Magnetic resonance imaging method and apparatus for guiding invasive therapy

Claims

I claim:

1. A method for guiding invasive therapy in a patient comprising:

positioning at least a region of patient anatomy containing a tissue upon which said therapy is to be performed within a magnetic resonance imaging apparatus;

introducing an instrument into said patient and guiding said instrument to said tissue by reference to at least one magnetic resonance image of said instrument within said region of patient anatomy acquired during the course of said guiding and to a previously prepared representation of the path to be followed by said instrument during the course of said guiding; and

carrying out said therapy on said tissue by positioning said instrument to perform said therapy.

2. The method according to claim 1, wherein said therapy is carried out on tissue containing a tumor.

3. The method according to claim 1, further comprising acquiring at least one additional magnetic resonance image of said instrument within said region of patient anatomy upon which said therapy is to be performed after said therapy has been commenced; and

monitoring the course of said therapy by reference to said at least one additional magnetic resonance image.

4. The method according to claim 1, wherein said instrument introduced into said patient comprises means for delivering a therapeutic chemical to said region of patient anatomy upon which therapy is to be performed; and

wherein said carrying out said therapy comprises delivering said therapeutic chemical to said region of patient anatomy.

5. The method according to claim 4, wherein said carrying out said therapy comprises delivering said therapeutic chemical to a tumor within said region of patient anatomy.

6. The method according to claim 5, wherein said delivering said therapeutic chemical comprises delivering oxygen to said region of patient anatomy.

7. The method according to claim 1, wherein said instrument introduced into the patient comprises means for delivering light to said region of patient anatomy upon which said therapy is to be performed; and

wherein said carrying out said therapy comprises delivering light to said region of patient anatomy.

8. The method according to claim 7, further comprising:

acquiring and utilizing at least one magnetic resonance image of said region of patient anatomy upon which said therapy is to be performed after said delivering light to establish that an adequate distribution of the light within said region of patient anatomy has been achieved.

9. The method according to claim 1, wherein said instrument introduced into said patient comprises means for delivering heat to said region of patient anatomy upon which said therapy is to be performed; and

wherein said carrying out said therapy comprises delivering heat to said region of patient anatomy.

10. The method according to claim 9, further comprising:

acquiring and utilizing at least one magnetic resonance image of said region of patient anatomy upon which said therapy is to be performed after said delivering heat to establish that an adequate distribution of the heat within said region of patient anatomy has been achieved.

11. The method according to claim 1, wherein said instrument introduced into said patient comprises means for applying radiation to said region of patient anatomy upon which therapy is to be performed; and

wherein said carrying out said therapy comprises delivering radiation to said region of patient anatomy.

12. The method according to claim 11, further comprising:

acquiring and utilizing at least one magnetic resonance image of said region of patient anatomy upon which said therapy is to be performed after said delivering radiation to establish that an adequate distribution of the radiation within said region of patient anatomy has been achieved.

13. The method according to claim 1, wherein said instrument introduced into said patient comprises means for surgically excising tissue, and said carrying out said therapy comprises surgically excising tissue.

14. A method according to claim 13, wherein said carrying out said therapy comprises excising diseased tissue using said surgical excision means.

15. The method according to claim 14, wherein said carrying out said therapy comprises excising diseased tissue plus any additional normal tissue as needed to effect an optimum treatment.

16. The method according to claim 1, further comprising acquiring at least one preliminary magnetic resonance image of said region of patient anatomy containing said tissue upon which said therapy is to be performed while said patient is in position for said therapy and prior to initiating said therapy.

17. The method according to claim 1, further comprising:

acquiring during the course of said guiding at least one magnetic resonance image of said region of patient anatomy; and

displaying said at least one magnetic resonance image of said instrument on said at least one magnetic resonance image of said region of patient anatomy, said image of said instrument being positioned on said image of said region of patient anatomy according to the actual position of said instrument relative to the anatomy of said patient.

18. The method according to claim 17, wherein said displaying said image of said instrument comprises forming said instrument image smaller than said at least one magnetic resonance image of said region of patient anatomy.

19. The method according to claim 17, wherein said displaying of said image of said instrument is more frequent than the displaying of said at least one magnetic resonance image of said region of patient anatomy.

20. A method of treating a tumor of a patient in vive by a catheter, comprising:

positioning a region of patient anatomy containing said tumor within a magnetic resonance imaging apparatus;

displaying a previously prepared representation of a path to be followed by said catheter through said patient to said tumor;

guiding said catheter through said patient to said tumor;

acquiring and displaying at least one magnetic resonance image of said catheter as it is being guided through said patient;

comparing said at least one magnetic resonance image of said catheter to said previously prepared representation of the path;

correcting for deviations in the position of said catheter from the previously prepared representation based on said comparison; and

delivering a therapeutic chemical to said tumor with said catheter.

21. The method according to claim 20, further comprising:

monitoring said tumor by magnetic resonance imaging to determine whether a desired degree of tumor perfusion by said therapeutic chemical has occurred.

22. The method according to claim 20, further comprising:

monitoring the effect of said therapeutic chemical on said tumor by magnetic resonance imaging.

23. The method according to claim 20, further comprising:

delivering an unactivated therapeutic chemical to said tumor through said catheter; and

activating said unactivated therapeutic chemical.

24. The method according to claim 20, wherein said delivering of said therapeutic chemical comprises:

sequentially delivering a first therapeutic chemical component and at least one second therapeutic chemical component which together comprise a therapeutic chemical for tumor treatment; and

monitoring the delivering of said first therapeutic chemical component by magnetic resonance imaging before delivering said at least one second therapeutic chemical component.

25. The method according to claim 20, further comprising acquiring at least one preliminary magnetic resonance image of said tumor while said patient is in position for said treatment and prior to initiating said treatment.

26. A method for surgically treating a joint of a patient, comprising:

positioning at least a region of joint anatomy of said patient which is to undergo surgical treatment within a magnetic resonance imaging apparatus, said joint of said patient being in position for said surgical treatment;

introducing an instrument into said patient;

displaying a pre-surgically prepared representation of the course of said instrument to said region of joint anatomy:

acquiring and displaying at least one magnetic resonance image of said instrument as it is being guided to said region of joint anatomy;

guiding the course of said instrument to said region of joint anatomy which is to undergo surgical treatment by comparing said at least one magnetic resonance image of said instrument to said representation of the course and correcting the position of said instrument for deviations from the previously prepared representation based on said comparison.

27. The method according to claim 26, further comprising:

acquiring at least one preliminary magnetic resonance image of said joint anatomy which is to undergo surgical treatment prior to positioning said joint for said surgical treatment; and

positioning said joint to undergo said surgical treatment by reference to said at least one preliminary magnetic resonance image.

28. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the knee of said patient by reference to at least one magnetic resonance image of the knee of said patient.

29. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the shoulder of said patient by reference to at least one magnetic resonance image of the shoulder of said patient.

30. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the hip of said patient by reference to at least one magnetic resonance image of the hip of said patient.

31. The method according to claim 26, wherein said guiding step comprising guiding the course of said instrument to the ankle of said patient by reference to at least one magnetic resonance image of the ankle of said patient.

32. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the temporomandibular joint of said patient by reference to at least one magnetic resonance image of the temporomandibular joint of said patient.

33. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the elbow of said patient by reference to at least one magnetic resonance image of the elbow of said patient.

34. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the wrist of said patient by reference to at least one magnetic resonance image of the wrist of said patient.

35. The method according to claim 26, wherein said guiding step comprises guiding the course of said instrument to the spine of said patient by reference to at least one magnetic resonance image of the spine of said patient.

36. The method according to claim 26, further comprising acquiring at least one preliminary magnetic resonance image of said region of joint anatomy while said joint of said patient is in position for said surgical treatment and prior to initiating said surgical treatment.

37. A method for surgically treating the spine of a patient, comprising:

positioning said patient in a prone position, a region of spinal anatomy of said patient which is to undergo surgical treatment being positioned within a magnetic resonance imaging apparatus while the spine of said patient is in said prone position;

introducing an instrument into said patient;

displaying a pre-surgically prepared representation of the course of said instrument to said region of spinal anatomy;

acquiring and displaying at least one magnetic resonance image of said instrument as it is being guided to said region of spinal anatomy;

guiding the course of said instrument to said region of spinal anatomy which is to undergo surgical treatment by comparing said at least one magnetic resonance image of said instrument to said representation of the course and correcting the position of said instrument for deviations from the pre-surgically prepared representation based on said comparison.

38. The method according to claim 37, further comprising: acquiring at least one magnetic resonance image of said region of spinal anatomy and orienting said at least one magnetic resonance image to contain a particular nerve root within said region of spinal anatomy.

39. The method according to claim 37 further comprising: acquiring at least one magnetic resonance image of said region of spinal anatomy and orienting said at least one magnetic resonance image to contain a particular nerve root and the intervertebral foramen through which said particular nerve root extends within said region of spinal anatomy.

40. The method according to claim 37, wherein said guiding the course of said instrument comprises controlling the path of said instrument to enter said spine of said patient through an intervertebral foramen.

41. The method according to claim 37, further comprising acquiring at least one preliminary magnetic resonance image of said region of spinal anatomy while said patient is in said prone position for said surgical treatment and prior to initiating said surgical treatment.

42. An apparatus for carrying out surgery, comprising:

a magnetic resonance imaging system, including a magnet having a patient-receiving volume to receive a patient upon whom said surgery is to be performed;

display means for displaying at least one magnetic resonance image of an anatomical region of said patient within said patient-receiving volume and upon which said surgery is to be performed;

means for performing said surgery upon said patient within said anatomical region of said patient, said display means displaying at least one magnetic resonance image of said means for performing said surgery within said anatomical region of said patient, said at least one magnetic resonance image being acquired during the course of said surgery, and said display means further comprising means for receiving a previously prepared representation of the path to be followed by said means for performing said surgery in said anatomical region of said patient and displaying said previously prepared representation, said means for performing said surgery being remotely operable; and

operating means for operating said means for performing said surgery from an operating position exterior to said patient by reference to said at least one magnetic resonance image and said previously prepared representation.

43. The apparatus according to claim 42

wherein said operating means receives said path representation for controlling the previously prepared of said means for performing said surgery to coincide with said previously prepared representation.

44. The apparatus according to claim 42, wherein said display means comprises:

an interactive display screen for displaying said at least one magnetic resonance image of said region of patient anatomy upon which said surgery is to be performed, and

means for displaying said previously prepared representation on said interactive display screen while an image of said region of patent anatomy is being displayed.

45. The apparatus according to claim 42, wherein said display means comprises:

a plurality of display screens each for displaying an image along a different orientation of said region of patient anatomy upon which said surgery is to be performed;

means for receiving said previously prepared representation; and

means for displaying said previously prepared representation on at least one of said display screens.

46. The apparatus according to claim 45, wherein said operating means receives said previously prepared representation for controlling the path of said means for performing said surgery to coincide with said previously prepared representation.

47. The apparatus according to claim 45, wherein said means for performing surgery includes a control means for automated advancement of a surgical probe along said previously prepared representation.

48. The apparatus according to claim 42, wherein said means for performing said surgery acquires at least one preliminary magnetic resonance image of said anatomical region of said patient while said patient is within said patient-receiving volume and prior to performing said surgery.

49. A method for guiding invasive therapy, comprising:

positioning a region of patient anatomy containing a tissue upon which said therapy is to be performed within a magnetic resonance imaging apparatus;

introducing an instrument into said patient and guiding said instrument to said tissue by reference to at least one three-dimensional magnetic resonance image of said instrument within said region of patient anatomy acquired during the course of said guiding and a previously prepared three-dimensional representation of the path of said instrument; and

carrying out said therapy on said tissue by positioning said instrument to perform said therapy.

50. The method according to claim 49, wherein said carrying out said therapy is carried out on tissue of the spine.

51. The method according to claim 49, further comprising acquiring at least one preliminary three-dimensional magnetic resonance image of said plurality of regions of patient anatomy containing said tissue while said patient is in position for said therapy and prior to initiating said therapy.

52. An apparatus for carrying out surgery, comprising:

a magnetic resonance imaging system, including a magnet having a patient-receiving volume to receive a patient upon whom said surgery is to be performed;

display means for displaying at least one magnetic resonance image of an anatomical region of said patient within said patient-receiving volume and upon which said surgery is to be performed;

means for performing said surgery upon said patient within said patient-receiving volume by reference to at least one magnetic resonance image of said anatomical region acquired during the course of said surgery, said means for performing said surgery being remotely operable and comprising at least a probe portion which can be detected on said magnetic resonance image; and

operating means for operating said means for performing said surgery from an operating position exterior to said patient,

said display means including means for receiving and displaying a pre-surgically prepared representation of the path to be followed by said means for performing surgery, said operating means receiving said path representation for controlling the path of said means for performing said surgery to coincide with said path representation, and said at least one magnetic resonance image of said anatomical region including paid probe portion obtained during the course of advancement of said means for performing surgery being compared to said pre-surgically prepared path representation and the actual course of said means for performing surgery being corrected during the course of said surgery for conformity to said pre-surgically prepared path representation.

53. An apparatus for carrying out surgery, comprising:

a magnetic resonance imaging system, including a magnet having a patient-receiving volume to receive a patient upon whom said surgery is to be performed;

display means comprising a plurality of screens each for displaying an image along a different orientation of said region of patient anatomy upon which said surgery is to be performed;

means for performing said surgery upon said patient within said patient-receiving volume by reference to at least one magnetic resonance image of said anatomical region acquired during the course of said surgery, said means for performing said surgery being remotely operable and comprising at least a probe portion which can be detected on said magnetic resonance image;

operating means for operating said means for performing said surgery from an operating position exterior to said patient,

said display means including means for receiving and displaying a representation of the path to be followed by said means for performing surgery on at least one of said display screens during the advancement of paid probe portion, said operating means receiving said path representation for controlling the path of said means for performing said surgery to coincide with said path representation and superimposing at least one magnetic resonance image of the actual path taken by said probe portion during the advancement of said probe portion in the course of said surgery on an image of said path representation on at least one of said display screens.

54. The apparatus according to claim 53, wherein said display means displays said path representation during the advancement of said probe portion, whereby said operating means corrects the actual path of said probe portion during the course of said surgery for conformity to said previously prepared representation.

BACKGROUND OF THE INVENTION

The present invention relates to improvements in magnetic resonance imaging, and more particularly to the development of treatment regimens and guidance of surgical procedures using magnetic resonance imaging.

The ability to produce excellent images of the internal anatomical structure of living beings using nuclear magnetic resonance signals has been well established. Magnetic resonance imaging is highly sensitive to the relaxation times of the nuclei emitting a magnetic resonance signal, and different relaxation times are manifested as different contrasts within the image. The tissues within the various organs and structures of a patient exhibit markedly different relaxation times. Diseased and injured issue changes in relaxation time relative to healthy tissue. Consequently, MRI produces very high contrast images of anatomical structure, in which injured and diseased tissues are clearly delineated from normal tissue.

It would be highly desirable to have techniques for making available high quality MRI images for use by a surgeon throughout the course of a surgical procedure in order to display the progress of the procedure. Presently, MRI has been largely constrained to pre-operative and post-operative imaging. Additionally, MRI has been used to perform MRI-guided fine-needle aspiration cytology and MRI-guided stereotactic neurosurgery. Experiments are also being carried out using MRI to monitor the delivery of laser light for medical purposes. However, none of these procedures involve continuous monitoring of the surgical procedure, including instrument guidance and control during the course of the procedure, by MRI guidance.

It would be an important advance in the art to have the capability of guiding an entire surgical procedure by reference to updated MRI images of the region of anatomy being operated upon. Many surgical procedures require a large incision for the purpose of exposing the anatomical region upon which the surgery is to be performed to the view of the surgeon. The surgical treatment aspect of the procedure, however, may be very localized and involve much less cutting of tissue or other disruption than that which is caused by the entire procedure. Thus, any techniques which reduce the amount of tissue damage necessary to reach the surgical site would be important in the field of surgical treatment.

Conventional CT scanning has limited application in guiding surgical procedures. First, limitations on patient exposure to X-ray prevents the unlimited use of CT scanning on any particular individual patient. In addition, a surgeon and other surgical team members must not be subjected to the repeated exposure to X-rays that would result with repeated operations on successive patients. Additionally, soft tissue imaging with CT scanning requires the use of contrast agents, and in many cases this would involve repeated and prolonged administration of contrast agents to the patient during the course of the surgical treatment.

CT also suffers from artifacts such as those that occur at the interface of bone and soft tissue. Additionally, MRI easily images an oblique plane of the patient so that the image plane orientation can be selected and changed during the course of the surgical procedure, as required. CT studies are limited to around the transaxial plane and might require patient repositioning for some surgical procedures. Moreover MRI also permits full three dimensional (3D) acquisition of images which is ideal for surgery by MRI guidance.

Related copending patent application, Ser. No. 07/993,072 filed Dec. 18, 1992, and commonly assigned herewith, discloses nuclear magnetic resonance magnets and apparatus which are suitable for MRI-guided surgery and discloses carrying out surgery within such magnets under MRI guidance. It would be desirable to use MRI guidance to the maximum degree possible, in order to minimize patient tissue damage which is caused only for the purpose of reaching the anatomical site where the surgery is to be carried out.

Notwithstanding the excellent image quality, resolution and contrast achieved in MRI images, the MRI technique has not become an integral part of the development of treatment regimens, and the identifying and development of therapeutic chemicals. MR imaging is presently applied like other traditional radiological techniques, for obtaining images representative of tissue structure. Magnetic resonance images of different anatomical portions of a patient are obtained, and the images are interpreted by a radiologist whose interpretations are reported back to a treating physician. For example, images of the internal structure of a patient's brain are obtained, the radiologist examines them for the presence of lesions, malformations or other pathology, and his interpretation is reported to the treating physician, e.g. a neurologist. The neurologist then determines a course of treatment based upon the radiologist's interpretation and other signs acquired by the neurologist.

It would be highly desirable to use MRI to acquire images of the actual course of a treatment, and not just tissue condition before and after drug treatment. Images obtained during the course of treatment could be used to alter a surgical procedure or drug therapy and drug dosage during the treatment. This use of MRI could find application in the selection of therapeutic chemicals and the selection of dosage, the development and modification of treatment regimens and for the verification of diagnosis accuracy.

Accordingly, it is an object of the invention to provide improved surgical instruments for use in MRI-guided surgery.

Another object of the invention is to provide improved MRI guided surgical procedures.

Another object of the invention is to provide new methods using MRI for developing treatment regimens and therapeutic chemicals.

Yet another object of the invention is to provide specific procedures for MRI guided treatment of tumors.

SUMMARY OF THE INVENTION

According to the invention MRI guided invasive therapy is carried out by positioning a patient for the therapy and acquiring at least one magnetic resonance image of the anatomy of the region of the patient upon which the therapy is to be performed, while that patient is in position for the therapy. An instrument is introduced into the patient and guided to the region where the therapy is to be performed by reference to the magnetic resonance image during the course of the guiding, and then the therapy is carried out with the instrument. The instrument may be fitted at its end with a surgical tool, for cutting, electrocautery, delivery of laser light, delivery of ionizing radiation, or delivery of non-ionizing radiation (e.f. rf, microwave) or a catheter for the localized delivery of drugs. In a preferred embodiment the instrument is a surgical instrument having a movable end which can follow a curved path to a treatment site within a patient and which can be monitored for proper positioning by MRI.

In another preferred embodiment of the invention a treatment regimen is identified using MRI. A plurality of therapeutic chemicals are administered, not necessarily through the instrument, directly to a tumor within a patient, and the tumor is continuously monitored by repetitive magnetic resonance imaging after the administration of the therapeutic chemicals. The resulting magnetic resonance images are examined to determine the effectiveness of the respective therapeutic chemicals. The amounts of selected ones of the therapeutic chemicals are adjusted to improve therapy effectiveness.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention are readily apparent from the detailed description of the preferred embodiments set forth below, in conjunction with the accompanying drawing in which:

FIG. 1 illustrates an apparatus according to the invention for carrying out MRI-guided invasive treatment;

FIGS. 2A-2D illustrate the operation of a surgical instrument according to the invention for use in MRI-guided surgery;

FIG. 3 illustrates details of the movable end of the surgical instrument shown in FIG. 2;

FIGS. 4A-4D are cross sections of the movable end of the instrument shown in FIG. 3;

FIGS. 5A-5E illustrate the steps in fabricating the movable end of the instrument shown in FIG. 3;

FIG. 6 illustrates another embodiment of a movable end of the instrument according to the invention;

FIGS. 7A and 7B illustrate a third embodiment of a movable end of the instrument according to the invention;

FIG. 8 is an anatomical drawing of a human lumbar vertebra showing a catheter implanted for carrying out MRI guided treatment;

FIG. 9 is a drawing of a magnetic resonance image of an anatomical model of a human lumbar spine with the spinal nerve root lying wholly within the image slice;

FIG. 10 is another drawing of a magnetic resonance image of an anatomical model of a human lumbar spine oriented as in FIG. 9 and showing a herniated intervertebral disc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an apparatus according to the invention for carrying out MRI-guided surgery. The apparatus includes a magnet 100 which is of the type disclosed in detail in copending U.S. application Ser. No. 07/993,072, filed Dec. 18, 1992 and commonly assigned. The detailed structure of the magnet is disclosed in the copending application. The magnet 100 is generally comprised of a ferromagnetic yoke 101 which provides a magnetic flux path for a magnetic flux generated by a source of magnetic flux 102. A pair of opposed pole surfaces 103, 104 define a gap 105 between them through which magnetic flux flows. A patient is positioned within the gap 105 for the acquisition of magnetic resonance imaging data, and to have surgical procedures carried out upon the patient under the guidance of the MRI images.

Block 110 represents means for performing surgery upon the patient. This structure can be dispensed with, and the surgery can be performed manually by a surgeon using appropriate surgical instruments. A preferred embodiment of the invention, and one of the novel features of the invention, utilizes a controllable apparatus for performing the surgery in lieu of manually performed surgery.

The surgery control unit 120 exercises control over the surgical performance unit 110 and receives position and other feedback signals for carrying out the surgical procedure. The surgical performance unit 110 is defined as the remote operating device wherein remote is defined as any region outside the body including all regions adjacent to the skin. The surgical performance unit can be either a manually operated or computer operated device for advancing the surgical instrument and/or therapeutic chemical delivery instrument into the patient's interior.

The scanner control unit and image display 130 interacts with the scanner magnet 100 and ancillary scanner subsystems for carrying out MRI on the patient and displaying the images on monitors for direct viewing. The scanner control unit and image display 130 can be used for carrying out repetitive scans during the course of the surgery to allow the surgeon and assisting personnel to continuously have current images of the anatomical site undergoing surgery on display to them.

The surgery control unit 120 and scanner control unit and image display 130 are shown as separate system elements, with a communication path 140 between them. In practice, these two units may be physically integrated such that they are operated from the same console and share operator controls and display screens. The surgery control unit 120 operated in concert with the scanner control unit enables the surgeon to plan the path of the surgical device from the outside of the patient towards the target tissue. The imaging display console may possess light pen capability so the surgeon may superimpose the line of the desired surgical path on the image or he may enter the path coordinates thru a keyboard. With the light pen path superimposed on the image the surgeon may then advance the surgical device (catheter, needle etc.) towards the target tissue and monitor its course with repeated images comparing the actual course with the planned light pen course to be certain the desired path is being achieved. However, they are functionally distinct and are separately represented in the drawing figure.

A novel feature of the present invention which is critical to carrying out generalized MRI-guided surgery is the provision of surgical instruments that can deviate from a linear path of travel through the human body while under MRI guidance. A preferred embodiment of such an instrument is shown in FIGS. 2A-2D.

A catheter and guide combination 200 shown in FIG. 2A is comprised of a tubular catheter body 201 having an open end 202. The open end 202 constitutes the leading end of the catheter body 201 which is inserted into the body of a patient. A guide wire 203 extends through the tubular catheter 201 along its length and is movable lengthwise through the catheter 201. The guide wire 203 terminates at a movable end portion 204 which is described below. The movable end portion 204 is the leading end of the guide wire 203 when it is advanced into the body of a patient.

The use of the catheter and guide structure is shown by the sequence of FIGS. 2A-2D. Initially the catheter 201 and guide wire 203 are straight. They are inserted into the patient's body as a pair and advanced together with the catheter open end 202 and the guide wire end portion 204 advancing together as the leading ends of the structure.

When it is desired to change the direction of advance of the catheter and guide wire the advancing of the catheter 201 is stopped while the guide wire 203 is advanced so that the guide wire end 204 extends beyond the open end 202 of the catheter 201. The end 204 of the guide wire 203 is caused to deflect toward the intended new direction of advance. This condition is shown in FIG. 2B.

Advancement of the catheter 201 is then resumed with the open end 202 of the catheter following along the curved end portion 204 of the guide wire 203. The deflected end portion 204 causes the advancing catheter 201 to change direction as it advances with a result that a bent portion 208 is induced in the normally straight catheter 201. This condition is shown in FIG. 2C.

Next, the guide wire 203 is advanced in the new direction. The catheter 201 is surrounded by body tissue so that the bend 208 will not relax and straighten, even after the end portion 204 of the guide wire is advanced out through the open end 202 of the catheter 201. Consequently, as the guide wire 203 is advanced into the patient's body it will change direction at a bend 210 which is a result of the guide wire advancing against the bent portion 208 of the catheter 201. This is shown in FIG. 2D.

If the tissue surrounding the catheter is sufficiently firm, the catheter can be advanced along with the guide wire without losing the change of direction achieved by the bent portion 208 of the catheter 201. Both the catheter 201 and the guide wire 203 should be resilient so that they can be bent, and so they will also return to their relaxed shape after any bending pressure has been removed. They must likewise be sufficiently stiff to allow them to be advanced axially by pushing on them at a location remote from the advancing end. Finally, the guide wire 203 should be nonferrous to avoid image artifacts caused by magnetic field homogeneity.

Details of the movable end portion 204 of the guide wire 203 are shown in FIG. 3. The movable end portion 204 is shown in longitudinal section and is comprised of a bimetallic structure having a lower 221 and an upper half 222. Lower half 221 and upper half 222 are each made from a different metal having a different coefficient of thermal expansion. The halves 221, 222 meet at a permanent junction 223 at the free end of the movable end portion 204.

A thin insulative layer 224 is disposed between the metal halves 221 and 222 of the movable end 204, except at the junction 223. For purposes of illustration the insulative layer 224 is shown thicker than it would be made in practice. The guide wire 203 is comprised of a coaxial conductor for providing a current path to the movable end 204. The center conductor