Apparatus and method for planning a stereotactic surgical procedure using coordinated fluoroscopy

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BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to an apparatus and method for planning and guiding insertion of an object along a linear trajectory into a body. More particularly, the present invention relates to an apparatus and method for coordinating two captured fluoroscope images to permit effective three-dimensional planning of the trajectory using only two-dimensional images.

Numerous medical interventions involve placing a needle, drill, screw, nail, wire or other device in the body. In some cases the angle and position of the device are both of critical importance, for example in the drilling of a hole for a screw along the axis of a spinal pedicle. In other cases, it is primarily the positioning of the end-point of the device which is important, for example in placing a biopsy needle into a suspected tumor. In still other cases, the objective is only to define a point rather than a line, for example in targeting a tumor for radiation therapy. Many other examples exist, especially in the field of orthopaedics.

The present invention is also relevant to the development of percutaneous technique. Executing a linear trajectory for the insertion of instrumentation into the body through the skin is more difficult than open surgical technique, but the reduced invasiveness and trauma of percutaneous placement makes it desirable.

Fluoroscopy is frequently used by surgeons to assist medical procedures. Continuous fluoroscopy during a surgical procedure is undesirable because it exposes the surgeon's hands to radiation. Furthermore, regardless of whether intermittent or continuous fluoroscopy is used, the resulting images are two-dimensional while insertion of the surgical instrument requires three-dimensional awareness by the surgeon.

The apparatus and method of the present invention involve acquisition and storage of two separate fluoroscopic images of the body, taken from two different angles. Typically, although not necessarily, these would be an anterior/posterior A/P) image taken front-to-back of the patient, and a sagittal image taken side-to-side. These two fluoroscopic images are displayed on two adjacent computer monitors. The surgeon uses a trackball or other computer input device to specify on the monitors an insertion point and an insertion trajectory.

A mechanical positioning device is then used to position a guide through which the surgeon performs the insertion of the surgical instrument. The positioning device may either be an active computer controlled manipulator such as a robot, or it may be a manually adjusted mechanical device which is set numerically in accordance with an output from the computer.

The apparatus and method of the present invention establish the projective geometric relationships relating each of two acquired fluoroscopic images to the three-dimensional workspace around and within the patient's body, despite essentially arbitrary positioning of the fluoroscope. The two images then become a coordinated pair, which permits three-dimensional planning that might otherwise be expected to require a computed tomography (CT) scan.

While the acquisition and display of two approximately orthogonal images may be expected to present the surgeon with the greatest ability to plan in three dimensions, two images are not strictly necessary. It is possible to use a single captured image for some procedures, particularly if the surgeon has adjusted the beam axis of the fluoroscope into alignment with the intended trajectory. Furthermore, more than two images could also be acquired and coordinated, should that be advantageous.

Several other approaches to stereotactic or robotic surgery, planned on a computer screen displaying medical images, have been described by other workers, and will be listed below. Some background is given here before discussing prior art. The method and apparatus of the present invention constitute a technique we call coordinated fluoroscopy. Coordinated fluoroscopy is a technique for REGISTRATION and for SURGICAL PLANNING. It allows registration based on the acquired fluoroscopic images themselves, without requiring any additional measuring devices. It allows three-dimensional surgical planning based on fluoroscopic views from two angles, without requiring three-dimensional imaging such as computed tomography (CT), and without requiring that the two fluoroscopic images be acquired from orthogonal fluoroscope poses.

Registration

Registration is a key step in any image-guided surgical system. Registration is the determination of the correspondence between points of the image upon which a surgical plan is prepared, and points of the workspace in the vicinity of (and within) the patient. If a numerically controlled tool (whether robotic or manual) is to be used, the coordinate system of that device must also be brought into registry with the image.

It is common to accomplish registration with the help of a global positioning device, usually optical, which can measure the three-dimensional coordinates of markers placed anywhere over a large volume of space. Coordinated fluoroscopy avoids the necessity for this expensive and inconvenient device, instead deriving registration directly from the acquired fluoroscopic images themselves. Coordinated fluoroscopy uses a "registration artifact" which is held in a fixed position relative to the patient while one or more fluoroscopic images are acquired from different angles (poses). There is no need to constrain the fluoroscope poses at which these various images are acquired, for instance to require that they be orthogonal, nor is there a need to instrument the fluoroscope so that the pose angles can be measured. Instead, pose information is extracted after-the-fact from the images. It is a substantial benefit of the present invention that surgeons can acquire fluoroscopic images using fluoroscope poses of their own choosing, as they are accustomed.

The registration artifact contains a plurality of features (fiducials) which are designed to be easily identifiable on a fluoroscopic image. The embodiment described here uses eight small steel spheres embedded in a radiolucent matrix. The positions of these fiducials are known relative to a coordinate system fixed in the artifact, either by design or by measurement.

From the two-dimensional locations of the projections of these fiducials in a fluoroscopic image, we can determine the geometric projections that carry a general three dimensional point anywhere in the vicinity of the artifact into a projected point on the image. This establishes registration between image and workspace. Several images can each be registered relative to the same registration artifact, thus also bringing all the images into registry with one another.

Identification of the geometric projections, as discussed above, would not be possible with raw fluoroscope images, which are highly nonlinear and distorted. It is necessary first to map and compensate for these distortions. It is useful to be aware of the necessity of distortion compensation when comparing the present invention to prior art.

Surgical Planning

Surgical planning is also a key step in image-guided surgery. Planning of three-dimensional surgical procedures might be expected to be done on a three-dimensional dataset, such as can be reconstructed from computed tomography (CT) data. However, surgeons are accustomed to planning on two-dimensional images: radiographs or fluoroscopic images. Indeed even when CT data is available, planning is usually done on individual two-dimensional CT "slices" rather than on a three-dimensional reconstruction.

The coordinates of the endpoints of a line segment representing an intended screw, biopsy needle, or drilled hole are of course three-dimensional, as are the coordinates of a single point within the body marking the present location of a tumor or a fragment of shrapnel. In surgical planning such points can be specified on a two-dimensional image, or on each of several two-dimensional images. Each such two-dimensional image is a projection of the same three-dimensional space.

It is necessary to convert the two-dimensional coordinates of specified points on each of several images into a three-dimensional coordinate which can be used to guide a tool along a desired trajectory or to a desired point within the body. To do so one must have knowledge of the geometric relationship of the projections that created the images.

In the absence of such geometric knowledge a point specified on one image and a point independently specified on another image may in fact not correspond to any single point within the body. This is so because a point specified on a two-dimensional image is the projection of a LINE in space. The implied point in three-dimensions is the intersection of two such lines, one implied by the point specified on each image. Two such lines created independently may be skew, intersecting nowhere. Similarly, line segments for an intended procedure can not be chosen independently on two images, otherwise they will in general not correspond to a well-defined three-dimensional line segment.

In coordinated fluoroscopy, the geometric projections that relate the two images to a single three-dimensional coordinate system are established before planning commences. The points chosen by the surgeon on two (or more) images can therefore be constrained by the software such that they DO correspond to a well-defined point in three-dimensions. In practice, as a surgeon adjusts an intended point or line segment on one image, the point or line segment displayed on the other image(s) continuously updates and adjusts as well. One cannot draw "arbitrary" points or line segments independently on the images; the software only allows one to draw points or line segments that correspond to a well-defined point or line segment in three-dimensions.

The benefits of planning on geometrically coordinated images as described above are threefold:

1) Once the surgeon has selected a point or a line segment on two images, the three-dimensional point or line segment to which the selections correspond is fully defined and ready to be executed.

2) An axial view such as could be attained from a CT slice is generally unattainable fluoroscopically. The angle that is most easily visualized in axial view, known as the transverse angle, is therefore difficult to select or execute under fluoroscopy. In coordinated fluoroscopy the transverse angle is implicitly specified by the surgeon by selecting line segments on two images. This may assist the surgeon in visualizing and planning the transverse angle for a procedure.

3) In conventional fluoroscopy, image dilation due to beam divergence is of unknown extent, making accurate measurement of anatomic distances difficult. In coordinated fluoroscopy the actual in-situ length of an intended line segment can be determined by the software. This is useful for selecting appropriate screw length, as well as for other purposes.

BACKGROUND

Lavalle et al. in Grenoble, France have developed a system for spinal surgery which uses computed tomography as an image source. The CT data is assembled into a three-dimensional data set which can then be resliced at will on orthogonal planes. Surgical planning proceeds on three mutually orthogonal planes simultaneously. Registration is performed by using an optical tracking device to digitize arbitrary surface points of the vertebrae, and matches those surface points to the CT data set.

Nolte et al. in Bern, Switzerland have developed a very similar spinal system to Lavalle et al. Registration differs in that the optical tracking device is used to digitize specific anatomic landmarks rather than general surface contours. The features are then pointed out manually in CT data, allowing a match to be made.

P. Finlay in High Wycombie, England has developed a fluoroscopic system for head-of-femur (hip) fractures. Accuracy requirements in this procedure are not very great, so fluoroscope distortion compensation is not needed. Its absence also precludes identification of the geometric projections from images as is done in the present invention. Instead, the two fluoroscope poses are required to be orthogonal and the C-arm must not be moved along the floor in between the two images. Registration is accomplished by noting various features of a surgical tool which appears in the images, and by highlighting a marker wire which also appears in the field of view of the fluoroscope.

Potamianos et al. in London, England have developed a system for kidney biopsy and similar soft-tissue procedures. It incorporates a digitizing mechanical arm to which a biopsy needle is attached, and which can be moved about manually by the surgeon. Surgical planning per se is absent; instead a line segment representing the present position of needle is displayed superimposed upon captured (static) fluoroscope images, as the needle is moved manually near and within the patient.

Phillips et al. in Hull, England have developed a system for orthopaedic procedures. It uses a optical tracking device as well as a fluoroscope. Registration is accomplished by instrumenting the fluoroscope with light emitting diodes and tracking them with the optical tracker. Surgical planning software is specific to the surgical procedure, and tends to offer medical opinion rather than just display a trajectory as in the present invention. For intramedullary nail placement, for instance, the surgeon outlines target holes in an intramedullary prosthetic, and software calculates a trajectory through them.

U.S. Pat. No. 4,750,487 (Zanetti) describes a stereotactic frame which overlays a patient. A single aterior/posterior fluorograph is then acquired, in which a crosshairs affixed to the frame is visible. By measuring the displacement of the crosshairs from the desired target, a motion of the frame can be accomplished which brings the two into alignment. This invention does not facilitate three-dimensional stereotaxy as does the present invention.

U.S. Pat. No. 5,078,140 (Kwoh) describes a stereotactic and robotic system for neurosurgery. It uses CT images.

ASPECTS OF THE INVENTION

According to the present invention, a method is provided for planning a stereotactic surgical procedure for a linear trajectory insertion of surgical instrumentation into a body using a fluoroscope for generating images of the body. The method includes placing adjacent to the body a registration artifact containing a plurality of fiducials; displaying on a computer monitor an image of the patient's body and the registration artifact; receiving a user or automatic algorithmic input to identify two-dimensional coordinates of the fiducials of the registration artifact displayed on the first monitor; and registering the image by creating a geometric model having parameters, said model projecting three-dimensional coordinates into image points, and numerically optimizing the parameters of the geometric model such that the projections of the known three-dimensional coordinates of the fiducials best fit the identified two-dimensional coordinates in the image.

The method further includes displaying on a second computer monitor a second image, taken of the patient's body and the registration artifact but from an angle different from that of the first image, and receiving a user or automatic algorithmic input to identify two-dimensional coordinates of the fiducials displayed on the second computer monitor; and registering the second image by creating a geometric model having parameters, said model projecting three-dimensional coordinates into image points, and numerically optimizing the parameters of the geometric model such that the projections of the known three-dimensional coordinates of the fiducials best fit the identified two-dimensional coordinates in the second image.

The method, whether one or two images have been acquired, further includes the step of receiving a user input to select on a computer monitor an entry point for a surgical instrument. In the case of two images, also receiving a user input to select on a computer monitor the position, length, and angles of a virtual guidewire representing the trajectory for the surgical instrument; and drawing a segment, to be known as a PROJECTED GUIDEWIRE, on the image(s). When there are two images, the projected guidewires are constrained to correspond geometrically to the same three-dimensional segment in space, to be known as the VIRTUAL GUIDEWIRE.

The method further includes receiving a user input to move either end of a projected guidewire, by revising the virtual guidewire of which the projected guidewire(s) are projections, and by redrawing the projected guidewires in correspondence with the revised virtual guidewire.

The method further includes receiving a user input to change the length of the virtual guidewire, and redrawing the projected guidewire(s) in correspondence with the revised virtual guidewire. A special case is that the length is zero, so that what is planned is a virtual targetpoint rather than a virtual guidewire.

The method further includes receiving a user input to change the sagittal, transverse, or coronal angle(s) of the virtual guidewire, updating the orientation of the virtual guidewire based on the new angles, and redrawing the projected guidewire(s) in correspondence with the revised virtual guidewire.

The method further includes producing an output to adjust the coordinates of a tool guide such that the projection of the axis of the guide in an image is brought into correspondence with the entry point displayed on the computer monitor.

The method further includes producing an output to adjust the coordinates of a tool guide such that it is brought into correspondence with the virtual guidewire; or producing an output to adjust the coordinates of a tool guide such that the position of the guide along its axis is offset by a preselected distance from one endpoint of the virtual guidewire, in order to control the location within the body of the surgical instrument to be inserted.

The method further includes transmitting said coordinates to a robot or other automatic mechanical device, or displaying said coordinates such that human operator may manually adjust a mechanical device.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figures in which:

FIG. 1 is a diagrammatic illustration of the stereotactic surgical apparatus of the present invention for coordinating images from a fluoroscope, planning a linear trajectory medical intervention, and controlling a robot to control the linear trajectory medical intervention;

FIG. 2 is a perspective view of a registration artifact and tool guide of the present invention;

FIG. 3a is a sample screen display of the user interface which includes an anterior/posterior (A/P) taken by the fluoroscope and displayed on a first computer monitor along with a number of the buttons and entry fields necessary to run the program;

FIG. 3b is a sample screen display which includes a sagittal image taken by the fluoroscope and displayed on a second computer monitor along with a number of the buttons and entry fields necessary to run the program;

FIGS. 3C,3D,3E, and 3F are flow charts of the steps performed by the computer during a main program loop;

FIG. 4 is a flow chart illustrating the steps performed by the computer to acquire an A/P image from the fluoroscope;

FIGS. 5A and 5B are flow charts illustrating the steps performed by the computer to acquire a sagittal image from the fluoroscope;

FIGS. 6A and 6B are flow charts illustrating the steps performed by the computer and the user to select or identify A/P fiducials from the A/P image displayed in FIG. 3a;

FIGS. 7A and 7B are flow charts of the steps performed by the computer and the user to select or identify sagittal fiducials displayed on the sagittal image of FIG. 3b;

FIG. 8 is a flow chart illustrating the steps performed by the computer to register the A/P image;

FIG. 9 is a flow chart illustrating the steps performed by the computer to register the sagittal image;

FIG. 10 is a flow chart illustrating the steps performed by the computer for changing a transverse angle of the virtual guidewire;

FIG. 11 is a flow chart illustrating the steps performed by the computer to change the length of the virtual guidewire used in the stereotactic surgical procedure;

FIG. 12 is a flow chart illustrating the steps performed by the computer to change a sagittal angle of the virtual guidewire;

FIG. 13 is a flow chart illustrating the steps performed by the computer to change the approach angle of the robot;

FIG. 14 is a flow chart illustrating the steps performed by the computer to move the robot illustrated in FIG. 1 to the planned position and orientation;

FIG. 15 is a flow chart illustrating the steps performed by the computer to move the end effector of the robot along the axis of the tool guide;

FIGS. 16A and 16B are flow charts illustrating the steps performed by the computer when the computer receives a user input based on a cursor in the A/P image area of FIG. 3a;

FIG. 16A and 16B are flow charts illustrating the steps performed by the computer when the computer receives a user input based on a cursor in the sagittal image area in FIG. 3b; and

FIGS. 18A and 18B are flow charts illustrating the steps performed by the computer when the computer receives a user input based on a cursor in the robot control areas of FIGS. 3a-b.

DETAILED DESCRIPTION OF DRAWINGS

Referring now to the drawings, FIG. 1 illustrates the stereotactic system 10 for linear trajectory medical interventions using calibrated and coordinated fluoroscopy. The apparatus and method of the present invention is designed to utilize images from a fluoroscope 12 such as a standard C-arm which generates fluoroscopic or x-ray images of a body on a surgical table 14. The imaging arm 16 is moveable so that both anterior/posterior (A/P) and sagittal or side images of the body can be taken.

A robot 18 is situated adjacent the surgical table 14. Illustratively, the robot is a PUMA-560 robot. The robot 18 includes a movable arm assembly 20 having an end flange 22. An alignment or registration artifact 24 is coupled to the end flange 22 of robot 18.

The registration artifact 24 is best illustrated in FIG. 2. The artifact 24 is X-ray and visually transparent with the exception of 8 opaque spheres or fiducials 26, and an aperture 30 to hold a tool guide 28 through the artifact 24. Initially, the artifact 24 is positioned roughly over the area of interest of body 32 and within the field of view of the fluoroscope 16. Therefore, the fiducials 26 show up as distinct dots on the A/P and sagittal images as discussed below. The shape of the artifact is designed so that the image dots from the fiducials 26 will not over shadow each other and is sensitive to any angular deviations. The robot arm 20 can adjust the artifact 24 in three-dimensions about X-axis 34, Y-axis 36, or Z-axis 38 illustrated in FIG. 1.

The coordinated fluoroscopic control system of the present invention is controlled by computer 40, which includes a microprocessor 42, and internal RAM 44, and a hard disk drive 46. Computer 40 is coupled to two separate graphics monitors 48 and 50. The first graphics monitor 48 displays a sagittal image taken by the C-arm 12. The second monitor 50 displays an A/P image taken by the C-arm 12. Computer 40 further includes a serial communication port 52 which is coupled to a controller 53 of robot 18. Computer 40 is also coupled to C-arm 12 for receiving the images from the C-arm 12 through an image acquisition card 54. Computer 40 is also coupled to an input device 56 which is illustratively a keyboard having a track ball input control 58. Track ball input 58 controls a cursor on both monitor 48, 50.

The displays on monitors 48 and 50 are illustrated in FIGS. 3a and 3b. Referring now to FIG. 3b, the sagittal image is displayed in area 62 on monitor 48. All eight fiducials 26 should appear in the sagittal image area 62. If not, the artifact 24 or the C-arm 12 should be adjusted. As discussed in detailed below, computer 40 displays a top entry point 64 and a bottom point 66 of a projected guidewire 68. The projected guidewire 68 is a line segment which is displayed on the sagittal image area representing the position of the instrumentation to be inserted during the stereotactic surgical procedure. A line of sight 70 is also displayed in the sagittal image area 62.

Various user option buttons are displayed on monitor 48. The surgeon or operator can access these options by moving the cursor to the buttons and clicking or by selecting the appropriate function keys (F1, F2, etc.) on the keyboard. The option buttons displayed on monitor 48 include button 72 (function F2) for acquiring the sagittal image, button 74 (F4) for selecting sagittal fiducials, and button 76 (F6) for registering the sagittal image. In addition, button 78 (F10) is provided for setting the sagittal angle, button 80 (F8) is provided for setting the screw length, and button 82 (F12) is provided for moving the robot along an axis of the tool guide. Finally, the display screen includes a robot control area 84. The operator can move the cursor and click in the robot control area 84 to control robot 18 as discussed below.

Referring to FIG. 3a, the A/P image displayed on the display screen of monitor 50 is illustrated. The A/P image is displayed in area 86 of the screen. Again, all eight fiducials 26 should appear within the A/P image area 86. The top insertion point of the virtual guidewire is illustrated at location 88, and the bottom point is located at location 90. The projection of the guidewire onto the A/P image is illustrated by line segment 92.

Computer 40 also displays various option buttons on monitor 50. Button 94 (F1) is provided for acquiring the A/P image. Button 96 (F3) is provided for selecting the A/P fiducials. Button 98 (F5) is provided for registering the AP image. Button 100 (F7) is provided for setting a transverse angle of the virtual guidewire, and button 102 (F9) is provided for setting an approach angle for the robot. Button 104 (F11) is provided for moving the robot. Computer 40 also displays a robot control area 84. The operator can move the cursor and click in the robot control area 84 to control robot 18 as discussed in detail below.

The present invention allows the surgeon to select the point of entry for the surgical instrument by moving the top point of the projected guidewire 88 in the A/P image area 86. The operator can also adjust the bottom point of the projected guidewire 90 to specify the transverse and sagittal angle. In addition, the operator can adjust the top point of the projected guidewire 64 to specify the position on the line of sight and bottom point of the projected guidewire 66 to specify the sagittal and transverse angle in the sagittal image area 62. Therefore, the surgeon can select the desired position and orientation of the surgical instrument into the body.

The computer 40 is programmed with software to correct spatial distortions from the optics of the fluoroscope 12. The system of the present invention permits effective three-dimensional planning of the stereotactic surgical procedure using only a pair of two dimensional fluorographic images displayed on the adjacent monitors 48 and 50. It is not required to use a CT slice in order to fully specify the location of the surgical instrument. The computer 40 establishes the direct geometric relationship between the A/P and sagittal images, despite image distortions and the essentially random or free-hand positioning of the C-arm 12, to establish the A/P and sagittal images. The improved system of the present invention can establish this exact geometric relationship within sub-millimeter accuracy.

Once the sagittal and A/P images are registered, points or lines chosen by the surgeon on one of the A/P image or the sagittal image are immediately displayed by computer 40 as corresponding projections on the other image. Therefore, using the sagittal image on monitor 48 and the A/P image on monitor 50, the surgeon can stereotactically plan the linear trajectory without the requirement of CT scan slice. Accordingly, the procedure of the present invention can be performed without the very expensive CT scan devices which can cost in excess of $1 million.

Details of the operation of the software for controlling the system of the present invention are illustrated in FIGS. 3C-18B.

All of the notations, subscripts and mathematical formulae, equations, and explanations are included in the attached Appendix. Throughout the flow charts described FIGS. 4-18B, reference will be made to the Appendix and to the numbered Sections [1] through [15] set forth in the Appendix.

The main program begins at block 110 of FIG. 3c. Computer 40 creates a parent window at block 112 and then draws buttons on a main window as illustrated at block 114. Computer 40 then creates a sagittal child window on monitor 48 as illustrated at block 116. Computer 40 also creates an A/P child window on monitor 50 as illustrated at block 118. Computer 40 then determines whether a button or key has been pressed at block 120. If not, computer 20 waits as illustrated at block 122 and then returns to block 120 to wait for a button or key to be pressed.

If a button or key was pressed at block 120, computer 40 determines whether the Acquire A/P Image button 94 or the F1 key was pressed at block 124. If so, computer 40 advances to block 166 of FIG. 4. If not, computer 40 determines whether the Acquire Sagittal Image button 94 or the F2 key was pressed at block 126. If so, the computer 40 advances to block 200 of FIG. 5A. If not, computer 40 determines whether the Select A/P Fiducial button 96 or the F3 key was pressed at block 128. If so, computer 40 advances to block 234 of FIG. 6A. If button 96 or the F3 key was not pressed at block 128, computer 40 determines whether the Select Sagittal Fiducial button 74 or the F4 key was selected as illustrated at block 130. If so, computer 40 advances to block 276 of FIG. 7A. If not, computer 40 advances to block 132.

In block 132, computer 40 determines whether the Register A/P Image button 98 or the F5 key was pressed. If so, computer 40 advances to block 324 of FIG. 8. if not, computer 40 determines whether the Register Sagittal Image button 76 or the F6 was pressed as illustrated at block 134. If so, computer 40 advances to block 350 of FIG. 9. If not, computer 40 advances to block 136.

From block 136, computer 40 determines whether the Transverse Angle button 100 or the F7 key was pressed as illustrated at block 138. If so, computer 40 advances to block 376 of FIG. 10. If not, computer 40 determines whether the screw Length button 80 or F8 key was pressed as illustrated at block 140. If so, computer 40 advances to block 388 of FIG. 11. If not, computer 40 determines whether the Sagittal Angle button 78 or the F10 key was pressed as illustrated at block 142. If so, computer 40 advances to block 400 of FIG. 12. If not, computer 40 determines whether the Approach Angle button 102 or the F9 key was pressed as illustrated at block 144. If so, computer 40 advances to block 412 of FIG. 13. If not, computer 40 advances to block 146.

In block 146, computer 40 determines whether the Move Robot button 104 or the F11 key was pressed. If so, computer 40 advances to block 422 of FIG. 14. If not, computer 40 determines whether the Move Robot Along Axis button 82 or the F12 key was pressed as illustrated at block 148. If so, computer 40 advances to block 452 of FIG. 15. If not, computer 40 determines whether the A/P Image area of monitor 50 has been selected by clicking when the cursor is in the A/P image area 86 as illustrated at block 150. If so, computer 40 advances to block 476 of FIG. 16A. If not, computer 40 then determines whether the Sagittal Image area was selected by positioning the cursor in the sagittal image area 62 on monitor 48 and clicking. If so, computer 40 advances to block 506 of FIG. 17A. if not, computer 40 advances to block 154.

From block 154, computer 40 determines whether the robot control area 54 or 106 was selected by moving the cursor and clicking in the Robot Control area 84 on monitor 48 or the Robot Control area 106 on monitor 50. If the Robot Control was selected, computer 40 advances block 536 of FIG. 18A. If the Robot Control was not selected, computer 40 advances to block 158 to determine whether the "Q" key was pressed indicating the operator desires to quit the main program. If the "Q" button was pressed, then computer 40 frees all allocated memory as illustrated at block 160 and ends the main program as illustrated at block 162. If the "Q" button was not pressed at block 158, computer 40 advances back to block 122, waiting for a another button or key to be pressed.

The various functions performed by the system of the present invention will be described in detail. If the Acquire A/P Image button 94 or the F1 key is pressed the, computer 40 advances to block 166 of FIG. 4. Computer 40 then determines whether the image acquisition card is in a passthrough mode at block 168. Button 94 and the F1 key are toggle buttons. When the button 94 or the F1 key is initially pressed, the card is in passthrough mode and images from the C-arm 12 are transmitted directly to the monitor 50. Whatever image is being taken by the C-arm is seen on the monitor 50 in the A/P image area 86. Therefore, if the card is not in the pass-through mode at block 168, pressing button 94 or the F1 key sets the pass-through mode at block 170. Computer 40 then returns to wait for the next command as illustrated at block 172. When the button 94 or the F1 key is pressed again after the image acquisition card within the computer 40 is in pass-through mode, it freezes the live image and captures the A/P image as illustrated at block 174. This captured image is then displayed on monitor 50 as illustrated at block 176. Computer 40 then disables and dims buttons F11, F12 and F5, and enables and brightens button 96 and key F3 as illustrated at block 178. In other words, after the A/P image has been captured, computer 40 allows the operator to have the option to select the A/P fiducials through button 96 or key F3.

Computer 40 then assigns a NULL tool as illustrated at block 180. The NULL tool of the robot is the three-dimensional location of end flange 22 of robot 18. In other words, the end flange 22 establishes a three-dimensional position for the robot, without depending on the particular surgical instrumentation which may be attached to the end flange 22. Computer 40 determines whether the NULL tool was properly assigned at block 182. If not, computer 40 generates an error message "Tool Not Assigned!" as illustrated at block 184. Computer 40 then waits for the next command as illustrated at block 186. If the NULL tool is assigned properly at block 182, computer 40 gets the current position of the end flange from the robot controller 53 as illustrated at block 188. Computer 40 then determines whether the sagittal image is displayed on monitor 48 as illustrated at block 190. If not, computer 40 sends a message of "Acquire Sagittal Image" as illustrated at block 192, and then returns to wait for the next command at block 194. If the sagittal image is displayed at block 190, computer 40 sends the message "Select the Fiducials" as illustrated at block 196. Computer 40 then returns to wait for the next command at block 198.

If the Acquire Sagittal Image button 72 or the F2 key is pressed, computer 40 advances to block 200 of FIG. 5A. Computer 40 then determines whether the image acquisition card is in a pass-through mode at block 202. Button 72 and the F2 key are toggle buttons. If the card is not in the pass-through mode at block 202, pressing button 72 or the F2 key sets the pass-through mode at block 204. Computer 40 then returns to wait for the next command as illustrated at block 206. When the button 72 or the F2 key is pressed again after the image acquisition card within the computer 40 is in pass-through mode, it freezes the live image and captures the sagittal image as illustrated at block 208. This captured image is then displayed on monitor 48 as illustrated at block 210. Computer 40 then disables and dims buttons F11, F12 and F6, and enables and brightens button 74 and key F3 as illustrated at block 212. In other words, after the sagittal image has been captured, computer 40 allows the operator to have the option to select the sagittal fiducials through button 74 or key F4.

Computer 40 then assigns a NULL tool as illustrated at block