METHOD AND APPARATUS FOR ACQUIRING MAGNETIC RESONANCE DATA WITH ACCELERATED ACQUISITION OF NAVIGATOR DATA

Abstract

In a method and apparatus for acquiring magnetic resonance data of an acquisition region of a patient, in particular at least a part of the head of the patient, navigator data are acquired for motion correction, between diagnostic data acquisition time windows, in navigator time windows by execution of a fat navigator sequence. The fat navigator sequence has a fat-selective excitation module with at least one radio-frequency pulse and a readout module undersampling in a respective navigator slice. Motion data for the motion correction of the diagnostic data are determined from the navigator data. The navigator data are acquired simultaneously from multiple excited fat navigator slices in the fat navigator sequence using simultaneous multislice imaging, after the excitation module acts on a number of fat navigator slices to be acquired in the readout module.

Claims

1. A method for acquiring magnetic resonance (MR) data from an acquisition region of a patient, said method comprising: operating an MR data acquisition scanner, while the patient is situated therein, in order to acquire MR diagnostic data from the acquisition region of the patient in respective diagnostic acquisition time windows; also operating said MR data acquisition scanner in order to acquire navigator data, for motion correction of said MR diagnostic data, from the acquisition region in respective navigator time windows between said diagnostic acquisition time windows, by executing, in each navigator time window, a fat navigator sequence comprising a fat-selective excitation module having at least one radio-frequency (RF) pulse and a readout module in which the navigator data are acquired from respective slices of the acquisition region with undersampling; and in each navigator time window, acquiring said navigator data simultaneously from a plurality of fat navigator slices that were excited by the fat-selective excitation module, using simultaneous multislice imaging in said readout module, after said fat-selective excitation module has acted on said plurality of fat navigator slices.

2. A method as claimed in claim 1 comprising acquiring said navigator data with said undersampling being defined by an in-plane undersampling factor of more than two.

3. A method as claimed in claim 1 comprising acquiring said navigator data with said undersampling being defined by an in-plane undersampling factor of more than ten.

4. A method as claimed in claim 1 comprising acquiring said navigator data from at least three fat navigator slices simultaneously.

5. A method as claimed in claim 1 comprising acquiring said navigator data from at least eight fat navigator slices simultaneously.

6. A method as claimed in claim 1 comprising acquiring said navigator data from at least eight fat navigator slices simultaneously, with said undersampling being defined by an in-plane undersampling factor of more than ten.

7. A method as claimed in claim 1 comprising operating said MR data acquisition scanner so as to execute a binomial pulse sequence for fat-selective excitation in said fat-selective excitation module.

8. A method as claimed in claim 7 comprising executing said binomial pulse sequence with a flip angle that is less than 30.

9. A method as claimed in claim 7 comprising executing said binomial pulse sequence with a flip angle that is less than 20.

10. A method as claimed in claim 7 comprising acquiring respective sets of said navigator data from different simultaneously excited fat navigator slices by emitting said binomial pulse sequence in said MR data acquisition scanner chronologically offset for the respective navigator slices so that one RF pulse of the respective binomial pulse sequence for a respective navigator slice is radiated between respective RF pulses of preceding and following binomial pulse sequences for other navigator slices in the plurality of simultaneously acquired navigator slices.

11. A method as claimed in claim 1 comprising providing the acquired navigator data to a processor and, in said processor, determining motion data from the acquired navigator data with respect to reference data that represents a previous motion state of the acquisition region of the patient, and bringing the acquired navigator data into registration with the reference data.

12. A method as claimed in claim 11 comprising using, as said reference data, navigator data acquired from the acquisition region of the patient in at least one preceding navigator time window with respect to time window for the navigator data from which said motion data are determined.

13. A method as claimed in claim 12 comprising, in said at least one preceding navigator time window, simultaneously acquiring navigator data from a number of fat navigator slices that is larger than a number of fat navigator from which the navigator data are simultaneously acquired in the time window from which said motion data are determined.

14. A method as claimed in claim 12 comprising acquiring said reference data from a plurality of preceding navigator time windows.

15. A method as claimed in claim 1 comprising, in at least some of said navigator time windows, acquiring said navigator data from a different group of navigator slices than in others of said navigator time windows.

16. A method as claimed in claim 15 comprising acquiring said navigator data from two different slice groups that alternate in successive navigator time windows.

17. A method as claimed in claim 15 comprising acquiring the navigator data from two different slice groups that are interleaved with each other.

18. A method as claimed in claim 15 comprising acquiring said navigator data from two different slice groups, with one of said different slice groups having a slice orientation that is rotated with respect to an orientation of another of said slice groups.

19. A method as claimed in claim 1 comprising using said navigator data to produce motion data for motion-correcting said MR diagnostic data by a motion correction technique selected from retrospective motion correction and prospective motion correction.

20. A method as claimed in claim 1 comprising acquiring said MR diagnostic data by executing a magnetic resonance diagnostic data acquisition sequence in which fat signals are corrected.

21. A method as claimed in claim 20 wherein said magnetic resonance diagnostic data acquisition sequence is an EPI sequence.

22. A method as claimed in claim 1 comprising executing, as said fat navigator sequence, an EPI sequence without refocusing pulses.

23. A magnetic resonance apparatus comprising: an MR data acquisition scanner; a computer configured to operate said MR data acquisition scanner, while a patient is situated therein, in order to acquire MR diagnostic data from the acquisition region of the patient in respective diagnostic acquisition time windows; said computer being configured to also operate said MR data acquisition scanner in order to acquire navigator data, for motion correction of said MR diagnostic data, from the acquisition region in respective navigator time windows between said diagnostic acquisition time windows, by executing, in each navigator time window, a fat navigator sequence comprising a fat-selective excitation module having at least one radio-frequency (RF) pulse and a readout module in which the navigator data are acquired from respective slices of the acquisition region with undersampling; and said computer being configured to operate said MR data acquisition scanner in each navigator time window, so as to acquire said navigator data simultaneously from a plurality of fat navigator slices that were excited by the fat-selective excitation module, using simultaneous multislice imaging in said readout module, after said fat-selective excitation module has acted on said plurality of fat navigator slices.

24. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a control computer of a magnetic resonance (MR) apparatus comprising an MR data acquisition scanner, and said programming instructions causing said control computer to: operate said MR data acquisition scanner, while a patient is situated therein, in order to acquire MR diagnostic data from an acquisition region of the patient in respective diagnostic acquisition time windows; also operate said MR data acquisition scanner in order to acquire navigator data, for motion correction of said MR diagnostic data, from the acquisition region in respective navigator time windows between said diagnostic acquisition time windows, by executing, in each navigator time window, a fat navigator sequence comprising a fat-selective excitation module having at least one radio-frequency (RF) pulse and a readout module in which the navigator data are acquired from respective slices of the acquisition region with undersampling; and in each navigator time window, acquire said navigator data simultaneously from a plurality of fat navigator slices that were excited by the fat-selective excitation module, using simultaneous multislice imaging in said readout module, after said fat-selective excitation module has acted on said plurality of fat navigator slices.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a flowchart for the acquisition of navigator data by execution of a fat navigator sequence.

[0041] FIG. 2 shows a usable binomial pulse sequence.

[0042] FIG. 3 shows the effects of the binomial pulse sequence of FIG. 2 on magnetizations of water and fat spins.

[0043] FIG. 4 shows time-staggered switching of binomial pulse sequences.

[0044] FIG. 5 is a schematic illustration of a first exemplary embodiment of the method according to the invention.

[0045] FIG. 6 is a schematic illustration of a second exemplary embodiment of the method according to the invention.

[0046] FIG. 7 is a schematic illustration of a third exemplary embodiment of the method according to the invention.

[0047] FIG. 8 is a schematic illustration of a fourth exemplary embodiment of the method according to the invention.

[0048] FIG. 9 is a schematic illustration of a fifth exemplary embodiment of the method according to the invention.

[0049] FIG. 10 schematically illustrates a magnetic resonance apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] FIG. 1 is a flowchart for acquiring and determining navigator data by execution of a fat navigator sequence in the method according to the invention. In this process, magnetic resonance signals from a number of fat navigator slices are to be acquired simultaneously through the use of SMS imaging in order to determine the navigator data therefrom. For this purpose, in a step S1, a radio-frequency pulse sequence which acts on the number of fat navigator slices and selectively excites fat spins is radiated in an excitation module. For this purpose, in the present example, binomial pulse sequences are used in the excitation module in step S1.

[0051] A fat-selective binomial pulse sequence shall be explained in more detail with reference to FIG. 2 and FIG. 3. The binomial pulse sequence 7 shown there is a 1-2-1 binomial pulse sequence, where the specified binomial coefficients relate to the flip angle, in the figure. Firstly, a radio-frequency (RF) pulse 1 is radiated into the acquisition region, specifically the fat navigator slices, the acquisition region having two different tissue components or spin types, in the present example fat spins and water spins. The first radio-frequency pulse 1 acts both on the fat spins and on the water spins and, as is shown in FIG. 2, deflects the magnetization by the flip angle , 22.5 in the example, out of the longitudinal direction, the fat spins and the water spins initially still being in phase. After a time delay t.sub.OPP, however, the fat and water spins have an opposite phase angle, as is indicated in FIG. 2 by the magnetization 5 and 6, respectively.

[0052] If the radio-frequency pulse 2 is now radiated with a doubled flip angle and reversed phase angle, the magnetizations 5 and 6 are tilted further, as shown in FIG. 3. After a further time delay t.sub.OPP, i.e. after the time interval t.sub.in in total, the third radio-frequency pulse 3 is radiated in once again at the flip angle , such that overall the magnetization vector 5 for water exhibits no transverse magnetization, whereas the magnetization vector 6 for fat lies in the transverse plane.

[0053] A binomial pulse sequence 7 of this type, or, as the case may be, the magnetizations 5, 6 resulting after this binomial pulse sequence 7, can then be combined in step S2 with a conventional signal readout, in particular by a readout train of an EPI sequence. Since only the magnetization 6 lies in the transverse plane, the other magnetization 5, in this case the water signal, has no signal component. It should be noted that the waveform 4 of the slice selection gradient G.sub.S is also shown in FIG. 3.

[0054] For a number of fat navigator slices, in which event, by reason of the spatial sparsity of fat, in particular in acquisition regions relating to the head of the patient, a large number can be chosen, it would in principle be necessary to overlay binomial pulse sequences 7 according to the multiband factor (number of fat navigator slices for a fat navigator sequence), as a result of which excessively high peak power levels can occur, in particular in respect of the central radio-frequency pulse 2. In order to counteract this, two measures can be provided according to the invention. One way is to reduce the flip angle , so that for example only a smaller total flip angle, for example of 20, is produced for the magnetization 6 instead of a total flip angle of 90. Since fat signals have a high intensity, an adequate excitation is also given at smaller flip angles.

[0055] Another way is to use a temporal displacement of radio-frequency pulses which excite different fat navigator slices, as is explained in more detail in relation to FIG. 4.

[0056] FIG. 4 shows a first binomial pulse sequence 8 and a second binomial pulse sequence 9, plus, once again, the waveform 10 of the slice selection gradient G.sub.S. The first binomial pulse sequence 8 and the second binomial pulse sequence 9 are once again 1-2-1-binomial pulse sequences comprising radio-frequency pulses 11, 12, 13 and 14, 15, 16, respectively, where the first binomial pulse sequence uses a flip angle , and the second binomial pulse sequence a flip angle . The time intervals are always chosen such that the magnetization of the water spins after their application in the corresponding fat navigator slices equates to zero. As can be seen, the radio-frequency pulses 11 to 13 and 14 to 16 are switched (activated) offset in time relative to one another, such that with respect to time the radio-frequency pulse 14 comes to lie between the radio-frequency pulses 11 and 12, and the radio-frequency pulse 15 between the radio-frequency pulses 12 and 13. Peak power levels can be further reduced in this way.

[0057] In a step S2, cf. once again FIG. 1, a readout module then follows, an EPI readout train accelerated by means of an in-plane acceleration technique being used in the present case in order to avoid refocusing pulses being applied at high peak power levels. In a step S3, the acquired navigator data are then assigned to the various fat navigator slices by application of a slice-GRAPPA algorithm, while in a step S4 an in-plane-GRAPPA algorithm is used with respect to the undersampling in the slice plane. After step S4 navigator data are thus present in the respective fat navigator slices.

[0058] FIGS. 5 to 9 show specific embodiments of the method according to the invention during the acquisition of magnetic resonance data by execution of at least one magnetic resonance sequence. The top section in all these figures shows the temporal succession of navigator time windows 17 and acquisition time windows 18. The use of two repetitions in the acquisition time windows is to be understood purely by way of example; it is also possible for more repetitions to take place within the acquisition time windows.

[0059] In the first exemplary embodiment shown in FIG. 5, in each navigator time window 17 shown, magnetic resonance signals are acquired simultaneously in the same slice group 39 of fat navigator slices, and the navigator data 20 are reconstructed in respective steps 19, as described in relation to FIG. 1. In this case the navigator data 20 acquired in the respective preceding navigator time window 17 also serves in the present example as reference data, since a registration with the previously acquired navigator data 20 is carried out in each case in step 21 in order to determine the motion data, cf. arrow 22. According to step 23, the thus determined motion data is used for correction purposes, and moreover either for retrospective correction or, preferably, as indicated by the dashed arrow 24, for prospective motion correction, which has immediate repercussions on the imaging in the next acquisition time window 18.

[0060] The simple first exemplary embodiment shown here is exceptionally suitable for use when a great number of fat navigator slices is contained in the slice group 39.

[0061] FIG. 6 illustrates a second exemplary embodiment of the method according to the invention, wherein, in contrast to the first exemplary embodiment, in the first navigator time window 17, indicated with extended temporal duration on the extreme left, magnetic resonance signals are acquired in multiple repetitions from a larger number of fat navigator slices of a first slice group 25, from which reference data 26 is determined in step 19. For instance, it is conceivable, in the example of eight navigator slices of the first slice group 25 illustrated by way of example, to perform two SMS imaging repetitions for two slice groups of fat navigator slices that are offset with respect to one another, which together result in the first slice group 25. It is, however, also possible to acquire these without SMS imaging if the reference data 26 is also intended to serve as calibration data, in particular for kernels of the GRAPPA algorithms used.

[0062] The reference data 26, which completely cover the acquisition region, therefore also contain information concerning fat components lying between fat navigator slices of the subsequently to be acquired second slice groups 27. This increases the reliability of the registration, performed once again in a step 21, of the navigator data 20 obtained in subsequent navigation time windows 17 with the reference dataset 26 in each case, cf. arrows 28. The additional information in the reference data 26 is moreover useful in particular in the case of movements in the slice direction.

[0063] Should excessively strong movements have occurred at a point in time during the overall acquisition time, it is furthermore possible to perform a reacquisition of the reference data 26 in a further extended navigator time window 17.

[0064] The third exemplary embodiment illustrated by FIG. 7 differs from the second exemplary embodiment in that the same four slices of the slice group 27 are not acquired in each of the further navigator time windows 17, but instead four of the eight slices of the slice group 25 are acquired in alternation in each case, which means that the fat navigator slices of the slice group 27 and the fat navigator slices of the slice group 29 formed from the remaining fat navigator slices of the slice group 25 are acquired alternately. This has the advantage that relaxation effects are attenuated because different fat navigator slices are excited every time.

[0065] In the fourth exemplary embodiment according to FIG. 8, the slice groups 27 and 29, which can furthermore be understood as interleaved, are likewise measured in alternation, although in that case it is provided to acquire reference data 26, not at the start, but in the second navigation time window 17 when navigator data 20 from both slice groups 27 and 29 is present, to determine from this, in a step 30, reference data for the slice group 25 combining the slice groups 27 and 29. The reference data 31 are updated according to step 32 with each navigation time window 17. Furthermore, the latest reference data 31 at a given time are used for the registration, cf. arrow 33, with the current navigator data 20, in order to enable the motion data to be derived accordingly and a corresponding correction to be performed.

[0066] It should be noted that the alternating acquisition of two slice groups 27, 29 is not limiting, so it is also possible to perform passes cyclically through more than the two slice groups 27, 29, for example through three or four different slice groups.

[0067] FIG. 9 shows a fifth exemplary embodiment of the method according to the invention, which differs from the third exemplary embodiment in that slice groups 27, 34 having a different orientation of the fat navigator slices are acquired in alternation. This has the advantage of an improved spatial registration, specifically in the case of a smaller number of fat navigator slices that are read out simultaneously.

[0068] It should be noted that the embodiment according to FIG. 8, relating to the regular updating of the reference data 31, can, of course, also be combined with the fifth embodiment of FIG. 9.

[0069] FIG. 10 schematically illustrates a magnetic resonance apparatus 35 according to the invention. This has, as is generally known, an MR data acquisition scanner 36 having a patient receiving zone 37 defined therein, into which a patient can be introduced by a patient table (not shown). A radio-frequency coil array and a gradient coil array are provided in the scanner 36, surrounding the patient receiving zone 37.

[0070] The operation of the magnetic resonance apparatus 35 is controlled by a control computer 38, which is also designed to carry out the method according to the invention. For this purpose, the control computer 38 can have an appropriately adapted sequence controller, an appropriately adapted evaluation processor, and a motion correction processor.

[0071] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.