U.S. patent application number 11/369903 was filed with the patent office on 2007-02-15 for method and device for locating magnetic implant source field.
Invention is credited to Francis M. IV Creighton, Bevil J. Hogg, Rogers C. Ritter, Peter R. Werp.
Application Number | 20070038074 11/369903 |
Document ID | / |
Family ID | 21801431 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070038074 |
Kind Code |
A1 |
Ritter; Rogers C. ; et
al. |
February 15, 2007 |
Method and device for locating magnetic implant source field
Abstract
An apparatus and method for locating a magnetic implant in a
surgical application using the field of a source magnet for the
implant guiding field. The source magnet is an electromagnet having
a separate calibrated magnetic field component in addition to the
guiding field, so that both the magnitude and orientation of the
magnetic field as a function of position around the magnet are
known. A magnetic implant is provided with a sensor, such as a
three-axis Hall effect sensor, to provide an indication of the
magnitude and orientation of an applied magnetic field when the
implant is surgically implanted in a patient. After implantation,
the source magnet is energized with a current having a modulated
component. The modulated component is received and filtered from
the signal received from the Hall effect sensor in the implant, and
provided to a processor that computes the location of the implant
relative to the electromagnet based upon the detected magnitude and
orientation of the modulated component of the magnetic field, and
the location and orientation of the electromagnet. Localizers may
be used to supply the relative locations of the patient and the
electromagnet to the processor. A display may be provided to
display a representation of the location of the implant in the
patient, which may also be superimposed over a preoperative image
of the patient.
Inventors: |
Ritter; Rogers C.; (St.
Louis, MO) ; Hogg; Bevil J.; (St. Louis, MO) ;
Werp; Peter R.; (St. Louis, MO) ; Creighton; Francis
M. IV; (St. Louis, MO) |
Correspondence
Address: |
Bryan K. Wheelock
Suite 400
7700 Bonhomme
St. Louis
MO
63105
US
|
Family ID: |
21801431 |
Appl. No.: |
11/369903 |
Filed: |
March 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10337237 |
Jan 6, 2003 |
7010338 |
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11369903 |
Mar 7, 2006 |
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09020942 |
Feb 9, 1998 |
6505062 |
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10337237 |
Jan 6, 2003 |
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Current U.S.
Class: |
600/411 |
Current CPC
Class: |
A61B 5/066 20130101;
A61B 5/06 20130101; A61B 5/743 20130101 |
Class at
Publication: |
600/411 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1. A magnetic surgical implant comprising a flexible probe having a
magnetic seed mounted at a distal end thereof and a magnetic field
sensor mounted near the magnetic seed and in a fixed relationship
thereto so that as an external magnetic field acts on said magnetic
seed to guide or propel it through a patient's body, the magnetic
field sensor is also acted upon and provides an output which can be
correlated with the position of the magnetic seed.
2. The magnetic surgical implant of claim 1 wherein the magnetic
field sensor comprises a 3-axis magnetometer probe.
3. The magnetic surgical implant of claim 2 wherein the magnetic
field sensor is mounted within the flexible probe.
4. The magnetic surgical implant of claim 3 further comprising at
least one sensor conductor connected to the magnetic field sensor
and extending through the flexible probe for carrying signals from
the magnetic field sensor to a receiver.
5. The magnetic surgical implant of claim 4 wherein the flexible
probe is an endoscope.
6. The magnetic surgical implant of claim 4 wherein the flexible
probe is a catheter.
7. The magnetic surgical implant of claim 4 wherein the magnetic
seed is also mounted within the flexible probe and at the tip
thereof.
8. The magnetic surgical implant of claim 4 wherein the sensor
conductor comprises at least one electrical conductor.
9. A device for providing position data for a magnetic surgical
implant having a magnetic field sensor which is guided or propelled
through a patient's body by an external magnet, said device
comprising a current source for supplying a modulated electrical
current to the external magnet to thereby generate an oscillating
magnetic field component, a demodulator coupled to the magnetic
field sensor and responsive to a magnetic field direction and
magnitude of the oscillating magnetic field component to provide a
signal indicative of the oscillating magnetic field component at
the location of the magnetic field sensor, a memory containing a
representation of a relationship of spatial locations to a magnetic
field pattern produced by the external magnet, and a data processor
coupled to the demodulator and the memory and configured to compute
a spatial location of the magnetic field sensor as a function of
the signal indicative of the oscillating magnetic field
component.
10. The device of claim 9, wherein the representation of the
relationship of spatial locations to the magnetic field pattern
produced by the electromagnet is a representation in a reference
frame, the electromagnet is moveable, and the device further
comprises a localizer configured to provide a signal representative
of a position and orientation of the electromagnet to the
processor, and the processor is configured to apply a correction
based upon the position and orientation provided by the
locator.
11. The device of claim 10 wherein the localizer comprises a sensor
mounted on the electromagnet and a first plurality of fiducial
markers positioned on the patient and a second plurality of
fiducial markers positioned on the electromagnet.
12. The device of claim 9 and further comprising a display coupled
to the processor, the processor further being configured to display
on said display an image representing the spatial location of the
magnetic field sensor and a preoperative image of a patient in
which the magnetic field sensor is implanted, the location image
and the preoperative image being superimposed on one another.
13. The device of claim 9, in which the electromagnet is configured
to generate a hemispherically symmetrical field.
14. The device of claim 9, in which the electromagnet is configured
to generate a hemispherically asymmetric field.
15. The device of claim 9 in which the electromagnet comprises a
plurality of separate coils.
16. A method for locating a magnetic surgical implant, comprising
the steps of: (a) surgically implanting a magnetic implant
including an associated magnetic field probe in a patient; (b)
applying a modulated magnetic field from an external electromagnet;
(c) detecting signals from the magnetic probe resulting from the
application of the modulated magnetic field; and (d) computing the
relative location of the magnetic probe, and therefore, the
magnetic implant, with respect to the electromagnet, from the
detected signals from the magnetic probe.
17. The method of claim 16, and further comprising the steps of
locating the patient and electromagnet with a localizer and
displaying an indication of the location of the magnetic probe
relative to the patient on a display.
18. The method of claim 17, and further comprising the step of
superimposing a preoperative image of the patient on the display
with the indication of the location of the magnetic probe.
19. The method of claim 18 wherein the step of applying a modulated
magnetic field from an external electromagnet comprises the steps
of selecting a current required by the external electromagnet to
guide the magnetic implant and modulating the selected current with
a modulating signal; and further comprising the step of guiding the
magnetic probe with the modulated magnetic field.
20. The method of claim 16 wherein the step of applying a modulated
magnetic field from an external electromagnet comprises the steps
of selecting a current required by the external electromagnet to
guide the magnetic implant 5 and modulating the selected current
with a modulating signal; and further comprising the step of
guiding the magnetic probe with the modulated magnetic field.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] This invention relates to methods for locating a magnetic
implant, and more specifically, to a method for locating a magnetic
object being guided, in a surgical application, by the field of a
source magnet.
[0003] (2) Description of Related Art
[0004] In the filed of surgery, there exists a need to control the
orientation, forces, and/or motion of internally implanted devices.
One method that has been used to control such implanted devices is
the application of a magnetic field from an external magnet. In
this method, the magnetic field acts upon the implanted device,
which itself comprises magnetic material, which may be in the form
of a permanent magnet. In accordance with prior art practice, a
physician surgically implants the device comprising magnetic
material and then guides the position of the implanted device by
moving an external permanent magnet and observing the resultant
movement directly with an X-ray fluoroscope. Examples of the prior
art may be found in a review article by Gillies et al., "Magnetic
Manipulation Instrumentation for Medical Physics Research," Rev.
Sci. Instrum. 65, 533 (1994). See also McNeil et al., "Functional
Design Features and Initial Performance Characteristics of a
Magnetic-Implant Guidance System for Stereotactic Neurosurgery,"
IEEE Trans. Biomed. Engrg., 42, 793 (1995); Tillander, "Magnetic
Guidance of a Catheter with Articulated Steel Tip," Acta Radiologa
35. 62 (1951);
[0005] Frei et al, "The POD (Para-Operational Device) and its
Applications," Med. Res. Eng. 5, 11 (1966); U.S. Pat. No. 3,358,676
to Frei et al., issued Dec. 19, 1967, entitled "Magnetic Propulsion
of Diagnostic or Therapeutic Elements Through the Body Ducts of
Animal or Human Patients"; Hilal et al., "Magnetically Guided
Devices for Vascular Exploration and Treatment," Radiology 113, 529
(1974); Yodh, et al., "A New Magnet System for Intravascular
Navigation," Med. & Biol. Engrg., 6, 143 (1968); Montgomery et
al., "Superconducting Magnet System for Intravascular Navigation,"
Jour. Appl. Phys. 40, 2129 (1969); U.S. Pat. No. 3,674,014 to
Tillander, issued Jul. 4, 1972, entitled "Magnetically Guidable
Catheter-Tip and Method"; and U.S. Pat. No. 3,794,041 to Frei et
al., issued Feb. 26, 1974, entitled "Gastrointestinal Catheter."
The full content of each of the cited documents are herein
incorporated by reference in their entirety.
[0006] Obviously, the above-described technique requires the
physician to react to the movement of the implanted device.
Determination of this movement can be a problem, because the
implanted device can, in general, move in three-dimensional space
inside the patient. With prior art hand-held magnets, the only
feedback the surgeon could have was his observation of motion of a
magnetic implant by x-ray or ultrasonic imaging in response to his
movement of the magnet. Usually, fluoroscopic imaging is employed.
However, fluoroscopic imaging can be subject to interference from
the magnet, itself. In difficult interference situations, it is
difficult without proper imaging guidance to provide even a
reasonable guess as to a correct direction for the magnet axis to
obtain field alignment with the intended path. The large
electromagnet of Yodh et al. (supra) is one attempt to minimize the
"blindness" of the approach just described, but the Yodh et al.
approach still relies on operator judgment and vision, and is
subject to such error. While multiple coil arrangements such as the
magnetic stereotaxis system (MSS) described in McNeil et al.
(supra) can be used to provide such guidance, it is difficult in
such systems to provide a combined guiding force and force-applying
field gradient in the same desired direction.
[0007] U.S. Pat. No. 5,558,091 issued Sep. 24, 1996 to Acker et
al., which is hereby incorporated by reference in its entirety,
discloses a magnetic position and orientation determining system
using magnetic fields. By monitoring field components detected at a
probe during application of the fields, the position and
orientation of the probe in the field can be determined. A
representation of the probe can be superimposed on a separately
acquired image of the subject to show the position and orientation
of the probe with respect to the subject. Although the devices and
methods disclosed in this patent can determine the location of an
implant, the magnetic fields used are so small as to not exert any
significant, perceptible forces on magnetic materials in the
sensing region. There is no disclosure or suggestion to use a
magnetic field to both align and/or guide an implanted probe as
well as determine its location via an externally applied magnetic
field, nor is there any suggestion to place strongly magnetizable
materials or permanent magnets in a seed or on a probe, adjacent to
a magnetic sensor, in such a manner as to allow accurate
determination of location using the magnetic sensor in the
immediate vicinity of the seed.
[0008] Clearly, both operation time and risk to a patient could be
reduced if an apparatus and method were available to more
accurately and reliably locate, as well as guide, orient, and/or
move a magnetic surgical implant. (For present purposes, when
reference is made to "guiding" an implant, it should be assumed
that this may also refer to "orienting" an implant, as well.)
Preferably, while such apparatuses and methods may allow the use of
x-ray, ultrasonic, or fluoroscopic imaging devices, they should not
require such imaging devices to provide the location of the
implant. In addition, it would be advantageous if the location
method would not require the addition of magnetic field creating
devices, such as is required by the Acker et al. patent, and which
might further increase the interference with the location and
operation of a guiding magnet. If the location can be obtained
without the use of such additional imaging and/or locating devices,
the external magnet or electromagnet (or magnets or electromagnets)
used for guiding the magnetic implant may be provided with a
larger, unobstructed range of motion. It would also be advantageous
if the location can be obtained without being subject to
interference from the magnet itself, as occurs or can occur with
many common fluoroscopic imaging systems.
SUMMARY OF THE INVENTION
[0009] There is thus provided, in accordance with a first aspect of
the invention, a magnetic surgical implant comprising a flexible
probe having a magnetic seed mounted at a distal end thereof and a
magnetic field sensor mounted near the magnetic seed and in a fixed
relationship thereto so that as an external magnetic field acts on
the magnetic seed to guide or propel it through a patient's body,
the magnetic field sensor is also acted upon and provides an output
that can be correlated with the position of the magnetic seed.
[0010] This embodiment of the invention may include a '-axis
magnetometer probe as the magnetic field sensor, which may be
mounted within the flexible probe. Signals from the sensor may be
conducted from the magnetic field sensor through the flexible probe
by at least one sensor conductor. The probe itself may be a
catheter or an endoscope.
[0011] There is also provided, in accordance with another aspect of
the invention, a device for providing position data for a magnetic
surgical implant having a magnetic field sensor which is guided or
propelled through a patient's body by an external magnet, said
device comprising a current source for supplying a modulated
electrical current to the external magnet to thereby generate an
oscillating magnetic field component, a demodulator coupled to the
magnetic field sensor and responsive to a magnetic field direction
and magnitude of the oscillating magnetic field component to
provide a signal indicative of the oscillating magnetic field
component at the location of the magnetic field sensor, a memory
containing a representation of a relationship of spatial locations
to a magnetic field pattern produced by the external magnet, and a
data processor coupled to the demodulator and the memory and
configured to compute a spatial location of the magnetic field
sensor as a function of the signal indicative of the oscillating
magnetic field component.
[0012] In accordance with yet another aspect of the invention, a
method for locating a magnetic implant is provided, comprising the
steps of (a) surgically implanting a magnetic implant including an
associated magnetic probe in a patient; (b) applying a modulated
magnetic field from an external electromagnet; (c) detecting
signals from the magnetic probe resulting from the modulated
magnetic field; and (d) calculating the relative location and
orientation of the magnetic probe, and therefore, the magnetic
implant, with respect to the electromagnet from the detected
signals from the magnetic probe.
[0013] Additional steps to the above-described method may be added,
in accordance with the invention. Such steps may include locating
the patient and the electromagnet so that an indication of the
magnetic probe, and therefore, the magnetic implant, is displayed.
An additional step of superimposing this indication over a
preoperative image of the patient may also be included.
[0014] It is thus an object of this invention to provide a method
and apparatus for locating a magnetic object which is being guided
by the field of a source magnet, using the field of the source
magnet itself to locate the object.
[0015] It is a further object of this invention to provide a method
and apparatus for accurately and reliably locating a magnetic
surgical implant without requiring x-ray, ultrasonic, or
fluoroscopic imaging devices.
[0016] It is another object of this invention to provide a method
and apparatus for accurately and reliably locating a magnetic
surgical implant by using a magnet that simultaneously serves the
functions of guiding a magnetic implant and locating it.
[0017] It is yet a further object of the invention to provide a
method and apparatus for locating a magnetic surgical implant that
permits a wide range of unobstructed motion to be provided to an
external magnet or electromagnet for guiding the magnetic surgical
implant.
[0018] It is a still further object of the invention to provide a
method and apparatus for locating a magnetic surgical implant that
is not subject to interference caused by the presence of the
external magnet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of the magnetic probe of the
present invention; and
[0020] FIG. 2 is a perspective view of the field generating coil,
along with equipment needed to use it for the location of the
implant.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 is a diagram illustrating an embodiment 100 of an
inventive device in accordance with a first aspect of the
invention. This device comprises a flexible probe 102, which may be
an endoscope or a catheter tube. A magnetic seed 104 of permanent
magnetic, or at least a permeable material, is located near an end
106 of the flexible probe. Seed 104 preferably comprises a
samarium-cobalt (SmCo) permanent magnet, or even more preferably a
neodymium-boron-iron (NdBFe) permanent magnet. Typically, seed 104
may be about 0.7 mm in diameter and length, but may range up to
about 4-5 mm in diameter and up to 7-10 mm long, depending upon the
surgical application of probe 102. It is not intended to exclude
seeds of other sizes, whether larger or smaller, from the scope of
the invention. Preferably, seeds 104 of the sizes given as examples
would have magnetic fields in the immediate vicinity of seed 104 of
no more than 0.4 T. It is contemplated that the magnetic seed 104
is fixedly held in place in a suitable configuration inside or
outside the probe, so that when the end 106 of probe 102 is
implanted in a patient, an externally-applied magnetic field can be
used to guide, direct and/or pull end 106 of probe 102 through a
desired path in the patient's body by means of magnetic forces
applied to seed 104, so that medicaments or therapy can be
delivered to a selected location in the patient's body.
[0022] A magnetic field sensor 108, such as a 3-axis Hall effect
probe, is also attached near the end 106 of probe 102, so that it
is fixed in physical relationship to, and closely proximate
magnetic seed 104 and the end of probe 102. Typically, the magnetic
field from seed 104 at the location of sensor 108 would be about
half of that indicated above in the immediate vicinity of the seed.
Sensor or signal wires 110 from sensor 108 are led out an end 112
of probe 102 opposite end 106. Sensor wires 110 provide a signal or
signals indicative of the magnitude and orientation of a magnetic
field at or near the seed 104, and/or the end 106 of probe 102. If
probe 102 is an endoscope, an optical fiber (not shown) may be
provided to pass through the probe to provide a view of a region
ahead of the endoscope. Other passages, not shown, may be provided
for other surgical purposes.
[0023] Referring now to FIG. 2, probe 102 is shown implanted in a
patient 200 resting on a table 202. A calibrated electromagnet 204
generating a magnetic field having a known magnitude as a function
of current, and having a known orientation as a function of
location relative to electromagnet 204 is provided. Electromagnet
204 is used for locating implant 100, or more precisely, the field
sensor 108 which is part of implant 100. Electromagnet 204 may also
be used to generate forces or field lines that may be used to move
or to guide the movement of implant 100 through patient 200.
Electromagnet 204 may be attached to a moveable arm 206 for this
purpose. Typically, electromagnet 204 is capable of generating a
field of between 0.1-0.3 T at the location of seed 104, or where it
is intended that seed 104 is to be moved.
[0024] In a preferred configuration, a low frequency generator 208
provides a signal to a computer 210, which adds or mixes this
signal with its own computation of the current required by
electromagnet 204 to generate a magnetic field for purposes of
guiding implant 100. This combined signal is sent to an
amplifier/power supply 212, which provides current to coil 204.
Coil 204 generates magnetic field lines such as 214, 216, 218, 220,
and 222 shown in FIG. 2. Sensor 108 of implant 100 lies on one of
these field lines, shown as 222 in FIG. 2, and sensor 108 senses
the strength and direction of the field at the location of implant
100. Wires 110 from the probe travel through the probe 102 (which
is usually an endoscope or catheter), to instrument module 224,
which converts the signals to a magnetic field direction and
magnitude as sensed by the probe. This information is sent to a
computer 226, which may be, but need not be the same as computer
212. Computer 226 has also received information from mounting arm
228, which enables it to know the location and orientation of
magnet 204. As long as no rotation of the magnet about its axis has
occurred, there is no ambiguity of the field lines in question. For
the front hemisphere of magnet 204, the field lines are known to
computer 226 (for example, by having a representation thereof in a
reference frame of the magnet 204 stored in a memory 230), so that
the computer 226 can match the data received from instrument module
224 with data from the known field lines in memory 230, to
determine the location and orientation of probe 106 relative to
magnet 204. This information may be provided to a display 232, or
it may be provided to computer 210 (or used by computer 210, if
computers 226 and 210 are one and the same computer) to provide a
signal to the magnet power supply 212 in a manner according to
"Method and Apparatus Using Shaped Field of Repositionable Magnet
to Guide Implant," U.S. Provisional Patent App. No. 60/065,107 to
W. M. Blume et al., filed Nov. 12, 1997, and a non-provisional
application by the same title as the provisional application, also
to W. M. Blume et al., and filed on even date with the present
application (both the provisional and non-provisional applications
referred to being herein incorporated by reference in their
entirety), or otherwise as may be known in the art.
[0025] The use of a modulated signal to generate the magnetic field
of magnet 204 allows various signal processing methods known to
those skilled in the art to achieve better accuracy in determining
the location of probe 102. With such modulation, for example, it
becomes less necessary, or unnecessary, to account for a varying DC
current to coil 204, which would otherwise have t taken into
account by computer 226 in correlating the measured magnitude of
the field with the stored representation of the relationship of
spacial locations to the magnetic field pattern produced by
electromagnet 204 that is stored in memory 230. Of course, the
varying DC current could be taken into account by communicating
this information from computer 210 to computer 226, or if computers
210 and 226 are one and the same computer.
[0026] It is thus seen that, in this invention, a low frequency
oscillating signal is added--electronically summed--into the signal
from a computer that sets a current from the amplifier--power
supplies to one or more coils. The present invention is used with a
single field generating coil, although accurately known fields from
any coil assembly could be used if they do not have duplicate
fields at more than one location. The oscillating signal is
detected by a very small 3-axis magnetic field sensor, for example
a 3-axis Hall probe, attached to the magnetic implant. The signal
wires for the probe are led out of the body by being in the
catheter or endoscope which contains or is attached to the guided
magnetic implant, and which is being moved in the body duct.
[0027] The three Hall voltages from the magnetic field sensor
provide a direction and magnitude for the oscillating magnetic
field at the location of the probe. The oscillating signals are
separated from the contributions from the steady guiding field by
the use of dc isolation or controlled biasing, and are made
accurate by the well known technique of synchronous detection, or
some other electronic noise reducing means. Since the desired
guidance field is computer generated and the signal is then sent to
the magnet power supply--amplifier, this signal can easily be used
to insert an appropriate biasing voltage in each of the three
sensor leads so as not to overload sensitive synchronous detection
(or other) circuity for the locating function. The signal from the
proximate magnetic seed will either be a DC signal (if the seed is
a permanent magnet) that can be biased out, or an AC signal (if the
seed is a permeable material) that introduces a known or calculable
multiplicative factor that can be removed through simple techniques
such as gain adjustment, if the seed is operated in its linear
range of permeability. (More complicated corrections would be
required if the seed is a permeable material operated at or near
saturation, but it is not envisioned that such operation would
normally be necessary.) The subsequent output from this circuitry
becomes the functional locator signals, sent to analysis in the
computer.
[0028] The locator signals from the magnetic field sensor, with the
aid of the computer which has either a lookup table or other means
to know the source magnet field pattern (in conjunction with
knowledge of any deviation of the source magnet from its standard
position and direction), provide a unique direction and magnitude
of the oscillating magnetic field component of the source magnet,
at the position of the probe, and as measured in the frame of the
probe, based on its orientation. For the front half of the field
generating magnet, for a given current there is a unique pattern of
the magnetic field direction and strength. If this pattern is
stored in the operating computer, then knowledge of a measured
field at any location near the magnet would locate the point in
relationship to the coil. If, in addition, the angle of the axis of
the magnet is known to the computer, and the location of the coil
center is known also, in relation to some fixed reference frame,
then the computer, reading the signals from the implant, can
provide the location and direction of the implant continuously in
that reference frame. If the patient is located relative to that
reference frame, then the implant position and direction is known
relative to the patent. This can be accomplished, for example, by
providing fiducial markers 242 and 244 on a patient's body and on
the magnet and/or magnet arm and locators, as shown in FIG. 2, and
by providing a localizer 240 to provide a signal representative of
the location of these fiducial markers to processor 226. Processor
226 can then provide a correction to the locations sensed --i.e.,
provide spatial coordinates in the coordinate system of the room,
rather than relative to the magnet 204, based upon the signals from
localizer 240. The use of localizers is more fully described in
U.S. Provisional Application No. 60/065,107, Filed Nov. 12, 1997,
and incorporated by reference in its entirety above.
[0029] Thus, the invention provides a location and direction of the
implant relative to the magnet, for the purpose of allowing correct
orientation of the magnet to guide it in a desired direction, and a
location and direction of the implant relative to the patient,
which for some purposes can be useful to the physician in knowing
where in a body duct the implant is located, without the need for
real time imaging apparatus in place if that is not otherwise
needed for medical purposes. It can also be envisioned, however,
that some medically useful preoperative image of the patient could
be put on a computer screen, and some icon of the implant
superimposed on that image as the implant moves.
[0030] In another embodiment of the invention, a further use of the
invention may be made in which information from instrument module
or demodulator 224 is used on a display 232 and combined with
preoperative images which may be supplied to computer 226 in some
conventional way. There are many different locations around the
front hemisphere of the magnet 204 which have the same field line
directions, but these locations all differ in the magnitude of the
field. As the field line leaving the magnet in a given plane, and
from a more frontmost point bends, it can have the same direction
that another field line had, in that plane, which left the magnet
further back. But it will be further out as it reaches that angle,
and the field will be weaker. This describes the nature of the
magnetic field by which there is a unique field line pattern in
each hemisphere of the magnet. (Preferably, magnet 204 will
generate a hemispherically symmetrical field. If the magnet is not
symmetric end-to-end, there will not be the hemispheric symmetry.
Nevertheless, such magnets can be made to function in this
invention by merely adapting the "map" of this field for use in the
analysis.)
[0031] In the use of this invention, the location and orientation
of the magnetometer probe, and hence the implant, is made available
to the physician executing the procedure, and this location is made
known using the same magnet (or perhaps magnets) as is used to
guide the probe. This location can be made known and updated
essentially simultaneously with the actual guidance of the probe.
The physician can then know, for example, the direction of his
vision in an endoscope with an optical fiber system viewing the
region ahead of the endoscope.
[0032] When location information is displayed and combined with
preoperative imaging, the system must be "calibrated"
preoperatively, with some localizing method, such as has been
described in "Method and Apparatus Using Shaped Field of
Repositionable Magnet to Guide Implant," incorporated by reference
in its entirety above. The display may be of any type useful for
medical imaging. For example, the display could be a volume
rendered MRI image, so that the seed can be located in its
physiological context at each point of the path.
[0033] In use, the basic operation of the inventive apparatus is
accomplished by a method comprising the steps of surgically
implanting a magnetic implant 100 including an associated magnetic
field probe 108 in a patient 200; applying a modulated magnetic
field from a calibrated electromagnet 204; detecting signals from
the magnetic probe resulting from the application of the modulated
magnetic field using demodulator 224; and computing the relative
location of the magnetic probe, and therefore, the magnetic
implant, with respect to the electromagnet, from the detected
signals from the magnetic probe. Optionally, the patient and
electromagnet may be located with a localizer and an indication of
the location of the magnetic probe relative to the patient may be
displayed on a preoperative image of the patient with the
indication of the location of the magnetic probe superimposed on
the display.
[0034] Many modifications and variations of the inventive concept
would be evident to one skilled in the art upon reading and
understanding this disclosure, and the embodiments described herein
are intended to be exemplary rather than exclusive. For these
reasons, the scope of the invention should be determined with
reference to the claims appended below, and the full legal range of
equivalents permitted under applicable law.
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