Method and Device for Position Determination in a Magnetic Resonance Tomography Unit

Abstract

A method for determining a position of a mobile device relative to a B0 field magnet along a z-coordinate axis, and a device and a magnetic resonance tomography unit for performing the method are provided. The device includes a magnetic field strength sensor arranged in a fixed relative position. A characteristic magnetic field strength B.sub.ref of the B0 field magnet that emerges for a plurality of x-y coordinate pairs with a same reference z-coordinate z.sub.ref is ascertained. The device is moved along the z-coordinate axis until the magnetic field strength sensor measures the characteristic magnetic field strength B.sub.ref.

Claims

1. A method for determining a position of a mobile device relative to a B0 field magnet along a z-coordinate axis that is defined by a symmetry axis of the B0 field magnet in a preferred direction of a B0 field, by a magnetic field strength sensor arranged in a fixed relative position on the mobile device, the method comprising: ascertaining a characteristic magnetic field strength B.sub.ref of the B0 field magnet that emerges for a plurality of x-y coordinate pairs at different distances from the z-coordinate axis having essentially a same reference z-coordinate z.sub.ref, wherein the characteristic magnetic field strength B.sub.ref lies in a range of between 20% and 80% of a magnetic field strength of the B0 field magnet at an isocenter; and moving the mobile device along the z-coordinate axis until the magnetic field strength sensor measures the characteristic magnetic field strength B.sub.ref.

2. The method of claim 1, wherein the mobile device is moved along the z-coordinate axis by a predetermined distance by a position determination unit relative to the B0 field magnet.

3. The method of claim 1, wherein the magnetic field strength sensor includes three sensor elements that record a field strength of three components of the B0 field in three directions that span an area, and the magnetic field strength sensor ascertains the magnetic field strength as an amount of a B0-field vector determined by three components of the B0 field.

4. The method of claim 1, further comprising: ascertaining B.sub.xy-z curves of a dependence of the magnetic field strength on a z-coordinate for a majority of x-y coordinate pairs, wherein the x-y-coordinate pairs indicate points at different distances from the z-coordinate axis; recording a majority of magnetic field strength values using the magnetic field strength sensor for different positions along the z-coordinate axis with constant x-y coordinates; selecting a B.sub.xy-z curve with the aid of the ascertained magnetic field strength values by an error minimization method; and determining the z-coordinate using the selected B.sub.xy-z curve and a measured magnetic field strength values.

5. The method of claim 4, wherein the magnetic field strength sensor measures the characteristic magnetic field strength B.sub.ref when the measured magnetic field strength value differs by less than 10 percent from the characteristic magnetic field strength B.sub.ref.

6. The method of claim 4, wherein the mobile device also includes an orientation sensor in predetermined alignment with the magnetic field strength sensor, wherein the orientation sensor is configured to ascertain a relative alignment to the B0 field magnet, and wherein the method further comprises: ascertaining a relative alignment of the magnetic field strength sensor to the B0 field magnet using the orientation sensor; and ascertaining an x-y coordinate pair as a function of the selected B.sub.xy-z curve, the ascertained z-coordinate, and the ascertained relative alignment.

7. The method of claim 1, wherein the B0-field magnet encloses a patient tunnel, and the reference z-coordinate z.sub.ref is arranged with regard to the z-coordinate axis outside the patient tunnel.

8. The method of claim 1, wherein the mobile device is a local coil.

9. The method of claim 1, wherein the mobile device includes a shim coil, and wherein the method further comprises setting a current through the shim coil as a function of a magnetic field strength value recorded by the magnetic field strength sensor.

10. The method of claim 1, further comprising correcting a my-Map in MR-PET as a function of a magnetic field strength value recorded by the magnetic field strength sensor.

11. The method of claim 2, wherein the magnetic field strength sensor includes three sensor elements that record a field strength of three components of the B0 field in three directions that span an area, and the magnetic field strength sensor ascertains the magnetic field strength as an amount of a B0-field vector determined by three components of the B0 field.

12. The method of claim 11, further comprising: ascertaining B.sub.xy-z curves of a dependence of the magnetic field strength on a z-coordinate for a majority of x-y coordinate pairs, wherein the x-y-coordinate pairs indicate points at different distances from the z-coordinate axis; recording a majority of magnetic field strength values using the magnetic field strength sensor for different positions along the z-coordinate axis with constant x-y coordinates; selecting a B.sub.xy-z curve with the aid of the ascertained magnetic field strength values by an error minimization method; and determining the z-coordinate using the selected B.sub.xy-z curve and a measured magnetic field strength values.

13. The method of claim 12, wherein the magnetic field strength sensor measures the characteristic magnetic field strength B.sub.ref when the measured magnetic field strength value differs by less than 10 percent from the characteristic magnetic field strength B.sub.ref.

14. A local coil comprising: a magnetic field strength sensor and an orientation sensor that are arranged on the local coil, wherein the magnetic field strength sensor is configured to record an amount and a direction of a magnetic field vector, and wherein the orientation sensor is configured to record a relative alignment of the orientation sensor to a predetermined spatial direction.

15. A shim coil comprising: a magnetic field strength sensor and an orientation sensor that are arranged on the shim coil, wherein the magnetic field strength sensor is configured to record an amount and a direction of a magnetic field vector; and wherein the orientation sensor is configured to record a relative alignment of the orientation sensor to a predetermined spatial direction.

16. A magnetic resonance tomography unit comprising: a B0 field magnet; a device; a mobile patient couch; a controller; and a storage unit, wherein a magnetic field strength sensor is arranged on the device that is in signal connection with the controller, wherein the patient couch is configured, parallel to a z-coordinate axis that is defined by a symmetry axis of the B0 field magnet in a preferred direction of a B0 field, to be moved into a scanning area of the B0 field magnet by the controller, wherein a characteristic magnetic field strength B.sub.ref of the B0 field magnet in a range of between 20% and 80% of magnetic field strength of the B0 field magnet at an isocenter is stored in the storage unit, and wherein the controller is configured to determine a reference z-coordinate z.sub.ref by moving the patient couch with the device parallel to the z-coordinate axis relative to the B0 field magnet and magnetic field strength values being recorded by the magnetic field strength sensor via a signal line and compared with the characteristic magnetic field strength B.sub.ref until a difference between the recorded magnetic field strength values and the characteristic magnetic field strength B.sub.ref is smaller than a predetermined tolerance value.

17. The magnetic resonance tomography unit of claim 16, wherein the controller is also configured to provide B.sub.xy-z curves of a dependence of the magnetic field strength on the z-coordinate for a majority of x-y coordinate pairs, wherein the x-y coordinate pairs indicate points at different distances from the z-coordinate axis, wherein the controller is configured to: record magnetic field strength values by the magnetic field strength sensor for different positions along the z-coordinate axis with constant x-y coordinates; select a B.sub.xy-z curve with the aid of the ascertained magnetic field strength values using an error minimization method; and determine a z-coordinate using the selected B.sub.xy-z curve and a measured magnetic field strength value.

18. The magnetic resonance tomography unit of claim 17, wherein the device also includes an orientation sensor in predetermined alignment with the magnetic field strength sensor, wherein the orientation sensor is configured to ascertain a relative alignment with the B0 field magnet, and wherein the controller is configured to: ascertain a relative alignment of the magnetic field strength sensor with the B0 field magnet using the orientation sensor; and ascertain an x-y coordinate pair as a function of the selected B.sub.xy-z curve, the ascertained z-coordinate, and the ascertained relative alignment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 shows an exemplary diagrammatic view of a magnetic resonance tomography unit with a device;

[0046] FIG. 2 shows a diagrammatic cross section through a field magnet of a magnetic resonance tomography unit according to an embodiment;

[0047] FIG. 3 shows a depiction of exemplary functions of the magnetic field strength of a field magnet as a function of the z-coordinate for different x-y coordinates;

[0048] FIG. 4 shows a diagrammatic view of a local coil according to an embodiment;

[0049] FIG. 5 shows a diagrammatic flow chart of one embodiment of a method; and

[0050] FIG. 6 shows a diagrammatic flow chart of a method according to an embodiment.

DETAILED DESCRIPTION

[0051] FIG. 1 shows a diagrammatic view of one embodiment of a magnetic resonance tomography unit 1 with a local coil 50 according to one or more of the present embodiments.

[0052] The magnetic unit 10 includes a field magnet 11 that produces a static magnetic field B0 for alignment of nuclear spin of samples or patients 40 in a scanning area. The scanning area is arranged in a patient tunnel 16 that extends in a longitudinal direction 2 through the magnetic unit 10. Usually, the field magnet 11 is a superconducting magnet that may provide magnetic fields with a magnetic flux density of up to 3T, in the case of the latest devices, even more. For lower field strengths, however, permanent magnets or electromagnets with normally conducting coils may also be used.

[0053] The magnetic unit 10 also includes gradient coils 12 that are configured to overlay the variable magnetic fields in three spatial directions over the magnetic field B0 to spatially differentiate the captured display ranges in the examination volume. The gradient coils 12 may be coils including normally conducting wires that may produce orthogonal fields to each other in the examination volume.

[0054] The magnetic unit 10 also includes a body coil 14 that is configured to emit a high-frequency signal supplied via a signal line, into the examination volume. The body coil 14 is also configured to receive resonance signals emitted by the patient 40 and to emit these via a signal line. However, replacement of the body coil 14 with local coils 50 that are arranged in the patient tunnel 16 close to the patient 40 for sending and/or receiving the high-frequency signal may be provided. In one embodiment, however, the local coil 50 is configured for sending and receiving, and a body coil 14 may therefore be omitted.

[0055] A control unit 20 (e.g., a controller) supplies the magnetic unit 10 with the various signals for the gradient coils 12 and the body coil 14 and evaluates the received signals.

[0056] The controller 20 therefore has gradient activation 21 that is configured to supply the gradient coils 12 with variable currents that provide the desired gradient fields in the examination volume in a chronologically coordinated fashion via supply lines.

[0057] The controller 20 includes a high-frequency unit 22 configured to produce a high frequency pulse with a predetermined chronological sequence, amplitude, and spectral power distribution for the excitation of the magnetic resonance of the nuclear spin in the patient 40. Pulse power in the range of kilowatts may be achieved in the process.

[0058] The magnetic resonance tomography unit 1 includes a local device that may be arranged on the patient (e.g., in the exemplary embodiment shown, the local coil 50). The local coil 50 is configured to receive magnetic resonance signals of excited nuclear spin and to transmit the magnetic resonance signals to the controller 20 via the signal connection 33, for a local coil 50, inter alia, to the high-frequency unit 22. Details of the device or local coil 50 are shown in FIG. 3.

[0059] In FIG. 1, the coordinate axes, which are used hereinafter, are also shown. The designations x, y and z are selected at random. The z-coordinate axis runs centrally through the patient tunnel 16 and simultaneously provides a symmetry axis for the magnetic unit 10 and the field magnet 11 that, as shown in the cross section in FIG. 2, has an almost cylindrical form that is symmetrical to an axis of rotation at the center. The y-coordinate axis is arranged perpendicularly to the z-axis and vertically upwards on the plane of the drawing in FIG. 1. Only shown in the cross section of FIG. 2, the x-coordinate axis is arranged perpendicularly both to the z-coordinate axis and to the y-coordinate axis. Due to the rotational symmetry of the field magnet 11, the x-y coordinate axes may be freely rotated around the z-coordinate axis without the following description changing.

[0060] FIG. 3 specifies the paths of the magnetic field strength B0 measured by a magnetic field strength sensor 60 as a function of the z-coordinate. The zero point is located 200 cm before the isocenter of the B0 field magnet on the z-coordinate axis. Curves with different x-y coordinates or distances to the z-coordinate axis are specified. As described below regarding the method, such curves of the B0 field magnet may be measured by moving the magnetic field strength sensor 60 with the same orientation and a fixed x-y coordinate along the z-coordinate axis through the B0 field and the measured values for magnetic field strength and z being recorded in the process.

[0061] In doing so, the curve 71 is directly assigned to the z-coordinate axis (x=0, y=0) with a magnetic field strength sensor, while the curve 72 corresponds to a path at the greatest distance from the z-coordinate axis. One or more of the present embodiments are based, inter alia, on the knowledge that the curves for a plurality of x-y coordinates at different distances from the z-coordinate axis intersect or come close to each other at an intersection 70. The intersection 70 is at a value B.sub.ref of the magnetic field strength, which for the 3T magnet shown, is approximately 50% of the magnetic field strength B0.sub.iso at the isocenter. This may vary (e.g., the value may lie within an interval of between 20% and 80% of the magnetic field strength in the isocenter).

[0062] The curve paths have a high gradient around the point of intersection (z.sub.ref, B.sub.ref), which distinguishes this point, for example, from the curve paths at z=0 or z=200. For example, a value of the gradient greater than 0.2*B0.sub.iso divided by the length of the B0 field magnet in a z-direction may be considered a high gradient.

[0063] Even with erroneous measurement of the magnetic field strength indicated by the horizontal bar 73 around the value B.sub.ref, the high gradient enables the reference z-coordinate z.sub.ref to be determined with great accuracy, indicated by the narrow vertical bar 74.

[0064] FIG. 4 shows an exemplary embodiment of a device (e.g., a local coil 50). The local coil 50 includes one or more antenna coils 52 in a housing 51.

[0065] In the local coil 50, there is a magnetic field strength sensor 60 that is configured to measure a magnetic field strength. In one embodiment, the magnetic field strength sensor 60 includes three sensor elements 61, 62, 63 that each records the strength of a spatial component of the magnetic field vector (e.g., Hall elements). The sensor elements 61, 62, 63 are arranged such that the three Spatial components span a three-dimensional area. For example, the sensor element 62 may be arranged perpendicularly to the sensor element 61 and both in turn perpendicularly to the sensor element 63. The amount may then be ascertained from the three components of the vector, for example, with a perpendicular arrangement of the sensor elements to each other by forming the root from the sum of the squares of the components. This may be done either by signal processing 53 in the local coil 50 and transmission of the result by signal processing 53 to the control unit 20, or signal processing 53 transmitting the measured values of the sensor elements 61, 62, 63 via the signal connection 33 to the controller 20 for calculation. However, the magnetic field strength sensor 60 may be configured to ascertain the amount of magnetic field strength directly, for example, via the nuclear magnetic resonance of a test material, comparable to a field probe.

[0066] In the embodiment shown in FIG. 4, the local coil 50 also includes an orientation sensor 54. The orientation sensor 54 is configured to ascertain alignment with regard to the field magnet 11. For example, a sensor that determines the direction of gravity, for example, of a test mass, by mechanical devices may be involved. As the alignment of the field magnet 11 with the force of gravity is known, two angles of the orientation sensor 54 and thus the local coil 50 may also be ascertained with regard to the field magnet 11. A third angle may be determined, for example, with reference to the preferred direction of the magnetic field at the isocenter or on the z-coordinate axis. Such orientation sensors are, for example, micromechanically available in silicon technology (MEMS).

[0067] However, in one embodiment, the orientation sensor 54 includes a camera and ascertains the orientation of optical features of the magnetic unit 10 (e.g., markings or light sources on the housing). The camera may also conversely be arranged on the magnetic unit and corresponding markers on the local coil 50.

[0068] If the orientation in several axes is already known, for example, by a fixed arrangement of the local coil 50 on a patient couch, or if lower precision of the z-coordinate is sufficient, the orientation sensor 54 may also be omitted.

[0069] In one embodiment, the device is not a local coil 50, as shown in FIG. 4, but is a shim coil, for which instead of the antenna coils 52, there are one or more coils for the production of a compensation field for interferences in the B0-magnetic field. The other components of the device are then as described in FIG. 4. This applies to all possible devices in connection with a magnetic resonance tomography unit 1 for which precise position determination is to be provided, at least in the z-coordinate.

[0070] FIG. 5 shows a diagrammatic flow chart of one embodiment of a method.

[0071] In act S10, a characteristic magnetic field strength B.sub.ref of the B0 field magnet is ascertained. The characteristic magnetic field strength B.sub.ref is characterized in that, as shown in FIG. 3, a value of the magnetic field strength B.sub.ref emerges as a result of a magnetic field strength sensor 60 for the majority of x-y coordinate pairs with the same reference z-coordinate z.sub.ref. As aforementioned in FIG. 3, the characteristic magnetic field strength B.sub.ref lies in the range of between 20% and 80% of a maximum magnetic field strength of the B0 field magnet and consequently differs from the magnetic field strength for large distances from the field magnet 11, which approaches zero or the magnetic field strength at the isocenter, which approaches 100% of B0.sub.iso.

[0072] Ascertainment may, for example, be performed by the controller 20, which moves a magnetic field strength sensor 60 arranged at different x-y coordinates by the patient couch 30 parallel to the z-coordinate axis. In doing so, the magnetic field strength sensor 60 records a curve for each x-y coordinate pair. The x-y coordinate pairs are at different distances from the z-coordinate axis. The intersection 70 at which all the curves intersect indicates the pair of values z.sub.ref, B.sub.ref to the controller 20. In one embodiment, the act S10 is performed once for each magnetic unit 10, is ascertained for all the magnetic units 10 of the same type, or is repeated for a magnetic unit 10 after changes to the operating parameters or the environment. The value B.sub.ref or the pair of values z.sub.ref, B.sub.ref may be stored in the controller 20.

[0073] In one embodiment, in act S10, the controller 20 calculates the B.sub.xy(z) curves for the magnetic unit 10 as a function of current operating parameters (e.g., the superconducting current) by a numerical approximation method and determines the pair of values z.sub.ref, B.sub.ref from the point of intersection of the curves.

[0074] In act S20 of the method, the controller 20 moves the device using the position determination unit 36 of the patient couch 30 (e.g., the local coil 50) along the z-coordinate axis until, with the aid of the magnetic field strength sensor 60, the controller 20 measures a magnetic field strength that is essentially similar to the characteristic magnetic field strength B.sub.ref (e.g., differs by less than 15%, 5% or 1% from B.sub.ref). The z-coordinate on which the patient couch 30 is located at this time is stored in a storage unit 35 in the control unit 20 as a reference z-coordinate z.sub.ref.

[0075] In act S30 of the method, the mobile device is moved a predetermined distance along the z-coordinate axis by a position determination unit 36 relative to the B0 field magnet. The z-coordinate of the magnetic field strength sensor 60 and the device or local coil 50 with regard to the field magnet 11 is then determined by the reference z-coordinate z.sub.ref plus the predetermined distance. This is executed by a z-movement of the patient couch 30 by the control unit 20 using the position determination unit 36.

[0076] The method shown in FIG. 5 is suitable if only the z-coordinate axis is to be calibrated with regard to the isocenter of the field magnet 10, and the coordinates x and y of the magnetic field strength sensor 60 and the device are irrelevant.

[0077] FIG. 6 shows a diagrammatic flow chart of a further possible embodiment of the method. The method of FIG. 6 enables still more coordinates of the magnetic field strength sensor 60 and the device to be established.

[0078] In act S11, first different B.sub.xy(z) curves (e.g., B(z) curves for different x-y coordinates) are ascertained. Ascertainment may also take place by a controller 20 that moves a magnetic field strength sensor arranged at different x-y coordinates using the patient couch 30 parallel to the z-coordinate axis. In the process, the magnetic field strength sensor records a curve for each x-y coordinate pair. Due to the rotational symmetry of the field magnet, the x-y coordinate pairs are each at different distances from the z-coordinate axis. The intersection 70, at which all the curves intersect, indicates, as aforementioned, indicates the pair of values z.sub.ref, B.sub.ref to the controller 20. The act S11 may also be performed once for each magnetic unit 10, ascertained for all magnetic units 10 of the same type, or may be repeated for a magnetic unit 10 after changes to operating parameters or the environment, respectively. The B.sub.xy(z) curves recorded in act S11 are stored in the storage unit 35 in the controller 20.

[0079] In act S21, magnetic field strength values are then recorded for different positions along the z-coordinate axis with constant x-y coordinate by the magnetic field strength sensor 60. For example, the local coil 50 is positioned on the patient 40 with the magnetic field strength sensor 60, as a result of which an x-y coordinate is predetermined for the magnetic field strength sensor 60 but not yet recorded as a coordinate pair. The patient 40 is then moved with the local coil 50 in an unchanged position by the controller 20 on the patient couch 30 along the z-coordinate axis, and the magnetic field strength is recorded by the magnetic field strength sensor 60 in the process.

[0080] In act S31, a B.sub.xy-z curve is selected by the controller 20 from the B.sub.xy-z curves in act S11 with the aid of the magnetic field strength values ascertained in act S21 using an error minimization method. The error minimization method thus selects the curve from the curve stored in act S10 that most closely resembles the current curve path. For example, Least Mean Square (LMS) may be applicable here. In one embodiment, a B.sub.xy-z curve for the current x-y coordinates is determined in act S31 through interpolation of the nearest B.sub.xy-z curves stored. With the aid of the measured values recorded during movement in a z-direction in the course of act S21, the relative position may be determined in a z-coordinate direction to the isocenter of the field magnet by fitting the measured values to the selected B.sub.xy-z curves and determining a corresponding z-coordinate in the sense of an inverse function from the currently measured magnetic field strength value B.

[0081] In one embodiment, a z.sub.ref is determined by the value B.sub.ref as in the method in FIG. 5. On account of the known path of the selected B.sub.xy-z curve, however, the vertical bar 74 and the horizontal bar 73 in FIG. 3 may be reduced, enabling z.sub.ref to be determined with greater accuracy (e.g., having a tolerance of less than one millimeter).

[0082] Information about the distance of the magnetic field strength sensor 60 from the z-coordinate axis is also linked to the choice of B.sub.xy-z curves. Clear x-y coordinates or the angle of a polar coordinate system around the z-coordinate axis may not be ascertained without additional parameters.

[0083] In an extended embodiment of the method in FIG. 6, in act S32, the controller 20 receives additional information about the relative orientation of the orientation sensor 54 to the magnetic unit 10 via signal processing 53 and the signal connection 33. For example, the orientation sensor 54 may specify information about the alignment of the orientation sensor 54 to the vertical, which is given by the direction of gravity. In connection with the direction of the B0 field, which runs in the z-direction at the isocenter and on the z-coordinate axis, the controller 20 ascertains the alignment of the magnetic field strength sensor from the additional three orientation parameters and consequently also the x and y coordinates from the distance from the z-coordinate axis. This enables the precise position and alignment of the local coil 50 to be determined using the magnetic field strength sensor 60 and the orientation sensor 54.

[0084] If some parameters are already known (e.g., the y-coordinate and the alignment to the vertical), when the local coil is arranged on or in the patient couch, the orientation sensor 54 may be simplified or omitted completely, and the position nonetheless determined in full. Alternatively, using all the parameters, a method for error minimization that improves the accuracy of position determination may be used.

[0085] Although the invention was illustrated and described in more detail by the exemplary embodiments, the invention is not limited by the disclosed examples. Other variations may be derived by a person skilled in the art without departing from the scope of the invention.

[0086] The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

[0087] While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.