Method and system for detecting small structures in images

Abstract

The invention is a method and apparatus for automated detection of small structures in images. One specific use is to detect malignant microcalcification clusters in mammograms. A digitized and filtered mammogram image is stored in a computer. Seed pixels, which are pixels that are brighter than their immediate neighbors, are identified to indicate candidate structures and used to construct two regions. Various features are then measured using the two regions around each seed point. The features characterize each candidate structure and are input to a classifier, such as a neural network. The classifier then distinguishes between structures of interest and background. The structures detected by the classifier are then presented to a clustering algorithm. A detected structure that is less than a threshold distance away from the nearest structure and a cluster is included in that cluster. Finally, the results are displayed, either on a monitor or on hard copy, with a frame around the detected cluster.

We claim:  
     
         1 . A method for detecting a structure in an image, the method comprising the steps of: 
 digitizing the image;    storing the digitized image in a memory of a data processing means;    detecting automatically the structure using the data processing means and the digitized image comprising the steps of: 
 segmenting the digitized image into a candidate structure to be classified;  
 extracting features from the candidate structure; characterizing the candidate structure using the features; and  
 classifying the candidate structure, using the extracted features, as structure or as background; and  
   indicating the location of the structure in the image.    
     
     
         4 . The method as recited in    claim 1   , wherein the candidate structure is classified using a neural network.  
     
     
         5 . The method as recited in    claim 4   , wherein the neural network is a feedforward neural network.  
     
     
         6 . The method as recited in    claim 1   , further comprising the step of clustering the classified structures that are a threshold distance from each other in the digitized image.  
     
     
         7 . The method as recited in    claim 1   , the segmenting the digitized image step comprising the steps of: 
 locating a seed pixel in the digitized image, the seed pixel having a higher intensity value than its neighboring pixels;    forming a gradient image of the digitized image;    constructing a first region beginning with the seed pixel and its neighboring pixels by adding to the first region all adjacent pixels having both a gradient value higher and a gray level lower than the pixels already in the first region that the adjacent pixels adjoin; and    constructing a second region beginning with the seed pixel and its neighboring pixels by adding to the second region all adjacent pixels having a gray level value lower than the pixels already in the second region that the adjacent pixels adjoin.    
     
     
         8 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring the standard deviation of pixel values along the edge of the second region (estd).  
     
     
         9 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring a gray level value of the seed pixel (sv).  
     
     
         10 . The method as recited in    claim 8   , the extracting features step further comprising the step of measuring a gray level value of the seed pixel (sv).  
     
     
         11 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring an area of the first region in pixels (a 1 ).  
     
     
         12 . The method as recited in    claim 10   , the extracting features step further comprising the step of measuring an area of the first region in pixels (a 1 ).  
     
     
         13 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring an area of the second region in pixels (a 2 ).  
     
     
         14 . The method as recited in    claim 12   , the extracting features step further comprising the step of measuring an area of the second region in pixels (a 2 ).  
     
     
         15 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring a local contrast obtained by subtracting the mean value of pixels along the edge of the first region from the seed pixel value (ac 1 ).  
     
     
         16 . The method as recited in    claim 14   , the extracting features step further comprising the step of measuring a local contrast obtained by subtracting the mean value of pixels along the edge of the first region from the seed pixel value (ac 1 ).  
     
     
         17 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring a local contrast obtained by subtracting the mean value of pixels along the edge of the second region from the seed pixel value (ac 2 ).  
     
     
         18 . The method as recited in    claim 16   , the extracting features step further comprising the step of measuring a local contrast obtained by subtracting the mean value of pixels along the edge of the second region from the seed pixel value (ac 2 ).  
     
     
         19 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring the mean value of the higher fifty percentile of the gradient values of the pixels along the edge of the first region (mxs 1 ).  
     
     
         20 . The method as recited in    claim 18   , the extracting features step further comprising the step of measuring the mean value of the higher fifty percentile of the gradient values of the pixels along the edge of the first region (mxs 1 ).  
     
     
         21 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring the mean value of the higher fifty percentile of the gray level differences between the seed pixel value and each pixel along the edge of the second region (mxc 2 ).  
     
     
         22 . The method as recited in    claim 20   , the extracting features step further comprising the step of measuring the mean value of the higher fifty percentile of the gray level differences between the seed pixel value and each pixel along the edge of the second region (mxc 2 )  23 .  
     
     
         23 . The method as recited in    claim 7   , the extracting features step comprising the step of measuring the mean angle index of the structure (aa).  
     
     
         24 . The method as recited in    claim 22   , the extracting features step further comprising the step of measuring the mean angle index of the structure (aa).  
     
     
         25 . The method as recited in    claim 24   , the extracting features step further comprising the step of dividing the area of the first region by the area of the second region (a 1 /a 2 ).  
     
     
         26 . The method as recited in    claim 25   , the extracting features step further comprising the step of dividing the local contrast of the first region by the local contrast of the second region (ac 1 /ac 2 ).  
     
     
         27 . The method as recited in    claim 26   , the characterizing the candidate structure comprising the step of selecting the sv, a 2 , a 1 /a 2 , ac 1 , ac 1 /ac 2 , mxs 1 , mxc 2 , estd, and aa features to characterize the candidate structure.  
     
     
         28 . The method as recited in claim  27 , wherein the image is a mammogram and the structure is a microcalcification.