Method and system for determining an orientation and an X-coordinate of a moving object relative to a B0 field magnet

Abstract

A method for determining an orientation of a movable object (200) relative to a B0 field magnet (110) of a magnetic resonance imaging scanner (100) in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor (220) arranged on the object (200) in a fixed relative position, wherein the method comprises at least the following steps: providing (50) B0 reference data of the B0 field magnet (110) with characteristic magnetic field strengths for a plurality of XYZ coordinates; positioning (51) the object (200) with respect to a Z coordinate axis (10) at a first position (Z1) outside the B0 field magnet (110); providing (52) first measured values ​​of the magnetic field strength sensor (220) at the first position (Z1), wherein the first measured values ​​comprise at least field strength components of the B0 field;Determining (53) a first angle value (Yaw1) using the first measured values ​​of the magnetic field strength sensor (220) at the first position (Z1) and the B0 reference data; positioning (54) the object (200) with respect to the Z coordinate axis (10) at a second position (Z2) within the B0 field magnet (110); providing (55) second measured values ​​of the magnetic field strength sensor (220) at the second position (Z2), wherein the second measured values ​​comprise at least field strength components of the B0 field; determining (56) a second angle value (Yaw2) using the second measured values ​​of the magnetic field strength sensor (220) at the second position (Z2) and the B0 reference data; correcting (57) the first angle value (Yaw1) using the second angle value (Yaw2) and providing a corrected first angle value (Yaw1').

Description

[0001] The present invention relates to a method and a system for determining an orientation of a movable object relative to a B0 field magnet of a magnetic resonance imaging scanner in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor arranged on the object in a fixed relative position.

[0002] Magnetic resonance imaging scanners are imaging devices that, to create an image of a subject, align the nuclear spins of the subject with a strong external magnetic field and then excite them to precess around this alignment using an alternating magnetic field. The precession, or return, of the spins from this excited state to a lower-energy state, in turn generates a response alternating magnetic field, which is received via antennas.

[0003] Using magnetic gradient fields, a spatial coding is imprinted on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated, and a three-dimensional image of the object under examination is provided.

[0004] Spatial coding is typically based on an XYZ coordinate system. The Z coordinate axis is usually defined as an axis of symmetry of the B0 field magnet through a patient tunnel of the B0 field magnet in the preferred direction of the B0 field. In the usual setup of an MRI scanner, the Z coordinate axis is aligned horizontally and runs centrally through the opening of the windings of the B0 field magnet and through an acquisition area of ​​the B0 field magnet. The object to be acquired is usually placed on a patient couch in the patient tunnel parallel to the Z coordinate axis. Together with the Z coordinate axis, an X coordinate axis and a Y coordinate axis span a space. The coordinate axes are preferably orthogonal to one another, with the X coordinate axis being aligned horizontally and the Y coordinate axis vertically.

[0005] To receive the signal, local antennas, so-called local coils, are preferably used. These are arranged directly on the examination subject to achieve a better signal-to-noise ratio. Their position is therefore not fixed in relation to the rest of the magnetic resonance imaging system, in particular to the B0 field magnet, and must be recorded separately. In this regard, it is known, for example, to calculate the orientation of a local coil in the horizontal plane, i.e., in the XZ coordinate plane, using the trigonometric equation Yaw = atan2 (x-field strength components, z-field strength components). The field strength components are preferably recorded using a magnetic field strength sensor. The magnetic field strength sensor is arranged in a fixed relative position on the local coil.The orientation of the magnetic field strength sensor thus determined is then used to calculate the X-coordinate of the magnetic field strength sensor and thus of the local coil.

[0006] However, the actual orientation value of the local coil in the XZ coordinate plane can only be determined where the magnetic field lines run parallel to the Z coordinate axis. This is only the case within the patient tunnel, i.e., in the so-called isocenter of the B0 magnet, or outside the patient tunnel only directly on the Z coordinate axis. If the local coil is not positioned in the patient tunnel and offset from the Z coordinate axis, the orientation determined in this way and the X coordinate of the local coil in the XZ coordinate plane are subject to a systematic error. The orientation error increases the further away from the Z coordinate axis and the closer to the B0 magnet the magnetic field strength sensor is positioned.

[0007] From DE 10 2009 021 026 A1 a device is known for determining a position of a local coil for a magnetic resonance imaging device.

[0008] From DE 10 2016 203 255 A1 a method is known for determining a position of a movable device relative to a B0 field magnet by means of a magnetic field strength sensor.

[0009] In this context, it has become apparent that there is a need to provide a method and a system with which an orientation and thus also an X-coordinate of a local coil can be provided with higher accuracy, even if the local coil is arranged outside the patient tunnel and offset from the Z-coordinate axis.

[0010] It is therefore an object of the present invention to provide a solution with which an orientation and thus an X-coordinate of a local coil can be provided with higher accuracy, even if the local coil is arranged outside the patient tunnel and offset from the Z-coordinate axis.

[0011] These and other objects, which will be mentioned upon reading the following description or which may be recognized by a person skilled in the art, are achieved by the subject matter of the independent claims. The dependent claims develop the central idea of ​​the present invention in a particularly advantageous manner.

[0012] According to the invention, a method for determining an orientation of a movable object relative to a B0 field magnet of a magnetic resonance imaging scanner in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor arranged on the object in a fixed relative position is disclosed, wherein the method comprises at least the following steps: Providing B0 reference data of the B0 field magnet with characteristic magnetic field strengths for a variety of XYZ coordinates; Positioning the object with respect to a Z coordinate axis at a first position outside the B0 field magnet; Providing first measured values ​​of the magnetic field strength sensor at the first position, wherein the first measured values ​​comprise at least field strength components of the B0 field; Determining a first angle value using the first measured values ​​of the magnetic field strength sensor at the first position and the B0 reference data; Positioning the object with respect to the Z coordinate axis at a second position within the B0 field magnet; Providing second measured values ​​of the magnetic field strength sensor at the second position, wherein the second measured values ​​comprise at least field strength components of the B0 field; Determining a second angle value using the second measured values ​​of the magnetic field strength sensor at the second position and the B0 reference data; Correcting the first angle value using the second angle value and providing a corrected first angle value.

[0013] In other words, the present invention proposes, in a first step, determining the orientation of the object based on the field strength components of the B0 field at a first position. The first position is located outside the B0 field magnet. As explained above, the orientation of the object thus determined is subject to a systematic error if the object is not positioned on the Z coordinate axis. The X coordinate of the object derived from this erroneous orientation of the object therefore also exhibits a systematic error.

[0014] In a second step, the object's orientation is determined based on the field strength components of the B0 field at a second position. The second position is located within the B0 field magnet, i.e., at the isocenter of the B0 field magnet. The object's orientation determined here can then be used to correct the erroneous orientation of the object outside the B0 field magnet. Using the corrected object orientation, the actual X-coordinates of the object in the first position can then be determined.

[0015] Preferably, the angle values ​​in the first and second positions are determined using the following equation: Angle value (Yaw) = atan2 (x-field strength component, z-field strength component). The angle values ​​thus determined are preferably assigned to a 90° quadrant in the XZ coordinate plane. Such assignment is preferably performed using a modulo function.

[0016] Preferably, a three-dimensional acceleration sensor is arranged in a fixed relative connection to the object, wherein the acceleration sensor is configured to provide an orientation of the object to a Y coordinate axis in the first position and the second position, and wherein the measured values ​​of the magnetic field strength sensor are leveled to the XZ coordinate plane by means of the orientation of the object in the first position and the second position. In other words, the acceleration sensor makes it possible to create a rotation matrix to align the measured values ​​of the magnetic field strength sensor, more precisely the measured field vectors, as if the magnetic field strength sensor were located in the horizontal XZ coordinate plane.

[0017] Preferably, to provide the corrected first angle value, the first measured values, more precisely the field vectors that have been leveled to the XZ coordinate plane, are rotated about the Y coordinate axis by means of the second angle value in the XZ coordinate plane such that the field vector is arranged parallel to the X coordinate axis.

[0018] Preferably, corrected measured values ​​of the magnetic field strength sensor in the first position are provided by means of the corrected first angle value, wherein a corrected X-coordinate of the object in the first position is provided by comparing the corrected measured values ​​with the B0 reference data.

[0019] Preferably, the B0 field magnet encloses a patient tunnel of a magnetic resonance imaging scanner, wherein the Z coordinate axis is defined by an axis of symmetry of the B0 field magnet in the preferred direction of the B0 field, wherein the coordinate axes are preferably provided orthogonal to one another and wherein the X coordinate axis is preferably aligned horizontally and the Y coordinate axis is preferably aligned vertically.

[0020] Preferably, the magnetic field strength sensor is configured to detect a field strength of three components of the B0 field in three directions spanning a space, and the magnetic field strength sensor determines the magnetic field strength as the magnitude of a B0 field vector determined by the three components of the B0 field.

[0021] The present invention further relates to a system for determining an orientation of a movable object relative to a B0 field magnet in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor arranged on the object in a fixed relative position, the system comprising: a first interface configured to receive B0 reference data of the B0 field magnet having characteristic magnetic field strengths for a plurality of XYZ coordinates; a second interface configured to receive first measured values ​​of the magnetic field strength sensor at a first position, wherein the first measured values ​​comprise at least field strength components of the B0 field; a third interface configured to receive second measured values ​​of the magnetic field strength sensor at a second position, wherein the second measured values ​​comprise at least field strength components of the B0 field; a computing unit connected to the interfaces and configured to carry out the above-mentioned method.

[0022] Preferably, the system further comprises a three-dimensional acceleration sensor arranged in a fixed relative connection to the object, wherein the acceleration sensor is configured to provide an orientation of the object to a Y-coordinate axis in the first position and the second position, and wherein the measured values ​​of the magnetic field strength sensor are leveled to the XZ-coordinate plane by means of the orientation of the object in the first position and the second position.

[0023] Preferably, the system further comprises at least one storage means in which B0 reference data of the B0 field magnet with characteristic magnetic field strengths for a plurality of XYZ coordinates and / or a plurality of angle values ​​and X coordinates of the object in the first position and corresponding corrected angle values ​​in the first position are stored.

[0024] Furthermore, the present invention relates to a computer program element with instructions which, when executed on data processing devices of a data processing environment, are configured to carry out the steps of the above-mentioned method in an above-mentioned system.

[0025] All embodiments described herein can be combined with one another, unless explicitly stated otherwise. Further features, advantages, and possible applications of the present invention will become apparent from the following description, the exemplary embodiment, and the figures. In these figures: Fig. 1 shows a plan view of a system according to the invention, in the form of a magnetic resonance imaging scanner; Fig. 2 shows a side view of the Fig. 1 shown magnetic resonance imaging scanner; Fig. 3 shows a schematic representation of a method according to the invention for determining an orientation of a movable object relative to a B0 field magnet of a magnetic resonance imaging scanner; and Fig. Figure 4 shows a schematic representation of the magnetic resonance imaging system on the Fig. 1 and Fig. 2, with an object in a first position and a second position.

[0026] The Fig. 1 shows a top view and Fig. 2 shows a side view of a system 100 according to the invention, in the form of a magnetic resonance imaging device 100. The magnetic resonance imaging device 100 comprises, in particular, a B0 field magnet 110 and a patient bed 120.

[0027] As in the Fig. 1 and Fig. As shown in Figure 2, the spatial coding is based on an XYZ coordinate system. The Z coordinate axis 10 is defined, as usual, as a symmetry axis of the B0 field magnet 110 through a patient tunnel 130 of the B0 field magnet 100 in the preferred direction of the B0 field. In the typical setup of the magnetic resonance imaging system 100 shown, the Z coordinate axis 10 is oriented horizontally and runs centrally through the opening of the windings of the B0 field magnet 110 through the patient tunnel 130 of the B0 field magnet 110. The subject to be scanned is usually placed on the patient couch 120 into the patient tunnel 130 parallel to the Z coordinate axis 10. Together with the Z coordinate axis 10, an X coordinate axis 20 and a Y coordinate axis 30 span a space, wherein the XYZ coordinate axes are preferably provided orthogonal to one another and wherein the X coordinate axis is aligned horizontally and the Y coordinate axis is aligned vertically.

[0028] In Fig. 1 also shows the orientation of an object 200, here in the form of an exemplary local coil 200, in the XZ coordinate plane, which was calculated using the trigonometric equation Yaw = atan2 (x-field strength components, z-field strength components). The dashed representation of the local coil 200 represents the calculated orientation of the local coil 200, the representation of the local coil 200 shown with a solid line represents the actual orientation of the local coil 200. If the orientation determined according to the above equation is subsequently used to calculate the X-coordinate of the local coil 200, the resulting Fig. 1 illustrates the error in the X-coordinate determination. The dashed representation of the X-coordinate (see reference numeral 210) of the local coil 200 is based on the incorrectly determined orientation of the local coil 200; the X-coordinate shown in a solid line (see reference numeral 210') represents the actual X-coordinate of the local coil 200.

[0029] Fig. 3 shows a schematic representation of a method according to the invention for determining an orientation of a movable object relative to a B0 field magnet of a magnetic resonance imaging scanner in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor arranged on the object in a fixed relative position.

[0030] In a first step 50, B0 reference data of the B0 field magnet 110 with characteristic magnetic field strengths for a plurality of XYZ coordinates are provided. These can be used to determine the position or location of the magnetic field sensor 220.

[0031] In a further step 51, the object 200 is arranged with respect to the Z-coordinate axis 10 at a first position outside the B0 field magnet 110. Such a first position is, for example, in Fig. 4a shown.

[0032] In a further step 52, first measured values ​​of the magnetic field strength sensor 220 at the first position are provided or acquired, wherein the first measured values ​​comprise at least field strength components of the B0 field. In a further step 53, a first angle value Yaw1 is determined using the first measured values ​​of the magnetic field strength sensor 220 at the first position and the B0 reference data.

[0033] In a further step 54, the object 200 is positioned with respect to the Z-coordinate axis at a second position within the B0 field magnet 110. Such a second position is, for example, in Fig. 4b. In a further step 55, second measured values ​​of the magnetic field strength sensor 220 are recorded or provided at the second position, wherein the second measured values ​​in turn comprise at least field strength components of the B0 field at the second position. In a further step 56, a second angle value Yaw2 is determined using the second measured values ​​of the magnetic field strength sensor 220 at the second position and the B0 reference data. Finally, in a further step, the first angle value Yaw1 is corrected using the second angle value Yaw2, and a corrected first angle value Yaw1' is provided. Using this corrected angle value Yaw1', an actual X-coordinate of the object 200 in the first position can be provided by comparing it with the B0 reference data.

[0034] A particularly preferred embodiment of a method according to the invention is described below: In a first position Z1 of the object 200 (cf. Fig. 4a) and thus in a first position of the magnetic field sensor 220 and the acceleration sensor 230, the position of the object 200 is determined from the measured values ​​of the magnetic field sensor 220 and the acceleration sensor 230. The measured values ​​of the acceleration sensor are first used to level the B0 data measured by the magnetic field sensor 220, i.e., a rotation matrix is ​​created that rotates the field vectors of the magnetic field sensor 220 as if the magnetic field sensor 220 were located in the horizontal XZ coordinate plane.

[0035] Now the angle value Yaw1 for the first position Z1 is calculated as follows: Yaw1 = atan2 (Hall.X, Hall.Z) and applied for an alignment in the horizontal XZ coordinate plane.

[0036] As explained, however, it should be noted that the angle value Yaw1 does not correspond to the actual orientation of the magnetic field sensor 220 due to the curvature of the field lines at the first position Z1 (cf. Fig. 1). Therefore, even a rotation by this angle does not result in the X-coordinate axis of the magnetic field sensor being aligned parallel to the X-coordinate axis.

[0037] The angle Yaw1 can now be normalized by assigning the angle value to a 90° quadrant in the XZ coordinate plane. Preferably, the assignment is performed using a modulo function. This allows the angle value Yaw1 to be assigned to one of the angle ranges. This information can already be used to determine a rough orientation of object 200 and, if necessary, to provide the B0 data for the corresponding quadrant.

[0038] Now the object 200 and thus the magnetic field sensor 220 and the acceleration sensor 230 are moved to the second position Z2 (cf. Fig. 4b), for example by moving the patient couch 120 into the patient tunnel 130. In the second position Z2, the above steps and an angle value Yaw2 for the second position Z2 are determined.

[0039] The second angle value Yaw2 is significantly more accurate than the first angle value Yaw1, since the field lines in the patient tunnel, i.e., in the isocenter, run parallel to the Z coordinate axis 10. The imported B0 reference data thus correspond exactly to the actual orientation of the object 200.

[0040] Using the second angle value Yaw2, the X-coordinate of object 200 in the first position can now be precisely determined. For this purpose, the first measured values, which were leveled to the XZ coordinate plane, are rotated around the Y-coordinate axis 30 in the XZ coordinate plane using the second angle value Yaw2, so that the field strength components of the measured values ​​are arranged parallel to the X-coordinate axis 20, corresponding to the X-coordinate axis. By comparing these corrected measured values ​​with the B0 reference data, the actual X-coordinate of object 200 in the first position Z1 can now be determined.

[0041] The present invention is not limited to the embodiment described above as long as it is encompassed by the subject matter of the following claims.

[0042] Additionally, it should be noted that the terms "comprising" and "having" do not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to the above embodiments may also be used in combination with other features.

[0043] Furthermore, it is pointed out that regardless of the grammatical gender of a particular term, persons with male, female or other gender identity are included.

Claims

[1] Method for determining an orientation of a movable object (200) relative to a B0 field magnet (110) of a magnetic resonance imaging device (100) in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor (220) arranged on the object (200) in a fixed relative position, the method comprising at least the following steps: Providing (50) B0 reference data of the B0 field magnet (110) with characteristic magnetic field strengths for a plurality of XYZ coordinates; Positioning (51) the object (200) with respect to a Z coordinate axis (10) at a first position (Z1) outside the B0 field magnet (110); Providing (52) first measured values ​​of the magnetic field strength sensor (220) at the first position (Z1), wherein the first measured values ​​comprise at least field strength components of the B0 field; Determining (53) a first angle value (Yaw1) using the first measured values ​​of the magnetic field strength sensor (220) at the first position (Z1) and the B0 reference data; Positioning (54) the object (200) with respect to the Z coordinate axis (10) at a second position (Z2) within the B0 field magnet (110); Providing (55) second measured values ​​of the magnetic field strength sensor (220) at the second position (Z2), wherein the second measured values ​​comprise at least field strength components of the B0 field; Determining (56) a second angle value (Yaw2) using the second measured values ​​of the magnetic field strength sensor (220) at the second position (Z2) and the B0 reference data; Correcting (57) the first angle value (Yaw1) by means of the second angle value (Yaw2) and providing a corrected first angle value (Yaw1'), wherein, in order to provide the corrected first angle value (Yaw1'), the first measured values ​​which were levelled to the XZ coordinate plane are rotated about the Y coordinate axis (30) by means of the second angle value (Yaw2) in the XZ coordinate plane. [2] Method according to claim 1, wherein the angle values ​​(Yaw1, Yaw2) are determined by applying the following equation: angle value (Yaw) = atan2 (x-field strength components, z-field strength components). [3] Method according to claim 2, wherein the angle value is assigned to a 90° quadrant in the XZ coordinate plane, preferably the assignment is carried out by means of a modulo function. [4] Method according to one of the preceding claims, wherein a three-dimensional acceleration sensor (230) is further arranged in a fixed relative connection to the object (200), wherein the acceleration sensor (230) is configured to provide an orientation of the object (200) to a Y-coordinate axis (30) in the first position (Z1) and the second position (Z2), and wherein by means of the orientation of the object (200) in the first position (Z1) and the second position (Z2) the measured values ​​of the magnetic field strength sensor (220) are leveled to the XZ-coordinate plane. [5] Method according to one of the preceding claims, wherein, in order to provide the corrected first angle value (Yaw1'), the first measured values ​​which have been levelled to the XZ coordinate plane are rotated about the Y coordinate axis (30) in the XZ coordinate plane by means of the second angle value (Yaw2) in such a way that the field strength component of the measured values ​​to the X coordinate axis is arranged parallel to the X coordinate axis (20). [6] Method according to one of the preceding claims, wherein corrected measured values ​​of the magnetic field strength sensor (220) in the first position (Z1) are provided by means of the corrected first angle value (Yaw1'), and an X-coordinate of the object (200) in the first position (Z1) is provided by comparing the corrected measured values ​​with the B0 reference data. [7] Method according to one of the preceding claims, wherein the B0 field magnet (110) encloses a patient tunnel (130) of the magnetic resonance imaging device (100), wherein the Z coordinate axis (10) is defined by an axis of symmetry of the B0 field magnet (110) in the preferred direction of the B0 field, wherein the coordinate axes are preferably provided orthogonal to one another and the X coordinate axis (20) is preferably aligned horizontally and the Y coordinate axis (30) is preferably aligned vertically. [8] Method according to one of the preceding claims, wherein the magnetic field strength sensor (220) is arranged to detect a field strength of three components of the B0 field in three directions spanning a space, and the magnetic field strength sensor (220) determines the magnetic field strength as the magnitude of a B0 field vector determined by the three components of the B0 field. [9] Method according to one of the preceding claims, wherein the object (200) is a local coil (200). [10] System (100) for determining an orientation of a movable object (200) relative to a B0 field magnet (110) of a magnetic resonance imaging device (100) in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor (220) arranged on the object (200) in a fixed relative position, the system comprising: a first interface configured to receive B0 reference data of the B0 field magnet (110) having characteristic magnetic field strengths for a plurality of XYZ coordinates; a second interface configured to receive first measured values ​​of the magnetic field strength sensor (220) at a first position (Z1), wherein the first measured values ​​comprise at least field strength components of the B0 field; a third interface configured to receive second measured values ​​of the magnetic field strength sensor (220) at a second position (Z2), wherein the second measured values ​​comprise at least field strength components of the B0 field; a computer unit connected to the interfaces and configured to carry out the method according to one of claims 1 to 9. [11] The system of claim 10, further comprising a three-dimensional acceleration sensor (230) arranged in fixed relative connection to the object (200), wherein the acceleration sensor (230) is configured to provide an orientation of the object (200) to a Y-coordinate axis (30) in the first position (Z1) and the second position (Z2), and wherein the measured values ​​of the magnetic field strength sensor (220) are leveled to the XZ-coordinate plane by means of the orientation of the object (200) in the first position (Z1) and the second position (Z2). [12] The system of claim 10 or claim 11, wherein the system further comprises at least one storage means in which B0 reference data of the B0 field magnet (110) having characteristic magnetic field strengths for a plurality of XYZ coordinates and / or a plurality of angle values ​​(Yaw1) in the first position and corresponding corrected angle values ​​(Yaw1') in the first position are stored. [13] A computer program element comprising instructions adapted, when executed on data processing devices of a data processing environment, to carry out the steps of the method according to any one of claims 1 to 9 in a system according to any one of claims 10 to 12.

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