REDUCING INTERFERENCE FROM MAGNETIC RESONANCE TOMOGRAPHY UNITS

Abstract

Systema and methods to improve the suppression of interference fields outside a magnetic resonance tomography unit. A radiofrequency alternating electromagnetic field of the magnetic resonance tomography unit is generated and measured. A step series is repeated multiple times. The step series includes generating an electromagnetic interference-reduction field for reducing the magnetic field strength at at least one defined location on the basis of a product of a weighting factor and a defined interference-reduction field strength; measuring a magnetic field strength of the generated interference-reduction field; determining an adjustment factor for the weighting factor in such a way as to minimize a sum of the measured field strength of the alternating electromagnetic field and the product of the adjustment factor and the measured interference-reduction field strength; and updating the weighting factor by multiplying by the adjustment factor.

Claims

1. A method for operating a magnetic resonance tomography unit, the method comprising: generating a radiofrequency alternating electromagnetic field; measuring a magnetic field strength of the radiofrequency alternating electromagnetic field; and repeating a step series multiple times, the step series comprising: generating an electromagnetic interference-reduction field for reducing the magnetic field strength of the radiofrequency alternating electromagnetic field at at least one defined location on the basis of a product of a weighting factor and a defined interference-reduction field strength; measuring a magnetic field strength of the generated interference-reduction field; determining an adjustment factor for the weighting factor in such a way as to minimize a sum of the measured field strength of the radiofrequency alternating electromagnetic field and the product of the adjustment factor and the measured field strength of the magnetic interference-reduction field; and updating the weighting factor by multiplying by the adjustment factor.

2. The method of claim 1, wherein the particular field strength is measured by a plurality of sensors, with a measured value acquired from each sensor, and the measured values together yield a measurement vector for the particular field strength, and the measurement vector corresponds to the measured field strength at the location of the respective sensor.

3. The method of claim 1, wherein the interference-reduction field is generated by a plurality of coils or antennas and generating the interference-reduction field and measuring the field strength of the interference-reduction field are performed separately for each of the plurality of coils or antennas.

4. The method of claim 1, wherein the step series is repeated until a defined break criterion is fulfilled.

5. The method of claim 1, wherein the method is not performed until after an object under examination has been brought into the magnetic resonance tomography unit.

6. The method of claim 1, wherein the method is performed automatically as soon as an object under examination is brought into the magnetic resonance tomography unit.

7. The method of claim 1, wherein the method is repeated at defined time intervals or according to a defined state or sequence step of the magnetic resonance tomography unit.

8. A non-transitory computer implemented storage medium, including machine-readable instructions stored therein, that when executed by at least one processor, cause the processor to: generate a radiofrequency alternating electromagnetic field; measure a magnetic field strength of the radiofrequency alternating electromagnetic field; and repeat a step series multiple times, the step series comprising: generating an electromagnetic interference-reduction field for reducing the magnetic field strength of the radiofrequency alternating electromagnetic field at at least one defined location on the basis of a product of a weighting factor and a defined interference-reduction field strength; measuring a magnetic field strength of the generated interference-reduction field; determining an adjustment factor for the weighting factor in such a way as to minimize a sum of the measured field strength of the radiofrequency alternating electromagnetic field and the product of the adjustment factor and the measured field strength of the magnetic interference-reduction field; and updating the weighting factor by multiplying by the adjustment factor.

9. The non-transitory computer implemented storage medium of claim 8, wherein the particular field strength is measured by a plurality of sensors, with a measured value acquired from each sensor, and the measured values together yield a measurement vector for the particular field strength, and the measurement vector corresponds to the measured field strength at the location of the respective sensor.

10. The non-transitory computer implemented storage medium of claim 8, wherein the interference-reduction field is generated by a plurality of coils or antennas and generating the interference-reduction field and measuring the field strength of the interference-reduction field are performed separately for each of the plurality of coils or antennas.

11. The non-transitory computer implemented storage medium of claim 8, wherein the step series is repeated until a defined break criterion is fulfilled.

12. The non-transitory computer implemented storage medium of claim 8, wherein the machine-readable instructions are not performed until after an object under examination has been brought into a magnetic resonance tomography unit.

13. The non-transitory computer implemented storage medium of claim 8, wherein the machine-readable instructions are performed automatically as soon as an object under examination is brought into a magnetic resonance tomography unit.

14. The non-transitory computer implemented storage medium of claim 8, wherein the machine-readable instructions are repeated at defined time intervals or according to a defined state or sequence step of a magnetic resonance tomography unit.

15. A magnetic resonance tomography unit comprising: an excitation device configured for generating a radiofrequency alternating electromagnetic field; a measuring device configured for measuring a magnetic field strength of the radiofrequency alternating electromagnetic field; an interference-reduction device configured for generating an electromagnetic interference-reduction field for reducing the magnetic field strength of the radiofrequency alternating electromagnetic field at at least one defined location on the basis of a product of a weighting factor and a defined interference-reduction field strength; and a control device configured for repeating multiple times a step series comprising: generating by the interference-reduction device the electromagnetic interference-reduction field; measuring by the measuring device a magnetic field strength of the generated interference-reduction field; determining by a processing unit of the control device an adjustment factor for the weighting factor in such a way as to minimize a sum of the measured field strength of the radiofrequency alternating electromagnetic field and the product of the adjustment factor and the measured field strength of the electromagnetic interference-reduction field; and updating by the processing unit the weighting factor by multiplying by the adjustment factor.

16. The magnetic resonance tomography unit of claim 15, wherein the measuring device comprises a plurality of sensors, wherein a measured value is acquired from each sensor, and the measured values together yield a measurement vector for the particular field strength, and the measurement vector corresponds to the measured field strength at the location of the respective sensor.

17. The magnetic resonance tomography unit of claim 15, wherein the interference-reduction device comprises a plurality of coils or antennas wherein generating the interference-reduction field and measuring the field strength of the interference-reduction field are performed separately for each of the plurality of coils or antennas.

18. The magnetic resonance tomography unit of claim 15, wherein the step series is repeated until a defined break criterion is fulfilled.

19. The magnetic resonance tomography unit of claim 15, wherein the step series is not performed until after an object under examination has been brought into the magnetic resonance tomography unit.

20. The magnetic resonance tomography unit of claim 15, wherein the step series is performed automatically as soon as an object under examination is brought into the magnetic resonance tomography unit.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0032] The following description of the embodiments will clarify and elucidate the above-described properties, features and advantages, and the manner in which they are achieved, which embodiments are explained in greater detail in conjunction with the drawings, in which:

[0033] FIG. 1 depicts a schematic diagram of an embodiment of a magnetic resonance tomography unit.

[0034] FIG. 2 depicts a schematic diagram of a magnetic resonance tomography unit having an interference-suppression transmitter according to an embodiment.

[0035] FIG. 3 is a schematic flow diagram of an embodiment of a method.

[0036] FIG. 4 is a diagram illustrating the principle of iterative weight optimization according to an embodiment.

DETAILED DESCRIPTION

[0037] The embodiments described in greater detail below constitute one or more embodiments among possible embodiments. The same reference numbers denote identical or similar elements in the figures. In addition, the figures are schematic representations of various embodiments. The elements depicted in the figures are not necessarily shown to scale. Instead, these are depicted in a way that makes their function and purpose clear to a person skilled in the art. The connections shown in the figures between functional units or other elements may also be implemented as indirect connections, where a connection may be wireless or wired. Functional units may be implemented as hardware, software or a combination of hardware and software.

[0038] FIG. 1 depicts a schematic diagram of an embodiment of a magnetic resonance tomography unit 1.

[0039] A magnet unit 10 of the magnetic resonance tomography unit 1 includes a field magnet 11, that produces a static magnetic field B0 for aligning nuclear spins of samples or of the patient 100 in an acquisition region. The acquisition region is characterized by an extremely homogeneous static magnetic field B0, the homogeneity relating, for example, to the magnetic field strength or magnitude. The acquisition region is approximately spherical and located in a patient tunnel 16, that extends through the magnet unit 10 in a longitudinal direction 2. A patient couch 30 may be moved inside the patient tunnel 16 by the travel unit 36. The field magnet 11 is usually a superconducting magnet, that may provide electromagnetic fields of magnetic flux density of up to 3 T or even higher in the latest equipment. For lower field strengths, however, permanent magnets or electromagnets having normal-conducting coils may also be used.

[0040] The magnet unit 10 also includes gradient coils 12, that are configured to superimpose variable magnetic fields in three spatial dimensions on the magnetic field B0 for the purpose of spatial discrimination of the acquired imaging regions in the volume of interest. The gradient coils 12 are usually coils made of normal-conducting wires, that may generate mutually orthogonal fields in the volume of interest.

[0041] The magnet unit 10 also includes a body coil 14, that is configured to radiate into the volume of interest a radiofrequency signal supplied via a signal line, and to receive resonance signals emitted by the patient 100 and to output the resonance signals via a signal line.

[0042] A control device 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14 and analyzes the received signals.

[0043] Thus, the control device 20 includes a gradient controller 21, that is configured to supply the gradient coils 12 via supply lines with variable currents that provide, coordinated in time, the desired gradient fields in the volume of interest.

[0044] In addition, the control device 20 includes a radiofrequency unit 22, that is configured to produce a radiofrequency pulse having a defined variation over time, amplitude, and spectral power distribution for the purpose of exciting magnetic resonance of the nuclear spins in the patient 100. Pulse powers may reach in the region of kilowatts here. The excitation pulses may be emitted via the body coil 14 or via a local transmit antenna into the patient 100.

[0045] A controller 23 communicates, for instance via a signal bus 25, with the gradient controller 21 and the radiofrequency unit 22.

[0046] Arranged on the patient 100 is a local coil 50 as a first receive coil, that is connected via a connecting line 33 to the radiofrequency unit 22 and its receiver. The body coil 14 may also be implemented as a first receive antenna.

[0047] At an edge of the opening of the patient tunnel 16 may be arranged, for instance, four second receive antennas 60, that may be arranged at the corners of a square circumscribed by the circular opening, with the result that the corners lie on the edge of the opening. The four second receive antennas 60 have a signal connection to a receiver 70 of the radiofrequency unit 22. It is conceivable, because there are a plurality of second receive antennas 60, that these do not all have an omnidirectional receive characteristic but, for example, are dipoles, and complement each other by the different orientation to form an omnidirectional characteristic. It would also be conceivable, however, to provide a crossed dipole, for example, as a single second antenna having an omnidirectional characteristic.

[0048] It is also possible that alternatively or additionally a second receive antenna 60 is arranged in the patient couch 30.

[0049] FIG. 2 depicts a schematic diagram of an embodiment of a magnetic resonance tomography unit 1 including an interference-suppression transmitter 80. Electric waves or alternating fields may also be suppressed by electric fields having the same frequency and amplitude level but opposite polarity or a 180-degree phase-shift. If amplitude levels and/or phases do not match precisely, then the destructive interference at least achieves a reduction. For the purpose of generating these alternating fields for interference suppression or interference reduction (both terms are used here synonymously), a magnetic resonance tomography unit 1 includes an interference-reduction device including one or more interference-suppression antennas 81 arranged around the source of the fields, in this case the patient tunnel 16. The interference-suppression antennas 81 may cover all spatial directions around the opening, and symmetry is employed, for instance equal distances from the opening of the patient tunnel 16 and/or distribution at equal angular spacings with respect to the opening, to simplify controlling the individual interference-suppression antennas 81. Any distribution may be used, however, by an amplitude and phase that may be adjusted individually for each interference-suppression antenna 81. Depending on the type of the alternating field, the antennas may be antennas with electric field, for instance dipoles, or with electromagnetic field, for instance transmit coils. The orientation of the antennas, or the polarization of the generated field, may be aligned with the field directions of the alternating fields to be suppressed.

[0050] The signal emitted by the interference-suppression antennas 81 is intended to reduce the emitted radiation of the excitation pulse and hence must have a predefined amplitude and phase relationship to the excitation pulse. Thus, if applicable, for rudimentary interference reduction or interference suppression, the signals are derived in analog form from the excitation pulse or else from the digital pulse generation. Separate units may provide the signals independently of the pulse generation, as long as the necessary amplitude and phase relationship is established.

[0051] FIG. 2 indicates symbolically a connecting line between the body coil 14, as the source of the electromagnetic waves, and the interference-suppression transmitter. A direct connection via a power splitter or, for instance, a directional coupler, may be used and also a sensor in the patient tunnel for direct detection of the electromagnetic field would be possible. It would also be possible, however, to extract from a power amplifier or a pulse generator a reference signal for generating the interference-suppression signal.

[0052] The reference signal derived from the excitation pulse for the basic interference suppression may then be delayed, or phase-shifted, by adjustable phase shifters 82 for the individual interference-suppression antennas 81, and then amplified in amplitude by adjustable amplifiers 83 before being emitted via the interference-suppression antennas 81.

[0053] The adjustment of the phase shifters 82 and the amplifiers 83 is performed by an interference-suppression controller 84 via a signal connection. The interference-suppression controller 84 may adjust phase shifts and amplitudes to predefined values that are determined, for example, during installation of the magnetic resonance tomography unit 1.

[0054] The adjustment, or generally the interference reduction, may be performed or initiated by a (calibration) measurement. A calibration receiver 85 may use one or a plurality of spatially distributed sensors or calibration elements 86 of a measuring device to capture the alternating field to be suppressed. Simultaneously, the calibration receiver 85 detects the signals fed to the interference-suppression antennas 81 and transfers the detected values to the interference-suppression controller 84. The interference-suppression controller 84 may then adjust, for instance the interference suppression, the phases, and amplitudes of the individual interference-suppression antennas by a linear optimization method such as LSR in such a way that the field strength becomes zero at the location of the sensors or calibration antenna 86. If the n calibration elements 86 are distributed over the solid angle, then the resultant alternating field from body coil 14 and interference-suppression antennas 81 may be changed into a multipole field having n nulls or lobes, that decrease with distance at a raised power and allow effective suppression.

[0055] In principle, the propagation of the fields is reversible. Thus, for the calibration the calibration element(s) 86 emit a signal, and the body coil 14 and the interference-suppression antennas 84 receive the signal, and then the interference-suppression controller 84 determines a suitable phase relationship and amplitudes.

[0056] Under ideal conditions, complete Tx cancellation of the BC emission, i.e., the cancellation of the radiofrequency magnetic field penetrating to the outside, is achieved by solving the linear problem of least squares:

Minimize∥{right arrow over (H.sub.BC)}+H.sub.TXAux.Math.{right arrow over (V)}∥

[0057] where {right arrow over (H.sub.BC)} is a BC emission vector obtained from N measured values that are measured at N sensor points.

[0058] H.sub.TXAux is an emission matrix containing N×M field strength values, where N is the number of sensor points and M is the number of AUX antennas that together radiate an interference-reduction field. The vector {right arrow over (V)} represents an interference-suppression or interference-reduction weighting vector for the M AUX antennas. A relevant minimum value of the sum shown above may be found my numerical optimization. In the ideal case, {right arrow over (H.sub.BC)}=−H.sub.TXAux.Math.{right arrow over (V)}.

[0059] The aforementioned real factors mean that the resultant numerically or theoretically optimized AUX interference-reduction vector {right arrow over (H.sub.TxC Opt)}=H.sub.TXAux.Math.{right arrow over (V.sub.opt)} is very different from the actually measured interference-reduction vector {right arrow over (H.sub.TxC Meas)}. This leads to insufficient cancellation of the radiofrequency alternating electromagnetic field and the interference-reduction field. In practice, this may be expressed by a varying measured cancellation weighting vector {right arrow over (V.sub.meas)}.

[0060] In an embodiment, an iterative calibration and optimization approach is used to overcome quasi-static imperfections in the overall interaction of AUX and BC signal-chain in the real environment. This optimization may be actuated with every patient measurement, for example.

[0061] A pseudocode for the optimization is presented below:

TABLE-US-00001  1 Measure the BC emission{right arrow over (H.sub.BC)} {right arrow over (H.sub.BC)}  2 {right arrow over (V)}.sub.0 = {right arrow over (V)}.sub.init  3 While (i=1; i++; i< maximum number of iterations ∥ break criterion reached)  4 Measure each individual TxAUX signal response with the last optimization weight:  5 For m=1:M  6 H.sub.TXAux meas i(:,m) = measured value from TxAUX channel m with weight .Math. {right arrow over (V)}.sub.i−1(m)  7 End  8 Actual measured TxAUX interference-reduction field with the last interference- reduction weighting vector (simple sum of all individual TxAUX signal responses)  9 {right arrow over (H.sub.TxCMeas i)} = Σ.sub.m=1.sup.M H.sub.TXAux meas i(;,m) 10 Determine new adjustment vector for the interference reduction - V.sub.opt i argmin.sub.V.sub.opt i∥{right arrow over (H.sub.BC)} + H.sub.TXAux meas i .Math. {right arrow over (V.sub.opt .Math.)}∥ 11  Theoretical TxAUX interference-reduction field - {right arrow over (H.sub.BTxCOpt .Math.)} = H.sub.TXAux meas i .Math. {right arrow over (V.sub.opt .Math.)} 12  Update overall weighting vector {right arrow over (V)}.sub..Math. = {right arrow over (V.sub.opt .Math.)} .* {right arrow over (V)}.sub.i−1 13 End

[0062] According to line 1, the radiofrequency alternating electromagnetic field is measured at N sensor sites, yielding the vector {right arrow over (H.sub.BC)} containing N entries. In line 2, the weighting vector is initialized and is given a defined initialization value for i=0. Lines 3 and 13 describe the start and end of a loop, that is cycled through until a defined maximum number of iterations or a break criterion is reached. In lines 4 to 7, for each individual AUX antenna m individually is measured an associated signal response at the N sensor sites. In order to form the matrix in line 6, these measured values are multiplied by the last optimization weight {right arrow over (V)}.sub.i-1(m) for the given antenna m. Here the operator “:” means that measured values from all N sensors are determined separately.

[0063] Then, according to lines 8 and 9, the actually measured TxAUX interference-reduction field is found as a sum of all the TxAUX signal responses obtained in line 6.

[0064] In line 10, a new adjustment vector {right arrow over (V.sub.opt 1)} is formed, that is obtained from the minimum of the sum of the measured BC emission field and the actually measured interference-reduction matrix multiplied by the corresponding weighting vector. According to line 11, a theoretical, optimum TxAUX interference-reduction field is obtained from each measured interference-reduction field by multiplying by the new adjustment vector {right arrow over (V.sub.opt 1)}. Finally, according to line 12, the new weighting vector is obtained from the new adjustment vector and the last weighting vector, where the individual entries in the vectors are multiplied by one another, expressed by the operator “.*”.

[0065] FIG. 3 depicts schematically the method flow of an embodiment. In a first step S1, a radiofrequency alternating electromagnetic field is generated. For example, this alternating field is generated by the body coil 14. In a second step S2, the magnetic field strength of the radiofrequency alternating electromagnetic field is measured. This measurement may be performed by one or more sensors or calibration elements. The measurement locations are usually situated where as large a cancellation as possible of the radiofrequency alternating electromagnetic field is meant to take place. The sensor(s) 86 may be arranged in the environment of the magnet part 10 of the magnetic resonance tomography unit 1.

[0066] In a step S3, an electromagnetic interference-reduction field for interference suppression or for reducing the magnetic field strength of the radiofrequency alternating electromagnetic field is generated at at least one defined location on the basis of a product of a weighting factor and a defined interference-reduction field strength. The interference-reduction field may be generated by one or more antennas or coils 81, that are arranged at the exit or entrance of the patient tunnel 16. Step S4 includes measuring a magnetic field strength of the generated interference-reduction field. This measurement of the interference-reduction field may be performed using the same sensors or calibration elements 86 as the measurement of the radiofrequency alternating electromagnetic field to be suppressed.

[0067] In a further step S5, an adjustment factor for the weighting factor is determined in such a way as to minimize a sum of the measured field strength of the radiofrequency alternating electromagnetic field and the product of the adjustment factor and the measured field strength of the electromagnetic interference-reduction field. Finally, in a step S6, the weighting factor is updated by multiplying by the adjustment factor.

[0068] The steps S3 to S6 constitute a step series that is repeated multiple times: these repetitions always take as a starting point a latest interference-reduction field, that is then optimized by a weighting factor such that the interference field, for example the radiofrequency alternating electromagnetic field, is reduced further.

[0069] Therefore, in step S7, a check is performed to ascertain whether a defined number of iteration steps or a break criterion is fulfilled. If the break criterion or the defined iteration count is not yet reached, the method jumps back to step S3, and an improved interference-reduction field is generated. Else, if the break criterion or the required number of iteration steps is reached, the method jumps to step S8, in which the optimized interference-reduction field is used to suppress the radiofrequency alternating electromagnetic field outside the magnetic resonance tomography unit.

[0070] Incorrect settings and environmental influences are implicitly captured by the iterative approach, and the interference-reduction or cancellation weights progressively approach the optimum interference-reduction solution. The difference between the numerically optimized and the measured interference-reduction weights decreases with every iteration. The residual precision is then primarily limited by dynamic effects and the stability of the N sensors or antenna measurement points. Therefore, the number of iterations may either be hard-coded or a suitable break criterion may be used, for instance a difference threshold between expected and measured interference-reduction or cancellation vector.

[0071] FIG. 4 depicts the optimization in a diagram by way of example. The starting point is an interference-reduction field {right arrow over (H.sub.TxCMeas 1)} measured in a real situation. In this situation it may be seen that the interference-reduction field does not fully suppress the interference field. The optimum interference-reduction field lies in a solution space 40 at an optimum 41. In this optimum 41, the product {right arrow over (H.sub.TxAux)}.Math.{right arrow over (V)}=−{right arrow over (H.sub.BC)}. Now an additional theoretical interference-reduction field {right arrow over (H.sub.TxCOpt 1)} may be determined by the above optimization algorithm. This theoretical interference-reduction field, however, leads in the next iteration step to an actually measured interference-reduction field {right arrow over (H.sub.TxCMeas 2)}, from which in turn an additional theoretical interference-reduction field {right arrow over (H.sub.TxCOpt 2)} may be determined. The latter, however, leads in the third iteration step to a measured interference-reduction field {right arrow over (H.sub.TxCMeas 3)}, from which in turn an additional optimized interference-reduction field {right arrow over (H.sub.TxCOpt 3)} is determined. In the fourth iteration step, this results in the measured interference-reduction field {right arrow over (H.sub.TxCMeas 4)} and so on. It may be seen that the interference-reduction field measured in reality gets ever closer to the optimum 41 with each iteration step.

[0072] By selecting a suitable calibration RF pulse in line 6 of the above pseudocode for the iterative approach, the iterative interference-reduction weighting vector may be extended in multiple dimensions. For example, the RF pulse may cover a plurality of frequencies and amplitude levels, and an individual calibration weight may be determined accordingly iteratively. It is hence advantageously possible to provide a short and efficient approach to overcoming/correcting hardware imperfections and environmental factors that impair the interference-reduction performance. In addition, a method may thereby be provided for instantaneously determining optimized interference-reduction weights for various amplitude and frequency levels of the Cx chain.

[0073] 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 invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that the 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.

[0074] While the present invention 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.