Abstract:
Methods and systems for generating computed tomographic (CT) images from image data acquired during different biological cycles are provided. A computer is programmed to receive a plurality of scan data acquired during a gated acquisition window of each of a plurality of biological cycles, blend the scan data acquired during a first of the plurality of biological cycles with the scan data acquired during a second of the plurality of biological cycles, and construct a final image from the blended data.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to computed tomography (CT) imaging and more particularly, to reducing banding artifacts in CT cardiac reformatted images.  
         [0002]     At least some known Multi-slice CT electrocardiogram (EKG)-gated cardiac reconstruction techniques produce a set of images at a given phase of a cardiac cycle. Data from a range of locations is acquired over a series of heartbeats or cardiac cycles. Images from different cardiac cycles are combined to represent the whole heart through image reformation. The inherently discontinuous sampling in time can give rise to gray scale non-uniformities (banding) in reformatted images.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0003]     In one embodiment, a computer programmed to generate computed tomographic (CT) images from image data acquired during different biological cycles is provided. The computer is programmed to receive a plurality of scan data acquired during a gated acquisition window of each of a plurality of biological cycles, blend the scan data acquired during a first of the plurality of biological cycles with the scan data acquired during a second of the plurality of biological cycles, and construct a final image from the blended data.  
         [0004]     In another embodiment, a method of reconstructing images from image data acquired during different biological cycles is provided. The method includes receiving a plurality of scan data acquired during a gated acquisition window of each of a plurality of biological cycles, blending the scan data acquired during a first of the plurality of biological cycles with the scan data acquired during a second of the plurality of biological cycles, and constructing a final image from the blended data.  
         [0005]     In yet another embodiment, a computed tomographic (CT) imaging system for reconstructing an image of an object is provided. The imaging system includes a detector array, at least one radiation source, and a computer coupled to the detector array and the radiation source wherein the computer is configured to receive a plurality of scan data acquired during a gated acquisition window of each of a plurality of biological cycles, blend the scan data acquired during a first of the plurality of biological cycles with the scan data acquired during a second of the plurality of biological cycles, and construct a final image from the blended data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a pictorial view of a multi slice volumetric CT imaging system;  
         [0007]      FIG. 2  is a block schematic diagram of the multi slice volumetric CT imaging system illustrated in  FIG. 1 ;  
         [0008]      FIG. 3  is a schematic diagram of two exemplary gated cardiac cycles, associated gated acquisition windows, and a plurality of imaging locations along a z axis that are associated with the gated acquisition window;  
         [0009]      FIG. 4  is a flow chart of an exemplary reconstruction-based technique of blending of image data for CT cardiac applications that reduces gray scale non-uniformities in the reformatted images; and  
         [0010]      FIG. 5  is a flow chart  500  of an alternative reconstruction-based technique of blending projection data for CT cardiac applications that reduces gray scale non-uniformities in the reformatted images. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     In some known CT imaging system configurations, a radiation 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 an “imaging plane”. The radiation 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 radiation beam received at the detector array is dependent upon the attenuation of a radiation 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.  
         [0012]     In third generation CT systems, the radiation source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that an angle at which the radiation beam intersects the object constantly changes. A group of radiation 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 includes a set of views made at different gantry angles, or view angles, during one revolution of the radiation source and detector.  
         [0013]     In an axial scan, the projection data is processed to reconstruct 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 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 display device.  
         [0014]     To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved 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.  
         [0015]     As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.  
         [0016]     Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated but a viewable image is not. Therefore, as used herein the term “image” broadly refers to both viewable images and data representing a viewable image. However, many embodiments generate (or are configured to generate) at least one viewable image. Additionally, although described in detail in a CT medical setting, it is contemplated that the benefits accrue to all imaging modalities including, for example, ultrasound, Magnetic Resonance Imaging, (MRI), Electron Beam CT (EBCT), Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and in both medical settings and non-medical settings such as an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning CT system for an airport or other transportation center.  
         [0017]      FIG. 1  is a pictorial view of a CT imaging system  10 .  FIG. 2  is a block schematic diagram of system  10  illustrated in  FIG. 1 . In the exemplary embodiment, a computed tomography (CT) imaging system  10 , is shown as including a gantry  12  representative of a “third generation” CT imaging system. Gantry  12  has a radiation source  14  that projects a cone beam  16  of X-rays toward a detector array  18  on the opposite side of gantry  12 .  
         [0018]     Detector array  18  is formed by a plurality of detector rows (not shown) including a plurality of detector elements  20  which together sense the projected X-ray beams 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 radiation beam and hence the attenuation of the beam as it passes through object or patient  22 . An imaging system  10  having a multislice detector  18  is capable of providing a plurality of images representative of a volume of object  22 . Each image of the plurality of images corresponds to a separate “slice” of the volume. The “thickness” or aperture of the slice is dependent upon the thickness of the detector rows.  
         [0019]     During a scan to acquire radiation projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 .  FIG. 2  shows only a single row of detector elements  20  (i.e., a detector row). However, multislice detector array  18  includes a plurality of parallel detector rows of detector elements  20  such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan.  
         [0020]     Rotation of gantry  12  and the operation of radiation source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes a radiation controller  28  that provides power and timing signals to radiation 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 radiation 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 .  
         [0021]     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated display system  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 , radiation 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 .  
         [0022]     In one embodiment, computer  36  includes a device  50 , for example, a floppy disk drive or CD-ROM drive, for reading instructions and/or data from a computer-readable medium  52 , such as a floppy disk or CD-ROM. In another embodiment, computer  36  executes instructions stored in firmware (not shown). Generally, a processor in at least one of DAS  32 , reconstructor  34 , and computer  36  shown in  FIG. 2  is programmed to execute the processes 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. In one embodiment, Computer  36  is programmed to perform functions described herein, accordingly, as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits. As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.  
         [0023]      FIG. 3  is a schematic diagram  300  of two exemplary gated cardiac cycles  302  and  304 , associated gated acquisition windows  306  and  308 , and a plurality of imaging locations  311 - 323  along a z axis  325  that are associated with gated acquisition windows  306  and  308 . Image locations  311  through  318  include image data acquired during gated acquisition window  306  of cardiac cycle  302 . Image locations  316  through  323  include image data acquired during gated acquisition window  308  of cardiac cycle  304 . Image locations  316  through  318  include image data acquired during gated acquisition windows  306  and  308 . Images from locations  316  through  318  may be reconstructed from data acquired during cardiac cycle  302  and/or cardiac cycle  304 .  
         [0024]     In the exemplary embodiment, redundant data from multiple heartbeats is used soften the transition between images acquired in different cardiac cycles. Although, such a method does not change temporal resolution as defined by the fraction of the cardiac cycle used for reconstruction, it may cause blurring due to cycle-to-cycle variation in cardiac motion. However, the method does not have the potential loss in resolution associated with methods based on spatial filters.  
         [0025]     In the exemplary embodiment, a cardiac scan is a low pitch acquisition that is retrospectively gated by using data from only a portion of each cardiac cycle. To acquire sufficient data for the necessary slice coverage, data is acquired over multiple heartbeats, with each cardiac cycle providing data for several slices. In the exemplary embodiment, each gated acquisition window  306  and  308  include data for eight images. In an alternative embodiment, each gated acquisition window  306  and  308  include data for other than eight images. Banding artifacts occur when the data from one cardiac cycle does not match the data from the next.  
         [0026]     In the exemplary embodiment, the helical pitch is set so there is some overlap in the slices that can be reconstructed from each heart beat. Gated acquisition window  306  from cardiac cycle  302  includes data for image locations  311  through  318 , and gated acquisition window  308  from cardiac cycle  304  includes data for images locations  316  through  323 . Accordingly, there are two different data sets for image locations  316  though  318 . In this “transition” region  330 , data from one or both of gated acquisition windows  306  and  308  is used to reconstruct an image.  
         [0027]      FIG. 4  is a flow chart  400  of an exemplary reconstruction-based technique of blending of image data for CT cardiac applications that reduces gray scale non-uniformities in the reformatted images.  
         [0028]     In the exemplary embodiment, images in transition area  330  are reconstructed twice, once from each data set corresponding to gated acquisition windows  306  and  308 . Averages or blends of the two reconstructions are then used to produce the final image. Blending details, including the number of blended images per transition region and the type of blending including a selectable choice of weight factors and/or additional filtering, are used to facilitate optimization based on additional information such as scan pitch, acquisition parameters, EKG regularity, and artifact tolerance. In various embodiments of the present invention, if no redundant data is available, processing is modified to increase available data by using a larger fraction of the cardiac cycle (reduce the temporal resolution of the scan), or to skip the blending to maintain temporal resolution with an increased risk of banding artifacts.  
         [0029]     In the exemplary embodiment, a scan data set  402  is received from first gated acquisition window  306 . Corrections are selectively applied  406  to scan data set  402  and the data is view weighted  410 . The view weighted data is then fan to parallel rebinned and filtered  414 . The data is backprojected  418  to generate an image  422  using the data only from gated acquisition window  306 . A scan data set  404  is also received from second gated acquisition window  308 . Corrections are selectively applied  408  to scan data set  404  and the data is view weighted  412 . The view weighted data is then fan to parallel rebinned and filtered  416 . The data is backprojected  420  to generate an image  424  using the data only from gated acquisition window  308 . Image  422  and image  424  are combined using blending  426  with a determined difference threshold that facilitates reducing banding, and a pixel weighting to generate a final image  428 .  
         [0030]      FIG. 5  is a flow chart  500  of an alternative reconstruction-based technique of blending projection data for CT cardiac applications that reduces gray scale non-uniformities in the reformatted images.  
         [0031]     In the exemplary embodiment, a scan data set  502  is received from first gated acquisition window  306 . Scan data corrections  506  are selectively applied to scan data set  502 . A scan data set  504  is also received from second gated acquisition window  308 . Scan data corrections  508  are selectively applied to scan data set  504 . Corrected scan data  507  and  509  are combined using at least one of view and z-axis dependent blending and cardiac segment view weighting  510  to generate a set of blended projection data that is then fan to parallel rebinned  514  and filtered  516 . The data is backprojected  518  to generate a final image  520 .  
         [0032]     In various embodiments of the present invention, adaptive blending is performed using weighting factors that change from pixel to pixel, depending on the difference between the multiple images reconstructed from different cardiac cycles instead of a set of pre-determined weighting functions. For example, at a particular pixel location, two of the pixel values may be approximately equal and a third pixel value may be largely different from the two. To facilitate minimizing the contribution from the third pixel, a weighting function is applied during the blending operation. In an alternative embodiment, the algorithm is implemented by weighting projection data to optimize reconstruction speed rather than implementation flexibility.  
         [0033]     The above-described embodiments of an imaging system provide a cost-effective and reliable means for reducing banding artifacts in images. More specifically, blending image data or projection data that include redundant data for at least a portion of the images facilitates reducing the banding artifacts in the final reconstructed image. As a result, the described embodiments of the present invention facilitate imaging a patient in a cost-effective and reliable manner.  
         [0034]     Exemplary embodiments of imaging system methods and apparatus are described above in detail. The imaging system components illustrated are not limited to the specific embodiments described herein, but rather, components of each imaging system may be utilized independently and separately from other components described herein. For example, the imaging system components described above may also be used in combination with different imaging systems. A technical effect of the various embodiments of the systems and methods described herein include at least one of facilitating imaging a patient with images wherein the banding artifacts have been substantially eliminated.  
         [0035]     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.