METHOD FOR DETERMINING A RADIO-FREQUENCY TRANSMISSION PULSE FOR A MAGNETIC RESONANCE SCAN, A MAGNETIC RESONANCE APPARATUS, AND A COMPUTER PROGRAM PRODUCT

Abstract

A method, a magnetic resonance apparatus, and a computer program product are disclosed. In particular, a method is provided for determining an RF transmission pulse for a magnetic resonance scan by a magnetic resonance apparatus including a gradient coil unit. The method includes a provision of a deviation information item, wherein the deviation information item characterizes a position-dependent deviation from a target state, caused by the gradient coil unit, in an imaging region of the magnetic resonance apparatus. The RF transmission pulse is determined taking account of the deviation information item.

Claims

1. A computer-implemented method for determining a radio-frequency (RF) transmission pulse for a magnetic resonance scan by a magnetic resonance apparatus comprising a gradient coil unit, the method comprising: providing a deviation information item, wherein the deviation information item characterizes a position-dependent deviation from a target state, caused by the gradient coil unit, in an imaging region of the magnetic resonance apparatus; and determining the RF transmission pulse taking into account the deviation information item.

2. The method of claim 1, wherein the position-dependent deviation is caused by a non-linearity of a gradient magnetic field generated by the gradient coil unit during the magnetic resonance scan.

3. The method of claim 2, further comprising: transmitting the RF transmission pulse, by a radio frequency antenna unit of the magnetic resonance apparatus, during the magnetic resonance scan.

4. The method of claim 3, further comprising: generating the gradient magnetic field, according to a gradient trajectory, during the transmitting of the RF transmission pulse.

5. The method of claim 4, wherein the gradient magnetic field is generated in a phase encoding direction and/or a readout direction.

6. The method of claim 1, further comprising: transmitting the RF transmission pulse, by a radio frequency antenna unit of the magnetic resonance apparatus, during the magnetic resonance scan.

7. The method of claim 6, further comprising: generating a gradient magnetic field, according to a gradient trajectory, during the transmitting of the RF transmission pulse.

8. The method of claim 7, wherein the gradient magnetic field is generated in a phase encoding direction and/or a readout direction.

9. The method of claim 1, wherein the RF transmission pulse is a dynamic pulse and/or a pTx-pulse.

10. The method of claim 1, wherein the determining of the RF transmission pulse also takes into account a B0 map and/or a B1 map.

11. The method of claim 1, wherein the deviation information item comprises a deformation map, and wherein the deformation map defines a spatial displacement of at least one image point of a magnetic resonance image.

12. The method of claim 1, wherein the determining of the RF transmission pulse comprises: providing an initial target state in a position space; determining a modified target state in the position space based on the deviation information item; determining a transformed target state in a k-space by transformation of the modified target state in the k-space; and determining the RF transmission pulse based on the transformed target state.

13. The method of claim 12, wherein the determining of the transformed target state is performed by a Fourier transform.

14. The method of claim 12, wherein the initial target state comprises a spatial frequency distribution of the RF transmission pulse.

15. A magnetic resonance apparatus comprising: a gradient coil unit, wherein the magnetic resonance apparatus is configured to: provide a deviation information item, wherein the deviation information item characterizes a position-dependent deviation from a target state, caused by the gradient coil unit, in an imaging region of the magnetic resonance apparatus; and determine a radio-frequency (RF) transmission pulse taking into account the deviation information item.

16. A non-transitory computer program product comprising a program configured to be loaded directly into a memory store of a programmable computing unit of a magnetic resonance apparatus, wherein, when the program is executed in the computer unit of the magnetic resonance apparatus, the program is configured to cause the magnetic resonance apparatus to: provide a deviation information item, wherein the deviation information item characterizes a position-dependent deviation from a target state, caused by a gradient coil unit of the magnetic resonance apparatus, in an imaging region of the magnetic resonance apparatus; and determine a radio-frequency (RF) transmission pulse taking into account the deviation information item.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1 depicts an example of a magnetic resonance apparatus in a schematic representation.

[0060] FIG. 2 depicts examples of parts of a gradient coil unit.

[0061] FIG. 3 depicts an example of a method sequence.

[0062] FIG. 4 depicts an example of a schematic representation of a deformation map.

[0063] FIG. 5 depicts an example of a portion of a magnetic resonance sequence during the transmission of an RF transmission pulse.

DETAILED DESCRIPTION

[0064] FIG. 1 shows a magnetic resonance apparatus 10 in a schematic representation. The magnetic resonance apparatus 10 includes a magnet unit 11 that has a main magnet 12 for generating a strong and, in particular, temporally constant main magnetic field B.sub.0. In addition, the magnetic resonance apparatus 10 includes a patient receiving region 14 to accommodate a patient 15. In the present exemplary embodiment, the patient receiving region 14 is configured cylindrical and is surrounded cylindrically in a circumferential direction by the magnet unit 11. Parallel to the direction of the main magnetic field B.sub.0, the main magnet field direction 13, the cylindrical axis of the patient receiving region 14, the Z-axis, extends in the spatial direction Z. The patient 15 may be moved by a patient positioning apparatus 16 of the magnetic resonance apparatus 10 into the patient receiving region 14. For this purpose, the patient positioning apparatus 16 has a patient table 17 which is designed to be movable within the patient receiving region 14.

[0065] The magnet unit 11 also has a gradient coil unit 18 (with a plurality of gradient coils as shown in FIG. 2) for generating gradient magnetic fields that are overlaid on the main magnet field B.sub.0 13 and are used, in particular, for position encoding during imaging. The gradient coil unit 18 includes a gradient control unit (not shown here) of the magnetic resonance apparatus 10. The magnet unit 11 further includes a radio frequency antenna unit 20 that, in the present exemplary embodiment, has a body coil which is permanently integrated into the magnetic resonance apparatus 10. The radio frequency antenna unit 20 may include a plurality of transmitting coils so that pTx pulses may also be transmitted with it. The radio frequency antenna unit 20 includes a radio frequency antenna control unit (not shown here) and transmits, in particular, RF transmission pulses into a patient receiving region 14 of the magnetic resonance apparatus 10. Situated in the patient receiving region 14 is an imaging region of the magnetic resonance scan, in which an excitation of atomic nuclei takes place. By way of relaxation of the excited atomic nuclei, magnetic resonance signals are generated. The radio frequency antenna unit 20 is designed to receive the magnetic resonance signals.

[0066] For control of the main magnet 12, the gradient coil unit 18 and the radio frequency antenna unit 20, the magnetic resonance apparatus 10 has a system control unit 22. The system control unit 22 centrally controls the magnetic resonance apparatus 10, for example, the execution of a pre-determined imaging gradient echo sequence. In addition, the system control unit 22 includes an evaluation unit (not shown in detail) for evaluating the magnetic resonance signals captured during the magnetic resonance examination. The system control unit 22 may further include a computing unit configured to determine an RF-transmission pulse for a magnetic resonance scan (in particular with a method according to FIG. 3), which may be transmitted with the radio frequency antenna unit 20.

[0067] Furthermore, the magnetic resonance apparatus 10 includes a user interface 23 connected to the system control unit 22. Control information, (e.g., imaging parameters and reconstructed magnetic resonance images), may be displayed on a display unit 24, (e.g., on at least one monitor of the user interface 23), for medical operating personnel. In addition, the user interface 23 has an input unit 25 by which the information and/or parameters may be input by the medical operating personnel during a scanning procedure.

[0068] FIG. 2 shows three gradient coils 18x, 18y, 18z of the gradient coil unit 18. The gradient coil 18x generates a gradient magnetic field with a gradient of the value of the magnetic field in the X-direction. The gradient coil 18y generates a gradient magnetic field with a gradient of the value of the magnetic field in the Y-direction. The gradient coil 18z generates a gradient magnetic field with a gradient of the value of the magnetic field in the Z-direction. The vector fields of the three gradient magnetic fields are ideally (only) oriented in the Z-direction. The gradients may have non-linearities that may lead, in particular, to deformations, in particular, displacements of image points (shown schematically in FIG. 4) in magnetic resonance images.

[0069] During the transmission of a dynamic pulse and/or pTx pulse, a rapid sequence of gradient trajectories may be played out with the gradient coil unit 18. These gradient trajectories are also subject to any imperfections of the gradient coil unit 18 which lead to position-dependent deviations from a target state in an imaging region of the magnetic resonance apparatus 10.

[0070] FIG. 3 illustrates a method for determining an RF transmission pulse in order to correct such imperfections at least partially as early as during the magnetic resonance scan. In S10, a deviation information item is prepared which characterizes a position-dependent deviation from a target state, caused by the gradient coil unit 18, in an imaging region of the magnetic resonance apparatus 10.

[0071] The position-dependent deviation may be caused by a non-linearity of a gradient magnetic field generated by the gradient coil unit during the magnetic resonance scan.

[0072] In S20, the RF transmission pulse, (e.g., a pTx pulse), is determined taking account of the deviation information item. The determining of the radio frequency pulse may also be carried out taking account of a B0 map and/or a B1 map. In S30, the RF-transmission pulse may be transmitted with the radio frequency antenna unit 20 into the imaging region during the magnetic resonance scan.

[0073] The determination of the RF-transmission pulse taking account of the deviation information item in S20 may include, in particular: in S21, an initial target state is provided in a position space. The initial target state may include, in particular, a spatial frequency distribution of the RF-transmission pulse.

[0074] In S22, a modified target state is determined in the position space based on the deviation information item. In S23, a transformed target state is determined in a k-space by transformation, in particular by a Fourier transform, of the modified target state in the k-space. Such a k-space may also be designated the excitation k-space. In S24, the RF transmission pulse is determined based on the transformed target state.

[0075] Advantageously, a correction of distorting effects may be taken into account by way of gradient non-linearities in the excitation k-space of a dynamic or pTx pulse. The target state of the excitation may be subjected to a distortion that corresponds to the distortion characteristic of the magnetic resonance apparatus 10. In particular, the method may make it possible for dynamic pulses to be performed in off-center positions and therein to take account of the influences of the gradient coil unit 18.

[0076] In particular, the deviation information item may be a deformation map that defines a spatial displacement of at least one image point of a magnetic resonance image. This is now described in greater detail by reference to FIG. 4. The solid points connected with continuous lines herein represent the allocation of a plurality of image points of a magnetic resonance image that arises if the position encoding of the image points were to take place with perfectly linear gradients. Non-linearities of the gradient coil unit 18 may lead to the position encoding of the image points (represented by hollow points connected by dashed lines) being faulty so that at least one part of the image points has a spatial displacement. With the aid of such a deformation map which may be measured, for example, with the aid of a phantom and/or may be calculated with the aid of a simulation of the gradient coil unit 18, the RF transmission pulse may be determined in S20.

[0077] FIG. 5 shows a short portion along the time axis t from a sequence diagram according to which a gradient magnetic field is generated during the transmission of the RF transmission pulse. The RF transmission pulse shown here may be regarded, in particular, as a dynamic pulse. Therein, RF.sub.A and RF.sub.P are the amplitude and phase of the RF transmission pulse. G.sub.RO represents the strength of the gradient on the readout axis, GP.sub.PE represents the strength of the gradient on the phase encoding axis and G.sub.SS represents the strength of the gradient on the slice selection axis. Therein, the readout axis, phase encoding axis and slice selection axis are virtual gradient axes; depending upon the orientation of the slice to be scanned, the readout axis, the phase encoding axis and the slice selection axis may coincide completely or partially with the physical axes of the gradient coils 18x, 18y, 18z (that is the X-direction, the Y-direction and the Z-direction), or not. According to this example, a gradient magnetic field is generated in the phase encoding direction and the readout direction during the transmission of the RF transmission pulse. By contrast, no gradient magnetic field is generated in the slice selection direction. Such dynamic gradient magnetic fields are suitable, in particular, for encoding the excitation k-space during the transmission of dynamic pulses and/or pTx pulses.

[0078] Finally, it should again be noted that the method described above in detail and the magnetic resonance apparatus disclosed are merely exemplary embodiments which may be modified by a person skilled in the art in a broad variety of ways without departing from the scope of the disclosure. Furthermore, the use of the indefinite article “a” or “an” does not preclude the possibility that the relevant features may also be present plurally. Similarly, the expression “unit” does not preclude the relevant components including a plurality of cooperating sub-components that may also be spatially distributed, if relevant.

[0079] It is to be understood that 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 disclosure. Thus, whereas the dependent claims appended below depend on 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, and that such new combinations are to be understood as forming a part of the present specification.

[0080] While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may 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.