Abstract:
A system and method for detecting central pulmonary embolisms in a subject&#39;s vasculature is provided. In some aspects, the method includes receiving, using the input, a set of images representing a vasculature of the subject&#39;s lungs, automatically analyzing the set of images to segment the main arteries associated with the subject&#39;s lungs and separate the main arteries from surrounding tissues. The method also includes automatically extracting central pulmonary embolism candidates from the set of images after segmenting and separating the main arteries, and automatically evaluating the central pulmonary embolism candidates in three-dimensional (3D) space by applying a series of rules. The method further includes automatically displaying a report indicating evaluated central pulmonary embolism candidates on a display.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on, claims priority to, and incorporates herein by reference in its entirety, U.S. Provisional Application Ser. No. 62/202,562, filed Aug. 7, 2015, and entitled “Systems and Methods for Detecting Central Pulmonary Embolism in CT Pulmonary Angiography Images.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    N/A 
       BACKGROUND 
       [0003]    Acute pulmonary embolisms (PEs) occurs when venous thrombi are dislodged and travel through the vascular system and partially or completely occlude the pulmonary artery. There are no clinical signs or symptoms that are specific for PEs because many of the presenting features are common among patients without PEs, rendering this diagnosis challenging. PEs represent a relatively common cardiovascular emergency with about 600,000 cases occurring annually, causing approximately 200,000 deaths in the US. According to one retrospective study covering a 25-year span, the annual incidence of pulmonary embolism is 69 per 100 thousand. In another study, the PE incidence rate in the Worcester Statistical Metropolitan Area was reported to be 27%. Pulmonary embolism was thought to be the cause of death in 239 of 2388 autopsies performed. The unsuspected or undiagnosed ante mortem PE rate at autopsy may be as high as 78% (1902/2448). With early diagnosis and treatment, the mortality rate can be reduced to less than 11%. 
         [0004]    Pulmonary embolisms do not have consistent sizes or morphologies, and the affected pulmonary arteries may be obstructed completely or partially. When embolic obstruction is complete, the entire arterial cross-section is occluded with embolic material causing the normally enhanced vascular lumen to show low attenuation. When partially occlusive embolic disease is present, enhanced blood partially surrounds intravascular emboli creating the appearance of low-attenuation intravascular material either seen floating within the vessel lumen or in contact with the lumen wall. 
         [0005]    Central PEs may be located in the main, left or right pulmonary arteries. Detecting central PEs is critical because the major pulmonary arterial obstruction can significantly compromise pulmonary perfusion and impair right ventricular function and result in sudden death. In one study, 579 patients were evaluated and, in 516 hemodynamically stable patients, central localization of emboli was an independent predictor of all-cause death or clinical deterioration while distal emboli were inversely associated with these outcome events. In a study on detection of central PE on unenhanced CT, of the 88 studies positive for pulmonary emboli, 64% had central involvement. In another study, clinically “silent” PE—an unexpected postmortem finding—was found in 32% of patients and may affect the central pulmonary arteries. Saddle PE, which may be in the main artery, was diagnosed in 17 (5.2%) cases of 289 patients with acute PE confirmed by spiral CT. In one study, 12.1% of 214 examinations had central clots. In a landmark study that initially described the CT technique now widely used for suspected pulmonary embolism assessment, 37% partially and 46% completely occlusive pulmonary arterial filling defects were found in  42  patients with central PE. 
         [0006]    The primary imaging technique utilized for PE diagnosis is computed tomography pulmonary angiogram (CTPA), where PEs appear as a low-attenuation filling defects within the normally highly attenuating, enhanced pulmonary arteries. In particular, a “dark” or hypo-attenuating region within a pulmonary artery represents a region of the vessel where intravenous contrast was excluded, indicating the presence of an PE candidate. Analysis by a radiologist typically involves the careful tracing each branch of the pulmonary vasculature in order to look for and identify suspected PEs. Although it is easier to detect central PEs compared to sub-segmental PEs, for instance, central PEs are often overlooked by human readers. This is because of the large number of arteries that have to be tracked as well as complexity and number of the images, with typical CTPA datasets having up to  500  individual images. Hence, PE diagnosis often requires extensive reading time whose accuracy depends on a clinician&#39;s experience, attention span, eye fatigue, as well as sensitivity to the visual characteristics of different PEs. 
         [0007]    Computer aided diagnosis (CAD) systems for PE detection have been utilized as a “second pair of eyes” in the clinical setting, in the attempt to improve the radiologists&#39; performance. However, computer-aided detection is a challenging task because the images are complex and include artifacts, and vary in imaging conditions. CAD systems for PE detection are typically focused on segmental and sub-segmental emboli, with only very few being capable of detecting central emboli. This is because such systems generally apply a lung segmentation that excludes the main, left and right pulmonary arteries and, as a result, central emboli are likely missed. For example, the CAD product developed for Philips is unlikely to be capable of detecting large or central emboli. The system developed by Siemens detects 88.5% of emboli affecting central pulmonary arteries. 
         [0008]    In one study, a computationally expensive methodology was used to segment the pulmonary trunk, left and right pulmonary artery to detect central PE from 33 cases, achieving a 14.4 FP/ds rate with 95.1% sensitivity. In another study, an active contour model was used to extract the pulmonary arterial tree, employing a k-means algorithm and the Gaussian mixture models for main vessel searching and using a skeleton model for vessel tracking. The detection system employed a relatively sophisticated algorithms, and achieved a 3.8% FP/ds overall performance rate, but with a small sample size of only 16 cases. In yet another study, an expectation-maximization analysis was used. The vessel tree was reconstructed and a rule-based method was used to detect PE. An initial seed point was used to initiate vessel tracking. Only 14 cases were tested and the detection sensitivity was reported as 67% for the pulmonary main, lobar, and segmental arteries. As appreciated, these studies were limited to sample sizes, hindering a solid performance evaluation. 
         [0009]    Thus, there remains a continuing need for new technologies capable of detecting pulmonary arterial emboli with high sensitivity and reliability. 
       SUMMARY 
       [0010]    The present disclosure overcomes the aforementioned drawbacks by providing systems and methods that facilitate tracing the pulmonary trunk, and left and right arteries, and detecting and marking central PE candidates. A rule-based approach is provided that effectively and efficiently guides analysis. 
         [0011]    In accordance with one aspect of the disclosure, a system for detecting pulmonary embolisms in a subject&#39;s vasculature is provided. The system includes an input configured to receive images acquired from a subject, and a processor configured to process the images with steps including receiving, using the input, a set of images representing a vasculature of the subject&#39;s lungs, and analyzing the set of images to segment the main arteries associated with the subject&#39;s lungs and separate the main arteries from surrounding tissues. The processor is also configured to carry out steps for extracting pulmonary embolism candidates from the set of images after segmenting and separating the main arteries, evaluating the pulmonary embolism candidates in three-dimensional (3D) space by applying a series of rules, and generating a report indicating evaluated candidate pulmonary embolisms. The system further includes an output for displaying the report. 
         [0012]    In accordance with another aspect of the disclosure, a method for detecting central pulmonary embolisms in a subject&#39;s vasculature. The method includes receiving, using the input, a set of images representing a vasculature of the subject&#39;s lungs, automatically analyzing the set of images to segment the main arteries associated with the subject&#39;s lungs and separate the main arteries from surrounding tissues. The method also includes automatically extracting central pulmonary embolism candidates from the set of images after segmenting and separating the main arteries, and automatically evaluating the central pulmonary embolism candidates in three-dimensional (3D) space by applying a series of rules. The method further includes automatically displaying a report indicating evaluated central pulmonary embolism candidates on a display. 
         [0013]    The foregoing and other advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a flow chart setting forth some non-limiting examples of steps for detecting a central pulmonary embolisms in accordance with the present disclosure. 
           [0015]      FIG. 2  is a graphic illustration of a process for generating an initial seed point in accordance with the present disclosure. 
           [0016]      FIG. 3  is a non-limiting example image illustrating segmented right and left pulmonary arteries in accordance with the present disclosure. 
           [0017]      FIG. 4  is a non-limiting example image illustrating detection of central PE in pulmonary arteries in accordance with the present disclosure. 
           [0018]      FIG. 5  is a block diagram of a system in accordance with the present disclosure and designed to perform the steps described with respect to  FIG. 1  and deliver results, such as described with respect to  FIGS. 2-4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A system and method for detecting pulmonary embolisms (PEs) in a subject&#39;s vasculature are provided. In particular, a novel approach is herein described, based on tracing the pulmonary trunk, and left and right arteries, that allows the detection and marking of central pulmonary arterial emboli candidates. A rule-based implementation is also utilized providing efficient and effective guidance. The system and method may be used alone or may be a part of a broader computer aided diagnosis (CAD) system for PE detection to help improve a radiologist performance. 
         [0020]    There are several challenges preventing current technologies from reliably detecting central pulmonary arterial emboli. First, certain tissues, such as the superior vena cava, may come in close proximity to pulmonary arteries on contiguous images, hence obscuring or interfering with the visibility of PEs. Therefore, the present disclosure recognizes the importance of separating pulmonary arteries from such adjacent non-arterial enhancing structures. Second, structures near the pulmonary arteries can also create a relatively hypointense, or “dark” region between the structure in question and the adjacent pulmonary artery, which may cause a false positive detection. Third, certain tissues surrounding the pulmonary artery, such as lymph nodes, may have attenuation values similar to PE. 
         [0021]    Recognizing these constraints, the present disclosure provides a system and method capable of differentiating PEs from adjacent tissues, particularly in the case when an artery is fully occluded by a PE. A similar situation can occurs when a PE is in contact with the vessel wall and needs to be differentiated if a vascular narrowing or an embolus exists. The system and method of the present disclosure can also facilitate differentiation in this situation. Lastly, irregular borders, poor image quality and various artifacts can further cause false positives. The system and method of the present disclosure are designed to overcome these challenges that plague traditional CAD or analysis systems, allowing accurate PE detection. 
         [0022]    Referring to  FIG. 1 , some non-limiting steps are provided for an automated process  100  for detecting of central pulmonary emboli, in accordance with aspects of the present disclosure. The process  100  may be carried out using a system as described with reference to  FIG. 5 , or any other suitable system. The process  100  may begin with acquiring or accessing image data, and in particular image data representing a subject&#39;s vasculature. In some aspects, the acquired image data may include a set of three-dimensional (3D) images, such as computed tomography pulmonary angiogram (CTPA) images. 
         [0023]    In processing an acquired or accessed image to identify the presence of central pulmonary emboli, an initial point detection may be performed at process block  10 . The initial point detection may be performed automatically, or semi-automatically using input from a user. This is illustrated in the example image  12  and further shown in  FIG. 2 . Referring specifically to  FIG. 2 , to perform initial point detection, an initial seed point  14  described by coordinates (x 0 ,y 0 ,z 0 ) can be identified or selected using an initial image  15 . By way of example, the seed point  14  may a point located just cranial to the bifurcation of the main pulmonary artery, as illustrated  FIG. 2 . To facilitate the identification or selection of the initial seed point  14 , in some aspects, the initial image  15  may be further processed. For instance, as shown in  FIG. 2 , a masking image or boundary image  16 , illustrating pulmonary artery tissue, for example, may be generated by processing the initial image  15  using a number of segmentation, thresholding and/or boundary or edge detection techniques. 
         [0024]    Referring again to  FIG. 1 , with an initial seed point in place, segmentation of the main arteries may then be performed at process block  18  using the acquired or accessed image. In particular, in a first step for detecting central PEs, the pulmonary trunk and main pulmonary arteries can be can be identified and segmented. As shown in  FIG. 1 , a first segmented image  20  identifying these arteries may be generated at process block  18 . In some aspects, the contrast or intensity level of the pulmonary trunk may be used to determine an appropriate threshold for segmentation. That is, rather than using a preselected threshold value, the threshold value may be dynamically adapted for each image, or image set. In one non-limiting example, a mean contrast or intensity value computed using approximately 10 pixel squares around the initial seed point determined at process block  10  may be used for the adaptive threshold selection. 
         [0025]    As the tissues surrounding the pulmonary arteries may have a similar intensity range to the pulmonary trunk and the main pulmonary arteries, they may appear connected to the regions of interest on the image, and may cause false positives. Some of these tissues may create the appearance of a “dark” region between the main arteries and the tissue in question, and be interpreted as a PE by the system. Thus, it may be desirable to perform error checking in an automated or other fashion. 
         [0026]    The first segmented image  20  obtained at process block  18  may then be used to perform a separation of surrounding tissue, as indicated by process block  22 . Specifically, the main arteries, namely the left and right arteries, may be extracted and separated from their surrounding tissues by using vertical and horizontal line from the initial seed point determined at process block  10 . In some aspects, a vessel edge tracking algorithm may be used. In particular, high value changes on the edges, which accounts for slices, are considered as connected tissues. New edges are generated after removing these tissues. In some aspects, a connected component analysis can be performed to determine the connectivity of the segmented vessels. 
         [0027]    The separation performed at process block  22  may yield a second segmented image  24  that is appropriate for PE candidate extraction at process block  26 , as shown in the example of  FIG. 1 . Based thereon, 3D space may analyzed at process block  28 . That is, the algorithm may continue to run iteratively through candidate extraction at process block  26  and 3D space analysis at process  28  until it reaches the bifurcation point of each artery as shown in  FIG. 3 . 
         [0028]    That is, as illustrated in  FIG. 3 , right and left pulmonary arteries have been segmented. Morphologic opening and filling operations can be performed to disconnect some of the surrounding tissues like the superior vena cava and fill the holes in the region. The pulmonary trunk and the main arteries can be extracted using connected component labeling obtained from the 2D images and reconstructed in 3D space for analyses at process block  28  of  FIG. 1 . 
         [0029]    More particularly, the pulmonary trunk and the main arteries can be searched separately for a hypo-attenuated or “dark” region either connected to the vessel wall or floating within the vessel lumen. The pulmonary trunk and left main artery can be traced from the cranial to the caudal direction, with the edges compared on each side of vessel. The right main artery can be traced from right to left with the cranial and caudal vessel edges compared. The sum of intensity values in the binary image can be compared to a variety of thresholds, such as with the number of pixels between the two edges. If the difference is greater than a predetermined threshold, such as a threshold selected based on a meaningful difference (such as a 5 pixel difference), then it may be determined that there is a hypo-attenuating focus within the vessel. Any such hypo-attenuating regions can be considered to be embolic candidates, and moved from the 2D image to be analyzed in 3D space. 
         [0030]    Any small pieces appearing on only one image can be removed using a logical operator. In this way these pieces can also be evaluated against rules or thresholds. For example, pieces that are more than a rule or threshold, such as being more than two voxels in height and/or width, may be automatically selected as candidates. As another example of a rule or threshold that may be applied, if the average intensity value of the candidate region is between, as a non-limiting example, −100 HU and 200 HU, then the region may be automatically chosen as a pulmonary embolus candidate. Using this process, a candidate detection can be automatically performed, as indicated by process block  30 . 
         [0031]    A report may then be provided or displayed either intermittently or in substantially real time, and may include a variety of information, including information associated with PEs, and particularly central PEs, detected. In some aspects, the report may be in the form of one-dimensional, two-dimensional, such as example image  32  in  FIG. 1 , or three-dimensional images that highlight various regions or volumes of interest, including detected PE candidates and PEs, as well as non-PE structures, such as vessels, lungs, and other tissues or organs. More particularly, the report may identify or mark the locations of identified PEs candidates, as illustrated in the example shown in  FIG. 4 . The report may also indicate the probabilities or confidence scores for identified objects being PEs. The report may further provide an audio alert or signal to an operator upon identification of PEs. 
         [0032]    It can be a challenge for a computer program to distinguish an embolus in contact with the vessel wall from vascular narrowing. Vascular edges normally narrow as they approach bifurcation points, but the degree of change on consecutive CTPA images in the caliber of a normal vessel is slight for a given pulmonary artery. In contrast, if the vascular caliber changes rapidly, it is then it can be assumed an embolus candidate may be present. The vessel edge changes may be kept in an array and the vessel edge analysis was performed in 3D to differentiate these possibilities. 
         [0033]    One non-limiting example study applying the above-described process used 55 cases with 24 central emboli. Each case consisted of 400-650 DICOM (Digital Imaging and Communications in Medicine) formatted images with 512×512 voxels and slice thickness is approximately 0.67 mm. Among the cases, 13 cases had suboptimal image quality, but they were included in the study in order to evaluate the robustness of our proposed algorithm. That is, cases with poor image quality and artifacts were not discarded. The sensitivity of the system was 96%, the FP/ds rate is 0.59, and only one false negative (FN) occurred where the vessel was completely obstructed by emboli. The central PE locations marked by the proposed system were confirmed by an experienced reviewer. 
         [0034]    Turning now to  FIG. 5 , a block diagram of an example system  500  for detecting pulmonary embolisms in a subject&#39;s vasculature is provided. The system  500  may generally include a processor  102 , an input  104 , an output  106 , and a memory  108  having software  110  and data  112  stored therein. For example, the system  500  may be a workstation, a notebook computer, a personal digital assistant (PDA), a multimedia device, a network server, a mainframe or any other general-purpose or application-specific computing device. In some aspects, the system  500  may be portable, such as a mobile device, laptop, tablet, or other portable device or apparatus. The system  500  may operate as part of, or in collaboration with a computer, system, device, machine, mainframe, or server, and so forth. In this regard, the system  500  may be any computing device, apparatus or system configured for carrying out instructions in accordance with aspects of the present disclosure. 
         [0035]    The system  500  may integrate a variety of software, hardware capabilities and functionalities, and operate autonomously or semi-autonomously. In some aspects, the system  500  may read executable instructions, stored in the memory  108  as transitory and/or non-transitory computer-readable media. The system  500  may also receive instructions, information or data from a user, or any other source logically connected thereto, such as another system, device, apparatus, networked computer, server, database, the internet, a cloud, and so forth. 
         [0036]    In some implementations, as shown in  FIG. 5 , the processor  102  may be in communication with an imaging system  114  and/or an external storage  118 , and configured to exchange a wide variety of data and information, including imaging data, therebetween. By way of example, the imaging system  114  may be a computed tomography (CT) system, a fluoroscopy system, a magnetic resonance (MR), system, an ultrasound (US) system, and so forth. The system  500  may be configured to acquire image data and analyze the data in substantially real-time. Alternatively, the system  500  may also be configured to access, retrieve and analyze image data already stored in memory  108 , external storage  116 , or other data location. 
         [0037]    In addition to being configured to carry out steps for operating the system  500 , the processor  102  may be configured to acquire and/or process image data, including image data obtained during a medical procedure, such as a pulmonary angiography. As such, the processor  102  may read and execute instructions stored on a non-transitory computer-readable medium. In particular, the processor  102  may be configured to acquire or access image data, such as two-dimensional and/or three-dimensional CT pulmonary angiography (CTPA) data representing a vasculature of the subject, and analyze the image data as described above. In some implementations, a user may provide various selections for different regions or volumes of interest using the input  104 , such as a mouse, keyboard, touchpad, touch screen, buttons, and others. 
         [0038]    In accordance with aspects of the disclosure, the processor  102  may transform and process acquired or accessed image data, in accordance with the present disclosure. As described, such representation affords many advantages, including consistent PE appearance that allows for accurate detection. 
         [0039]    A report may then be generated by the processor  102 , and provided via the output  106 , which may include an audio and/or visual display system. The report may be provided either intermittently or in substantially real time, and may include a variety of information, including information associated with pulmonary embolisms detected. For example, the report may include CTPA images that highlight various regions or volumes of interest, including detected PE candidates and PEs, as well as non-PE structures, such as vessels, lungs, and other tissues or organs. The report may also indicate the probabilities or confidence scores for identified objects being PEs. The report may also include an audio alert or signal to an operator upon identification of PEs. 
         [0040]    The present invention has been described in terms of one or more embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. The appended document describes additional features of the present invention and is incorporated herein in its entirety by reference.