Abstract:
A method for reconstructing an image from imaging data using an imaging system configured to accept input from an imaging system operator. The method includes steps of generating a first model from the imaging data; accepting as input an operator-specified region of interest based on the first model; and generating a second model from the imaging data based on the specified region of interest. This method simplifies entry of parameters for retrospective image reconstruction by using the operator-entered region of interest as input for automated parameter selection.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to imaging systems and, more particularly, to methods and apparatus for retrospectively reconstructing an image using a region of interest specified by an imaging system operator. 
     Imaging systems include a source that emits signals (including but not limited to x-ray, radio frequency, or sonar signals), and the signals are directed toward an object to be imaged. The emitted signals and the interposed object interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the object. 
     For example, in computed tomography (CT) imaging, 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. 
     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 while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one-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. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast. 
     It is known to use an imaging system, e.g. a CT imaging system, to retrospectively reconstruct images, e.g. axial images, from raw scan data. Retrospectively reconstructed images can differ in various respects from original scan images. For example, axial slices can be reconstructed closer together or farther apart than those of the original scan, and the scan field of view can be reduced to concentrate reconstruction within a smaller scan region in order to resolve more detail. 
     When prescribing retrospective image reconstruction, an operator enters a series of parameters into the imaging system in order to specify image reconstruction characteristics such as a new field of view. Having to determine and enter sequences of parameters, however, can increase chance for error. 
     It would be desirable to allow an imaging system operator to select parameters specifying retrospective image reconstruction without having to determine and enter a lengthy parameter sequence. It also would be desirable to provide the operator with a way to use a selection of a region of interest within a scan to specify retrospective image reconstruction. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment, a method for reconstructing an image from imaging data using an imaging system configured to accept input from an imaging system operator, the method including the steps of generating a first model from the imaging data; accepting as input an operator-specified region of interest based on the first model; and generating a second model from the imaging data based on the specified region of interest. 
     The above-described method simplifies entry of parameters for retrospective image reconstruction by using the operator-entered region of interest as input for automated parameter selection. 
    
    
     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; and 
     FIG. 3 is a flow diagram of an embodiment of a method for selecting retrospective reconstruction parameters. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 , for example an x-ray tube, 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  that 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, or origin,  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 . Control mechanism  26  also includes 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  that stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator (not shown) 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  that controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  along a z-axis through gantry opening  48 . 
     Referring to FIG. 3, a method for retrospectively reconstructing an image in one embodiment includes generating  100  a first three-dimensional model (not shown) from a set of imaging data. The first model is displayed on, for example, cathode ray tube display  42  and typically is reconstructed using original scan parameters. Generated as part of the first three-dimensional model are data spacing parameters SP xy  and SP z , further described below and determined by original image size, slice thickness, and slice spacing. Parameters SP xy  and SP z  are stored in system  10  as part of the first three-dimensional model. 
     An imaging system  10  operator then selects a region of interest (ROI) (not shown) within the first model. Specifically, the operator inputs  102  to imaging system  10 , e.g. via console  40 , beginning and ending points of each dimension of the ROI, expressed in the first model coordinate system. Thus the operator enters, for example, a beginning point S xbegin , and an ending point S xend  of the ROI in an x-direction of the first model, a beginning point S ybegin  and an ending point S yend  of the ROI in a y-direction of the first model, and a beginning point S zbegin  and an ending point S zend  of the ROI in a z-direction of the first model. The operator then requests  104  via console  40  that ROI reconstruction begin. 
     Computer  36  accepts  106  the operator-specified ROI parameters and uses them to determine  108  a new display field of view (DFOV) for the retrospective reconstruction. More specifically, a new DFOV is determined using a relationship written as: 
     
       
           DFOV=MAX (| S   xbegin   −S   xend   |,|S   ybegin   −S   yend |)* SP   xy   
       
     
     where data spacing parameter SP xy  represents the first model imaging data spacing in the x- and y-directions. 
     Computer  36  then determines  110  starting and ending points for image reconstruction in a z-axis direction. More specifically, a starting slice Z begin  and an ending slice Z end  for reconstruction are determined using relationships written as: 
     
       
           Z   begin   =INT ( S   zbegin   *SP   Z ) 
       
     
     
       
           Z   end   =INT ( S   zend   *SP   z ) 
       
     
     where data spacing parameter SP z  represents the first model imaging data spacing in the z-directions. 
     An x-offset O x  and a y-offset O y  then are determined  112  with respect to a reconstruction origin, e.g. center of rotation  24 , for the retrospective reconstruction using relationships written as: 
     
       
         
           O 
           x 
           =S 
           xbegin 
           *SP 
           xy 
         
       
     
     
       
         
           O 
           y 
           =S 
           ybegin 
           *SP 
           xy 
         
       
     
     where SP xy  represents the first model imaging data spacing in the x- and y-directions. 
     Reducing the DFOV for a retrospective reconstruction increases the aspect ratio AR of data spacing in the x-y plane to data spacing along the z-axis. Increasing AR requires additional slices to be interpolated in the z-directions to reconstruct a three-dimensional model, which can result in imaging artifacts. Therefore in one embodiment the operator of system  10 , using console  40 , selects  114  an option to specify an AR of data spacing in the x-y plane (i.e. SP xy ) to data spacing along the z-axis (i.e. SP z ). More specifically, the operator specifies a constant AR, or chooses to limit the AR to a maximum value. Overlapping reconstructions then can be prescribed to generate a number of slices appropriate for minimizing artifacts. 
     Computer  36  accepts  116  the operator-specified AR and uses it to determine  118  a new slice thickness in the z-directions. More specifically, a new slice thickness T z  is determined using a relationship written as: 
     
       
           T   z =( DFOV/SL   res )/ AR   
       
     
     where SL res  is a resolution of a needed slice, AR is the operatorspecified aspect ratio, and DFOV is the new display field of view. Slice resolution SL res  is a parameter stored in imaging system  10 , or, in one embodiment, is another input specified by the operator via console  40 . 
     Computer  36  then determines  120  a number of needed slices based on the desired AR. More specifically, a number of needed slices N slice  is determined using a relationship written as: 
     
       
           N   slice (( Z   begin   −Z   end )* SP   z )/ T   z   
       
     
     where T z  represents the new slice thickness. 
     Computer  36  then passes the above-described parameters to image reconstructor  34 , which generates  122  a second model (not shown) from the imaging data based on the operator-specified region of interest. More specifically, image reconstructor  34  generates a new series of axial slices (not shown) which are used to form another three-dimensional volume concentrating on the ROI specified by the operator based on the first model. 
     The above-described method increases automation of reconstruction parameter entry and thus allows an imaging system operator to more easily prescribe retrospective reconstruction. Furthermore, the above-described method can be used for any three-dimensional model representation technique including, but not limited to, volume rendering, maximum intensity projection, and surface rendering. 
     It also should be understood that, although the above method is described herein with respect to a CT imaging system, the invention can be practiced in connection with other types of 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.