TIME-SAVING GENERATION OF A B0 MAP BASED ON A DUAL ECHO SEQUENCE WITH STIMULATED ECHOES

Abstract

The disclosure relates to a method for generating a B.sub.0 map for a magnetic resonance examination of an examination subject, a magnetic resonance device, and a computer program product for executing the method. The method provides for the application of at least two preparatory RF pulses during a preparatory stage and at least one readout RF pulse during an acquisition stage. At least one stimulated echo signal is acquired after the readout RF pulse. A B.sub.0 map that shows the actual spatial distribution of the magnetic field strength of the main magnetic field is derived from the at least one acquired FID echo signal and the at least one acquired stimulated echo signal.

Claims

1. A method for generation of a B.sub.0 map for a magnetic resonance examination of an examination subject, the method comprising: generating a main magnetic field; applying to an examination space in which the examination subject is located of an imaging sequence having a preparatory stage and a subsequent acquisition stage, wherein the imaging sequence comprises: applying at least two preparatory radio-frequency (RF) pulses during the preparatory stage, wherein a first preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.1, wherein a second preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.2, wherein times t.sub.1 and t.sub.2 are separated by a period TS; applying at least one readout RF pulse at a time t.sub.3 during the acquisition stage; acquiring at least one free induction decay (FID) echo signal after the readout RF pulse at a time t.sub.4, wherein times t.sub.3 and t.sub.4 are separated by a period TE.sub.FID; and acquiring at least one stimulated echo signal after the at least one readout RF pulse at a time t.sub.5, wherein times t.sub.3 and t.sub.5 are separated by a period TE.sub.STE; and deriving a B.sub.0 map that shows the actual spatial distribution of a magnetic field strength of the main magnetic field from the at least one FID echo signal and the at least one stimulated echo signal, wherein the period TS is chosen as a function of the periods TE.sub.FID and TE.sub.STE so that, between times t.sub.4 and t.sub.5, a signal component of the echo signals from protons bound in water has a same phase difference as a signal component of the echo signals from protons bound in fat.

2. The method of claim 1, wherein the choosing of the period TS comprises: specifying the periods TE.sub.FID and TE.sub.STE so that these are as short as possible; and determining the period TS as a function of the specified periods TE.sub.FID and TE.sub.STE.

3. The method of claim 1, wherein: $TS=N*\frac{1}{{\delta }_{WF}*\left(\gamma /2\pi \right)*B_{0,\mathrm{targe}t}}+T{E}_{STE}-T{E}_{\mathrm{FID}}$ wherein: N is an integer>0, δ.sub.WF indicates a chemical shift of water and fat, and γ indicates a gyromagnetic ratio of protons bound in water.

4. The method of claim 3, wherein N=1.

5. The method of claim 1, wherein the main magnetic field has a target magnetic field strength B.sub.0,target of less than 2 T.

6. The method of claim 1, wherein TE.sub.FID<5 ms and TE.sub.STE<6 ms.

7. The method of claim 1, wherein TE.sub.FID<2.5 ms and TE.sub.STE<3.5 ms.

8. The method of claim 1, wherein the at least one FID echo signal and the at least one stimulated echo signal are not suitable for use in deriving a B.sub.1 map that shows a spatial distribution of flip angles of the at least two preparatory RF pulses.

9. The method of claim 1, wherein, to adjust the periods TE.sub.STE and TE.sub.FID: at least one gradient pulse G.sub.prep is applied to the examination space between the at least two preparatory RF pulses during the preparatory stage, at least one gradient pulse G.sub.deph is applied to the examination space after the applying of the readout RF pulse and before the acquiring of the at least one FID signal and the at least one stimulated echo signal during the acquisition stage, and at least one gradient pulse G.sub.reph is applied to the examination space during the acquisition of the at least one FID signal and the at least one stimulated echo signal during the acquisition stage.

10. The method of claim 1, wherein TE.sub.FID<TE.sub.STE.

11. The method of claim 1, wherein the acquisition stage comprises a gradient echo train with multiple readout RF pulses, and wherein at least one FID signal triggered by the readout RF pulse and at least one stimulated echo signal triggered by the readout RF pulse are acquired after each readout RF pulse of the gradient echo train.

12. The method of claim 11, wherein one k-space row is recorded with each readout RF pulse of the multiple readout RF pulses.

13. The method of claim 11, wherein multiple FID signals triggered by the readout RF pulse and multiple stimulated echo signals triggered by the readout RF pulse are acquired after each readout RF pulse of the gradient echo train.

14. The method of claim 1, wherein the imaging sequence comprises at least two successive sequence units, wherein each sequence unit of the sequence units comprises a preparatory stage and a subsequent acquisition stage so that at least one acquisition stage of a sequence unit is directly followed by a preparatory stage of a subsequent sequence unit, and wherein at least one eddy current compensation gradient pulse is applied to the examination space between the acquisition stage and the preparatory stage that directly follows the acquisition stage.

15. A magnetic resonance device comprising: a main magnet configured to generate a main magnetic field; and an examination space in which the examination subject is configured to be positioned, wherein the magnetic resonance device is configured to generate an imaging sequence comprising: applying at least two preparatory radio-frequency (RF) pulses during a preparatory stage, wherein a first preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.1, wherein a second preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.2, wherein times t.sub.1 and t.sub.2 are separated by a period TS; applying at least one readout RF pulse at a time t.sub.3 during an acquisition stage; acquiring at least one free induction decay (FID) echo signal after the readout RF pulse at a time t.sub.4, wherein times t.sub.3 and t.sub.4 are separated by a period TE.sub.FID; and acquiring at least one stimulated echo signal after the at least one readout RF pulse at a time t.sub.5, wherein times t.sub.3 and t.sub.5 are separated by a period TE.sub.STE, wherein the magnetic resonance device is further configured to derive a B.sub.0 map that shows the actual spatial distribution of a magnetic field strength of the main magnetic field from the at least one FID echo signal and the at least one stimulated echo signal, and wherein the period TS is chosen as a function of the periods TE.sub.FID and TE.sub.STE so that, between times t.sub.4 and t.sub.5, a signal component of the echo signals from protons bound in water has a same phase difference as a signal component of the echo signals from protons bound in fat.

16. A computer program product comprising a program configured to be loaded directly into a memory of a programmable system control unit of a magnetic resonance device program resources, wherein the program, when executed in the system control unit of the magnetic resonance device, causes the magnetic resonance device to: generate a main magnetic field; apply to an examination space in which the examination subject is located of an imaging sequence having a preparatory stage and a subsequent acquisition stage, wherein the imaging sequence comprises: applying at least two preparatory radio-frequency (RF) pulses during the preparatory stage, wherein a first preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.1, wherein a second preparatory RF pulse of the at least two preparatory RF pulses is applied at a time t.sub.2, wherein times t.sub.1 and t.sub.2 are separated by a period TS; applying at least one readout RF pulse at a time t.sub.3 during the acquisition stage; acquiring at least one free induction decay (FID) echo signal after the readout RF pulse at a time t.sub.4, wherein times t.sub.3 and t.sub.4 are separated by a period TE.sub.FID; and acquiring at least one stimulated echo signal after the at least one readout RF pulse at a time t.sub.5, wherein times t.sub.3 and t.sub.5 are separated by a period TE.sub.STE; and derive a B.sub.0 map that shows the actual spatial distribution of a magnetic field strength of the main magnetic field from the at least one FID echo signal and the at least one stimulated echo signal, wherein the period TS is chosen as a function of the periods TE.sub.FID and TE.sub.STE so that, between times t.sub.4 and t.sub.5, a signal component of the echo signals from protons bound in water has a same phase difference as a signal component of the echo signals from protons bound in fat.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Additional advantages, features, and details of the disclosure emerge from the exemplary embodiments described hereinbelow and with reference to the drawings. Corresponding parts have the same reference signs in all the figures, in which:

[0066] FIG. 1 depicts an example of a magnetic resonance device in a schematic diagram.

[0067] FIG. 2 depicts an example of a diagram of a dual echo gradient echo sequence for the generation of a B.sub.0 map according to the prior art.

[0068] FIG. 3 depicts an example of a flowchart of a method for the generation of a B.sub.0 map.

[0069] FIG. 4 depicts an example of a progression over time of a phase accumulation generated by off-resonance in a preparatory stage and a subsequent acquisition stage.

[0070] FIG. 5 depicts an example of a diagram of an imaging sequence for the generation of a B.sub.0 map.

[0071] FIG. 6 depicts an example of a diagram of an imaging sequence with an eddy current compensation gradient pulse.

DETAILED DESCRIPTION

[0072] FIG. 1 is a schematic view of a magnetic resonance device 10. The magnetic resonance device 10 includes a magnet unit 11 that has a main magnet 12 for the generation of a strong main magnetic field 13 with a target magnetic field strength B.sub.0,target that is, in particular, constant over time. The magnetic resonance device 10 additionally includes an examination space 14 for receiving a patient 15. The examination space 14 in the present exemplary embodiment is cylindrical and is surrounded in a circumferential direction by the magnet unit 11. It is though perfectly conceivable for the examination space 14 to be realized in a different form. The patient 15 may be introduced into the examination space 14 using a patient positioning device 16 of the magnetic resonance device 10. The patient positioning device 16 has for this purpose a patient table 17 that may be moved within the examination space 14.

[0073] The magnet unit 11 additionally has a gradient coil unit 18 to generate magnetic field gradients by applying gradient pulses that are used, inter alia, for spatial encoding during imaging. The gradient coil unit 18 includes three gradient coils, for example, each for one spatial direction. The gradient coil unit 18 is controlled using a gradient control unit 19 of the magnetic resonance device 10. The magnet unit 11 additionally includes a radio frequency antenna unit 20, which in the present exemplary embodiment takes the form of a body coil permanently integrated into the magnetic resonance device 10. The radio frequency antenna unit 20 is controlled by a radio frequency antenna control unit 21 of the magnetic resonance device 10 and applies radio frequency pulses to an examination volume that is formed by an examination space 14 of the magnetic resonance device 10. The main magnetic field 13 generated by the main magnet 12 excites atomic nuclei by this method. The relaxation of the excited atomic nuclei generates magnetic resonance signals, in particular echo signals. The radio frequency antenna unit 20 is configured to receive the echo signals.

[0074] The magnetic resonance device 10 has a system control unit 22 for controlling the main magnet 12, the gradient control unit 19, and the radio frequency antenna unit 21. The system control unit 22 controls the magnetic resonance device 10 centrally, such as for the performance of a predefined imaging gradient echo sequence, for example. The system control unit 22 also includes an analysis unit, not shown in any greater detail, to analyze the echo signals acquired during the magnetic resonance examination. The analysis unit is in particular able to generate, with reference to the echo signals, a B.sub.0 map showing the actual spatial distribution of the magnetic field strength of the main magnetic field 13. The magnetic resonance device 10 additionally includes a user interface 23 that is connected to the system control unit 22. Control information such as imaging parameters, for example, and reconstructed magnetic resonance images may be displayed for a medical operator on a display unit 24 of the user interface 23, for example, on at least one monitor. The user interface 23 also has an input unit 25 using which information and/or parameters may be input by the medical operator during a measurement operation.

[0075] One possible problem forming a basis for the disclosure is the measurement of B.sub.0 field distributions, also known as B.sub.0 mapping. This may be used for the patient-specific optimization of shim currents, for the determination of a local resonant frequencies or for certain image correction procedures. As shown in FIG. 2, this is done according to the prior art by measuring multiple (e.g., two but possibly more) echo signals (e.g., during the readout window shown on the ADC axis) after application of an RF pulse (shown on the RF axis), each of which echo signals is generated with the aid of a preceding dephasing gradient and a subsequent rephasing gradient (shown on the G.sub.RO axis, which represents a readout direction).

[0076] The spatial distribution of the resonance frequency f is calculated from the phase difference of the measured echo signals. The resonant frequency f, in turn, is proportionate to the B.sub.0 field strength, so the result is a B.sub.0 map. In the straightforward and frequently applied instance, two echoes are measured (for example, in a gradient echo sequence) whose echo times differ by ΔTE. Once the measured data belonging to the two echoes has been reconstructed, the resonant frequency for the B.sub.0 field is calculated from their phase difference ΔΦ according to Equation 1.

[0077] Consideration of the chemical shift between fat and water leads to the long measurement times already presented above when creating a B.sub.0 map, especially in the case of low field strengths of the main magnetic field 13.

[0078] FIG. 3 shows a procedure for a method for the generation of a B.sub.0 map for a magnetic resonance examination of an examination subject. Different acts of the method are explained with reference to FIGS. 4 and 5.

[0079] In act S1, a main magnetic field 13 with a target magnetic field strength B.sub.0,target is generated. In acts S2 to S5, an imaging sequence including a preparatory stage and a subsequent acquisition stage is applied to an examination space 14 in which the examination subject 15 is located.

[0080] In act S2, at least two preparatory RF pulses are applied during the preparatory stage, with a first of the at least two preparatory RF pulses being applied at a time t.sub.1, a second of the at least two preparatory RF pulses being applied at a time t.sub.2, and the times t.sub.1 and t.sub.2 being separated by a period TS.

[0081] FIG. 4 shows that during a preparatory stage PS, a first preparatory RF pulse PHF1 is applied at a time t.sub.1 and a second preparatory RF pulse PHF2 is applied at a time t.sub.2. The two preparatory RF pulses are separated in time by the period TS.

[0082] A phase Φ builds up to a value Φ.sub.TS during the period TS as a result of off-resonance. The stronger the off-resonance is, the steeper will be the rise in the phase Φ and also in the value Φ.sub.TS. The two preparatory RF pulses PHF1 and PHF2 prepare longitudinal magnetization. Generated transverse magnetization is after the second preparatory RF pulse PHF2 with the spoiler gradient G.sub.spoil, which is shown in FIG. 5.

[0083] In act S3, at least one readout RF pulse is applied at a time t.sub.3 during the acquisition stage.

[0084] As may be seen in FIG. 4, the application of the readout RF pulse AHF1 causes the phase to be inverted from Φ.sub.TS to −Φ.sub.TS. The phase Φ.sub.STE then continues to increase. The phase accumulation Φ.sub.FID of the FID echo signal also begins.

[0085] In act S4, at least one FID echo signal is acquired after the readout RF pulse at a time t.sub.4, where times t.sub.3 and t.sub.4 are separated by a period TE.sub.FID.

[0086] In act S5, at least one stimulated echo signal is acquired after the readout RF pulse at a time t.sub.5, where times t.sub.3 and t.sub.5 are separated by a period TE.sub.STE.

[0087] In act S6, a B.sub.0 map that shows the actual spatial distribution of the magnetic field strength of the main magnetic field 13 is derived from the at least one acquired FID echo signal and the at least one acquired stimulated echo signal, the period TS being chosen so that echo signals from protons bound in water and echo signals from protons bound in fat experience the same dephasing.

[0088] As shown in FIG. 4, the phase accumulation Φ.sub.FID at the time t.sub.4 of the acquisition of the FID echo signal has a different value to the phase accumulation Φ.sub.STE at the time t.sub.5 of the acquisition of the stimulated echo signal. The difference ΔΦ.sub.eff thus is not equal to zero. The resonant frequency or the B.sub.0 map may be calculated from the phase difference ΔΦ.sub.eff according to Equation 1 by replacing ΔTE in the equation with an effective dephasing time ΔTE.sub.eff.

[0089] The effective dephasing time ΔTE.sub.eff here is the difference between the periods during which the signals of the two echoes each accumulate a phase due to off-resonance.

[0090] The FID echo signal accumulates a phase during the period TE.sub.FID. The stimulated echo signal accumulates a phase during the period TS and during the time TE.sub.STE so that the phase accumulation period for the stimulated echo signal amounts to −TS+TE.sub.STE. Consequently, the difference in the phase accumulation periods is ΔTE.sub.eff=TE.sub.FID−(−TS+TE.sub.STE).

[0091] Equation 3 may apply so that the echo signals from protons bound in water and echo signals from protons bound in fat experience the same dephasing.

[0092] The time TS between the RF pulses of the preparatory stage PS is to this end chosen in particular so that the effective dephasing time ΔTE.sub.eff meets the fat/water in-phase condition. The period TS is specifically extended by a certain amount at low field strengths<2 T, e.g., <1 T. This means that for lower field strengths, an additional waiting time is inserted between the RF pulses of the preparatory stage PS to realize a suitable effective dephasing time ΔTE.sub.eff.

[0093] The extension of TS at low field strengths has the effect that with customary sequence timing, T2* compensation of the two echoes no longer happens, which makes this parameterization unsuitable for the reconstruction of a B.sub.1 map. The method proposed here thus optimizes the parameters for the reconstruction of a B0 map without considering any possible suitability of the sequence for the creation of a B.sub.1 map. It is thus possible that the acquired FID echo signals and stimulated echo signals will not be suitable for use in deriving a B.sub.1 map that shows a spatial distribution of the flip angles of the at least two preparatory RF pulses.

[0094] To this end, the periods TE.sub.FID and TE.sub.STE may be defined first, with the period TS only being defined subsequently as a function of the periods TE.sub.FID and TE.sub.STE previously defined.

[0095] It is also conceivable for act S5 to come before act S4, that is to say TE.sub.FID<TE.sub.STE. The order of the echoes in the acquisition stage is in particular chosen so that with the sequence timing may be achievable for the two echoes, fat and water are in phase as far as possible. Signal losses due to fat and water components in phase opposition may otherwise occur in voxels in which both chemical bonding types are present. Accordingly, the stimulated echo is advantageously to be measured first before the FID gradient echo at field strengths of around 3 T or more. At lower field strengths, in contrast, the option presents itself to measure the FID gradient echo first before the stimulated echo, as shown in FIGS. 4 and 5.

[0096] As shown in FIG. 5, a gradient pulse G.sub.prep is applied to the examination space 14 between the two preparatory RF pulses PHF1 and PHF2 during the preparatory stage PS to adjust the periods TE.sub.STE and TE.sub.FID. A dephasing gradient pulse G.sub.reph is additionally applied to the examination space 14 after the application of the readout RF pulse AHF1 and before the acquisition of the FID signal and the stimulated echo signal during the acquisition stage AS. A rephasing gradient pulse G.sub.reph is additionally applied to the examination space 14 during the acquisition of the FID signal and the stimulated echo signal during the acquisition stage AS.

[0097] The respective echo signal may occur when the rephasing gradient moment is equal to a preceding dephasing gradient moment. The times t.sub.4 and t.sub.5 of the FID echo signal and the stimulated echo signal may be adjusted accordingly by the timing and form of the gradient pulses G.sub.prep, G.sub.deph, and G.sub.reph.

[0098] As shown in FIG. 5, the acquisition stage AS may include a gradient echo train with multiple readout RF pulses AHF1, AHF2, . . . , at least one FID signal triggered by the readout RF pulse and at least one stimulated echo signal triggered by the readout RF pulse being acquired after each readout RF pulse of the gradient echo train. In particular, one k-space row may be recorded with each of the multiple readout RF pulses AHF1, AHF2, . . . . This makes it possible, for example, to measure an entire slice with just one preparatory stage PS.

[0099] FIG. 5 shows that it is only ever one FID signal and one stimulated echo signal that are recorded after a readout RF pulse. It is also conceivable, however, for multiple FID signals triggered by the readout RF pulse and multiple stimulated echo signals triggered by the readout RF pulse to be acquired after each readout RF pulse of the gradient echo train. These may then be used to increase the value range of the reconstructed B.sub.0 map via a suitable reconstruction. The data from these additional echoes may then be used to separate the fat and water signals using a Dixon reconstruction, for example.

[0100] FIG. 6 illustrates a sequence that includes multiple successive sequence units, each having a preparatory stage and an acquisition stage. In particular, each sequence unit measures one slice of the patient 15. Shown is the end of the m.sup.th acquisition stage AS.sub.m of the m.sup.th sequence unit. This is followed by the m+1.sup.th preparatory stage PS.sub.m+1 and the start of the m+1.sup.th acquisition stage AS.sub.m+1 of the m+1.sup.th sequence unit. An eddy current compensation gradient pulse G.sub.ECC,RO in the readout direction and an eddy current compensation gradient pulse G.sub.ECC,SS in the slice-selection direction are applied to the examination space 14 between the acquisition stage AS.sub.m and the subsequent preparatory stage PS.sub.m+1.

[0101] Every gradient pulse may induce eddy currents. Assuming that the time constant for the decay of the eddy currents is long relative to the duration of the gradient pulse, it may be shown that the strength of the eddy currents approximately corresponds to the moment of the gradient pulse causing the eddy current. Such eddy currents may cause a disturbance of the static main magnetic field 13 that changes over time. In certain examples, only the static main magnetic field 13 is to be measured.

[0102] Here, the eddy current compensation gradient pulses G.sub.ECC,RO and G.sub.ECC,SS are applied at the end of the acquisition stage ASm, (e.g., of a gradient echo train), for this purpose. Their moment is advantageously such that the eddy currents they generate at least partially offset the eddy currents caused by the preceding gradient pulses. Eddy currents of a higher spatial order, that is to say second order and higher, may be offset by this method. This means that any B.sub.0 maps determined in S6 will be less distorted by eddy currents.

[0103] The eddy current compensation gradient pulses may be calculated on the basis of time constants of the decay of the eddy currents. These time constants may be calculated individually for each magnetic resonance device 10 or average values may be calculated for magnetic resonance devices of the same type.

[0104] The eddy current compensation gradient pulses may be applied only in the slice-selection direction G.sub.SS and the readout direction G.sub.RO. The overall moment of the phase-encoding gradient pulses is advantageously approximately zero.

[0105] It may be stated in summary that the method shown in FIG. 3 in particular makes it possible to realize a low measurement time. Suitable effective dephasing periods ΔTE.sub.eff (e.g., fat and water are in phase for both echoes) may be realized, in particular for low field strengths as well, without any significant increase in the overall measurement time. It is sufficient that additional “waiting time” to realize the effective dephasing time with decreasing field strength is applied just once per measured slice, for example, and not (as in the conventional method according to FIG. 1) with every sequence element that measures a k-space row.

[0106] Finally, it is emphasized once again that the methods described in detail above and the magnetic resonance device are just exemplary embodiments that may be modified in various ways by the person skilled in the art without moving beyond the scope of the disclosure. Furthermore, the use of the indefinite article “a” does not preclude there actually being present more than one of the features/attributes concerned. Similarly, the term “unit” does not preclude the components concerned being composed of multiple interacting sub-components that may also be spatially distributed.

[0107] 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 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, and that such new combinations are to be understood as forming a part of the present specification.

[0108] Although the disclosure has been illustrated and described in detail with the exemplary embodiments, the disclosure is not restricted by the examples disclosed and other variations may be derived therefrom by a person skilled in the art without departing from the protective scope of the disclosure.