     i   k     ]       N   ×   1       =           [   G   ]       N   ×   N       -   1            [     I   j     ]         N   ×   1                              
     Accordingly, the N signals i k , k=1, . . . N have been recovered. 
     Referring to FIGS. 4 and 5, a detector element  20  includes, in one embodiment, a scintillator  50  and first and second photodiodes  56  and  58 . Scintillator  50  is optically coupled to photodiodes  56  and  58 . The signal transmitted by first photodiode  56  is combined, i.e., summed, at adder  54  with the gain-adjusted signal transmitted by second photodiode  58 , and the combined signal is transmitted to DAS  32 . DAS  32  samples the summed intensity projection signals. 
     For example, if the number of sub-intervals N is selected to be 2, gains dg 1 (t 1 ) and dg 2 (t 1 ) are set equal to δg, gains dg 1 (t 2 ) and dg 2 (t 2 ) are set equal to −δg, T represents the standard single slice sampling time, and, with the following time dependency for dg(t):            d                 g     =     δ                 g       ,     0   &lt;   t   &lt;       1   2        T                   d                 g     =       -   δ                   g       ,         1   2        T     &lt;   t   &lt;   T                            
     two intensity measurements I 1  and I 2  are collected over time T. Measurements I 1  and I 2 , to the first order, relate to the intensity I that would have been collected over T should δg=0 by:                  I   1     ≈         1   2        I     +       1   4        δ                   g        (     I   +     Δ                   zI   z   ′         )                  
            I   2     ≈         1   2        I     -       1   4        δ                   g        (     I   +     Δ                   zI   z   ′         )                     (   1   )                                
     where Δz is a geometric factor that depends on cell length. Accordingly, by linear combination, the following relationships are obtained:                I   ≈       I   1     +     I   2              
            Δ                   zI   z   ′       ≈         (       I   1     -     I   2       )         1   2        δ                 g       -     (       I   1     +     I   2       )                 (   2   )                                
     An estimate of the z-derivative of the intensity signal therefore is obtained by a processor, e.g., the image reconstructor  34  processor. In the reconstruction process, and by applying a higher order interpolation, it is possible to obtain either increased patient coverage by imaging at higher helical pitch while maintaining image quality, or improved image quality without increasing pitch. 
     Referring to FIG. 6, and in an alternative embodiment, a detector element pair  60  includes a first detector element  62  and a second detector element  64 . The DAS sampling interval is kept at time T, but alternate gains of detector element pair  60  are determined as indicated in FIG.  6 . Specifically, a positive gain dg is applied to intensity projection signals transmitted by first photodiode  56  of first detector element  62 , and a negative gain dg is applied to intensity projection signals transmitted by first photodiode  56  of second detector element  64 . 
     Benefits of using the above described embodiment include the absence of a need for a higher DAS sampling frequency and minimal hardware incremental costs. In one embodiment, gains −dg and +dg are fixed for the duration of the scan. 
     Since a sloped straight line is not affected by a z-convolution with an even kernel, to the extent that imaging occurs at the center z 0  of scintillator  50 , and that the scintillator gain profile is even with respect to z 0 , the fact that light photons are spread over several millimeters in z should not affect the ability to estimate the slope. A Taylor expansion of the signal around z 0  demonstrates that the foregoing proposition extends to all odd terms. Although even terms have their magnitude modified (symmetrically with respect to z 0 ), such terms do not introduce an error when modulated by linearly varying DAS gains. 
     The present invention is useful in improving image quality in axial scans, particularly for large apertures, and the signal z-slope information can be used to correct for partial volume errors. Gains (1−dg) and (1+dg) also can be applied respectively to intensity projection signals transmitted by two photodiodes of one detector element over the first T/2 sampling interval and switched respectively to (1+dg) and (1−dg) over the second T/2 sampling interval. In another embodiment, wherein DAS  32  sets dg=0, system  10  reverts to the usual single-slice operating mode. By keeping the DAS  32  double sampling rate, twice as many views are collected. From the z-derivative of the intensity signal, the z-derivative of the line-integral 1(z) is: 
     
       
           l′ ( z )= I′ ( z )| I ( z ). 
       
     
     The algorithms described herein could be implemented in computer  36  or in image reconstructor  34 . Also, it should be understood that system  10  is described herein by way of example only, and the invention can be practiced in connection with other types of imaging systems. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.