WO2011158113A1

WO2011158113A1 - A device for magnetic localization and tracking

Info

Publication number: WO2011158113A1
Application number: PCT/IB2011/001682
Authority: WO
Inventors: Stéphane Lavallee
Original assignee: Minmaxmedical
Priority date: 2010-06-16
Filing date: 2011-06-15
Publication date: 2011-12-22

Abstract

The invention relates to a device (1) for localization of a magnetic sensor comprising: [a] a post configured to be fixed to a patient table (3) and to maintain a patient in predefined lying position on said patient table, [b] said post comprising a magnetic emitter, for the localization of at least magnetic sensor embedded in an instrument or in said patient's body.

Description

A device for magnetic localization and tracking TECHNICAL FIELD:

[01 ] The invention relates to the field of computer assisted surgery, and more particularly to a device and a method for tracking sensors using a magnetic localizer.

BACKGROUND OF THE INVENTION:

[02] A highly important issue in surgical navigation systems is related the localization device that can track trackers attached to instruments or to structures of a patient body.

[03] Optical localization devices are highly popular because they are accurate and robust against major perturbations. However, they present the drawback that the line of sight must remain visible between the camera of the optical localization device and the trackers. In order to compensate this problem, magnetic localization devices have been introduced. This type of device comprises a magnetic emitter, made in general of at least three coils, and trackers, which are magnetic sensors made of at least one coil. The magnetic emitter generates some magnetic field along several dimensions. In general, each coil of the magnetic emitter is generating magnetic fields that can be separated from each other, using time synchronization such as in the DC systems manufactured by Ascension Technologies (Burlington, VT, USA), or using different frequencies such as in the AC systems manufactured by Northern Digital Inc. (Waterloo, Canada).

[04] Recent magnetic sensors are made of one, two or three miniature coils that can be inserted at the tip of small instruments such as needles, in order to compensate for bending of the needle since only the tip of the needle matters for localization. Indeed, in an optical system, the tracker is large and must be fixed to the back part of the needle, it cannot be fixed to the needle tip. This compensation of bending is another advantage of magnetic localization systems over optical localization systems. The measurements made by the magnetic sensor for each activation of the individual emitter coils are used to compute the six degrees of freedom that characterize the position and orientation of the sensor. This represents the position and orientation of the magnetic sensor with respect to the magnetic emitter. This computation can be done using well known methods, in which the emitters are modeled by dipoles and the measurement made by a sensor coil is the projection of the magnetic field generated by the dipole on the sensor coil axis. In this method, the emitter coils, on one hand, and the sensor coils, on the other hand, have perfectly known relative geometry.

[05] This technology would be particularly suitable for applications of computer assisted surgery to arthroscopic procedures, wherein minimally invasive access is not perfectly compatible with optical localization devices that require the placement of large trackers. An optimal application would be hip arthroscopy procedures. [06] However, the main issue with magnetic localization devices is that the magnetic field is distorted by the presence of some metallic or motorized objects in the vicinity of the magnetic emitter and/or sensors. This can lead to errors of several centimeters, which makes the magnetic localization very inaccurate and even dangerous in the field of computer assisted.

[07] The invention aims at providing a magnetic localization device that overcomes these problems.

SUMMARY OF THE INVENTION:

[08] An object of the invention is a device for localization of a magnetic sensor comprising:

[a] a post configured to be fixed to a patient table and to maintain a patient in a predefined lying position on said patient table,

[b] said post comprising a magnetic emitter, for the localization of at least a magnetic sensor embedded in an instrument or in said patient's body.

[09] The magnetic emitter may comprise a plurality of magnetic coils, e.g. at least three magnetic coils.

[10] According to an embodiment of the invention, the post comprises:

[a] a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first magnetic emitter, and

[b] a second part, rigidly linked to said first part and comprising a second magnetic emitter.

[1 1 ] According to another embodiment of the invention, the post comprises:

[a] a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first plurality of magnetic emitters, and

[b] a second part, linked to said first part and comprising a second plurality of magnetic emitters.

[12] According to another embodiment, the post comprises:

[b] a second part consisting of an extension bar rigidly linked to said first part and comprising at its extremity a second plurality of magnetic emitters.

[13] The post advantageously comprises: [a] a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first set of at least three magnetic coils, and

[b] a second part consisting of an extension linked to said first part and comprising a second set of at least three magnetic coils.

[14] The post is preferably selected from the list consisting of a pubis post, a lateral decubitus post, and a patient supine post.

[15] In addition, the post may be adjustable in height.

[16] According to a preferred embodiment, the extension has a shape of an arch adapted to fit the patient's thigh anatomy.

[17] Another object of the invention is a system for localizing a medical instrument, comprising:

[a] a medical instrument, comprising at least one embedded magnetic sensor, and

[b] a localizing device as described above.

[18] Another object of the invention is a system for localizing a magnetic sensor embedded in a patient's body, comprising

[a] a magnetic sensor, which is adapted to be embedded in a patient's body,

[b] a localizing device as described above.

[19] The magnetic sensor preferably comprises a plurality of magnetic coils.

[20] Another object of the invention is a medical imaging system comprising the localizing device as described above.

[21 ] The invention further relates to a method for localizing a magnetic sensor during a surgical intervention, comprising the steps of:

[a] maintaining a patient in a predefined lying position on a patient table using a post fixed to said patient table, said post comprising a magnetic emitter,

[b] emitting a magnetic field with said magnetic emitter,

[c] measuring a magnetic field at the level of at least a magnetic sensor embedded in a medical instrument or the patient's body, and

[d] computing from the measured magnetic field the position of the magnetic sensor in a reference system of the localizing device.

[22] According to a preferred embodiment:

[a] the post comprises:

i. a first part fixed to said patient table and comprising a first magnetic emitter comprising a first plurality of magnetic coils, and ii. a second part consisting of an extension linked to said first part and comprising a second magnetic emitter comprising a plurality of magnetic coils, [b] the magnetic field is emitted by the magnetic coil of the first or second emitters for which the distance between said magnetic coil and said magnetic sensor is the shortest.

[23] According to an embodiment of the invention, the magnetic sensor is rigidly fixed to a patient's bone.

[24] The post advantageously contacts the portion of the patient's body wherein the surgical intervention is carried out.

[25] The distance between the magnetic emitter and the magnetic sensor is preferably of less than 10 cm.

DESCRIPTION OF THE DRAWINGS:

[26] Fig. 1 is a perspective view of one embodiment of the device during hip arthroscopy procedure.

[27] Fig. 2 is a detailed perspective view of another embodiment of the device during hip arthroscopy procedure.

[28] Fig. 3 is a perspective view of magnetic sensors attached to bones and instruments.

[29] Fig. 4 is a perspective view of a magnetic emitter and a magnetic sensor embedded in an instrument.

[30] Fig. 5 is a perspective view of a magnetic emitter with integrated coils.

[31 ] Fig. 6 is a perspective view of an embodiment of the device comprising an extension.

[32] Fig. 7 is a schematic view of a shaver with a miniature sensor.. DETAILED DESCRIPTION OF THE INVENTION:

[33] Errors of magnetic localization devices are mainly due to the presence of metallic or motorized objects in the vicinity of the magnetic emitter or sensor. Since the magnetic field of a dipole measured by another dipole depends on the inverse of the cube of the distance between the emitter and the sensor, when a metallic object is present, the relative distance between the emitter/sensor on one hand and emitter/object or sensor/object on the other hand plays a very significant role. Small distances between the emitter and the sensor relative to distances to metallic objects are required. The emitter must be as close as possible to the surgical areas where sensors will be present.

[34] As shown in Figure 1 , it is proposed to create a device 1 that has two main functions. A first function of the device is to act as a post to maintain the patient in a fixed position. Thus, the post is configured to be fixed to a patient table and to maintain the patient in a predefined lying position on said patient table. The post can be fixed to the patient table 3 like any conventional post (e.g. by screws, quick release, clip mechanism, or any known attachment means). The materials used to manufacture the post and the attachment means of the post to the patient table are non magnetic metals or plastic, that do not generate artefacts on the measurements. Traction is usually applied to the patient leg to create a pressure on the post that maintains the patient position fixed. A second function of the device 1 is to embed a magnetic emitter that constitutes the emitter of a localization device used to measure the localization and orientation of magnetic sensors, such as miniature sensors, which are embedded in an instrument or in the patient body. Thus, the post 1 comprises a magnetic emitter, for the localization of at least a magnetic sensor embedded in an instrument or in a patient body.

[35] In general, the magnetic emitter comprises at least a magnetic coil. In one embodiment, the magnetic emitter comprises a plurality of coils, preferably at least three coils. Each magnetic sensor comprises in general at least one coil, preferably two or three coils. For arthroscopic surgery, sensors can be miniature sensors mounted at the extremity of an instrument 6 (arthroscope, probe, shaver, canula), as shown in Figure 2. Thus the magnetic sensor can be in this embodiment embedded in an instrument, such as a medical instrument.

[36] In a preferred embodiment, as shown in Figure 3 and in Figure 7, placing at least one pair of miniature coils 12 at the tip 16 of a shaver 6 mounted on a motor 7 is performed in order to locate the shaver tip position in real time with respect to a reference system attached to the device 1 comprising the magnetic emitter.

[37] Miniature sensors can be also directly placed inside the bones in order to track the bone. In this case, each magnetic sensor can be made of at least two coils embedded in a shell that can be fixed rigidly to the bone. In this embodiment, the magnetic sensor is adapted to be embedded in a patient body.

[38] One magnetic sensor 1 1 can be placed in the femur bone 8, for instance in the area of the greater trochanter, and one magnetic sensor 10 can be placed in the pelvis bone 9, for instance in the area of the iliac spine. Emitters and sensors have cables that link them to a computer control unit. For clarity, the cables and the computer control unit are not represented.

[39] Different known techniques can be used to register the bone with a 3D image of the patient, which means to compute a transform between the coordinate of the 3D image and the coordinate of a reference system attached to a patient reference tracker. This 3D image includes Computed Tomography (CT) or Magnetic Resonance (MR) images. Once registration has been performed, the position of the tip of the instrument 16 is localized with respect to the bone magnetic sensor using the main function of the magnetic localization device, both being localized in the same reference system attached to the localizer device 1 , such that the tip of the instrument is localized and displayed on the 3D image in order to assist the surgeon in performing the surgery with accuracy and safety. [40] In conventional surgical navigation systems, the localization device comprises a magnetic emitter contained in a plastic housing with size ranging from one cm x one cm x one cm up to ten cm x ten cm x ten cm. Finding a good location for the localization device made of the emitter in hip arthroscopy is a difficult problem since metal objects or obstacles are present all around the operated area of the hip. No satisfactory solution have been proposed in the prior art. Indeed, below the patient is the operating table 4. Above the patient is the image intensifier of the x-ray imaging device 2 that is mounted on a C-arm. On the external side of the patient, the shaver motor is very often an electric motor that generates significant artefacts in the magnetic localization device. Towards the head of foot of the patient it is not possible either to place the device, since the space is busy by the patient. The internal side is therefore the preferred position to place the device, as shown in Figure 1 , that is to say between the legs of the patient. However, this space is usually busy in the prior art because of a conventional post that maintains the patient position. The device 1 solves this problem because it integrates a magnetic emitter to the post, for instance coils necessary for the magnetic emission.

[41 ] In this preferred embodiment, the distance between the emitter coils included in the post 1 and the magnetic sensors is around two to ten centimeters which is sufficiently small to reach a good accuracy and obtain reliable measurements not affected by metal objects and motors in the surroundings. Those distances are very easy to reproduce from one patient to another for the surgeon and the medical staff. The installation of the device 1 on the operating table 4 is done exactly as in the way a conventional post is installed. The only difference for the user is that a cable, not represented on the figures, must be installed between the device 1 and a computer control unit in order to transmit electrical signals to the coils of the magnetic emitter.

[42] As shown in Figure 4, in a preferred embodiment, the device 1 has a cylindrical shape that includes at least three coils with arbitrary positions. A calibration process can be applied to measure the exact location of the center and axis of each individual coil with respect to a reference frame attached to the magnetic emitter. Such calibration can be performed after manufacturing by using a sensor with a known geometry and varying the sensor position along different dimensions using calibrated linear displacement actuators, for instance in three directions.

[43] In a preferred embodiment, the emitter contains four coils in order to provide redundant measurements. Indeed, each coil generates a magnetic field and each sensor measures one value which is the projection of the local magnetic field on its axis. Using four coils and an instrument made of three miniature embedded coils provide twelve measurements, that are sufficient to track the sensor in position and orientation in a three dimensional space. Using four coils in the emitter to track a sensor made of two miniature coils shall provide eight measurements that provide redundancy to locate the six degrees of freedom of the sensor, which is a very interesting feature since it reinforces the accuracy and safety of measurements. Least squares techniques are used to take into account those redundant measurements. The methods for locating a sensor in the reference system of an emitter are not described in this invention since they are well known by the one ordinary skilled in the art.

[44] According to one embodiment of the invention, a patient is maintained in a predefined lying position on a patient table using a post fixed to said patient table, said post comprising a magnetic emitter. The magnetic emitter then emits a magnetic field, or a plurality of magnetic fields, and the magnetic field at the level of the magnetic sensor embedded in an instrument or the patient body is measured, in order to infer the localization of said magnetic sensor.

[45] As shown in Figure 5, in another preferred embodiment, the coils that constitute the emitter 1 are integrated together. For instance, this is achieved by rolling wires all around a cube along the three directions X, Y, and Z of the cube.

[46] In another preferred embodiment, the post 1 comprises a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first magnetic emitter, and a second part, linked to said first part and comprising a second magnetic emitter. Of course, the first magnetic emitter can comprise a plurality of coils, such as three coils or more, and the second magnetic emitter can comprise a plurality of coils, such as three coils or more. In a preferred embodiment, said first part and second parts are linked rigidly together with a known geometrical relationship. In another preferred embodiment, the first part and second parts are linked rigidly together with an unknown geometrical relationship. In the latter case, a calibration step is required at the beginning of the procedure, which consists in placing a set of magnetic sensors in the vicinity of the first and second part, for multiple positions, retain the measurement values of the relative sensor to emitter positions that are consistent together and average them.

[47] As shown in the embodiment of Figure 6, the device 1 can contain as a second part an extension 5 that contains additional coils 14. The coils 14 can be inside the extension 5. In another preferred embodiment, the extension is a long bar that contains additional coils 14 at its extremity. The shape of said bar is designed to be above the patient with minimal size. The extension offers several advantages, since it implements the concept of using multiple magnetic emitters to improve the accuracy and reliability of measurements. First, it generates more volume to embed more coils, and having more coils improves the accuracy of measurements. Second, the coils 14 generate independent measurements of the coils 13 included in the post 1. For a given sensor, in a preferred embodiment, it is then possible to select the measurements for which the distance between the sensor and the coils 14 or 13 is the shortest, since a shortest distance usually provides more reliable and accurate measurements. Measurements using magnetic emitters 13 and 14 can also be compared for check. More generally the extension can contain more than just three coils.

[48] According to a preferred embodiment, the coils that are the closest from the magnetic sensors will be used at a given instant. In this case, a magnetic field is emitted with the coil of the first plurality of coils 13 or second plurality of coils 14 for which the distance between said coil and the magnetic sensor embedded in an instrument or the patient body is the shortest. Then, the magnetic field at the level of the magnetic sensor is measured.

[49] Having an extension is therefore a significant advantage for the accuracy of measurements. However it is not easy to implement this extension because of all constraints due to the patient and the environment.

[50] In a preferred embodiment, the extension 5 is placed above and on the side of the first part of the post, which has for instance a cylindrical shape, such that it goes above the junction between the body and the thigh. In a preferred embodiment, the extension 5 has a shape like an arch that fits with the anatomy in this area of the body, for instance the thigh's anatomy. This design makes it possible to perform a flexion, extension, abduction, rotation of the leg with no impingement between the leg and the device. An adjustment TZ of the height of the post 1 is then proposed to fit the size of patients such that the emitter is as close as possible to the patient whilst leg motions are still possible. Thus according to one embodiment, the post 1 is adjustable in height.

[51 ] As shown in Figure 7, a miniature sensor is embedded in the tip of the instrument which is most often a shaver. The rotating shaft 15 of a shaver blade is attached to the motor 7. The tip of the shaver blade can have several shapes such as a sphere 16. The sphere 16 can be partially covered by the housing in one direction to create a protection, not represented on the figures. It acts like a milling tool. The housing 6 of the shaft 15 is containing two holes. The first one 17 is a long hollow used to pass the shaft 15. The second one 18 is a small and long hollow used to pass the sensor and the wires that go to the basis of the shaver. The section of the housing 6 has a pear shape. This design makes it possible to bring the sensor mounted on the instrument quite close to the emitter for hip arthroscopy procedures in the embodiment described in this invention and the location of a shaver is predominantly important for navigation of this type of surgery since it is one of the most widely used tools.

[52] The device described above can be used for many applications. The invention also provides a medical imaging system comprising the device previously described, such as a CT scan.

[53] In the set up shown in Figure 1 , the post is adapted for a patient supine, lying on the back on operating table. The exact same device can be used when the patient is lying on the table in lateral decubitus, lying on the side. In that case, the axis of the cylindrical shape of the post will be horizontal instead of vertical. In an application such as total hip arthroplasty, a pubis post is also commonly used with a more squared external shape. The device 1 can be easily adapted to fit this particular embodiment by placing the emitters in the pubis post device. Thus, the post can be notably selected from the list consisting of a pubis post, a lateral decubitus post, a patient supine post.

[54] More generally, the device can be used for computer assisted surgery wherein a mechanical fixation device such as a post is used. ADVANTAGES:

[55] The invention offers accurate and reliable measurements in a magnetic localization system used for computer assisted surgery, whilst the invention is not more cumbersome than the conventional set up of surgical devices for patient fixation. The invention is compact, can be easily integrated in conventional devices, such as medical imaging system, patient table, and provides a flexible solution.

Claims

A device for localization of a magnetic sensor comprising:

i) a post configured to be fixed to a patient table and to maintain a patient in a predefined lying position on said patient table,

ii) said post comprising a magnetic emitter, for the localization of at least a magnetic sensor embedded in an instrument or in said patient's body.

The device of claim 1 , wherein the magnetic emitter comprises a plurality of magnetic coils.

The device of one of claims 1 or 2, wherein the magnetic emitter comprises at least three magnetic coils.

The device of one of claims 1 to 3, wherein the post comprises:

i) a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first magnetic emitter, and

ii) a second part, rigidly linked to said first part and comprising a second magnetic emitter.

The device of claim 1 , wherein the post comprises:

i) a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first plurality of magnetic emitters, and

ii) a second part, linked to said first part and comprising a second plurality of magnetic emitters.

The device of claim 1 , wherein the post comprises:

ii) a second part consisting of an extension bar rigidly linked to said first part and comprising at its extremity a second plurality of magnetic emitters.

The device of claim 6, wherein the post comprises:

i) a first part, configured to be fixed to the patient table and to maintain the patient in the predefined lying position on said patient table, said first part comprising a first set of at least three coils, and

ii) a second part consisting of an extension linked to said first part and comprising a second set of at least three coils.

The device of one of claims 1 to 7, wherein the post is selected from the list consisting of a pubis post, a lateral decubitus post, and a patient supine post. The device of one of claims 1 to 8, wherein the post is adjustable in height.

10. The device of claim 6, wherein the extension has a shape of an arch adapted to fit the patient's thigh anatomy.

1 1 . A system for localizing a medical instrument, comprising:

i) a medical instrument, comprising at least one embedded magnetic sensor, and

ii) a localizing device according to one of claims 1 to 10.

12. A system for localizing a magnetic sensor embedded in a patient's body, comprising

i) a magnetic sensor, which is adapted to be embedded in a patient's body, ii) a localizing device according to one of claims 1 to 10.

13. The system according to one of claims 1 1 or 12, wherein the magnetic sensor comprises a plurality of magnetic coils.

14. Medical imaging system comprising the device of one of claims 1 to 10.

15. Method for localizing a magnetic sensor during a surgical intervention, comprising the steps of:

i) maintaining a patient in a predefined lying position on a patient table using a post fixed to said patient table, said post comprising a magnetic emitter, ii) emitting a magnetic field with said magnetic emitter,

iii) measuring a magnetic field at the level of at least a magnetic sensor embedded in a medical instrument or the patient's body, and

iv) computing from the measured magnetic field the position of the magnetic sensor in a reference system of the localizing device.

16. Method according to claim 15, wherein:

i) the post comprises:

i. a first part fixed to said patient table and comprising a first magnetic emitter comprising a first plurality of magnetic coils, and ii. a second part consisting of an extension linked to said first part and comprising a second magnetic emitter comprising a plurality of magnetic coils,

ii) the magnetic field is emitted by the magnetic coil of the first or second emitters for which the distance between said magnetic coil and said magnetic sensor is the shortest.

17. Method according to one of claims 15 or 16, wherein the magnetic sensor is rigidly fixed to a patient's bone.

18. Method according to one of claims 15 to 17, wherein the post contacts the portion of the patient's body wherein the surgical intervention is carried out.

19. Method according to one of claims 15 to 18, wherein the distance between the magnetic emitter and the magnetic sensor is of less than 10 cm.