Patent Publication Number: US-2006004284-A1

Title: Method and system for generating three-dimensional model of part of a body from fluoroscopy image data and specific landmarks

Description:
RELATED APPLICATION DATA  
      This application claims priority of U.S. Provisional Application No. 60/588,898 filed on Jul. 16, 2004, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to a method for generating a three-dimensional model of a part of a body with the aid of a medical and/or surgical navigation system and, more particularly, to generating such a model without preceding tomographic imaging.  
     BACKGROUND OF THE INVENTION  
      Currently used techniques for computer tomographic-free navigation mainly involve navigation with the aid of x-ray images obtained from a fluoroscopy apparatus. Using the fluoroscopy apparatus, images are acquired and, after calibration and distortion correction, landmarks are determined in the images. This purely fluoroscopic navigation is awkward and often requires many x-ray recordings in order to obtain all the necessary data available at any desired point in time. Additionally, the numerous x-ray recordings cause an increased radiation load on the patient and on the operating staff.  
      European Patent No. EP 1 329 202 B1 describes a method and apparatus for assigning digital image information to navigation data of a medical navigation system, wherein image data produced using a digital C-arc x-ray apparatus are incorporated into navigation. Using this technique, numerous x-ray recordings are taken in succession, which, like above, causes a corresponding radiation load on the patient and staff.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method for generating a three-dimensional model of a part of the body that overcomes disadvantages of the prior art. In particular, the invention enables three-dimensional navigation using simple means without acquiring tomographs, specifically computer tomograph (CT) recordings, prior to performing the navigation. The invention can produce a three-dimensional model of a part of the body, thereby permitting navigation without successively obtaining new fluoroscopy recordings.  
      A method in accordance with the invention uses fluoroscopy image data sets in conjunction with positional identification of characteristic landmarks on a body part to generate a model of the body part. Combining fluoroscopy with positional identification of characteristic landmarks to generate the model of the part of the body results in much simpler and less elaborate calculations than using fluoroscopy image data sets alone.  
      The identified landmarks reproduce absolute spatial points which, in conjunction with fluoroscopic data, facilitate production of the model. In other words, the present invention combines two methods of detecting body features, each of which can be performed independently, in such a way so as to produce a three-dimensional model.  
      In accordance with the invention, two fluoroscopy image data sets obtained from different detection directions for each of particular, individual and delimited region of a part of the body, are used to produce the three-dimensional model. In addition, the positional data acquired from each technique can supplement data obtained from the other technique. For example, points that cannot be tapped by a pointer can be determined from fluoroscopic transillumination images by performing a symmetry calculation on symmetrically or substantially symmetrically formed parts of the body.  
      The characteristic body part data can include lengths of body part sections and angles of body part sections with respect to each other. In accordance with the invention, joint rotation center points also can be determined by positionally identifying characteristic landmarks (or a navigation reference array, such as a reference star, on a movable joint bone) at a number of angular positions of the joint and then calculating back to the rotational center from the obtained trajectory points.  
      The part of the body to be modeled may be the femur, pelvis, etc.  
      The model of the part of the body may be supplemented or completed with the aid of generic body part data, in particular by using a generic model of the part of the body that has been adapted on the basis of information already ascertained for the model of the part of the body to be generated.  
      An image output may display, before a landmark is identified or the fluoroscopy image data sets are produced, which landmark is to be identified (e.g., tapped with a pointer) next in succession and/or which fluoroscopy image is to be obtained next in succession.  
      Accordingly, a method for generating a three-dimensional model of a part of the body with the aid of a medical and/or surgical navigation comprises the steps of identifying to the navigation system landmarks on the part of the body that are characteristic of the model of the part of the body, obtaining at least two fluoroscopy image data sets for each of one or more predetermined, individual and delimited regions of the part of the body, and ascertaining characteristic body part data by processing and combining the landmark positions and parameters of the fluoroscopy data sets. A three-dimensional and positionally determined model of the part of the body then may be generated from the characteristic body part data.  
      Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a femur and pelvis.  
       FIG. 2  is a schematic representation in accordance with  FIG. 1 , additionally displaying regions for fluoroscopy recordings in accordance with an embodiment of the invention.  
       FIG. 3  is a screen shot of a program for assisting in recording fluoroscopy image data sets in accordance with an embodiment of the invention.  
       FIG. 4  is a block diagram of a computer system that can be used to implement the method of the present invention. 
    
    
     DETAILED DESCRIPTION  
      The invention will now be described in more detail on the basis of generating a model that can be used in hip operations. It should be appreciated, however, that the present invention can be applied to other medical procedures, and reference to hip operations is not intended to be limiting in anyway.  
      The invention provides a navigation method based on a three-dimensional model generated from two-dimensional fluoroscopic image recordings and specific landmarks of a part of the body. As a result, the three-dimensional orientation of the human pelvis is improved with respect to prior techniques, thereby enabling an implant to be suitably positioned in surgical total hip replacement procedures. This includes not only the hip bone but also the femur.  
      Two orientation parameters are important when positioning a cavity implant: the cavity anteversion and the cavity inclination. These two parameters relate to a hip coordinate system that is defined by the anatomy of the hip bone, i.e., by a frontal pelvic plane and a mid-sagittal plane. These two planes represent the basis for all angular calculations used to place the cavity implant in replacement of the anatomical hip joint.  
      Two relevant orientation parameters also are defined for placing a femur implant, namely, the shaft axis and the neck axis (neck axis of the femur). These two axes represent the basis for all calculations for placing the shaft implant and/or the femur implant when replacing the anatomical hip joint.  
      In accordance with the present invention, specific landmarks may be acquired using a navigation pointer of a medical navigation system, and information relating to the landmarks is combined with information obtained from fluoroscopy images. The medical navigation system can include a navigation system, such as is described in co-owned U.S. Pat. No. 6,351,659, which is incorporated herein by reference in its entirety. The navigation pointer includes reference elements, which allow the navigation system to track the location of the pointer within a medical workspace.  
      The fluoroscopy images can be obtained using image processing algorithms, for example. As will be described in more detail below,  FIG. 1  illustrates points that are acquired using a navigation pointer  1 , such as the spina iliaca anterior superior, the epicondylus lateralis and the epicondylis medialis. The navigation pointer  1  includes reference markers  1   a,  which permit the pointer  1  to be tracked by the medical navigation system.  FIG. 2  indicates regions  20 - 26  for which fluoroscopy recordings are obtained, wherein the two circles shown for each region indicate that two fluoroscopy recordings are obtained from different angles for each region. The two images for each region enable the detected features to be three-dimensionally reconstructed. Image processing then can be performed either by the fluoroscopy apparatus itself or via an image processing unit of the medical navigation system. As used herein, image processing is defined as the computer manipulation of images using one or more image processing algorithms, including, but not limited to, convolution, Fast Fourier Transform, Discrete Cosine Transform, thinning (or skeletonization), edge detection and contrast enhancement.  
      Determining the coordinate system of the pelvis is difficult when specific landmarks are not physically acquired or acquirable. In the absence of landmarks, it is necessary to fall back on fluoroscopy images that describe the bone structure (pelvis or femur). Symmetry of the bone structures can be utilized here. For example, the mid-sagittal plane  2  of the pelvis can be determined without the navigation pointer  1  having access to both spina iliaca anterior superior  4  points. The mid-sagittal plane  2  can be determined by obtaining x-ray recordings of the tuberculum pubis region. An automatic algorithm then can calculate the image&#39;s center axis of symmetry, and another automatic algorithm can calculate the orientation of the center axis of symmetry. This is likewise possible for the femur shaft axis  6  and the neck axis  8 .  
      A spina iliaca anterior superior point  4  usually can be tapped relatively simply using the navigation pointer  1 . This also applies to the epicondylus lateralis  10  and the epicondylus medialis  12  on the femur  14 . As used herein, to tap or tapping a landmark refers to positioning one end of a trackable pointer  1  substantially on the landmark such that the landmark position can be recorded by the medical navigation system.  
      The rotational center point  16  for the femur can be calculated by positionally tracking the landmarks  4 ,  10 ,  12  or by tracking a reference array  17  ( FIG. 2 , e.g., a reference star) on the bone as the bone is pivoted in the medical navigation system. As the bone is pivoted, the landmarks move on a spherical surface and the center point of this surface defines the rotational center point of the femur. Using software, a mechanical axis  18 , which is determined by two points, e.g., by the rotational center point and the center of the epicondular axis, then can be defined.  
      Once this pre-registration has been completed, eight fluoroscopy images, e.g., four pairs of fluoroscopy images in specific and delimited regions, are produced as indicated in  FIG. 2  by the reference numerals  20  to  26 . The specific regions for the fluoroscopy images are: the proximal femur  20 ; the neck of the femur  22 ; the pubis  24 ; and the spina iliaca anterior superior  26 . Two fluoroscopy images are made for each region in order to obtain three-dimensional information from the two-dimensional images.  
      The image information from the pubic region allows, among other things, the mid-sagittal plane  2  to be ascertained, the contralateral spina point  26  to be calculated, and the frontal pelvic plane (not shown) to be ascertained. For example, the frontal pelvic plane can be defined by a pubic bone point and both spina points  4  and  26 . The pubic bone point also can be calculated from the images.  
      The image information for the acetabulum/neck of the femur allows, among other things, the neck axis  8  to be ascertained by means of active contours, a predetermined value for the neck axis to be used on the basis of the angle between the neck  28  and the shaft  30  (about 130°), the head of the femur to be automatically ascertained (the size of the cavity equals the diameter of the head plus eight millimeters), and a leg length (LL) calculation from the position of the cavity and the femur implant.  
      The neck axis can be determined in a manner similar to that of the shaft axis (discussed below). Since the shaft axis is already known, as well as an estimation of the center of rotation of the shaft axis, a rough estimation of the location of the neck contours can be identified in the images. The two neck contours and the “round” portion of the femur head can be determined in both images (e.g., by active contours of the images in  FIGS. 1 and 2 ). This information can be propagated to three dimensions, thus yielding the neck axis, improved center of rotation and the size of the femur head.  
      The image data for the proximal femur enable, among other things, the femur shaft axis  6  to be detected (the anatomical axis by means of active contours). The angle between the anatomical axis  6  and mechanical axis  18  of the femur can be presupposed to be 7° and, using this information, the size of the femur implant can be determined.  
      For example, a center of rotation of the femur can be accurately determined (e.g., within 3 mm) by pivoting the femur about the rotational center point  16 , while points on the medial and lateral condyle  10 ,  12  can be determined via a pointer. The mechanical axis  18  runs through center of rotation and mid-point of the condyle points. Utilizing the 7° assumption, the direction of the shaft axis is roughly known (e.g., within 5°), and the bone contours of the shaft are detected in both images (e.g., using an active contour method of the images in  FIGS. 1 and 2 ). The direction of these contours is similar to the shaft axis (which is roughly known). Hence, contour detection is very stable since the “search region” is quite small. The shaft axis can be determined as the line that is most “central” to the two contours (in the least squares sense). This is done in both images. The two dimensional axes are back projected to three dimensions, thus creating three dimensional planes. The three dimensional shaft axis is at the intersection of these planes.  
      The image information in the region of the spina iliaca anterior superior  4  allows, among other things, the crista iliaca  32  to be automatically detected. The contour of the crista iliaca, for example, can be identified in both images ( FIGS. 1 and 2 ). Again, an active contour method can be used to automatically find the contours in the images (the position and orientation is already roughly known). This provides two two-dimensional curves, which are back projected to three dimensions to obtain two surfaces. The intersection of the two surfaces yields the three dimensional curve of the crista iliaca.  
      Using the information thus obtained, a three-dimensional model for the pelvis and femur can be defined, said model enabling navigation and allowing planning of implant placement.  
      Correctly acquiring the fluoroscopy images can be simplified via a software assistant. The software assistant guides the operating team through the acquisition of fluoroscopy images, and is sub-divided into various sections. The main part of the assistant includes a model with two circles, which is shown in the screen shot  40  in  FIG. 3 . The broken circle  42  describes the actual position of the C-arc fluoroscopy apparatus which for this purpose can be tracked by the medical navigation system. The continuous circle  44  shows the target position for producing a fluoroscopy image of a specific region. In the lower part  46  of the display  40 , a status indicator displays which images have already been acquired and which are still to be produced.  
      At the top left of the display  40 , reference arrays  48  relating to the software tools are displayed using color markings. The reference arrays assist the user in setting the angle from which the images will be obtained. A target image  50  helps find the best image for a specific region. These images can be compared to the then current image  52  shown below the target image. The final sector  54 , bottom left, displays the angle of the C-arc.  
      With the aid of such an assistant, acquiring the fluoroscopy images is made simple and quick. Since the positions of the landmarks also can be acquired quickly via the pointer, a three-dimensional model of a part of the body, according to the invention, can be generated in a simple and quick way. The model can be used to plan and/or navigate surgical instruments before and during a surgical procedure. Alternatively, the model can provide supplemental data to the surgeon.  
      Moving to  FIG. 4 , a computer system  60  for executing a computer program in accordance with the present invention is illustrated. The computer system  60  includes a computer  62  for processing data, and a display  64  for viewing system information. The technology used in the display is not critical and may be any type currently available, such as a flat panel liquid crystal display (LCD) or a cathode ray tube (CRT) display, or any display subsequently developed. A keyboard  66  and pointing device  68  may be used for data entry, data display, screen navigation, etc. The keyboard  66  and pointing device  68  may be separate from the computer  62  or they may be integral to it. A computer mouse or other device that points to or otherwise identifies a location, action, etc., e.g., by a point and click method or some other method, are examples of a pointing device. Alternatively, a touch screen (not shown) may be used in place of the keyboard  66  and pointing device  68 . A touch screen is well known by those skilled in the art and will not be described herein.  
      Included in the computer  62  is a storage medium  70  for storing information, such as application data, screen information, programs, etc. The storage medium  70  may be a hard drive, for example. A processor  72 , such as an AMD Athlon 64™ processor or an Intel Pentium IV® processor, combined with a memory  74  and the storage medium  70  execute programs to perform various functions, such as data entry, numerical calculations, screen display, system setup, etc. A network interface card (NIC)  76  allows the computer  62  to communicate with devices external to the computer system  60 .  
      The actual code for performing the functions described herein can be readily programmed by a person having ordinary skill in the art of computer programming in any of a number of conventional programming languages based on the disclosure herein. Consequently, further detail as to the particular code itself has been omitted for sake of brevity. As will be appreciated, the various computer codes for carrying out the processes herein described can be embodied in computer-readable media.  
      Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.