Abstract:
Methods for registering a three-dimensional model of a body volume to a real-time indication of a sensor position that involve analyzing scanned and sensed voxels and using parameters or thresholds to identify said voxels as being either tissue or intraluminal fluid. Those voxels identified as fluid are then used to construct a real-time sensed three-dimensional model of the lumen which is then compared to a similarly constructed, but previously scanned model to establish and update registration.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61,058,470 filed Jun. 3, 2008 entitled Feature-Based Registration Method, which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Breakthrough technology has emerged which allows the navigation of a catheter tip through a tortuous channel, such as those found in the pulmonary system, to a predetermined target. This technology compares the real-time movement of a sensor against a three-dimensional digital map of the targeted area of the body (for purposes of explanation, the pulmonary airways of the lungs will be used hereinafter, though one skilled in the art will realize the present invention could be used in any body cavity or system: circulatory, digestive, pulmonary, to name a few). 
         [0003]    Such technology is described in U.S. Pat. Nos. 6,188,355; 6,226,543; 6,558,333; 6,574,498; 6,593,884; 6,615,155; 6,702,780; 6,711,429; 6,833,814; 6,974,788; and 6,996,430, all to Gilboa or Gilboa et al.; and U.S. Published Applications Pub. Nos. 2002/0193686; 2003/0074011; 2003/0216639; 2004/0249267 to either Gilboa or Gilboa et al. All of these references are incorporated herein in their entireties. 
         [0004]    Using this technology begins with recording a plurality of images of the applicable portion of the patient, for example, the lungs. These images are often recorded using CT technology. CT images are two-dimensional slices of a portion of the patient. After taking several, parallel images, the images may be “assembled” by a computer to form a three-dimensional model, or “CT volume” of the lungs. 
         [0005]    The CT volume is used during the procedure as a map to the target. The physician navigates a steerable probe that has a trackable sensor at its distal tip. The sensor provides the system with a real-time image of its location. However, because the image of the sensor location appears as a vector on the screen, the image has no context without superimposing the CT volume over the image provided by the sensor. The act of superimposing the CT volume and the sensor image is known as “registration.” 
         [0006]    There are various registration methods, some of which are described in the aforementioned references. For example, point registration involves selecting a plurality of points, typically identifiable anatomical landmarks, inside the lung from the CT volume and then using the sensor (with the help of an endoscope) and “clicking” on each of the corresponding landmarks in the lung. Clicking on the landmarks refers to activating a record feature on the sensor that signifies the registration point should be recorded. The recorded points are then aligned with the points in the CT volume, such that registration is achieved. This method works well for initial registration in the central area but as the sensor is navigated to the distal portions of the lungs, the registration becomes less accurate as the distal airways are smaller and move more with the breathing cycle. 
         [0007]    Another example of a registration method is to record a segment of an airway and shape-match that segment to a corresponding segment in the CT volume. This method of registration suffers similar setbacks to the point registration method, though it can be used in more distal airways because an endoscope is not required. The registration should be conducted more than once to keep the registration updated. It may be inconvenient or otherwise undesirable to require additional registration steps from a physician. Additionally, this method requires that a good image exists in the CT volume for any given airway occupied by the sensor. If for example, the CT scan resulted in an airway shadowed by a blood vessel, for example, the registration will suffer because the shape data on that airway is compromised. 
         [0008]    An alternative registration method known as “Adaptive Navigation” was developed and described in U.S. Published Application 2008/0118135 to Averbuch et al., incorporated by reference herein in its entirety. This registration technique operates on the assumption that the sensor remains in the airways at all times. The position of the sensor is recorded as the sensor is advanced, thus providing a shaped historical path of where the sensor has been. This registration method requires the development of a computer-generated and automatically or manually segmented “Bronchial Tree” (BT). The shape of the historical path is matched to a corresponding shape in the BT. 
         [0009]    Segmenting the BT involves converting the CT volume into a series of digitally-identified branches to develop, or “grow,” a virtual model of the lungs. Automatic segmentation works well on the well-defined, larger airways and smaller airways that were imaged well in the CT scans. However, as the airways get smaller, the CT scan gets “noisier” and makes continued automatic segmentation inaccurate. Noise results from poor image quality, small airways, or airways that are shadowed by other features such as blood vessels. Noise can cause the automatic segmentation process to generate false branches and/or loops—airways that rejoin, an occurrence not found in the actual lungs. 
         [0010]    It would be advantageous to provide a registration method that is automatic and continuous, and has an increased accuracy potential that is achieved without requiring any steps to be taken by a physician. 
       SUMMARY OF THE INVENTION 
       [0011]    In view of the foregoing, one aspect of the present invention provides a feature-based registration method. When the CT scans are taken, the CT machine records each image as a plurality of pixels. When the various scans are assembled together to form a CT volume, voxels (volumetric pixels) appear and can be defined as volume elements, representing values on a regular grid in three dimensional space. Each of the voxels is assigned a number based on the tissue density Housefield number. This density value can be associated with gray level or color using well known window-leveling techniques. 
         [0012]    One aspect of the present invention relates to the voxelization of the sensing volume of an electromagnetic field by digitizing it into voxels of a specific size compatible with the CT volume. Each voxel visited by the sensor can be assigned a value that correlates to the frequency with which that voxel is visited by the sensor. The densities of the voxels in the CT volume are adjusted according to these values, thereby creating clouds of voxels in the CT volume having varying densities. These voxels clouds or clusters thus match the interior anatomical features of the lungs. 
         [0013]    Another aspect of the present invention is to provide a plurality of parameters that a particular voxel of the CT volume must meet before being considered as a candidate for matching to a corresponding voxel in the sensor sensing volume. For example, the voxel could be required to meet parameters such as: 1) falls within a particular density range, 2) falls within a predefined proximity from a currently accepted (registered) voxel, 3) fits within a specific template such as a group of continuous densities corresponding to air next to a plurality of densities corresponding to a blood vessel. This may be useful when it is known that, for example, a particular airway runs parallel to a pulmonary artery, so, for a given length, the airway voxels should be in specified proximity to pulmonary artery voxels. 
         [0014]    One aspect of the present invention provides an iterative approach to registration. In other words, registration is continually updated and restarted, such that previous registration is being constantly discarded. This may be advantageous when, for example, navigating to a very distal portion of the lungs. Because the distal lungs move considerably with the breathing cycle, registration that occurred closer to the main carina may not be relevant to the distant areas. Additionally, using this iterative approach, the potential inaccuracy is not cumulative. 
         [0015]    Another aspect of the present invention provides a continuous approach, as an alternative to the iterative approach, to registration. The continuous approach involves the step-by-step correction of the previously performed transformation of voxel-based cavity features to geometry-based structures and shapes. 
         [0016]    Another aspect of the present invention is that, by using a voxel-based approach, registration is actually accomplished by comparing anatomical cavity features to cavity voxels, as opposed to anatomical shapes or locations to structure shapes or locations. An advantage of this approach is that air-filled cavities are of a predictable, constant density. Conversely, tissue, especially lung tissue, is of a variable, less predictable density. One skilled in the art will see that all of the technology described herein applies equally well to the vasculature of a patient. Blood-filled cavities, like air-filled cavities, are of a predictable, constant density. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]      FIG. 1  is a flowchart of a method of the present invention; and 
           [0018]      FIG. 2  is a flowchart of a more specific example of an embodiment of the method of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Generally, the present invention includes a system and method for registering a three-dimensional model of a body volume, such as a CT volume, to a real-time image of a sensor. This registration method compares anatomical cavity features to cavity voxels, as opposed to anatomical shapes or locations to structure shapes or locations. 
         [0020]    Referring now to the flowchart of  FIG. 1 , it is shown that the method of the present invention begins at  20  with a collection of reference data. This step involves the acquisition of a plurality of CT scans, which are then assembled into a CT volume. During the procedure, the sensor is inserted into the lungs of the patient and a data stream is established between the sensor and a system processor. 
         [0021]    At step  22 , the data acquired is processed, which involves de-cluttering and digitization. Each of the voxels is assigned a number based on the tissue density Housefield number. This density value can be associated with gray level or color using well-known window-leveling techniques. The density is proportional to a probability that the sensor will occupy a given voxel. The data is also filtered as desired. For example, if the sensor is advanced slowly rather than quickly, it will necessarily result in higher densities as any one voxel is going to be occupied for a longer period of time while the sensor takes longer to pass through. Hence, an advancement rate may be noted and used to normalize the densities by speed, accordingly. After filtering, the voxels with higher densities are given higher weight in registration than voxels having lower densities. 
         [0022]    At step  24  the desired parameters are defined. By way of example only, the voxel could be required to meet parameters such as: 1) falls within a particular density range, 2) falls within a predefined proximity from a currently accepted (registered) voxel, 3) fits within a specific template such as a group of continuous densities corresponding to air next to a plurality of densities corresponding to a blood vessel. 
         [0023]    At  26 , a compare and fit function is performed. This step includes multiple sub-steps, beginning with step  30 . These steps are performed iteratively and repeatedly until the target is reached. 
         [0024]    Step  30  involves an initial guess and is based on assumptions or known landmark techniques. For example, the main carina is relatively easy to match to the main carina of a BT. 
         [0025]    At  32 , the CT volume is registered to the sensor data using the initial guess and a difference between the two is calculated. 
         [0026]    At  34 , for each real voxel visited by the sensor, the registration software finds the closest voxel in the CT volume that matches specific parameters. The registration is then updated accordingly. If the process is iterative, the matched voxels may be aligned completely (ideally). If the process is continuous, a density function is used to weight the importance of that particular voxel match and the registration is adjusted, using frequency and/or density, a degree that is proportional to the weighted importance. 
         [0027]    Referring now to  FIG. 2  for illustration purposes, there is shown a more specific example of an embodiment of the method of  FIG. 1 , which represents a binary voxel-based approach. At  60  a collection of reference data is taken, similar to the data acquisition step  20  described above. This step involves the acquisition of a plurality of CT scans, which are then assembled into a CT volume. The voxels representing internal lung air are then segmented from the CT volume using a known segmentation algorithm, obviating the need to extract the geometry, surfaces, or structures of the lung. During the procedure, the sensor is inserted into the lungs of the patient and a data stream is established between the sensor and a system processor. 
         [0028]    At step  62 , the data acquired from the sensor is processed, which involves de-cluttering and digitization. Each of the voxels is assigned a number based on the tissue density Housefield number. This density value can be associated with gray level or color using well known window-leveling techniques. The density is proportional to a probability that the sensor will occupy a given voxel. The data is also filtered as desired. For example, if the sensor is advanced slowly rather than quickly, it will necessarily result in higher densities as any one voxel is going to be occupied for a longer period of time while the sensor takes longer to pass through. Hence, an advancement rate may be noted and used to adjust the densities accordingly. After filtering, the voxels with higher densities are given higher registration importance than voxels having lower densities. 
         [0029]    At step  64  a threshold value is set for the sensing volume voxels. For example, if the density of a given voxel is higher than the threshold value, that voxel is considered to be tissue and is given a value of zero. If the density of the voxel is below the threshold, that voxel is considered to be air and is given a value of 1. Hence the voxel space now becomes a binary voxel space. This function is performed both on the CT volume as well as on the sensor data. 
         [0030]    At step  66  a compare and fit function is performed. Because a binary system is being used, it is possible to use a variety of matching methods to register the two binary volumes. For example, a subtraction method could be used. A subtraction method superimposes a segment of the sensor data over a corresponding segment of the binary CT volume. The registration is effected by subtracting the binary values of the one volume from the other. For example for any given voxel, if the values are both 1, when the aligned voxels are subtracted the value for that matched voxel space is zero. If they are not the same, however, subtraction results in either a 1 or a −1. All values are converted to their absolute values and totaled. The registration of that particular segment of sensor data is adjusted until a minimum subtracted total is acquired. One advantage of this method is that a minimum may be acquired regardless of image quality. 
         [0031]    Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.