METHOD FOR RECORDING A MAGNETIC RESONANCE IMAGE DATASET, DATA MEDIUM, COMPUTER PROGRAM PRODUCT, AND MAGNETIC RESONANCE INSTALLATION

Abstract

A method for recording a magnetic resonance image dataset includes providing a magnetic resonance sequence with a series of sequence blocks, and providing at least one correction term to compensate for a magnetic field change. The magnetic field change is produced as a change of an actual magnetic field compared to a setpoint magnetic field by gradient pulses. The magnetic field change is established via a transfer characteristic of the gradient system of the magnetic resonance installation. The at least one correction term is used to compensate for the magnetic field change, and at least one magnetic resonance image dataset is recorded with the magnetic resonance sequence using the correction term.

Claims

1. A method for recording a magnetic resonance image dataset with a magnetic resonance installation having a gradient system, the method comprising: providing a magnetic resonance sequence for carrying out a magnetic resonance measurement, wherein the magnetic resonance sequence has a series of sequence blocks and an excitation section, and at least one detection section is present in each sequence block of the series of sequence blocks, and wherein the magnetic resonance sequence has at least one gradient pulse; providing at least one correction term to compensate for a magnetic field change, wherein the magnetic field change is produced as a change of an actual magnetic field compared to a setpoint magnetic field by the at least one gradient pulses, wherein the magnetic field change is established via a transfer characteristic of the gradient system of the magnetic resonance installation, and the transfer characteristic represents the system characteristic of the gradient system in amplitude, phase, or amplitude and phase of different frequencies; compensating for the magnetic field change using the at least one correction term; and recording at least one magnetic resonance image dataset with the magnetic resonance sequence using the at least one correction term.

2. The method of claim 1, wherein a Steady State Free Precession (SSFP) measurement sequence is used as the magnetic resonance sequence.

3. The method of claim 1, further comprising creating a compensation field using the at least one correction term, wherein the compensation field is applied at least in part after at least one detection section of the magnetic resonance sequence.

4. The method of claim 1, wherein the magnetic field change is a magnetic field change of the basic magnetic field, and the correction term is used to create a compensation phase or a basic compensation field.

5. The method of claim 4, wherein the magnetic field change of the basic magnetic field is computed as:
ΔB.sub.k,l=0(t)=.sup.1({Gnom,k(t)}.Math.GSTFk,l(f)}), wherein l refers to the 0th order of a transfer function of the gradient system of the magnetic resonance installation, k refers to a gradient axis, refers to a Fourier transform, and G.sub.nom,k refers to the at least one gradient pulse applied to the gradient axis.

6. The method of claim 1, wherein the magnetic field change is a magnetic field change of the at least one gradient pulse, and the correction term is used to create a compensation gradient moment.

7. The method of claim 6, further comprising storing the correction term for the compensation moment in a pre-emphasis filter for the gradient system of the magnetic resonance installation.

8. The method of claim 7, wherein the pre-emphasis filter is used exclusively for correction of the magnetic field change of the at least one gradient pulse.

9. The method of claim 6, further comprising adding the compensation gradient moment to a gradient pulse after the setting of parameters of the magnetic resonance sequence.

10. The method of claim 1, wherein the magnetic field change is a magnetic field change of a field term of an order greater than 1, and the correction term is determined to create a compensation field moment.

11. The method of claim 1, wherein a transfer function or a Fourier-transform of the transfer function is used as the transfer characteristic.

12. The method of claim 1, wherein the magnetic resonance installation comprises a transfer characteristic-based pre-emphasis filter for the gradient system, and the transfer characteristic-based pre-emphasis filter is used to compensate for deviations of the gradient pulses.

13. A non-transitory computer-readable storage medium that stores instructions executable by a control facility to record a magnetic resonance image dataset with a magnetic resonance installation having a gradient system, the instructions comprising: providing a magnetic resonance sequence for carrying out a magnetic resonance measurement, wherein the magnetic resonance sequence has a series of sequence blocks and an excitation section, and at least one detection section is present in each sequence block of the series of sequence blocks, and wherein the magnetic resonance sequence has at least one gradient pulse; providing at least one correction term to compensate for a magnetic field change, wherein the magnetic field change is produced as a change of an actual magnetic field compared to a setpoint magnetic field by the at least one gradient pulses, wherein the magnetic field change is established via a transfer characteristic of the gradient system of the magnetic resonance installation, and the transfer characteristic represents the system characteristic of the gradient system in amplitude, phase, or amplitude and phase of different frequencies; compensating for the magnetic field change using the at least one correction term; and recording at least one magnetic resonance image dataset with the magnetic resonance sequence using the at least one correction term.

14. The non-transitory computer-readable storage medium of claim 13, wherein a Steady State Free Precession (SSFP) measurement sequence is used as the magnetic resonance sequence.

15. The non-transitory computer-readable storage medium of claim 13, wherein the instructions further comprise creating a compensation field using the at least one correction term, and wherein the compensation field is applied at least in part after at least one detection section of the magnetic resonance sequence.

16. The non-transitory computer-readable storage medium of claim 13, wherein the magnetic field change is a magnetic field change of the basic magnetic field, and the correction term is used to create a compensation phase or a basic compensation field.

17. A magnetic resonance installation comprising: a gradient system; and a control facility, wherein the control facility is configured to record a magnetic resonance image dataset with a magnetic resonance installation having a gradient system, the recordation of the magnetic resonance image dataset comprising: provision of a magnetic resonance sequence for carrying out a magnetic resonance measurement, wherein the magnetic resonance sequence has a series of sequence blocks and an excitation section, and at least one detection section is present in each sequence block of the series of sequence blocks, and wherein the magnetic resonance sequence has at least one gradient pulse; provision of at least one correction term to compensate for a magnetic field change, wherein the magnetic field change is produced as a change of an actual magnetic field compared to a setpoint magnetic field by the at least one gradient pulses, wherein the magnetic field change is established via a transfer characteristic of the gradient system of the magnetic resonance installation, and the transfer characteristic represents the system characteristic of the gradient system in amplitude, phase, or amplitude and phase of different frequencies; compensation for the magnetic field change using the at least one correction term; and recordation of at least one magnetic resonance image dataset with the magnetic resonance sequence using the at least one correction term.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] FIG. 1 shows one embodiment of a magnetic resonance installation;

[0079] FIG. 2 shows an exemplary bSSFP sequence diagram with Cartesian k-space sampling;

[0080] FIG. 3 shows an exemplary first gradient pulse with added gradient correction term;

[0081] FIG. 4 shows an exemplary bSSFP sequence diagram with spiral-shape k-space sampling;

[0082] FIG. 5 shows the read gradient impulses of FIG. 4 with added compensation field;

[0083] FIG. 6 shows an exemplary transfer function for the order 0;

[0084] FIG. 7 shows magnitude values of a transfer function for the order 1;

[0085] FIG. 8 shows phase values of a transfer function for the order 1;

[0086] FIG. 9 shows a first exemplary execution scheme for recording a magnetic resonance dataset; and

[0087] FIG. 10 shows a second exemplary execution scheme for recording a magnetic resonance dataset.

DETAILED DESCRIPTION

[0088] FIG. 1 shows one embodiment of a magnetic resonance installation 1 with a scanner 2 and a control facility 3. The scanner 2 includes a gradient system 4 consisting of three separate coils 5, 6, and 7 for creation of three gradient fields. The gradient fields able to be created are aligned in the x-, y-, and z-direction and are at right angles to one another.

[0089] Arranged at the scanner 2 is also a transmit coil arrangement 8 and a receive coil arrangement 9. The transmit coil arrangement 8 may be embodied as a whole-body coil. The receive coil arrangement 9 may be embodied as a coil array. The receive coil arrangement 9 is used as a local coil.

[0090] Basically, however, the transmit coil arrangement 8 may also be used for signal receipt. The use of a receive coil arrangement 9, however, increases not only the signal-to-noise ratio SNR per se, but may also be used to carry out parallel imaging. This enables the measurement time to be reduced.

[0091] The control facility 3 of the magnetic resonance installation 1 has a data medium 10 on which a computer program product 11 for carrying out the described method is stored. Stored on the data medium 10 may be a bSSFP measurement sequence 12 with Cartesian k-space sampling, a bSSFP measurement sequence 13 with spiral-shaped sampling, and other magnet resonance sequences 14.

[0092] As well as this, at least one pre-emphasis filter p, p1 and/or p2 may be stored on the data medium.

[0093] For the sake of clarity, additional usual components of the magnetic resonance installation 1, such as a patient couch, etc., are not shown.

[0094] FIG. 2 shows a bSSFP sequence diagram 15 for bSSFP measurement sequence 12 with a Cartesian sampling scheme. The imaging gradient axes are labeled as usual with G.sub.R for the read direction, G.sub.P for the phase encoding direction, and G.sub.S for the slice selection direction. These may match the axes x, y and z from FIG. 1, as described in the introduction, but do not have to do so.

[0095] Even if the axes match, a fixed assignment is not mandatory, and the axis of the read direction may thus lie in the direction of the x-, y-, or z-axis. The axis of the phase encoding direction and the axis of the slice selection direction are distributed between the remaining axes from the group x-axis, y-axis, and z-axis.

[0096] Usually, however, the axes do not match. The gradients in the directions G.sub.R, G.sub.P, and G.sub.S are then formed by two or three of the coils 5, 6 and 7, and thus, the gradients are formed in the direction of the axes x, y, and z.

[0097] ACQ refers to the axis for the radio-frequency impulses and acquisition window.

[0098] In order to excite only one slice with the radio-frequency impulses 16, a slice selection gradient 17 is present at the same time as the radio-frequency impulse 16 in the slice selection direction G.sub.S. In order to compensate for a dephasing effect on the magnetization in the transversal plane, a slice rephasing gradient 18 follows on directly from the slice selection gradient 17.

[0099] The radio-frequency impulses of two consecutive sequence blocks may have two alternating RF-phases α and −α in this case.

[0100] In phase encoding direction G.sub.P, a phase encoding gradient 19 is used. This is provided from sequence block to sequence block (e.g., in each of the n.sub.pe repetitions, with varying strengths) in order to create a spatial encoding in phase encoding direction G.sub.P. This is indicated by the arrow 20.

[0101] The read dephasing gradient 21 and the read gradient 22 lie in direction G.sub.R. An acquisition window 23 is open at the same time as the read gradient 22 in order to record the echo signal 24.

[0102] In addition to these gradients, which are present for imaging in all magnetic resonance sequences in which a Cartesian sampling of the k-space is undertaken, a slice rewind gradient 25 and a further slice selection gradient 17 are present in the slice selection direction G.sub.S. The slice selection gradient 17 is assigned to the next radio frequency impulse 16 of the next sequence block. These provide that the bSSFP measurement sequence 11 is “balanced” over a repetition time T.sub.R in the slice selection direction G.sub.S. For example, the sums of the gradient moments in slice selection direction G.sub.S are equal to zero over a repetition time T.sub.R.

[0103] With the same purpose, a phase rewind gradient 26 and a read rewind gradient 27 are also present. In the phase rewind gradient 26, an arrow 28 is arranged in the opposite direction to the arrow 20. This shows the opposite polarity of the phase encoding gradient 19 compared to the phase rewind gradient 26 when the phase encoding gradient 19 and the phase rewind gradient 26 each have the same strength.

[0104] The gradients in read direction G.sub.R and slice selection direction G.sub.S are thus arranged symmetrically as regards the point in time 29, but the phase encoding gradient 19 and the phase rewind gradient 26 are arranged antisymmetrically. The gradients are thus arranged time-symmetrically with regard to the point in time 29.

[0105] A sequence block extends from one radio-frequency impulse to the next and has the length TR. The excitation section includes the excitation impulse 16 and the slice selection gradient 17. The detection section contains the read gradients 26. Provided this is present, an acquisition window is also open.

[0106] The bSSFP measurement sequence 12 is this case is representative of all SSFP measurement sequences, in which a steady state obtains between the excitation and the relaxation of the magnetization. Between the excitation pulses, the nuclear spins precess (e.g., free precession). The steady magnetization state is decisive for the strength of the MR signal.

[0107] In order to obtain the desired dynamic equilibrium, the gradient moment for all SSFP measurement sequences in each TR is to be the same; for the bSSFP measurement sequence, the gradient moment is to produce the value zero as well as possible over a repetition time TR. Since gradients may have a bipolar effect on the nuclear spins, it is possible to revise their effect by opposing gradients. This applies independently of the sampling type of the k-space (e.g., also for the embodiment according to FIG. 4).

[0108] FIG. 3 shows a gradient pulse (e.g., a phase encoding gradient in a predetermined sequence block). Without taking account of the self terms and cross terms, the gradient pulse 30 shown as a dashed line would be applied as a phase rewind gradient. For correction of the gradient moment, however, a compensation moment ΔGM is added to the gradient moment, where both a positive and also a negative compensation moment ΔGM may be added. In general terms, the compensation moment is then added or subtracted.

[0109] In this case, the compensation moment ΔGM may have an additional portion ΔGM.sub.z and a free portion ΔGM.sub.f. The free portion is applied after the gradient pulse 30. The free portion compensates for oscillations in the magnetic resonance installation. What is involved here is a continuous correction, which is undertaken for as long as the oscillations are present.

[0110] The gradient pulse shown by a solid line (e.g., the phase rewind gradient 26) is then applied in the magnetic resonance measurement. The free portion ΔGM.sub.f and the additional portion ΔGM.sub.z may be taken into account via a pre-emphasis filter. Both portions ΔGM.sub.z and ΔGM.sub.f may be implemented independently of one another. Both, or just one of the two, may be implemented via the pre-emphasis filter. The other portion may be not taken into account at all, or the other portion may also be permanently implemented in the magnetic resonance sequence. Then, however, the respective value may possibly have to be adapted manually.

[0111] Illustrated in FIG. 3 is a form of embodiment, in which, at a predetermined point in time per sequence block (per TR), a correction of the gradients is achieved by applying an additional gradient moment (e.g., compensation field or compensation moment ΔGM). In FIG. 3 in this case, purely by way of example, the phase rewind gradient 26 is shown. For example, a compensation moment ΔGM in the form of a free portion ΔGM.sub.f and/or an additional portion ΔGM.sub.z may also be provided for any other gradient pulse and, for example, for a read rewind gradient 27 via a correction term.

[0112] FIG. 4 shows a bSSFP sequence diagram 31 for bSSFP measurement sequence 13 with a spiral-shaped sampling scheme. The explanations as given for FIG. 2 apply, provided the reference characters match.

[0113] By contrast with the Cartesian sampling scheme, in a spiral sequence of phase encoding gradients, two read encoding directions are used instead, which in FIG. 4, for the sake of clarity, are labeled G.sub.R and G.sub.P. In order to achieve a spiral-shaped k-space trajectory, oscillating gradients 32 and 33 are applied in each case in both read encoding directions.

[0114] To balance out the gradient moments, a read rewind gradient 34 and a phase rewind gradient 35 are located at the end of the oscillating gradients 32 and 33 applied. The uncorrected gradient pulses are shown as dashed lines. Because of the correction terms of the present embodiments, the gradient pulses 34, 35 are corrected; however, a compensation field (e.g., additional compensation field) is applied, for example, so that the rewind gradient pulses 34 and 35 are ultimately output as shown by the solid line. Through this, another gradient moment (e.g., corrected gradient moment) is obtained in both read encoding directions G.sub.R and G.sub.P. The bSSFP measurement sequence 13 is thereby “fully balanced” over a repetition time T.sub.R (e.g., the sums of the gradient moments in all directions are equal to zero over a repetition time T.sub.R).

[0115] A sequence block extends, as in the embodiment according to FIG. 2, from one radio-frequency impulse 16 to the next and has the length T.sub.R. The excitation section includes the excitation impulse 16 and the slice selection gradient 17. In the detection section, the measurement signals 36 are recorded. However, in this process, no k-space rows are recorded; instead a spiral-shaped sampling takes place.

[0116] Instead of the bSSFP measurement sequence, any other SSFP measurement sequence with a spiral-shaped k-space sampling may be used.

[0117] FIG. 5 once again shows a spiral-shaped sampling sequence, as in FIG. 4, with particular focus on the two read encoding gradients G.sub.R and G.sub.P In a similar way to the modification of the rewind gradient pulses 34 and 35, the oscillating gradient pulses 32 and 33 may be corrected in accordance with the present embodiments (e.g., provided with a pre-emphasis). In FIG. 5, the gradient curves 32, 33, which have been applied uncorrected, are shown by dashed lines. The gradient pulses taking account of the correction term of the present embodiments (e.g., with pre-emphasis) are shown by solid lines. FIG. 5 shows that a correction term may also be implemented by a variable modification of the gradient strength of an oscillating gradient. As shown in FIG. 5, a continuous correction may be involved here (e.g., during the entire duration of the gradient pulse 32, 33 a corrected gradient (the uncorrected gradient plus a compensation field) is applied). The correction of the oscillating gradients 32, 33 may be brought about with a pre-emphasis filter.

[0118] In FIG. 5, purely by way of example, the gradients 32 and 33 in the read encoding direction are shown. For example, a compensation field or a compensation moment ΔGM in the form of a free portion ΔGM.sub.f and/or an additional portion ΔGM.sub.z, may also be provided for any other impulse and, for example, for a read gradient 32 and a read rewind gradient 34 and/or for a slice rewinder 18 or slice prewinder 15.

[0119] The gradients 17, 18, 19, 21, 22, 25, 26, 27, 30, 32, 33, 34, 35 shown in the FIGS. 2 to 5 are each a gradient pulse.

[0120] The gradients 17, 18, 19, 21, 22, 25, 26, 27, 30, 32, 33, 34, 35 shown in FIGS. 2 to 5 as solid lines may be the nominal gradient G.sub.nom (e.g., that this form should be present in the magnetic resonance measurement). However, because of imperfections, this is not the case when the nominal gradients are also arranged in this way. However, if the pre-emphasis is calculated-in beforehand, the nominal gradient G.sub.nom may be obtained by energizing a gradient coil to create a pre-emphasized gradient G.sub.pre at the end. The pre-emphasized gradient G.sub.pre may be computed by the formula given above.

[0121] The use of the gradient pulse 30 to add the compensation moment □GM is possible, as already described. However, basically a compensation field or a part of the compensation field may be added to any of the gradient pulses shown. In one embodiment, a continuous or partly continuous correction may be provided, since further interactions may be avoided thereby. The gradient moments in each sequence block where possible are balanced out at the point in time directly before the excitation pulses, since then, a steady state will be obtained. A balancing-out at the times at which the signals are read out improves the image quality further.

[0122] The transfer function GSTF used for determination with 0 components GSTF.sub.x, GSTF.sub.y and GSTF.sub.z, is shown with a magnitude portion in FIG. 6. The frequency in kHz is plotted on the axis 38, and a standardized magnitude value on the axis 39. The line 40 shows the component for x-gradients, line 41 shows the components for y-gradients, and line 42 shows the component for z-gradients. The components may be measured with the field camera method, for example.

[0123] Using the formula already presented

ΔB.sub.k,l=0(t)=.sup.−1({G.sub.nom,k(t)}.Math.GSTF.sub.k,l(f)}).

from the transfer function GSTF or components GSTF.sub.x, GSTF.sub.y and GSTF.sub.z of the transfer function GSTF, a change in the magnetic field ΔB.sub.0,x, ΔB.sub.0,y and ΔB.sub.0,z may be established in each case. From this, a phase change Δϕ is produced.

[0124] FIG. 7 shows the magnitude portion of the components GSTF.sub.xx, GSTF.sub.yy and GSTF.sub.zz of the transfer function GSTF, and thereby of the 1st order. Once again, the frequency in kHz is plotted on the axis 38, and a standardized magnitude value is plotted on the axis 39. In this case, the gradient system may be embodied as a lowpass filter: while low frequencies are transmitted almost in a ratio of 1:1, the portion of higher frequencies is greatly reduced.

[0125] The line 44 shows the component GSTF.sub.xx, line 45 shows the component GSTF.sub.yy, and line 46 shows the component GSTF.sub.zz. These components too may be measured with the field camera method, for example.

[0126] FIG. 8 shows the phase portion of the components GSTF.sub.xx, GSTF.sub.yy and GSTF.sub.zz of the transfer function GSTF, and thereby of the 1st order. Once again, the frequency in kHz is plotted on the axis 38, and a standardized phase value is plotted on the axis 47.

[0127] The line 48 shows the phase values of the component GSTF.sub.xx, the line 49 shows the phase values of the component GSTF.sub.yy, and the line 50 shows the phase values of the component GSTF.sub.zz. These components too may be measured with the field camera method.

[0128] The components shown in FIGS. 7 and 8 may be used for correction of the GIRF-based artifacts of the self terms of the gradient pulses. The 0th order in accordance with FIG. 6 may be employed to establish the basic magnetic field change. The computation of pre-emphasized gradients may be established using the relationship already described

G.sub.pre,k(t)=.sup.1{({G.sub.nom,k(t)}/GSTF.sub.k.sup.l=1(f)}).

[0129] FIG. 9 shows a first flow diagram for recording a magnetic resonance image dataset.

[0130] In act S1, a transfer function GSTF, or, more precisely, at least the 0th order of the components GSTF.sub.x, GSTF.sub.y and GSTF.sub.z, is provided. This provision may be undertaken independently of the following acts.

[0131] This may be used in order to establish an effect of the gradient pulses and compensate for the effect via a B0 or phase correction or via a correction of the gradient pulses (e.g., by application of compensation fields and/or compensation gradient moments).

[0132] In act S2, a magnetic resonance sequence (e.g., an SSFP magnetic resonance sequence and a bSSFP magnetic resonance sequence 12 or 13) may be provided.

[0133] Usually, after the loading of a magnetic resonance sequence in act S3, parameters such as the location of the slice or the slices, the resolution, the number of image elements, etc., are set. Only after this is the exact configuration of the gradients fixed.

[0134] The sequence of gradients or gradient pulses produced by the magnetic resonance sequence 12 may either be used as a whole in order to compute the pre-emphasized gradient G.sub.pre1 taking into account all effects in act S4. The effects taken into account are at least self terms of the 1st order, in a few forms of embodiment cross terms, and/or magnetic field change of the basic magnetic field B.sub.0. Then, in act S5, with the magnetic resonance sequence, a magnetic resonance image dataset is recorded. This may be a 2D image dataset with one or more slices, a 3D image dataset, or also a 4D image dataset. In this case, the pre-emphasis filter p is also employed, via which the correction takes place. Using the pre-emphasis filter p, a compensation moment in the form of an additional portion ΔGM.sub.z and also of a free portion ΔGM.sub.f may be created as a correction term.

[0135] FIG. 10 shows a second embodiment of an execution scheme for recording a magnetic resonance dataset. After the acts S1 to S3, the provision of the transfer function and of the magnetic resonance sequence as well as the setting of the parameters of the magnetic resonance sequence, this has the act S6. In this act, section-by-section (e.g., for a repetition time T.sub.R), the gradient pulses are collectively sent as a digital gradient execution signal to a control facility, a pre-emphasis filter p1 is applied to the gradient pulses, and the filtered gradient pulses are transferred as a pre-emphasized gradient signal G.sub.pre1 to the magnetic resonance installation.

[0136] In this case, the pre-emphasis filter p1 is simple to realize, since from short sections, as described above, nominal gradient pulses G.sub.nom may be converted into pre-emphasized gradient pulses G.sub.pre1.

[0137] The repeated computation is necessary since in different sequence blocks at least the phase encoding gradient 19 and then also the phase rewind gradient 26 have different values. This form of embodiment is, for example, preferred for a Cartesian sampling with ongoing sequence blocks.

[0138] 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.

[0139] 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.