1. FIELD OF THE INVENTION

[0001] This invention relates to an apparatus for navigating medical devices within the body to sites of treatment delivery, and methods of using this apparatus to achieve this navigation. More specifically, this invention relates to the use of a magnetic field from an MR imaging device to navigate a magnetic medical device within the body.

2. BACKGROUND OF THE INVENTION

[0002] The need for improved surgical navigation techniques stimulated the development of magnetic stereotaxis as a novel means for guiding a surgical implant, such as a catheter, along nonlinear paths within a body part. In particular, it is useful in intraparenchymal applications within the brain, where linear stereotactic techniques (either framed or frameless) do not permit the probe to follow single-pass curvilinear paths to a target location deep within the brain, as first taught by Howard et al. in U.S. Pat. No. 4,869,247 incorporated herein by reference. Howard et al. subsequently taught magnetic stereotactic techniques for volume-contoured therapy delivery within the brain and elsewhere in the human body in succeeding U.S. Pat. Nos. 5,125,88, 5,707,334, and 5,779,694 incorporated herein by reference. Advanced versions of magnetically guided surgical systems capable of performing magnetic stereotactic procedures in the brain and in other body parts have been disclosed in U.S. patents by Werp et al., U.S. Pat. No. 5,9331,818; Blume et al., U.S. Pat. No. 6,014,580; Werp et al., U.S. Pat. No. 6,015,414; Ritter et al., U.S. Pat. No. 6,128,174; and Blume et al., U.S. Pat. No. 6,157,853. In all of these approaches, as well as in any of the other known techniques for magnetic manipulation of a probe mass or implant located within the body (see Gillies et al., “Magnetic manipulation instrumentation for medical physics research,” Review of Scientific Instruments, pp. 533-562 (USA 1994)), incorporated herein by reference, the controlled movement of the probe mass or implant is actuated by a magnetic field created external to the body. In all such arrangements the magnetic component of the implant (typically located at the tip of a catheter) is a passive ferromagnetic or permanent magnetic element of a geometry consistent with that of the catheter's form and function, and within which there either exists or can be made to exist, adequate magnetic moment to create the forces and torques needed to steer and/or guide the implant within the body part into which it has been inserted.

[0003] Magnetic stereotaxis is particularly useful for navigation of medical devices throughout body tissues, cavities, and vessels. Discussion of applications to catheter navigation within the chambers of the heart for electrophysiologic mapping and ablation can be found in Hall et al., U.S. patent application Ser. No. 09/405,314, incorporated herein by reference. Disclosure of navigation of catheters within the myocardial tissue of the heart can be found in Sell et al., U.S. Pat. Application Serial No. 09/398,686, incorporated herein by reference. Removal of tissues from body lumens and cavities via magnetic navigation of atherectomy tools is disclosed in Hall et al., U.S. Pat. Application Serial No.09/352,161, incorporated herein by reference. Catheters for magnetic navigation within the blood vessels of the brain and other body parts are disclosed in Garibaldi, U.S. Pat. Application Serial No. 60/153,307, incorporated herein by reference.

[0004] Four inherent limitations to this general design of magnetic stereotaxis system are the following. First, it is generally unsafe to perform magnetic resonance (MR) imaging studies during or after a magnetic stereotaxis procedures in which the magnetic element of the implant is still resident within the patient, as might be contemplated in situations where updated MR data might be needed for ongoing magnetic stereotaxis navigation requirements. This is because the large fields intrinsic to all types of MR scanners (either standard bore-type systems or the lower-field interventional-style systems) are large enough to cause otherwise uncontrolled displacement of the implant within the patient. The nature of this particular problem is discussed in the broader context of MR-driven forces on implants, by Planert et al., “Measurements of magnetism-related forces and torque moments affecting medical instruments, implants, and foreign objects during magnetic resonance imaging at all degrees of freedom,” Medical Physics, pp. 851-856 (USA 1996) and by Manner et al., “MR Imaging in the presence of small circular metallic implants,” Acta Radiological, pp. 551-554 (Denmark 1996), the disclosures of both of which are incorporated herein by reference.

[0005] A second limitation of the existing art is that relatively complex arrangements of magnetic field sources external to the patient must be assembled and controlled in order to carry out magnetic stereotactic movement of the implant. A single static background field is virtually always inappropriate for effecting controlled movement of the magnetic element in the implant used in existing magnetic stereotaxis procedures. A third limitation, related to the second, is that a magnetic element left in the brain or another body part can create a significant imaging artifact when that body part is imaged by an MR scanner, most typically rendering that imaging data set useless or of greatly reduced diagnostic and therapeutic value to the clinician and patient.

[0006] A fourth limitation is that appreciably and clinically precious time could be lost when carrying out a sequential and reciprocal process of conducting a magnetic stereotaxis procedure that must be interleaved with intra-operative MR imaging studies for diagnostic, therapeutic or navigational purposes. These limitations are not traversed by Kucharczyk et al. in their U.S. patent application Ser. No. 09/174,189 and in their International Application No. PCT/US99/24253, (the disclosure of both of which are incorporated herein by reference), which teach means for serial and reciprocal movement of the patient from a magnetic stereotaxis system to an MR scanner for purposes of updating the imaging information used for the reference portion of the magnetic stereotaxis procedure.

[0007] A more nearly ideal situation would arise if it were possible to integrate the form and function of a MR scanner and a magnetic stereotaxis system in such a way that magnetic stereotaxis procedures could be carried out within an MR scanner (or vice versa), and all done in such a way that the form and function of the MR scanning process would not interfere with those of the magnetic stereotaxis process, but that the respective forms and functions would instead complement and/or enhance each other. The subject of the present invention is a means and technique that accomplishes this goal and circumvents the existing limitations by incorporating a triaxial arrangement of miniature electromagnets as the magnetic element at the tip of the medical device or catheter. By externally regulating the electrical currents that pass through each of the independent coils, the torque and force on the tip of the medical device or catheter can be made to react to a static magnetic field of a MR scanner in such a way that the tip of the medical device or catheter can be guided along a preferred path to reach a target location within a brain or other body part. The resulting means and technique will thus exhibit all of the advantages of conventional magnetic stereotaxis (primarily the ability to navigate the medical device or catheter along complex curvilinear paths), while incorporating the further advantages of rapid sequential MR imaging of the patient, without introducing imaging artefacts on the MR images, since imaging is performed during periods when no currents flow through the triaxial coil components.

[0008] Medical devices with one or more miniaturized coils on them have been disclosed for a variety of other purposes, but none have been designed for use as the actuator in a combined magnetic stereotaxis and MR imaging process such as the type that is the subject of the present invention. Instead, such coil systems have been limited in function to identifying the location of the probe (in which they are housed) in relation to the body part into which the probe is inserted. Examples of such disclosures include Grayzel, U.S. Pat. No. 4,809,713; Dumoulin et al., U.S. Pat. No. 5,211,165; Twiss et al., U.S. Pat. No. 5,375,596; Acker et al., U.S. Pat. No. 5,558,091; Martinelli, U.S. Pat. No. 5,592,939; Calhoun et al., U.S. Pat. No. 5,606,980; Golden et al. U.S. Pat. No. 5,622,169; Shapiro et al., U.S. Pat. No. 5,645,065; Heruth et al., U.S. Pat. No. 5,713,858; Watkins et al., U.S. Pat. No. 5,715,822; Saad, U.S. Pat. No. 5,727,553; Weber et al., U.S. Pat. No. 5,728,079; Acker, U.S. Pat. No. 5,729,129; Darrow et al., U.S. Pat. No. 5,730,129; Young et al., U.S. Pat. No. 5,735,795; Glantz, U. S. Pat. No. 5,749,835; Acker et al., U. S. Pat. No. 5,752,513; Slettenmark, U.S. Pat. No. 5,758,6670; Polvani, U.S. Pat. No. 5,762,064; Kelly et al., U.S. Pat. No. 5,787,886; Vesely et al., U.S. Pat. No. 5,797,849; Ferre et al. U.S. Pat. No. 5,800,352; Kuhn, U.S. Pat. No. 5,810,728; Young et al., U.S. Pat. No. 5,817,017; Young et al., U.S. Pat. No. 5,819,737; Kovacs, U.S. Pat. No. 5,833,603; Crowley, U.S. Pat. No. 5,840,031; Webster, Jr. et al., U.S. Pat. No. 5,843,076; Johnston et al., U.S. Pat. No. 5,843,153; Lemelson, U.S. Pat. No. 5,845,646, Lemelson, U.S. Pat. No. 5,865,744, Glowinski et al., U.S. Pat. No. 5,868,674; Horzewski et al., U.S. Pat. No. 5,873,865; Haynor et al., U.S. Pat. No. 5,879,297; Daum et al., U.S. Pat. No. 5,895,401; Ponzi, U.S. Pat. No. 5,897,529; Golden et al., U.S. Pat. No. 5,902,238; Vander Salm et al., U.S. Pat. No. 5,906,579; Weber et al., U.S. Pat. No. 5,908,410; Lee et al., U.S. Pat. No. 5,911,737; Bladen et al., U.S. Pat. No. 5,913,820; Snelten et al., U.S. Pat. No. 5,916,162; Lemelson, U.S. Pat. No. 5,919,135; Chen et al., U.S. Pat. No. 5,921,244; Navab, U.S. Pat. No. 5,930,329; Rasche et al., U.S. Pat. No. 5,938,599; Lloyd, U.S. Pat. No. 5,938,602; Ponzi, U.S. Pat. No. 5,938,603; Johnson, U.S. Pat. No. 5,941,858; Cerrnak. U.S. Pat. No. 9,941,889; Johnson et al., U.S. Pat. No. 5,944,023; Derbyshire et al., U.S. Pat. No. 5,947,900; Beisel, U.S. Pat. No. 5,947,940; Van Vaals et al., U.S. Pat. No. 5,951,472; Lev, U.S. Pat. No. 5,951,566; Rogers et al., U.S. Pat. No. 5,951,881; Wan, U.S. Pat. No. 5,952,825; Rosenberg et al., U.S. Pat. No. 5,959,613; Ponzi, U.S. Pat. No. 5,964,757; Ferre et al., U.S. Pat. No. 5,967,980; Wittkampf, U.S. Pat. No. 5,983,126; Taniguchi et al., U.S. Pat. No. 5,997,473; Mouchawar et al., U.S. Pat. No. 6,002,963; Van Der Brug et al., U.S. Pat. No. 6,006,127; Pflueger, U.S. Pat. No. 6,013,038; Vesely et al., U.S. Pat. No. 6,019,725; Webb, U.S. Pat. No. 6,019,726; Murata, U.S. Pat. No. 6,019,737; Wendt et al., U.S. Pat. No. 6,023,636; and Holdaway et al., U.S. Pat. No. 6,083,166. The disclosures of all of the foregoing are incorporated herein by reference. Other uses for miniature coils or microcoils on catheters include the controlled introduction of local electromagnetic fields during the MR imaging process for the purpose of improving imaging contrast in the tissues adjacent to the catheter or probe, as taught for instance by Truwit et al., U.S. Pat. No. 5,964,705, incorporated herein by reference. Miniature triaxial arrangements for field sensing in medical probes have been disclosed by Acker, U.S. Pat. No. 5,833,608, incorporating herein by reference. Additional publications that document related uses for microcoils on catheters for either tracking or imaging purposes include the papers of Wildermuth et al., “MR Imaging-guided intravascular procedures: initial demonstration in a pig model,” Radiology, 578-583 (USA 1997), Bakker et al., “MR-guided endovascular interventions: susceptibility-based catheter and near-real-time imaging technique,” Radiology, pp. 273-276 (USA 1997), Rasche et al., “Catheter tracking using continuous radial MRI,” MRM, pp. 963-968 (USA 1997), Worley, “Use of a real-time three-dimensional magnetic navigation system for radiofrequency ablation of accessory pathways,” PACE, pp. 1636-1645 (USA 1998), Burl et al., “Twisted-pair RF coil suitable for locating the track of a catheter,” MRM, pp. 636-638 (USA 1999) and Coutts et al, “Integrated and interactive position tracking and imaging of interventional tools and internal devices using small fiducial receiver coils,” MRM, pp. 908-913 (USA 1998). The disclosures of which are incorporated by reference. Coils in the catheter tip can be used to both locate the tip, and to measure the orientation of the tip in three dimensional space, as discussed by Shapiro et al, U.S. Pat. No. 5,645,065, and Haynor et al, U.S. Pat. No. 5,879,297, the disclosures of which are incorporated by reference.

SUMMARY OF THE INVENTION

[0009] The invention relates to the interaction between the static magnetic field of an MR scanner and one or more independent magnetic dipole moments created by a plurality of electromagnetic elements that are located within a medical device or catheter within a patient. The concept of utilizing a variable magnetic moment in the tip of a catheter for navigation in a static magnetic field was disclosed by Garibaldi et al., U.S. patent application Ser. No. 09/504,835, which is incorporated herein in its entirety by reference. Garibaldi et al. discusses a variety of permanent and electromagnetic means for generating a variable moment at the catheter tip for navigation in a static field, which may be energized for the purpose of navigation. The present invention employs the static field of an MRI imager, and the combined sequential processes of navigation and MRI imaging. Our discussion focuses on the static field of an MR imager which is always on, and for practical purposes cannot be turned off or otherwise changed or interrupted. For this reason, the present invention cannot employ permanent or inducible magnetic materials within the medical device. Consequently, the variable moment must be generated by coils, and preferably air-core coils.

[0010] If a static magnetic field H is acting along the z direction of the bore of a MR imaging, and a magnetic dipole moment m is present in the magnetic element of an implanted probe or catheter, then the vectors representing H and m in a three-dimensional space can be written H Hk along the z-axis and m=m_xi+m_yj+m_zk and the torque experienced by the dipole moment in the field of the MR scanner is τ=m×H. The x, y, and z components of the moment m are controlled independently, so that the vector m can point in any arbitrary direction in three dimensional space.

[0011] Evaluation of the vector cross product produces the components τ_x=m_yH, τ_y=−m_xH, and τ_z=0. The torque acting on the dipole is perpendicular to m and H and in this case has no component about the z-axis along which the magnetic field lies. It is possible, however, to navigate a catheter to points lying in the plane perpendicular to H via successive small displacements of the moment out of this plane. This process can be referred to as compound rotation about the H axis. The first step in the process is to rotate the catheter tip upward out of the x-y plane at an angle which advances the catheter projection on the x-y plane, followed by a second rotation which rotates the catheter back down to the x-y plane, while once again advancing the catheter orientation angle. The net rotation of the catheter tip about the z-axis thus follows a sawtooth or triangular trajectory, each step in the advancement being made up of two allowed out-of-plane rotations.

[0012] One can make numerical estimates of the sizes of the torques that can act on the dipole in the presence of the MR scanner field by noting that the magnitude of the torque is given by expanding the cross product τ=m×H to obtain τ=mH cos θ where θ is the angle between m and H. τ is a maximum when θ=90°. If the dipole moment is produced by a coil that has N turns of wire windings carrying an electrical current I, and has a cross-sectional area A, then the expression for the torque acting on the coil can be written as τ=NIAB where B=μ_oH defines the relationship between the magnetic induction B and the magnetic field strength H, with μ_o being the permeability of free space. As a practical example of the application of these principles in a clinically realistic setting, the mechanical torque required to rotate a dipole moment produced by 350 turns of wire carrying 1.4 A of current and having a diameter of 2 mm with associated cross-sectional area of 3.14×10⁻⁶ m² would be 2.3×10⁻³ N-m or 230 gram-mm in a MR scanner field of 1.5 T. If this coil is 5 mm long, the effective force couple producing the torque is 230/5=46 grams, which is adequately large for catheter navigation.

[0013] In the preferred embodiment of the present invention, three miniaturized coils of appropriate length, radius and current carrying capacity are assembled into a triaxial configuration in which the cross-sectional planes of each are orthogonal to those of the others. There are a number of different ways of doing this. In one preferred embodiment, the coils are wound on a common hollow coil form or mandrel in the shape of a rectangular parallelepiped that is made from non-conducting, non-susceptible materials which are MR compatible. In one embodiment, the coils are approximately 1 cm in length, have a diameter of 2 mm and a thickness of 0.25 mm. One transverse coil is wound around a long axis of the parallelepiped mandrel, a second transverse coil is wound along the other long axis perpendicular to the first, and the third (axial) coil is wound around the other two. The latter coil has a similar cross sectional area times number of turns as the transverse coils, so that all three coils have approximately equal dipole moment magnitude for equal energizing currents. This assembly is fixed inside the tip of a supple catheter with the leads from the coils brought down the internal length of the catheter tube to an exit point at which they are connected to three independent power supplies, one for each coil.

[0014] Cooling water or some other heat exchange medium can be made to flow through an internal jacket inside the catheter, thus bathing the triaxial coil assembly and carrying away a substantial amount of the heat generated during operation of the independent coils. The amount of cooling power available to the coils limits the level of current flow and ohmic heating that they can sustain; for example 16 W of cooling would establish a limit of 2.3 A maximum per coil assuming that the total resistance of a given coil is approximately 3Ω. The pressure driving the flow would ideally be 100 psi or less, with the flow entering the catheter at body temperature and leaving it at some higher temperature governed by considerations of patient safety and comfort.

[0015] With the coils being open circuit at the beginning of a procedure, the patient is imaged, and the location of the catheter noted. The coils themselves, rather than being passive open circuit coils, could actually serve as pick up coils for the MRI rf signal, providing an enhanced image at the site of the catheter, as discussed in the cited literature. Following this essentially real time imaging, the coils are energized, following the predictions of a coil-current vs. torque algorithm that would determine the catheter's directional advancement within the MR scanner's field, thus permitting the tip of the catheter to be steered along a desired direction. Subsequent imaging sequences are then carried out to verify the new location of the catheter's tip, and the next movement sequence is then planned and executed.

[0016] In some modem MRI machines, the patient can actually be rotated during a procedure relative to a transverse MRI magnetic field residing in a gap between magnets. Such patient rotations may be employed to further enhance navigation. In particular, the patient may be rotated in these machines to ensure that maximum torque can always be applied about directions that would otherwise be parallel with the MRI field.

[0017] Using this means and technique, catheters can be manipulated through a body part, for example the brain, and positioned such that the lumen of the catheter is left along a curvilinear path that might be optimized for contoured drug delivery for the treatment of a neurodegenerative disorder intrinsic to the brain. Many other possible scenarios can also be achieved in the same way, for instance the nonlinear stereotactic guidance of an electrode for recording of potentials, ablation of a zone of tissue or deep brain stimulation for pain or tremor control. Likewise, electrophysiological mapping and ablation procedures can be carried out within the chambers of the heart. We note that the cooling required to adequately energize the coils can also cool the electrode tip of an ablation catheter. Such cooling results in larger ablative lesions, with fewer complications associated with clot formation on the electrode tip. Steering of catheters and devices can also be carried out within the endovascular system, for diagnostic and therapeutic purposes.

[0018] A host computer can coordinate and control the power supplies used to drive the individual coils, interpreting instruction from a clinician operating an intuitive interface such as a joy stick. The control algorithm requires as input the present orientation of the triaxial coil assembly within the patient, the direction and magnitude of the MR scanner field at the location of the triaxial coil assembly, the desired new angular orientation or curvilinear displacement that is to be taken in the next step in the movement sequence, and any related anatomical or physiological information about the patient as might be required to safely and efficaciously carry out the procedure.