Abstract:
A system and method for detecting the aortic valve is provided. The method comprises: (a) casting rays on a slice of a computed tomography (CT) dataset of an aorta; (b) computing a Gaussian model for voxels on the slice, wherein the Gaussian model produces a threshold; (c) growing a circle from a point within the aorta until control points of the circle reach the threshold; (d) computing a repulsion vector for each control point reaching the threshold; (e) repositioning the circle according to an average of the repulsion vectors, wherein if the circle is within the aorta, repeating steps (c-e) until the circle is not within the aorta; (f) calculating a statistical value for the circle; (g) projecting a copy of the circle onto an adjacent slice; (h) reducing the radius of the copy of the circle; and (i) repeating steps (c-h) on remaining CT slices until the aortic valve is detected.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/549,458, filed Mar. 2, 2004, the disclosure of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to image segmentation, and more particularly, to image segmentation performed on multi-slice computed tomography (CT) angiography datasets. 
     2. Discussion of the Related Art 
     Cardiovascular disease, principally heart disease and stroke, is Western society&#39;s leading killer for both men and women. For example, almost one million Americans die of cardiovascular disease each year, which adds up to more than forty percent of all deaths. Moreover, heart disease does not just kill the elderly, it is the leading cause of death for all Americans age thirty-five and older. Although recent initiatives to encourage healthier lifestyles and increase early detection can prevent cardiovascular disease for those who are healthy and improve the health of people who have experienced this disease, proper diagnosis of cardiovascular disease is imperative for those afflicted. 
     Cardiovascular disease is typically diagnosed by a doctor who reviews a patient&#39;s medical history, health behaviors, family history, and other risk factors. If the patient has symptoms associated with, for example, heart disease, the doctor may perform a physical examination of the patient&#39;s lungs, heart and all of the blood vessels near and around the heart. Once it is determined that the patient has or is at risk of heart disease, an electrocardiogram, chest x-ray or echocardiogram is performed to determine the extent of the disease. 
     Although these techniques have been used for many years, they only provide a doctor with limited amounts of information. For example, x-ray and echocardiograms produce images in two-dimensions (2D) rather than in three-dimensions (3D) and the electrocardiogram records the electrical activity of the heart. Recently, however, computed tomography (CT), which is capable of producing images in 3D and four-dimensions (4D), has been used to diagnose cardiovascular disease. For example, a CT angiography may be performed to identify aneurysms in the aorta or in other major blood vessels. 
     A CT angiography is an examination that uses x-rays to visualize blood flow in arterial vessels throughout the body, from arteries serving the brain to those bringing blood to the lungs, kidneys, arm and legs. CT combines the use of x-rays with computerized analysis of the x-ray images. For example, beams of x-rays are passed from a rotating device through the area of interest in the patient&#39;s body from several different angles to create cross-sectional images, which are then assembled by a computer into a 3D picture of the area being studied. 
     In order to obtain measurements of cardiac parameters such as wall thickness of the coronary artery and ventricular volumes, segmentation of the CT images is required. For example, in order to determine the position of the aortic valve in the heart, a region growing segmentation technique in the left ventricle is performed. However, when performing a region growing in the left ventricle, the segmentation will leak through the aortic valve into other parts of the aorta. This will cause additional portions of the aorta to be segmented thus reducing the quality of the image. Although such leakage may be constrained by drawing a region around the aortic valve to restrict the segmentation algorithm from entering other parts of the aorta, this requires user interaction. 
     Accordingly, there is a need for an image segmentation technique that prevents leakage and that requires minimal to none user interaction thus enabling a quicker and more accurate diagnosis of cardiovascular disease. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the foregoing and other problems encountered in the known teachings by providing a system and method for detecting the aortic valve using a model-based segmentation technique. 
     In one embodiment of the present invention, a method for detecting an aortic valve, comprises: (a) casting a plurality of rays from a seed point within a slice of a computed tomography (CT) dataset of an aorta; (b) computing a Gaussian model for voxels in the slice, wherein the Gaussian model produces a threshold; (c) growing a circle on the slice from a point within the aorta until control points of the circle reach the threshold; (d) computing a repulsion vector for each control point reaching the threshold; and (e) repositioning the circle according to an average of the repulsion vectors, wherein if the circle is repositioned within the aorta, repeating steps (c-e) until the circle is not within the aorta. 
     The method further comprises: (f) calculating a statistical value for the circle; (g) projecting a copy of the circle onto an adjacent slice of the CT dataset; (h) reducing the radius of the copy of the circle; and (i) repeating steps (c-h) on remaining slices of the CT dataset until the aortic valve is detected. The aortic valve is detected by a decrease in the radius of the copy of the circle. The method further comprises: (j) segmenting the aorta using one of a region growing technique and an intensity-based segmentation algorithm constrained by the location of the aortic valve. 
     The radius of the copy of the circle is reduced by fifty percent. The seed point is selected by a user clicking on a portion of the slice. The length of each of the plurality of rays is defined by an estimated aorta radius. The CT dataset is acquired by one of a CT and helical CT imaging technique. 
     In another embodiment of the present invention, a system for detecting an aortic valve, comprises: a memory device for storing a program; a processor in communication with the memory device, the processor operative with the program to: (a) cast a plurality of rays from a seed point within a slice of a CT dataset of an aorta; (b) compute a Gaussian model for voxels in the slice, wherein the Gaussian model produces a threshold; (c) grow a circle on the slice from a central point within the aorta until control points of the circle reach the threshold; (d) compute a repulsion vector for each control point reaching the threshold; and (e) reposition the circle according to an average of the repulsion vectors, wherein if the circle is repositioned within the aorta, repeating steps (c-e) until the circle is not within the aorta. 
     The processor is further operative with the program code to: (f) calculate a statistical value for the circle; (g) project a copy of the circle onto an adjacent slice of the CT dataset; (h) reduce the radius of the copy of the circle; (i) repeat steps (c-h) on remaining slices of the CT dataset until the aortic valve is detected; and (j) segment the aorta using one of a region growing technique and an intensity-based segmentation algorithm constrained by the location of the aortic valve, wherein the aortic valve is detected by a decrease in the radius of the copy of the circle. The radius of the copy of the circle is reduced by fifty percent. The CT dataset is acquired by one of a CT and helical imaging device. 
     In yet another embodiment of the present invention, a method for detecting an aortic valve in a multi-slice CT dataset of an aorta, comprises: (a) casting rays from an initial point within a two-dimensional (2D) slice of the CT dataset; (b) computing a maximum median deviation for all voxels in the 2D slice, wherein the maximum median deviation produces a threshold value for determining which voxels of the 2D slice belong to the aorta; (c) growing a circle comprised of control points from a central point within the aorta and identifying where the control points cross the threshold value; (d) computing a repulsion vector for each point where the control points crossed the threshold value; and (e) repositioning the central point of the circle according to an average of the repulsion vectors to maximize an average radius of the circle, wherein if the circle is repositioned within the aorta, repeating steps (c-e) until the circle is not within the aorta. 
     The method further comprises: (f) calculating average voxel intensities for the circle; (g) projecting a copy of the circle onto an adjacent 2D slice of the CT dataset by translating the central point onto the adjacent slice; (h) reducing the radius of the copy of the circle by fifty percent; and (i) repeating steps (c-h) on remaining slices of the CT dataset until the aortic valve is detected. The aortic valve is detected by one of a decrease in the radius of the reduced copy of the circle and a decrease in the average voxel intensities of the circle. The method further comprises: (j) segmenting the aorta using one of a region growing technique and an intensity-based segmentation algorithm constrained by the location of the aortic valve. The initial point is selected by a user. The method further comprises: (k) analyzing variances of profile data given by each ray; and (l) selecting a cluster of minimum-variance rays from a portion of the 2D slice that belongs exclusively to the aorta prior to computing the maximum median deviation. 
     In another embodiment of the present invention, a method for detecting an aortic arch, comprises: (a) casting a plurality of rays from a seed point within a slice of a CT dataset of an aorta; (b) computing a Gaussian model for voxels in the slice, wherein the Gaussian model produces a threshold; (c) growing a circle on the slice from a point within the aorta until control points of the circle reach the threshold; (d) computing a repulsion vector for each control point reaching the threshold; (e) repositioning the circle according to an average of the repulsion vectors, wherein if the circle is repositioned within boundaries of the aorta, repeating steps (c-e) until the circle is not within the boundaries of the aorta; (f) calculating a statistical value for the circle; (g) projecting a copy of the circle onto an adjacent slice of the CT dataset; (h) reducing the radius of the copy of the circle; and (i) repeating steps (c-h) on remaining slices of the CT dataset until the aortic arch is detected, wherein the aortic arch is detected by one of a decrease in the radius of the copy of the circle and a decrease in the average voxel intensities of the circle. 
     The foregoing features are of representative embodiments and are presented to assist in understanding the invention. It should be understood that they are not intended to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. Therefore, this summary of features should not be considered dispositive in determining equivalents. Additional features of the invention will become apparent in the following description, from the drawings and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system for detecting the aortic valve using a model-based segmentation technique according to an exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  illustrate a flowchart of a method for detecting the aortic valve using a model-based segmentation technique according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates a plurality of rays cast from a seed point constrained by a predetermined radius in an aorta in accordance with an exemplary embodiment of the present invention; 
         FIG. 4  is a graph illustrating a radius of the aorta as it decreases in computed tomography (CT) image slices as the CT image slices approach the aortic valve; and 
         FIGS. 5A and 5B  illustrate an axial view and a sagittal view, respectively, of an aorta segmented in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a block diagram of a system  100  for detecting the aortic valve using a model-based segmentation technique according to an exemplary embodiment of the present invention. As shown in  FIG. 1 , the system  100  includes, inter alia, a scanning device  105 , a personal computer (PC)  110  and an operator&#39;s console  115  connected over, for example, an Ethernet network  120 . The scanning device  105  may be a magnetic resonance imaging (MRI) device, a computed tomography (CT) imaging device, a helical CT device, a positron emission tomography (PET) device, a two-dimensional (2D) or three-dimensional (3D) fluoroscopic imaging device, a 2D, 3D, or four-dimensional (4D) ultrasound imaging device, or an x-ray device. 
     The PC  110 , which may be a portable or laptop computer or a personal digital assistant (PDA), includes a central processing unit (CPU)  125  and a memory  130 , which are connected to an input device  150  and an output device  155 . The CPU  125  includes a model-based segmentation module  145  that includes one or more methods for detecting the aortic valve using a model-based segmentation technique. 
     The memory  130  includes a random access memory (RAM)  135  and a read only memory (ROM)  140 . The memory  130  can also include a database, disk drive, tape drive or a combination thereof. The RAM  135  functions as a data memory that stores data used during execution of a program in the CPU  125  and is used as a work area. The ROM  140  functions as a program memory for storing a program executed in the CPU  125 . The input device  150  is constituted by a keyboard or mouse and the output device  155  is constituted by a liquid crystal display (LCD), cathode ray tube (CRT) display or printer. 
     The operation of the system  100  is typically controlled from the operator&#39;s console  115 , which includes a controller  165  such as a keyboard, and a display  160  such as a CRT display. The operator&#39;s console  115  communicates with the PC  110  and the scanning device  105  so that 2D image data collected by the scanning device  105  can be rendered into 3D data by the PC  110  and viewed on the display  160 . It is to be understood that the PC  110  can be configured to operate and display information provided by the scanning device  105  absent the operator&#39;s console  115 , using, for example, the input device  150  and output device  155  to execute certain tasks performed by the controller  165  and display  160 . 
     The operator&#39;s console  115  further includes any suitable image rendering system/tool/application that can process digital image data of an acquired image dataset (or portion thereof) to generate and display 2D and/or 3D images on the display  160 . More specifically, the image rendering system may be an application that provides 2D/3D renderings and visualizations of medical image data, and which executes on a general purpose or specific computer workstation. Moreover, the image rendering system may enable a user to navigate through a 3D image or a plurality of 2D image slices. The PC  110  may also include an image rendering system/tool/application for processing digital image data of an acquired image dataset to generate and display 2D and/or 3D images. 
     The model-based segmentation module  145  may also be used by the PC  110  to receive and process digital medical image data, which as noted above, may be in the form of raw image data, 2D reconstructed data (e.g., axial slices), or 3D reconstructed data such as volumetric image data or multiplanar reformats, or any combination of such formats. The data processing results can be output from the PC  110  via the network  120  to an image rendering system in the operator&#39;s console  115  for generating 2D and/or 3D renderings of image data in accordance with the data processing results, such as segmentation of organs or anatomical structures, color or intensity variations, and so forth. 
       FIGS. 2A and 2B  illustrate a flowchart of a method for detecting the aortic valve using a model-based segmentation technique according to an exemplary embodiment of the present invention. As shown in  FIG. 2A , prior to performing the method for detecting the aortic valve, image data is acquired from a patient&#39;s heart ( 205 ). This is accomplished by using the scanning device  105 , for example a CT scanner, operated by a user at the operator&#39;s console  115 , to scan the heart thereby generating a series of 2D images associated therewith. The 2D images may then be converted or transformed into a 3D rendered image using the model-based segmentation module  145 . 
     After the image data is acquired, the user located at the operator&#39;s console  115 , selects a seed point in the image data in the aorta ( 210 ). The seed point may selected by the user clicking on a portion of a volume of the 3D image data or a portion of a 2D slice of the 3D image data as it is illustrated on the display  160 . The selected seed point is typically located near the center of the aorta or in between the aortic arch and valve. 
     Once the seed point is selected, rays are cast from the seed point within the selected 2D slice ( 215 ). The rays are cast using a conventional raycasting technique and are typically limited to a length that is smaller than a smallest known aorta radius, which may be, for example, less than two centimeters. The goal of this step is to find a portion of the 2D slice that is exclusively within the aorta. This is done by analyzing the variances of profile data given by each ray and choosing a cluster of minimum-variance rays from a portion of the 2D slice that belongs exclusively to the aorta. 
     After the rays have been cast, a maximum deviation or a Gaussian model for all voxels in the 2D slice is computed ( 220 ). The Gaussian model may be computed, for example, by assuming that n number of rays are cast in step  215 . Next, assume that n sets of samples are collected along the n number of rays where n variances of the n samples are computed. From this, a number of x rays having minimum variances are selected. The number of x rays are then merged into a single sample S and a variance V and the median M of S are computed. Thus, a final voxel intensity acceptance window or Gaussian model [imin, imax], may be determined using the following:
 
imin= M−V *sigma
 
imax= M+V *sigma
 
where sigma is a parameter also used to determine the Gaussian model. In particular, sigma is used to adjust the sensitivity of the Gaussian model. For example, if sigma is set low the acceptance window will be narrow and if sigma is set high the acceptance window will be wide. The computed Gaussian model may then be used in the following steps to determine which voxels of the 2D slice belong to the aorta.
 
     In steps  215  and  220 , an initial gathering of statistical information associated with the 2D slice takes place. More specifically, the steps are used to detect a drop in the intensities of the voxels near the boundaries of the aorta. This gathering of statistical information is illustrated, for example, in  FIG. 3 . In particular,  FIG. 3  illustrates a 2D slice of CT image data that includes a user selected seed point  310  with a plurality of rays  320  cast therefrom. The seed point  310  is surrounded by a circle  330  that is defined by a minimum expected aorta radius. By performing steps  215  and  220 , a shaded area  340  within the circle  330  is determined as belonging to the aorta and this will be the area of the 2D slice to be further analyzed. 
     Subsequent to the computation of the Gaussian model for all voxels in the 2D slice, a circle is grown within an area of the 2D slice determined to belong to the aorta ( 225 ). In particular, the circle, which is comprised of a plurality of control points, is grown from an initial point within the aorta until one of the control points reaches a threshold. In other words, the circle is grown until it comes into contact with one of the colliding points determined by the threshold of the Gaussian model. As the circle comes into contact and crosses the colliding points, a set of repulsion vectors for each of the colliding points is computed ( 230 ). 
     The repulsion vectors are calculated, for example, by defining the circle as having n control points p 1 , p 2  . . . pn uniformly spaced around the circle. The control points p 1 , p 2  . . . pn have n associated angles a 1 , a 2  . . . an, each having n associated cosines x 1 , x 2  . . . xn and sins y 1 , y 2  . . . yn. When one of the control points p 1 , p 2  . . . pn exceeds the threshold, an escape or repulsion vector is computed as follows: 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 xv = 0 
               
               
                   
                 yv = 0 
               
               
                   
                 for i in [1 . . . n] 
               
               
                   
                 { 
               
               
                   
                  if pi is not within the threshold 
               
               
                   
                  { 
               
               
                   
                   xv = xv + xi 
               
               
                   
                   yv = yv + yi 
               
               
                   
                  } 
               
               
                   
                 } 
               
               
                   
                 normalize(xv, yv). 
               
               
                   
                   
               
             
          
         
       
     
     Using the computed repulsion vectors, the circle is repositioned ( 235 ). This is done to maximize the average radius of the circle. Assuming that c(x, y) is the current center of the circle and v (xv, yv) is the escape or repulsion vector, the circle may be re-centered according to the following procedure: 
     
       
         
               
             
           
               
                   
               
             
             
               
                 i = 0; 
               
               
                 while (i &lt; max_escape_iterations) and some control points do not fit 
               
               
                 the vessel intensity model { 
               
               
                   x = x + xv * x_speed 
               
               
                   y = y + yv * y_speed 
               
               
                   ++i. 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     Next, it is determined if the circle is correctly repositioned within the aorta ( 240 ). The circle is correctly repositioned in the aorta if all the control points are on valid voxels. In other words, the circle is within the boundaries of the aorta. If the circle is correctly repositioned within the aorta, the method returns to step  225 , another circle is grown and the process is repeated until the circle does not fit in the aorta. If the circle is not correctly positioned within the aorta, a reduced copy of the circle is projected onto an adjacent 2D slice of the CT image dataset ( 245 ). This occurs when all the voxels in the circle are less than max_escape_iterations. This may be accomplished by assuming that c (x, y, z) are the coordinates of the center of the circle and r is its radius. The circle model is initialized by adding or subtracting (depending on the direction of the image acquisition) the thickness of the 2D slice from the z coordinate of the circle, thus reducing the radius of the copy of the circle by, for example, 50%. It is then determined if the radius of the projected circle is below the threshold ( 250 ). 
     If the radius is below the threshold, statistical measurements associated with the circle will be used to determine the location of the aortic valve. The statistical measurements associated with the circle may be calculated average intensities of each circle or observed variations of the center point for each circle. It is to be understood that the statistical measurements associated with each circle on each 2D slice may be taken at, during or after any step or steps. 
     An example of a statistical measurement used to determine the location of the aortic valve is shown in  FIG. 4 . As shown in  FIG. 4 , when the reproduced circles near the aortic valve (at the right hand side of the graph), their radii will drop off precipitously. This occurs because as the aorta enters the left ventricle the radii of the reproduced circles that correspond to this area converge to a minimum. This observation can then be used to stop the method and deduce the location of the aortic valve. 
     Referring back to step  250 , if the radius is not below the threshold, the method again returns to step  225  and the process is repeated until a circle having a radius below the threshold is found. This enables the location of the aortic valve to be known. Using the data associated with the location of the aortic valve, a segmentation may be performed on the aorta constrained by the location of the aortic valve. In other words, the aorta may be segmented and the segmentation will stop at the aortic valve. Thus, the segmentation will not leak into other regions of the aorta. 
     An example an aorta segmented in accordance with an exemplary embodiment of the present invention is shown in  FIGS. 5A and 5B . In particular,  FIG. 5A  illustrates an axial view and  FIG. 5B  illustrates a sagittal view of an aorta  510  segmented in accordance with the present invention. As shown in  FIG. 5B , the segmentation stops or is precluded from entering other parts of the aorta at the aortic valve  520 . 
     Thus, in accordance with an exemplary embodiment of the present invention, the location of the aortic valve may be determined. This location may then be used as a constraint when segmenting the aorta, the coronary arteries or the left ventricle. Therefore, this technique provides a quick (e.g., about 0.7 seconds on a 1 GHz i686 machine for a 100 CT slice aorta) and relatively non-computationally intensive pre-segmentation process, which can be used in conjunction with other segmentation processes to detect plaques, aneurysms and abnormal configurations of the aorta, the coronary arteries or the left ventricle. Using the same techniques for locating the aorta valve, the location of the aortic arch may also be determined. This location may then be used, for example, to determine where the coronary arteries branch out from the aorta thus preventing a segmentation from leaking into the coronary arteries. 
     It is to be understood that because some of the constituent system components and method steps depicted in the accompanying figures may be implemented in software, the actual connections between the system components (or the process steps) may differ depending on the manner in which the present invention is programmed. Given the teachings of the present invention provided herein, one of ordinary skill in the art will be able to contemplate these and similar implementations or configurations of the present invention. 
     It is to be further understood that the present invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or a combination thereof. In one embodiment, the present invention may be implemented in software as an application program tangibly embodied on a program storage device (e.g., magnetic floppy disk, RAM, CD ROM, DVD, ROM, and flash memory). The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. 
     It should also be understood that the above description is only representative of illustrative embodiments. For the convenience of the reader, the above description has focused on a representative sample of possible embodiments, a sample that is illustrative of the principles of the invention. The description has not attempted to exhaustively enumerate all possible variations. That alternative embodiments may not have been presented for a specific portion of the invention, or that further undescribed alternatives may be available for a portion, is not to be considered a disclaimer of those alternate embodiments. Other applications and embodiments can be straightforwardly implemented without departing from the spirit and scope of the present invention. 
     It is therefore intended that the invention not be limited to the specifically described embodiments, because numerous permutations and combinations of the above and implementations involving non-inventive substitutions for the above can be created, but the invention is to be defined in accordance with the claims that follow. It can be appreciated that many of those undescribed embodiments are within the literal scope of the following claims, and that others are equivalent.