Abstract:
A system for use during a medical or surgical procedure on a body. The system generates a display representing the position of two or more body elements during the procedure based on a reference image data set generated by a scanner. The system produces a reference image of a body elements, discriminates the body elements in the images and creates an image data set representing the images of the body elements. The system produces a density image of the body element. The system modifies the image data set according to the density image of the body element during the procedure, generates a displaced image data set representing the position and geometry of the body element during the procedure, and compares the density image of the body element during the procedure to the reference image of the body element. The system also includes a display utilizing the displaced image data set generated by the processor to illustrate the position and geometry of the body element during the procedure. Methods relating to the system are also disclosed.

Description:
This application is a continuation of U.S. patent application Ser. No. 09/398,313, filed on Sep. 20, 1999, now U.S. Pat. No. 6,434,415, which is a continuation of U.S. patent application Ser. No. 08/931,654 filed on Sep. 16, 1997, now U.S. Pat. No. 6,347,240, which is a continuation of U.S. patent application Ser. No. 08/319,615, filed on Oct. 7, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to systems which generate images during medical and surgical procedures, and in particular, a system for generating images during medical and surgical procedures based on a scan taken prior to the procedure. 
     Image guided medical and surgical procedures comprise a technology by which images, obtained either pre-procedurally or intra-procedurally (i.e., prior to or during a medical or surgical procedure), are used to guide a doctor during the procedure. The recent increase in interest in this field is a direct result of the recent advances in imaging technology, especially in devices using computers to generate three dimensional images of parts of the body, such as computed tomography (CT) or magnetic resonance imaging (MRI). 
     The majority of the advances in imaging involve devices which tend to be large, encircle the body part being imaged, and are expensive. Although the images produced by these devices depict the body part under investigation with high resolution and good spatial fidelity, their cost usually precludes the dedication of a unit to the performance of procedures. Therefore, image guided surgery is usually performed using images taken preoperatively. 
     The reliance upon preoperative images has focused image guidance largely to the cranium. The skull, by encasing the brain, serves as a vessel which inhibits changes in anatomy between imaging and surgery. The skull also provides a relatively easy point of reference to which a localization system may be attached so that registration of pre-procedural images to the procedural work space can be done simply at the beginning of the procedure. Registration is defined as the process of relating pre-procedural images of anatomy to the surgical or medical position of the corresponding anatomy. For example, see U.S. Ser. No. 07/909,097, now U.S. Pat. No. 5,383,454, the entire disclosure of which is incorporated herein by reference. 
     This situation of rigid fixation and absence of anatomical movement between imaging and surgery is unique to the skull and intracranial contents and permits a one-to-one registration process as shown in FIG.  1 . The position during a medical procedure or surgery is in registration with the pre-procedural image data set because of the absence of anatomical movement from the time of the scan until the time of the procedure. In almost every other part of the body there is ample opportunity for movement which degrades the fidelity of the pre-procedural images in depicting the intra-procedural anatomy. Therefore, additional innovations are needed to bring image guidance to the rest of the body beyond the cranium. 
     The accuracy of image guided surgery is based on the identification of structures within the body that do not change shape, do not compress, nor deform between the process of imaging and surgery. Such structures are termed “rigid bodies,” and the bones of the skeleton satisfy this definition for a rigid body. Bones are commonly a target for medical or surgical procedures either for repair, fusion, or biopsy. Therefore, a technique is needed whereby registration can be performed between the bones or bone fragments (skeletal elements) as depicted pre-procedurally on scans and the position of these same skeletal elements as detected intra-procedurally. This technique must take into account that movement can occur between portions of the skeleton which are not rigidly joined, such as bones connected by a joint, or fragments of a broken bone. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a system which allows registration between multiple skeletal elements depicted in pre-procedural images and detected during surgery. 
     It is a further object of this invention to provide a system which can localize multiple rigid bodies that move with respect to each other between imaging and a procedure and provide a display during the procedure of the bodies in their displaced positions. 
     It is another object of this invention to provide a system for use during a medical or surgical procedure on the body, the system generating a display representing the position of two or more body elements during the procedure based on an image data set generated by a scanner prior to the procedure. 
     It is another object of this invention to provide a system for use during a medical or surgical procedure on a body which modifies the image data set according to the identified relative position of each of the elements during the procedure. 
     It is another object of this invention to provide a system which generates a display representative of the position of a medical or surgical instrument during a procedure in relation to body elements. 
     It is a further object of this invention to provide a system for use during image guided medical and surgical procedures which is easily employed by the doctor or surgeon conducting the procedure. 
     It is another object of this invention to provide a system which determines the relative position of body elements based on the contour of the body elements which, in some cases, avoids the need for exposing the body elements. 
     It is still another object of this invention to provide a system which employs the projected fluoroscopic images of body elements to determine their relative position. 
     It is yet a further object of this invention to describe a surgical or medical procedure which employs a display representing the position of body elements during the procedure based on an image data set of the body elements generated prior to the procedure. 
     It is a further object of this invention to provide a system and method for medical or surgical procedures which allows repositioning of body elements during the procedure and still permits the generation of a display showing the relative position of the body elements. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
     The invention comprises a system for use during a medical or surgical procedure on a body. The system generates a display representing the position of one or more body elements during the procedure based on one or more reference images. The system comprises a first means for producing the reference image of the one or more body elements. The system includes a means for discriminating a body element in a reference image and creating an image data set representing the position and geometry of the reference image of the one or more body elements. A second means produces, during the procedure, a density image of the one or more body elements. A processor modifies the image data set according to the density image. The processor generates a displaced image data set representing the position and geometry of the body elements during the procedure. A display utilizes the displaced image data set to illustrate the position and geometry of the body elements during the procedure. 
     The invention also comprises a system for displaying relative positions of the body elements during a procedure on a body. The system includes a processor to discriminate body elements of a reference image data set to create an image data subset defining position and geometry the one or more body elements. The image data subset has a plurality of data points correlatable to a plurality of reference points for the body elements, the position of reference points of a particular body element relative to the data points for that particular body element is known. The system includes a memory for storing the image data subset. A reference system determines, during the procedure, the position of the reference points of the body element relative to the reference points of the other body elements. A radiographic device produces a two-dimensional radiographic image of the body elements during the procedure which includes the identification of reference points of the body elements. The processor further digitizes the radiographic image. The processor also generates a displaced image data set representing the position of the body elements during the procedure by modifying the image data subset using an iterative process such that a two-dimensional projection through the displaced image data set matches the one or more radiographic images. The system also includes a display utilizing the displaced image data set to display the relative position of the body element during the procedure. 
     In another embodiment, the invention includes a system for use during a medical or surgical procedure on a body. The system generates a display from a displaced image data set representing the position of a body element during the procedure based on reference images taken of the body element by a scanner. The invention includes a means for producing an image data set from the reference image of a body element. Another means substantially discriminates the body element in the image data set and creates an image data subset representing the position and geometry of the body element. Another means produces, during the procedure, images of the body element to be displayed. A processor modifies the image data subset of the body element such that a two-dimensional projection through a displaced image data set matches the images during the procedure of the body elements as produced by the producing means. The processor generates the displaced image data set representing the position and geometry of the body element during the procedure. A display utilizes the displaced image data set and illustrates the position and geometry of the one or more body elements during the procedure. 
     In another embodiment the invention includes a system for use during a medical or surgical procedure on a body. The system generates a display from a displaced image data set representing an image of a body elements during the procedure based on reference images taken by a scanner, wherein the reference images have contours for the body elements. The system includes a processor which substantially discriminates the contour of the body element of the reference image as represented by an image data set and creates an image data subset which defines the position, geometry and contours of the one or more body element. The system includes a determining system which is configured to determine, during the procedure, images and contours of images of the body elements to be displayed. The processor further modifies the image data subset according to the determined contour for body element, during the procedure, as determined by the determining system. The processor generates a displaced image data set representing the position and geometry of the contour of the body element during the procedure. A display utilizes the displaced image data set to illustrate the position and geometry of the body element during the procedure. 
     In another embodiment, the invention provides a method for use during a procedure to generate a display representing the position of body elements during the procedure based on a reference image data set. The method includes the steps of creating a reference image of body element and creating a corresponding image data set. Next the method discriminates the body element from the soft tissue in the image data set. The invention then produces a two-dimensional image based on density of the body element. Next the method produces a displaced image data set by modifying the image data set such that a two-dimensional projection through the displaced image data set matches the two-dimensional image during the procedure. Then a display is generated based on the displaced image data set which illustrates the position of the body elements during the procedure. 
     In another embodiment, the invention provides a method for use during a medical or surgical procedure on a body. The method generates a display representing the position of body elements during the procedure based on reference images taken of the body elements. First, a reference image of the body element is produced. Then the method discriminates the one or more body elements in the reference images and creates an image data set representing the reference image, position and geometry of the body elements. During the procedure, a density image of the body element to be displayed is produced. The method then generates a displaced image data set representing the position and geometry of the body element during the procedure. To generate the displaced image data set, the method compares the density image of the body element during the procedure to the image data set of the body elements and modifies the image data set according to the density image of the body element during the procedure. Finally, the displaced image data set is displayed thereby illustrating the position and geometry of the body elements during the procedure. 
     In yet another embodiment of the present invention, a system displays the relative positions of body elements during a procedure on a body. The system includes a memory for storing an image data set. The image data set represents the position of the body elements based on reference images of the body and has a plurality of data points correlatable to a plurality of reference points for each of the body elements. The position of reference points of a particular body element relative to the data points for that particular body element are known. The invention also includes a reference system for identifying, during the procedure, the position of the reference points of each of the body elements relative to the reference points of the other body elements. The reference system also determines the density of each of the body elements during the procedure. A processor compares the density of each of the body elements during the procedure as determined by a device to the density of each of the body elements as represented by the image data set. The processor modifies the spatial relation of the data points of one body element relative to the data points of another body element according to the identified relative position of the reference points during the procedure as identified by the reference system. The processor also generates a displaced image data set representing the position of the body elements during the procedure. A display utilizes the displaced image data set to display the relative position of the body elements during the procedure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of the prior art system in which rigid fixation and absence of movement between imaging and surgery permits a one-to-one registration process between the pre-surgical image data set and the position in surgery. 
         FIG. 2A  is an illustration of operation of the invention in which the pre-procedural image data set is modified in accordance with the intra-procedural position in order to generate a displaced data set representative of the intra-procedural position. 
         FIG. 2B  is a block diagram of one preferred embodiment of a system according to the invention. 
         FIG. 3  is an illustration of the pre-procedural alignment of three body elements during scanning. 
         FIG. 4  is an illustration of the intra-procedural alignment of the three body elements of  FIG. 3  during surgery. 
         FIG. 5  is an illustration of three body elements, one of which has a reference frame attached thereto, in combination with a registration probe. 
         FIG. 6  is an illustration showing ultrasound registration according to the invention in which emitters are attached to the patient&#39;s body. 
         FIG. 7  is an illustration of a fluoroscopic localizer according to the invention for providing projections of an image of the body elements. 
         FIG. 8  is an illustration of a drill guide instrument of the invention wherein the position of a drill guide relative to the body elements may be displayed. 
         FIGS. 9 and 10  illustrate a clamped reference frame and a wired reference frame, respectively. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2A , an overview of operation of one preferred embodiment of the system according to the invention is illustrated. Prior to a particular procedure, the body elements which will be part of the procedure are scanned to determine their alignment. For example, the alignment may be such as illustrated in  FIG. 3  wherein body elements  10 ,  20 , and  30  are more or less aligned in parallel. These body elements may be bones or other rigid bodies. In  FIG. 3 , three-dimensional skeletal elements  10 ,  20 ,  30  are depicted in two dimensions as highly stylized vertebral bodies, with square vertebra  11 ,  21 ,  31 , small rectangular pedicles  12 ,  22 ,  32 , and triangular spinous processes  13 ,  23 ,  33 . During imaging, scans are taken at intervals through the body parts  10 ,  20 ,  30  as represented in  FIG. 3  by nine straight lines generally referred to be reference character  40 . At least one scan must be obtained through each of the body elements and the scans taken together constitute a three-dimensional pre-procedural image data set. 
       FIG. 2B  is a block diagram of the system according to the invention. A scanner interface  102  allows a processor  104  to obtain the pre-procedural image data set generated by the scanner and store the data set in pre-procedural image data set memory  106 . Preferably, after imaging, processor  104  applies a discrimination process to the pre-procedural image data set so that only the body elements  10 ,  20 ,  30  remain in memory  106 . If a discrimination process is employed, processor  104  may execute the discrimination process while data is being transferred from the scanner through the scanner interface  102  for storage in memory  106 . Alternatively, memory  106  may be used for storing undiscriminated data and a separate memory (not shown) may be provided for storing the discriminated data. In this alternative, processor  104  would transfer the data set from the scanner through scanner interface  102  into memory  106  and then would discriminate the data stored in memory  106  to generate a discriminated image data set which would be stored in the separate memory. 
     Once the body elements  10 ,  20 ,  30  are discriminated from the soft tissue and each defined as a single rigid body, they can be repositioned by software algorithms, well known in the art, to form the displaced image data set. Each of the body elements  10 ,  20 ,  30  must have at least three reference points which are selected by the doctor or surgeon and which are visible on the pre-procedural images. These reference points must be able to be indicated with accuracy during the procedure. For body part  10 , reference points  10 A,  10 B, and  10 C are located on the spinous process  13 ; for body part  20 , reference points  20 A and  20 C are located on the vertebra  21  and reference point  20 B is located on spinous process  23 ; and for body part  30 , reference points  30 A and  30 B are located on the spinous process  33  and reference point  30 C is located on the vertebra  31 . More than one reference point can be selected on each scan through the bone, although the maximal accuracy of registration is achieved by separating the reference points as far as possible. For example, in the case of posterior spinal surgery, it may be preferable to select reference points  10 A,  10 B, and  10 C on the spinous process which is routinely exposed during such surgery. It is contemplated that work station software may allow the manual or automated identification of these same points on the images of the body elements  10 ,  20 ,  30 . As  FIG. 3  is a two-dimensional simplification of a three-dimension process, the reference points will not necessarily be limited to a perfect sagittal plane, as depicted. 
     After imaging, the skeletal body elements  10 ,  20 ,  30  may move with respect to each other at the joints or fracture lines. In the procedure room, such as an operating room or a room where a medical procedure will be performed, after positioning the patient for surgery, the body elements will assume a different geometry, such as the geometry depicted in FIG.  4 . 
     As a result of this movement, the pre-procedural image data set stored in memory  106 , consisting of the scans through the skeletal elements, does not depict the operative position of the skeletal elements, as shown in FIG.  4 . However, the shape of the skeletal elements, as depicted by the scans through the element, is consistent between imaging and procedure, as indicated by the lines  40  through each element in FIG.  4 . Therefore, the image data set must be modified to depict the current geometry of the skeletal elements. This modification is performed by identifying the location of each reference point of each skeletal element in procedure space. As diagrammatically illustrated in  FIG. 2B , a localizer  108  identifies the location and provides this information so that the pre-procedural data set may be deformed or re-positioned into the displaced data set. As a result, the displaced data set is in registration with the intra-procedural position of the elements  10 ,  20 ,  30 . Once the locations of the reference points are determined by the localizer  108 , processor  104 , which is a part of the work station, can execute software which re-positions the images of the skeletal elements to reflect the position of the actual elements in the procedure room thus forming the displaced set and the registration between the displaced set and the intra-procedural position. 
     Preferably, a three-dimensional digitizer may be used as the localizer  108  to determine the position and space of the elements  10 ,  20 ,  30  during the procedure. In general, the digitizer would include a reference array  110  which receives emissions from a series of emitters. Usually, the emissions consist of some sort of energy, such as light, sound or electromagnetic radiation. The emitters are applied to and positioned in coordination with the elements being localized and the reference array  110  is distant therefrom, determining the position of the emitters. As is apparent, the emitters may be placed distant to the elements and the reference array  110  may be attached to the elements being localized. 
     According to one preferred embodiment of the invention as shown in  FIG. 5 , a reference frame  116  is attached to one of the skeletal elements  10  at the beginning of the procedure. Reference frame  116  is equipped with a plurality of emitters  114  which together define a three-dimensional procedural coordinate system with respect to the skeletal element  10 . Emitters  114  communicate with sensors  112  on a reference array  110  located in the procedure room and remote from the reference frame  116  and patient. If the body of the patient is not immobilized during surgery, then multiple reference frames may be required. The three-dimensional procedural coordinate system may alternatively be defined by rigid fixation of the frame emitters  114  directly (or indirectly, for example, to the skin) to the skeletal elements  10 ,  20 , or  30 . In either case, the emitters  114  emit a signal which is received by the sensors  112 . The received signal is digitized to compute position, for example, by triangulation. Through such information, the localizer  108  or a digitizer which is part of the localizer  108  can determine the exact three-dimensional position of the frame emitters  114  relative to the sensors  112 . The sensors  112  are in a fixed position throughout the procedure, as the reference array  110  is fixed in the procedure room to the ceiling or other support. Thereby, localizer  108  or the processor  104  can exactly determine the position of the reference frame  116  relative to the array. The reference frame  116  is free to move except during localization, e.g., activation of the emitters  114  on the reference frame  116  and activation of the probe emitters  120 . Emitters  114  of the reference frame  116  are energized to provide radiation to the sensors  112 , which radiation is received and generates signals provided to the localizer  108  for determining the position of the frame  116  relative to the array  110 . 
     Next, it is necessary to determine the position of the skeletal element  10  to which the reference frame  116  is affixed. In particular, the position of the skeletal element  10  relative to the reference frame  116  must be determined. After exposure of the reference points  10 A,  10 B,  10 C by surgical dissection, the reference points are touched by the tip of a registration probe  118  equipped with emitters  120 . As each of the reference points  10 A,  10 B,  10 C is touched by the tip of the probe  120 , the emitters are energized to communicate with the sensors  112  of reference array  110 . This communication permits the localizer  108  to determine the position of the registration probe  120 , thereby determining the position of the tip of the probe  120 , thereby determining the position of the reference point  10 A on which the tip is positioned. By touching each of the reference points  10 A,  10 B,  10 C on each skeletal element  10 ,  20 ,  30  involved in the procedure, and relating them to their corresponding reference points on the images of the same elements, an intra-procedural position data is generated and stored in memory  121 . This data is used to derive a transformation which allows the determination of the exact procedural position and orientation of each skeletal element. Using the intra-procedural position of the skeletal elements  10 ,  20 ,  30 , localizer  108  and processor  104  employ software which manipulates the pre-procedural image data set stored in memory  106  to produce a displaced image data set which is stored in memory  122 . The displaced image data set in memory  122  reflects the geometry of the actual elements  10 ,  20 ,  30  during the procedure. Processor  104  displays the displaced image data set on display  124  to provide a visual depiction of the relative position of the skeletal elements  10 ,  20 ,  30  during the procedure. This image is used by the doctor during the procedure to assist in the procedure. In addition, it is contemplated that an instrument which would be used during the procedure may be modified by the addition of emitters. This modified instrument when moved into the area of the skeletal elements  10 ,  20 ,  30  would be activated so that its emitters would communicate with the reference array  110  thereby permitting localizer  108  to determine the instrument&#39;s position. As a result, processor  104  would modify display  124  to indicate the position of the instrument, such as by positioning a cursor. 
     Reference frame  116  allows the patient to be moved during the procedure without the need for re-registering the position of each of the body elements  10 ,  20 ,  30 . It is assumed that during the procedure, the patient is immobilized so that the body elements are fixed relative to each other. Since the reference frame  116  is affixed to skeletal element  10 , movement of the patient results in corresponding movement of the reference frame  116 . Periodically, or after each movement of the patient, array emitters  114  may be energized to communicate with the sensors  112  of reference array  110  in order to permit localizer  108  to determine the position of the reference frame  116 . Since the reference frame  116  is in a fixed position relative to element  10  and since we have assumed that elements  20  and  30  are in fixed relation to element  10 , localizer  108  and/or processor  104  can determine the position of the elements. From this position, a displaced image data set memory can be created for display on display  124 . 
     An alternative to touching the reference points A, B, C with the tip of the probe  118  would be to use a contour scanner  126 . Such a device, using some form of energy such as sound or light which is emitted, reflected by the contour and sensed, would allow the extraction of a contour of the skeletal elements  10 ,  20 ,  30 , thus serving as a multitude of reference points which would allow registration to occur. The registration process is analogous to the process described for ultrasound extracted contours below. 
     In certain situations, markers may be used on the skin surface as reference points to allow the transformation of the pre-procedural image data set into the displaced image data set. Reciprocally, skin surface fiducials applied at the time of imaging can be used to re-position the body to match the geometry during imaging and is described below. 
     Localization of skeletal elements  10 ,  20 ,  30  may be desired without intra-procedural exposure of the reference points A, B, C on those skeletal elements. Examples wherein the spine is minimally exposed include percutaneous biopsy of the spine or discectomy, spinal fixation, endoscopy, percutaneous spinal implant insertion, percutaneous fusion, and insertion of drug delivery systems. In this situation, localization of reference points on the skeletal elements must be determined by some form of imaging which can localize through overlying soft tissue. There are currently two imaging techniques which are available to a surgeon in the operating room or a doctor in a procedure room which satisfy the needs of being low cost and portable. Both imaging techniques, ultrasonography and radiography, can produce two- or three-dimensional images which can be employed in the fashion described herein to register a three-dimensional form such as a skeletal element. 
     As described in U.S. patent application Ser. Nos. 07/858,980 and 08/053,076, the entire disclosures of which are incorporated herein by reference, the coupling of a three-dimensional digitizer to a probe of an ultrasound device affords benefits in that a contour can be obtained which can be related directly to a reference system that defines three-dimensional coordinates in the procedural work space. In the context of the present invention, a patient is imaged prior to a procedure to generate a pre-procedural image data set which is stored in memory  106 . In the procedure room, the patient&#39;s body is immobilized to stabilize the spatial relationship between the skeletal elements  10 ,  20 ,  30 . A reference system for the body is established by attaching a reference array  110  to one of the skeletal elements or by otherwise attaching emitters to the patient or skeletal elements as noted above. For example, this could be performed by using the percutaneous placement of a reference system similar to the one described above, radiopaque markers screwed into the elements or by placing emitters  130  directly on the skins, as illustrated in  FIG. 6 , based on the assumption that the skin does not move appreciably during the procedure or in respect to the axial skeleton. 
     An ultrasound probe  128  equipped with at least three emitters  130  is then placed over the skeletal element of interest. The contour (which can be either two- or three-dimensional) of the underlying bone/soft tissue interface is then obtained using the ultrasound probe  128 . This contour of the underlying bone can be expressed directly or indirectly in the procedural coordinates defined by the reference system. Emitters  130  communicate with sensors  112  of reference array  110  to indicate the position of the ultrasound probe  128 . An ultrasound scanner  131  which energizes probe  128  determines the contour of the skeletal element of interest being scanned. This contour information is provided to processor  104  for storage in contour memory  132 . 
     The intra-procedural contour stored in memory  132  is then compared by a contour matching algorithm to a corresponding contour extracted from the pre-operative image data set stored in memory  106 . Alternatively, a pre-procedural contour data set may be stored in memory  134  based on a pre-procedural ultrasound scan which is input into memory  134  via scanner interface  102  prior to the procedure. This comparison process continues until a match is found for each one of the elements. Through this contour matching process, a registration is obtained between the images of each skeletal element and the corresponding position of each element in the procedural space. 
     In certain instances, the ultrasound registration noted above may not be applicable. For example, ultrasound does not penetrate bone, and the presence of overlying bone would preclude the registration of an underlying skeletal element. Further, the resolution of ultrasound declines as the depth of the tissue being imaged increases and may not be useful when the skeletal element is so deep as to preclude obtaining an accurate ultrasonically generated contour. In these circumstances, a radiological method is indicated, which utilizes the greater penetrating power of x-rays. 
     Pre-operative imaging occurs as usual and the skeletal elements are discriminated from the soft tissue in the image data set as above. In particular, a CT scan of the skeletal elements  10 ,  20 ,  30  is taken prior to the procedure. Processor  104  may then discriminate the skeletal elements. Next, the patient is immobilized for the procedure. A radiograph of the skeletal anatomy of interest is taken by a radiographic device equipped with emitters detectible by the digitizer. For example, a fluoroscopic localizer  136  is illustrated in FIG.  7 . Localizer  136  includes a device which emits x-rays such as tube  138  and a screen  140  which is sensitive to x-rays, producing an image when x-rays pass through it. In general, this screen is referred to as a fluoroscopic plate. Emitters  142  may be positioned on the tube  138 , or on the fluoroscopic plate  140  or on both. For devices in which the tube  138  is rigidly supported relative to the plate  140 , emitters need only be provided on either the tube or the plate. Alternatively, the reference array  110  may be attached to the tube or the plate. By passing x-rays through the skeletal element  141  of interest, a two-dimensional image based on bone density is produced and recorded by the plate. The image produced by the fluoroscopic localizer  136  is determined by the angle of the tube  138  with respect to the plate  140  and the position of the skeletal elements therebetween. Fluoroscopic localizer  136  includes a processor which digitizes the image on the plate  140  and provides the digitized image to processor  104  for storage in memory  106 . Processor  104  may simulate the generation of this two-dimensional x-ray image by creating a two-dimensional projection of the three-dimensional skeletal elements that have been discriminated in the image data set stored in memory  106 . In order to form the displaced data set and thus achieve registration, an iterative process is used which re-positions the images of the skeletal elements such that a two-dimensional projection through the displaced data set matches the actual radiographic image. The described process can utilize more than one radiographic image. Since the processor  104  is also aware of the position of the fluoroscopic localizers because of the emitters  142  thereon, which are in communication with localizer  108 , the exact position of the skeletal elements during the procedure is determined. 
     The above solutions achieve registration by the formation of a displaced image data set stored in memory  122  which matches the displacement of the skeletal elements at the time of the procedure. An alternative technique to achieve registration is to ensure that the positions of the skeletal elements during the procedure are identical to that found at the time of imaging. This can be achieved by using a frame that adjusts and immobilizes the patient&#39;s position. In this technique, at least three markers are placed on the skin prior to imaging. These markers have to be detectible by the imaging technique employed and are called fiducials. A multiplicity of fiducials is desirable for improving accuracy. 
     During the procedure, the patient&#39;s body is placed on a frame that allows precise positioning. Such frames are commonly used for spinal surgery and could be modified to allow their use during imaging and could be used for repositioning the patient during the procedure. These frames could be equipped with drive mechanisms that allow the body to be moved slowly through a variety of positions. The fiducials placed at the time of imaging are replaced by emitters. By activating the drive mechanism on the frame, the exact position of the emitters can be determined during the procedure and compared to the position of the fiducials on the pre-procedural image data set stored in memory  106 . Once the emitters assume a geometry identical to the geometry of the fiducials of the image data set, it is considered that the skeletal elements will have resumed a geometric relationship identical to the position during the pre-procedural scan, and the procedure can be performed using the unaltered image data set stored in memory  106 . 
     In general, instrumentation employed during procedures on the skeleton is somewhat different than that used for cranial applications. Rather than being concerned with the current location, surgery on the skeleton usually consists of placing hardware through bones, taking a biopsy through the bone, or removing fragments. Therefore, the instrumentation has to be specialized for this application. 
     One instrument that is used commonly is a drill. By placing emitters on a surgical drill, and by having a fixed relationship between the drill body and its tip (usually a drill bit), the direction and position of the drill bit can be determined. At least three emitters would be needed on the drill, as most drills have a complex three-dimensional shape. Alternatively, emitters could be placed on a drill guide tube  800  having emitters  802 , and the direction  804  of the screw being placed or hole being made could be determined by the digitizer and indicated on the image data set (see FIG.  8 ). The skeletal element  806  would also have emitters thereon to indicate its position. 
     Besides modification of existing instrumentation, new instrumentation is required to provide a reference system for surgery as discussed above. These reference frames, each equipped with at least 3 emitters, require fixation to the bone which prevents movement or rotation. 
     For open surgery, a clamp like arrangement, as depicted in  FIG. 9 , can be used. A clamp  900  is equipped with at least two points  902 ,  904 ,  906 ,  908  which provide fixation to a projection  910  of a skeletal element. By using at least two point fixation the clamp  900 , which functions as a reference frame, will not rotate with respect to the skeletal element. The clamp includes emitters  912 ,  914 ,  916  which communicate with the array to indicate the position of the skeletal element as it is moved during the procedure. 
     Many procedures deal with bone fragments  940  which are not exposed during surgery, but simply fixated with either wires or screws  950 ,  952  introduced through the skin  954 .  FIG. 10  depicts a reference platform  956  attached to such wires or screws  950 ,  952  projecting through the skin  954 . The platform  956  includes a plurality of emitters  958 ,  960 ,  962 ,  964  which communicate with the array to indicate the position of the bone fragment  940  as it is moved during the procedure. 
     The reference frame can be slipped over or attached to the projecting screws or wires to establish a reference system. Alternatively, the frame can be attached to only one wire, as long as the method of attachment of the frame to the screw or wire prevents rotation, and that the wire or screw cannot rotate within the attached skeletal element. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.