Abstract:
A method for reducing slice thickness of a computed tomography imaging system including a source configured to direct an x-ray beam through an object toward a plurality of rows of detector elements configured to collect projection data in slices. The method includes steps of obtaining imaging data from a pair of adjacent rows, each of the adjacent rows having an outer edge; deconvolving at least a portion of the imaging data obtained from an area bounded by the adjacent row outer edges; and combining the deconvolved imaging data to obtain a slice sensitivity profile for the adjacent row pair. This method allows a multi-slice imaging system user to implement imaging data deconvolution to reduce slice thickness to less than one millimeter. Thus image resolution is improved without having to modify hardware in existing multi-slice imaging systems.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to tomographic imaging, and more particularly to methods and apparatus for generating computed tomographic imaging data using a multi-slice imaging system. 
     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 known CT systems the x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray-absorbing material with an aperture therein for restricting the x-ray beam. 
     CT imaging systems typically provide image resolution within limitations imposed by such factors as collimator aperture size and slice thickness. A minimum slice thickness for at least one CT system is 1.25 millimeters, as determined primarily by detector element pitch size. In order to improve image resolution, it is desirable to reduce slice thickness to less than 1 millimeter, and to achieve such reduction with minimal impact on imaging system hardware. 
     It is known to reduce slice thickness of a single-slice imaging system by irradiating a portion of a detector element and deconvolving imaging data, for example, projection data or image data, to reduce the full-width-at-half-maximum (FWHM) interval of a reconstructed slice profile. It is desirable to achieve similar slice-width reductions on multi-slice systems without impacting system hardware. Difficulties can arise, however, in implementing this approach for a multi-slice imaging system, because multi-slice sampling is limited, for example, by joints between adjacent detector rows. 
     It would be desirable to use double-slice imaging data collection and deconvolution techniques to reduce slice thickness on a multi-slice imaging system without having to change system hardware. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment, a method for reducing slice thickness of a computed tomography imaging system including a source configured to direct an x-ray beam through an object toward a plurality of rows of detector elements configured to collect projection data in slices, the method including the steps of obtaining imaging data from a pair of adjacent rows, each of the adjacent rows having an outer edge; deconvolving at least a portion of the imaging data obtained from an area bounded by the adjacent row outer edges; and combining the deconvolved imaging data to obtain a slice sensitivity profile for the adjacent row pair. 
     The above-described method allows a multi-slice imaging system user to implement imaging data deconvolution to reduce slice thickness to less than one millimeter. Thus image resolution is improved without having to modify hardware in existing multi-slice imaging systems. 
    
    
     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 diagram illustrating geometry of an x-ray beam striking a typical multi-slice detector array; 
     FIG. 4 is an illustration of a typical slice sensitivity profile for a left center detector element row when irradiated as shown in FIG. 3; 
     FIG. 5 is an illustration of a typical slice sensitivity profile for a right center detector element row when irradiated as shown in FIG. 3; 
     FIG. 6 is a diagram of a multi-slice sampling pattern according to one embodiment; 
     FIG. 7 is an illustration of the left center detector slice sensitivity profile of FIG. 4 deconvolved in accordance with one embodiment; 
     FIG. 8 is an illustration of the right center detector slice sensitivity profile of FIG. 5 deconvolved in accordance with one embodiment; and 
     FIG. 9 is a graph of a combined slice sensitivity profile obtained according to one embodiment. 
    
    
     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 an object  22 , for example, a medical patient. Detector array  18  may be fabricated in a single slice or multi-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. One such parameter supplied by the operator is a nominal slice thickness for data acquisition. 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 . 
     In a typical multi-slice system and referring to FIG. 3, x-ray beam  16  emanates from a focal spot  50  of source  14  and is projected through a pre-patient collimator  52  that defines beam  16  profile in the z-axis. Collimated beam  16  is projected toward detector array  18 , which includes a plurality of rows  54 , for example, four rows  54  of detector elements  20 . Adjacent rows  56  and  58  are detector array  18  center rows having outer edges  60  and  62  respectively. An inner edge  64  of row  56  is adjacent to an inner edge  66  of row  58 . 
     In one embodiment, a method for reducing imaging system  10  slice thickness includes obtaining imaging data (e.g. projection data or image data generated from projection data) from a pair of adjacent rows  54 , for example, center rows  56  and  58 . More particularly, beam  16  is directed to irradiate a portion of an area  68  bounded by left center detector row  56  outer edge  60  and right center detector row  58  outer edge  62 . For example, beam  16  is collimated by collimator  52  such that beam  16  is contained within a portion of area  68 . 
     Directing beam  16  onto center rows  56  and  58  and obtaining imaging data as above described effectively reduces slice thickness for some clinical applications. For example, where a full-width-at-half-maximum (FWHM) interval of system  10  is 1.25 millimeters, slice thickness is reduced to a FWHM of 0.8 to 0.9 millimeters. Further reductions are limited by x-ray focal spot  50  size and system  10  geometry (i.e. distance between collimator  52  and focal spot  50  and distance between detector elements  20  and focal spot  50 ). 
     For many clinical applications it is desirable to achieve a slice thickness of 0.5 millimeters. Thus in another embodiment, at least a portion of imaging data obtained from area  68  is deconvolved. More particularly, FIG. 4 illustrates a typical slice sensitivity profile  70  for a left center row such as row  56  when irradiated as shown in FIG.  3 . Sensitivity profile  70  closely approximates a step function at a distance  72  where a joint, i.e. inner edges  64  and  66  respectively of rows  56  and  58 , defines a beam  16  boundary. Where defined by collimator  52 , beam.  16  boundary falls off gradually, for example, around a distance  74 , as outer edge  60  is approached from the right. Similarly and referring to FIG. 5, a typical slice sensitivity profile  76  for a right center row such as row  58  closely approximates a step function at a distance  78  where inner edge  64  meets inner edge  66 . Where defined by collimator  52 , beam  16  boundary falls off gradually, for example, around a distance  80 , as outer edge  62  is approached from the left. 
     The above-described dissimilarities between the step-function-approximated and collimator-defined beam  16  boundaries make it difficult to compensate effectively for undershoot if deconvolution is applied to multi-slice imaging data as known for single-slice applications. Thus in one embodiment, deconvolution is applied to a portion of the imaging data, e.g. deconvolution is applied separately to each adjacent row  56  and  58 . More particularly and for example, deconvolution for left center row  56  is applied toward left outer edge  60 , and deconvolution for right center row  58  is applied toward right outer edge  62 , using relationships written as:                  P     1      A     ′          (   i   )       =       ∑     k   =   0     N            w   kA            P     1      A            (     i   -   k     )                   (   1   )                   P     1      B     ′          (   i   )       =       ∑     k   =   0     N            w   kB            P     1      B            (     i   +   k     )                   (   2   )                                
     where P 1A  and P 1B  are original imaging data samples for a left center detector row  1 A and a right center detector row  1 B respectively, P′ 1A  and P′ 1B  are modified imaging data samples for left center detector row  1 A and right center detector row  1 B respectively, and w kA  and w kB  are deconvolution kernel points. 
     Because original slice sensitivity profiles of, e.g., rows  56  and  58  typically are asymmetric, deconvolution results are improved when all data samples used in a deconvolution are from the same row  54 , for example, as described above in Equations (1) and (2). Thus in one embodiment a sampling pattern for, e.g., an axial scan is performed as shown in FIG.  6 . After an imaging data sample  90  is obtained, detector array  18  is incremented in the z-axis by a uniform interval, for example, one half of the nominal slice thickness of a row  54 , such that new samples  92  and  94 , for example, from row  56  at least partially overlap, e.g. straddle, previous samples taken from row  56 . 
     FIGS. 7 and 8 respectively illustrate slice sensitivity profiles  70  and  76  deconvolved in the above-described manner. In the embodiment shown in FIGS. 7 and 8, deconvolved sensitivity profiles  82  and  84  are obtained using a three-point deconvolution kernel. In other embodiments, kernels of different sizes are used. 
     When one-sided deconvolution is used as described above, centroids of deconvolved slices are shifted compared to original slice profiles. (Thus, for example, an apparent overlap shown in FIG. 6 of first sample  90  of row  58  over third sample  94  of row  56  is changed through deconvolution.) As shown in FIGS. 7 and 8, deconvolution according to one embodiment shifts a row  56  centroid  86  in a rightward direction while a row  58  centroid  88  is shifted leftward. 
     Thus in one embodiment and referring to FIGS. 7 and 8, the deconvolved imaging data for row  56  is shifted rightward by a difference  96  in row  56  centroid  86  location before and after deconvolution. Similarly, deconvolved data for row  58  is shifted leftward by a difference  98  in row  58  centroid  88  location before and after deconvolution. The deconvolved and shifted imaging data from rows  56  and  58  is combined to obtain a slice sensitivity profile  100  as shown in FIG. 9. A combined slice profile  102  from rows  56  and  58  before deconvolution also is indicated in FIG.  9 . 
     Thus the above-described method allows a multi-slice system user to achieve FWHM intervals as small as 0.64 millimeters on a system with an original FWHM of 1.25 millimeters. Thus slice thickness is reduced and image resolution is improved without hardware changes. 
     Although particular embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. In addition, the CT system described herein is a “third generation” system in which both the x-ray source and detector rotate with the gantry. Many other CT systems including “fourth generation” systems wherein the detector is a full-ring stationary detector and only the x-ray source rotates with the gantry, may be used if individual detector elements are corrected to provide substantially uniform responses to a given x-ray beam. Furthermore, the present invention can be practiced with other imaging systems besides CT imaging systems. In some embodiments, the methods described herein are implemented by software, firmware or a combination thereof controlling either computer  36 , image reconstructor  34 , or both. Furthermore, the invention can be practiced using other processors besides computer  36  and image reconstructor  34 . 
     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.