Abstract:
Image processing techniques may include a methodology for normalizing medical image and/or voxel data captured under different acquisition protocols and a methodology for suppressing selected anatomical structures from medical image and/or voxel data, which may result in improved detection and/or improved rendering of other anatomical structures. The technology presented here may be used, e.g., for improved nodule detection within computed tomography (CT) scans. While presented here in the context of nodules within the lungs, these techniques may be applicable in other contexts with little modification, for example, the detection of masses and/or microcalcifications in full field mammography or breast tomosynthesis based on the suppression of glandular structures, parenchymal and vascular structures in the breast.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional patent application deriving priority from U.S. Provisional Patent Application No. 61/971,042, filed on Mar. 27, 2014, and incorporated by reference herein. 
     
    
     FIELD OF ENDEAVOR 
       [0002]    The present disclosure may relate generally to suppressing unwanted structures within a medical image and/or voxel data using feature extraction and successive model-based prediction methods and may further relate to techniques that may improve detection of lung nodules within computed tomography (CT) scans. Model-based prediction in the context of this disclosure is defined as the use of an analytical model(s), empirical model(s) or combination thereof, e.g., neural network, to predict a value (e.g., a pixel or voxel) based on measures, either computed or derived from pixels/voxels. 
       BACKGROUND 
       [0003]    It is widely recognized that object detection is a challenging problem. There are many aspects that make object detection difficult for computer vision systems, including factors such as variations in image acquisition, complexity of object appearance, and significant variability in object backgrounds (usually referred to as clutter), to name just a few. In the domain of medical imaging, an “object” might refer to a particular component of normal anatomy, the location of a non-anatomical object, or the presence of disease such as a tumor. 
         [0004]    One important application of object detection in medical imaging is the detection of lung nodules, or masses, in CT scans of the chest. Despite more than two decades of effort, the general problem of machine nodule detection remains unsolved, and human detection remains limited. We argue that a significant reason for this is a failure to address one significant component of what makes the problem difficult: the complex interaction of nodules with pulmonary vessels, and the variation in appearance due to varying acquisition protocols. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    Various aspects of this disclosure may include an approach for normalizing medical image and/or voxel data captured under different acquisition protocols, and/or a method for suppressing selected non-nodule structures within medical image and/or voxel data of the chest. Most non-nodule structure is vascular content, and therefore, the term “vessel suppression” will be used in this disclosure as a general term for such non-nodule structure suppression. However, the disclosed techniques may also apply to structures other than “vessels”/vascular structures, e.g., bronchial walls and fissure lines in the thorax, as well as, on occasion, man-made objects that take on tubular-like properties. This may further extend to other body parts (e.g., the breast, the heart, the head), other modalities (e.g., ultrasound, tomosynthesis, etc.) and/or other domains (e.g., video surveillance, military targeting). The techniques may be used for the purposes of improved nodule detection, nodule characterization and/or improved rendering of selected anatomically suppressed or enhanced image data. While the techniques are described herein with specific reference to nodules within the lungs, similar methodologies may be applied in other contexts. 
         [0006]    Additional features and advantages of various aspects of this disclosure will be apparent from the detailed description that follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of various aspects of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    In the accompanying drawings: 
           [0008]      FIG. 1  shows an example of a process diagram, e.g., for CT acquisition normalization, according to an aspect of the disclosure; 
           [0009]      FIG. 2  shows a further process diagram, showing further processing according to various aspects of the disclosure; 
           [0010]      FIGS. 3A and 3B  show an example of a slice of a volume before and after nodule simulation;  FIG. 3C  shows the same slice shown in  FIG. 3B  after vessel suppression; 
           [0011]      FIG. 4  shows a process diagram of an example of a prediction phase, according to an aspect of the disclosure; 
           [0012]      FIG. 5  provides a conceptual depiction of tissue separation, according to an aspect of the disclosure; 
           [0013]      FIGS. 6A-6C  show an image example of a ground glass nodule with vessels removed; and 
           [0014]      FIG. 7  shows a conceptual diagram of a system in which various aspects of the disclosure may be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    There are several approaches that may be taken to nodule detection, which may include template matching (e.g., Q. Li, S. Katsuragawa, and K. Doi, “Computer-aided diagnostic scheme for lung nodule detection in digital chest radiographs by use of a multiple-template matching technique,”  Medical Physics,  2001, 28(10): 2070-2076; hereinafter “Li et al. 2001”), multi-level thresholding (e.g., S. Armato, M. Giger, and H. MacMahon, “Automated detection of lung nodules in CT scans: preliminary results,”  Medical Physics,  2001, 28, 1552-1561; hereinafter “Armato et al. 2001”), enhancement filters (e.g., A. Frangi, W. Niessen, K. Vincken, and M. Viergever, “Multiscale vessel enhancement filtering,” MICCAI,  1998, 130-137; hereinafter “Frangi et al. 1998;” and Q. Li, S. Sone. and K. Doi, “Selective enhancement filters for nodules, vessels, and airway walls in two- and three-dimensional CT scans,”  Medical Physics  2003, 30(8): 2040-2051; hereinafter “Li et al. 2003”), and voxel classification (e.g., D. Wu, L. Lu, J. Bi, Y. Shinagawa, K. Boyer, A. Krishnan, and M. Salganicoff, “Stratified learning of local anatomical context for lung nodules in CT images,”  CVPR,  2000, 2791-2798.hereinafter “Wu et al. 2000”). 
         [0016]    Template matching (e.g., as in Li et al. 2001) may involve measuring the similarity about each voxel to a set of templates. The more varied the object appearance, the more templates may be needed for reasonable performance. This scaling behavior may make template matching inefficient in difficult domains. 
         [0017]    Multi-level thresholding may be used in CT (e.g., as in Armato et al. 2001), in part because voxel values, which may be defined in Hounsfield units (HU), may have a meaningful interpretation. Knowledge of nodule density, for example, may be used for setting the thresholds. This technique may encounter some difficulties, one of which may be proper measurement of object morphology for a given threshold. Nodules may be connected to surrounding structures, which may make accurate morphological assessment difficult. To compensate, such approaches may embed a morphological post-processing step to remove connected structures. The morphological post-processing may, however, alter the nodule to such an extent that it may become undetectable. One might adapt this process by using more elaborate post-processing, e.g., rule-driven adaptation, but such rule driven adaption may lead to brittleness, and the method may begin to look more like template matching. 
         [0018]    Filter enhancement methods (e.g., as in Frangi et al. 1998 and Li et al. 2003) may improve upon template matching by being adaptive to local structures. One such approach may be to estimate local structure derived from local tensor information. Two example tensors are the 3×3 Hessian matrix and the 3×3 structure tensor, where the “3” refers to the number of spatial dimensions. The eigenvalues from these tensors may be used to quantify the degree of “tubeness”, “blobness” or “plateness” at each voxel. These indicators may be combined to derive a composite feature index. While such analysis may be simple and analytically neat, it may encounter some limitations. First, the expressions used to combine the information may be based on idealizations of nodules and vessels that may not be true in reality. For example, nodules are not generally perfect spheres, and vessels are not generally perfect cylinders. As the methods may only use information captured by first- and second-order derivatives, the performance may falter in more complex regions such as where vessels bifurcate or where nodules become attached to vascular structure. Lastly, as the methods are based on idealizations, combining indicators across multiple scales may not be easy. 
         [0019]    Voxel classification, e.g., as in Wu et al. 2000, may involve extracting features from a CT scan that may then be used by a classification method to produce probabilities, or other outputs, that may indicate if a nodule is present or not. Voxel classification may need large amounts of manually labeled data, which may be impractical. The voxel classification method may also suffer from sample bias, which means that it may be specifically tuned to the types of nodules collected to train it, which may result in missed nodules. 
         [0020]    Another method is found in R. Wiemker, T. Buelow and T Klinder, “Visual Suppression of Selective Tissue in Image Data,” U.S. Patent Application Publication No. 2015/0063669 (hereinafter “Wiemker et al. 2015”). In Wiemker et al., the inventors describe a method for suppressing vascular structure as a weighted combination of original data with suppressed data. This type of methodology may often be referred to as “inpainting” in the literature. One such approach to object removal may be pyramid blending. The weight used for blending may be derived from a local likelihood of “vesselness,” which may correspond to a value between zero and one. How the measure is actually derived is never described in Wiemker et al. 2015; only its use as a means to blend image data with and without suppression is described. Wiemker et al. describes various modes of modification of the likelihood for emphasizing or deemphasizing the weight. The mechanism for getting the “opacity mappings” of Wiemker et al. is less clear. The description seems to imply that the density of voxels is decreased by a type of look-up table. 
         [0021]    It has also been recognized that it may be useful to develop automated methods for nodule detection and segmentation, but which may also provide adjunctive information that may be in the form, e.g., of secondary volumes that can be used by human experts (see, e.g., B. Van Ginneken, S. Armato, et al., “Comparing and combining algorithms for computer-aided detection of pulmonary nodules in computed tomography scans: The ANODE09 study,”  Med. Image Analysis,  2010, 14(6), 707-722, and E. M. Van Rikxoort, B. Van Ginneken, “Automated segmentation of pulmonary structures in thoracic computed tomography scans: a review,”  Physics in Medicine and Biology,  2013, 58, 187-220). 
         [0022]    Various aspects of the present disclosure may be related to frameworks built by the present inventors for normal anatomy suppression (see, e.g., U.S. Patent Application Publication No. 2009/0290779 to Knapp et al. (hereinafter, “Knapp et al. 2009”) and U.S. Patent Application Publication No. 2013/0223711 to Knapp et al. (hereinafter, “Knapp et al. 2013”), both of which are incorporated by reference herein). In Knapp et al. 2009), models were built by predicting an alternative image where the density of bones is removed. In Knapp et al. 2013, a pectoral muscle suppression technique may predict image data where the bias associated with the pectoral muscle is removed. 
         [0023]    In the context of the present disclosure, use of the term “suppression” implies that anatomical structures (such as vessels) are actually removed from an image and are not simply made less dense. Aspects of the present disclosure may relate to building a prediction model that may “predict out” undesired density, as will be described further below. 
         [0024]    Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
         [0025]    In identifying nodules in a CT scan, one may ideally like to take the CT scan and to suppress structures other than nodules. To start this process each scan may be normalized to account for variations associated with acquisition. This may enhance robustness and simplify further processes. The main steps in this normalization process, according to an aspect of this disclosure, may be seen in  FIG. 1 . The normalization process may begin with “body segmentation”  10 , in which a patient&#39;s body may be segmented from other structures within the field-of-view of the input CT image, and may subsequently segment the air regions associated with the patient&#39;s respiratory system. Following this, the air region may be calibrated  11  to a fixed value so that the lung density remains near a fixed value. After density calibration  11 , the CT scan&#39;s noise properties may be analyzed, and an adaptive local averaging method may be used in order to suppress noise artifacts  12 . Following noise suppression  12 , the image may be processed so that the contrast detail is as consistent as possible from one scan to the next  13 . This may be achieved, e.g., using techniques similar to those used in Knapp et al. 2009 and U.S. Patent Application Publication No. 2010/0266189 to Knapp et al. (hereinafter, “Knapp et al. 2010;” Knapp et al. 2010 is also incorporated by reference herein), in which histogram matching was used on a multi-scale representation in order to calibrate contrast detail. Lastly, the volume may be resized (resampled to a specified size, e.g., in millimeters), which may be performed in-plane (within a CT image) and/or out of plane (across CT images), which may involve using interpolation, so that slice spacing and thickness may be made as consistent as possible from one scan to the next  14 ; this may, again, use techniques described in Knapp et al. 2009 and/or Knapp et al. 2010. 
         [0026]    In order to perform vessel suppression, given a volume, one may wish to generate a volume with the vessels predicted out, while being careful not to remove other structures such as nodules. This may be achieved by a strategy of “forward through simulation, inversion with prediction,” which will be clarified in the subsequent discussion. 
         [0027]      FIG. 2  shows an example of an overall process flow according to an aspect of this disclosure. The process may begin with case selection, i.e., a set number of CT volumes of the thorax that capture a representative amount of normal (anticipated) variability. Using these selected cases, which may have been normalized for acquisition variation, a vessel suppressed volume may be generated for the purpose of model construction, shown in  FIG. 2  as target formation  20 . In one technique that may be used to create a vessel-suppressed volume (to which the invention is not limited), one may first compute a local minimum intensity projection (min-ip) along the axial direction of the CT volume. This min-ip operation may serve to suppress all, or substantially all, content within the lung fields. The min-ip volume may be blended with the CT volume using a slightly smoothed version of the segmented vessel mask, or in order to suppress all structures, may blended with the CT volume with a mask of known nodule locations. The vessel and/or nodule masks may be generated using an automated algorithm or a semi-automated method; or they may be derived from manual outlines. As these masks are to be used solely for the offline process of creating target data, the actual mechanism used to create them is not essential; however, the more accurate they are, the better. In order to have a sufficient number of examples of nodules interacting with pulmonary vessels, one may use nodule simulation, e.g., as in Knapp et al. 2009. That is, synthetic nodules may be inserted into the unsuppressed and/or vessel-suppressed volumes. The result may be a pair of volumes with and without vessels, but both with unaltered, or substantially unaltered, nodules.  FIG. 3A  is a slice from a represented example, while in  FIG. 3B  we see the same slice, only with simulated nodules added. In  FIG. 3C  we see the same slice from  FIG. 3B , only with vessels suppressed, and in which the nodules have been left unaltered. 
         [0028]    Once the pairs of input-target volumes are created  20 , one may process each image by passing it through a feature generation process  21 . This process may be used to extract voxel-level features, where an example set of features may be as follows:
       Multi-scale Gaussian derivatives with a range of orders;   Local minimum and maximum intensity projections;   In-plane features computed from and/or on the local minimum and maximum intensity projections.       
 
         [0032]    Other features may be derived based on location or the derivative features themselves (shape indices, curvature, etc . . . ) or learned using model-based approaches, such as patch analysis or deep neural networks. Given the large collection of data, a model, or a set of models, may be generated  22 . According to one aspect of this disclosure, multi-layer feed-forward neural networks may be used for this purpose. These models may subsequently be able to predict suppressed data, without the use of a segmentation mask or object indicators, similar to methods found in Knapp et al. 2013. Once a model or set of models have been built, their performance on validation data can be assessed  23 . This may lead to the selection of a particular subset of models and/or could be used to further guide the training process by the selection of additional cases for training or the creation of more simulated nodules. 
         [0033]      FIG. 4  depicts an example of a high-level flow diagram from a normalized CT scan to a suppressed scan, according to aspects of this disclosure. The extracted features  40  may be the same as those used in model generation  22 . The prediction phase  41  may involve applying a set of prediction models, e.g., neural networks, whose outputs may be combined. The combining may be performed, e.g., by averaging, by using a cost function to select an optimal one of the outputs, or both (i.e., selecting an optimal set of outputs to average). In this aspect of the disclosure, multiple model outputs may be obtained for multiple image zones, where each image zone may be identified by its anatomical location and/or by its pixel density. Post-processing  42  may be used to adjust for the normalization process so that the suppressed volume is in-line with the original image data, if desired for display purposes, as it may not be needed for computer-aided detection. 
         [0034]      FIG. 5  provides a conceptual example of how the overall technique may operate on a real nodule and may generate two representations. An image  50  may undergo anatomy suppression  51 , to result in two representations  52 . The upper representation in  52  may be a suppressed volume, which may allow for easier detection and segmentation of nodules; and the lower representation in  52  may be a vessel volume, which may simply be the difference between the CT data and the suppressed data. The vessel volume may be useful, e.g., for vessel segmentation or registration with prior scans. 
         [0035]      FIG. 6 , consisting of  FIGS. 6A-6C  is an up-close view of an example of a real ground glass nodule with apparent vessel superposition.  FIG. 6A  shows the CT image.  FIG. 6B  shows an up-close view of the ground glass nodule image, and  FIG. 6C  shows a vessel-suppressed image. As can be seen, the algorithm may remove the vascular structure while preserving the nodule content. 
         [0036]    Various embodiments of the invention may comprise hardware, software, and/or firmware.  FIG. 7  shows an exemplary system that may be used to implement various forms and/or portions of embodiments according to various aspects of this disclosure. Such a computing system may include one or more processors  72 , which may be coupled to one or more system memories  71 . Such system memory  71  may include, for example, RAM, ROM, or other such machine-readable media, and system memory  71  may be used to incorporate, for example, a basic I/O system (BIOS), operating system, instructions for execution by processor  72 , etc. The system may also include further memory  73 , such as additional RAM, ROM, hard disk drives, or other processor-readable media. Processor  72  may also be coupled to at least one input/output (I/O) interface  74 . I/O interface  74  may include one or more user interfaces, as well as readers for various types of storage media and/or connections to one or more communication networks (e.g., communication interfaces and/or modems), from which, for example, software code may be obtained or provided (e.g., by downloading or uploading). 
         [0037]    It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised as described in the usage and extension to other applications and domains sections without departing from the spirit and scope of the present invention. 
         [0038]    To elaborate, generation of a vessel-suppressed volume may have many uses outside nodule detection and characterization. These may include:
       Simpler and more robust segmentation of vessels, which can be used as inputs to other processes, such as pulmonary embolism detection;   Improved vessel tree navigation to biopsy nodules attached to the vessels based on improved visualization of the vascular tree;   Improved detection of disease related to the vascular structure that leads the thickening and/or constriction of the vessels;   Use of the vessel volume, or segmented mask, as an input to a scan registration process, thus allowing for assessment of any potential abnormal changes, such as nodule growth;   Use of the suppressed volume to generate a more-informative display, such as the generation of a maximum intensity projection of the lung (which may, in effect, provide a high-level indication of where nodules may be located);   Two-dimensional reconstructions with both bones and vessels suppressed.       
 
         [0045]    Furthermore, the present techniques may be applied in other applications/domains. Many extensions of the framework presented above go beyond vessel suppression, and the following is merely a list (which is not intended to be exhaustive) of some further applications:
       Vessel suppression in the brain for highlighting cerebral aneurysms;   Vessel suppression in CT liver scans for highlighting nodules and improving the ability to segment the vascular structure, perhaps for registration purposes;   Vessel suppression in fundus images of the eye for highlighting micro-aneurysms and other diseases;   Vessel suppression in the breasts (volumes or images acquired via tomosynthesis or mammograms, respectively) for detection or removal of benign calcification, for use in registration or other processes.       
 
         [0050]    While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.