Method and apparatus for performing stereotactic surgery

I claim:

1. A method for performing stereotactic surgery with a medical instrument upon a target within a skull, comprising the steps of:

(a) establishing a first, predetermined geometric relationship between a positioning fixture attached to both the skull and to a support surface upon which the skull is disposed;

(b) scanning the skull to produce an image of the target within the skull with respect to the positioning fixture;

(c) transferring at least a portion of the positioning fixture to a phantom fixture and disposing the positioning fixture portion with respect to the phantom fixture to establish a second, predetermined geometric relationship therebetween, which is identical to the first, predetermined geometric relationship, whereby the slope of the skull where the positioning fixture is attached to the skull is duplicated within the phantom fixture;

(d) disposing a phantom target within the phantom fixture at a location which corresponds to the location of the target within the skull;

(e) determining the trajectory and distance of a medical instrument extending from the positioning fixture portion to the phantom target;

(f) attaching the portion of the positioning fixture upon the skull in the same location it was originally attached to the skull; and

(g) inserting the medical instrument through the positioning fixture in the trajectory determined from the phantom fixture, whereby the medical instrument will intersect the target in the skull.

2. The method of claim 1, wherein the positioning fixture is attached to the support surface by an attachment member having first and second end portions, and the first, predetermined geometric relationship is the attachment member disposed coplanar with the support surface; and the second, predetermined geometric relationship is the attachment member disposed coplanar with the phantom fixture.

3. The method of claim 2, including the steps of: moveably associating the first end portion of the attachment member with respect to the positioning fixture; and securing the attachment member with respect to the positioning fixture prior to scanning the skull.

4. The method of claim 3, including the steps of: moveably associating the attachment member with respect to a portion of the positioning fixture by using a positioning ball secured to the first end portion of the attachment member; and the positioning ball is rotatably received within the positioning fixture.

5. The method of claim 4, including the steps of using as the attachment member an elongate rod, which includes a means for identically orientating the second end portion of the attachment member with respect to both the support surface and the phantom fixture.

6. The method of claim 5, including the steps of: using a mating tongue and groove connection as the means for identically orientating the second end portion of the attachment member; and associating the mating tongue and groove connection with the second end of the attachment member and an end of an elongate rod disposed coplanar with the support surface and the phantom fixture.

7. The method of claim 1, including the steps of: using an outer and inner gimbal in the phantom fixture; and disposing the positioning fixture portion within the inner gimbal in the second geometric relationship.

8. The method of claim 7, wherein the positioning fixture is attached to the support surface by an attachment member having first and second end portions; the first, predetermined geometric relationship is the attachment member disposed coplanar with the support surface; the second geometric relationship is the attachment member disposed coplanar with the phantom fixture; and, while the positioning fixture portion is fixed with respect to the attachment member, the outer and inner gimbals are adjusted to receive the positioning fixture portion.

9. The method of claim 8, including the steps of locking the inner and outer gimbal with respect to the phantom fixture; securing the positioning fixture portion within the inner gimbal; and determining the trajectory and distance of the medical instrument by: inserting the medical instrument through the positioning fixture until it intersects the phantom target; and locking the medical instrument with respect to the positioning fixture at the desired trajectory and distance.

10. The method of claim 9, wherein the medical instrument is locked with respect to the positioning fixture portion by: associating a trajectory ball with the positioning fixture portion; inserting the medical instrument through the trajectory ball until it intersects the phantom target; and locking the trajectory ball with respect to the positioning fixture portion.

11. The method of claim 10, including the step of indicating the distance from the positioning fixture to the phantom target when the medical instrument has intersected the phantom target.

12. The method of claim 11, wherein the distance is indicated by associating a depth stop member with the medical instrument after the trajectory ball has been locked.

13. A system for performing stereotactic surgery with a medical instrument upon a target within a skull, comprising:

(a) a positioning fixture, having associated therewith, a means for attaching the positioning fixture to both the skull and to a support surface upon which the skull is disposed; the positioning fixture including means for establishing a first, predetermined geometric relationship between the positioning fixture and the support surface; and

(b) a phantom fixture, including: means for receiving at least a portion of the positioning fixture; means for establishing a second predetermined geometric relationship between the positioning fixture portion and the phantom fixture, wherein the second geometric relationship is identical to the first geometric relationship, whereby the slope of the skull where the positioning fixture is attached to the skull may be duplicated within the phantom fixture.

14. The system of claim 13, wherein the phantom fixture includes a phantom target which may be disposed within the phantom fixture at a location which corresponds to the location of the target within the skull with respect to the positioning fixture.

15. The system of claim 13, wherein the means for attaching the positioning fixture includes an attachment member having first and second end portions; in the first, predetermined geometric relationship, the attachment member is disposed coplanar with the support surface; and in the second, predetermined geometric relationship, the attachment member is disposed coplanar with the phantom fixture.

16. The system of claim 15, wherein the means for establishing the first, predetermined geometric relationship, includes the first end portion of the attachment member being moveably associated with respect to the positioning fixture; and the positioning fixture includes means for locking the attachment member with respect to at least a portion of the positioning fixture.

17. The system of claim 16, wherein the first end portion of the attachment member has a positioning ball associated therewith; and a portion of the positioning fixture includes means for rotatably receiving the positioning ball therein.

18. The system of claim 17, wherein the means for locking the attachment member with respect to a portion of the positioning fixture comprises at least one lock screw, which is associated with the rotatable receiving means, and is engageable with the positioning ball.

19. The system of claim 17, wherein the rotatable receiving means comprise a ball socket member which permits the first positioning ball to rotate therein; the ball socket member being rotatably mounted with respect to at least a portion of the positioning fixture and includes means for locking the ball socket member with respect to at least a portion of the positioning fixture.

20. The system of claim 19, wherein the ball socket member locking means comprises at least one thumb screw which contacts the ball socket member to lock its position with respect to a portion of the positioning fixture.

21. The system of claim 17, wherein the attachment member is an elongate rod having a means for identically orientating the second end portion of the attachment member with respect to both the support surface and the phantom fixture.

22. The system of claim 21, wherein the orientation means is a tongue and groove connection associated with the second end portion of the attachment member and both the support surface and the phantom fixture.

23. The system of claim 22, wherein the tongue and groove connection is associated with the support surface and the phantom fixture at an end of an elongate rod which is disposed coplanar with both the support surface and the phantom fixture.

24. The system of claim 13, wherein the means for establishing the second, predetermined geometric relationship, includes an inner and outer gimbal associated with the phantom fixture; the means for receiving at least a portion of the positioning fixture being associated with the inner gimbal.

25. The system of claim 24, wherein the means for receiving the positioning fixture portion includes means for locking the positioning fixture portion with respect to the inner gimbal.

26. The system of claim 24, wherein the phantom fixture includes means for locking the inner and outer gimbals with respect to the phantom fixture.

27. The system of claim 19, wherein the ball socket member has a trajectory ball disposed therein, upon the removal of the positioning ball.

28. The system of claim 27, wherein the second trajectory ball has a passageway therethrough adapted to permit a medical instrument to pass through the trajectory ball.

29. An apparatus, useful for performing stereotactic surgery, comprising:

(a) a skull plate member, adapted to be secured to a skull;

(b) a secondary plate member, adapted to overlie the skull plate member;

(c) a ball socket member rotatably received within the secondary plate member;

(d) means for aligning the skull plate member and secondary plate member with respect to one another; and

(e) means for locking the ball socket member with respect to the secondary plate member.

30. The apparatus of claim 29, including a ball disposed within the ball socket member, the ball having an elongate attachment member secured thereto, the attachment member being adapted to be attached to an imaging table.

31. The apparatus of claim 30, including means for locking the ball within the ball socket member.

32. The apparatus of claim 29, wherein the ball socket member is an annular disk member having an upwardly extending cup-shaped flange member.

33. The apparatus of claim 32, wherein sections of the upwardly cup-shaped flange member are removed.

34. A phantom fixture, useful in performing stereotactic surgery, comprising:

a frame;

a pair of rotatable gimbals, the first gimbal being disposed within the frame and rotatable about a first axis; the second gimbal being disposed within the first gimbal and rotatable about a second axis, the first and second axis being perpendicular to one another; and

the second gimbal including means for receiving at least a portion of a stereotactic positioning fixture.

35. A stanchion clamp, useful in performing stereotactic surgery, comprising:

an elongate member having upper and lower ends;

a bracket disposed at the lower end and including means for locking the bracket to at least a portion of a stereotactic positioning fixture; and

means for securing a medical instrument to the elongate member, whereby stereotactic surgery procedures can be performed.

Description Submit all comments and votes

FIELD OF THE INVENTION

The invention relates to a method and apparatus for performing stereotactic surgery with a medical instrument upon a target within a skull.

DESCRIPTION OF THE PRIOR ART

One of the ongoing interests of neurosurgeons is the practice of stereotactic surgery: gaining precise access to a specific point in the cranium through the application of an external three-dimensional coordinate system. Much time and effort has gone into the development of instrumentation for implementing such an approach to the human brain. With the development of computerized tomographic ("CT") scanning, and its precise imaging, stereotactic surgery is becoming the diagnostic and therapeutic procedure of choice for many disorders involving the intracranial cavity.

CT scanning produces an image representing a "slice" of brain tissue displayed with anatomical accuracy. The series of "slices", which constitute the complete CT study, represent a three-dimensional picture of the brain, defining the relationship of neurological structures or accurately localizing lesions. CT scanning has allowed physicians to visualize the brain directly, thus making identification of anatomical and pathological areas of interest much more precise, and thus much more accessible to the precise mechanics of stereotactic surgery. Mating CT scanning and stereotactic surgery involves a coordinate transformation from the two-dimensional space of CT scanning to the three-dimensional space of stereotactic surgery.

Although there has been a wide range of methods and devices designed to implement such a coordinate conversion, most of the devices have had a similar conceptual approach, wherein the resulting devices have left stereotactic surgery as being perceived as an esoteric, cumbersome, expensive, and time consuming procedure.

These prior art devices and methods typically utilize a frame mounted to the patient's skull by four pins or screws. Such devices have been found to be quite accurate and reliable and have allowed targets within a skull to be accessed with an accuracy of 1 mm. or less. They have allowed small, relatively inaccessible tumors to be biopsied with minor morbidity and practically absent mortality. These devices have also given surgeons a means of biopsying accessible tumors that are radiosensitive without the need for a formal craniotomy, a procedure that carries a much higher mortality and morbidity than stereotactic surgical procedures. In addition, such devices have provided a means for implementing new modalities for treating hematomas and abscesses, as well as the placement of radioisotopes and chemotherapeutic agents in the treatment of malignant brain tumors.

Despite these advances, there are characteristics of current stereotactic instruments which have severely limited their potential widespread application. The performance of careful stereotactic procedures on a regular basis with the prior art systems available requires much operating room time to be wasted during the procedure. Processing of X-ray pictures, target point calculations, and cumbersome mechanical adjustments on stereotactic frames add time to the operation. The inability of these systems to be reused on the same patient without recalculating target points also adds to their inefficiency. Although the prior art stereotactic instruments are adequate for reaching a single intracranial target point, rapid access to multiple targets during a procedure is inconvenient. Furthermore, the prior art devices are extremely expensive and are quite complicated to employ, thereby making their appeal to the surgeon in private practice quite limited. Some of the prior art systems require modifications of existing CT scanning software, or alternatively, require software generated coordinates determined from a hand-held calculator as part of the system. The frame required by these prior art devices requires fixation to the skull of the patient, typically via four screws, whereby the frame is quite cumbersome and uncomfortable. Additionally, the frame cannot be left on the patient's head if the same procedure is to be repeated at a later date. If subsequent stereotactic procedures are to be performed, the frame must be reapplied at the time of the second procedure, including the step of again using a CT scanner to calculate the coordinates of the target point within the skull.

Accordingly, prior to the development of the present method and apparatus for performing stereotactic surgery, there has been no method and apparatus for performing stereotactic surgery which: is compact, inexpensive, easy to use, precise, and comfortable; permits reaccessing of a target within the skull without additional CT scanning and coordinate recalculation; and does not require a bulky skull mounted frame. Therefore, the art has sought a method and an apparatus for performing stereotactic surgery which: does not require a skull mounted frame; is compact, inexpensive, easy to use, precise and comfortable; and permits reaccessing a target within the skull without re-scanning and coordinate recalculation.

SUMMARY OF THE INVENTION

In accordance with the invention, the foregoing advantages have been achieved through the present method and apparatus for performing stereotactic surgery. The method for performing stereotactic surgery, in accordance with the present invention, includes the steps of: establishing a first, pre-determined geometric relationship between a positioning fixture attached to both the skull and to a support surface upon which the skull is disposed; scanning the skull to produce an image of the target within the skull with respect to the positioning fixture; transferring at least a portion of the positioning fixture to a phantom fixture and disposing the positioning fixture portion with respect to the phantom fixture to establish a second, predetermined geometric relationship therebetween, which is identical to the first, predetermined geometric relationship, whereby the slope of the skull where the positioning fixture is attached to the skull is duplicated within the phantom fixture; disposing a phantom target within the phantom fixture at a location which corresponds to the location of the target within the skull; determining the trajectory and distance of a medical instrument extending from the positioning fixture portion to the phantom target; attaching the portion of the positioning fixture to the skull in the same location it was originally attached to the skull; and inserting the medical instrument through the positioning fixture in the trajectory determined from the phantom fixture, whereby the medical instrument will intersect the target in the skull.

Another feature of the present invention is that the positioning fixture may be attached to the support surface by an attachment member having first and second end portions and the first predetermined geometric relationship is the attachment member disposed coplanar with the support surface; and the second, predetermined geometric relationship is the attachment member disposed coplanar with the phantom fixture. A further feature of the present invention includes the steps of: moveably associating the first end portion of the attachment member with respect to the positioning fixture; and securing the attachment member with respect to the positioning fixture prior to scanning the skull.

A further feature of the present invention may include the steps of: moveably associating the attachment member with respect to at least a portion of the positioning fixture by using a first positioning ball secured to the first end portion of the attachment member; and the first positioning ball may be rotatably received within the positioning fixture.

Another feature of the present invention may include the steps of: using an outer and inner gimbal in the phantom fixture; and disposing the positioning fixture portion within the inner gimbal in the second, predetermined geometric relationship. A further feature of the present invention is that the positioning fixture may be attached to the support surface by an attachment member having first and second end portions; the first, predetermined geometric relationship is the attachment member disposed coplanar with the support surface; the second geometric relationship is the attachment member orthogonally disposed with respect to the phantom fixture; and while the positioning fixture portion is fixed with respect to the attachment member, the outer and inner gimbals are adjusted to receive the positioning fixture portion.

In accordance with the invention, the foregoing advantages have been achieved through the present system for performing stereotactic surgery. The system for performing stereotactic surgery, in accordance with the present invention, may include: a positioning fixture having associated therewith, a means for attaching the positioning fixture to both the skull and to a support surface upon which the skull is disposed; the positioning fixture including means for establishing a first, predetermined geometric relationship between the positioning fixture and the support surface; and a phantom fixture, which may include: means for receiving at least a portion of the positioning fixture; means for establishing a second, predetermined geometric relationship between the positioning fixture portion and the phantom fixture, wherein the second geometric relationship is identical to the first geometric relationship, whereby the slope of the skull where the positioning fixture is attached to the skull may be duplicated within the phantom fixture.

Another feature of the system in accordance with the present invention is that the means for attaching the positioning fixture may include an attachment member having first and second end portions; in the first, predetermined geometric relationship, the attachment member is disposed coplanar with the support surface; and in the second, predetermined geometric relationship, the attachment member is disposed coplanar with the phantom fixture.

An additional feature of the system of the present invention is that the means for establishing the first, predetermined geometric relationship, may include the first end portion of the attachment member being moveably associated with respect to the positioning fixture; and the positioning fixture may include means for locking the attachment member with respect to at least a portion of the positioning fixture. A further feature of the system of the present invention is that the means for establishing the second, predetermined geometric relationship, may include an inner and outer gimbal associated with the phantom fixture; the means for receiving at least a portion of the positioning fixture being associated with the inner gimbal.

The method and apparatus for performing stereotactic surgery of the present invention, when compared with previously proposed prior art methods and apparatus, have the advantages of being: compact, inexpensive, easy to use, precise, and comfortable for the patient; does not require a skull mounted frame; and permits reaccessing target areas within the skull without recalculating coordinates or rescanning the skull.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a front view of a patient (shown in dotted lines) with the positioning fixture of the present invention disposed upon the patient's skull;

FIG. 2 is a top view of a patient (shown in dotted lines) being disposed upon a support surface, with the positioning fixture of the present invention disposed on the patient's skull;

FIG. 3 is a side view of a patient (shown in dotted lines) disposed upon a support surface with the positioning fixture of the present invention disposed on the patient's skull;

FIG. 4 is a front view of a phantom fixture in accordance with the present invention;

FIG. 5 is another front view of a phantom fixture in accordance with the present invention;

FIG. 6 is a front view of a patient (shown in dotted lines) with the positioning fixture of the present invention being used to direct a medical instrument toward a target within the patient's skull;

FIG. 7 is a front view of a positioning fixture in accordance with the present invention, illustrating the intracranial area accessibility of a positioning fixture of the present invention;

FIG. 8 is a perspective view of a stanchion clamp accessory for use with the present invention;

FIG. 9 is a partial cross-sectional view of the positioning fixture taken along line 9--9 of FIG. 10;

FIG. 10 is a partial cross-sectional view of the positioning fixture taken along line 10--10 of FIG. 9;

FIG. 11 is a partial cross-sectional view of the positioning fixture of the present invention taken along line 11--11 of FIG. 10;

FIG. 12 is a perspective, exploded view of the positioning fixture of the present invention shown in FIGS. 9-11;

FIG. 13 is a perspective view of a phantom fixture of the present invention;

FIG. 14 is a partial cross-sectional view of a positioning fixture of the present invention, including a medical instrument disposed therein; and

FIG. 15 is a perspective, exploded view of the positioning fixture of FIG. 14.

While the invention will be described in connection with the preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

With respect to FIGS. 1-6, the method for performing stereotactic surgery with a medical instrument upon a target within a skull will be generally described. Throughout FIGS. 1-6, the patient's skull 160 is shown in dotted lines, and target 161 is illustrated to represent the abnormality which is desired to be treated. For example, target 161 could be a hematoma, abscess or tumor. With reference to FIG. 1, a positioning fixture 162 is attached to skull 160 in a manner to be hereinafter described in greater detail. Preferably, positioning fixture 162 is constructed in accordance with the present invention, as will be hereinafter described in greater detail. The location at which positioning fixture 162 is attached to skull 160 may be determined by the location of target 161 within skull 160, as well as by a trial fit of positioning fixture 162 to skull 160. As seen in FIGS. 1-3, positioning fixture 162 may have an attachment member 163 associated therewith. Attachment member 163 has a first end portion 164 moveably associated with respect to positioning fixture 162, as will be hereinafter described in greater detail.

With reference to FIGS. 2 and 3, the patient is laid upon a support surface 165 and skull 160 is thus disposed upon support surface 165. Typically, support surface 165 is a conventional, planar imaging table 166 which is used in connection with a conventional scanning device, such as a CT scanner. As seen in FIGS. 2 and 3, attachment member 163 is moved with respect to positioning fixture 162, so that attachment member 163 may mate with an upright bracket 167 which is secured to imaging table 166 in any suitable fashion, such as by a clamp or screws 168. With the positioning fixture 162 and attachment member 163 in the positions illustrated in FIGS. 2 and 3, a first, predetermined geometric relationship has been established between the positioning fixture 162 and the support surface 165 upon which skull 160 is disposed. Preferably, this first, pre- determined geometric relationship is with attachment member 163 being disposed coplanar with support surface 165. By use of the term "coplanar" it is meant that the longitudinal axis of attachment member 163 is caused to lie in a plane which is parallel with the longitudinal axis of the support surface 165, as seen in FIG. 3, and in a plane which is perpendicular to the longitudinal axis of support surface 165, as seen in FIG. 2.

With the patient's skull 160 disposed upon support surface 165, as shown in FIGS. 2 and 3, and the attachment member disposed in the first, predetermined geometric relationship with respect to support surface 165 and attached therebetween, the skull is then scanned by any suitable scanning device, such as a CT scanner, in a conventional manner. A radiopaque marker (not shown) is disposed in the center of the positioning fixture 162, whereby the scanning procedure produces a series of images of the target 161 with respect to the positioning fixture 162. After the scanning of skull 160 has been performed, at least a portion 171 of the positioning fixture 162 may be removed from the patient's skull 160 and transferred to a phantom fixture 172, as shown in FIG. 4. In this regard, preferably only a portion 171 of positioning fixture 162 is removed from skull 160; however, it is possible that the entire positioning fixture 162 could be removed from the patient's skull 160. Further, it should be noted that in contrast to the prior art methods and devices previously described, if a portion of positioning fixture 162 remains on skull 160, such positioning fixture portion is much more compact with respect to the patient's skull 160, as well as being much more comfortable for the patient.

Further, with regard to FIGS. 1-3, it is seen that the first end portion 164 of attachment member 163 may be moveably associated with respect to positioning fixture 162, as by securing the first end portion 164 of attachment member 163 to a positioning ball 169, and positioning ball 169 is rotatably received within the positioning fixture 162. Accordingly, attachment member 163 may be freely moved with respect to positioning fixture 162, whereby attachment member 163 can be disposed in the geometric relationship shown in FIGS. 2 and 3. Positioning fixture 162 may also be provided with a means for securing, or locking, 170 the attachment member 163 with respect to positioning fixture 162. Prior to scanning the skull 160, when attachment member 163 and positioning fixture 162 are disposed in the geometric relationship shown in FIGS. 2 and 3, positioning ball locking means 170, to be hereinafter described in greater detail, is preferably engaged, whereby skull 160, attachment member 163 and positioning fixture 162 will remain disposed in the geometric relationship shown in FIGS. 2 and 3 throughout the scanning procedure. After the scanning procedure has been performed, the portion 171 of positioning fixture 162 is transferred to phantom fixture 172. Preferably, positioning fixture portion 171 includes attachment member 163, positioning ball 169, and positioning ball locking means 170, as shown in FIG. 4, and as will be hereinafter described in greater detail.

With reference to FIG. 4, the positioning fixture portion 171, including attachment member 163 and positioning ball 169, which remains in the locked position shown in FIGS. 2 and 3, has been transferred to phantom fixture 172, and is disposed with respect to phantom fixture 172 in a second, predetermined geometric relationship therebetween. The second, predetermined geometric relationship is shown as being attachment member 163 being disposed coplanar with phantom fixture 172. In this regard, the term "coplanar" is defined as meaning that the attachment member 163 lies in planes which are all parallel with respect to the longitudinal axis 173 (as shown in dotted lines in FIGS. 4 and 13), of the phantom fixture 172. Thus, the second, predetermined geometric relationship between positioning fixture portion 171 and phantom fixture 172 is identical to the first, predetermined geometric relationship between positioning fixture 162 and support surface 165. Since the positioning fixture portion 171 disposed within phantom fixture 172 was originally disposed upon skull 160, the slope of skull 160 at the location where positioning fixture 162 was attached to skull 160 has been duplicated within phantom fixture 172. In summary, with the attachment member 163, including positioning fixture portion 171 secured thereto, disposed within phantom fixture 172 in the same position as shown in FIGS. 2 and 3, the second geometric relationship between the attachment member 163 and the phantom fixture 172 is the same as the first geometric relationship between the attachment member and support surface 165. Since the disposition of attachment member 163 with respect to the positioning fixture portion 171 is identical in FIGS. 2 and 3, and FIG. 4, and since positioning ball 169 has been locked with respect to positioning fixture 162, the slope of the skull 160 where the positioning fixture 162 has been attached to skull 160 is duplicated within the phantom fixture 172.

Still with reference to FIG. 4, a phantom target 174 is disposed within the phantom fixture 172 at a location which corresponds to the location of the target 161 within skull 160. The three-dimensional spatial coordinates of phantom target 174, representative of target 161 within skull 160, are determined in a conventional manner from the scanning procedure. The x and y coordinates of the target 161 within skull 160 may be read from the generated images from the scanning procedure and the z coordinate may be obtained by noting the difference in imaging table 166 displacement between the image slice containing the positioning fixture 162 and the image slice containing the target 161, insofar as the series of generated images disclose the target with respect to positioning fixture 162. In this regard, a radiopaque marker (not shown) is preferably disposed at the center of positioning ball 169, and, as will hereinafter be described in greater detail, the positioning fixture 162, including positioning ball 169 are made of a radiolucent material, such as LEXAN.RTM., so as not to interfere with the images produced by the scanning procedure. Thus, the coordinates of the target 161 are generated in relation to the positioning fixture 162. They can then be transferred to the phantom fixture 172 in a conventional manner, so that phantom target 174, which is preferably the tip of a phantom target rod 175, may be disposed within phantom fixture 172 at a spatial location which corresponds to the location of the target 161 within skull 160. Target rod 175, and in turn phantom target 174, may be disposed anywhere within phantom fixture 172 in a conventional manner, as will hereinafter be described in greater detail in connection with FIG. 13.

With reference to FIGS. 4 and 13, phantom fixture 172 preferably utilizes an outer and inner gimbal 176, 177. As seen in FIGS. 4 and 13, the positioning fixture portion 171 is disposed within the inner gimbal 177. Preferably, outer and inner gimbals 176, 177 are constructed as an outer gimbal ring 178 and an inner gimbal ring 179. Outer gimbal 176 may be preferably disposed between two upright frame members 180 which form a portion of a frame 181 of phantom fixture 172. Outer gimbal 176 may be rotated about an axis defined by two rod members 182 journaled within uprights 180. Likewise, inner gimbal 177 is mounted within outer gimbal 176 as to be freely rotatable with respect to outer gimbal 176, as by mounting inner gimbal 177 upon rod members 183 which are rotatably mounted in outer gimbal 176. Outer and inner gimbals 176, 177 may be rotated independently, the inner gimbal 177 rotating within outer gimbal 176. Thus, by adjusting the rotation of the outer and inner gimbals 176, 177 within phantom fixture 172, the inner gimbal 177 can be adjusted to receive the positioning fixture portion 171 when positioning fixture portion 171 is transferred, along with attachment member 163 in its locked position with respect to positioning fixture 171, and thus disposed within phantom fixture 172. Therefore, positioning fixture portion 171 will lie within inner gimbal 177 of phantom fixture 172, so as to be disposed in the second, predetermined geometric relationship previously described. It should of course be understood that the configuration of inner and outer gimbals 177, 176 could be any other configuration other than inner and outer gimbal rings 179, 178, so long as inner gimbal 177 is free to rotate within outer gimbal 176, and outer gimbal 176 can freely rotate within phantom fixture 172. For example, inner gimbal and outer gimbal 177, 176 could have a square configuration, wherein the inner square (not shown) is sized so as to be able to rotate within the outer square (not shown).

Still with reference to FIGS. 4 and 13, phantom fixture 172 may preferably include means for locking 184 the inner and outer gimbals 177, 176 with respect to phantom fixture 172. Preferably, locking means 184 for gimbals 176, 177 may be any suitable device which can lock rod member 182 with respect to uprights 180 of phantom fixture 172, and lock rod members 183 with respect to outer gimbal 176. For example, locking means 184 can comprise two thumb nuts 185, 186. Thumb nut 185 can engage upright 180, thus securing rod member 182, and in turn outer gimbal 176 in a fixed position. Thumb nut 186 can engage outer gimbal 176 which in turn locks the position of rod member 183 with respect to outer gimbal 178. Further, the inner gimbal 177 includes a means for locking 187 the positioning fixture portion 171 with respect to inner gimbal 177. In this regard, as seen in FIG. 4, inner gimbal 177 may be provided with an interior recessed flange member 188 upon which positioning fixture portion 171 rests upon when it is received within inner gimbal 177. Locking means 187, which preferably comprises at least one lock nut 189, and preferably two lock nuts 189, can be threaded downwardly to bear upon a small plate 190 which in turn bears against inner gimbal 177 and positioning fixture portion 171 as shown in FIG. 4. After positioning fixture portion 171, including attachment member 163, are disposed within phantom fixture 172, inner and outer gimbals, 177, 176 are adjusted as previously described. Locking means 184, 187 may then be engaged to lock inner and outer gimbals 177, 176 in the position shown in FIG. 4, and positioning fixture portion 171 is then secured within inner gimbal 177.

After the positioning fixture portion 171 and phantom target 174 have been disposed within phantom fixture 172 as previously described and as illustrated in FIG. 4, it is necessary t