Method for locating a magnetic field-measuring assembly using a variable magnetic field

Abstract

The invention relates to a method for locating a magnetic field-measuring assembly comprising at least one magnetoresistive sensor and / or Hall sensor, comprising the steps of: a: / generating, by way of an electromagnetic field-generating device (1), a magnetic field of variable amplitude and / or shape such that the position of a predetermined value of the amplitude of the magnetic field varies along at least two directions (X, Y) at different frequencies; b / measuring the magnetic field along each of the directions at the frequency corresponding to each direction by way of the magnetic field-measuring assembly; c / when the magnetic field-measuring assembly detects the predetermined value of the magnetic field along each of the directions, determining the position of the magnetic field-measuring assembly.

Description

[0001]Description Title: Method for locating a magnetic field measuring assembly by a variable magnetic field Technical field The present invention relates to the field of electromagnetic localization of an object in space, in particular when direct visualization of this object is impossible. The invention relates more particularly to a localization method comprising the detection by a magnetic field measuring assembly of an electromagnetic signal coded in space by its amplitude and / or its shape, allowing the three-dimensional localization of an object. The measuring assembly may in particular comprise one or more magnetoresistive sensors. The main field of application envisaged relates to the localization and / or guidance of a medical device, in particular a catheter during a surgical / endoscopic intervention. The invention applies, however, to any field requiring the localization of an object,including robotics or microrobotics, the positioning of industrial equipment or even indoor positioning systems known as indoor localization systems. Prior art It is known to implement guidance of medical devices by radiative imaging, including in particular X-ray tomography and computed tomography. Existing guidance methods make it possible to determine the position of the medical device in space and are currently used for surgical applications. However, the invasive aspect of the emitted radiation severely limits the use of these methods. The use of electromagnetic coding fields and their detection by coils have been proposed. Thus, the company NDI markets a device under the name Aurora allowing the implementation of such a method. This type of method, however, has several limitations. First of all,It is necessary to use detection coils with a ferrous core in order to obtain sufficient sensitivity to electromagnetic fields, which makes it difficult to apply these methods to the location of very small objects. Coils with ferrous cores are difficult to miniaturize. In addition, it is necessary to detect electromagnetic fields at their emission frequency. Consequently, if the object to be located is connected, for example, to a cable, as is the case with a catheter, the cable contributes to part of the detected signal, inducing noise and uncertainty in the measurement. The article "A pilot study on an electromagnetic tracking system using tunneling magnetoresistance (TMR) sensors applicable to a 4F catheter (1.4 mm in diameter)", Nagano, R., Hara, K., Kobayashi, E. et al.. Int J CARS 18,17–27 (2023) describes a localization method using tunnel effect magnetoresistance sensors to detect a magnetic field. However, the device described requires direct coupling between the sensors and the excitation system. There is therefore a need to improve existing localization methods. The aim of the invention is to meet at least part of this need. Disclosure of the invention To do this, the invention relates in one of its aspects to a method for localizing within a volume a magnetic field measurement assembly comprising at least one magnetoresistive sensor and / or a Hall effect sensor, comprising the steps of: a / generating in the volume, by means of an electromagnetic field generation device,a magnetic field of variable amplitude and / or shape such that the position of a predetermined value of the amplitude of the magnetic field varies along each of at least a first direction (X) and a second direction (Y), the magnetic field having a first frequency and a second frequency respectively along the first direction and the second direction, the first and second frequencies being different from each other; b / measuring the magnetic field along each of the directions at the frequency corresponding to each direction by means of the magnetic field measuring assembly; c / when the magnetic field measuring assembly detects the predetermined value of the magnetic field along each of the directions, determining the position of the magnetic field measuring assembly. Preferably,a magnetic field of variable amplitude and / or shape is generated such that the position of a predetermined value of the amplitude of the magnetic field varies along each of a first direction (X), a second direction (Y) and a third direction (Z), the magnetic field having respectively a first frequency, a second frequency and a third frequency along the first direction, the second direction and the third direction, the first,second and third frequencies being different from each other. The method according to the invention performs a coding of the space within the volume of interest by a magnetic field whose shape and / or amplitude are variable over time in a known manner. A scan of the volume of interest or at least of a surface contained in the volume of interest is carried out with a predetermined value. The magnetic field measuring assembly can be precisely located by measuring the magnetic field in the volume of interest until this predetermined value is measured. Any object can be located by attaching a magnetic field measuring assembly to the object. The volume of interest is coded by generating an electromagnetic field along the three directions of space X, Y and Z in such a way that the local amplitude of the magnetic field varies over time,preferably at any point in the volume of interest. When the local magnetic field reaches a target value on the measuring assembly, the position of the measuring assembly is recognized because it is linked to the time coding of the magnetic field. The time coding of the magnetic field therefore makes it possible to determine the position of the measuring assembly. For example, a magnetic field can be generated along the X direction with an amplitude having a predetermined value, preferably an amplitude maximum or zero, which moves along the X direction over time. When this predetermined value is measured with the magnetic field measuring assembly,the measuring assembly is located at the corresponding X coordinate. The same operation can be performed along each of the Y and Z directions to locate the measuring assembly in space. The scanning of the volume can also be performed along all three directions at the same time. The electromagnetic field used to encode the space is applied at three different frequencies along the three directions X, Y, Z and is measured at these three frequencies with the measuring assembly to discriminate between the three directions. Alternatively, a surface contained in the volume of interest is encoded by generating a magnetic field in the volume in such a way that the local amplitude of the magnetic field varies over time in the surface of interest defined by the first and second directions X, Y,preferably at any point on this surface. The surface of interest is advantageously planar. The electromagnetic field used to encode the plane is applied at two different frequencies in the first and second directions X, Y. The position in the third direction Z is preferably determined based on the absolute value of the magnetic field in the Z direction. The measuring assembly is sensitive to the magnetic field in the three directions X, Y, Z and may comprise one or more magnetic field sensors. A magnetic field sensor may be sensitive to the magnetic field in one or more directions. For example, the measuring assembly may comprise three magnetic field sensors each sensitive in one direction X, Y or Z, or two magnetic field sensors each sensitive in two directions in space, for example X, Y and Y, Z,or a magnetic field sensor sensitive in one direction of space and a sensor sensitive in two directions of space. If a magnetic field sensor is sensitive in several directions, the same sensor can be used to detect the magnetic field in these directions by demodulating the signal at the excitation frequencies corresponding to these directions. The magnetic field sensors may in particular comprise one or more Hall effect sensors and / or one or more magnetoresistive sensors, which may be giant magnetoresistance sensors (GMR sensors), tunneling magnetoresistance sensors (TMR sensors) and / or anisotropic magnetoresistance sensors (AMR sensors). These sensors can advantageously be easily miniaturized, which improves the positioning accuracy. Advantageously, the magnetoresistance and Hall effect sensors can be powered by a current of frequency fcap,measure a magnetic field of frequency fem and carry out an in situ modulation, so as to produce a reading current of frequency f, cap ± f emfor reading. Thus, these sensors make it possible to overcome the noise and uncertainties induced by their cables, as well as direct couplings by the cables. Advantageously, a measurement set using two GMR or TMR sensors typically allows detection of weak signals, of the order of a few nanoteslas, with good spatial resolution. Preferably, the sensitive surface of each sensor is greater than or equal to 1 µm², 10 µm² or 20 µm² and / or less than or equal to 1000 µm², 500 µm², 100 µm², 50 µm² or 20 µm². The length of the active zones of the sensors along their sensitivity directions is preferably less than or equal to 500 µm, 200 µm, 100 µm, 50 µm or 10 µm and / or greater than or equal to 1 µm, 5 µm, 10 µm, 50 µm or 100 µm. Preferably, the first, second and third directions are linearly independent, in particular orthogonal two by two.According to an advantageous embodiment, the electromagnetic field generating device comprises three pairs of coils each aligned along one of the first, second and third directions. Alternatively, the electromagnetic field generating device comprises a first pair of coils extending in a first plane and configured to generate a first variable magnetic field along the first direction, a second pair of coils extending in a second plane parallel to the first plane and configured to generate a second variable magnetic field along the second direction, and a fifth coil extending in the second plane, arranged between the coils of the second pair of coils and configured to generate a magnetic field along a direction orthogonal to the first and second directions.According to an alternative embodiment, the electromagnetic field generating device comprises three pairs of permanent magnets each aligned and movable along one of the first, second and third directions. Preferably, the permanent magnets are each mounted on a movable support. The movement of the permanent magnets makes it possible to create a variable magnetic field. Preferably, each coil is powered by an alternating current of the form. where i = {1, 2} identifies the coils of a pair, j = {X, Y, Z} identifies a pair of coils according to the alignment direction of that pair, t is time and f jis the frequency of the current in the coil. The frequencies fX, fY and fZ are different from each other. Ip, i, j, which can be positive or negative, represents the maximum amplitude of the current flowing through the coil i, j. This amplitude Ip is preferably constant for a duration allowing the magnetic field measuring system to make a measurement, typically of the order of a millisecond, but varies over longer durations depending on a quantity t A which defines the coding of the magnetic field. The values ​​of I p, i, j follow a specific profile, such as the profiles shown in Figure 5. These profiles advantageously allow the position of the zero of the magnetic field to be varied linearly between the coils. Thus, the magnetic field ^^ ^^(^^) generated by the pair of coils j varies depending on the value of Ip, i, j and therefore varies over time. Since the amplitude of the magnetic field at a given point varies over time, it is possible to vary the position of a predetermined value of the amplitude of the magnetic field. Depending on the values ​​of the current flowing through the two coils of a pair, the field B j can therefore reach a predetermined characteristic value for a specific torque I 1,j (t ref ), I 2,j (t ref ). Detecting this field value, for example ^^^^ = ^^^^^^^^ or ^^^^ = 0, for ^^ = ^^^^^^^^ allows to identify and therefore the position of the measuring assembly^^ = ^^^^^^^^. According to an advantageous variant, the predetermined value of the magnetic field is a maximum or a zero and this value is detected when the derivative of the value of the ^^^^ magnetic field with respect to time t A ^^^^ ^^is zero or maximum, respectively. This method of measuring the predetermined value of the magnetic field is more accurate if the predetermined value of the magnetic field is poorly resolved spatially. The frequencies fj are typically between 1 kHz and 100 kHz. However, they can be higher if the temporal resolution for the intended application requires it. For example, a temporal resolution of the order of a millisecond is required for a cardiovascular endoscopic monitoring application. The temporal resolution is determined in particular by the signal acquisition speed and the current scanning speed to encode the space.According to an advantageous characteristic, the determination of the position of the magnetic field measuring assembly is carried out successively for each direction, the magnetic field being generated so that the position of the predetermined value varies successively along the first direction, along the second direction and then along the third direction. Alternatively, the determination of the position of the magnetic field measuring assembly is carried out simultaneously for each direction, the magnetic field being generated so that the position of the predetermined value varies simultaneously along the first, second and third directions. Preferably, the electromagnetic field generating device comprises three pairs of coils, each pair being aligned along one of the three directions.More preferably, the magnetic field measuring assembly comprises a first magnetoresistive sensor, preferably being a first GMR, TMR or AMR sensor extending in a plane. The plane may in particular be formed by two of the three directions. The magnetic field measuring assembly may advantageously comprise a second magnetoresistive sensor, preferably being a second GMR, TMR or AMR sensor extending in a plane orthogonal to the plane in which the first magnetoresistive sensor extends. The magnetic field measuring assembly may further comprise a third magnetoresistive sensor, preferably being a third GMR, TMR or AMR sensor extending in a plane orthogonal to the planes in which the first and second magnetoresistive sensors extend. The magnetic field measuring assembly may also comprise a Hall effect sensor extending parallel to the first magnetoresistive sensor.Hall effect sensors are sensitive in a direction perpendicular to the plane in which they extend while magnetoresistive sensors are sensitive in the plane in which they extend. The invention also relates to a method for tracking an object, in particular a catheter in a human or animal body, comprising attaching a magnetic field measuring assembly to the object and locating the magnetic field measuring assembly by the localization method as described above. Preferably, the tracking method comprises attaching one or more reference measuring assemblies in a volume of interest and locating them by the localization method as described above.Ultimately, the invention provides numerous advantages, including: - separation of the components of the magnetic field in the three directions of space X, Y, Z according to the frequencies of the magnetic field, - insensitivity to radiofrequency disturbances because the signal is demodulated locally at the object, - insensitivity to static fields possibly creating a measurement offset on the magnetic field measurement assembly when the derivative of the magnetic field is used to detect the predetermined value, - use of magnetic fields of variable frequencies depending on the intended application. For example, for catheter guidance in the context of cardiovascular endoscopic monitoring, low frequencies (less than 100 kHz) are advantageously used, which avoid signal distortions in the body or in the presence of metallic objects such as implants or medical instruments and which have no induced effects.Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings [Fig 1] Figure 1 represents an electromagnetic field generation device that can be used in the context of the invention. [Fig 2] Figure 2 represents a first configuration of coils that can be used to generate a magnetic field as implemented in the context of the invention. [Fig 3] Figure 3 represents a second configuration of coils that can be used to generate a magnetic field. [Fig 4] Figure 4 represents the temporal evolution along the axis of a pair of coils of a magnetic field generated in the context of a method according to the invention.[Fig 5] Figure 5 is a graph showing an example of the intensity profile of the currents flowing through a pair of coils so as to generate a magnetic field, the position of the zero of the magnetic field being determined by these currents. [Fig 6] Figure 6 shows a third configuration of coils that can be used to generate a magnetic field. Detailed description Figure 1 illustrates a magnetic field generation device 1 suitable for generating a magnetic field as implemented according to the invention. For reasons of clarity, only one pair of coils is shown in this figure. The device 1 comprises a computer 2 connected to a frequency generator 3 which generates one or more alternating currents at one or more determined frequencies. Each generated current is transmitted to two amplifiers 4 which amplify it. At the output of each amplifier 4, the amplified current supplies a coil of the pair of coils 5, 6.The two coils 5, 6 are arranged opposite each other. The currents flowing in the coils 5, 6 create a magnetic field whose frequency and amplitude are controlled by the computer 2 by means of the frequency generator 3 and the amplifiers 4. A magnetic field measuring assembly 7 is arranged in the volume delimited by the three pairs of coils. The magnetic field measuring assembly 7 is advantageously configured to be fixed to an object in order to allow the latter to be located. One or more reference measuring assemblies 8 may also be arranged in the volume delimited by the three pairs of coils. The reference measuring assemblies 8 may in particular be used when the magnetic field measuring assembly 7 is arranged within an object likely to move. This object is for example a human or animal body, in which case the reference measuring assemblies 8 may advantageously be placed on the skin of the body.The reference measuring assemblies 8 then make it possible to measure any movement of the body during the localization process so that the position of the magnetic field measuring assembly 7 within the body can be tracked accurately, even in the event of movement or deformation of the latter. The magnetic field measuring assembly 7 and the reference measuring assemblies 8 are configured to measure a magnetic field in three linearly independent spatial directions, preferably orthogonal in pairs. The reference measuring assemblies 8 may be identical to the magnetic field measuring assembly 7. Their localization may be carried out in the same way. The device 1 further comprises an acquisition system 9 preferably comprising an acquisition card connected to the magnetic field measuring assembly 7 and to the reference measuring assemblies 8.The acquisition system 9 transmits the collected data to the computer 2 which performs the data processing. The device 1 is shown in Figure 1 with a single pair of coils 5, 6. A pair of coils makes it possible to locate the magnetic field measuring assembly in one direction of space. Thus, to achieve the location of a measuring assembly in the three directions of space, three pairs of coils 5, 6, 15, 16, 25, 26 are used. Each pair of coils is arranged along an axis orthogonal to the other two. Preferably, the frequency generator 3 generates a signal for each of the three pairs of coils, the generation of these signals being multiplexed. The frequency generator 3 is preferably a synchronous multi-channel frequency generator controllable in amplitude and phase. However, each pair of coils can also be associated with a natural frequency generator.More preferably, each coil is associated with its own amplifier 4. Preferably, the amplifiers 4 generate currents of intensity greater than or equal to 5 A, 10 A or 20 A and / or less than or equal to 20 A or 10 A, and of frequency greater than or equal to 1 kHz, 2 kHz or 5 kHz and / or less than or equal to 200 kHz, 100 kHz or 50 kHz. The magnetic field measuring assembly 7 and / or the reference measuring assemblies 8 may comprise one or more magnetoresistive and / or Hall effect sensors, each sensor making it possible to measure the magnetic field in one or two directions in space. The device 1 may also comprise, if necessary, a power supply configured to power the magnetic field measuring assembly 7 and / or the reference measuring assemblies 8.The device 1 may also comprise electronic means for processing the signal from the sensors 7 and 8, such as an amplifier, a filter and a demodulation means, before acquisition by the acquisition card. The acquisition card of the acquisition system 9 is preferably adapted to acquire data at a frequency greater than or equal to twice the highest frequency of the currents supplying the coils. The acquisition card advantageously has as many acquisition channels as there are magnetoresistive and / or Hall effect sensors in the magnetic field measuring assembly 7 and the reference measuring assemblies 8 and six additional channels intended to sample the currents flowing in the coils. Figure 2 shows a possible configuration of three pairs of coils 5, 6, 15, 16, 25, 26 adapted to generate a magnetic field allowing the location of sensors in the three directions of space.The coils of the same pair face each other and are aligned on the same axis, the axes of the three pairs of coils being orthogonal to each other. In the example of Figure 2, the coils are circular in shape in a plane orthogonal to the axis of their pair of coils. Figure 3 represents an alternative configuration of the three pairs of coils 5, 6, 15, 16, 25, 26, the coils having a rectangular shape with rounded corners. Such a configuration is particularly suitable for the case of locating a magnetic field measurement assembly inside a human or animal body. Figure 4 represents the evolution of the magnetic field between two coils 5, 6 as a function of the evolution over time of the current flowing through these two coils. I1(t) and I2(t) represent respectively the intensity of the current flowing through coil 5 and that of the current flowing through coil 6.Curves B1, B2, …, BN each represent the amplitude between the two coils 5, 6 of the magnetic field generated by currents I1 and I2 at times t1, t2, …, t. Nrespectively. The acquisition of the magnetic field measurements is carried out at the frequency of the currents supplying the coils 5, 6. The points X1, X2, …, XN represent the points on the axis of the pair of coils where the value of the magnetic field is equal to 0 at times t1, t2, …, tN respectively. As illustrated in Figure 4, the evolution of the currents supplying the coils modifies the profile of the magnetic field generated between these coils. Preferably, the profile of evolution of the currents supplying the coils is chosen so that the amplitude of the generated magnetic field has a linear profile in the axis of the corresponding pair of coils, as shown in Figure 4. The predetermined value of the magnetic field which makes it possible to locate the magnetic field measurement assembly is preferably chosen to be equal to 0 or to be equal to the maximum or minimum of the amplitude of the magnetic field.Figure 5 shows an example of a profile of the amplitudes of the currents I1, I2, allowing a linear profile of variation of the amplitude of the magnetic field to be obtained. In the case of a pair of circular coils of diameter d separated by a distance 2a, the magnetic field generated along the axis of the pair of coils is given ^^ ^^ ^^² ^^ ^^ 1 =. 0 1 4√^^ 2 +4∗(^^−^^)² for the first coil and ^^2=0^^² by ^^24√^^ 2 +4∗(^^+^^)² for the second coil, where x is the position along the axis with x = 0 at the midpoint between the coils, and µ0 is the magnetic permeability of vacuum. At position x = 0, the field is zero when I1= I2. For any position x, the magnetic field is zero when and so when Thus, it is possible to generate a magnetic field whose amplitude is zero at an arbitrary point along the axis of the coil pair between the two coils, up to the position of the coils themselves. The currents allowing to obtain a linear time variation of the zero position of the magnetic field (i.e. x = vt where t is the time and v is the rate of variation of the zero position) are determined by The greatest position resolution is obtained when the currents are maximum. We therefore preferably set I1= I max when x is negative and I2= I max when x is positive, where I maxis the maximum current intensity supplying the coils. For a pair of coils of 1 m diameter separated by a distance of 1 m, the profiles of the currents I1 and I2 shown in Figure 5 are obtained. The abscissa of the graph in Figure 5 indicates the position of the zero of the magnetic field along the axis of the pair of coils for each pair of currents (I1, I2). Figure 6 shows a third configuration of coils that can be used to generate a magnetic field. A first pair of coils 30, 31 extends in a first plane. The coils 30, 31 are preferably adjacent. A second pair of coils 32, 33 extends in a second plane parallel to the first plane. Preferably, the second pair of coils 32, 33 is adjacent to the first pair of coils 30, 31 along the vertical direction Z and is arranged below the first pair of coils.A fifth coil 34 is arranged between the two coils 32, 33 of the second pair of coils. The fifth coil 34 is configured to generate a magnetic field along the vertical direction Z in the volume of interest. The coils 32, 33 of the second pair are electrically powered in phase and are configured to generate a variable magnetic field in a direction orthogonal to the vertical direction, for example the X direction. Preferably, this field is canceled at the center of the volume of interest when the two coils are powered by identical currents. The coils 30, 31 of the first pair are configured to generate a variable magnetic field in a direction orthogonal to those of the fields generated by the second pair of coils and by the fifth coil, for example the Y direction. Preferably, this field is canceled at the center of the volume of interest when the two coils are powered by identical currents.The configuration shown in Figure 6 makes it possible to generate in the volume of interest a magnetic field whose amplitude and / or shape varies in the XY plane by adjusting the supply currents of the first and second pairs of coils. It is thus possible to determine the position of the measuring assembly in the XY plane by searching for a predetermined value of the magnetic field. The magnetic field in the volume of interest is not adjustable in the Z direction. An estimate of the position of the measuring assembly in this direction can be obtained from a measurement of the absolute value of the field in this direction. Advantageously, the configuration of Figure 6 is compact and makes it possible to position the coils without occupying space in all directions around the volume of interest. Such a configuration is particularly advantageous when seeking to determine the position of the measuring assembly in a patient.The coils can in particular be arranged under a table on which the patient must lie down. Example In a particular embodiment, the method according to the invention is implemented to carry out endoscopic cardiovascular monitoring on a person. In this example, the electromagnetic field generation device comprises three pairs of coils, each oriented along an axis orthogonal to the other two axes. The coils are arranged around the person while leaving sufficient space to allow access to the person. The coils have a typical maximum length of the order of one meter and each comprise between 100 and 1000 turns of copper wire. The magnetic field measurement assembly comprises two GMR type sensors arranged orthogonally so as to be able to measure the magnetic field along the three axes of the pairs of coils of the magnetic field generation device.The sensitivity of GMR sensors is of the order of 1 nT.Hz. -1 / 2 , which may in particular be less than or equal to 10 nT.Hz -1 / 2 and / or greater than or equal to 0.5 nT.Hz -1 / 2The magnetic field measuring assembly is attached to a catheter. It can in particular be attached inside the catheter. The length of the active zones of the sensors along the axes of the pairs of coils is less than or equal to 100 µm. This length limits the resolution of the localization and is in particular chosen according to the intended application. The three pairs of coils are powered at distinct frequencies, greater than or equal to 1 kHz and / or less than or equal to 10 kHz. The amplitude of the currents flowing through the coils of each pair varies between +I and -I according to a well-chosen profile such as that shown in Figure 5, where I is a maximum value of the current, typically of the order of 10 A. This makes it possible to vary the position of the 0 value of the magnetic field along the axis of the pair of coils. The volume between the pairs of coils in which the zero of the magnetic field is displaced is typically of the order of 1 mx 1 mx 1 m.The amplitude of the magnetic field is then varied along the three directions X, Y, Z one after the other in order to detect a predetermined value of the magnetic field on the magnetic field measuring assembly. The sensors of the measuring assembly acquire a measurement for a typical duration of approximately 20 ms. The analysis of this measurement makes it possible to determine the value of the magnetic field at the level of the magnetic field measuring assembly. This analysis may in particular include carrying out a Fourier transform of the measurement made and measuring the amplitude of the signal at the frequency (emf – fcap) where emf represents the frequency of the magnetic field along the measurement direction and f. caprepresents the frequency of the supply signal of the sensor which measures the magnetic field in this direction. Then, the same sensor of the magnetic field measuring assembly 7 acquires a signal, still for a typical duration of the order of 20 ms, for another value of t A . The currents supplying the coils are then different compared to the first signal acquisition and the position of the predetermined value of the magnetic field is also different. The value of the magnetic field at the measuring assembly is again determined. The process is repeated by varying the currents supplying the coils to change the position of the predetermined value of the magnetic field until the magnetic field measuring assembly measures this predetermined value. The position of the magnetic field measuring assembly along the X direction is then determined. Preferably, the choice of amplitudes I is optimized. p, i,jcurrents to accelerate the determination of the position of the measuring assembly. For example, the magnetic field value can first be measured at a limited number of points distributed between two coils, for example five points. From these measurements, the approximate position of the predetermined magnetic field value is estimated, for example by means of linear regression. The magnetic field value is then measured in a restricted interval around this estimated position. If necessary, the process can be repeated. Typically, two or three iterations are sufficient to obtain maximum positioning accuracy of the measuring assembly, i.e. a position whose accuracy is limited by the signal-to-noise ratio of the measuring assembly.The same steps are then carried out with the pairs of coils aligned along the second direction Y and the third direction Z to determine the position of the magnetic field measuring assembly in space. If reference measuring assemblies 8 are used, the same localization method is then implemented to determine their positions. Tracking the position of the reference measuring assemblies makes it possible to correct for any movement of the object in which the magnetic field measuring assembly 7 is arranged. The computer 2 is configured to control the device 1 and to reconstruct the position of the measuring assembly, for example in relation to a previously acquired anatomical image of the body. Other variants and improvements may be provided without departing from the scope of the invention.

Claims

Claims 1. Method for locating within a volume a magnetic field measuring assembly (7) comprising at least one magnetoresistive sensor and / or a Hall effect sensor, comprising the steps of: a / generating in the volume, by means of an electromagnetic field generating device (1), a magnetic field of variable amplitude and / or shape(s) so that the position of a predetermined value of the amplitude of the magnetic field varies along each of at least a first direction (X) and a second direction (Y), the magnetic field having respectively a first frequency and a second frequency along the first direction and the second direction,the first and second frequencies being different from each other; b / measuring the magnetic field along each of the directions at the frequency corresponding to each direction by means of the magnetic field measuring assembly; c / when the magnetic field measuring assembly detects the predetermined value of the magnetic field along each of the directions, determining the position of the magnetic field measuring assembly.

2. The location method according to claim 1, wherein the magnetic field generated in step a / is of variable amplitude and / or shape such that the position of a predetermined value of the amplitude of the magnetic field also varies along a third direction (Z), the magnetic field having a third frequency along the third direction, the first, second and third frequencies being different from each other.

3. The location method according to claim 2,the determination of the position of the magnetic field measuring assembly being carried out successively for each direction, the magnetic field being generated so that the position of the predetermined value varies successively along the first direction, along the second direction and then along the third direction.

4. A location method according to claim 2, the determination of the position of the magnetic field measuring assembly being carried out simultaneously for each direction, the magnetic field being generated so that the position of the predetermined value varies simultaneously along the first, second and third directions., 5. A localization method according to one of the preceding claims, the electromagnetic field generating device comprising three pairs of coils (5, 6, 15, 16, 25, 26), each pair being aligned along one of the three directions.

6. A localization method according to claim 1, the electromagnetic field generating device comprising a first pair of coils (30, 31) extending in a first plane and configured to generate a first variable magnetic field along the first direction, a second pair of coils (32, 33) extending in a second plane parallel to the first plane and configured to generate a second variable magnetic field along the second direction, and a fifth coil (34) extending in the second plane, arranged between the coils of the second pair of coils and configured to generate a magnetic field along a direction orthogonal to the first and second directions. 7.

8. Location method according to one of the preceding claims, the magnetic field measurement assembly comprising a first magnetoresistive sensor, preferably being a first GMR, TMR or AMR sensor extending in a plane.

9. Location method according to the preceding claim, the magnetic field measurement assembly comprising a third magnetoresistive sensor, preferably being a third GMR, TMR or AMR sensor extending in a plane orthogonal to the planes in which the first and second magnetoresistive sensors extend.A method of localization according to claim 7, the magnetic field measuring assembly comprising a Hall effect sensor extending parallel to the first magnetoresistive sensor.

11. A method of tracking an object, in particular a catheter in a human or animal body, comprising attaching a magnetic field measuring assembly to the object and locating the magnetic field measuring assembly by the localization method according to one of the preceding claims.

12. Monitoring method according to the preceding claim, comprising fixing one or more reference measurement sets (8) in a volume of interest and locating them by the locating method according to one of claims 1 to 10.

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