Abstract:
A method for creating a variable slice thickness for displaying an imaged object is disclosed. The method includes acquiring a plurality of projection images from a plurality of different projection angles within a defined sweep angle, reconstructing a plurality of object images from the plurality of projection images, each object image having a first slice thickness, and applying a function rule to combine images, whole images or portions thereof or attributes thereof, of the plurality of projection images, of the plurality of object images, or of both, thereby providing for the display of the object utilizing a second slice thickness that varies from the first slice thickness.

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to medical imaging, and particularly to the generation of object image slices. 
     Conventional (projection) X-ray imaging does not allow for viewing of detailed cross-sections of tissue structures at a predetermined depth. Tomosynthesis is an advanced application in X-ray radiographic imaging that allows retrospective reconstruction of an arbitrary number of tomographic planes of anatomy from a set of low-dose projection images acquired during a defined translation of an x-ray source, and provides for depth information relating to the projection images. The use of a digital flat panel, which may measure 40 centimeters (cm)×40 cm for example, allows large amounts of data to be collected with each exposure. The depth information carried by these tomographic planes is unavailable in conventional (projection) x-ray imaging. 
     With the introduction of tomosynthesis, it is possible to encode the depth information of the overlapping/underlying anatomical structures with the images. A minimum slice thickness (which is also referred to as a nominal slice thickness) of tomosynthetic image slices is determined primarily by a sweep angle of an x-ray source. Nominal slice thickness is usually defined by the full-width-half-maxima (FWHM) of the slice sensitivity profile (SSP), because the slice orientation is perpendicular to the x-ray detector panel. Although the nominal slice thickness may provide the maximum z-resolving power, thicker slices may provide practical benefit in many clinical settings. 
     Accordingly, the art of tomosynthesis imaging may be advanced by providing a method and system that is capable of generating and managing image slices of variable thickness. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An embodiment of the invention includes a method for creating a variable slice thickness for displaying an imaged object. The method includes acquiring a plurality of projection images from a plurality of different projection angles within a defined sweep angle, reconstructing a plurality of object images from the plurality of projection images, each object image having a first slice thickness, and applying a function rule to combine images, whole images or portions thereof or attributes thereof, of the plurality of projection images, of the plurality of object images, or of both, thereby providing for the display of the object utilizing a second slice thickness that varies from the first slice thickness 
     Another embodiment of the invention includes a user interface for displaying an imaged object, the imaged object having associated therewith a plurality of reconstructed object images each having a first slice thickness, the reconstructed object images having been reconstructed from a plurality of projection images. The user interface includes means for a user to select a function rule or a function rule parameter, means for applying the function rule or function rule parameter to combine images, thereby providing for the display of the object utilizing a second slice thickness that varies from the first slice thickness, and means for displaying a portion of the object at the second slice thickness. The function rule may combine whole images or portions thereof or attributes thereof, of the plurality of projection images, of the plurality of object images, or of both. 
     Another embodiment of the invention includes a system for imaging an object. The system includes an image detector, an imaging source capable of angular movement relative to the object, and a processing device in signal communication with the image detector and the imaging source. The imaging source is disposed to direct imaging radiation toward the image detector. In response to movement of the imaging source, a plurality of projection images from a plurality of different projection angles within a defined sweep angle is acquired at the image detector. The processing device is configured to reconstruct a plurality of object images from the plurality of projection images, each object image having a first slice thickness. The processing device is also configured to apply a function rule to combine images, whole images or portions thereof or attributes thereof, of the plurality of projection images, of the plurality of object images, or of both, thereby providing for display of the object utilizing a second slice thickness that varies from the first slice thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures: 
         FIG. 1  depicts an exemplary block schematic tomosynthesis system in accordance with an embodiment of the invention; 
         FIG. 2  depicts a graph illustrating exemplary slice sensitivity profiles for different x-ray source sweep angles in accordance with embodiments of the invention; 
         FIGS. 3A and 3B  depict exemplary embodiments of two digitized images of a medical data display in accordance with embodiments of the invention; 
         FIG. 4  depicts a graph illustrating an exemplary weighting coefficient function in accordance with embodiments of the invention; 
         FIG. 5  depicts an exemplary embodiment of a method for optimizing the transformation of image slices in accordance with embodiments of the invention; 
         FIG. 6  depicts an exemplary embodiment of a dialog window to input image parameters in accordance with embodiments of the invention; and 
         FIG. 7  depicts an exemplary embodiment of a user interface to input image parameters and simultaneously observe parameter effects on image data in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention provides a radiologist with an interface to take advantage of flexibility to tailor tomosynthesis image slice thickness to best suit the diagnostic requirements of an application. Although a minimum (nominal) image slice thickness may provide maximum resolution within a direction of slice thickness, thicker slices may provide practical benefit in many clinical settings. 
     First, there are a number of clinical applications that favor thicker slices. For example, to diagnose interstitial diseases, the slice thickness of at least 1 cm may be preferred because vessel continuation is much better visualized. Another example is mammography, where the slice thickness of about 1 cm is advantageous to diagnose a presence of clustered micro calcifications. Second, image noise and artifacts are reduced during the forming of thicker slices. This is because of improved data consistency with thicker slices relative to thinner slices. In certain clinical applications, this reduction of image noise and artifacts is more valuable than the loss of local contrast and image sharpness that may accompany thicker image slices. Third, thicker slices may improve radiologist productivity. Clinical feedback has repeatedly emphasized that the large amount of images generated by tomosynthesis may have a significant impact on radiologist productivity and financial considerations. 
     For all of the above reasons, it is advantageous to create images of variable slice thickness via the combination of thin image slices into thicker image slices. The optimum slice thickness is dependent upon the diagnostic application and user preference, including trade-offs between coverage, slice thickness, and artifacts. An embodiment of the invention will allow a user to select the desired slice thickness based on the application and his/her preference. 
     Referring to  FIG. 1 , a schematic of an exemplary embodiment of a tomosynthesis system  50  is depicted. An x-ray source (also herein referred to as an imaging source)  100 , projects an x-ray beam (also herein referred to as imaging radiation)  101 , which is directed through an imaging object  120 , such as a portion of human anatomy for example, toward a panel detector (also herein referred to as an image detector)  150 , which is typically stationary and in an embodiment is two-dimensional. As the x-ray source  100  translates along either a defined arc trajectory  105  or a defined linear trajectory  110  from a first position (depicted by the disposition of x-ray source  100  in  FIG. 1 ) to a second position (depicted by the disposition of x-ray source  100 ′ in  FIG. 1 ), the x-ray beam  101  travels through the imaging object  120 . As the x-ray beam  101  passes through the imaging object  120 , components of varying densities within the imaging object  120  provide for differential x-ray attenuation. An attenuated x-ray Beam  102  is received by the panel detector  150 , which produces an electrical signal responsive to the intensity of the attenuated x-ray beam  102 . 
     A processing device  160  communicates with the x-ray source  100  to provide power and timing signals. The processing device  160  is also in communication with a motor (not depicted in  FIG. 1 ) to drive the translation of the x-ray source  100 , the panel detector  150  to receive the electrical signal data for subsequent processing, a data storage device  156 , an input device  157 , and an output device  169 . The processing device  160  reconstructs the electrical signal data, which represents a plurality of projection images, from the panel detector  150  into a plurality of individual image slices  125  of the imaging object  120 . As used herein, reference in general to image slices will be to one of a group of image slices  121 , as depicted in  FIG. 1 . Each image slice  121  represents a 3-D slice containing depth data, including relative positions and sizes of internal components with varying densities. Each image slice  121  has a depth-of-view in a z-direction  127  defined by a minimum (also herein referred to as a nominal or first) slice thickness  126 , which will be described further below. The processing device  160  stores the image slices  121  in the data storage device  156  and displays the data signals as an image via the output device  169 . In accordance with an exemplary embodiment, the image slices  121  are each individually viewable via a display screen  170  of the output device  169 . 
     As the x-ray source  100  translates through a sweep angle θ from the first position of the x-ray source  100  to the second position of the x-ray source  100 ′, a plurality of radiographic projection images are acquired by the panel detector  150  from a plurality of projection angles within the defined sweep angle θ. The sweep angle θ determines a slice sensitivity profile  130  and the nominal slice thickness  126 . While an embodiment of the invention has been described employing the stationary flat panel detector  150 , it will be appreciated that the scope of the invention is not so limited, and that the invention also applies to tomosynthesis systems  50  utilizing a panel detector which may have alternate shapes, such as a concave profile for example, and may also be capable of movement. 
     Referring to  FIG. 2 , a set of curves, each representing the different slice sensitivity profiles  130  for the corresponding sweep angle θ value of an embodiment of the invention is represented. These curves demonstrate a full width half maxima determination of the nominal slice thickness  126 . The X-Axis represents a number of pixels, and the Y-Axis represents a pixel value. For example, an outermost curve  200  represents the slice sensitivity profile  130  when θ is equal to five degrees. The maximum pixel value is approximately 30,000, therefore the half maxima value is approximately 15,000. Referring to the curve  200 , two points  201 ,  202  along the curve representing a Y-axis pixel value of approximately 15,000 are depicted. The points  201  and  202  correspond to X values of approximately 19 and 43, respectively. Therefore, the minimum (nominal) slice thickness  126  for the embodiment described by  FIG. 2  in response to the sweep angle θ of 5 degrees is approximately 43 minus 19, or 24 pixels. If the pixel spacing of the detector panel  150  is known, the value for minimum slice thickness  126  can be determined. It may be appreciated from the set of curves and the graph legend of  FIG. 2  that as the sweep angle θ increases, the nominal slice thickness  126  decreases. It may also be appreciated that while the minimum (nominal) slice thickness  126  is primarily determined by a physical constraint (sweep angle θ), and may not be reduced further (without increasing the sweep angle θ), there is no such physical constraint upon combining slices  121  to provide a slice  125  with greater thickness in the z-direction  127 . 
     The selection of appropriate slice thickness is dependent upon the application requirements as well as the radiologist preference. Use of the nominal slice thickness  126  may provide the maximum sharpness, contrast, and resolution for the z-dimension  127  within a given image slice  121 . However, use of image slices  125  that are thicker than the nominal slice thickness  126  provide practical benefits. As used herein, image slice  126  is referred to as a nominal slice thickness defined by sweep angle θ, while image slice  125  is referred to as a given slice thickness that may be thicker than the nominal slice thickness  126 . If tomosynthesis is to be used for the detection of breast cancer for example, the objective is to detect the presence of micro calcification clusters. Although detailed evaluation of small objects, such as individual micro calcifications may be enhanced by the increased resolution of the nominal slice thickness  126 , quantification of micro calcifications within the cluster and cluster size determination may be improved with the selection of an increased image slice thickness  125  to enlarge the field of view, thereby surrounding the boundaries of the cluster. In a similar way, the larger field of view provided by a thick image slice  125  enhances diagnosis of interstitial diseases. Thicker imaging slices  125  can allow visualization of the entire vessel including a potential blockage, as distinguished from image slices of nominal thickness  126 , which are only able to visualize a portion of the vessel. 
     Referring now to  FIGS. 3A and 3B , an image is depicted on the left ( 3 A) that has been generated from the nominal thickness image slice  126 , and an image is depicted on the right ( 3 B) that has been generated via transformation of multiple nominal thickness image slices  126  to create one, thicker image slice  125 . Use of the thicker image slice  125  will improve data consistency, resulting in the reduction of image artifacts, ringing, and a higher signal to noise ratio. These benefits may be seen by comparing the image on the left ( FIG. 3A ) to the image on the right ( FIG. 3B ). 
     An additional benefit to the use of thicker image slices  125  generated from multiple image slices of nominal thickness  126  relates to the workflow of the radiologist. For example, if an embodiment of the imaging object  120  has a thickness in the z-direction  127  of 10 cm, and the nominal slice thickness  126  is 1 mm, one hundred image slices having with the nominal (also herein referred to as a first) slice thickness  126  will be generated. Alternatively, if the radiologist chooses to transform the image slice to a second thicker slice thickness  125  of 1 cm, the number of image slices will be reduced from one hundred to ten, allowing the radiologist to review the condition of the imaging object  120  more quickly. 
     A function rule to create a second set of image slices  125  having increased thickness may be represented by the following form: 
                     G   j     =         S   start_index     *     W   start_index       +       ∑     i   =     start_index   +   1         end_index   -   1       ⁢       S   i     *     W   i         +       S   end_index     *     W   end_index                 Equation   ⁢     -     ⁢   1               
where:
 
     Gj (j=0, 1, . . . M) represents the second set of image slices  125  having the second (user-selected) slice thickness, 
     Si (i=0, 1, . . . N) represents a first set of image slices having the first (nominal) slice thickness  126 , 
     start_index, end_index define the first and last slices, respectively of the first set of images at the nominal slice thickness  126 , 
     Wi are weighting coefficients determining the contribution from each nominal slice  126 , 
     Sstart_index represents the first slice from the first set of image slices having the first (nominal) slice thickness  126 , 
     Wstart_index represents the weighting coefficient determining the contribution from the first slice from the first set of image slices having the first (nominal) slice thickness  126 , 
     Send_index represents the last slice from the first set of image slices having the first (nominal) slice thickness  126 , 
     Wend_index represents the weighting coefficient determining the contribution from the first slice from the first set of image slices having the first (nominal) slice thickness  126 , and 
     M and N are integers, and M&lt;N. 
     Referring back to  FIG. 1 , the processing device  160  reconstructs the plurality of projection images into the first set of object image slices  126  at the nominal slice thickness. In response to the radiologist providing a set of parameters  301 ,  306 ,  311 , described further below with reference to  FIG. 6 , via the input device  157 , the processing device  160  applies the aforementioned function rule to transform the first set of image slices  126  into the second set of image slices  125  having the increased slice thickness using the preceding formula, Equation-1. Equation-1 describes a function rule utilizing a weighted summation to transform the first set of nominal thickness image slices  126  to the second set of image slices  125 , having a greater thickness. As used herein, the term weighted or weighting refers to a function rule for adjusting the value of a given variable. The weighting coefficients may be a function of any form. One embodiment of a weighting function is depicted in  FIG. 4 , which provides greater weight (influence) to the central image slices  126  than those at the ends of the first set of image slices  126 . The exemplary weighting function depicted in  FIG. 4  and utilized within Equation-1 is defined such that the weighting coefficients sum to 1.0, wherein W 1 +W 2 + . . . WN=1.0. While an embodiment of the invention has been described employing a linear weighting function possessing a triangular shape to bias the central image slices  126  as depicted in  FIG. 4 , it will be appreciated that the scope of the invention is not so limited, and that the invention also applies to other weighting functions, such as a polynomial or exponential function, with or without a central bias, which may also be applied to projection images, for example. It will be further appreciated that while  FIG. 4  may imply that the weighting function is to be applied over the entire range of projection images or of nominal thickness image slices  126 , the weighting function may also be utilized over a subset of the projection images, or of the nominal thickness image slices  126 . 
     While an embodiment of the invention has been described employing the function rule for slice thickness transformation via the weighted summation of the nominal slice thickness image slices  126 , (within the image domain, following reconstruction of projection images into object images) it will be appreciated that the scope of the invention is not so limited. Additional function rules may utilize other image attributes, such as frequency components, signal strength, pixel value, brightness, or contrast, for example, to transform image slice thicknesses. For example, an alternate function rule may provide image slice  121  thickness transformations via weighted frequency band summation (also within the image domain). With weighted frequency band summation, the first set of image slices  126  are broken into discrete frequency bands, which are then weighted, summed, and transformed into the second set of thicker image slices  125 . Another image slice  121  transformation method is weighted projection summation (within the projection domain), wherein the projection images acquired via the panel detector  150  are weighted, summed, and processed prior to reconstruction into the first set of image slices  126 , for example. 
     It will be appreciated however, that there is a practical limit to the benefits provided by thicker image slices  125 . As greater numbers of image slices  121  are combined, there is increased averaging, or loss, of depth information. For example, if all of the nominal thickness image slices  126  were to be transformed into a single, thick image slice  125 , (assuming a perfect transformation function rule), it would provide no beneficial information beyond a two-dimensional radiographic projection image. 
     Referring now to  FIGS. 5 and 6  collectively, an embodiment of a method  299  to optimize the selection of image slice  121  thickness by the radiologist is depicted. Block  300  represents selection of a range of a volume of interest (also herein referred to as a volume range)  301 . The volume range  301  describes the dimension of the volume of interest within the imaging object  120  in the z-direction  127 , and is established by selecting a start height and an end height (see  FIG. 6 ) of the volume of interest. Block  305  represents selection of an image slice thickness  306 , and block  310  represents selection of an image slice spacing (overlap)  311 . Slice thickness  306  defines the dimension in the z-direction  127 , as selected by the radiologist, of the image slice  125  with the second, greater thickness. Image slice spacing (overlap)  311  describes how much image information each image slice  125  having the second, user-selected thickness will share with the adjacent image slices  121 . Block  315  represents transformation of the first set of nominal thickness image slices  126  to the second set of thicker imaging slices  125  by the processing system  160 . Block  320  represents determination by the radiologist whether the second set of thicker image slices  125  fulfills the diagnostic objectives. If the second set of thicker image slices  125  does not fulfill the diagnostic objectives of the radiologist, the method  299  is repeated until the radiologist is satisfied with the results of the second set of thicker image slices  125 . 
     The volume range selection  300 , slice thickness selection  305 , and slice spacing (overlap) selection  310  may be may accomplished via direct input of the image parameters  301 ,  306 ,  311  into an ASCII or binary computer configuration file. However, the editing of such files required by repeated iterations of changes may become time consuming.  FIG. 6  depicts an embodiment of a dialog window  419  to allow the radiologist to input the parameters  301 ,  306 ,  311 . The volume range  301  may be input via a pair of dialog boxes  420 ,  421 , the slice spacing (overlap)  311  via a dialog box  422 , and the slice thickness  306  via a drop-down box  423  by the radiologist. Note that subsequent to the input of the parameters  301 ,  306 ,  311  via the dialog window  419 , the radiologist must exit the dialog window  419  to view and evaluate the effects of these parameters  301 ,  306 ,  311 . A significant amount of time may be required to switch between the dialog window  419  and the image if multiple iterative loops of the parameter selection  300 ,  305 ,  310  are necessary to obtain a satisfactory image for diagnostic purposes. 
     Referring now to  FIG. 7 , an embodiment of a user interface for the input of the image parameters  301  (via dialog boxes  420 ,  421 ),  306 , and  311  (via dialog box  422 ) is depicted. In the embodiment depicted, the display screen  170  has been divided into three zones. A first input area  400 , a second input area  405 , and an image viewing area  410  are depicted. The viewing area  410  is configured to display the image slices  121  at either the first (nominal)  126  or second (transformed) thickness  125 . The first input area  400  may provide the radiologist access to a variety of image viewing and analysis tools, which will be well known to one skilled in the art. In the embodiment depicted, the first input area  400  also allows the radiologist to input the volume range  301  and slice spacing (overlap)  311  function rule parameters via the dialog boxes  420 ,  421 ,  422 . A button  425  within the first input area  400  applies the function rule and parameters  301 ,  306 ,  311  to create and display an image slice  121  from the second set of image slices  125  within the image viewing area  410 . The second input area  405  contains a first arrow  430 , a second arrow  440 , and a set of tick marks  435  arranged proximate to a slider bar  445  to represent the image slice thickness  306  and/or volume range  301 . The radiologist may utilize the input device  157  to position the first arrow  430  and the second arrow  440  to represent the desired slice thickness  306  and/or volume range  301  function rule parameters. 
     In the embodiment of a user interface depicted in  FIG. 7 , it may be seen that nine tick marks  435  are depicted between the arrows  430 ,  440  (inclusive). This may be interpreted to indicate that the image displayed within the image viewing area  410  represents a transformed, thicker image slice  125 , which has been created from eight image slices of nominal slice thickness  126 . The image viewing area  410  allows the radiologist to view the effects of parameter  301 ,  306 ,  311  changes without the need to close or open any additional dialog windows  419 . The image resulting from the parameters  301 ,  306 ,  311  selected by the radiologist may be reviewed in the image viewing area  410  to determine if the result is acceptable. If it is not acceptable, one of the parameters  301 ,  306 ,  311  may be changed, and the effect simultaneously observed in the image viewing area  410 . By incorporating the parameter selection  300 ,  305 , and  310 , and the display image, within the same user interface of the display screen  170 , the amount of discrete steps (and therefore, time) to determine the appropriate slice thickness for a specific diagnostic application may be reduced. While an embodiment has been described depicting the image viewing area  410  disposed between the first input area  400  and the second input area  405 , it will it will be appreciated that the scope of the invention is not so limited, and that the invention also applies to other arrangements of the display screen  170 , such as having both the first input area  400  and the second input area  405  combined into one input area located above, below, to the left of, or, to the right of the image viewing area  410 , for example. 
     As disclosed, some embodiments of the invention may include some of the following advantages: the ability to modify image slice thickness to suit radiologist preference and the diagnostic needs of the application; the ability to reduce radiologist workflow by minimizing the total number of images for review; the ability to enhance image quality by reducing ringing, image artifacts, and increasing the signal to noise ratio; and, the ability to observe effects of slice thickness modification in a single user interface without switching between different windows. 
     An embodiment of the invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention may also be embodied in the form of a computer program product having computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, USB (universal serial bus) drives, or any other computer readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. A technical effect of the executable instructions is to provide for the display of an object utilizing a second image slice thickness that varies from a first, original slice thickness, the object having been imaged via X-ray tomography. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.