Sensor system for detecting a medium

Abstract

The invention relates to a magnetic field sensor arrangement (1) for detecting a cardiac magnetic field caused by excitation of the Tawara bundle branches (8, 9) of the heart, comprising at least three magnetic field sensors (2, 3, 4). A first magnetic field sensor (2) of the at least three magnetic field sensors (2, 3, 4) is arranged on a first, imaginary circular line (17), and a second and third magnetic field sensor (3, 4) of the at least three magnetic field sensors (2, 3, 4) are arranged on a second, imaginary circular line (18), wherein the first circular line (13) delimits a first circle, the second circular line (7) delimits a second circle concentric with the first circle, and a first radius (17) of the first circle is smaller than a second radius (18) of the second circle.

Description

[0001] The present invention relates to a magnetic field sensor arrangement for detecting a cardiac magnetic field caused by excitation of the Tawara bundle branches and / or the Purkinje fibers of the heart, as well as a method for operating such a magnetic field sensor arrangement. State of the art

[0002] To date, numerous arrangements of magnetic field sensors for measuring biomagnetic signals have been described. These include, for example, gradiometers for magnetoencephalography (MEG) or magnetocardiography (MCG). These typically use SQUID-based systems, but their arrangement is spatially fixed. In contrast, magnetic field sensor arrangements based on optically pumped magnetic field sensors allow adaptation to the external physiognomy, for example, of the skull and the thorax. However, previous gradiometer systems for thorax diagnostics only partially exploit the radial symmetry of the cardiac magnetic field during the excitation phase of the ventricles.

[0003] Conventional magnetic field sensors, such as SQUID-based magnetic field sensors, have edge lengths significantly greater than 1 cm, thus limiting the ability to exploit the underlying anatomy to increase the signal-to-noise ratio. Furthermore, conventional SQUID magnetic field sensors are sensitive only in the xy direction, i.e., within the coronal plane, or in the z direction. Optically pumped magnetic field sensors also only provide a maximum of two-dimensional information, and their directional dependence is significantly limited.

[0004] Magnetic field sensors based on NV diamonds have been the subject of research for several years. These are based on diamond crystals whose crystal lattices contain defects in the form of NV centers. In an NV center, a nitrogen atom occupies the lattice site of a carbon atom, with a defect located directly adjacent to the nitrogen atom - again on the lattice site of a carbon atom. If such a crystal lattice is irradiated with excitation radiation with a wavelength between 490 nm and 575 nm, an electronic transition from a ground state 3 A2 into an excited state 3 E induced. From the excited state 3 E relaxes the NV center back to the ground state by emitting fluorescence radiation in a wavelength range between 650 nm and 750 nm 3 A2.

[0005] The ground state 3A2 has three magnetic substates with m s =0, m s =±1. The states with m s =0 and m s =±1 are distinguished by an energy difference of 2.87 GHz (zero field splitting). The excited state 3 E also has three magnetic substates with m s =0, m s =±1. If the NV center in the ground state 3 A2 exposed to microwave radiation with a frequency of 2.87 GHz, the NV center oscillates between the m s =0, 3 A2 - Ground state and the m s =±1, 3 A2 -ground state. Upon irradiation with the excitation radiation, the NV center is now partially removed from the m s =±1, 3 A2 ground state in the excited m s =±1, 3E state. From there, it relaxes back to the ground state, predominantly without radiation. If the intensity of the fluorescence radiation is measured as a function of the frequency of the microwave radiation, a sudden drop in the intensity of the fluorescence radiation (a so-called peak) occurs at a frequency of 2.87 GHz. The drop in the intensity of the fluorescence radiation can be explained by the fact that - when microwave radiation with a frequency of 2.87 GHz is applied - fewer NV centers in the m s =0, 3 A2 ground state are available, which are optically excited and emit fluorescence radiation into the m s =±1, 3 A2 ground state can relax.

[0006] In an external magnetic field, the m s =±1, 3 A2 -ground state into two states with spin quantum number m s =1 and m s=-1 (Zeemann effect). If the intensity of the fluorescence radiation is now measured while changing the frequency of the microwave radiation, two peaks are obtained. The frequencies at which these peaks occur depend on the size of the splitting of the m s =±1, 3 A2 ground state and thus on the magnetic flux density of the external magnetic field.

[0007] NV magnetic field sensors are vector-valued and have a compact design.

[0008] It is an object of the invention to provide a magnetic field sensor arrangement for detecting a magnetic field caused by excitation of the bundle branches of Tawara and / or the Purkinje fibers of the heart, which has an improved signal-to-noise ratio. Disclosure of the invention

[0009] The present invention relates to a magnetic field sensor arrangement for detecting an excitation phase of the heart muscle according to claim 1, as well as a method for operating such a magnetic field sensor arrangement according to claim 10. Advantageous embodiments of the invention are the subject of the subclaims and the description.

[0010] The invention provides a magnetic field sensor arrangement for detecting a cardiac magnetic field evoked during an excitation phase of the heart. This excitation phase is, in particular, a period during early systole. Cardiac excitation typically begins with atrial excitation and then progresses via the atrioventricular node and the two bundle branches of Tawara to the apex of the heart. For the purposes of this application, it is assumed that the bundle branches of Tawara are so close to one another in the cardiac septum that the magnetic field lines of the cardiac magnetic field resulting from the excitation of the bundle branches of Tawara during the excitation phase of the bundle branches of Tawara are circular or approximately circular with respect to a central axis arranged between the two bundle branches of Tawara. The magnetic field sensor arrangement comprises at least three magnetic field sensors.A first magnetic field sensor of the at least three magnetic field sensors is arranged on a first, imaginary circular line that delimits a first circle. A second and third magnetic field sensor of the at least three magnetic field sensors are arranged - at different positions - on a second, imaginary circular line that delimits a second circle that is arranged concentrically to the first circle. A first radius of the first circle is smaller than a second radius of the second circle. Such a magnetic field sensor arrangement can now be arranged near the heart, so that the cardiac magnetic field caused by the excitation of the Tawara branches can be measured at the location of the first magnetic field sensor. The first magnetic field sensor is arranged in particular closer to the central axis and thus also closer to the Tawara branches than the second and third magnetic field sensors.Analogous to the second and third magnetic field sensors, additional magnetic field sensors can also be arranged along the second circular line. If the at least three magnetic field sensors are sensitive in only one direction or in two directions, i.e., if the at least three magnetic field sensors have one or two sensitivity directions, they are aligned along the first or second circular line such that one of their sensitivity directions points tangentially to the first or second circular line. The first and second circular lines correspond at least approximately to the field lines of the cardiac magnetic field.

[0011] The magnetic field sensor array with at least three magnetic field sensors forms a gradiometer. This utilizes the fact that the magnitude of the magnetic flux density drops sharply in the vicinity of the cardiac magnetic field. At the same time, the distance between the first magnetic field sensor and the Tawara limbs is, in most cases, small compared to the distance between the magnetic field sensor array and the interference source. It is further assumed that distant interference fields have a small gradient at the location of the magnetic field sensor array. In contrast, the cardiac magnetic field in the near field of the magnetic field sensor array has a large gradient.If the interference magnetic field does not have a gradient, a first difference between the magnitude of the magnetic flux density measured at the location of the first magnetic field sensor and the magnitude of the magnetic flux density measured at the location of the second magnetic field sensor is equal to a second difference between the magnitude of the magnetic flux density measured at the location of the first magnetic field sensor and the magnitude of the magnetic flux density measured at the location of the third magnetic field sensor. If the interference magnetic field has a gradient, even if a small one, the magnitude of the magnetic flux density measured at the location of the second magnetic field sensor differs from the magnetic flux density measured at the location of the third magnetic field sensor, and thus also the respective differences to the magnitude of the magnetic flux density measured by the first magnetic field sensor.The difference with the smallest magnitude yields the magnitude of the magnetic flux density at the location of the first magnetic field sensor—and thus of the cardiac magnetic field at that location—that most closely approximates the value that would be measured in the absence of interfering magnetic fields. Ideally, the magnetic field sensor whose measured values ​​yield this smallest difference is positioned on the same magnetic field line of the interfering field as the first magnetic field sensor, so that the first magnetic field sensor and this magnetic field sensor measure the same magnitude of the magnetic flux density of the interfering magnetic field. Such a magnetic field sensor arrangement exhibits a very favorable signal-to-noise ratio.

[0012] In a further development of the invention, a fourth magnetic field sensor is provided, which is arranged in a sensor plane defined by the positions of the first, second, and third magnetic field sensors. The provision of the fourth magnetic field sensor enables even more flexible detection of the cardiac magnetic field.

[0013] In a further development of the invention, the sensor plane is arranged perpendicular or approximately perpendicular to the central axis. The cardiac magnetic field detected by the magnetic field sensor arrangement is time-dependent, with the cardiac magnetic field running at least approximately in a plane perpendicular to the central axis at any point in time during the excitation phase under consideration. If the sensor plane is not arranged perpendicular to the central axis, the result is an arrangement in which the at least three magnetic field sensors are arranged offset from one another along the central axis. Due to the time dependence of the cardiac magnetic field, the magnetic field sensors, each arranged offset along the central axis, detect the cardiac magnetic field with a time delay. This can lead to measurement errors. In contrast, if the sensor plane runs perpendicular to the central axis, the first magnetic field sensor is arranged on a first field line of the cardiac magnetic field, which in particular corresponds to the first circular line.The second and third magnetic field sensors are arranged, in particular, on a second field line of the cardiac magnetic field emanating from the same location on the Tawara branches. The second field line corresponds, in particular, to the second circular line. The first and second field lines run concentrically or approximately concentrically to each other and can be traced back to the cardiac magnetic field emanating from the Tawara branches at the same time and location. A magnetic field sensor arrangement designed in this way achieves a very good signal-to-noise ratio.

[0014] The arrangement of the second and third magnetic field sensors on the same circular line allows the second and third magnetic field sensors to measure the same amount of the magnetic flux density of the cardiac magnetic field—albeit a small amount due to the comparatively large distance from the central axis. This prevents contributions to the magnetic flux density of the cardiac magnetic field from being attributed to the magnetic flux density of the interference magnetic field at the location of the second or third magnetic field sensors. A magnetic field sensor arrangement designed in this way exhibits a very good signal-to-noise ratio.

[0015] In a further development of the invention, the second and third magnetic field sensors are arranged outside a straight line passing through an intersection point between the central axis and the sensor plane and the position of the first magnetic field sensor. Typically, interfering magnetic fields are circular or elliptical in shape, so that arranging the second and / or third magnetic field sensors at an angle to the straight line through the first magnetic field sensor and the intersection point increases the probability that—for geometric reasons—the first and the second or third magnetic field sensors, which are not arranged on the straight line, are located on the same magnetic field line of the interfering magnetic field.

[0016] In particular, a fourth magnetic field sensor is arranged on the straight line through the intersection of the sensor plane with the central axis and the first magnetic field sensor, and on the second circular line. This allows the interference magnetic field to be detected at different positions and thus any existing, direction-dependent gradient of the interference magnetic field to be detected and taken into account, thus enabling a particularly favorable signal-to-noise ratio and thus a particularly accurate measurement of the cardiac magnetic field.

[0017] In a further development of the invention, the second and third magnetic field sensors are arranged symmetrically to the straight line. Arranging the magnetic field sensors symmetrically to the straight line enables the magnetic field sensor arrangement to measure the same magnetic flux densities in any first orientation and a second orientation rotated by 180° relative to the first orientation. This facilitates the use of the magnetic field sensor arrangement because a source of error—a user-incorrect orientation of the magnetic field sensor arrangement by 180°—is eliminated.

[0018] In a further development of the invention, at least one of the magnetic field sensors, in particular the first magnetic field sensor, is configured to determine the direction of the cardiac magnetic field. If the second, third, and / or fourth magnetic field sensors are also configured to determine the direction of the magnetic field, further conclusions can be drawn regarding the direction(s) of possible interfering magnetic fields. This makes it possible to detect possible cardiac anomalies.

[0019] In a further development of the invention, at least one of the magnetic field sensors, in particular the first magnetic field sensor, is designed as an NV magnetic field sensor. NV magnetometers have a very high spatial resolution. In particular, it can be provided that the first magnetic field sensor is designed as a vector magnetic field sensor, e.g., an NV magnetic field sensor, while the remaining magnetic field sensors arranged on the second circular line can only detect an amount of the magnetic flux density. In particular, the magnetic field sensors arranged on the second circular line are sensitive in only one or two directions and are aligned tangentially to the field lines of the cardiac magnetic field in (one of) these sensitivity directions. This enables a cost-effective, compact, and robust design of the magnetic field sensor arrangement, while the magnetic field sensor arrangement is simultaneously very sensitive.

[0020] In one development of the invention, the magnetic field sensor arrangement has an evaluation device which is set up to determine the first difference and the second difference, wherein the first difference results from the amount of the magnetic flux density measured by the first magnetic field sensor and the amount of the magnetic flux density measured by the second magnetic field sensor, and the second difference results from the amount of the magnetic flux density measured by the first magnetic field sensor and the amount of the magnetic flux density measured by the third magnetic field sensor. The evaluation device is set up, in particular, to compare the first and the second difference with one another. If further magnetic field sensors, such as e.g.If the fourth magnetic field sensor is provided, the evaluation device can be configured to calculate a third difference between the magnitude of the magnetic flux density measured by the first magnetic field sensor and the magnitude of the magnetic flux density measured by the fourth magnetic field sensor and to compare this third difference with the first and second differences. The provision of the evaluation device enables a very compact magnetic field sensor arrangement.

[0021] In a further development of the invention, the at least three magnetic field sensors are arranged on a common base plate. This ensures in a simple manner that the at least three magnetic field sensors are arranged in a single plane, enabling a simple design.

[0022] The invention also includes a method for operating the magnetic field sensor arrangement with one or more of the features mentioned.

[0023] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0024] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows a schematic representation of the human heart in a front view Fig. 2 shows a magnetic field sensor arrangement according to an embodiment of the invention; Fig. 3 shows a magnetic field sensor arrangement according to an embodiment of the invention, in relation to a human. Embodiments of the invention

[0025] Fig. Figure 1 shows a schematic representation of the human heart. Cardiac excitation typically begins with the excitation of the sinus node and the atria (14, 15) and then progresses via the atrioventricular node (16) and the right (8) and left bundle branches (9) of Tawara to the apex of the heart. The bundle branches (8, 9) of Tawara lie close together in the cardiac septum, so that a cardiac magnetic field resulting from the excitation of the bundle branches (8, 9) can be assumed to be circular or approximately circular, i.e., circular or approximately circular, around a central axis (6) running between the bundle branches (8, 9).

[0026] Now let us consider a plane (10) which is perpendicular to the central axis (6). In this plane (10) which is perpendicular to the central axis (6), the Fig. 2 shown magnetic field sensor arrangement (1). In Fig. 2 shows magnetic field lines (7, 12, 13) of the cardiac magnetic field generated at a specific time by the excitation of the Tawara branches (8, 9). A first magnetic field sensor (2) is arranged at a first distance (17) from the central axis (6). A second magnetic field sensor (3), a third magnetic field sensor (4), and a fourth magnetic field sensor (5) are arranged at a second distance (18). The fourth magnetic field sensor (5) is arranged on a straight line (19) that runs through the intersection point (22) of the central axis (6) with the plane (10) and the position of the first magnetic field sensor (2). The third magnetic field sensor (4) and the second magnetic field sensor (3) are arranged symmetrically to the straight line (19).

[0027] In Fig.3 shows a magnetic field sensor arrangement (1) with a total of five magnetic field sensors (2, 3, 4, 5, 20). The magnetic field sensor arrangement (1) comprises an evaluation device (21) that is operatively connected to each of the magnetic field sensors (2, 3, 4, 5, 20). The evaluation device (21) is configured to receive and evaluate signals from the respective magnetic field sensors (2, 3, 4, 5, 20). The signals can, for example, be a magnitude or a direction of a magnetic flux density. The evaluation device (21) can then be configured to calculate the respective differences between a magnitude of a magnetic flux density detected by the first magnetic field sensor (2) and the magnitudes of a magnetic flux density detected by the remaining magnetic field sensors (3, 4, 5, 20) and to compare these differences with one another.The smallest difference in magnitude then corresponds to the magnitude of the magnetic flux density of the cardiac magnetic field at the location of the first magnetic field sensor (2).

Claims

[1] Magnetic field sensor arrangement (1) for detecting a cardiac magnetic field which is caused by an excitation of the Tawara bundle branches (8, 9) and / or the Purkinje fibers of the heart, with at least three magnetic field sensors (2, 3, 4), wherein a first magnetic field sensor (2) of the at least three magnetic field sensors (2, 3, 4) is arranged on a first, imaginary circular line (13) and a second and third magnetic field sensor (3, 4) of the at least three magnetic field sensors (2, 3, 4) are arranged on a second, imaginary circular line (7), wherein the first circular line (13) delimits a first circle and the second circular line (7) delimits a second circle which is concentric with the first circle and a first radius (17) of the first circle is smaller than a second radius (18) of the second circle. [2] Magnetic field sensor arrangement (1) according to claim 1, wherein the at least three magnetic field sensors (2, 3, 4, 5) are arranged in a plane (10) perpendicular to an imaginary central axis (6) arranged between the Tawara legs (8, 9). [3] Magnetic field sensor arrangement (1) according to one of the preceding claims, with a fourth magnetic field sensor (5), wherein the first, second, third and fourth magnetic field sensors (2, 3, 4, 5) are arranged in a sensor plane (23) and the sensor plane (23) corresponds in particular to the plane (10). [4] Magnetic field sensor arrangement (1) according to one of the preceding claims, wherein the second and third magnetic field sensors (3, 4) are arranged outside a straight line (19) passing through the intersection point (22) between the sensor plane (23) and the central axis (6) and the position of the first magnetic field sensor (2). [5] Magnetic field sensor arrangement (1) according to claim 4, wherein the second and third magnetic field sensors (3, 4) are arranged symmetrically to the straight line (19). [6] Magnetic field sensor arrangement (1) according to one of the preceding claims, wherein at least one of the magnetic field sensors (2, 3, 4), in particular the first magnetic field sensor (2), is designed to determine a direction of the cardiac magnetic field. [7] Magnetic field sensor arrangement (1) according to one of the preceding claims, wherein at least one of the magnetic field sensors (2, 3, 4), in particular the first magnetic field sensor (2), is designed as an NV magnetic field sensor. [8] Magnetic field sensor arrangement (1) according to one of the preceding claims, comprising an evaluation device (21) which is configured to determine and compare a first difference between an amount of the magnetic flux density measured by the first magnetic field sensor (2) and an amount of the magnetic flux density measured by the second magnetic field sensor (3) and a second difference between the amount of the magnetic flux density measured by the first magnetic field sensor (2) and an amount of the magnetic flux density measured by the third magnetic field sensor (4). [9] Magnetic field sensor arrangement (1) according to one of the preceding claims, wherein the at least three magnetic field sensors (2, 3, 4) are arranged on a common base plate. [10] Method for operating a magnetic field sensor arrangement (1) according to one of claims 1 to 9.

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