Patent Publication Number: US-8994747-B2

Title: Superimposed display of image contours

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Patent Application No. 60/986,647 titled “Superimposed Display of Image Contours,” which was filed on Nov. 9, 2007, and the contents of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The description generally relates to image processing and display and more specifically to superimposed displays of image contours. 
     BACKGROUND 
     Digital images often contain objects of interest and can be displayed at various resolutions and at various points in time. In addition, digital images often contain noise or less relevant features in various amounts depending on the device used to collect the data associated with the digital image and the level of resolution. In particular, obtaining a digital image using inferential techniques such as radiographic, echographic, or ultrasonic techniques can be especially susceptible to blurring between the object of interest and noise (or less relevant features) due to, e.g., electromagnetic effects. In some applications, an ultrasound technician captures an image using an image capture device that captures still images or sequences of still image frames (e.g., video). The ultrasound technician can annotate the captured image using an input device to mark, for example, landmarks within the image, distances between points in the image, or other features. 
     In certain circumstances, useful information can be obtained by comparing an image of a moving object captured at one time t 1  with an image of the object captured at a different, e.g., later, time t 2 . For example, the effectiveness of a treatment regimen can be monitored by comparing an ultrasound image of the transgastric short axis view (TGSAV) of a human heart under observation at time t 1  with an ultrasound image of the TGSAV of the same heart at time t 2 . Typically, comparisons between such ultrasound images are facilitated by displaying an image and objects therein at time t 1  alongside a second image at time t 2  on a single display monitor (e.g., the images are displayed side-by-side). 
     One drawback to displaying ultrasound images side-by-side is that differences between the images or objects therein can be obscured due to noise or presence of less relevant features in the images. Additionally, side-by-side comparison does not readily facilitate comparison between objects present in each image when the image capture device or the objects may have moved between time t 1  and time t 2  or when the objects appear out of scale between the images. Moreover, visually comparing two objects from different images in different fields of view can be time consuming for the ultrasound technician or medical or healthcare personnel (e.g., a doctor) seeking to visually assess the efficacy of a course of treatment. 
     SUMMARY 
     The concepts described herein address the drawbacks associated with presenting two images side-by-side by displaying information extracted about the objects of interest from each of the images at substantially the same time in a field of view free from noise or less relevant features in the images. For example, where the object is human heart, and the images are ultrasound images, a contour of the heart is determined and extracted from each image, thereby producing two contours. These two contours are displayed, centered about a common value, in a different field of view from either of the two images. Superimposing the contours of objects from the image in a common field of view allows differences between the contours (and thus, the objects the contours represent) to be assessed visually and relatively quickly. 
     In general, in one aspect, the invention relates to a computer-implemented method. The method involves calculating a first centroid or center-of-mass of a first contour that is extracted from a first image that depicts a moving object as the object appeared at a first time. The method also involves calculating a second centroid or center-of-mass of a second contour that is extracted from a second image that depicts the object as the object appeared at a second time. The second image depicts the image in substantially the same plane as the first image. The method also involves displaying the first and second contours with the respective first and second centroids or centers-of-mass positioned at a common location. 
     In some embodiments, the first and second contours are extracted automatically by a contour-finding algorithm operating on the respective first and second images. The first and second images can be ultrasound images. In some embodiments, the moving object is a chamber of a human heart (e.g., a ventricle or ventricle wall), and the ultrasound images can be transgastric short axis views of the chamber. 
     In some embodiments, the method further involves associating a first identifying feature with the first contour and associating a second identifying feature with the second contour. The identifying features can be, for example, colors, line thickness, or types of lines. Some embodiments of the invention feature the first and second contours being associated with a left ventricle end diastolic area (LVEDA) or a left ventricle end systolic area (LVESA) of a human heart. The first and second contours, in some implementations, are displayed over a background pattern that is different from a first or second background pattern of the respective first or second images. An example of a background pattern is a solid color (e.g., black). 
     Some implementations involve a method where displaying the first and second images involves determining a value for the common location, transforming a first set of data representative of the first contour using the value of the common location, and illustrating the first contour based on the first set of transformed data. Similarly, a second set of data representative of the second contour is transformed using the value of the common location, and the second contour is illustrated based on the value of the transformed data. Illustrating the first and second contours can involve processing respective first and second cineloops to display the contours within a user interface window. In some embodiments, the method also involves displaying the first and second images in a first display window and displaying the first and second contours (superimposed) in a second display window. 
     In another aspect, the invention relates to a system. The system includes a computing apparatus that includes a processor, a memory, and an image capturing module. The system also includes a calculation module to calculate a first centroid or center-of-mass of a first contour extracted from a first image that depicts a moving object as the object appeared at a first time and to calculate a second centroid or center-of-mass of a second contour extracted from a second image that depicts the object as the object appeared at a second time. The second image depicts the object in substantially the same plane as the first image. The system also includes a first display window in communication with the processor to display images from the image capturing module and a second display window in communication with the processor and the calculation module to display the first and second contours of the object with the respective first and second centroids or centers-of-mass positioned at a common location. 
     In some embodiments, the system includes a transesophageal echocardiography probe (e.g., an imaging probe) in communication with the processor. The system can also include a transformation module to transform the first and second contours based on the value of the common location. In the first display window, the first and second images can be displayed side-by-side. Some implementations feature the first and second contour displayed superimposed in the second display window. The second display window can be superimposed over the first display window. The second display window can also be displayed in response to a command by or issued by a user. Such a command is available in response to a first cineloop and a second cineloop being populated with a respective first set of data associated with the first contour and a second set of data associated with the second contour. 
     In still another aspect, the invention relates to a computer program product, tangibly embodied in an information carrier. The computer program product includes instructions operable to cause data processing apparatus to calculate a first centroid or center-of-mass of a first contour extracted from a first image that depicts a moving object as the object appeared at a first time and to calculate a second centroid or center-of-mass of a second contour extracted from a second image that depicts the object as the object appeared at a second time. The second image depicts the object in substantially the same plane as the first image. The computer program product also includes instructions operable to cause data processing apparatus to display the first and second contours of the object with the respective first and second centroids or centers-of-mass positioned at a common location. 
     In some implementations, any of the above aspects can include all of the above features and/or advantages. These and other features will be more fully understood by reference to the following description and drawings, which are illustrative and not necessarily to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a block diagram of a system that embodies the invention. 
         FIG. 2  is an exemplary display window illustrating images and superimposed contours of an object shown in the images. 
         FIG. 3  is an enlarged view of the display window of  FIG. 2 , illustrating superimposed contours of the object. 
         FIG. 4A  is a flow chart illustrating an exemplary process by which the display window of  FIG. 3  is displayed. 
         FIG. 4B  is an exemplary user interface of a prompt that embodies and illustrates the process of  FIG. 4A . 
         FIG. 5  is a flow chart illustrating an exemplary method for displaying superimposed image contours. 
         FIG. 6  is a flow chart illustrating a process for displaying image contours with a common centroid or center-of-mass. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a block diagram of a system  100  that embodies the invention. The system  100  includes a computing apparatus or computer  105  that is in communication with a display  110 . The computer  105  includes a processor  115 , a memory  120 , and an image capturing module  125 . The image capturing module  125  is in communication with the memory  120  and the display  110  for storing and displaying images (and data associated therewith), respectively. 
     The system includes an imaging probe  130  in communication with the processor  115 . The imaging probe  130  can be an ultrasound probe. In some embodiments, the imaging probe is a transesophageal echocardiography probe. An example of a suitable transesophageal echocardiography probe is disclosed in U.S. patent application Ser. Nos. 10/997,059 and 10/996,816, each titled “Transesophageal Ultrasound Using a Narrow Probe” by Roth et al., filed Nov. 24, 2004, the entire contents of which are hereby incorporated herein by reference. 
     The system  100  includes an input device  135  in communication with the processor  115  for communicating information to the processor  115 . For example, the input device  135  can include a mouse or pointer to identify and mark information displayed on the display  110 , and the information can be used for further processing, as discussed in more detail below. The input device  135  can also control the processor  115 , memory  120 , image capturing module  125 , and/or the display  110  by facilitating a user issuing commands the processor  115  carries out. 
     The image capturing module  125  collects and processes image data from the imaging probe  130  via the processor  115 . For example, the image capturing module  125  captures still images and/or video images (e.g., sequences of still images at a particular shutter speed). The processor  115  can store information captured by the image capturing module  125  in the memory  120 , e.g., as a data file, an image file, a cineloop file, or other type of suitable data structure. A cineloop file is a file containing multiple images, stored digitally as a sequence of individual frames. In some embodiments, the number of images or frames in a cineloop is pre-determined (e.g.,  150  frames), but this is not required. 
     The processor  115  can include sub-modules or sub-routines for carrying out, implementing, or performing various operations and tasks. As illustrated, the processor  115  includes a calculation module  140  and a transformation module  145  in communication with the calculation module  140 . The processor  115  utilizes the calculation module  140  and the transformation module  145  to facilitate display of image contours on the display  110 , as discussed in more detail below. 
     The display  110  displays different types of information about the system  100 . For example, the display  110  displays image data captured from the imaging probe  130  by the image capturing module  125 . The display  110  also displays information calculated or determined by the processor  115 , the calculation module  140 , and/or the transformation module  145 . The display  110  can also display information from the input device  135 . In some embodiments, the display  110  displays a cursor, landmarks, and/or menus called or selected by the input device  135 . In some embodiments, the display  110  features a user interface, e.g., a Windows-based user interface. Those of skill will appreciate other user interfaces suitable for carrying out the principles described herein. 
       FIG. 2  is an exemplary display window  200  illustrating image data and superimposed contours of an object within the images. In some embodiments, the window  200  is displayed, for example, on the display  110  of the system  100  of  FIG. 1 . The window  200  includes a first area  204   a  and a second area  204   b . The first area  204   a  and the second area  204   b  are displayed side-by-side in the window  200 . The first area  204   a  includes a first image  208   a , and the second area  204   b  includes a second image  208   b.    
     As illustrated, the first image  208   a  depicts a moving object (e.g., a patient&#39;s human heart) as the object appeared at a first time t 1 . The first area  204   a  displays the time t 1  within the region  212   a  of the first area  204   a . The time t 1  at which the first image  208   a  depicts the human heart is labeled “11:42:53 AM” on “Nov. 5, 2007” as shown in the first region  212   a . The second image  208   b  depicts the moving object (e.g., the same human heart depicted in the first image  208   a ) as the object appeared at a second time t 2 . The second area  204   b  displays the time t 2  within the region  212   b  of the second area  204   b . The time t 2  at which the second image  208   b  depicts the human heart is labeled “03:14:05 PM” on “Nov. 6, 2007” as shown in the second region  212   b . Though  FIG. 2  is described with reference to a human heart, one of skill will appreciate that the concepts described herein apply to other types of images or images of other objects as well. 
     The first image  208   a  and the second image  208   b  represent a projection of the object in substantially the same plane, which allows, for example, meaningful comparison between the images. In some embodiments, an imaging probe (e.g., the imaging probe  130  of  FIG. 1 ) emits a fan- or conical-shaped beam (not shown) such that the plane of the beam defines the image plane of the images  208   a - 208   b . Where the imaging probe is aimed upwardly through the left ventricle of the human heart (e.g., by positioning the imaging probe in the fundus of a human stomach), the imaging probe can provide a trans-gastric short axis view of the heart represented in the images  208   a - 208   b . The trans-gastric short axis view of the left ventricle is useful for monitoring the heart&#39;s operation because the view allows medical personnel to directly visualize the left ventricle, which is the main pumping chamber of the heart. Positioning the imaging probe in other locations, as would be apparent to one of skill, allows visualization of the majority of the relevant cardiac anatomy (e.g., other chambers of the heart, blood vessels, and chest cavity). The positioning of the imaging probe does not affect the operation of the principles herein since the images  208   a - 208   b  depict the object projected onto substantially the same image plane. 
     A comparison between the images of the human heart at time t 1  and time t 2  can provide valuable information, for example, to a healthcare professional such as a doctor. For example, the comparative information can be used to assess the efficacy or effectiveness of a course of treatment and/or to monitor the heart&#39;s reaction to administration of treatment. However, the first image  208   a  and the second image  208   b  include less relevant features (or even noise) associated with ultrasonic imaging and/or electromagnetic phenomena. 
     One way to effectively compare the first image  208   a  of the patient&#39;s heart with the second image of the patient&#39;s heart  208   b  is to determine a contour of the heart appearing in the first and second images  208   a - 208   b . The first image  208   a  includes a first contour  216   a  that approximates the outline or contour of the patient&#39;s heart (or chamber or portion thereof), as the heart appeared at the time t 1  the first image  208   a  was captured. The first contour  216   a  includes a set of interconnected points  220   a  positioned relative to the first image  208   a . The second image  208   b  includes a second contour  216   b  that approximates the outline or contour of the patient&#39;s heart, as the heart appeared at the time t 2  the second image was captured. Like the first contour  216   a , the second contour  216   b  includes a set of interconnected points  220   b  positioned relative to the second image  208   b.    
     The first set of points  220   a  is used to create or draw the first contour  216   a , e.g., by connecting the points  220   a  to form a first closed polygon representative of the outline of the patient&#39;s heart in the first image  208   a , at time t 1 . The first closed polygon has N 1  vertices, each vertex representative of one of the points  220   a . Similarly, the second set of points  220   b  is used to create or draw the second contour  216   b , e.g., by connecting the points  220   b  to form a second closed polygon representative of the outline of the patient&#39;s heart in the second image  208 , at time t 2 . The second closed polygon has N 2  vertices, each vertex representative of one of the points  220   b . In some embodiments, the number of points  220   a  used to extract the first contour  216   a  (e.g., N 1 ) is the same as the number of points  220   b  used to extract the second contour  216   b  (N 2 ), but this is not required. 
     In some embodiments, the location (and thus coordinate values) of the first and second sets of points  220   a - 220   b  are determined by a user. For example, an ultrasound technician or healthcare provider can use the input device  135  of the system  100  of  FIG. 1  to identify the first and second sets of points  220   a - 220   b  (e.g., by “clicking” on the image to position the points). In some embodiments, the ultrasound technician manually connects the points  220   a - 220   b  within the respective first and second sets, e.g., using the input device  135  to draw lines between points or by allowing the user to drag a point to its location on the image. Alternatively, an algorithm can connect the points  220   a - 220   b  within the respective first and second sets, e.g., using a connection algorithm that draws lines (e.g., straight-line segments) between adjacent points until the algorithm generates a closed polygon where the first vertex (x 1 , y 1 ) is also the Nth vertex (x N , y N ). 
     In some embodiments, the need for an ultrasound technician or healthcare professional to determine of the sets of points  220   a - 220   b  that generates the first and second contours  216   a - 216   b  is eliminated by using an automated contour-finding algorithm operating on the respective first and second images  208   a - 208   b . For example, an edge or feature detector can be applied to the first and second images  208   a - 208   b  determine sets of the respective contours  216   a - 216   b , either before or after the first or second image  208   a - 208   b  is filtered or sub-sampled if such pre-processing steps occur. The algorithm can operate either by locating the set of points  220   a - 220   b  within the respective images  208   a - 208   b  and drawing the respective contours  216   a - 216   b  from the points  220   a - 220   b , or, alternatively, by drawing the respective contours  216   a - 216   b  and locating/selecting points along the contours  216   a - 216   b  to populate the respective sets  220   a - 220   b . A combination of these algorithms is also contemplated. 
     The sets of points  220   a - 220   b  are, in some embodiments, data structures that can be stored (e.g., in the memory  120  of the system  100  of  FIG. 1 ). In some embodiments, the sets of points  220   a - 220   b  are associated with the respective images  208   a - 208   b , e.g., as cineloop data. In some embodiments, a data extraction algorithm generates a separate data structure representative of the first and second contours  216   a - 216   b  based on the sets of points  220   a - 220   b , and the data structure representative of the first and second contours  216   a - 216   b  is stored (e.g., rather than points  220   a - 220   b  themselves). According to any of the described examples, the first and second contours  216   a - 216   b  are extracted from the respective first and second images  208   a - 208   b  for subsequent processing. 
     After the contours  216   a - 216   b  have been extracted from the respective images  208   a - 208   b , the centroid or center-of-mass (not shown) for each contour is calculated, as discussed in more detail below. The window  200  includes a second window  230 . The second window  230  displays the first contour  216   a  and the second contour  216   b  with the respective centroids or centers-of-mass positioned/aligned at the common location. The second window  230  is discussed in more detail below with respect to  FIG. 3 . The second window  230  is superimposed over the first and second images  208   a - 208   b  in the window  200 . 
       FIG. 3  is an enlarged view of the display window  230  of  FIG. 2 , illustrating superimposed contours  216   a - 216   b  of the object. The window  230  includes a background  234  on which the contours  216   a - 216   b  are displayed. As illustrated, the background  234  is a solid color, e.g., black. Presenting the background  234  as a solid color provides greater contrast with the contours  216   a - 216   b  than the background (not shown) of the respective images  208   a - 208   b . The greater contrast is due, in part, to the absence in the background  234  of the less relevant features present in the images  208   a - 208   b . After the contours  216   a - 216   b  have been extracted from the respective images  208   a - 208   b , the centroid or center-of-mass (not shown) of each contour  216   a - 216   b  is determined, as discussed in more detail with respect to  FIGS. 5-6 . 
     Within the window  230 , the centroid or center-of-mass of the first contour  216   a  is positioned at a common location, illustrated as  238 . The centroid or center-of-mass of the second contour  216   b  is also positioned at the common location  238 . Aligning the centroids or centers-of-mass of the first and second contours  216   a - 216   b  allows the contours  216   a - 216   b  to be displayed superimposed relative to each other in the window  230 . Superimposing the contours  216   a - 216   b  using the same location for the centroid or center-of-mass for each contour  216   a - 216   b  allows for quick visual comparison between the contours  216   a - 216   b , and correspondingly, between the images  208   a - 208   b  of the moving object as it appeared at time t 1  and time t 2 . 
     In some embodiments, the first contour  216   a  and/or the second contour  216   b  are transformed to a common scale. Examples of transformations include scale transformations, aspect transformations, skew transformations, translation transformations, rotational transformations, or any combination thereof of the contours  216   a - 216   b . Some of these transformations fall into a category of transformations known as affine transformations. 
     The window  230  features a description of the view or plane represented by the contours  216   a - 216   b . For example, as illustrated, the contours  216   a - 216   b  represent an end diastole transgastric short axis view (“TGSAV”) of the left ventricle (or ventricular wall) of a moving heart, as indicated by the words “End Diastole”  242  in the window  230 . The window  230  also includes an identifying characteristic or feature  246   a  associated with the first contour  216   a . The feature  246   a  in window  230   a  indicates “Left” and the text is represented as a particular color that corresponds to a line color with which the first contour  216   a  is displayed. The “Left” designation refers to the relative position of the image  208   a  from which the contour  216   a  was extracted. Specifically, “Left” indicates that the contour  216   a  represents the image  208   a  in the first area  204   a  of the window  200  of  FIG. 2  (i.e., the image on the left-side of the side-by-side display). 
     Similarly, the window includes an identifying characteristic or feature  246   b  associated with the second contour  216   b . The feature  246   b  in window  230  indicates “Right” and a particular color (different from the color associated with feature  246   a ) that corresponds to a line color with which the second contour  216   b  is displayed in the window  230 . The “Right” designation refers to the relative position of the image  208   b  from which the contour  216   b  was extracted; specifically, “Right” indicates that the contour  216   b  represents the image  208   b  in the second area  204   b  of the window  200  in  FIG. 2 . 
     In some embodiments, the features  246   a - 246   b  are not line color, but some other characteristic for visually distinguishing the respective contours  216   a - 216   b  and/or indicating a correspondence between the contour  216   a - 216   b  and the underlying image  208   a - 208   b  from which the respective contour  216   a - 216   b  was extracted. Examples of suitable identifying characteristics include line-type (e.g., dotted or dashed, of varying lengths and frequency) or line thickness. In another implementation, the contours  216   a - 216   b  can be distinguished using labeled arrows (not shown) pointing to the contour  216   a  or  216   b  that corresponds to the respective image  208   a - 208   b.    
     The window  230  also includes a first button  250   a  and a second button  250   b . The first button  250   a  is labeled “ED,” which is representative of “End Diastole.” The second button  250   b  is labeled “ES,” which is representative of “End Systole.” A user (not shown) can toggle between the end diastole view and the end systole view of the heart by selecting the first button  250   a  or the second button  250   b , respectively. In some embodiments, if sufficient data does not exist to populate the window  230  for a particular view (e.g., the end systole view), the respective button (e.g., the second button  250   b ) will not be available for user selection. The availability of the first or second button  250   a - 250   b  for user selection indicates sufficient data exists to superimpose the contours  216   a - 216   b  of the end diastole view or the contours (not shown) of the end systole view in the window. 
     In some embodiments, the first and second contours  216   a - 216   b  are extracted from a sequence or series of images, rather than from a single image  208   a - 208   b . Accordingly, the first and second contours  216   a - 216   b  can change shape and evolve over time, appearing as an animation. For each such evolution or change to the contours  216   a - 216   b , the centroid or center-of-mass is calculated and positioned at the common location  238 . In such an embodiment, the first and second contours  216   a - 216   b  will give the appearance of moving (e.g., via animation or time-evolution) in the window  230 , while the centroid or center-of-mass of each moving centroid appears at the same location  238  in the window. Such embodiments facilitate comparison between moving images of the object and permit assessment of the time evolution of the object, e.g., to visually determine a rate of volumetric change in the ventricle of a heart over a period of time T 1  relative to a second period of time T 2 . 
       FIG. 4A  is a flow chart  400  illustrating an exemplary process by which the display window  230  of  FIG. 3  is displayed. In general, the window  230  of  FIG. 3  is displayed in response to a user command or directive via, e.g., a user menu or prompt. The window  230  is populated with contours (e.g., contours  216   a - 216   b ) extracted from images (e.g., images  208   a - 208   b ). To illustrate the contours  216   a - 216   b  in the window, sufficient data about the extracted contour  216   a - 216   b  is used to control the display (e.g., data associated with the contour is accessed by a display algorithm to illustrate the contour on the background  234  in the window  230 ). 
     At step  405 , the process queries and determines whether image data and contour data exists for a first image and contour, for example, as the currently displayed cineloop or as a previously-displayed and stored cineloop. If image and/or contour data for the first image and/or contour does not exist, the prompt for displaying the window  230  is unavailable (step  410 ). If, on the other hand, the first image and/or contour data does exist, the process (step  415 ) then queries and determines whether image data and contour data for a second image and contour exists. If image and/or contour data for the second image and/or contour does not exist, the prompt for displaying the window  230  is unavailable (step  410 ). If data representative of both the first and second images and the first and second contours exists, the user prompt is available (step  420 ). 
     When the user prompt is available (step  420 ), the user can select whether to open the window  230  (step  425 ) by, for example, issuing a command through an input device such as the input device  135  of the system  100  of  FIG. 1 . If the user does not issue a command, query or request (step  425 ), the process is terminated (step  430 ), and the window  230  is not displayed. If the user, in response to prompt, opts to open the window  230  (step  425 ), the user indicates the preference, and the window  230  opens (step  435 ). Upon the window opening, the data (e.g., cineloop data) associated with the first and second contours (e.g., contours  216   a - 216   b ) is loaded and displayed in the window  230 . When the window  230  opens, the window is superimposed over the existing display (e.g., over the window  200  of  FIG. 2 ). 
       FIG. 4B  is an exemplary user interface  450  of a prompt that embodies the process of  FIG. 4A . The user interface  450  includes a command  455  called “Compare ED/ES” that corresponds to step  420  of the process of  FIG. 4A . “Compare” is associated with a visual comparison between image contours, while “ED” refers to the end diastole view and “ES” refers to the end systole view, as discussed above. In some embodiments, the user interface  450  appears in response to action by the user (e.g., a right mouse-click). The user interface  450  can also appear automatically in response to the existence of data associated with the first and second images and first and second contours. Similarly, in the absence of data associated with either the first or second image or first or second contour (e.g., step  410  of  FIG. 4A ), the command  455  is unavailable for selection by the user. 
       FIG. 5  is a flow chart  500  illustrating an exemplary method for displaying superimposed image contours. The method involves capturing a first image at a first time t 1  (step  504 ). As discussed above, a first contour of an object of interest (e.g., a moving object, captured as the object appeared at time t 1 ) is extracted from the first image (step  508 ). After the contour is extracted, the centroid or center-of-mass of the first contour is determined or calculated (step  512 ). Optionally, data associated with the centroid or center-of-mass and the first contour is stored in a computer memory (step  516 ). 
     The centroid or center-of-mass of the first contour is determined using a centroid-finding or center-finding algorithm applied to the contour. As discussed above, the first contour includes a set of points of the image. In some embodiments, the points are pixel addresses or values in a pixel grid, and as such, are represented as an ordered pair or coordinate value (x i , y i ). The contour forms a closed polygon of connected vertices where each vertex is a point or ordered coordinate pair in the set of points that define the contour. 
     Various algorithms for determining the centroid of a closed polygon can be used. In some embodiments, the area of the polygon (e.g., the first contour) is calculated, and the area is used to determine the coordinates of the centroid or center-of-mass (e.g., by treating the polygon as a plate of uniform mass density). 
     The area, A, of a non-overlapping, closed polygon that is defined by N vertices (x i , y i ), is governed by Equation 1, below. In Equation 1, the Nth or notional vertex (e.g., (x N , y N )) is set equal to the zeroth vertex (e.g., (x 0 , y 0 )), indicating a closed polygon. 
     
       
         
           
             
               
                 
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                           i 
                           + 
                           1 
                         
                       
                       ⁢ 
                       
                         y 
                         i 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     After the area, A, of the contour has been determined, the x and y coordinates of the centroid or center-of-mass (C x , C y ) are determined using Equations 2A and 2B, where “A” in Equations 2A and 2B are determined from Equation 1. Like Equation 1, the Nth or notional vertex (e.g., (x N , y N )) is set equal to the zeroth vertex (e.g., (x 0 , y 0 )). 
     
       
         
           
             
               
                 
                   
                     C 
                     x 
                   
                   = 
                   
                     
                       1 
                       
                         6 
                         ⁢ 
                         A 
                       
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           0 
                         
                         
                           N 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               x 
                               i 
                             
                             + 
                             
                               x 
                               
                                 i 
                                 + 
                                 1 
                               
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           ( 
                           
                             
                               
                                 x 
                                 i 
                               
                               ⁢ 
                               
                                 y 
                                 
                                   i 
                                   + 
                                   1 
                                 
                               
                             
                             - 
                             
                               
                                 x 
                                 
                                   i 
                                   + 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 y 
                                 i 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                   ⁢ 
                   A 
                 
               
             
             
               
                 
                   
                     C 
                     y 
                   
                   = 
                   
                     
                       1 
                       
                         6 
                         ⁢ 
                         A 
                       
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           0 
                         
                         
                           N 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               y 
                               i 
                             
                             + 
                             
                               y 
                               
                                 i 
                                 + 
                                 1 
                               
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           ( 
                           
                             
                               
                                 x 
                                 i 
                               
                               ⁢ 
                               
                                 y 
                                 
                                   i 
                                   + 
                                   1 
                                 
                               
                             
                             - 
                             
                               
                                 x 
                                 
                                   i 
                                   + 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 y 
                                 i 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                   ⁢ 
                   B 
                 
               
             
           
         
       
     
     Mathematically, a centroid (or barycenter) of an object in an n-dimensional space represents the intersection of all hyperplanes that divide the object into two parts of equal moment about the hyperplane. The centroid is sometimes referred to as the “average” of all points in the object. In situations in which the object has a uniform density, the location of the centroid coincides with the object&#39;s center-of-mass. As discussed herein, the “centroid” and “center-of-mass” are used interchangeably since the calculation involves determination of a centroid of a polygonal shape that is deemed or assumed to have uniform mass density. 
     In some embodiments, the centroid or center-of-mass is calculated using the so-called “triangle method.” According to the triangle method, the polygon is partitioned into triangles (or “triangulated”), each having three vertices: (x 1 , y 1 ), (x 2 , y 2 ), and (x 3 , y 3 ). The centroid of each triangle is the average of the vertices, so the coordinate of the centroid (x c , y c ) is given by Equation 3: 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         x 
                         c 
                       
                       , 
                       
                         y 
                         c 
                       
                     
                     ) 
                   
                   = 
                   
                     ( 
                     
                       
                         ( 
                         
                           
                             
                               x 
                               1 
                             
                             + 
                             
                               x 
                               2 
                             
                             + 
                             
                               x 
                               3 
                             
                           
                           3 
                         
                         ) 
                       
                       , 
                       
                         ( 
                         
                           
                             
                               y 
                               1 
                             
                             + 
                             
                               y 
                               2 
                             
                             + 
                             
                               y 
                               3 
                             
                           
                           3 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     The coordinate of the centroid (x c , y c ) of each of the triangles into which the polygon was partitioned is calculated. The coordinates of the centroids are added or summed, weighted by the area of each triangle. Specifically, the abscissa (x c ) values are summed and weighted to determine an abscissa value for the centroid (x c ), and the ordinate (y c ) values are summed and weighted to determine an ordinate value for the centroid (y c ). The centroid&#39;s abscissa and ordinate values are then normalized using the total polygonal area to determine the coordinate of the centroid (C x , C y ). In some embodiments, the coordinate of the centroid is rounded to the nearest pixel value for each of the abscissa and/or ordinate. The coordinate can also be rounded to the nearest sub-pixel value where pixels are partitioned (e.g., in quarter-pixel increments). In some embodiments, the coordinate of the centroid is a real-valued number and is not rounded to the nearest integer pixel. 
     The method involves capturing a second image at a second time t 2  (step  520 ). A second contour is extracted from the second image (step  524 ) as discussed above. Similarly, a second centroid of the second contour is calculated and determined (step  528 ) (e.g., using one of the centroid-finding techniques discussed above). Optionally, data associated with the second contour and centroid can be stored (step  532 ). 
     In some embodiments, the steps of the process can be completed in a different order. For example, extracting the first contour (step  508 ) does not necessarily precede capturing the second image at the second time t 2 . Specifically, the contours can be extracted (step  508  and  524 ) near the same time, after both images have been captured (steps  504  and  520 ). 
     After the centroids for the first and second contours have been determined, the position of a common location for displaying the contours is determined (step  536 ). In some embodiments, the common location is selected to be an integer pixel coordinate or sub-pixel coordinate in a pixel grid that will be displayed in a window (e.g., the window  230  of  FIG. 2 ). The common location can also be a real-valued coordinate pair. The common location is used as a reference point or reference value for both the first contour and the second contour. As discussed above, when the first and second contours are displayed in the window, the respective first and second centroids (or centers-of-mass) are positioned at the common location. The first and second contours are redrawn or illustrated in the window centered about the common location since the respective centroids are aligned at the central location (steps  540  and  544 ). In some embodiments, the centroids are assigned a value that is the same as the value of the common location, and the values of the points that define the contour are transformed based on the value of the common location. 
       FIG. 6  is a flow chart  600  illustrating a process for displaying image contours with a common centroid or center-of-mass. The process involves determining the position of the common location (step  536  of  FIG. 5 ). After the position of the common location has been determined, data representative of the first contour is transformed based on the coordinate value of the common location (step  604 ). In some embodiments, each point in the set of points that define the contour are associated with a vector that identifies a distance and a direction of the particular point relative to the centroid. When the centroid (C x , C y ) is positioned at the common location (x CL , y CL ), the centroid undergoes a linear transformation L (x CL -C x , y CL -C y ) that relates the common location to the centroid. The linear transformation L can be applied, e.g., via matrix transformation, to each of the vectors representative of the points defining the first contour. In some embodiments, more complicated transformations can be applied to the set of vectors, e.g., scale transformations, rotational transformations, skew transformations, aspect transformations, or combinations of these. The transformed contour can be stored for later use, or subsequently processed and/or displayed without storage. Other representations of the points that define a contour (e.g., other than a vector representation) will be apparent to one of skill. 
     After the vectors associated with the points defining the first contour have been transformed, the first centroid is positioned at the common location (step  608 ), and the first contour is illustrated, displayed, drawn, or redrawn with the first centroid at the common location (step  612 ). Specifically, the transformed data is used to illustrate the contour in the window about the common location (e.g., rather than using the nontransformed data). 
     A similar operation occurs with respect to the second contour. Specifically, data associated with or representative of the second contour is transformed based on information about the common location (step  616 ). The second centroid is positioned at the common location (step  620 ), and the second contour is illustrated, displayed, drawn, or redrawn with the second centroid at the common location (step  624 ), based on the transformed data. In this way, the image contours can be displayed in a common field of view (e.g., against a solid-color background) in a window for quick assessment and comparison. In some embodiments, illustrating the first and second contours (steps  612  and  624 ) involves processing a cineloop associated with the respective contours. The cineloops can have the transformation data stored or associated therewith, or the transformation calculation can be implemented on-the-fly as the contour is illustrated. 
     In some embodiments, the transformation steps (steps  604  and  616 ) are carried out by a transformation module (e.g., the transformation module  145  of  FIG. 1 ), based on the value of the common location. 
     The above-described techniques can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The implementation can be as a computer program product, e.g., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
     Method steps can be performed by one or more programmable processors executing a computer program to perform functions of the technology by operating on input data and generating output. Method steps can also be performed by, and apparatus can be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). Modules can refer to portions of the computer program and/or the processor/special circuitry that implements that functionality. 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor receives instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Data transmission and instructions can also occur over a communications network. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry. 
     The terms “module” and “function,” as used herein, mean, but are not limited to, a software or hardware component which performs certain tasks. A module may advantageously be configured to reside on addressable storage medium and configured to execute on one or more processors. A module may be fully or partially implemented with a general purpose integrated circuit (“IC”), FPGA, or ASIC. Thus, a module may include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided for in the components and modules may be combined into fewer components and modules or further separated into additional components and modules. Additionally, the components and modules may advantageously be implemented on many different platforms, including computers, computer servers, data communications infrastructure equipment such as application-enabled switches or routers, or telecommunications infrastructure equipment, such as public or private telephone switches or private branch exchanges (“PBX”). In any of these cases, implementation may be achieved either by writing applications that are native to the chosen platform, or by interfacing the platform to one or more external application engines. 
     To provide for interaction with a user, the above described techniques can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer (e.g., interact with a user interface element). Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. 
     The above described techniques can be implemented in a distributed computing system that includes a back-end component, e.g., as a data server, and/or a middleware component, e.g., an application server, and/or a front-end component, e.g., a client computer having a graphical user interface and/or a Web browser through which a user can interact with an example implementation, or any combination of such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communications, e.g., a communications network. Examples of communications networks, also referred to as communications channels, include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet, and include both wired and wireless networks. In some examples, communications networks can feature virtual networks or sub-networks such as a virtual local area network (“VLAN”). Unless clearly indicated otherwise, communications networks can also include all or a portion of the PSTN, for example, a portion owned by a specific carrier. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communications network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     Various embodiments are depicted as in communication or connected by one or more communication paths. A communication path is not limited to a particular medium of transferring data. Information can be transmitted over a communication path using electrical, optical, acoustical, physical, thermal signals, or any combination thereof. A communication path can include multiple communication channels, for example, multiplexed channels of the same or varying capacities for data flow. 
     Multiple user inputs can be used to configure parameters of the depicted user interface features. Examples of such inputs include buttons, radio buttons, icons, check boxes, combo boxes, menus, text boxes, tooltips, toggle switches, buttons, scroll bars, toolbars, status bars, windows, or other suitable icons or widgets associated with user interfaces for allowing a user to communicate with and/or provide data to any of the modules or systems described herein. In some embodiments, a touchscreen is used to facilitate an interface or interaction between components hereof and the user. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.