Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is directed to a registration method for navigation-guided medical interventions of the type wherein a coordinate transformation that is employed for navigation is determined between a coordinate system of a position acquisition system and a measurement volume of an X-ray device. The invention is also directed to an apparatus for the implementation of the method. The method and apparatus of the invention do not require patient-associated markers. 
   2. Description of the Prior Art 
   Navigation is increasingly being used for supporting medical interventions at living subjects, this being understood as the guidance of a medical instrument relative to a subject or relative to a tissue region of subject being treated, that is supported by means of optical image information. An image of the instrument is mixed into a 2D or 3D image of the subject acquired with the X-ray device. In this way, an operator can guide an instrument that has at least partially penetrated into the subject so that its tip is no longer directly visible, for example due to the penetration into body tissue, relative to the tissue region of the subject on the basis of the image information without a risk of unintentionally harming the subject. 
   In order to enable such a navigation-guided intervention, i.e. in order to be able to mix an image of the instrument into image information of a subject in a manner that is accurate as to position and orientation, it is necessary to produce a mathematical relationship in the form of a coordinate transformation between a coordinate system of the image information of the subject or a coordinate system of the reconstructed volume of the subject and a coordinate system with respect which the positions of the instrument are indicated. To this end, artificial markers are sometimes arranged at the subject or anatomical markers, for example distinct bone structures, are defined. The anatomical or artificial markers (patient-associated markers) must be clearly visible in the image information of the subject registered with the X-ray device and must be easily accessible at the subject. For example, the artificial markers are secured to the skin surface of the subject order to be able to undertake a registration, which is understood to mean the determination of the spatial transformation rule between the coordinate system wherein the positions of the instrument to be navigated are defined and the coordinate system of the image information or of the reconstructed volume of the subject. The markers usually must be individually approached with the instrument in a specified sequence in order to be able to determine the coordinate transformation between the two coordinate systems. In extremely precise medical interventions, the markers are secured to the body of the subject so as to be immobile. The attachment of a stereotactic frame to the head of a patient and the attachment of markers in bones or at the spinal column of a patient are as examples. The attachment of the markers partly ensues in a separate operation since the markers must already be applied before a pre-operative imaging that is frequently employed for navigation. 
   The attachment and registration of the markers, accordingly, is a relatively unpleasant procedure for a patient and is also relatively time-consuming for an operator in preparing a navigation-guided intervention. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a simplified method for markerless determination of the transformation rule in a navigation-guided medical intervention as well as an apparatus for the implementation of the method. 
   This object is inventively achieved by making a series of 2D projections from different projection directions of an X-ray calibration phantom on a carrying arm is secured, via a support mount, in a defined way to the X-ray device for alignment relative to the X-ray device. A coordinate transformation between a coordinate system allocated to the X-ray calibration phantom and a coordinate system allocated to a measurement volume of the X-ray device is determined from the 2D projections. Since the coordinate transformation between the coordinate system allocated to the X-ray calibration phantom and a coordinate system allocated to the support mount is known, due to the defined arrangement of the carrying arm at the support mount and the defined arrangement of the X-ray calibration phantom at the carrying arm, the coordinate transformation between the coordinate system allocated to the support mount and the coordinate system allocated to the measurement volume of the X-ray device can be unproblemmatically determined. When, after the determination of this coordinate transformation, a marker plate provided with markers is finally arranged, for example, at the support mount of the X-ray device, the marker plate being detectable by a position acquisition system, then coordinates of the coordinate system allocated to the measurement volume can be transformed into coordinates of a coordinate system allocated to the position acquisition system and vice versa. For medical applications, for example, coordinates of a medical instrument acquired with respect to the coordinate system of the position acquisition system can be transformed into coordinates of the coordinate system of the measurement volume. Mixing of images of the instrument into images of a subject produced with the X-ray device thus is possible without registration, i.e. without employing markers to be attached to a subject. 
   In a version of the invention, the X-ray device is a C-arm X-ray device, the C-arm of which is preferably isocentrically adjustable. The C-arm X-ray device has a support for the C-arm in which the C-arm is adjustable along its circumference, and the support mount for the calibration phantom is arranged at the support. When the carrying arm provided with the X-ray calibration phantom is secured to the support mount arranged at the support, then, given an adjustment of the C-arm around its orbital axis, the phantom can remain at the support mount during the entire acquisition of the series of 2D projections from different projection directions. In another version of the invention the support mount is arranged at the X-ray receiver of the X-ray device that moves relative to the X-ray calibration phantom during the acquisition of the series of 2D projections. The phantom is releasably connected to the carrying arm. An adjustable stand, preferably placeable on the floor, to which the X-ray calibration phantom can be secured is provided. The stand is preferably adjustable in five degrees of freedom. After arranging the carrying arm provided with the X-ray calibration phantom in a defined position at the X-ray receiver, the X-ray calibration phantom is arranged on the stand without changing its position and that the connection between the X-ray calibration phantom and the carrying arm is released. The X-ray calibration phantom is then only held at the stand. For acquiring the series of 2D projections, finally, the carrying arm is removed from the support mount of the X-ray device, and the series of 2D projections of the X-ray calibration phantom, now arranged only at the stand, is acquired from different projection directions. 
   In a further embodiment, a first series of 2D projections of an X-ray calibration phantom is acquired with the X-ray device and projection matrices for the X-ray device are determined therefrom. Subsequently, at least one further 2D projection in at least one defined position of the carrying device for the X-ray source and an X-ray receiver is acquired from the same or from some other X-ray calibration phantom that is secured in a defined way to a carrying arm securable via a support mount to the X-ray device, for alignment relative to the X-ray device. A second projection matrix is determined from this (at least one) further 2D projection. The coordinate transformation between the coordinate system allocated to the support mount and the coordinate system allocated to the X-ray calibration phantom is known due to the defined arrangement of the carrying arm at the support mount and the defined arrangement of the X-ray calibration phantom at the carrying arm. Therefore, the coordinate transformation between the coordinate system allocated to the support mount and a coordinate system allocated to a measurement volume of the X-ray device can be determined on the basis of a first projection matrix that is allocated to the defined position of the carrying device and that was determined during the course of determining the first projection matrix and on the basis of the second projection matrix, which is allocated to the defined position of the carrying device. As mentioned above, a marker plate that, for example, can be detected by a position acquisition system, is arranged at the support of the X-ray device after the determination of this coordinate transformation, then images of medical instruments can be mixed without registration into the images of a subject generated with the X-ray device. 
   In this embodiment as well, the X-ray device can be a C-arm X-ray device with a C-arm that is preferably isocentrically adjustable, and the support mount can be arranged at the X-ray source of the X-ray device. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a C-arm X-ray device constructed with a support mount arranged at a bearing part of the C-arm X-ray device and to which a carrying arm provided with an X-ray calibration phantom is secured; 
       FIG. 2  is a side view of a C-arm X-ray device constructed and operating in accordance with the invention, with a support mount arranged at the X-ray reception device at which a carrying arm provided with an X-ray calibration phantom is arranged. 
       FIG. 3  is a side view of a C-arm X-ray device constructed and operating in accordance with the invention, with a support mount arranged at the X-ray source, to which a carrying arm provided with an X-ray calibration phantom is secured. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A C-arm X-ray device  1  shown in  FIG. 1  has components arranged in a known manner, with the differences in accordance with the invention described below. The C-arm X-ray device has a support  2  at which a C-arm  5  provided with an X-ray source  3  and an X-ray receiver  4  is seated. In the exemplary embodiment, the C-arm  5  is isocentrically adjustable (see the double arrow ‘a’) along its circumference around its isocenter IZ and its orbital axis O. Together with the support  2 , the C-arm  5  is also isocentrically pivotable around its angulation axis A in the directions of the double arrow ‘b’. 
   2D and 3D images of subjects, for example patients, can be acquired with the C-arm X-ray device  1  and presented on a display device  6 . The devices required for this purpose, particularly an image computer, are implemented in a known way and are therefore not shown in FIG.  1  and need not be explicitly described. 
   Particularly for medical applications, navigation-guided interventions at a patient (not shown) are to be implemented with the C-arm X-ray device  1 . For this reason, it is necessary to determine a coordinate transformation between a coordinate system K M  that is allocated to a measurement of the C-arm X-ray device  1 , and has its origin in the isocenter IZ of the C-arm X-ray device  1  in the exemplary embodiment, and a coordinate system K P  allocated to a position acquisition system  10  (schematically shown in  FIG. 1 ) wherein the coordinates of an instrument  11  to be navigated relative to a patient are defined. 
   To this end, a support mount  20  to which a coordinate system K H1  is allocated is arranged at the support  2  of the C-arm X-ray device  1 . A carrying arm  21  that is removable from the support mount  20  is secured to the support mount  20 . An X-ray calibration phantom  22  is secured to the carrying arm  21 , with a coordinate system K R1  being allocated to the X-ray calibration phantom  22 . The carrying arm  21  is arranged in a defined manner at the support mount  20  and the X-ray calibration phantom  22  is arranged in a defined manner at the carrying arm  21  so that the coordinate transformation between the coordinate system K H1  allocated to the support mount  20  and the coordinate system K R1  allocated to the X-ray calibration phantom  22  is known on the basis of the known dimensions of the carrying arm  21 . 
   For determining the coordinate transformation between the coordinate system K R1  allocated to the X-ray calibration phantom  22  and the coordinate system K M  allocated to the measurement volume of the C-arm X-ray device  1 , a series of 2D projections of the X-ray calibration phantom  22  is acquired from different projection directions by movement of the C-arm  5  around its orbital axis O. The X-ray calibration phantom  22  is arranged at the carrying arm  21  so that it is penetrated by an X-ray beam proceeding from the X-ray source  3  to the X-ray receiver  4 . The coordinate transformation between the coordinate system K R1  allocated to the X-ray calibration phantom  22  and the coordinate system K M  allocated to the measurement volume of the C-arm X-ray device  1  is determined from the acquired series of 2D projections of the X-ray calibration phantom  22 . To this end, moreover, the X-ray calibration phantom  22  has X-ray-positive marks in a known way that are imaged in the 2D projections. The orientation of the X-ray-positive marks in the coordinate system K R1  allocated to the X-ray calibration phantom  22  is thereby known. 
   Since, thus, the coordinate transformation between the coordinate system K H1  allocated to the support mount  20  and the coordinate system K R1  allocated to the X-ray calibration phantom  22 , and the coordinate transformation between the coordinate system K R1  allocated to the X-ray calibration phantom  22  and the coordinate system K M  allocated to the measurement volume of the C-arm X-ray device  1 , are known, the coordinate transformation between the coordinate system K H1  allocated to the support mount  20  and the coordinate system K M  allocated to the measurement volume can also be determined in a simple way. This latter transformation, for example, is stored in a memory (not shown in  FIG. 1 ) of the C-arm X-ray device  1 . 
   When the C-arm X-ray device  1  is to be utilized for navigation-guided interventions at a patient, the carrying arm  21  provided with the X-ray calibration phantom  22  is removed from the support mount  20 , and a marker plate (not shown in  FIG. 1  but well known) that is provided with markers is arranged in a defined manner at the support mount  20  of the C-arm X-ray device  1  thus the coordinate system K H1  allocated to the support mount  2  is also allocated to the marker plate in the exemplary embodiment. 
   The coordinate transformation between the coordinate system K H1  allocated to the marker plate  20  and the coordinate system K P  allocated to the position acquisition system can be determined from camera images acquired with cameras  12 ,  13  of the position acquisition system  10  wherein the marker plate is imaged, so that—overall—the coordinate transformation between the coordinate system K M  allocated to the C-arm X-ray device  1  and the coordinate system K P  allocated to the position acquisition system can be determined, for example with a computer (not shown in FIG.  1 ). This computer can be allocated to the position acquisition system  10  or can be the image computer (likewise not shown in  FIG. 1 ) of the C-arm X-ray device  1 . Camera images are acquired of the instrument  11 , provided with a marker plate  14  having markers during the course of a navigation-guided intervention. The positions of the instrument  11  with respect to the coordinate system K P  allocated to the position acquisition system  10  thus can be determined on the basis of the camera images, and these can be can be transformed into coordinates of the coordinate system K M  of the measurement volume on the basis of the identified coordinate transformation between the coordinate system K P  allocated to the position acquisition system  10  and the coordinate system K M  allocated to the measurement volume. For guidance of the instrument  11  relative to a patient, images of the instrument  11  can be mixed into X-ray images of the patient acquired with the C-arm X-ray device  1 . 
     FIG. 2  illustrates a second possibility for determining a coordinate transformation between a coordinate system allocated to a measurement volume of a C-arm X-ray device and a coordinate system allocated to the C-arm X-ray device itself without the use of markers in the registration. The C-arm X-ray device shown in  FIG. 2  essentially corresponds to the C-arm X-ray device shown in  FIG. 1 , so the components of the C-arm X-ray device are provided with the same reference characters. Differing from the C-arm X-ray device  1  shown in  FIG. 1 , the C-arm X-ray device  1  shown in  FIG. 2  has a support mount  30  arranged at the radiation receiver  4 . The support mount  30  has a coordinate system K H2  allocated to it. A carrying arm  31  at which an X-ray calibration phantom  32  is arranged in a defined way is arranged at the support mount  30  in a defined way. The X-ray calibration phantom  32  is releasably attached to the carrying arm  31 . Like the X-ray calibration phantom  22 , the X-ray calibration phantom  32  has X-ray-positive marks (not shown). The orientation of these marks relative to a coordinate system K R2  allocated to the X-ray calibration phantom  32  is known. Due to the defined arrangements of the carrying arm  31  at the support mount  30  and the X-ray calibration phantom  32  at the carrying arm  31 , as well as due to the known dimensions of the carrying arm  31 , the coordinate transformation between the coordinate system K H2  allocated to the support mount  30  and the coordinate system K R2  allocated to the X-ray calibration phantom  32  is known. 
   As can be seen from  FIG. 2 , the X-ray calibration phantom  32  is arranged at a stand  33  standing on the floor in addition to being arranged at the carrying arm  31 . The arrangement of the X-ray calibration phantom  32  at the stand  33  does not ensue until after the fastening of the carrying arm  31  provided with the X-ray calibration phantom  32  to the support mount  30 . In the present exemplary embodiment, the stand  33  is (in a way not shown in detail) height-adjustable, and is adjustable around ball-and-socket joints  34 ,  35  for this purpose. For acquiring 2D projections of the X-ray calibration phantom  32  from different projection directions, the carrying arm  31  is released from the X-ray calibration phantom  32  and thus from the support mount  30 , but the orientation of the X-ray calibration phantom  32  does not change relative to the support mount  30  nor relative to the C-arm X-ray device  1 , so that the transformation rule between the coordinate system K H2  allocated to the support mount  30  and the coordinate system K R2  allocated to the X-ray calibration phantom  32 , which is known due to the defined arrangement of the X-ray calibration phantom  32  at the carrying arm  31  and of the carrying arm  31  at the support mount  30 , is preserved. 
   For determining the coordinate transformation between the coordinate system K R2  allocated to the X-ray calibration phantom  32  and the coordinate system K M  allocated to the measurement volume of the C-arm X-ray device  1 , a series of 2D projections of the X-ray calibration phantom  32  from different projection directions is acquired−after the removal of the carrying arm  31 −by movement of the C-arm  5  around its orbital axis. The coordinate transformation between the coordinate system K R2  allocated to the X-ray calibration phantom  32  and the coordinate system K M  allocated to the measurement of the C-arm X-ray device  1  is then determined from the acquired series of 2D projections of the X-ray calibration phantom  32 . 
   Since, thus, the coordinate transformation between the coordinate system K H2  allocated to the support mount  30  and the coordinate system K R2  allocated to the X-ray calibration phantom  32  and the coordinate transformation between the coordinate system K R2  allocated to the X-ray calibration phantom  32  and the coordinate system K M  allocated to the measurement volume of the C-arm X-ray device  1  are known, the coordinate transformation between the coordinate system K H2  allocated to the support mount  30  and the coordinate system K M  allocated to the measurement volume can also be determined in a simple way. This latter transformation can be stored in a memory (not shown in  FIG. 2 ) of the C-arm X-ray device  1 . 
   A marker plate detectable by the position acquisition system  10  is arranged at the support mount  30  in a defined manner so that—as in the exemplary embodiment shown in FIG.  1 —the coordinate system K H2  allocated to the support mount  30  can also be allocated to the marker plate. A transformation rule between the coordinate system K M allocated  to the measurement volume and a coordinate system allocated to a position acquisition system thus can be determined. As a result, the pre-conditions are established for mixing images of an instrument into X-ray images of, for example, a patient acquired with the C-arm X-ray device  1 . 
     FIG. 3  illustrates a third possibility for determining a coordinate transformation between a coordinate system allocated to a C-arm X-ray device and a coordinate system allocated to the measurement volume of the C-arm X-ray device. The C-arm X-ray device shown in  FIG. 3  corresponds to the C-arm X-ray device  1  shown in FIG.  1  and  FIG. 2 , so that the components of the C-arm X-ray device shown in  FIG. 3  are provided with the same reference characters as the components of the C-arm X-ray device  1  shown in  FIGS. 1 and 2 . The C-arm X-ray device  1  shown in  FIG. 3  differs from the C-arm X-ray devices  1  shown in  FIGS. 1 and 2  by virtue of a support mount  40 , to which a coordinate system K H3  is allocated, being arranged at the X-ray source  3 . A carrying arm  41  is arranged at the support mount  40  in a defined way and an X-ray calibration phantom  42  is arranged at the carrying arm  41  in a defined way. Like the X-ray calibration phantoms  22  and  32 , the X-ray calibration phantom  42  has X-ray-positive marks whose orientation relative to a coordinate system K R3  allocated to the X-ray calibration phantom  42  is known. Due to the defined arrangements of the carrying arm  41  at the support mount  40  and the X-ray calibration phantom  42  at the carrying arm  41  as well as due to the known dimensions of the carrying arm  41 , the coordinate transformation between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K R3  allocated to the X-ray calibration phantom  42  is known. 
   For determining the coordinate transformation between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K M  allocated to the measurement volume, and before the carrying arm  41  provided with the X-ray calibration phantom  42  is arranged at the support mount  40 , the X-ray calibration phantom  42  or some other X-ray calibration phantom is arranged relative to the C-arm X-ray device  1  independently of the carrying arm  41 , for example on any kind of substrate, so that an X-ray beam emanating from the X-ray source  3  can penetrate the X-ray calibration phantom. The X-ray calibration phantom  42  is employed in the exemplary embodiment. A first series of 2D projections of the X-ray calibration phantom  42  is acquired from different projection directions by moving the C-arm  5  around the orbital axis O, for example along its circumference. First projection matrices for the C-arm X-ray device  1  are determined therefrom and deposited in a memory of the C-arm X-ray device  1 . Subsequently, the carrying arm  41  provided with the X-ray calibration phantom  42  is arranged at the support mount  40 , as shown in  FIG. 3. A  2D projection of the X-ray calibration phantom  42  is now acquired at an arbitrarily selectable but defined position of the C-arm  5 . The only requirement is that C-arm this C-arm position must be a position of the C-arm  5  that this assumed in the acquisition of a 2D projection of the first series of 2D projections. A second projection matrix belonging to this position of the C-arm  5  is determined from this 2D projection. Based on the known coordinate transformation between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K R3  allocated to the X-ray calibration phantom  42  as well as based on a first projection matrix that is allocated to the defined position of the C-arm  5 , determined during the course of determining the first projection matrices, and based on the second projection matrix that is allocated to the defined position of the C-arm  5 , the coordinate transformation between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K M  allocated to the measurement of the C-arm X-ray device  1  is calculated. The calculation thereby ensues on the basis of the following equation:
 
 P   1   =P   2   *T ( K   H3   ; K   R3 ) *T ( K   H3   ; K   M ),
 
wherein P 1  is the first projection matrix that is allocated to the defined position of the C-arm  5 , P 2  is the second projection matrix that is allocated to the defined position of the C-arm  5 , and T(K H3 ; K R3 ) is the known transformation rule between the coordinate systems K H3  and K R3 . T(K H3 ; K M ) is the sought transformation rule between the coordinate systems K H3  and K M .
 
   By resolution of the projection matrix P 1  into an extrinsic component and an intrinsic component as well as by resolution of the projection matrix that proceeds from a matrix multiplication of the projection matrix P 2  by the transformation rule T(K H3 ; K R3 ) into an intrinsic component and extrinsic component, the transformation rule T(K H3 ; K M ) can be composed of a rotary component and a translational component that are formed from the extrinsic components of the resolutions of the projection matrices. Accordingly, the transformation rule between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K M  allocated to the measurement volume can be determined in this way, so that the preconditions are established for being able to mix images of an instrument into X-ray images of, for example, a patient acquired with the C-arm X-ray device  1 , with the assistance of a position acquisition system, for example the position acquisition system  10  of FIG.  1 . 
   In the exemplary embodiment shown in  FIG. 3 , moreover, second projection matrices can be acquired in a number of defined positions of the C-arm  5  on the basis of 2D projections of the X-ray calibration phantom  42  acquired at these position in order to determine the transformation rule between the coordinate system K H3  allocated to the support mount  40  and the coordinate system K M  allocated to the measurement volume. It must be noted, however, that the second projection geometries of the C-arm X-ray device  1  may possibly be modified by the weight of the carrying arm  41  and of the X-ray calibration phantom  42 . This would then require an additional calibration of the C-arm X-ray device  1  with the carrying arm and the X-ray calibration phantom  42  arranged thereat. 
   In the exemplary embodiments, the C-arm  5  is moved around its orbital axis O in order to acquire 2D projections of the X-ray calibration phantom. The C-arm  5  alternatively can be moved around its angulation axis A for acquiring 2D projections. 
   Further, the X-ray device need not necessarily be a C-arm X-ray device. 
   The arrangements of the support mounts  20 ,  30  and  40  as described in the exemplary embodiments are examples. In the exemplary embodiment shown in  FIG. 1 , for example, the support mount  20  alternatively can be arranged in a component of the C-arm X-ray device  1  that is immobile during the acquisition of the 2D projections. In the case of the exemplary embodiment shown in  FIG. 2 , further, the support mount  30  alternatively can be arranged at the X-ray source  3  or at the C-arm  5 . The situation is the same in the exemplary embodiment shown in FIG.  3 . 
   Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

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