Abstract:
A method for linear structure detection in mammographic images, comprising: locating a plurality of microcalcification candidate clusters in digital mammographic images; extracting a region of interest that encloses each of the candidate clusters; processing the region of interest to generate feature points that reveal geometric properties in the region; applying a line detection algorithm to the feature points to produce a line model; and analyzing the line model to determine whether a true linear structure is present in the first region of interest.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of commonly assigned, copending U.S. patent application Ser. No. 11/834,243 filed Aug. 6, 2007 by Chen et al for LINE STRUCTURE DETECTION AND ANALYSIS FOR MAMMOGRAPHY CAD; and Ser. No. 12/058,803 filed Mar. 31, 2008 by Chen et al for LINEAR STRUCTURE VERIFICATION  1 N MEDICAL APPLICATIONS. The disclosures of these applications hereby are incorporated by reference into the present specification. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to image processing in medical applications, and in particular, to computer aided detection (CAD) in mammography. 
       BACKGROUND OF THE INVENTION 
       [0003]    Mammography is an effective method of screening for breast cancer, a leading cause of mortality among women. However, analyzing mammograms requires skilled radiologists whose performance can be degraded by the demand of viewing a large number of images in a finite amount of time. A CAD feature has been provided in many mammography systems to assist radiologists in capturing true-positives (TP) that might otherwise have been overlooked. For example, see “Automatic detection of clustered microcalcifications in digital mammograms using mathematical morphology and neural networks”, by Stelios Hakiotis et al., Signal Processing, Volume 87, Issue 7, July 2007. 
         [0004]    Abnormalities visible in mammograms include microcalcifications (MCs), which are tiny deposits of calcium in breast carcinoma. It is very difficult to distinguish between malignant and benign microcalcification clusters (MCCs), even for experienced radiologists, which may lead to a high rate of unnecessary biopsies. Therefore, it is desirable to design the CAD algorithm in such a way that a high TP rate can be achieved while the number of false-positives (FPs) is kept to a minimum. It has been noted that many FP MCC candidates as selected by known mammography CAD systems were found to fall on linear normal structures such as blood vessels in digital or film-based mammograms. Thus, it has been recognized that removing those MCC candidates that are associated with linear structures will significantly reduce the overall FP rate. 
         [0005]    Various methods for extracting linear structures from a mammographic image have been proposed. R. Zwiggelaar, T. C. Parr, and C. J. Taylor, in their article “Finding orientated line patterns in digital mammographic images,”  Proc.  7 th    Br. Machine Vision Conf.,  1996, pp. 715-724, have compared the performance of several different approaches (including orientated bin and line operator methods) to the detection of linear structures with synthetic mammographic images. Their results suggest significant differences between the different approaches. One approach has been implemented as a multi-scale line operator and gives intuitively convincing results. The output could be used for classifying linear structures. 
         [0006]    The work of a line operator can be described as follows: The line operator takes the average grey level of pixels lying on an orientated local line passing through a target pixel and subtracts the average intensity of all the pixels in the locally orientated neighborhood. The line strength is compared for n orientations. Line direction is obtained from the orientation producing the maximum line strength. Scale information can be obtained by applying the line operator to images that are resealed by Gaussian smoothing and sub-sampling. For each pixel, the scale producing the maximum line strength is taken as the detected line scale. 
         [0007]    N. Cerneaz and M. Brady, in their article “Finding Curvilinear Structures in Mammograms,”  Lecture Notes in Computer Science,  905, pp. 372-382, 1995, introduced a method that estimates the intensity profile of curvilinear structures (CLS) in mammograms in a single scale. In this method, the CLS are assumed to have circular cross section when the breast is not compressed. And the cross section of CLS in a mammogram is assumed to be elliptical. Candidate pixels for CLS are detected using the response of a second order difference operation which is applied in four directions. If there is a sufficient high response for one of the orientations the pixel will form part of a CLS. A measure of line strength is obtained by determining the contrast of the line profile at these pixels. 
         [0008]    Wai et al, in their article “A Multi-resolution CLS Detection Algorithm for Mammographic Image Analysis,”  Medical Imaging Computing and Computer - Assisted Intervention, MICCAU , pp. 865-872, 2004&#39; adopted the two-step method from the work of Cerneaz and Brady and devised a multi-resolution ridge detector for structures ranging from 1800 microns to 180 microns. Wai et al also enhanced the performance of the detector by using a local energy thresholding to suppress undesirable response from noise. The local energy is also used to determine CLS junctions. 
         [0009]    Alexander Schneider et al, in U.S. Patent Application Publication No. 2002/0159622, proposed a system and method for detecting lines in medical images. In their method, a direction image array and a line image array are formed by filtering a digital image with a single-peaked filter; convolving the regular array with second-order difference operators oriented along the horizontal, vertical and diagonal axes; and computing the direction image arrays and line image arrays as direct scalar functions of the results of the second order difference operations. As best understood by the present inventors, line detection based on the use of four line operator functions requires fewer computations than line detection based on the use of three line operator functions, if the four line operator functions correspond to the special orientations of 0, 45, 90 and 135 degrees. 
         [0010]    For the issue of FP reduction, Zhang et al, in their paper “A New False Positive Reduction Method for MCCs Detection in Digital Mammography,”  Acoustics, Speech and Signal Processing  2001 , Proc. IEEE Intl. Conf. on  (ICASSP), V. 2, Issue 2001, pp. 1033-1036, 2001, described a mixed feature multistage FP reduction algorithm utilizing eleven features extracted from spatial and morphology domains. These features include gray-level description, shape description and clusters description; but no feature is directly related to linear structures. Wai et al mention in their article that the results from the multi-resolution ridge detector could be beneficial to microcalcification FP reduction but the realization of the reduction is absent. Moreover, Wai et al say that it is computationally inefficient to generate actual linear structures just for the purpose of confirming the association of an MCC candidate with a linear structure in mammography CAD. 
         [0011]    Therefore, a need exists for an improved approach for image linear structure detection in mammography. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is designed to overcome the problems set forth above. More particularly, with the present invention, all MCC candidates are assumed to be associated with linear structures until verified otherwise. Therefore, the present invention provides a method for detection of linear structures in mammography CAD systems with the objective of reducing MCC FPs. 
         [0013]    According to one aspect, the present invention provides a method for linear structure detection in a digital mammographic image using microcalcification candidate clusters. The method uses a processor or computer to perform steps of locating a plurality of microcalcification candidate clusters in the image; extracting a first region of interest that encloses each of the plurality of clusters; processing the first region of interest to generate feature points that reveal geometric structures in the first region of interest; applying a line detection algorithm to the feature points to produce a line model; and analyzing the line model to determine whether a linear structure is present in the first region of interest. 
         [0014]    The processing step may comprise steps of generating a gradient magnitude region of interest from the first region of interest; and generating a binary region of interest containing the feature points from the gradient magnitude region of interest. The line detection algorithm may be a RANSAC algorithm. The analyzing step may comprise steps of: computing a first integral curve in a direction parallel to a line determined by the line model; computing a second integral curve in a direction perpendicular to the line determined by the line model; computing a first peak to peak value for the first integral curve; computing a second peak to peak value for the second integral curve; and computing an identifiable value from a ratio of the first peak value to the second peak value, the identifiable value being indicative of whether a true linear structure is present in the first region of interest. 
         [0015]    The identifiable value may be used in a classifier together with other identifiable intensity and geometrical values to detect microcalcifications in the image. The method may further comprise a step of training the classifier with the identifiable values. 
         [0016]    In the various embodiments of the inventive method, the mammographic image may be a digitized X-ray film mammogram, or a digital mammogram captured with a computerized radiography system, or a digital mammogram captured with a digital radiography system. 
         [0017]    These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other. 
           [0019]      FIG. 1  is an illustration of an embodiment of the method of the invention for detection of linear structures in mammographic images. 
           [0020]      FIGS. 2A and 2B  are illustrations of exemplary workflows of an embodiment of the method of the current invention. 
           [0021]      FIG. 3A  is an illustration of an embodiment of the method of the invention using an MCC candidate driven linear structure detection in mammographic images. 
           [0022]      FIG. 3B  is an illustration of an embodiment of computing an identifiable value. 
           [0023]      FIGS. 4A and 4B  respectively show a mammogram and an enlarged region of interest in the mammogram illustrating exemplary MCC candidate clusters. 
           [0024]      FIG. 5A  shows an exemplary ROI (region of interest) that contains a linear structure; and  FIG. 5B  shows an exemplary ROI that does not contain a linear structure. 
           [0025]      FIG. 6A  shows a simulated bROI (binary region of interest) that contains a linear structure; and  FIG. 6B  shows the line model associated with the bROI after applying a RANSAC line detection algorithm. 
           [0026]      FIG. 7  shows the result after applying a RANSAC line detection algorithm to an ROI that does not contain line structures. 
           [0027]      FIG. 8A  shows an ROI as in  FIG. 5A  that contains a linear structure;  FIG. 8B  shows a gROI (gradient magnitude region of interest) of the ROI of  FIG. 8A ; and  FIG. 8C  shows two integral curves and their associated features computed from the gROI of  FIG. 8B . 
           [0028]      FIG. 9A  shows an ROI as in  FIG. 5B  that does not contain a linear structure;  FIG. 9B  shows a gROI of the ROI of  FIG. 9A , and  FIG. 9C  shows two integral curves and their associated features computed from the gROI of  FIG. 9B . 
           [0029]      FIG. 10A  shows a chart that displays identifiable values for a plurality of ROIs that contain linear structures; and  FIG. 10B  shows a chart that displays identifiable values for a plurality of ROIs that do not contain linear structures. 
           [0030]      FIG. 11A  displays an exemplary gROI with ring-like structure; and  FIG. 11B  shows a graph illustrating a ring-like structure search method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The following is a detailed description of the preferred embodiments that use the current invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. 
         [0032]    Turning now to  FIG. 1 , the method of the present invention will be outlined.  FIG. 1  is a work flow chart  150  illustrating an embodiment that employs the linear structure detection method of the present invention. There are many variations in appearance of linear structures in mammograms in terms of contrast, brightness, texture and morphological shapes, among others. Therefore, in a practical image processing system it is desirable, as one measure, to synergistically integrate the skills of the human operator of the system with the power of the computer in the process of linear structure detection. A typical human excels in creativity, use of heuristics, flexibility and common sense; while a computer excels in speed of computation, strength and perseverance. This synergy of human and computer can be realized by incorporating a detection parameter control into the process. 
         [0033]    In  FIG. 1 , there are provided steps of parameter selection  158  and human intervention  162  that allow a human operator or user  168  to participate in the process loop. Parameter selection step  158  allows selection of a set of parameters either from a known constant parameter generator  154  or a known controllable parameter generator  152 . The process begins with an initial set of default parameters being selected automatically by the system. The results of selection are conveyed to the user preferably though a graphical display. The user responds to the display by indicating displayed results that need refinement. One way to accomplish this is for the user to graphically note on the display features that are not part of a linear structure. This makes use of the user&#39;s high level knowledge, and does not require the user to delve into directly setting parameters. Parameter selection step  158  sends the selected parameters to a configurable linear structure detection step  160  according to the invention, through a forward path  176 . Controllable parameter generator  152  is manipulated by operator  168  through a human intervention step  162 . Operator  168  receives information on mammographic images from input  156  or feedback on path  164  from detection step  160 . Then the operator makes a decision to use parameters either from constant parameter generator  154  or from controllable parameter generator  152 . For instance, the initial linear structures identified may contain pixels that the user can quickly and qualitatively determine should not be considered by the computer. These features ideally should be considered as outliers by the computer, but as misclassified and hence used in the parameter estimation. These features can be eliminated by the user for consideration by the computer and a new set of parameters may be estimated. 
         [0034]    A feature of the embodiment that uses the present invention is that it provides an integrated solution to the linear structure detection problem by constructing a configurable detection system with a configurable verifier. This configurable system can be configured or reconfigured by commands from operator  168  through a command path  174 . In general, the configurable verifier can be used in different positions in a workflow (or equivalently, forming different workflows). 
         [0035]    Exemplary workflows for detection step  160  are depicted in  FIGS. 2A and 2B . In  FIG. 2A , mammographic images on input  156  are first processed in pre-processing step  252  that applies various known image processing algorithms such as filtering, morphological processing, feature extraction, etc. to the incoming images to form MCCs. Referring to  FIG. 2A , for a first exemplary workflow from linear structure detection step  256  to MCC classification step  258 , the MCCs enter step  256  followed by step  258  that classifies the MCCs into malignant or benign types. The above described workflow provides linear structure detection prior to MCC classification. Referring to  FIG. 2B , for a second exemplary workflow, the MCCs enter the classification step  258  first, followed by the linear structure detection step  256 . The  FIG. 2B  work flow thus provides linear structure detection after MCC classification. The output from either of the exemplary workflows goes to a post-processing step  260  that consolidates cancer detection results from other detection modules (such as mass lesion detection, not discussed in the present invention) in the mammography CAD system. 
         [0036]    The use of the exemplary workflow  256  to  258  of  FIG. 2A  reduces the computational burden of step  258  in terms of the number of MCCs to be classified since step  256  removes a sizable quantity of MCCs from the candidate list. It does, however, require the CAD classifier in step  258  to be retrained if the linear structure detection  256  is an add-on step. The use of the exemplary workflow  258  to  256  of  FIG. 2B  enables a kind of ‘plug and play’ feature for the mammography CAD system if linear structure detection step  256  is an add-on step. It does not require the CAD classifier in step  258  to be retrained. Linear structure detection step  256  in  FIG. 2B  only processes those MCCs that are classified as true positives by MCC classification step  258 . 
         [0037]    Persons skilled in the art understand that a mammography CAD system can be designed in such a way that the functionalities provided by linear structure detection step  256  can be part of the functionalities provided by MCC classification step  258 . In other words, steps  256  and  258  can be combined into a single unit. Persons skilled in the art can design an MCC classifier or the equivalent that has the capability of verifying linear structures for use in accordance with the present invention. In other words, linear structure detection step  256  and MCC classification step  258  can be combined into a single process. 
         [0038]    In step  256 , the linear structure detection itself can be configured or reconfigured to have various functional combinations by commands from operator  168  through command path  174  via parameter selection step  158 . Step  158  sends the selected parameters to step  160  through path  176 . For example, to be detailed later, the linear structure verifier for step  160  can be configured so that a function of TP protection is activated in the linear structure detection process. The linear structure verifier can also be configured by commands from operator  168  so that different operational points can be realized. Exemplary operational points are numerical thresholds that are used in cascade rule based detection. 
         [0039]      FIG. 3A  depicts an embodiment of a method according to the invention of a workflow chart  301  for an MCC candidate driven, line structure detection step  256  for mammographic images. An exemplary linear structure verifier includes algorithm steps  302 ,  304 , and  306  and can be used as step  256  in step  160 . Step  302  receives mammographic images which can be digitized X-ray film mammograms, digital mammograms captured with computerized radiography systems or mammographic images captured with digital radiography systems. In Step  302 , a plurality of known image processing and computer vision procedures are applied to the input mammographic images to identify clusters of connected pixels that present characteristics which are similar to those of MCCs in the mammogram. Exemplary characteristics known to persons skilled in the art can be statistical moments derived from pixel values in intensity (including intensity derivatives), spatial frequency, and topological domains. Step  302  forms MCC candidates using the identified clusters of connected pixels. The clusters of connected pixels are also called MCC candidate spots. Each MCC candidate formed in Step  302  has a plurality of MCC candidate spots that are close to each other within a certain distance. 
         [0040]    In step  304 , a region of interest (ROI) is defined in the mammographic image. The center of an ROI is the center of an MCC candidate. The size of an ROI is selected appropriately to enclose all the MCC candidate spots in an MCC candidate. An exemplary size is 200 by 200 in image pixels. In step  306 , the ROI will be processed and an identifiable value to be used in step  258  or step  260  will be determined using the process of  FIG. 3B . 
         [0041]    Curvilinear structures appear in the mammographic images due to the curvilinear appearance of blood vessels, lactation ducts, and ligaments of the breast as projected on a mammogram. Vascular lines are large calcified blood vessels on a mammogram. MCCs appear as bright spots on mammograms and usually are the only signs indicating early breast cancers. MC spots appear usually in clusters, and malignant MCC spots usually have irregular shapes. The curvilinear structure is a net of massive and complex curvilinear markings on a given mammogram. Due to the projection of three-dimensional breast into a two-dimensional mammogram, different parts of the curvilinear structure may have different appearances with varying width, lengths and different contrasts to the surrounding breast tissue. The option of pre-classification global mask generation to extract curvilinear structure may be of limited use because global curvilinear marking is a massive structure, and accurate mask extraction is not an easy task. Global mask tends to extract only small portion of linear structure or extract too many “lines”. 
         [0042]    In accordance with the invention, an alternative to extracting curvilinear structures is, after initial MCC candidate classification as in Step  258 , to identify linear structures only in the neighborhood where the MCC candidates reside. For each of the MCC candidates that have passed initial classification, a region of interest (ROI) is defined that encloses the MCC candidate spots of each of the MCC candidates. Further, and in accordance with the invention, an MCC FP reduction procedure, or equivalently, an MCC candidate driven linear structure detection procedure, is applied to a small region (ROI) centered around each MCC candidate. Processing time thus is reduced since the number of MCC candidates is limited and only a small portion of the breast area needs to be processed, in comparison to global line mask approach.  FIG. 4A  shows an exemplary mammogram  402  with small triangles that indicate the MCC candidates identified as previously described in the pre-processing state, before any MCC classification.  FIG. 4B  shows an enlarged image  404  of the ROI that contains one of the MCC candidate clusters. The MCC FP reduction procedure will be applied to only a fraction of all MCC candidate clusters indicated in  FIG. 4A . 
         [0043]    The MCC candidate driven linear structure detection method of the present invention will be detailed next.  FIG. 5A  shows an exemplary ROI  502  that contains a linear structure  504 ; and  FIG. 5  B shows an exemplary ROI  503  that does not contain linear structures in area  505 . The corresponding gradient magnitude ROI (gROI)  804  of ROI  502  is shown in  FIG. 8B . As illustrated in  FIG. 3B , a gROI is computed in Step  310  using a gradient magnitude operation known to persons skilled in the art. In general, ROI and gROI are gray scale images. The gROI image is further processed by adaptive thresholding in Step  312  to produce a binary image bROI in which each pixel has one of two values, e.g. 0 or 1. In Step  312 , a threshold is determined dynamically based on the intensity values of all the pixels in the gROI image. If a pixel value exceeds the threshold in the gROI image then the corresponding pixel in bROI image is set to 1; otherwise, the corresponding pixel in bROI is set to 0. Non-zero pixels constitute a type of feature points to be used to detect the linear structures. Some non-zero pixels may correspond to MCCs. 
         [0044]    In Step  314  a line model is computed using the pixels in the bROI image. The preferred line detection algorithm to compute the line model is the RANSAC algorithm (“Random Sample Consensus: A Paradigm for Model Fitting with Applications to Image Analysis and Automated Cartography”, by Martin Fischler, et al, Graphics and Image Processing,  Commun. ACM , Vol. 24, No. 6, June 1981, pp. 381-395. Graph  602  in  FIG. 6A  displays a simulated bROI image with a line associated with random noises. Graph  604  in  FIG. 6B  shows the resulting image after applying the RANSAC algorithm to the simulated bROI image. The straight line  606  represents the line model generated by the RANSAC algorithm. A line model essentially has two parameters, one is the orientation of the line section and the other is the center of the line section. RANSAC is a robust algorithm for detecting line structures in an image. A difficulty is that the RANSAC algorithm always produces a line model even if there are no explicit line structures in an image. For example, the ROI image  702  in  FIG. 7  contains no obvious line structures, but the RANSAC algorithm still generates a line model between two indicative spots  704  and  706  that are generated by RANSAC. Mathematically, two points define a line (orientation) in a two dimensional space. Essentially, indicative spots  704  and  706  are not MC candidate spots although they may happen to be in the locations where MC candidate spots reside. Also, the algorithm removes from the list an MCC candidate, not individual MC candidate spots, although removing an MCC candidate removes its spots. Thus, in this example, ROI image  702  might be removed erroneously from the candidate list. 
         [0045]    A method is thus devised in this invention to determine if the line model produced by the RANSAC algorithm indicates a true line structure. The method essentially describes a model of a line guided linear structure detection. The inventive method now is explained. For an exemplary case of ROI  502  that contains a linear structure  504  as seen in  FIG. 5A , the method includes the following steps as illustrated in  FIG. 3B . 
         [0046]    In Step  316  an integration curve C 1  is computed in a direction parallel to the line detected by the RANSAC algorithm. The direction for computing C 1  is indicated by an arrow  808  in  FIG. 8B . The line is a collection of spots  806  which are part of the feature points. The integral curve C 1  is indicated by plot  814  in  FIG. 8C . 
         [0047]    In practice, a point on curve C 1  and plot  814  is the average value of all the pixels values on the corresponding line that is parallel to the line detected by the RANSAC algorithm (not shown explicitly but indicated by spots  806 ). 
         [0048]    In Step  318 , another integration curve C 2  is computed in a direction  810  shown in  FIG. 8B  that is perpendicular to the line detected by the RANSAC algorithm. In practice, a point on curve C 2  is the average value of all the pixels values on the corresponding line that is perpendicular to the line detected by the RANSAC algorithm (not shown explicitly but indicated by spots  806 ). The integral curve C 2  is indicated by plot  816  in  FIG. 8C . 
         [0049]    In Step  320 , the method computes a peak to peak value P 1  of plot  814  for curve C 1 . In Step  322 , a peak to peak value P 2  is computed of plot  816  for curve C 2 . 
         [0050]    In step  324  an identifiable value R is computed that is the ratio of P 1  to P 2  for the case ROI  502 . 
         [0051]    For another exemplary case of ROI  503  (shown in  FIG. 9A ) that does not contain a linear structure but does contain random spots (not illustrated), the method operates as follows: 
         [0052]    In Step  316  an integration curve C 1  is computed in a direction parallel to the line detected by the RANSAC algorithm. The direction for computing C 1  is indicated by an arrow  908  in  FIG. 9B . The line is a collection of candidate spots  906 . The integral curve C 1  is indicated by plot  914  in  FIG. 9C . 
         [0053]    In practice, a point on curve C 1  and plot  914  is the average value of all the pixels values on the corresponding line that is parallel to the line detected by the RANSAC algorithm (not shown explicitly but indicated by spots  906 ). 
         [0054]    In Step  318 , another integration curve C 2  is computed in a direction  910  as shown in  FIG. 9B  that is perpendicular to the line detected by the RANSAC algorithm. In practice, a point on curve C 2  is the average value of all the pixels values on the corresponding line that is perpendicular to the line detected by the RANSAC algorithm (not shown explicitly but indicated by spots  906 ). The integral curve C 2  is indicated by plot  916  in  FIG. 9C . 
         [0055]    In Step  320  the method computes a peak to peak value P 1  of plot  914  for curve C 1 . In Step  322 , a peak to peak value P 2  is computed of plot  916  for curve C 2 . 
         [0056]    In step  324  an identifiable value R is computed that is the ratio of P 1  to P 2  for the case ROI  503  in which RANSAC has generated indicative spots, not illustrated, in the manner previously discussed. 
         [0057]    Graph  1002  in  FIG. 10A  depicts a collection of R values computed by the method in this invention for a plurality of ROIs that contain no linear structures. Graph  1002  shows that for ROIs that contain no linear structures, their R values mostly stay below an exemplary value of 2. 
         [0058]    Graph  1004  in  FIG. 10B  depicts a collection of R values computed by the method in this invention for a plurality of ROIs that contain linear structures. Graph  1004  shows that for ROIs that contain linear structures, over half of their R values stay above an exemplary value of 2. This identifiable value R can be used directly in post processing step  260  to determine if a positively classified MCC is in fact associated with a linear structure. 
         [0059]    The inventors have observed that some of the TP MCCs may reside in an area where linear structure-like objects are present and can be verified as FPs. It also has been observed that TP MCCs may possess topological ring-like structures surrounding some of the spots. For example, see gROI  1102  in  FIG. 11A . A method in accordance with one embodiment of the invention is hence crafted to take advantage of the ring structure in a cluster to protect TPs. 
         [0060]    Using the spot as the origin, a search is performed along a ray that radiates from the origin until the ray hits the ring or reaches a predefined distance without a hit. The search process can be formulated as follows, referring to graph  1104  in  FIG. 11B : 
         [0061]    Denote a search ray by R kβ  and its opposite search ray by R kβ+π , where β=π/N R ; kε└1, . . . , N R ┘. N R  is a positive integer greater than one. Denote a hit array by T=         . The array elements t k  are initialized as zero. Only if both R kβ  and R kβ+π  hit a ring, the corresponding array element t k  will be set to one, otherwise, the corresponding array element t k  remains zero. 
         [0062]    A simple measure is therefore defined as the sum of hits ξ t  that a hit array has. The measure is simply computed as 
         [0000]    
       
         
           
             
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         [0063]    This measure as an identifiable value is also included in the MCC detector training process, together with the identifiable value R and other identifiable values in the intensity and geometrical spaces known to persons skilled in the art, in MCC classification step  258  to classify MCC candidates into malignant or benign types. The classifier is first trained on sample ground truths using these identifiable values. 
         [0064]    A plurality of normal (non-cancerous) mammographic images are processed in a pre-processing stage which applies various image processing algorithms such as filtering, morphological processing, and spot feature extraction, for example, to the incoming normal images to form normal clusters. The image data for the mammographic images are processed in a cluster selection step, based on a ground truth measure. The ground truth measure provides an indication of the degree of importance or involvement of a cancerous mammographic image or a true MCC cluster in MCC detection training. In embodiments of the present invention, a ground truth measure may be represented by different tests. One exemplary test is the number of true MCC clusters that are enclosed within a ground truth region that is defined by a radiologist. Exemplary cluster identifiable values may include density values, boundary values, morphological values, texture values, and distribution values, among others. 
         [0065]    These identifiable values are input for evaluation to a identifiable value selection and MCC detector training stage. Since each identifiable value has been carefully chosen to separate cancer cases from normal cases, it is reasonable to expect that there will be some separation in identifiable value space between a set of normal candidates and a set of cancer candidates. Exemplary requirements state that the linear separation of each identifiable value be at least 55%, which means that 55%, or other suitable threshold percentage of candidates be classified correctly with a linear classifier. The exemplary requirement leads to a single alternative: to use a linear classifier to evaluate the identifiable values. One alternative for a linear classifier is to assume a Gaussian distribution (with equal variances) and construct a classifier under this assumption. This alternative uses logic described in numerous basic statistics books, such as for a linear discriminator classifier, for example, and allows relatively straightforward implementation. 
         [0066]    In general, identifiable value selection and detector training involve manual operations. The user must typically select or de-select identifiable value from a list. The updated identifiable value list is automatically saved and can be compared to other identifiable value sets by classifying with a linear classifier or Neural Network (NN). The user also has the ability to view histograms of each identifiable value and scatter plots of identifiable values (in 2 dimensions), comparing cancerous lesions and normal areas, which also helps in assessing the relative quality of identifiable values. 
         [0067]    One alternative is to automatically analyze the identifiable values, selecting the best combination. A disadvantage of this approach is that there is no clear method to determine what the best combination of identifiable values might be. First, criteria must be defined (most likely including a classification or distance metric, separating normal regions from cancerous lesions). Secondly, a systematic procedure to combine identifiable values must be defined. At the extreme, every combination of n identifiable values can be processed. However, this may be very time consuming depending on the criteria. Another option is to start with an empty set of identifiable values and iterate, adding the ‘best’ identifiable value at each iteration, until the criteria metric no longer significantly improves. 
         [0068]    Here, detector training refers to the process involving ‘training’ data (a subset of the overall data that have been determined as normal or cancerous) which are selected identifiable values. In the case of NN logic, training involves using the training data to tune the NN&#39;s weights. 
         [0069]    A NN can be described as a system of weights connected by non-linear activation functions. The activation function is usually a “squashing” function, such as a hyperbolic tangent, which forces all inputs to the range of (−1, +1). To train, the NN uses the true class of each candidate to compute an error function. For example, the square error is the square of the difference between the output of the NN and the true class of the candidate. In the worst case, the square error is 4 and in the best case the square error is 0. Each candidate is processed through the NN and the error is computed. The error is then fed back through the network and each weight is updated in a manner that reduces the error. The degree to which each weight is adjusted is determined by a user parameter called the learning rate, which scales the weight update factor. The user also decides on the overall size of the NN, which in turn determines the degree of complexity of the NN. 
         [0070]    In the end, several parameters of the NN can be adjusted (such as the error function, the activation function, and the update procedure, for example), but the two mentioned earlier (learning rate and size) are the most important. The NN learning level can be evaluated by plotting the mean square error (MSE) of all the candidates as the NN is trained. Given a subset of candidates that is not used to train, but is evaluated each iteration, the user can observe how well the NN generalizes to new data. Generally, in a well trained NN, the test or evaluation data&#39;s MSE follows the training MSE closely. The test MSE will tend to be greater than the training MSE, but the relative closeness of the two is more important. As these diverge, it indicates that the NN is memorizing the training data. A trained MCC detector is used for MCC detection. 
         [0071]    The present invention is described as a method. However, in another preferred embodiment, the present invention comprises a computer program product for image linear structure detection in medical applications in accordance with the method described. In describing the present invention, it should be apparent that the computer program of the present invention can be utilized by any well-known computer system, such as the personal computer. However, many other types of computer systems can be used to execute the computer program of the present invention. Consequently, the computer system will not be discussed in further detail herein. 
         [0072]    It will be understood that the computer program product of the present invention may make use of image manipulation algorithms and processes that are well known. Accordingly, the present description will be directed in particular to those algorithms and processes forming part of, or cooperating more directly with, the method of the present invention. Thus, it will be understood that the computer program product embodiment of the present invention may embody algorithms and processes not specifically shown or described herein that are useful for implementation. Such algorithms and processes are conventional and within the ordinary skill in such arts. 
         [0073]    Additional aspects of such algorithms and systems, and hardware and/or software for producing and otherwise processing the images or co-operating with the computer program product of the present invention, are not specifically shown or described herein and may be selected from such algorithms, systems, hardware, components and elements known in the art. 
         [0074]    The computer program for performing the method of the present invention may be stored in a computer readable storage medium. This medium may comprise, for example; magnetic storage media such as a magnetic disk (such as a hard drive or a floppy disk) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program. The computer program for performing the method of the present invention may also be stored on computer readable storage medium that is connected to the image processor by way of the internet or other communication medium. Those skilled in the art will readily recognize that the equivalent of such a computer program product may also be constructed in hardware. 
         [0075]    It will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. The subject matter of the present invention relates to digital image processing and computer vision technologies, which is understood to mean technologies that digitally process a digital image to recognize and thereby assign useful meaning to human understandable objects, attributes or conditions, and then to utilize the results obtained in the further processing of the digital image. 
         [0076]    All articles and other publications mentioned in this specification are hereby incorporated by reference into this specification. 
         [0077]    The invention has been described in detail with particular reference to presently preferred embodiments, but it will be understood that variations and modifications can be effected within the scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein. 
       PARTS LIST 
       [0000]    
       
           150  workflow chart 
           152  controllable parameter generator 
           154  constant parameter generator 
           156  input for mammographic images 
           158  step of parameter selection 
           160  step of linear structure detection 
           162  step of human intervention 
           164  feedback path 
           168  human operator or user 
           174  command path 
           176  forward path 
           252  algorithm step for preprocessing images 
           256  algorithm step for linear structure detection 
           258  algorithm step for MCC classification 
           260  algorithm step for post processing 
           301  workflow chart 
           302  algorithm step for locating candidate clusters 
           304  algorithm step for defining region of interest 
           306  algorithm step of generating identifiable features and computing identifiable value R 
           310  algorithm step for computing gradient region of interest 
           312  algorithm step for computing binary region of interest 
           314  algorithm step for computing line model 
           316  algorithm step for computing integral curve C 1   
           318  algorithm step for computing integral curve C 2   
           320  algorithm step for computing peak to peak P 1  for C 1   
           322  algorithm step for computing peak to peak P 2  for C 2   
           324  algorithm step for computing identifiable value R 
           402  mammogram 
           404  region of interest (ROI) 
           502  exemplary region of interest 
           503  exemplary region of interest 
           504  linear structures in  502   
           505  area without linear structures in  503   
           602  graph of simulated binary ROI image 
           604  binary ROI image after applying RANSAC 
           606  straight line from RANSAC 
           702  ROI with no line structures 
           704  indicative spot generated by RANSAC 
           706  indicative spot generated by RANSAC 
           804  gradient magnitude ROI of ROI  504   
           806  candidate spots 
           808  arrow for direction of computing C 1   
           810  arrow for direction of computing C 2   
           814  plot of integral curve C 1   
           816  plot of integral curve C 2   
           906  candidate spots 
           908  arrow for direction of computing C 1   
           910  arrow for direction of computing C 2   
           914  plot of integral curve C 1   
           916  plot of integral curve C 2   
           1002  graph of R values for ROIs without linear structures 
           1004  graph of R values for ROIs with linear structures 
           1102  gradient magnitude ROI with ring-like structures 
           1104  graph of ring-like structure 
         C 1  integral curve for plot  814  or plot  914   
         C 2  integral curve for plot  816  or plot  916   
         P 1  peak to peak value of plot  814  or  914   
         P 2  peak to peak value of plot  816  or  916   
         R identifiable value, ratio of P 1  to P 2   
         R β  search ray 
         R β+π  opposite search ray