Abstract:
A method for reducing artifacts in computed tomographic (CT) images is provided that is particularly useful for CT applications requiring higher gantry rotation rates. The method includes selecting a set of thresholds for projection view data; utilizing a smoothing kernel in accordance with the selected set of thresholds to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; producing a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstructing images of the object utilizing the final projections.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to computed tomographic (CT) imaging methods and apparatus, and more particularly to methods and apparatus for reducing artifacts in computed tomographic imaging systems such as those used for medical imaging. 
     In recent computed tomographic (CT) imaging systems, scan speeds have been increased to sub-second gantry rotations to reduce patient motion, enable new applications, and to increase patient throughput. However, because of x-ray tube power limitations, x-ray currents cannot be kept as constant as at slower scan speeds, often causing image artifacts to be created. These artifacts appear in the images as noise and streaking, which are related to the generally lower signals levels available at higher gantry speeds. Corrections have been applied based upon a single signal threshold level to reduce image noise and streaking artifacts to some extent. However, there remain areas in which signals are slightly higher than the single threshold but for which corrections are still required. In addition, there remain areas in which a greater correction is needed than can be provided with such corrections. As a result, residual streaking artifacts often exist in images processed using this artifact reduction method. 
     SUMMARY OF INVENTION 
     Therefore, in one aspect of the present invention, a method for reducing artifacts in computed tomographic (CT) images is provided that is particularly useful for CT applications requiring higher gantry rotation rates. The method includes selecting a set of thresholds for projection view data; utilizing a smoothing kernel in accordance with the selected set of thresholds to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; producing a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstructing images of the object utilizing the final projections. 
     In another aspect of the present invention, there is provided a method for reducing artifacts in CT images that includes utilizing clinical image studies to select a set of thresholds, including thresholds T 1 , T 2 , and T 3 , for projection data, in accordance with a desired image resolution and noise, utilizing a smoothing kernel in accordance with the selected set of thresholds to produce a set of smooth projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; producing a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstructing at least one image of the object utilizing the final projections. 
     In yet another aspect of the present invention, there is provided a CT imaging system configured to acquire a set of original projections of an object; utilize a smoothing kernel to produce a set of smoothed projections from the set of original projections, wherein an amount of smoothing applied varies depending upon a relation of the original projections to the thresholds; produce a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstruct at least one image of the object utilizing the set of final projections. 
     In still another aspect of the present invention, there is provided a CT imaging system configured to: acquire a set of original projections; utilize a smoothing kernel in accordance with a selected set of thresholds T 1 , T 2 , and T 3  to produce a set of smoothed projections from the set of original projections, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; produce a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstruct images of the object utilizing the set of final projections, wherein the smoothing kernel is a three-point kernel for views that are below T 1  but greater than T 2 , a five-point smoothing kernel for views that are below T 2  but greater than T 3 , and a nine-point kernel for views that are below T 3 . 
     In yet another aspect of the present invention, there is provided a processor for reducing artifacts in scanned images. The processor is configured to: utilize a smoothing kernel in accordance with a selected set of thresholds to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; produce a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstruct an image of the object utilizing the final projections. 
     In still another aspect of the present invention, there is provided a processor for reducing artifacts in scanned images. The processor is configured to: utilize a smoothing kernel in accordance with a selected set of thresholds T 1  , T 2 , and T 3 , to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; producing a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstructing images of the object utilizing the final projections, wherein the smoothing kernel is a 3-point kernel for views that are below T 1  but greater than T 2 , a 5-point kernel for views that are below T 2  but greater than T 3 , and a 9-point kernel for views that are below T 3 . 
     In another aspect of the present invention, there is provided a computer-readable medium having recorded thereon instructions configured to instruct a processor to: utilize a smoothing kernel in accordance with a selected set of thresholds to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; produce a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstruct an image of the object utilizing the final projections. 
     And in yet another aspect of the present invention, there is provided A computer-readable medium having recorded thereon instructions configured to instruct a processor to: utilize a smoothing kernel in accordance with a selected set of thresholds T 1 , T 2 , and T 3 , to produce a set of smoothed projections from a set of original projections obtained from a scan of an object, wherein an amount of smoothing applied varies depending upon a relationship of the original projections to the thresholds; produce a set of final projections utilizing the set of original projections and the set of smoothed projections; and reconstruct images of the object utilizing the final projections, wherein the smoothing kernel is a 3-point kernel for views that are below T 1  but greater than T 2 , a 5-point kernel for views that are below T 2  but greater than T 3 , and a 9-point kernel for views that are below T 3 . 
     The above-described embodiments of the present invention are useful in reducing image noise and streaking compared to previously known artifact reduction methods, while retaining image resolution and sharpness. Thus, embodiments of the invention can be advantageously used in situations in which faster scans and/or lower dose scans are required. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system embodiment. 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG.  1 . 
     FIG. 3 is a flow chart of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In one embodiment and 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. 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 . Detector array  18  may be fabricated in a single slice or multi-slice configuration. In a multi-slice configuration, detector array  18  has a plurality of rows of detector elements  20 , only one of which is shown in FIG.  2 . 
     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  (or another suitable type of display) 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 . 
     To reduce artifacts in a signal-independent manner, image reconstructor  34  in one embodiment of the present invention reconstructs images utilizing a set of threshold values for projection views in scan data. The set of threshold values includes more than one threshold, and, in one embodiment, includes three threshold values. The thresholds are selected  50  based upon a study of clinical images. In one embodiment, one of the selected thresholds is greater and one of the selected thresholds is smaller than a threshold selected for use with the previously-known single-threshold method. This selection of thresholds reduces residual noise and streaking artifacts and provides stronger correction for signals that are extremely low. To minimize adverse effects of smoothing and artifact correction on image resolution and sharpness, different sets of smoothing kernels are associated  52  with the different thresholds. Less smoothing (i.e., shorter kernels) is applied to the highest threshold, and stronger smoothing (longer smoothing kernels) is applied to the smaller threshold. Thus, a smoothing kernel is used in accordance with the selected set of threshold to produce a set of smoothed projections from a set of original projections. The amount of smoothing varies depending upon the relationship of the original projections (i.e., the data comprising the original projections) to the thresholds. 
     In one exemplary embodiment, a set of three thresholds (T 1 , T 2 , and T 3 ) are selected using clinical image studies, in accordance with a compromise between image resolution and noise. 
     A scan  54  of an object  22  is performed utilizing CT system  10  to collect scan data, including projection views. In one embodiment of the present invention, the projection views are processed by image reconstructor  34 , which performs functions  56 ,  58 ,  60 ,  62 , and  64  described below. However, in another embodiment, these functions are performed in a stand-alone processor on scan data collected by imaging system  10 . 
     Smoothing  56  of projection data is performed with a 3-point kernel for views that are below T 1  but greater than T 2 ; with a 5-point kernel for views that are below T 2  but greater than T 3 ; and with a 9-point kernel for views that are below T 3 . Thus, a set of smoothed projections (i.e., projection views) is obtained. 
     Error projections are formed by subtracting  58  the smoothed projection views from the original projection views. 
     The error projections are then multiplied  60  by a signal-dependent smoothing gain to produce smoothed error projections. 
     Final projections are formed by subtracting  62  the smoothed error projections from the corresponding original projections. 
     The final projections are used to reconstruct  64  images suitable for display on cathode ray tube  42  or another display device. Smoothing  56 , subtracting  58 , multiplication  60 , and subtracting  62  are repeated for each of the angular views. The serial loop implementation represented in FIG. 3 may be replaced, as a design choice, by an equivalent implementation having more parallelism. 
     In tests performed on patient scan data, reduced image noise and streaking were observed compared to previously known artifact reduction methods, while image resolution and sharpness was found to be close to that of the images produced by the previously-known methods. Thus, embodiments of the invention will be found useful in situations where faster scans and/or lower dose scans are required. 
     The present invention has been described by reference to a CT imaging system  10 , in which ample computing power resides in a “processor” (e.g., one or more of image reconstructor  34  and computer  36 ) to perform the data computations described herein. However, in another embodiment, the processor resides in a different type of scanning imaging system. In one embodiment, the processor is separate from the imaging system, and inputs projections obtained from a separate scanning imaging system. Thus, scanning an object or patient  22  with an imaging system  10  can produce projection data that can be stored and later used in such a processor. Both CT imaging system embodiments and other processor embodiments can be provided with media readers, such as diskette drives and CD-ROM drives, that read computer-readable media having encoded instructions thereon for performing the methods and processes described. 
     The indefinite articles “a” or “an” preceding an element or step in the description or claims presented herein are intended to refer to one or more of the named elements or steps, unless such meaning is explicitly excluded. In addition, features described in connection with “one embodiment of the present invention,” should not be understood as implying that those features may not be found in other embodiments of the present invention. 
     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.