WO2002062196A2

WO2002062196A2 - Magnet for aneurysm filling

Info

Publication number: WO2002062196A2
Application number: PCT/US2001/048343
Authority: WO
Inventors: Francis M. Creighton
Original assignee: Stereotaxis, Inc.
Priority date: 2000-12-13
Filing date: 2001-12-12
Publication date: 2002-08-15
Also published as: AU2001297599A1; WO2002062196A3

Abstract

An optimal magnet (20) for guiding a magnetic embolic material into an aneurysm. The magnet (20) comprises a generally C-shaped structure having an interior opening adapted to receive a part of a patient's body containing the aneurysm. The structure is comprised of a plurality of separate segments, the magnetization direction of each segment generally optimizing the cross product of the magnetic gradient and the magnet field at an operating point (30) between the arms of the 'C', in effect providing a focus to that point of the desired projected magnetic properties of the magnet.

Description

MAGNET FOR ANEURYSM FILLING

FIELD OF THE INVENTION

This invention relates to aneurysm filling, and in particular to a magnet for directing magnetic embolic materials into aneurysms. BACKGROUND OF THE INVENTION

An aneurysm is a bulge in the wall of a blood vessel caused by a weakening and thinning of the vessel wall. In the past aneurysms have been treated by attaching a clip to the neck of the aneurysm to isolate it from the blood vessel. Attempts have been made to treat aneurysms by filling them with an embolic material.^' However, an important difficulty with the use of embolic materials has been keeping the embolic material within the aneurysm to prevent downstream occlusion and stroke. More recently, magnetic embolic materials have been developed that can be guided into aneurysms with a magnet.

Various magnets have been developed for applying a magnetic force on the embolic in the appropriate direction to direct magnetic embolic materials through the neck of an aneurysm and into the body. As a general rule it is desirable to use a magnetic force that is perpendicular to the local magnetic field. When the field direction is parallel to magnetic gradient, the magnetic embolic material forms needle-like structures that can protrude from the neck of the aneurysm. When the magnetic field is perpendicular to the magnetic gradient, the magnetic embolic material builds up in the aneurysm in smooth layers, closing the neck of the aneurysm without protruding into the blood vessel.

An embolic material must be carefully delivered to the patient, using fluoroscopic imaging with multiple viewing angles. Solid state imaging plates, such as amorphous silicon imaging plates, give undistorted views of the aneurysm even in the presence of large magnetic field and gradients near the magnet used to deliver the embolic materials. A magnet used to deliver magnetic embolic materials must be designed to integrate with imaging plates or other imaging modality, providing the appropriate force and field directions at the aneurysm while simultaneously providing multiple imaging views during the fill. SUMMARY OF THE INVENTION

The present invention relates to a compact, relatively light weight magnet adapted to apply a magnetic gradient with a generally perpendicular magnetic field to aneurysms in the head of a patient. Generally, the magnet of the present invention is relatively thin and C-shaped. The magnet is sized to allow it to be readily manipulated around the patient's head,. so that it can apply a magnetic gradient with a perpendicular magnetic field, perpendicular to the wall of the blood vessel in which the aneurysm is located. The magnet is adapted to apply a field at an operating point. The magnet is sized and shaped so that the magnet can be moved to position the magnet's operating point anywhere in an operating volume in the patient's head. The magnet is comprised of a number of segments. The magnetization direction of each segment is selected to provide the optimal magnetic gradient in a selected direction with a perpendicular magnetic field, based on selected design criterion. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is a perspective view of a magnet constructed according to the principles of this invention;

Fig. IB is a side elevation view of the magnet;

Fig. 1C is a back end elevation view of the magnet;

Fig. ID is a top plan view thereof;

Fig. 2A is a perspective view showing the magnet as it would be mounted on a support for use in manipulating the magnet of the present invention;

Fig. 2B is a side elevation view of the magnet and support;

Fig. 2C is a rear elevation view of the magnet and support;

Fig. 2D is a top plan view of the magnet and support;

Fig. 3 A is a front elevation view of a patient's head, showing the position of the operating region in which the magnet of the present invention operates;

Fig. 3B is a side elevation view of a patient's head, showing the position of the operating region;

Fig. 4A is a side elevation view of a patient's head, showing the excluded angles at which mechanical interference prevents the magnet from applying gradient in the correct direction;

Fig. 4B is a front elevation view of a patient's head, showing the excluded angles at which the magnet cannot apply a correct gradient;

Fig. 5A is a plan view of one half of a 3 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength;

Fig. 5B is a plan view of one half of a 6 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength;

Fig. 5C is a plan view of one half of an 8 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength;

Fig. 5D is a plan view of one half of a 10 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength;

Fig. 6 is a plot of a two-dimensional curve of constant contribution of local moments to the cross product of magnetic gradient and magnetic field at the origin, useful in determining the shape of magnets in accordance with this invention;

Fig. 7 is a plot of a three-dimensional curve of constant contribution to the cross product of magnetic gradient and magnetic field at the origin, useful in determining the shape of magnets in accordance with this invention.

Fig. 8 is a top plan view of a magnet constructed according to the principles of this, showing lines of constant field strength; Fig. 9 is a top plan view of a magnet constructed according to the principles of this invention, showing lines of constant field strength, and arrows indicating field direction;

Fig. 10 is a top plan view of a magnet constructed according to the principles of this invention, showing lines of constant field strength;

Fig. 11 is an end elevation view of a magnet constructed according to the principles of this invention; showing lines of constant field strength;

Fig. 12 is a graph of magnetic field strength versus position along the axis of symmetry through the operating point;

Fig. 13 is a graph of magnetic gradient versus position along the axis of symmetry through the operating point;

Fig. 14 is a graph of the product of magnetic field strength and magnetic gradient along the axis of symmetry through the operating point;

Fig. 15 is a top plan view of one half of a magnet constructed according to the principles of this invention, showing the magnetic field direction the magnetic gradient direction;

Fig. 16A is a side elevation view of a magnetic medical system incorporating a magnet and support constructed according to the principles of this invention;

Fig. 16B is an end elevation view of the magnetic medical system;

Fig. 16C is a top plan view of the magnetic medical system;

Fig. 16D is an enlarged perspective view of the magnetic medical system, showing the relative positioning of the magnet and the imaging; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A magnet constructed according to the principles of the present invention is indicated generally as 20 in Figs. 1A through ID. The magnet 20 is generally C-shaped, with a curved outer surface 22, a curved inner surface 24, and a gap 26 that communicates with a central opening 28 for receiving the head of the patient. As shown in the Figures, in this preferred embodiment the surfaces 22 and 24 are not parallel, and thus the width of the magnet 20 varies. The magnet has an operating point 30 between the legs of the "C" about which the design of the magnet 20 is preferably optimized. The magnet 20 preferably is symmetric about a line bisecting the "C" through the opening 26 and the operating point 28.

The inside surface 24 of the magnet is generally cylindrical, for ease of design and manufacture, and in the preferred embodiment has a diameter of approximately 14 inches. This is sufficiently large to allow the magnet to surround the patient's head, and to move sufficiently relative to the patient so that the operating point 30 can be positioned anywhere in an operating volume in the patient's head. This operating volume is illustrated in Figs. 3A and 3B, in which the preferred operating volume OV in a patient P, is shown as being approximately 4.2 inches wide, 1.5. inches high, and 3.5 inches deep. In a typical adult human being, this operating volume is located 4.0 inches from the top of the head, 1.3 inches from either side, and 3.0 inches from the front of the head and 3.1 inches from the back of the head. This OV includes the vast majority of head aneurysms that would be treated. The front and back faces of the magnet 20 are preferably flat, so that the magnet has a uniform thickness. In the preferred embodiment the magnet 20 has been designed to be 3 inches thick, which represents a compromise between providing a magnet that can be easily manipulated around the patient's head and does not interfere with imaging (e.g., bi-planar fluoroscopy) of the operating region, yet provides sufficient magnetic field and force to guide the magnetic embolic material. If the magnet 20 were designed to be thicker in a direction perpendicular to the plane of the "C", the magnet could be made narrower in the other dimension while still providing the same magnetic gradient and field at the operating point. In fact, the optimum weight (lightest) magnet may have a different aspect ratio, however for considerations of access to the patient and ease of imaging, the magnet of the preferred embodiment is heavier but thinner than the optimal from a weight standpoint. The only direction in which the magnet 20 cannot pull is vertically downwardly along the axis of the patient's body. This is illustrated in Figs. 4A and 4B which show that angles of 30° or less from vertically downwardly, cannot be achieved because the patients' body interferes with the positioning of the magnet 20.

Fig. 5 A is a plan view of one half of a 3 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength; Fig. 5B is a plan view of one half of a 6 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength; Fig. 5C is a plan view of one half of an 8 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength; and Fig. 5D is a plan view of one half of a 10 inch thick magnet constructed according to the principles of this invention (the other half being a mirror image thereof), with arrows indicating local field direction, and lines of constant field strength. As illustrated in Figs. 5A through 5D, as the thickness of the magnet in a direction perpendicular to the plane of the "C" increases, the width of the magnet can decrease. If optimally designed for a given cross product of the magnetic gradient and the magnetic field at operating point, the three inch thick magnet (Fig. 5A) would weigh about 74 pounds, the six inch thick magnet (Fig. 5B) would weigh about 49 pounds, the eight inch thick magnet (Fig. 5G) would weigh about 48 pounds, and the ten inch thick magnet (5D) would weigh about 51 pounds.

The outside side 26 of the magnet 20 is shaped in accordance with a surface of equal magnetic field-gradient product contribution. This surface can be determined in two dimensions, and a two dimensional curve is shown in Fig. 6. By "two-dimensions" it is meant that the calculations are made assuming that there is no variation in the magnetization direction within the magnetic material in the axis perpendicular to the plane of the "C" Fig. 6 is determined by calculating a line of constant contribution to maximizing the cross product of the magnetic gradient and the magnetic field at the operating point (the origin shown in Fig. 6). This curve preferably defines the outer surface 22, and as described above the inner surface 26 is a cylindrical surface of appropriate size. The shape of the outer surface 22 preferably conforms to a surface of equal contribution, because material outside such a surface contributes less to the maximized property (magnetic field gradient than material inside such a surface, and thus the most efficient design is one in which the material generally conforms to a surface of constant contribution. However, for ease of manufacturability, the actual magnet may simply conform closely, but not exactly to a surface of constant contribution, substituting flat faces or other easy to manufacture shapes for the exact shapes determined by curves of constant contribution. The shape of the magnet is shown superposed over the two- dimensional curve of Fig. 6. Other magnet designs exploiting the use of constant contribution curves are disclosed in co-pending, co-owned, U.S. Patent Application No. 09/546,840, filed April 11, 2000, U.S. Patent Application No. 09/497,467, filed February 3, 2000, the disclosures of which are incorporated herein by reference.

Alternatively, the shape of the outer surface can be deteπ ined in three dimensions, by determining the three dimensional surface of constant contribution. This method maximizes the desired magnetic property (the magnetic gradient magnetic field cross product in this preferred embodiment), taking into account that the local magnetization direction can vary in three-dimensions rather than just two as described above. One quadrant of the three dimensional surface of equal contribution to maximize the cross product of the magnetic gradient and the magnetic field is shown in Fig. 7. While the outside surface 22 of the magnet 20 would ideally be determined in three dimensions, conforming to a curve such as shown in Fig. 7, because of practical limitations in manufacturing, the cost of manufacturing the smooth, continuously curved surface resulting from three dimensional optimization may not be cost justified. The optimal shape of one half of the magnet according to three-dimensional constant contribution surface (the other half being a mirror image at the x-z plane) is shown in Fig. 7.

In accordance with the principles of this invention, the magnet 20 is comprised of a plurality of segments. In this preferred embodiment there are eleven segments, but there could be more or fewer segments, if desired. Eleven segments represents a preferred balance between an infinite number of elements each with the field direction to optimize the cross product of magnet gradient and magnetic field strength, and mmimizing the number of elements for ease of manufacturability. The size and shape of each segment can be determined in a variety of ways, for example, each segment may represent an equal angular arc, or the segments may be defined by the relatedness of the directions of magnetization. At present there are limitations on the size and shape of commercially available magnet material, and thus the size and shapes of the segments may be limited by the size and shape of the magnet material available. In order to optimize the magnetic properties of the magnet 20, the magnetization direction would vary with position within each segment. However, to facilitate manufacturability, the magnetization direction within each segment can be uniform. This uniform direction can be selected in a number of ways: It can be the calculated optimum direction at the center of mass of the segment. It can be a weighted average of the optimum direction at each point in the segment. The size and shapes of the segments, and the magnetization directions can be iteratively selected so that the resulting cross product of the magnetic gradient and the magnetic field from the selected segment sizes and shapes and magnetization directions is within some predetermined percentage of the calculated theoretical value if the magnetization direction varied on a microscopic level. For example, it may oe determined that a design that achieves 90% of the theoretical cross product of magnetic gradient and magnetic field is acceptable. Of course, some other design criteria could be used.

The magnet 20 is capable of providing a strong magnetic gradient perpendicular to the magnetic field at an operating point 30. In the preferred embodiment, the magnet is sized so that it can be moved sufficient relative to the patient to position the operating point anywhere in the operating zone of the patient. The magnet 20 can be manipulated relative to the patient to provide a pulling magnetic gradient in nearly any direction except angles within about 30° of the vertical axis of the patient. However, this is sufficient for the magnet to be useful in filling the majority of aneurysms in the head.

As shown and described herein the magnet has a field strength at the operating point (0, 0, 0) of 0.06T, and a gradient at the operating point (0, 0, 0) of 0.7 T/m. The field gradient product at the operating point (0, 0, 0,) is 0.04T²/m, and the field gradient ratio is 0.086m. The magnetic field strength increases along the axis of symmetry from the operating point to about 6.5 inches, and falls of slightly from 6.5 to the inner surface 24 of the magnet 20. This is shown in Fig. 12. The magnetic gradient also increases along the axis of symmetry from the operating point to about 5.5 inches, and falls off sharply at 6.25 inches. This is shown in Fig. 13. The product of the magnetic field and the magnetic gradient generally increases along the axis of symmetry, from the operating point to about 4.5 inches, and the product falls off sharply at about 6.5 inches.

The magnet provides a sufficiently strong magnetic force to guide magnetic embolic material into an aneurysm through its neck. The magnet can provide this guiding force in most of the directions likely to be required to fill most aneurysms. The preferred direction is perpendicular to the wall of the blood vessel in which the aneurysm is located, in a direction toward the back of aneurysm. At the operating point the magnetic field is perpendicular to the gradient, so that the magnetic material forms smooth layers, filling the aneurysm up to the neck without creating protrusions in the blood vessel.

The magnet is sufficiently light weight that it can be manipulated manually, or mounted so that it can be automated to allow the physician to quickly and easily orient the magnet in the appropriate direction to complete the aneurysm filling. It is to be recognized that the preferred sizes and shape of the magnet described are based upon optimization suing presently available magnetic materials, and that considerable changes could occur with improvements in these materials over time.

A manually operated support for the magnet 20 is indicated generally as 100 in Figs. 2A - 2D. The support comprises a base 102 with a generally vertical, telescoping stanchion 104. A generally horizontal rod 106 is rotatably mounted in the upper end of the telescoping stanchion 104. A U-shaped bracket 108 is mounted on one end of the rod 106, and a handle 110 is mounted on the other end of the rod. A mounting block 112 is pivotally mounted between the legs of the U-shaped bracket 108. The magnet 20 is rotatably mounted to the mounting block 112, and has a handle 114 for turning the magnet. The manually operated support allows the magnet 20 to be readily positioned about the head of a patient, to apply the desired magnetic force to draw and hold magnetic embolic materials into an aneurysm, yet the magnet can be adjusted in position to minimize interference with imaging equipment.

An aneurysm filling system incorporating a magnet constructed in accordance with the principles of this invention is indicated generally as 200 in Figs 16A through 16D. The system 200 includes a magnet 20 mounted on a support 100. The system 200 further comprises a patient support, such as bed 202, a C- arm mechanism 204, mounting bi-planar imaging equipment including imaging beams generators, such as x-ray generators 206 and imaging devices, such as solid state amorphous silicon imaging plates 208. The slim profile and open design of the magnet 20 allows the magnet to be readily positioned to apply the desired magnetic field and gradient to fill an aneurysm in the head of a patient on the bed 202, yet does not interfere with imaging of the entire procedure through the bi-planar imaging system mounted on the C-arm. The great flexibility of the positioning of the magnet, and of the bi-planar imaging on the C-arm which allows rotation of the imaging equipment about three axes, allows the physician to watch the procedure.

Claims

What is claimed is:

1. A magnet for guiding a magnetic embolic material into an aneurysm, the magnet comprising a generally C-shaped structure having an interior opening adapted to receive a part of a patient's body containing the aneurysm, the structure comprised of a plurality of separate segments, the magnetization direction of each segment generally optimizing the cross product of the magnetic gradient and the magnet field at an operating point between the arms of the "C".

2. The magnet according to claim 1 wherein the interior opening of the "C" is large enough to allow the magnet and the patient to be moved so that the operating point can be positioned anywhere inside an operative volume 4.2 inches wide, 1.5. inches high, and 3.5 inches deep in the portion of the patient in the interior opening.

3. The magnet according to claim 1 wherein the exterior surface of the generally C-shaped structure is shaped in accordance with a curve of equal contribution to the cross product of the magnetic gradient and the magnetic field at the operating point.

4. A method of treating an aneurysm in the blood vessel of a patient, the method comprising; providing a generally C-shaped magnet comprising a plurality of segments whose magnetization directions are selected to generally maximize the cross product of magnetic gradient and the magnetic field at an operating point between the legs of the "C"; placing the portion of the patient's body containing the aneurysm inside a generally C-shaped magnet so that the operating point is substantially at the location of the aneurysm; orienting the magnet so that the gradient of the magnet pulls a magnetic embolic material through the neck of the aneurysm to fill the aneurysm.