Abstract:
The invention relates to a method and device for referencing between a data set, which describes geometrically the spatial model of a body, and the real physical environment in which the body is placed. A three-dimensional position reference body is used on the real body. The position reference body has one or more elementary bodies or markers whose 3-dimensional position can be detected with sensors and which define a fixed geometric reference with respect to the center of gravity of the body or to other body reference volumes. For registration, the position reference body and/or its elementary bodies are correlated in the data model and in the physical world.

Description:
BACKGROUND OF THE INVENTION 
   A patient data set and an operating site during surgery are typically referenced to each other by using either anatomical indicia or—before an image data set is created—implants (bone or skin markers) that are applied to the patient. The implants are indicated simultaneously with an input device at a workstation and with a localization system on the patient. 
   A more general task relates to referencing between a data set (which describes geometrically the spatial model of a body) and the real physical environment in which the actual body is placed. For referencing, a three-dimensional position reference body is used on or applied to the real body. The position reference body consists of one or more elementary bodies (markers) whose 3-dimensional position can be detected with sensors and which define a fixed geometric reference with respect to the center of gravity of the body or to other reference volumes of the body. For the purpose of registration, the position reference body and/or its elementary bodies are correlated in the data model and in the physical world. 
   Unlike the present method, the German patent DE 197 47 427 describes a method and a device wherein the characteristic surface of bone structures is used for providing a reference between a data set and the operating site. DE 197 47 427 describes an individual template which carries 3-D localization markers and is applied to and/or screwed on a bone segment. 
   The method has the disadvantage that an expensive CAD-CAM model has to be produced from a patient data set before an individual template can be manufactured. With many surgical procedures, large areas of bone have to be exposed for applying the individual surface template, which makes the procedure unnecessarily invasive. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method and a device for instrument, bone segment, tissue and organ navigation, which operates without auxiliary devices, such as templates, and facilitates a safe and reproducible navigation. 
   By using optical measurements and by referencing the operating site via the characteristic surface of a soft tissue envelope or a bone surface, the cost can be reduced significantly and the surgical access path can be significantly less invasive. For a bone segment navigation with the present invention, the 3-D localization markers can be individually secured on a bone segment independent of a template—such as a screw, which opens additional possibilities for a minimally-invasive surgical procedure. 
   Optical referencing between the data set, the operating site and the 3-D localization markers is also faster and more precise than the aforedescribed referencing method that uses anatomical indicia and implants, because large surface structures in a patient data set (for example MRT or CT) can be imaged more exactly and reproduced than small, singular reference points. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are illustrated in the drawings and will be described hereinafter in more detail. 
     It is shown in 
       FIG. 1  a schematic diagram of the devices in an operating room, 
       FIG. 2  a detailed view of the 3-D reference markers with a device for detecting these markers by optical methods using 3-D scanners, 
       FIG. 3  a geometric device for optical detection of 3-D reference markers, which are affixed to a frame, 
       FIG. 4  a perspective view of the 3-D reference markers with associated geometric devices of different form (depression/sulcus, raised portion/crista, planar color-coded surface) for optical detection with the 3-D scanner, 
       FIG. 5  alternative examples of different geometric forms that have a known spatial association with the 3-D marker, 
       FIG. 6  additional coupled referencing markers, 
       FIG. 7  an embodiment with a different coupling between the scanner and position detection unit, 
       FIG. 8  referencing of bodies that do not necessarily have a stable form, and 
       FIG. 9  device for positioning bodies that do not necessarily have a stable form. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will be described hereinafter in more detail with reference to a first embodiment. 
   The entire system  1  is used for optical referencing between an operating site, a patient data set and 3-D markers. 
   An optical 3-D scanner  5  is attached to a position detection unit  4  via a coupling device  13 . The position detection unit  4  can acquire, for example, infrared signals, ultrasound signals or electromagnetic signals and allows the determination of three-dimensional coordinates of a corresponding 3-D marker  6  (for example: ultrasound transmitter, infrared transmitter, electromagnetic transmitter and/or reflectors  17  for all types of waves, ultrasound, infrared, radar, etc.). The 3-D scanner  5  (for example a 3-D laser scanner  5  or a radar unit  5   a ) can detect the shape and color of surfaces (for example  7 ), but not the signals from the 3-D markers  6 . The signals from the 3-D markers  6  can be transmitted actively, for example with an LED, or passively, for example by using reflectors. 
   The data measured by the position detection unit  4  and the 3-D scanner  5  or the radar unit  5   a  are transmitted via a connection  10  and  11  to a display and processing unit  2 . Since the position detection unit  4  and the 3-D scanner  5  are coupled via a connection  13  having a known geometrical relationship and/or are kinematically attached to each other via a connection  13 , all coordinates measured with the position detection unit  4  can also be expressed in the coordinate system of the 3-D scanner  5  and vice versa. 
   A planning unit  3  is connected via  12  to the display and processing unit  2 . Surgical procedures can be simulated on this planning unit  3 ; for example, resetting osteotomies can be planned before a bone segment navigation. 
   In this embodiment, at least three 3-D markers  6  are attached to the patient, which define a coordinate system on the patient. Geometric  FIGS. 7  which can be detetced by the 3-D scanner  5 , are arranged in a known, fixed spatial relationship to these 3-D markers  6 . These  FIGS. 7  can be implemented, for example, as a depression/sulcus  7   a,  a raised portion/crista  7   b,  as color-coded lines and fields  7   c  or as a bar code. The geometric  FIG. 7  can also be in the form of a base on which a 3-D marker  6  is placed. The geometric  FIG. 7  can also be formed directly by one or several 3-D markers  6 . 
   The coordinates of the 3-D markers  6  can be uniquely determined by processing unit  2  from the geometry of the devices  7  by an inverse transformation. The geometry of these devices  7  can be different ( 7 ′,  7 ″,  7 ′″); it is only necessary that the geometry can be detected by the 3-D scanner  5  and that the processing unit  2  can determine the coordinates of the 3-D markers  6  from the geometry of the devices  7 . 
   If the three 3-D markers  6  are fixedly connected with one another by a frame  14  in order to define a patient coordinate system, then the coordinates of the 3-D markers  6  can be determined by the processing unit  2  from the arrangement of the geometric  FIGS. 7  on the frame  14 . Alternatively, the scanner can also determine the coordinates directly by the analyzing the known geometries of the 3-D markers. 
   The operating site, the patient data set and the 3-D markers  6  are referenced to each other by first detecting with the 3-D scanner  5  the soft tissue (before the surgery, i.e., before the soft tissue swells or is displaced) or the bone surfaces  9  of the patient. The processing unit  2  processes the data from the 3-D scanner  5  and determines the most advantageous surface area fit between the patient and the patient data set. Thereafter, the patient and the patient data set can be referenced to each other by a coordinate transformation. 
   So far, however, the 3-D scanner  5  has not yet detected the 3-D markers  6 . However, since the geometric devices  7  surrounding the 3-D markers  6  were scanned together with the patient and since the spatial relationship between the 3-D markers  6  and the geometric devices  7  is known, the coordinates of the 3-D markers  6  can be imaged both in the coordinate system of the data supplied by the 3-D scanner  5  as well as in the coordinate system of the patient data set. 
   Additional 3-D markers  8  which are either attached directly on a bone segment  9  or on a work tool  15  or coupled to these through a kinematic measurement mechanism or a coordinate measurement device, can subsequently be imaged in the patient data set on the display and processing unit  2 . 
   In this way, a spatial displacement of a bone segment  9  that has been simulated on the planning unit  3 , can also be reproduced on the patient. 
   Instead of coupling the 3-D scanner  5  and the 3-D marker position detection unit  4  through a fixed connection, the 3-D scanner  5  can also be flexibly coupled to the position detection unit  4  so as to be movable relative to the 3-D marker position detection unit  4 , and can itself be provided with 3-D markers  8  for detection by the 3-D marker position detection unit  4 . 
     FIG. 6  shows a 3-D marker  16  embodied as an LED and embodied as a passive reflector  17 . The 3-D geometry of the bodies is sufficiently known and can therefore be used directly to uniquely determine the coordinates of the markers from the scanner data, without the need for additional encoding. The markers can be directly used as device geometries. 
     FIG. 7  shows an embodiment of a scanner  18  with a kinematic coordinate measurement device implemented as a measuring profile  19  and directly connected with the position detection unit. If necessary, the relative position of the scanner  18  can be determined by the second kinematic coordinate measurement device with significantly higher accuracy and measuring frequency. In an alternate embodiment, the base of the kinematic coordinate measurement device itself can be provided with a position reference body  20 . In the simplest case, the kinematic coordinate measurement device is a simple body (for example a rod) of known geometry. Advantageously, the kinematic coordinate measurement device can also be attached to a table or applied directly on the patient, depending of which relative accuracy between the markers and the body should be optimized. 
     FIG. 8  shows instead of a bone (hard tissue) a more typical situation involving tissue that does not necessarily have a stable form, and/or an arbitrary body  21 . In the simplest case, a relationship is established via a center of a gravity  22  of the body or another reference volume  23 . This is advantageous when the method is to be applied also to soft tissue, organs or implants during alignment, transplantation and implementation. Even if perfect dimensional stability is not achieved, the method and device can still assist with navigation. Elementary bodies  24  are arranged on the position reference body  20   b.    
     FIG. 9  shows a device for affixing the position reference bodies  20   b  to bodies  21  that may lack dimensional stability. The position reference body  20   b  is hereby attached to a mechanism that is disposed on the body  21  that lacks dimensional stability. In the depicted example, body tissue is drawn in by a reduced pressure process through a lumen  25  and through a membrane  26  and pressed into a predefined form. This form can advantageously have a shape that facilitates, for example, placement during transplantation or implantation. Other methods for affixing the tissue to the device, for example with adhesive, burrs or stitches, are also feasible.