Method and magnetic resonance apparatus to acquire magnetic resonance data of a target region including a metal object

Abstract

In a method and magnetic resonance (MR) apparatus to acquire MR data of a target region that includes a metal object, an MR sequence that includes at least one radio-frequency excitation to be emitted via a radio-frequency coil arrangement is used. A radio-frequency coil arrangement having multiple coil elements that can be controlled independently with different amplitude and/or phase is used. The amplitudes and/or phases of the coil elements that describe the polarization of the radio-frequency field are selected to at least partially reduce artifacts arising in the metal object due to the radio-frequency excitation, in comparison to a homogeneous, circular polarization of the radio-frequency field of the radio-frequency field in the target region.

Claims

1. A method for acquiring magnetic resonance (MR) data from a target region of an examination subject, which includes a metal object, comprising: operating an MR data acquisition unit, which comprises multiple, individually controllable radio-frequency (RF) coil elements, in which an examination subject is situated, said examination subject comprising a target region that contains a metal object, to acquire MR data from the target region using a magnetic resonance sequence that comprises at least one RF excitation of nuclear spins in the target region; in said MR sequence, radiating RF signals respectively from said multiple RF coils elements with respectively different amplitudes and/or phases, and setting the respectively different amplitudes and/or phases to produce a non-circular polarization of an RF field in said target region that reduces locally induced RF currents in said metal object due to said RF field, in comparison to RF currents induced by a homogenous, circular polarization of said RF field in said target region; and providing MR data acquired from the target region in said MR sequence to a processor and, in said processor, reconstructing an image of the target region from the MR data, said MR image comprising reduced image artifacts as a result of the reduced induced currents, and making said MR image available as a data file at an output of said processor.

2. A method as claimed in claim 1 comprising operating said MR data acquisition unit with a turbo spin echo sequence as said MR sequence.

3. A method as claimed in claim 1 comprising, prior to acquiring said MR data by operating said MR data acquisition unit with said MR sequence, operating said MR data acquisition unit to acquire MR calibration data from an additional metal object, which coincides with said metal object in said target region, that represent a parameter that describes said metal object, and, in said processor, using said calibration data to determine said amplitudes and/or said phases.

4. A method as claimed in claim 3 comprising, in said processor, using said calibration data to calculate a simulation of operation of said MR data acquisition unit according to said MR sequence to acquire said MR data from said target region containing said metal object, and using said simulation to determine said amplitudes and/or phases.

5. A method as claimed in claim 1 comprising, in said processor, determining said amplitudes and/or said phases by implementing an optimization procedure that starts from an initial setting describing said homogenous, circular polarization of said RF field, and wherein said basic setting is successively modified until a predetermined image quality, corresponding to a degree of said artifacts in said image, is obtained.

6. A method as claimed in claim 5 comprising implementing said optimization procedure using a model of said target region with said metal object therein in said MR data acquisition unit.

7. A method as claimed in claim 6 comprising using, as said model, a model wherein the metal object is assumed as a homogenous model object having a geometric shape, selected from the group consisting of a round rod, a polygonal rod, a sphere, and an ellipsoid, within an infinitely long cylinder or ellipsoid filled with a fluid.

8. A method as claimed in claim 7 wherein said fluid is water.

9. A method as claimed in claim 6 comprising determining at least one model parameter of said model from data obtained by operating said MR data acquisition unit in a preparation measurement that precedes operation of said MR data acquisition unit with said MR sequence.

10. A method as claimed in claim 9 comprising, in said processor, determining, as said at least one model parameter, a position of a patient in the MR data acquisition unit, from said data acquired in said preparation measurement.

11. A method as claimed in claim 9 comprising, in said processor, determining, as said at least one model parameter, a position of the position of the metal object in the patient, from said data acquired in said preparation measurement.

12. A method as claimed in claim 6 comprising implementing said optimization procedure to identify said amplitudes and/or said phases that produce a polarization of said RF field that minimizes an electrical field in a region of said model, in which a model object representing said metal object, is situated.

13. A method as claimed in claim 6 comprising implementing said optimization procedure to determine said amplitudes and/or said phases that produce a polarization of said RF field that optimally homogenizes a totality of said RF field adjacent to a model object, that represents said metal object, in said model.

14. A method as claimed in claim 5 comprising operating said MR data acquisition unit to acquire an RF field map, and adapting initial values for said amplitudes and/or phases in said optimization procedure dependent on said RF field map.

15. A method as claimed in claim 14 comprising operating each of said multiple RF coils with a transmitter voltage that contributes to a field strength of said RF field, and adapting the respective transmitter voltages in said optimization procedure to determine said amplitudes and/or said phases that produce a predetermined field strength of said RF field.

16. A method as claimed in claim 1 comprising providing said processor with an input that describes a parameter of said metal object and, via said processor, retrieving said amplitudes and/or said phases from a look-up table as values for said amplitudes and/or said phases that are stored in said look-up table corresponding to said parameter.

17. A method as claimed in claim 1 comprising operating said MR data acquisition unit before operation of said MR data acquisition unit according to said MR sequence, in an adjustment measurement to acquire at least two parameter sets of said amplitudes and/or said phases, and setting said amplitudes and/or said phases in said MR sequence dependent on said at least two parameter sets.

18. A method as claimed in claim 17 comprising operating said MR data acquisition unit in a projection measurement, as said adjustment measurement.

19. A method as claimed in claim 1 comprising operating said MR data acquisition unit in at least two repetitions of said MR sequence and, in each of said repetitions, acquiring MR data only from a sub-region of said target region, and thereby obtaining at least two data subsets, and combining said at least two data subsets dependent on a degree of said artifacts respectively contained therein, in order to reconstruct said image of said target region.

20. A magnetic resonance apparatus comprising: an MR data acquisition unit comprising multiple, individually controllable radio-frequency coil elements; a control unit configured to operate the MR data acquisition unit, which comprises multiple, individually controllable radio-frequency (RF) coil elements, in which an examination subject is situated, said examination subject comprising a target region that contains a metal object, to acquire MR data from the target region using a magnetic resonance sequence that comprises at least one RF excitation of nuclear spins in the target region; said control unit being configured to operate the MR data acquisition unit in said MR sequence to radiate RF signals respectively from said multiple RF coils elements with respectively different amplitudes and/or phases, and setting the respectively different amplitudes and/or phases to produce a non-circular polarization of an RF field in said target region that reduces locally induced RF currents in said metal object due to said RF field, in comparison to RF currents induced by a homogenous, circular polarization of said RF field in said target region; and a processor provided with MR data acquired from the target region in said MR sequence, said processor being configured to reconstruct an image of the target region from the MR data, said MR image comprising reduced image artifacts as a result of the reduced induced currents, and to make said MR image available as a data file at an output of said processor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the acquisition of MR data from a target region that contains a metal object according to the prior art.

(2) FIG. 2 illustrates, in a diagram corresponding to FIG. 1, the acquisition of MR data from a target region containing a metal object, with the method according to the invention.

(3) FIG. 3 schematically illustrates a magnetic resonance apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIG. 1 explains the prior art from which the present invention proceeds. Magnetic resonance (MR) data are to be acquired from a target region 1 in which a metal object 2 (an implant, for example) is present, using a magnetic resonance apparatus (here only portions of the radio-frequency transmission system thereof are shown). For acquisition of magnetic resonance data, it is known to control a radio-frequency coil arrangement 3 so that—at least in the target area 1—a homogeneous, circular polarization of the resulting radio-frequency field, with which the radio-frequency excitation should take place, is created, as indicated by the symbol 4. A circularly polarized B1 field thus exists in the target region 1. This is achieved by the radio-frequency signal generated by an amplifier device 5 being divided via a 90° hybrid device 6, and passed to the radio-frequency coil arrangement 3. Due to this radio-frequency field, induced radio-frequency currents occur within the metal object 2 that, in turn, result in radio-frequency fields 7 that cause artifacts (signal shadows, for example) in the acquired magnetic resonance data. Such artifacts generated by the radio-frequency field (B1 field) should be reduced by the method according to the invention.

(5) The present invention is based on the insight that the radio-frequency field can be modified (in particular with regard to its polarization) so that an essentially identical imaging result is nevertheless achieved, but the artifacts are at least reduced due to the modified radio-frequency field. In accordance with the invention as shown in FIG. 2, a radio-frequency coil arrangement 3′ is thus used that has multiple, independently controllable coil elements in which different amplitudes and phases can be selected (set) independently for the individual coil elements, so a variation range exists for the amplitudes and phases that can be utilized according to the invention in order to determine an artifact-reducing setting.

(6) This is depicted in FIG. 2 as an example of a radio-frequency coil arrangement 3′ with two transmission channels, wherein each transmission channel is controlled via a corresponding amplifier device 5a, 5b. The amplitude A.sub.1 and the phase P.sub.1 are used for the first transmission channel; the amplitude A.sub.2 and the phase P.sub.2 are used for the second transmission channel. The phases and amplitudes are now selected by a control device of the magnetic resonance device for the specific use case so that the artifacts are reduced. This is achieved by an elliptical polarization of the frequency field, as is indicated by the symbol 8. In this way, the induced radio-frequency currents in the metal object 2 are reduced so that reduced induced fields 7′ also result, and overall a reduction of artifacts in the magnetic resonance data occurs.

(7) The invention is naturally not limited to two transmission channels or two coil elements, but can also be used with radio-frequency coil arrangements 3′ having a greater number of independently controllable transmission channels.

(8) In a first exemplary embodiment of the method according to the invention, when MR data from a target region 1 should be acquired in which a metal object 2 (in particular an implant) is present, parameters describing the target region 1 (in particular the body region) and object parameters describing the metal object 2 are initially compiled. A turbo spin echo sequence is used as a magnetic resonance sequence in order to also largely avoid artifacts due to the disruption of the basic magnetic field (B0 field). Variations of this magnetic resonance sequence can be selectively used, with the sequence parameters that describe each variation being known in advance. All of these parameters can now be used in order to retrieve suitable amplitudes and phases for the current magnetic resonance data acquisition in a look-up table. The corresponding phases and amplitudes are then used to control the radio-frequency coil arrangement.

(9) The data sets (which include phases and amplitudes for the different transmission channels and are associated with object parameters and/or target region parameters and/or sequence parameters and/or additional input parameters) can be determined in different ways, but preferably within the scope of an optimization process. Measurements to acquire calibration data are possible and calculations, in particular using simulations, can be alternatively or additionally implemented. Measurements of calibration data can take place in that conditions defined by a specific values of object parameters and possible additional input parameters (for example with the addition of a specific implant) are established, and calibration data are acquired for different values of amplitudes and phases for the channels. It is advantageous that here a form of an optimization process is already used as a basis, which means that amplitudes and phases that are to be used for the measurement of calibration data are determined next under consideration of the calibration data acquired in a preceding acquisition. The procedure can be the same for simulations. Given such optimization processes, and in general given the implementation of calibration measurements and/or simulations, attention must be paid to the variation range so that a defined image quality is maintained. Results of simulations or the calibration data are assessed with regard to the presence of artifacts, for example by determining values of a cost function, so that values for the amplitude and the relative phases can be obtained that are optimal, or at least improved with regard to artifacts.

(10) As an alternative to the use of look-up tables, it is also possible to initially implement an adjustment measurement for at least two parameter sets of amplitudes and/or phases before the acquisition of the magnetic resonance data, wherein the adjustment data that are thereby measured are evaluated with regard to artifacts, and the amplitudes and/or phases are selected depending on the result of the adjustment measurements, wherein an optimization process can again be used. The adjustment measurements are then preferably measurements that can be implemented quickly, for example projection measurements. Shadow artifacts will appear in these projection measurements.

(11) In a further embodiment of the invention, it is also possible to determine the most suitable polarization (consequently the optimal phases and/or amplitudes), individualized to the patient, through a sequence of an optimization process with calculations in a model and at least one test measurement. However, the procedure described in the following is naturally also suitable to determine data sets for the aforementioned look-up such tables.

(12) In this approach, a model is used that describes the patient, the metal object 2 and the interaction with the magnetic resonance apparatus. Analytical calculations are possible using a simple geometric shape for the model object representing the metal object 2 in the model. If it is desired that the shape of the model object should correspond to the actual geometric shape of the metal object 2, a numerical simulation can be used for field calculations in the model.

(13) As a specific example for explanation, a metal rod is considered as a model object in an infinitely long cylinder that is filled with water. An ellipsoid is also conceivable to describe the model of the patient. Background concerning analytical calculations in this regard, can be found in the articles by John G. Sled and G. Bruce Pike, “Standing-Wave and RF Penetration Artifacts Caused by Elliptic Geometry: An Electrodynamic Analysis of MRI”, IEEE Transactions on Medical Imaging 17 (1998), P. 653-662, and by James Tropp, “Image brightening in samples of high dielectric constant”, Journal of Magnetic Resonance 167 (2004), P. 12-24.

(14) To complete the model, a preparation measurement is initially implemented in which the position of the metal object 2 in the patient and the position of the patient in the magnetic resonance device are determined. Corresponding measurement methods—for example the acquisition of localizers—are already sufficiently known in the prior art.

(15) In the following, a radio-frequency coil arrangement 3′ with two coil elements (thus two transmission channels) is again assumed, for example. A pair of linearly polarized radio-frequency fields B.sub.1.sup.x and B.sub.1.sup.y is generated along the x-axis and y-axis. The resulting vector potential A can be determined analytically in the cylinder (see the aforementioned article) and depends on the polarization of the two fields that yields the total polarization. A.sub.x is thereby induced by B.sub.1.sup.x, and A.sub.y is induced by B.sub.1.sup.y.
A.sup.t=real(A.Math.e.sup.iωt) with A=A.sub.x.Math.e.sup.idφ+A.sub.y

(16) The radio-frequency field undistorted by the model object then results as
B=rot A.sup.t
and the electrical field connected with this at the location r results as
E(r)=iω/2r×B(r),
which, in the model object (formed here as a metal rod) of length l.sub.R at the position p=(x.sub.p,y.sub.p), generates a current flow in the z-direction
I.sub.z≈E.sub.z(p)l.sub.R/(iωL.sub.R)
depending on its inductivity L.sub.R. This in turn now induces the interference field

(17) $B_x^{\mathrm{ind}}\left(r\right)=-\frac{{\mu }_0}{2\pi }\frac{\sin \left({\Psi }_r\right)}{r_p}{I}_z\mathrm{and}$$B_y^{\mathrm{ind}}\left(r\right)=\frac{{\mu }_0}{2\pi }\frac{\cos \left({\Psi }_r\right)}{r_p}{I}_z,$
wherein the angle Ψ.sub.r is defined as an azimuth of the metal rod, and r.sub.p designates the length between the considered point r and the metal rod at position p.

(18) The total effective radio-frequency field (total radio-frequency field) in the object can consequently be described as

(19) $B_1^{\mathrm{tot}}\left(r\right)=\left({\partial }_y+i{\partial }_x\right)A\left(r\right)+\frac{{\mu }_0{l}_R}{4\pi L_R}\frac{\sin \left({\Psi }_r\right)+i\cos \left({\Psi }_r\right)}{r_p}\left(x_p{\partial }_x+y_p{\partial }_y\right)A\left(p\right).$

(20) An infinitely fast propagation speed is hereby assumed.

(21) Different fields for different polarizations can now be determined with this, such that an optimization process can be realized. Two optimization goals are conceivable. The variation of the polarization can be used in the optimization method to set the electric field for all points belonging to the model object as close to zero as possible. Alternatively, the total radio-frequency field B.sub.1.sup.tot can be required to be as homogenous as possible in a region (preferably the entire region) adjacent to the model object. The polarization of A is varied by variation of the amplitudes and/or phases.

(22) The theory described here can naturally also be extended to radio-frequency coil arrangements with more than two transmission channels.

(23) Within the scope of a test measurement, after conclusion of the optimization process the usability of the determined amplitudes and phases can in practice be checked, for which a radio-frequency field map is measured (B1 map) on the basis of which an additional adaptation of the amplitudes and/or phases—or even of the model—can take place with new optimization.

(24) Particularly in cases in which an optimally low electrical field—or even an electrical field that is not present at all—is optimized in the region of the model object (but also otherwise), cases can occur in which the strength of the resulting total radio-frequency field (in particular determined within the scope of the test measurement) is too low or too high, which can be compensated via adaptation of the transmitter voltage so that the desired flip angle is achieved.

(25) Within the scope of the method according to the invention, it is also possible to acquire multiple data subsets when a sufficient reduction of artifacts cannot be achieved from the data set for the totality of the target region 1. By acquiring respective data subsets that each represent only a sub-region of the overall target region 1, a sub-region-by-sub-region reduction or spatial shift of the artifacts occurs. For the acquisition of the multiple data subsets different parameter sets of amplitudes and phases are used so that complete and wide-ranging, artifact-reduced magnetic resonance data can be obtained by a combination of these data subsets or the respective sub-images resulting therefrom.

(26) FIG. 3 shows a magnetic resonance device 9 according to the invention. As is basically known, this apparatus has a basic magnetic field unit 10 in which a cylindrical patient receptacle 11 is situated. A patient bed (not shown) is provided to move a patient into and out of the receptacle 11. Surrounding the patient receptacle 11 are a gradient coil arrangement 12 and a radio-frequency coil arrangement 3′ with independently controllable RF coil elements.

(27) The parameters (thus amplitudes and phases) with which the transmission channels of the radio-frequency coil arrangement 3′ can be controlled can be selected by a control device 13 as described above, in order to reduce artifacts due to the radio-frequency inductions in the metal object 2, which means that the control device 13 is designed to implement the method according to the invention. A memory to store the look-up table (if used) can be provided, preferably within the control device 13.

(28) Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.