Method for obtaining a magnetic resonance dataset, storage medium and magnetic resonance apparatus

Abstract

Storage medium, magnetic resonance apparatus and method for obtaining a magnetic resonance dataset including a pilot signal uses a magnetic resonance sequence. The pilot signal is generated at a first frequency range, and a magnetic resonance signal is generated at a second frequency range. The pilot signal and the magnetic resonance signal are acquired simultaneously. At least one parameter, in particular the phase and/or the frequency range, of the pilot signal is changed during the execution of the magnetic resonance sequence at least once.

Claims

1. A method for obtaining a magnetic resonance dataset, the method including a pilot signal using a magnetic resonance sequence, the method comprising: a) generating a pilot signal at a first frequency range, b) generating a magnetic resonance signal at a second frequency range, and c) acquiring the pilot signal and the magnetic resonance signal simultaneously, wherein at least one parameter of the pilot signal is changed during the execution of the magnetic resonance sequence at least once, wherein the at least one parameter of the pilot signal is changed when a predetermined sequence event occurs, the predetermined sequence event comprising a predetermined RF pulse being applied.

2. The method of claim 1, wherein the at least one parameter of the pilot signal is changed at a point of time within an excitation cycle.

3. The method of claim 1, wherein the at least one parameter of the pilot signal is changed during the application of an RF pulse.

4. The method of claim 1, wherein at least one parameter of the pilot signal is saved.

5. The method of claim 4, wherein parameters including the at least one parameter of the pilot signal and the pilot signal are saved in a data package.

6. The method of claim 4, wherein the current phase or phase change is impressed on the pilot signal.

7. The method of claim 1, wherein the at least one parameter of the pilot signal is changed at least once in each excitation cycle.

8. The method of claim 1, wherein the change of a phase of the pilot signal is calculated according to a quadratic phase cycle.

9. The method of claim 8, wherein the phase (Y) is chosen dependent on the magnetic resonance sequence.

10. The method of claim 8, wherein the phase act (Y) is 50° or 117°.

11. The method of claim 1, wherein a phase of the pilot signal is changed randomly.

12. The method of claim 1 wherein the at least one parameters is a frequency range.

13. A non-transitory computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer system of a magnetic resonance apparatus that comprises an MR data acquisition scanner having two RF transmitters, an RF receiver, a gradient coil arrangement, and a memory, said programming instructions causing said computer system to: generate a pilot signal at a first frequency range, generate a magnetic resonance signal at a second frequency range, and acquire the pilot signal and the magnetic resonance signal simultaneously, wherein at least one parameter of the pilot signal is changed during the execution of the magnetic resonance sequence at least once, wherein the at least one parameter of the pilot signal is changed when a predetermined sequence event occurs, the predetermined sequence event comprising a predetermined RF pulse being applied.

14. The non-transitory computer-readable data storage medium of claim 13, wherein the at least one parameter is a phase and/or a frequency range.

15. A magnetic resonance apparatus comprising: an MR data acquisition scanner comprising two RF transmitters, an RF receiver, and a gradient coil arrangement, a memory in which parameter sets are stored, a computer having access to said memory and being configured to read said parameter sets from said memory, and said computer being configured to generate a pilot signal at a different frequency range than a magnetic resonance signal and acquire the pilot signal and the magnetic resonance signal simultaneously, wherein at least one parameter of the pilot signal is changed during the execution of the magnetic resonance sequence at least once, wherein the computer is configured to change the at least one parameter of the pilot signal when a predetermined sequence event occurs, the predetermined sequence event comprising a predetermined RF pulse being applied.

16. The magnetic resonance apparatus of claim 15 wherein the at least one parameter comprises a phase.

17. The magnetic resonance apparatus of claim 16 wherein the change of the phase of the pilot signal is calculated according to a quadratic phase cycle.

18. The magnetic resonance apparatus of claim 15 wherein the at least one parameter comprises a frequency.

19. A method for obtaining a magnetic resonance dataset, the method including a pilot signal using a magnetic resonance sequence, the method comprising: a) generating a pilot signal at a first frequency range, b) generating a magnetic resonance signal at a second frequency range, and c) acquiring the pilot signal and the magnetic resonance signal simultaneously, wherein at least one parameter of the pilot signal is changed during the execution of the magnetic resonance sequence at least once, wherein the change of the pilot signal is a change in a phase of the pilot signal calculated according to a quadratic phase cycle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Parts that correspond to one another are labeled with the same reference characters in all figures.

(2) FIG. 1 shows an embodiment of a magnetic resonance apparatus,

(3) FIG. 2 shows an example sequence diagram of a magnetic resonance sequence,

(4) FIG. 3 shows a first example magnetic resonance image,

(5) FIG. 4 shows a second example magnetic resonance image,

(6) FIG. 5 shows a third example magnetic resonance image,

(7) FIG. 6 shows a fourth example magnetic resonance image,

(8) FIG. 7 shows an example phase change of the pilot signal,

(9) FIG. 8 shows example first and second frequency ranges, and

(10) FIG. 9 is a procedure diagram of changing phases of pilot signals, according to one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) FIG. 1 shows a magnetic resonance apparatus 1. The magnetic resonance apparatus 1 has a scanner 2. A first transmit coil arrangement 3 is part of the scanner 2. The first transmit coil arrangement 3 is used to generate the pilot signals in a first frequency range.

(12) Furthermore, the magnetic resonance apparatus 1 has a second transmit coil arrangement 4, which is usually designed as a body coil, and thus has a single coil. The second transmit coil arrangement is used to generate RF pulses in a second frequency range.

(13) The first frequency range and the second frequency range both lie within the receiver bandwidth but have no overlap.

(14) Furthermore, the magnetic resonance apparatus 1 has a reception coil arrangement 5. The reception coil arrangement 5 is a coil array with coils 6, 7, 8 and 9. To enable the coils 6, 7, 8 and 9 to be distinguished more easily, the transmit coil arrangements 3 and 4 are shown by a dashed outline.

(15) A control computer 10 controls the operation of the magnetic resonance apparatus 1.

(16) The magnetic resonance apparatus 1 also has a non-transitory data storage medium 11 as part of the control computer 10 or independent thereof, on which computer code for carrying out magnetic resonance measurements is stored.

(17) The coil array 5 is used only to read out the measurement signal, which can be an echo signal. The coils 6, 7, 8 and 9 of the coil array 5 read out the measurement signal at the same time. Instead of the coil array 5, an individual coil can also be used as the detection coil for individual embodiments.

(18) Further components of the magnetic resonance apparatus 1, such as gradient coils and a patient bed are not shown, for clarity.

(19) FIG. 2 shows an exemplary sequence diagram 12 of a FLASH sequence. Axis PS shows the pilot signal, axis RF the RF pulses and acquisition windows, axis G.sub.r the gradients in read direction, axis G.sub.pe the gradients in phase encoding direction and axis G.sub.s the gradients in slice selection direction.

(20) The pilot signals 13 may be applied all the time after the excitation pulse 14. The excitation pulse 14 is the only RF pulse applied in the shown sequence diagram 12. Slice selection gradient 15 is applied at the same time to select a defined slice in a patient. It is known to use an additional slice rephrasing gradient 16 to compensate the dephasing fraction of the slice selection gradient 15.

(21) A dephasing gradient 17 in readout direction may be applied along with the slice rephrasing gradient 16 to get a minimal echo time T.sub.E. Further, the phase encoding gradient 18 may be applied at the same time.

(22) In readout direction G.sub.r a readout gradient 19 is applied after the dephasing gradient 17. This generates a gradient echo 20, which is acquired. Simultaneously, the pilot signal 13 is measured.

(23) All acts beginning at one excitation pulse, here RF pulse 14, to the next excitation pulse are part of an excitation cycle 21. The length of an excitation cycle is the repetition time T.sub.R.

(24) The excitation cycle is repeated n.sub.pe times, one repetition for one k-line of k-space if no coil array is used for sampling the echo signals. In case of coil arrays, the number of excitation cycles is reduced.

(25) The FLASH sequence is only used to show the dependencies of changes of the parameters of the pilot signal 13 to sequence events. Of course, other sequences could be used as well.

(26) If the pilot signal 13 has a single frequency and the phase is held coherent to the acquisition phase of the receiver, this can result in a bright spot as shown in FIG. 3.

(27) FIG. 3 shows a first magnetic resonance image 22.

(28) Image 22 may have been acquired using the sequence according to FIG. 2. Basically, a pilot signal has to be used during the acquisition. In image 22, an exemplary water tube 23 is shown. If the phase of the pilot signal 13 is held coherent to the acquisition phase of the receiver and the frequency of the pilot signal is not changed either, a bright spot 24 may occur in image 22. The bright spot is a single pixel having a signal intensity higher than the surrounding water due to the pilot signal 13. The phase encoding direction named k.sub.y and the read direction k.sub.x are shown as well.

(29) FIG. 4 shows a second magnetic resonance image 25. During the acquisition of the raw data of image 25, the phase of the pilot signal 13 has been changed according to one of the alternatives shown above. E.g. a quadratic phase cycle for the pilot signal with Ψ.sub.p=50° and one for the receiver phase with Ψ.sub.r=117° has been used. Hence there is no bright spot but a brighter line 26. The difference between the bright line 26 and the water signal in the water tube 23 is much lower than between the bright spot 24 and the water signal.

(30) To change the phase of the pilot signal 13, the duration of the excitation pulse 14 has been used. In this duration no signal can be acquired. Therefore, the time can be used for the phase change without information loss.

(31) Changing the phase this way, there is a new phase for each excitation cycle 21.

(32) FIG. 5 shows a third magnetic resonance image 27 showing again the water tube 23. During the acquisition of the raw data of image 27, the transmission phase and the transmission amplitude of the pilot signal have been kept constant, but the frequency has been changed. Here again the frequency may be changed for each excitation cycle. This leads to a brighter line 28 which extends in read direction k.sub.x which is frequency encoded.

(33) FIG. 6 shows a fourth magnetic resonance image 29 showing again the water tube 23. During the acquisition of the raw data of image 29, only the transmission amplitude of the pilot signal has been kept constant, while the frequency and the phase of the pilot signal 13 have been changed. Here again change has happened in each excitation cycle. This leads to a brightening of the whole image 29.

(34) Of course, the acquired raw data have been processed in a known way, e.g. using a Fourier transform to get the images 22, 25, 27 and 29.

(35) FIG. 7 shows the realization of a phase-locked phase change. There, a reference phase 30 is used to track a reference phase position. During an RF pulse 14 the phase of the pilot signal 13 can be changed to a predetermined value. FIG. 7 shows a change of 117° and the first change in a quadratic phase cycle. These values can be calculated before an examination for typical parameters. The number of phase encoding acts often is 128, 256 or 512. Then lists of phase values for Ψ.sub.p=50° and Ψ.sub.r=117° and n.sub.pe=512 can be stored in the storage medium 11. It is not necessary to store lists for 128 or 256 phase encoding acts, because they are included as the first 128 and 256 values in the list with 512 acts.

(36) FIG. 8 shows two frequency ranges 31 and 32. The second frequency range 31 is the range of the magnetic resonance signal and is a simple band having a starting frequency 33 and a final frequency 34.

(37) The first frequency range 32, the range of the pilot signal 13, may have subranges 35 and 36 including a number of single frequencies 37. Alternatively, the second frequency range 32 may include only one of the subranges 35 or 36. Moreover the second frequency range 32 may only include a single frequency 37. In this case, the frequency of the pilot signal 13 is constant and only the phase of the pilot signal 13 is changed. During transmission, only one of the frequencies in the second frequency range 32 is used for the pilot signal 13. If the frequency is changed as described above the frequencies 37 of the second frequency range 32 are used one by one.

(38) Independent of the construction of the ranges 31 and 32, the ranges 31 and 32 do not overlap.

(39) FIG. 9 shows a procedure diagram of obtaining a magnetic resonance dataset including a pilot signal using a magnetic resonance sequence. In act S1, the sequence is chosen. For example, a FLASH sequence having a sequence diagram as shown in FIG. 2 is taken. This sequence uses RF spoiling.

(40) In act S2, the parameters of the FLASH sequence are adjusted. Parameters usually to be adjusted are the resolution, the echo time T.sub.E, the flip angle α of RF pulse 14, and so on.

(41) Depending on acts 1 and 2, in act 3, one of a number of stored lists with precalculated phases is loaded into the working memory of computer 10. For example, one of the lists has been calculated using Ψ=50°, a second list using Ψ=117° and a third list using pseudo random numbers.

(42) Depending on the choice of act 3, one of acts 4.1 to 4.3 is executed. In act 4.1, the first list is used, in act 4.2, the second list, and in act 4.3, the third list.

(43) After the acquisition of all MR signals and pilot signals including phase and/or frequency changes of the pilot signal, the data set is processed to an image as shown in FIGS. 4 to 6.

(44) Although the invention has been illustrated and described in detail by the preferred exemplary embodiment, it is not limited by the disclosed examples and a person skilled in the art can derive other variations here from without departing from the scope of the invention. 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.

(45) It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims can, 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.