Abstract:
Methods and apparatus for reducing motion-induced artifacts in CT imaging is described. The imaging system scans a patient&#39;s heart to obtain a plurality of projection views, a differential projection is determined from the projection views, a weighting function is applied to the differential projection to minimize motion artifacts, and an inconsistency index is determined from the differential projection, and the inconsistency index is used to locate an image reconstruction location. This method directly measures the mechanics of the heart, rather than an electrical signal and utilizes projection data to select the best locations to minimize image artifact.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to utilizing a method to minimize motion artifacts caused by cardiac motion.  
           [0002]    In at least some “computed tomography” (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as an “imaging plane”. The x-ray beam passes through an 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 a 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.  
           [0003]    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 the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. 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.  
           [0004]    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.  
           [0005]    To reduce the total scan time required for multiple slices, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.  
           [0006]    Known cardiac CT scanners utilize “electro-cardio-gram” (EKG) signals when acquiring scan data. Typically, a plurality of leads are connected to a patient to measure the EKG signal, which indirectly represents a cardiac cycle. The cardiac cycle includes a period of relaxation and dilation of the heart cavities known as diastole, and a period of contraction of the heart during which blood is ejected from the ventricles known as systole. A typical period of time for one cardiac cycle is slightly less than one second. Thus, a heart goes through a substantial portion of its cycle during one gantry revolution. Motion induced image artifacts result from heart motion.  
           [0007]    To suppress the image artifact, some cardiac CT scanners correlate the EKG electrical signals with a plurality of mechanical signals of the heart. The electrical signals and the mechanical signals, however, cannot be precisely correlated for each patient. Therefore, extra views of projection data are acquired based on EKG signals. A radiologist then visually selects a best image from the set of reconstructed images.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    Methods and apparatus for reducing motion-induced artifacts in computed tomography (CT) imaging are described. In one aspect, a method for imaging a heart is provided in which the heart is scanned to obtain projection data for a plurality of projection views. A differential projection is determined based on a first and a last projection view. A weighting function is applied to the differential projection to minimize motion artifacts, and an inconsistency index is generated from the differential projection, which is used to identify an image reconstruction location.  
           [0009]    In another aspect, a processor in the imaging system is programmed to acquire projection data for a plurality of projection views of the heart. The processor is programmed to determine a differential projection based on a first and a last projection set, apply a weighting function to the differential projection and generate an inconsistency index to determine an image reconstruction location.  
           [0010]    In yet another aspect, a computer-readable medium in the imaging system is provided which comprises a plurality of records of projection data. A program residing on the computer-readable medium utilizes a plurality of rules to generate a differential projection based on a first and a last projection view, define a weighting function that is applied to the differential projection to minimize motion induced artifacts, and utilize a plurality of rules to determine an inconsistency index to identify an image reconstruction location.  
           [0011]    This method directly measures the mechanics of the heart, rather than an EKG electrical signal. In addition, this method utilizes projection data to select a reconstruction location to minimize motion-induced image artifact. Further, implementing the method does not require that additional hardware be used or replaced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a pictorial view of a CT imaging system;  
         [0013]    [0013] 
         [0014]    [0014]FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1;  
         [0015]    [0015]FIG. 3 is a flow chart illustrating a sequence of steps executed by the CT system to determine diastole and systole phases of a human heart; and  
         [0016]    [0016]FIG. 4 is a chart illustrating view number versus inconsistency index used to determine diastole and systole phases of the heart. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring to FIGS. 1 and 2, a computed tomography (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, such as a medical patient  22 . 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 object or 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 . In one embodiment, and as shown in FIG. 2, detector elements  20  are arranged in one row so that projection data corresponding to a single image slice is acquired during a scan. In another embodiment, detector elements  20  are arranged in a plurality of parallel rows, so that projection data corresponding to a plurality of parallel slices can be acquired simultaneously during a scan.  
         [0018]    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 .  
         [0019]    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 .  
         [0020]    [0020]FIG. 3 is a flow chart  50  illustrating the steps executed to determine a reconstruction location where a motion induced artifact is minimum. The method illustrated in FIG. 3 can be practiced by DAS  32  (shown in FIG. 2), image reconstructor  34  (shown in FIG. 2), or computer  36  (shown in FIG. 2). Generally, a processor in at least one of DAS  32 , reconstructor  34 , and computer  36  is programmed to execute the process steps described below. Of course, the method is not limited to practice in CT system  10  and can be utilized in connection with many other types and variations of imaging systems.  
         [0021]    For a CT data set, artifacts caused by the motion of an object are introduced by the largest inconsistency present in adjacent projection views. For example, in a full scan, the discrepancy between the start and end of the scan typically represents a worst-case condition. When scanning an object with a cyclic motion, not necessarily periodic, the motion artifact is minimum when the object is roughly in the same motion state at the start and end of the scan. It is known that when the period of the motion matches exactly the cycle of the gantry speed for a half-scan and a full scan, the motion artifact is minimal.  
         [0022]    To minimize motion artifacts, a starting projection view is determined after collecting a plurality of projection views  52 . The differences between the first and the last views used in the reconstruction are determined and the starting view selected is the one which minimizes the difference. For example, to find the starting angle for a halfscan to minimize motion induced artifacts, a differential projection  54  is determined by the following relationship:  
           d (γ,β)=| P (γ,β+γ m −γ)− P (−γ,β+π+γ m +γ)|,  (1)  
         [0023]    where β is the projection angle, γ is the detector angle, and γ m  is the maximum detector angle. For a full scan, a differential projection  54  is determined by the following relationship:  
           d (γ,β)=| P (γ,β)− P (γ,β+2π)|,  (2)  
         [0024]    where β is the projection angle and γ is the detector angle.  
         [0025]    To minimize the influence of other motion induced differences, e.g., such as respiratory motion, the differential projection is multiplied by a weighting function  56 . An inconsistency index  58 , ξ(β), is determined according to the following relationship:  
         ξ(β)=∫ ym   ym   w (γ) d (γ,β) dγ,   (3)  
         [0026]    where d(γ,β) is a differential projection, ω(γ) is a weighting function, γ m  is a maximum detector angle, and −γ m  is a minimum detector angle. The diastole and systole phases of the heart are determined  60  based on a rate of change of inconsistency index  58  per view.  
         [0027]    Referring specifically to FIG. 4, which illustrates a chart  70  of balloon phantom (not shown). The balloon phantom is inflated and deflated at a rate of sixty-five “beats per minute” (bpm), and projection data is acquired at 0.8s by CT scanner  10  in cine mode. Inconsistency index  58 , ξ(β), is determined and plotted on an ordinate  72  and view numbers  74  are plotted on an abscissa  76 . In an exemplary embodiment, a first valley  78  of a first curve occurs at view number two-hundred and a second valley  80  occurs at view number six-hundred-and-seventy. Of course, other charts of inconsistency index versus view numbers are possible with the first valley and the second valley positioned at various other view numbers. First valley  78  corresponds to the halfscan acquisition centered at the end of deflating the balloon, e.g., analagous to the end of systole in a cardiac cycle. Second valley  80  corresponds to the halfscan acquisition centered at the end of inflating the balloon, e.g., analagous to the end of diastole in a cardiac cycle. The rate-of-change, e.g., slope, of the inconsistency index near the two valleys is different. For first valley  78 , e.g., the deflation case, the slope is five-thousand-eight-hundred-and-thirty-three, and for second valley  80 , e.g., the inflation case, the slope is three-thousand-six-hundred-and-forty. Therefore, by comparing the inconsistency index slope, the systole and diastole phases  62  of the heart are determined, where steeper slope values correspond to the systolic phase. The acquisition center at the end of diastole phase of the heart should provide minimum motion artifact.  
         [0028]    In another embodiment, a plurality of images are reconstructed based on views from the end of systole phase of the heart to the end of the diastole phase of the heart. These reconstructed images represent different phases of the heart and depict the cardiac cycle when viewed in sequence.  
         [0029]    In yet another embodiment, a CT system  10  includes a computer program residing on a computer-readable medium within mass storage  38  for reconstructing the image. A plurality of records of projection data for a plurality of projection views are stored on the computer-readable medium. A plurality of records of differential projections are generated from the records of projection data. A plurality of rules apply a weighting function to the records of differential projections, and a plurality of rules determine records of inconsistency index for each record of projection view.  
         [0030]    This method of imaging the heart is based on direct measurements of the mechanics of the heart, rather than an EKG electrical signal. In addition, this method utilizes projection data to select a reconstruction location to minimize motion induced image artifact. Further, implementing the method does not require that additional hardware be used or replaced.  
         [0031]    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.