System for indicating the position of a surgical probe within a head on an image of the head

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BACKGROUND OF THE INVENTION

Precise localization of position has always been critical to neurosurgery. Knowledge of the anatomy of the brain and specific functions relegated to local areas of the brain are critical in planning any neurosurgical procedure. Recent diagnostic advances such as computerized tomographic (CT) scans, magnetic resonance imaging (MRI) scanning, positron emission tomographic (PET) scanning, and magnetoencephotographic (MEG) scanning have greatly facilitated preoperative diagnosis and surgical planning. However, the precision and accuracy of the scanning technologies have not become fully available to the neurosurgeon in the operating room. Relating specific structures and locations within the brain during surgery to preoperative scanning technologies has previously been cumbersome, if not impossible.

Stereotactic surgery, first developed 100 years ago, consists of the use of a guiding device which channels the surgery through specific parts of the brain as localized by preoperative radiographic techniques. Stereotactic surgery was not widely used prior to the advent of modern scanning technologies as the injection of air into the brain was required to localize the ventricles, fluid containing chambers within the brain. Ventriculography carried a significant complication rate and accuracy in localization was marginal.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a system which can determine the position of a probe within an object and display an image corresponding to the determined position. It is a further object of this invention to provide a system which can determine the position of an ultrasound probe relative to an object and, still further, which can display scan images from other scanning technologies corresponding to the scan images produced by the ultrasound probe. It is a further object of this invention to provide a system which can relate scan images of an object produced with one technology to scan images of the same object produced with another technology.

The invention comprises a system for indicating a position within an object. The system includes reference points means in fixed relation to the object. Means generates images of the object, said images including reference images corresponding to the reference points means. The system also includes reference means located outside the object and a probe including a tip. First means determines the position of the tip of the probe relative to the reference means. Second means measures the position of the reference points means of the object relative to the reference means, so that the position of the tip relative to the reference points means of the object is known. Means translates the determined position of the tip of the probe into a coordinate system corresponding to the images of the object. Means displays an image of the object which corresponds to the translated position of the tip of the probe.

The invention also comprises a system for indicating a position within a body of a patient. The system includes reference points means in fixed relation to the body. Means generates images of the body, said images including reference images corresponding to the reference points means. The system further includes reference means located outside the body and a probe including a tip. First means determines the position of the tip of the probe relative to the reference means. Second means determines the position of the reference points means of the body relative to the reference means, so that the position of the tip relative to the reference points means of the body is known. Means translates the determined position of the tip of the probe into a coordinate system corresponding to the images of the body. Means displays an image of the body which corresponds to the translated position of the tip of the probe.

The invention also comprises a method for indicating a position of a tip of a probe which is positioned within an object such as a body on images of the body wherein the body and the images of the body include reference images corresponding to a reference point. The method includes the steps of determining the position of the tip of the probe relative to a reference means having a location outside the body; determining the position of the reference points of the body relative to the reference means so that the position of the tip relative to the reference points of the body is known; translating the determined position of the tip of the probe into a coordinate system corresponding to the images of the body; and displaying an image of the body which corresponds to the translated position of the tip of the probe.

The invention also comprises a system for determining a position of an ultrasound probe relative to a part of a body of a patient wherein the probe is positioned adjacent to and scanning the body part. An array is positioned in communication with the probe. First means determines the position of the ultrasound probe relative to the array. Second means determines the position of the body part relative to the array. Means translates the position of the ultrasound probe into a coordinate system corresponding to the position of the body part.

The invention also comprises a system for relating scan images of a body of a patient. The scan images are produced from first and second scanning technologies. The system includes reference points means in fixed relation to the body. Means relates the first scanned images to the reference points means. Means relates the second scanned images to the reference points means. Means selects a particular first scanned image. Means determines the position of the particular first scanned image relative to the reference points means. Means generates a second scanned image which has the same position relative to the reference points means as the determined position so that the generated second scanned image corresponds to the particular first scanned image.

The invention also comprises apparatus for indicating a position relative to a body of a patient. The apparatus comprises radiopaque markers and means for noninvasively supporting the markers on the surface of the skin of the body. The supporting means may comprise a sheet of material overlying the body, and means on the sheet of material for supporting the markers.

The invention may be used with a scanner for scanning a body part of a patient in order to generate images representative of the body part. The improvement comprises means for marking the surface of the skin on the body part with a radiopaque material, whereby the generated images include images of the marking means.

Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective illustration of a reference ring of the prior art which is mounted by uprights to a patient's head to support the cylindrical frame structure of FIG. 1B or the ring 306 of FIG. 3B.

FIG. 1B is a perspective illustration of a cylindrical frame structure of the prior art which is mounted around a patient's head during the scanning process.

FIG. 1C is a plan view according to the prior art of the rods of the cylindrical frame structure of FIG. 1B taken along a plane midway between the upper and lower rings.

FIG. 1D is a perspective illustration of the coordinate system of a three dimensional scanned image.

FIG. 2A is a perspective view of the caliper frame of the prior art used to target a position in the brain and to determine a position in the head relative to the phantom base.

FIG. 2B is a perspective view of the caliper frame of the prior art of FIG. 2A illustrating its angles of

FIG. 2C is a block diagram of the steps involved in the prior art process of determining the position of a probe relative to the scanned images so that the image corresponding to the probe position can be identified and viewed by the surgeon.

FIG. 2D is a perspective illustration of a three dimensional coordinate system of a probe.

FIG. 3A is a block diagram of one system of the invention for indicating the position of a surgical probe within a head on an image of the head.

FIG. 3B is a perspective schematic diagram of a microphone array, probe and base ring according to one system of the invention.

FIG. 3C is a block diagram of the steps involved in the process according to the invention for determining the position of a surgical probe relative to the scanned images so that the image corresponding to the probe position can be identified and viewed by the surgeon.

FIG. 3D is an illustration showing three reference points on a head for use as a frame of reference during preoperative scanning and surgery.

FIG. 4A is a perspective schematic diagram of an infrared detector array, probe, reference bar, clamp and optical scanner according to one system of the invention.

FIG. 4B is a block diagram of a system for use with the apparatus of FIG. 4A for determining the contour of a forehead.

FIG. 5 is a flow chart of the translational software for translating coordinates from the probe coordinate system to the scanned image coordinate system according to the invention.

FIG. 6A is a perspective schematic diagram of a detector array, reference bar, clamp and ultrasound probe according to one system of the invention;

FIGS. 6B and 6C illustrate ultrasound and scanned images, respectively.

FIG. 7 illustrates the orientation of the base ring with a scanning plane for relating the position of a probe with a scanned image or for interrelating the scanned images of different scanning technologies which correspond to a common position in the head according to one system of the invention.

FIG. 8 illustrates the use of a remote depth finder for determining the contour of a forehead.

FIGS. 9 through 11 illustrate apparatus including a cap and grommets for holding radiopaque markers during scanning.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With the advent of modern scanning equipment and techniques, several stereotactic systems have been developed and are presently available. These stereotactic systems allow a surgeon to localize specific points detected on CT, MRI, PET, or MEG scans which have been previously generated prior to the surgical procedure being performed. In Particular, the stereotactic systems allow the selection of specific points detected on the scans to be localized within the brain by the surgeon during the surgical procedure using a mechanical device.

In use, the prior art stereotactic systems often require a base such as a ring 120 (also known as a BRW head ring) in FIG. 1A. Ring 120 is firmly attached to the patient's skull via uprights 122 and sharp pins 124 throughout scanning and surgery.

During scanning, some form of localizing device, such as a cylindrical structure 100 in FIG. 1B, is attached to ring 120. Structure 100 comprises an upper circular ring 102 in parallel with a lower circular ring 104. Lower ring 104 is mounted to reference ring 120 so that the three rings 102, 104 and 120 are in parallel planes. Rings 102 and 104 are interconnected by six vertical rods 106 and three diagonal rods 108. These specific marking rods are also called fudicels. The three diagonal rods 108 diagonally interconnect rings 102 and 104. Any plane orthogonal to an axis 110 of structure 100 which passes through structure 100 will create a unique pattern of six cross sectional views of rods 106 and three cross sectional views of rods 108. The resultant spacing between the diagonal and upright rods defines a unique orthogonal plane within structure 100. FIG. 1C shows, for example, the spacing of the rods when the position of the scan plane 112 is parallel to and midway between rings 102 and 104 of structure 100.

After the scanning process, the images obtained are analyzed and the position of rods 106 and 108 shown in the images is measured. By knowing the position of rods 106 and 108, the specific location of a scan with respect to structure 100 and therefore with respect to base ring 120 can determined. As shown in FIG. 1D, the scans can be arranged within a scanned image coordinate system 125 with reference plane RP set in fixed relation to the position of ring 120. A scan plane SP can be defined within the scanned image coordinate system 125 by at least three reference points SP1, SP2 and SP3 located on the head of the patient. By associating a scan image with a scan plane SP in the scanned image coordinate system, a point on the scan can be identified with a point in the patient's head.

During surgery, the surgeon can use the stereotactic system to calculate a specific position within the brain corresponding to a scan image and then target that portion of the brain with a probe. First, the structure 100 used during scanning is removed from ring 120 and a specially designed caliper frame 200, as illustrated in FIG. 2A, is attached to ring 120. Frame 200 holds a surgical probe 202 which is positioned on an arch 206 for insertion into the patient's head. Frame 200 indicates the alpha, beta, gamma and delta angles on scales 208, 210, 212 and 214 for directing probe 202 to a particular target, as shown in FIG. 2B. The distance 216 from the tip of probe 202 to arch 206 is also determined. A computer is then used to correlate the position of the targeted scan image in the scanned image coordinate system with the corresponding angles alpha, beta, gamma and delta and distance 216 on frame 200 to enable the surgeon to apply the probe to the targeted area of the brain. A target picked out on a scan of a specific image can be approached with a fair degree of accuracy using this surgical procedure.

In the past, the surgeon has also used the stereotactic system in reverse in order to determine the position of the probe 202 in the brain relative to the scanned images so that the scan image corresponding to the probe position can be identified and viewed. To do this, the surgeon again attaches frame 200 to ring 120. Probe 202 is then positioned in frame 200 and inserted into the brain. Frame 200 is then removed from ring 120 and mounted to a phantom base 250 in a manner as illustrated in FIG. 2A. Phantom base 250 has a coordinate system (X.sub.1, Y.sub.1, Z.sub.1). Generally, caliper frame 200 identifies a point 201 over phantom base 250. A pointing device 252 is positioned to have its tip 254 at point 201. The X.sub.1 -Y.sub.1 plane of phantom base 250 is parallel to the plane in which the reference points RP1, RP2 and RP3 are located. The (X.sub.1, Y.sub.1, Z.sub.1) coordinates define the position of point 201. As a result, the position of point 254 with respect to the X.sub.1 -Y.sub.1 plane and, therefore, with respect to the reference plane RP is now known. A computer is used to calculate the specific position within the brain and the particular scan which corresponds to the calculated position can now be accessed and viewed on a scanning system. This prior art process is shown in diagram form in FIG. 2C.

After this cumbersome and time-consuming process, the surgeon has now

determined the position of the tip 201 of probe 202 with respect to the scanned images and can now view the image corresponding to the probe position to decide the next step in the surgical procedure. This entire process takes approximately ten to fifteen minutes and increases the risks of intraoperative contamination as the base of frame 200 is nonsterile. Because of these considerations, this surgical procedure is not commonly performed.

Although stereotactic surgery as performed with the apparatus of the prior art allows a surgeon to be guided to a specific point with accuracy, it has not been particularly useful in allowing the surgeon to identify the particular location of a probe within the brain at any point during the surgical process. Frequently in neurosurgery, brain tumors or other target points within the brain are indistinguishable from surrounding normal tissue and may not be detected even with the use of frozen sections. Moreover, with modern microsurgical techniques, it is essential that the neurosurgeon identify specific structures within the brain which are of critical functional importance to the patient. The boundaries of these structures must be accurately defined and specifically known to the surgeon during the surgical process. In this way, these tissues will not be disturbed or otherwise damaged during the surgical process which would otherwise result in injury to the patient. The minimal accuracy afforded by stereotactic surgery is generally insufficient for modern microsurgical techniques. Consequently, stereotactic surgery is not generally available to the majority of patients undergoing surgery.

The present invention solves these problems by allowing the surgeon to retrieve and display quickly the scanned image which corresponds to the current position of a tip 301 of a surgical probe 302. A cursor appears on the displayed scan to show the position of probe tip 301 within the displayed scan. FIGS. 3A-3C and 5 illustrate a system of the invention which includes sound emitters 360 and 370 and microphone detectors 350 and associated hardware to determine the position of probe tip 301 relative to a reference ring 306 on the patient's head. Because the position of the scanned images relative to reference ring 306 is known from the scanning procedure, the position of probe tip 301 relative to the scanned images is known and the relevant image can be displayed. FIGS. 3A and 4A-8 illustrate a system of the invention which includes infrared emitters 540 and 545 and detectors 550 in place of the sound emitters 360, 370 and microphone detector 350 for determining the position of a reference bar 548 and a probe tip 541. A computer 396 and an infrared scanner 380 relate the scanned images to the shape of the forehead and relate the shape of the forehead to the position of reference bar 548. Reference bar 548 is then associated with the scanned images through the forehead shape without the use of the cylindrical reference frame 100 during scanning. The use of the forehead shape as a reference point also allows the scanned images from different scanning technologies to be interrelated. As an alternative to reference ring 306 and reference bar 548 described above, FIG. 3D uses reference pins 307 affixed to the skull for determining the position of the patient's head during surgery. As a further alternative, FIGS. 9-11 use a removable cap for holding markers during scanning. The positions of the markers are marked on the head for later use during surgery in registering the surgical space with the scan images. FIG. 6 includes an ultrasound probe 500 for use during surgery. Other advantages are also provided as more fully described below.

In relating the position of a probe tip e.g., probe tip 301, to a scanned image, it can be seen in FIGS. 1D and 2D that the surgeon must know the specific location of tip 301 with respect to the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0) of the scans that were preoperatively performed. In other words, probe tip 301 has a particular coordinate system (X.sub.2, Y.sub.2, Z.sub.2) which is illustrated in FIG. 2D. Ideally, the surgical probe coordinate system (X.sub.2, Y.sub.2, Z.sub.2) must be related to the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0). The prior art as illustrated in FIG. 2B has suggested relating these coordinate systems via the phantom base coordinate system (X.sub.1, Y.sub.1, Z.sub.1). However, as noted above, this relational process is inaccurate, time-consuming and cumbersome. The invention uses a 3D digitizer system to locate the position of probe tip 301 within the probe coordinate system (X.sub.2, Y.sub.2, Z.sub.2) and to relate it to the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0).

FIGS. 3A and 3B show a microphone array 300, a temperature compensation emitter 304, a surgical probe 302, and a base ring 306. Microphone array 300 includes a plurality of microphones 350 which are preferably spaced one meter apart. Microphones 350 may be attached to the operating light above the patient's head in direct line of sight of all of the emitters 360 and 370. Microphones 350 thereby detect the sound emitted from the emitters. Surgical probe 302 preferably is a surgical coagulating forceps such as a bipolar coagulating forceps. Probe 302 could also be a drill, suction tube, bayonet cauterizing device, or any other surgical instrument modified to carry at least two sound emitters 360 thereon for determining position. Emitters 360 on probe 302 are essentially coaxial on an axis 362 with tip 301. Emitters 360 are in line and immediately below the surgeon's line of sight so that the line of sight is not blocked. Probe 302 has a bundle of wire 364 attached thereto or connection to an electrical power source. The wires required to energize emitters 360 are combined with bundle 364. The surgeon is familiar with handling such a probe connected to a wire bundle; therefore, this apparatus does not inconvenience the surgeon. During surgery, ring 306 is affixed to the reference ring 120 attached to the patient's head and is essentially coplanar with it. Ring 306 includes a plurality of emitters 370 which are preferably positioned 90 degrees apart with the center emitter being located at the anterior of the head. This permits ring 306 to be mounted around the head so that all three emitters are in line of sight with array 300.

In use, the position of each of emitters 360 and 370 is determined individually in order to determine the position of the devices to which the emitters are attached. This is accomplished by rapidly energizing the emitters one at a time in a predetermined sequence and measuring the time required for the individual sounds to reach each of microphones 350 in array 300. A 3D digitizer 312 controls this operation through a signal generator 308 and a multiplexer 310. Digitizer 312 may be an off-the-shelf Model GP-8-3D three dimensional sonic digitizer produced by Scientific Accessories Corporation. Under the control of digitizer 312, multiplexer 310 applies an energizing signal from signal generator 308 first to a temperature compensation emitter 304, then sequentially to emitters 370 on ring 306, then sequentially to emitters 360 on probe 302. During this time, digitizer 312 receives and digitizes the output signals produced by microphones 350 in response to the energizations of the emitters. The digitized output signals are output to a computer 314.

Computer 314, following the flow chart shown in FIG. 5 as more fully described below, is programmed with the predetermined pattern and timing for energizing emitters 360 and 370. Computer 314 includes a spatial acquisition and recording (SAR) program 316 which acquires and records spatial coordinates based on the digitized signals. For example, the SAR program 316 may be the SACDAC program licensed by PIXSYS of Boulder, Colo. SAR program 316 measures the time of transmission from each of the emitters to each of the microphones 350. By comparing these times, SAR program 316 calculates the position of each of emitters 360 and 370. Since ring 306 contains three emitters 370, SAR program 316 can calculate the position of ring 306 through standard geometric computations. This plane essentially defines the reference plane of the scan images because it is coplanar with the reference points RP1, RP2 and RP3 in the scanning coordinate system of FIG. 1D. Similarly, since probe 302 contains two emitters 360, SAR program 316 can calculate the position of probe tip 301 through standard geometric computations. After SAR program 316 determines the respective positions of ring 306 and probe tip 301 relative to array 300, it next determines the position of ring 306 relative to tip 301 within the probe coordinate system of FIG. 2D.

One consideration in using sound emitters to determine position is that the speed of the emitted sound Will vary with changes in the temperature of the air in the operating room. In other words, since the system is very accurate, the period of time that it takes from the instant a particular emitter 360 or 370 is energized to emit sound until the instant that each of microphones 350 of array 300 receives the sound will vary with air temperature. In order to calibrate the system for these changes, temperature compensation emitter 304 is located in a fixed position relative to array 300. Temperature compensation emitter 304 may be, for example, a sonic digitizer as is used in the Scientific Accessories Corporation Model GP-8-3D. SAR program 316 knows, through calibration, the distance between temperature compensation emitter 304 and each of the microphones 350 of array 300. The speed of sound transmitted from temperature compensation emitter 304 to microphones 350 is measured by the SAR program and compared against the known distance to determine the speed at which the sound is being transmitted through the air. Therefore, SAR program 316 can immediately calculate the reference standard, i.e., the velocity of the emitted sound through the air. This instantaneous reference is applied to the sound emitted from the other emitters 360 and 370 to determine accurately the position of the other emitters.

After SAR program 316 has accurately determined the position of probe tip 301 in the probe coordinate system shown in FIG. 2D, it outputs the coordinates to translational software 318 in computer 314. Translational software 318 then translates the coordinates from the surgical probe coordinate system of FIG. 2D into the scanned image coordinate system shown in FIG. 1D, as more fully described below. A memory 320 accessed through a local area network (LAN) 321 stores each of the images of the preoperative scan according to the respective positions of the scans within the scanned image coordinate system of FIG. 1D. The respective positions of the scans are known from the position of rods 100 and 109 in the scans, which information is stored in memory 320. The translated coordinates generated by translational software 318 are provided to stereotactic image display software 322, also resident within computer 314. Stereotactic image display software 322 actuates a stereotactic imaging system 324 to generate a scan image from the data stored in memory 320 corresponding to the translated coordinates. Stereotactic imaging system 324 displays the generated image on a high resolution display 326. Display 326 preferably displays the axial, saginal and coronal views corresponding to probe tip 301. Stereotactic image display software 322 and stereotactic image system 324 may be any off-the-shelf system such as manufactured by Stereotactic Image Systems, Inc. of Salt Lake City, Utah. This cycle of calibrating the system through temperature compensation emitter 304, sequentially energizing emitters 370 and 360 to determine the respective positions of ring 306 and probe 302, and generating and displaying a scan image corresponding to the position of probe tip 301 all occur each time the surgeon closes a switch to activate the system. The switch (not shown) may be positioned on probe 302, in a floor pedal (not shown), or wherever else may be convenient to the surgeon.

As seen above, ring 306 is one apparatus for determining and positioning the reference points RP1, RP2 and RP3 with respect to microphone array 300. An advantage of ring 306 is that, each time emitters 360 on probe 302 are energized, emitters 370 on ring 306 are also energized to redefine the reference plane. This allows the surgeon to move she patient's head during surgery.

Alternatively, as shown in FIG. 3D, the reference points RP1, RP2 and RP3 can be established with the 3D digitizer 312 and three reference pins 307. Reference pins 307 are radiolucent surgical screws with radiopaque tips. pins 307 are permanently affixed to the patient's skull before surgery and before the preoperative scanning. The radiopaque tips thereby provide a constant reference during scanning and throughout the stereotactic surgical procedure. During surgery, probe tip 301 is positioned on each of pins 307 and actuated to emit a signal which is detected by microphone array 300 and output to 3D digitizer 312. This allows the position of tip 301 to be determined at each of these points. This is performed during a reference mode of operation of 3D digitizer 312. At the end of the reference mode, SAR program 316 calculates the position of the reference points RP1, RP2 and RP3. The use of pins 307 requires that the reference points have to be reestablished before the position of probe 302 is determined to avoid changes in the reference plane due to movement of the head. A further variation contemplates that emitters 370 may each be separately mounted to pins 307 or other fixed structures positioned at each of the reference points.

In summary, this process according to the invention is illustrated in FIG. 3C and identifies the location of probe tip 301 for the surgeon. Initially, the reference plane is determined by energizing ring 306 or by positioning probe tip 301 at the reference points. Next, the emitters of probe 302 are energized so that the position of probe tip 301 in the head is determined in the probe coordinate system (X.sub.2, Y.sub.2, Z.sub.2). Translational software 318 then converts the probe coordinate system into the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0) so that the image corresponding to the position of probe tip 301 can be generated and displayed.

In another system of the invention as shown in FIG. 4A, infrared emitters 540 and 545 and an array 552 of detectors 550 are used respectively in place of sound emitters 360 and 370 and microphones 350 of FIG. 3B. Fixed reference bar 548, a surgical probe 542, and related components are used in place of ring 306, probe 302, and related components of FIG. 3B. A Mayfield clamp 570 of known construction is used in place of ring 120 for rigid attachment to the patient's head 394. Clamp 570 includes sharp pins 572 attached to adjustable jaws 574 and 576. Clamp 570 is thereby adjusted for rigid attachment to head 394. Reference bar 548 is rigidly attached to clamp 570 so that there is no relative movement between bar 548 and head 394. No temperature compensating emitter such as emitter 304 in FIG. 3B is required in FIG. 4A because the apparatus of FIG. 4A uses the position of emitters 540 and 545 as viewed by detectors 550 (as more fully explained below) to determine probe and ring positions instead of the time of transmission of the emitted signal as with the embodiment of FIG. 3B.

In use, infrared detectors 550 are attached to a mounting bar 551 in fixed relation to each other. Detectors 550 are generally positioned so that their views converge on a phantom point. For example, the two outer detectors 550L and 550R may view a field of two intersecting vertical planes and the center detector 550C would view a horizontal plane. This can be accomplished by employing vertical slits on the field of view of the outer detectors and a horizontal slit on the field of view of the center detector. The phantom point is set to be in the general vicinity of the patient's forehead 390. Mounting bar 551 is suspended from the operating room light in direct line of sight of the patient's forehead 390 and of emitters 540 and 545. Detectors 550 thereby detect the infrared light emitted from emitters 540 and 545. Detectors 550 include a large number of linear chip cameras such as CCD (charge coupled device) cameras or pixels. A cylindrical lens (not shown) may also be used behind the slits in detectors 550 to collimate the infrared light. By knowing which particular pixel of the large number pixels found in each of the three detectors 550 receives the infrared light from emitters 540 and 545, the angle to a particular emitter from each of detectors 550 can be determined and, therefore, the positions of each of emitters 540 and 545 can be determined using conventional mathematical analysis. Accordingly, the position of probe tip 541 within the scan image coordinate system is known.

The apparatus of FIGS. 4A, 4B, 6A, 7 and 8 may be controlled with the computer and other hardware shown in FIG. 3A using the software shown in FIG. 5. Apart from the use of infrared light in place of sound and the measurement of the position of the emitters through geometry instead of the timed delay of sound, the operation of this hardware and software parallels the operation disclosed above.

An advantage of using infrared light is that it allows for the use of the

contour of a portion of the patient's head 394, preferably the forehead 390 above and around the patient's eyes, to relate the position of the probe 542 to the scan images. This is accomplished with an optical scanner 380 which generates an infrared laser beam which is reflected off of the patient's forehead 390 in timed sequence with the firing of emitters 545 to determine the forehead contour relative to reference bar 548. Such optical scanning of the forehead allows preoperative scanning to occur well in advance of anticipated surgery and without intubation. Other benefits and features of the improvement are more fully explained below.

In particular, FIGS. 4A and 4B include infrared selector array 552, probe 542, reference bar 548 and optical scanner 380. Surgical probe 542 preferably is a surgical coagulating forceps such as a bipolar coagulating forceps. probe 542 could also be a drill, suction tube, bayonet cauterizing device, or any other surgical instrument modified to carry at least two infrared emitters 540 thereon for determining position. Emitters 540 on probe 542 are essentially coaxial on an axis 544 with tip 541. Emitters 540 are in line and immediately below the surgeon's line of sight so that the line of sight is not blocked. Probe 542 has a bundle of wire 364 attached thereto for connection to an electrical power source. The wires required to energize emitters 540 are combined with bundle 364. Bar 548 comprises a bar with a plurality of at least three infrared emitters 545 positioned thereon. During surgery, the line of sight between some of the emitters 545 and the array 552 may be blocked by a surgical hose or other object. This could temporarily prevent array 552 from detecting the position of bar 548. Accordingly, it is preferable to place more than three emitters (e.g., seven or eight emitters) on bar 548 so that the line of sight for at least three emitters is always maintained. Such additional emitters can also be used to more precisely locate the position of bar 548. Bar 548 which holds emitters 545 is also preferably positioned slightly away from head 394 for increased clearance around head 394 and to reduce the number of instances where the line of sight between emitters 545 and array 552 is blocked. Optical scanner 380 is generally located in front of the patient's forehead 390. Optical scanner 380 and its associated software to generate a forehead image are standard, off-the-shelf components such as those used to scan an object to determine its three-dimensional shape. For example, a limb scanner such as the PIXSYS Optical Scanner used to develop three-dimensional models for artificial limbs may be used.

During the preoperative scanning process, when the cross sectional scanned images of the patient's head 394 are created, head 394 is fastened securely in a cushioned cradle 392 with surgical straps (not shown). If the contour of forehead 390 appears in the scan images, then computer 396 employs forehead fitting software 398 to derive the forehead contour from the scan images and to database the scan images as a function of the forehead contour in memory 320. If the scan images do not show the forehead 390, then (as shown in FIG. 7) head 394 is firmly clamped in fixed relation with a reference source, such as a ring 590, having emitters 592 thereon. Optical scanner 380 is then used to determine the position of the forehead contour relative to ring 590 (as more fully described below). Because the position of the scan images relative to ring 590 is known from the scanning procedure, the position of the scan images relative to the forehead contour is known. This information is then databased in memory 320 and used during surgery to relate the position of probe 542 to the scan images.

Forehead scanning with optical scanner 380 is accomplished in the following way. During preoperative scanning, head 394 is rigidly attached to ring 590 in FIG. 7. This attachment may be accomplished with a base ring (not shown) such as ring 120 in FIG. 3B. Under the control of 3D digitizer 312, scanner 380 emits an infrared laser beam which bounces off a single point on forehead 390 and is detected by array 552. Computer 396 determines the position in space of this first point on forehead 390, such as by triangulation. Next, emitters 592 on ring 590 are energized sequentially. Array 552 detects these emissions and computer 396 determines the relation between the first detected position on forehead 390 and the position of ring 590. This process is repeated many times, with scanner 380 tracing a path across forehead 390. All of the data comprising the position of each point of reflection from forehead 390 and the related position of ring 590 is input into forehead fitting software 398 of computer 396. Computer 396 thereby determines the contour of forehead 390 and, thus, the position of the forehead contour relative to ring 590. Forehead fitting software 398 may be any off-the-shelf or custom software which graphs a set of points so that a curve defining the contour of the forehead can be calculated. Computer 396 then outputs data relating the position of the forehead contour with the position of ring 590 to translational software 318 of computer 314. During scanning, the position of the scan images relative to ring 590 is known so that the position of the scan images relative to the forehead contour is also known. Accordingly, the scan images are stored in memory 320 as a function of the forehead contour.

Prior to surgery, head 394 is clamped with a mechanism such as the Mayfield clamp 570 shown in FIG. 4A for maintaining head 394 in rigid position. Reference bar 548 is rigidly attached to clamp 570 with emitters 545 in line of sight with array 552. Optical scanner 380 next scans the forehead to determine the position of the forehead contour relative to bar 548. The forehead contour derived from this second optical scanning is matched to the forehead contour stored for the scanned images in memory 320 so that the current position of bar 548 with respect to the scanned images is known. The forehead contour matching between the stored forehead contour and the forehead contour derived from the second optical scanning is accomplished using the well known Pellazari Chen algorithm or any other suitable surface matching algorithm. Bar 548 used during surgery includes emitters 545 which communicate with array 552 to establish the position of bar 548. Since the position of probe 542 relative to bar 548 is known (because of communication via emitters 540 and 545 and array 552) and since the position of bar 548 relative to the scanned images is known, the position of probe 542 relative to the scanned images is known. Accordingly, a scanned image corresponding to the position of tip 541 of probe 542 is generated and displayed.

One advantage of using either optical scanner 380 or surgical pins 307 in establishing a reference is that the reference ring, such as ring 120, is removed after preoperative scanning and before surgery. This is advantageous because the patient can not be intubated while ring 120 is attached to the skull. In the prior art, where ring 120 can not be removed during the time between preoperative scanning and surgery, the patient must be intubated (and therefore anesthetized) prior to preoperative scanning. Thus, by using the contour of forehead 390 to define the reference point, the preoperative scanning is performed without the need for intubation and the anesthesia accompanying it. This is particularly advantageous during PET, MEG and any other type of functional scanning where the patient must be conscious to elicit behavior during scanning. It is also advantageous during any form of scanning where the medical equipment for providing intubation and anesthetic would otherwise interfere with the scanning technology, such as MRI scanning.

In summary, when CT scanning is used, the patient lies with the head held in place on a CT table during the preoperative scanning process. The scans are organized in memory 320 according to the forehead contour appearing in the scans. Prior to surgery, the patient's head 394 is rigidly held in a Mayfield clamp or similar clamp on which reference bar 548 is mounted. Optical scanner 380 is then used to determine the patient's forehead contour relative to bar 548. Since the position of the scan images relative to the forehead contour is already known, the position of bar 548 relative to the scan images is known. During surgery, the surgeon positions probe 542 in the position desired within head 394. Emitters 540 of probe 542 and emitters 545 of bar 548 are then energized so that the position of probe tip 541 relative to bar 548 and, therefore, relative to the scan images is known. This is accomplished through the translational software 318 which converts the probe coordinate system (X.sub.2, Y.sub.2, Z.sub.2) into the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0) so that the image corresponding to the position of probe tip 541 can be generated and displayed.

Further summarizing, when MRI, PET or MEG scanning is used, the patient lies on an MRI, PET or MEG table with head 394 rigidly attached to ring 590. Optical scanner 380 then scans forehead 390 to determine the position of the forehead contour relative to ring 590. The MRI, PET or MEG scanning is then performed and the scan images are produced in known relation to the position of ring 590 and, therefore, in known relation to the forehead contour. The scans are organized in memory 320 according to the forehead contour. Prior to surgery, head 394 is rigidly held in a Mayfield clamp or similar clamp on which reference bar 548 is mounted. Optical scanner 380 is then used to determine the patient's forehead contour relative to bar 548. Since the position of the scan images relative to the forehead contour is already known, the position of bar 548 relative to the scan images is known. During surgery, the surgeon positions probe 542 in the position desired within head 394. Emitters 540 of probe 542 and emitters 545 of bar 548 are then energized so that the position of probe tip 541 relative to bar 548 and, therefore, relative to the scan images is known. This is accomplished through translational software 318 which converts the probe coordinate system (X.sub.2, Y.sub.2, Z.sub.2) into the scanned image coordinate system (X.sub.0, Y.sub.0, Z.sub.0) so that the image corresponding to the position of probe tip 541 can be generated and displayed.

Referring to FIG. 5, a flow chart of the operation of translational software 318 is shown as it is used with the apparatus of FIG. 3B. Initially, the surgeon locates probe 542 in the position which is to be determined. (If ring 306 is not being used to identify the location of the reference plane, the initial step is for the surgeon to use the reference mode of 3D digitizer 312 to identify the reference plane by locating probe tip 541 at several points in the plane.) The system then initializes at a step 400 so that translational software 318 opens a window menu at a step 402 of a multitasking program such as DESQ VIEW distributed by Quarterdeck Office Systems of Santa Monica, Calif. Such software permits simultaneous execution of multiple software programs. In general, once a program is selected for actuation, it continues to run either in the foreground or in the background until deactuated.

Translational software 318 continues initializing by selecting stereotactic imaging system 324 through stereotactic image display software 322 and actuating stereotactic imaging system 324 in the foreground by opening the stereotactic window at a step 404. Thereafter, translational software 318 returns to the window menu at a step 406 moving stereotactic image display software 322 to the background and selects the digitizer window at a step 408 to actuate digitizer 312 in the foreground. Computer 314 is then ready to be actuated by the foot switch.

The surgeon then actuates a foot pedal or other switch which indicates that the system should perform a computation. Actuation of the foot switch is essentially the beginning of a start step 410. Upon actuation, if sound transducers 360 and 370 and microphones 350 of FIG. 3B are being used, digitizer 312 initiates calibration through temperature compensation emitter 304 to determine the velocity of the sound waves in the air, energizes emitters 370 of ring 306 to locate the reference plane and energizes emitters 360 of probe 302 to locate the position of probe tip 301. The signals detected by microphone array 300 are digitized so that SAR program 316 determines the coordinates of tip 301. At a step 412, translational software 318 selects the coordinates from SAR program 316.

Next, the window menu is again accessed at a step 414 and the window menu switches stereotactic image system software 322 to the foreground at a step 416 to specifically control the operation of stereotactic imaging system 324. At this point, translational software 318 issues an F1 command to stereotactic image display software 322 which in turn prepares stereotactic imaging system 324 to accept coordinates. At a step 420, the window menu is again selected so that at a step 422 computer 314 switches the digitizer window into the foreground. At a step 424, the digitizer window menu is accessed and coordinate translation is selected. At a step 426, digitizer 312 begins calculating the coordinates and at a step 428 the coordinate calculation is ended. Translational software 318 then returns to the digitizer window menu at a step 430, switches windows to place stereotactic image system software 3