Method and system for determining orientation and x-coordinate of movable object relative to B0 field magnet

Abstract

The invention relates to a method for determining the orientation of a movable object relative to a B0 field magnet of a magnetic resonance tomography scanner in an X-Z coordinate plane by means of a three-dimensional magnetic field strength sensor arranged at a fixed relative position on the movable object, comprising at least the following steps: providing B0 reference data; positioning the object at a first position outside the B0 field magnet with respect to the Z coordinate axis; providing a first measurement of the magnetic field strength sensor at the first location; ascertaining a first angle value by means of a first measured value of the magnetic field strength sensor at the first position and the B0 reference data; positioning the object at a second position within the B0 field magnet with respect to the Z coordinate axis; providing a second measurement of the magnetic field strength sensor at a second location; ascertaining a second angle value by means of a second measured value of the magnetic field strength sensor at the second position and the B0 reference data; the first angle value is corrected by means of the second angle value and a corrected first angle value is provided.

Description

Technical Field

[0001] The invention relates to a method and a system for determining the orientation of a movable object in an XZ coordinate plane relative to a B0 field magnet of a magnetic resonance tomograph using a three-dimensional magnetic field strength sensor arranged at a fixed relative position on the object. Background Art

[0002] A magnetic resonance tomography scanner is an imaging device that, to image the subject, aligns the subject's nuclear spins using a strong external magnetic field. An alternating magnetic field excites the nuclear spins to precess about the orientation. The precession of the spins from the excited state to a state with lower energy, or vice versa, in turn generates an alternating magnetic field that is received via an antenna.

[0003] The signals are subjected to a spatial encoding by means of gradient magnetic fields, which subsequently enables the received signals to be associated with volume elements. The received signals are then evaluated and a three-dimensional imaging representation of the examination object is provided.

[0004] Position encoding is typically based on an XYZ coordinate system. The Z coordinate axis is typically defined as the axis of symmetry of the B0 field magnet, extending through the patient tunnel of the B0 field magnet along the preferred direction of the B0 field. In a typical setup of a magnetic resonance tomograph, the Z coordinate axis is oriented horizontally and extends centrally through the opening of the B0 field magnet's windings through the B0 field magnet's receiving area. The recording object is typically placed on an examination bed in the patient tunnel parallel to the Z coordinate axis. Together with the Z coordinate axis, the X and Y coordinate axes define space, preferably with the axes arranged orthogonally to one another, with the X coordinate axis oriented horizontally and the Y coordinate axis oriented vertically.

[0005] To receive signals, local antennas, so-called local coils, are preferably used. These are positioned directly on the examination subject to achieve a better signal-to-noise ratio. Therefore, the position of these local antennas is not fixed relative to the rest of the magnetic resonance tomograph, particularly relative to the B0 field magnet, and must be determined individually. For example, it is known to calculate the orientation of the local coil in the horizontal plane, or the XZ coordinate plane, using the trigonometric equation Yaw = atan² (x-field strength component, z-field strength component). The field strength components are preferably detected using a magnetic field strength sensor. The magnetic field strength sensor is positioned at a fixed relative position to the local coil. The thus determined orientation of the magnetic field strength sensor is then used to calculate the X coordinate of the magnetic field strength sensor and, therefore, the X coordinate of the local coil.

[0006] However, this allows the actual orientation values ​​of the local coil in the XZ coordinate plane to be determined only at locations where the magnetic field lines run parallel to the Z coordinate axis. This is only true within the patient tunnel, i.e., at the so-called isocenter of the B0 magnet, or outside the patient tunnel, only directly on the Z coordinate axis. As long as the local coil is not positioned in the patient tunnel and is offset from the Z coordinate axis, the orientation and X coordinate of the local coil thus determined in the XZ coordinate plane are subject to systematic errors. The further the magnetic field strength sensor is positioned from the Z coordinate axis and the closer it is to the B0 magnet, the greater the orientation error.

[0007] In this context, it has been found that there is a need to provide methods and systems by means of which the orientation of the local coil and thus also its X coordinate can be determined with high accuracy even if the local coil is arranged outside the patient corridor and offset with respect to the Z coordinate axis. Summary of the Invention

[0008] The object of the present invention is therefore to provide a solution by which the orientation of the local coil and thus also its X coordinate can be determined with high accuracy even if the local coil is arranged outside the patient tunnel and offset to the Z coordinate axis.

[0009] This object and other objects which will still be mentioned or which can be recognized by a person skilled in the art upon reading the following description are achieved by the subject matter according to the invention.The embodiments further develop the central idea of ​​the invention in a particularly advantageous manner.

[0010] According to the present invention, a method for determining the orientation of a movable object in an XZ coordinate plane relative to a B0 field magnet of a magnetic resonance tomograph is disclosed by means of a three-dimensional magnetic field strength sensor arranged at a fixed relative position on the object, wherein the method comprises at least the following steps:

[0011] Providing B0 reference data of a B0 field magnet having representative magnetic field strengths for a plurality of XYZ coordinates;

[0012] Positioning the object at a first position outside the B0 field magnet about the Z coordinate axis;

[0013] Providing a first measurement value of the magnetic field strength sensor at a first position, wherein the first measurement value includes at least a field strength component of the B0 field;

[0014] Determining a first angle value by means of a first measurement value of the magnetic field intensity sensor at the first position and B0 reference data;

[0015] positioning the object at a second position within the B0 field magnet about the Z coordinate axis;

[0016] Providing a second measurement value of the magnetic field strength sensor at a second position, wherein the second measurement value includes at least a field strength component of the B0 field;

[0017] Determining a second angle value by means of a second measurement value of the magnetic field intensity sensor at the second position and the B0 reference data;

[0018] The first angle value is corrected with the aid of the second angle value, and the corrected first angle value is provided.

[0019] In other words, the present invention proposes that, in a first step, the orientation of the object is determined based on the field strength components of the B0 field at a first location. In this case, the first location is outside the B0 field magnet. As described above, if the object is not positioned on the Z coordinate axis, the thus determined orientation of the object is subject to systematic errors. Consequently, the object's X coordinate, derived from this erroneous orientation of the object, also has systematic errors.

[0020] In a second step, the orientation of the object is determined based on the field strength components of the B0 field at a second location. The second location is located within the B0 field magnet, i.e., at the isocenter of the B0 field magnet. The object's orientation determined in this manner can then be used to correct for errors in the object's orientation outside the B0 field magnet. The corrected orientation of the object can then be used to determine the actual X coordinate of the object at the first location.

[0021] The angle values ​​in the first and second positions are preferably determined by applying the following equation: Angle value (Yaw) = atan2 (x-field strength component, z-field strength component). The angle values ​​determined in this way are preferably associated with the 90° quadrant in the XZ coordinate plane. This association is preferably performed using a modulo function.

[0022] Preferably, the three-dimensional acceleration sensor is arranged in a fixed relative connection to the object, wherein the acceleration sensor is configured to provide the orientation of the object in the first position and the second position relative to the Y coordinate axis, and wherein the measured values ​​of the magnetic field strength sensor are leveled onto the XZ coordinate plane using 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 in order to orient the measured values ​​of the magnetic field strength sensor, more precisely the measured field vector, as if the magnetic field strength sensor were located in the horizontal XZ coordinate plane.

[0023] Preferably, in order to provide a corrected first angle value, the first measured value, or more precisely the field vector, leveled on the XZ coordinate plane is rotated about the Y coordinate axis in the XZ coordinate plane by means of a second angle value so that the field vector relative to the X coordinate axis is arranged parallel to the X coordinate axis.

[0024] Preferably, a corrected measurement value of the magnetic field strength sensor in the first position is provided by means of the corrected first angle value, and a corrected X coordinate of the object in the first position is provided by matching the corrected measurement value with the B0 reference data.

[0025] Preferably, the B0 field magnet surrounds a patient channel of the magnetic resonance tomography scanner, wherein the Z coordinate axis is defined by a symmetry axis of the B0 field magnet along a preferred direction of the B0 field, wherein the coordinate axes are preferably arranged orthogonally to each other, and wherein the X coordinate axis is preferably oriented horizontally and the Y coordinate axis is preferably oriented vertically.

[0026] Preferably, the magnetic field strength sensor is configured to detect the field strength of three components of the B0 field in three directions of the expanded space, and the magnetic field strength sensor determines the magnetic field strength as the absolute value of the B0 field vector determined by the three components of the B0 field.

[0027] The present invention also relates to a system for determining the orientation of a movable object in an XZ coordinate plane relative to a B0 field magnet by means of a three-dimensional magnetic field strength sensor arranged at a fixed relative position on the object, wherein the system comprises:

[0028] a first interface configured to receive B0 reference data of a B0 field magnet having representative magnetic field strengths for a plurality of XYZ coordinates;

[0029] a second interface configured to receive a first measurement value of the magnetic field strength sensor at a first position, wherein the first measurement value includes at least a field strength component of the B0 field;

[0030] a third interface configured to receive a second measurement value of the magnetic field strength sensor at a second position, wherein the second measurement value includes at least a field strength component of the B0 field;

[0031] A computer unit is connected to the interface and is configured to execute the above method.

[0032] Preferably, the system further comprises a three-dimensional acceleration sensor, which is arranged in a fixed relative connection to the object, wherein the acceleration sensor is configured to provide an orientation of the object in the first position and in the second position relative to the Y coordinate axis, and wherein the measured values ​​of the magnetic field strength sensor are leveled to the XZ coordinate plane with the aid of the orientation of the object in the first position and in the second position.

[0033] Preferably, the system further comprises at least one storage mechanism in which: B0 reference data of a B0 field magnet having representative magnetic field strengths for a plurality of XYZ coordinates; and / or a plurality of angle values ​​and X coordinates of the object in a first position; and corrected angle values ​​corresponding to the angle values ​​in the first position are stored.

[0034] Furthermore, the present invention relates to a computer program element having instructions which, when executed on a data processing device of a data processing environment, are configured to perform the steps of the above-described method in the above-described system. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Unless otherwise expressly stated, all embodiments described herein can be combined with one another. Further features, advantages and application possibilities of the present invention can be derived from the following description, examples and drawings. In the drawings:

[0036] Figure 1 A top view of a system according to the invention in the form of a magnetic resonance tomograph is shown;

[0037] Figure 2 Shown in Figure 1 A side view of a magnetic resonance tomography scanner is shown in FIG;

[0038] Figure 3 A schematic diagram showing a method for determining the orientation of a movable object relative to a B0 field magnet of a magnetic resonance tomograph according to the present invention; and

[0039] Figure 4 shows Figure 1 and Figure 2 Schematic diagram of a magnetic resonance tomography scanner with a subject in a first and a second position. DETAILED DESCRIPTION

[0040] Figure 1 A top view of a system 100 according to the invention in the form of a magnetic resonance tomograph 100 is shown, and Figure 2 The magnetic resonance tomograph 100 comprises, in particular, a B0 field magnet 110 and an examination table 120 .

[0041] As in Figure 1 and Figure 2As shown in FIG, the position encoding is based on an XYZ coordinate system. Here, the Z coordinate axis 10 is defined, as is conventional, as the axis of symmetry of the B0 field magnet 110, which extends through the patient channel 130 of the B0 field magnet 100 along the preferred direction of the B0 field. In the conventional arrangement of the magnetic resonance tomograph 100 shown, the Z coordinate axis 10 is oriented horizontally and extends centrally through the patient channel 130 of the B0 field magnet 110, through the opening of the windings of the B0 field magnet 110. The recording object is typically placed in the patient channel 130 on an examination bed 120 parallel to the Z coordinate axis 10. Together with the Z coordinate axis 10, the X coordinate axis 20 and the Y coordinate axis 30 define space, with the XYZ coordinate axes preferably being arranged orthogonally to one another, with the X coordinate axis being oriented horizontally and the Y coordinate axis being oriented vertically.

[0042] exist Figure 1 Also shown in FIG. 1 is the orientation of the object 200, here in the form of an exemplary local coil 200, in the XZ coordinate plane, which is calculated using the trigonometric equation Yaw=atan2(x-field strength component, z-field strength component). The dashed line representation of the local coil 200 here represents the calculated orientation of the local coil 200, while the solid line representation of the local coil 200 represents the actual orientation of the local coil 200. If the orientation determined according to the above equation is then used to calculate the X coordinate of the local coil 200, then Figure 1 The error in the X-coordinate determination is shown in FIG. The dashed illustration of the X-coordinate of the local coil 200 (see reference numeral 210 ) is based on an incorrectly determined orientation of the local coil 200 , while the X-coordinate shown with a solid line (see reference numeral 210 ′) represents the actual X-coordinate of the local coil 200 .

[0043] Figure 3 A schematic diagram shows a method according to the invention for determining the orientation of a movable object in an XZ coordinate plane relative to a B0 field magnet of a magnetic resonance tomograph using a three-dimensional magnetic field strength sensor arranged at a fixed relative position on the object.

[0044] In a first step 50 , B0 reference data of the B0 field magnet 110 with representative magnetic field strengths for a plurality of XYZ coordinates are provided. These B0 reference data can be used when determining the position or location of the magnetic field sensor 220 .

[0045] In a further step 51, the object 200 is positioned at a first position outside the B0 field magnet 110 with respect to the Z coordinate axis 10. This first position is for example Figure 4a Shown in.

[0046] In a further step 52, a first measurement value of magnetic field strength sensor 220 is provided or detected at a first position, wherein the first measurement value includes at least a field strength component of the B0 field. In a further step 53, a first angle value Yaw1 is determined using the first measurement value of magnetic field strength sensor 220 at the first position and the B0 reference data.

[0047] In a further step 54, the object 200 is positioned at a second position within the B0 field magnet 110 with respect to the Z coordinate axis. This second position is for example Figure 4b . In a further step 55, a second measurement value of magnetic field strength sensor 220 is detected or provided at the second location, wherein the second measurement value again includes at least the field strength component of the B0 field at the second location. In a further step 56, a second angle value Yaw2 is determined using the second measurement value of magnetic field strength sensor 220 at the second location 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. The corrected angle value Yaw1' can be used to provide the actual X coordinate of object 200 at the first location by matching it with the B0 reference data.

[0048] A particularly preferred embodiment of the method according to the present invention is described below:

[0049] In the first position Z1 of the object 200 (see Figure 4a ), so that in a first position of magnetic field sensor 220 and acceleration sensor 230, the position of object 200 is determined from the measured values ​​of magnetic field sensor 220 and acceleration sensor 230. The measured values ​​of the acceleration sensor are first used to level the B0 data measured by magnetic field sensor 220, i.e., a rotation matrix is ​​created that rotates the field vector of magnetic field sensor 220 as if magnetic field sensor 220 were in a horizontal XZ coordinate plane.

[0050] The angle value Yaw1 for the first position Z1 is now calculated as follows: Yaw1 = atan2 (Hall.X, Hall.Z) and is used for orientation in the horizontal XZ coordinate plane.

[0051] However, as explained, it should be noted that in this case, due to the curvature of the field lines at the first position Z1, the angle value Yaw1 does not correspond to the actual orientation of the magnetic field sensor 220 (see Figure 1 ). Consequently, a rotation by this angle also does not result in the X-coordinate axis of the magnetic field sensor being oriented parallel to the X-coordinate axis.

[0052] Angle Yaw1 can now be normalized by associating the angle value with a 90° quadrant in the XZ coordinate plane, preferably using a modulo function. This allows the angle value Yaw1 to be associated with one of the angle ranges. This information can already be used to determine the rough orientation of object 200 and optionally provide B0 data for the corresponding quadrant.

[0053] The object 200 and thus the magnetic field sensor 220 and the acceleration sensor 230 are now placed in the second position Z2 (see Figure 4b ), for example, by moving the examination bed 120 into the patient access 130. The above steps are carried out in the second position Z2 and the angle value Yaw2 for the second position Z2 is determined.

[0054] The second angle value Yaw2 is significantly more accurate than the first angle value Yaw1 because the field lines in the patient channel, ie in the isocenter, run parallel to the Z coordinate axis 10 . The read-in B0 reference data therefore corresponds exactly to the actual orientation of the object 200 .

[0055] The X coordinate of object 200 in the first position can now be precisely determined using second angle value Yaw2. To this end, the first measured value, which is leveled on the XZ coordinate plane, is rotated about Y coordinate axis 30 in the XZ coordinate plane using second angle value Yaw2 so that the field strength component of the measured value corresponding to the X coordinate axis is arranged parallel to X coordinate axis 20. The actual X coordinate of object 200 in first position Z1 can now be determined by matching these corrected measured values ​​with the B0 reference data.

[0056] The present invention is not limited to the embodiments described above as long as they are encompassed by the subject matter according to the present invention.

[0057] In addition, it should be pointed out that the term "comprising" or "having" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. It should also be pointed out that the features or steps described with reference to the above embodiments can also be used in combination with other features.

[0058] Furthermore, it is noted that regardless of the grammatical gender of a particular term, persons having both male and female identities are included.

Claims

1. A method for determining the orientation of a movable object (200) relative to a B0 field magnet (110) of a magnetic resonance tomograph (100) in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor (220), the magnetic field strength sensor (220) being arranged at a fixed relative position on the object (200), the method comprising at least the following steps: providing (50) B0 reference data of the B0 field magnet (110) having representative magnetic field strengths for a plurality of XYZ coordinates; Positioning (51) the object (200) at a first position (Z1) outside the B0 field magnet (110) about a Z coordinate axis (10); Providing (52) a first measurement value of the magnetic field intensity sensor (220) at the first position (Z1), wherein the first measurement value includes at least a field intensity component of the B0 field; Determining (53) a first angle value (Yaw1) by means of a first measurement value of the magnetic field intensity sensor (220) at the first position (Z1) and the B0 reference data; positioning (54) the object (200) at a second position (Z2) within the B0 field magnet (110) about the Z coordinate axis (10); Providing (55) a second measurement value of the magnetic field intensity sensor (220) at the second position (Z2), wherein the second measurement value includes at least a field intensity component of the B0 field; Determining (56) a second angle value (Yaw2) by means of a second measurement value of the magnetic field intensity sensor (220) at the second position (Z2) and the B0 reference data; The first angle value (Yaw1) is corrected (57) by means of the second angle value (Yaw2) and a corrected first angle value (Yaw1') is provided.

2. The method according to claim 1, wherein the angle value (Yaw1, Yaw2) is obtained by applying the following equation: Angle value (Yaw) = atan2 (x-field strength component, z-field strength component). 3 . The method according to claim 2 , wherein the angle value is associated with a 90° quadrant in the XZ coordinate plane, preferably by means of a modulo function.

4. A method according to any of the preceding claims, wherein a three-dimensional acceleration sensor (230) is also arranged at the object (200) in a fixed relative connection, wherein the acceleration sensor (230) is configured to provide the orientation of the object (200) relative to the 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 with the aid of the orientation of the object (200) in the first position (Z1) and the second position (Z2).

5. A method according to any one of the preceding claims, wherein, in order to provide the corrected first angle value (Yaw1'), the first measurement value leveled on the XZ coordinate plane is rotated around the Y coordinate axis (30) in the XZ coordinate plane by means of the second angle value (Yaw2).

6. A method according to any one of the above claims, wherein, in order to provide the corrected first angle value (Yaw1'), the first measured value leveled on the XZ coordinate plane is rotated around the Y coordinate axis (30) in the XZ coordinate plane by means of the second angle value (Yaw2) so that the field strength component of the measured value with respect to the X coordinate axis is arranged parallel to the X coordinate axis (20).

7. The method according to claim 1 , wherein a corrected measurement value of the magnetic field strength sensor ( 220 ) in the first position ( Z1 ) is 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 matching the corrected measurement value with the B0 reference data.

8. A method according to any of the above claims, wherein the B0 field magnet (110) surrounds a patient channel (130) of the magnetic resonance tomography scanner (100), wherein the Z coordinate axis (10) is defined by an axis of symmetry of the B0 field magnet (110) along a preferred direction of the B0 field, wherein the coordinate axes are preferably arranged orthogonally to each other, and the X coordinate axis (20) is preferably oriented horizontally, and the Y coordinate axis (30) is preferably oriented vertically.

9. A method according to any one of the above claims, wherein the magnetic field strength sensor (220) is configured to detect the field strength of three components of the B0 field in three directions in the expanded space, and the magnetic field strength sensor (220) determines the magnetic field strength as the absolute value of the B0 field vector determined by the three components of the B0 field.

10. The method according to any of the preceding claims, wherein the object (200) is a local coil (200).

11. A system (100) for determining the orientation of a movable object (200) relative to a B0 field magnet (110) of a magnetic resonance tomography scanner (100) in an XZ coordinate plane by means of a three-dimensional magnetic field strength sensor (220), the magnetic field strength sensor (220) being arranged at a fixed relative position on the object (200), the system comprising: a first interface configured to receive B0 reference data of the B0 field magnet (110) having representative magnetic field strengths for a plurality of XYZ coordinates; a second interface configured to receive a first measurement value of the magnetic field intensity sensor (220) at a first position (Z1), wherein the first measurement value comprises at least a field intensity component of the B0 field; a third interface configured to receive a second measurement value of the magnetic field intensity sensor (220) at a second position (Z2), wherein the second measurement value includes at least a field intensity component of the B0 field; A computer unit is connected to the interface and is configured to execute the method according to any one of claims 1 to 10.

12. The system according to claim 11 further comprises a three-dimensional acceleration sensor (230), which is arranged at the object (200) in a fixed relative connection, wherein the acceleration sensor (230) is configured to provide the orientation of the object (200) relative to the Y coordinate axis (30) in the first position (Z1) and the second position (Z2), and wherein the measurement values ​​of the magnetic field strength sensor (220) are leveled to the XZ coordinate plane with the help of the orientation of the object (200) in the first position (Z1) and the second position (Z2).

13. A system according to claim 11 or 12, wherein the system further comprises at least one storage device in which B0 reference data of the B0 field magnet (110) having representative 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.

14. A computer program element comprising instructions configured to, when executed on a data processing device of a data processing environment, perform the steps of the method according to any one of claims 1 to 10 in a system according to any one of claims 11 to 13.

Images