Method and apparatus for recording a magnetic resonance data record

Abstract

In a method and apparatus for recording a magnetic resonance (MR) data record using multiple reception coils, the data of the MR data record contain measurement signals of at least two image data records respectively from at least two slices. The MR data record is acquired in a sequence having an excitation phase, an evolution phase, readout of a first echo signal while a first read gradient is being applied, application of at least one shift gradient in a slice-selection direction, and readout of at least one further echo signal while a further read gradient is applied. The shift gradient is positioned so as to cause a shift of at least one further image data record generated from the further echo signal or signals relative to the first image data record generated from first echo signals.

Claims

1. A method for recording a magnetic resonance (MR) data record using multiple radio-frequency (RF) reception coils of an MR scanner, wherein measurement signals of the MR data record contain measurement signals of at least two image data records respectively from at least two slices of a subject, comprising operating said MR scanner to execute a measurement sequence comprising: a) an excitation phase; b) an evolution phase; c) a detection phase comprising: c1) readout of a first echo signal of a first image data record with said multiple RF coils while a first read gradient is applied, the first echo signal occurring at the first echo time, c2) application of at least one shift gradient in a slice-selection direction, and c3) readout of at least one further echo signal of a further image data record with said multiple RF coils while a further read gradient is applied, the at least one further echo signal occurring at a respective further echo time different from the first echo time, wherein c4) the shift gradient is positioned to cause a shift of at least one further image data record generated from the further echo signal or signals relative to the first image data record generated from first echo signals.

2. The method as claimed in claim 1, comprising applying the shift gradient only in every nth excitation cycle and/or in every nth detection phase, where n is a natural number greater than 1.

3. The method as claimed in claim 1 comprising applying the shift gradient with a same gradient moment when applied multiple times in different excitation cycles and/or in the same excitation cycles.

4. The method as claimed in claim 1, comprising performing steps b) to c4) are multiple times in an excitation cycle.

5. The method as claimed in claim 1, comprising applying a second read gradient and a third read gradient as further read gradients and reading out a second echo signal and a third echo signal.

6. The method as claimed in claim 5, comprising applying two shift gradients, wherein the first shift gradient causes a shift of the second image data record generated from second echo signals relative to the first image data record generated from first echo signals, and the second shift gradient causes a shift of the third image data record generated from third echo signals relative to the second image data record generated from second echo signals.

7. The method as claimed in claim 6, comprising applying the first shift gradient simultaneously with an end gradient ramp of the first read gradient and/or a start gradient ramp of the second read gradient.

8. The method as claimed in claim 6, comprising applying the second shift gradient simultaneously with an end gradient ramp of the second read gradient and/or a start gradient ramp of the third read gradient.

9. The method as claimed in claim 1, wherein water protons and fat protons are in-phase and/or out-of-phase during the readout of the echo signals.

10. The method as claimed in claim 1, comprising applying bipolar gradients as said read gradients.

11. The method as claimed in claim 1, comprising operating said MR scanner to execute a fast spin echo as the measurement sequence.

12. The method as claimed in claim 1, comprising recording in a gradient echo train comprising precisely three echo signals.

13. The method as claimed in claim 1, wherein the first image data record is one of an in-phase image data record and an opposed-phase image data record, and the at least one further image data record is the other of the in-phase image data record and the opposed-phase image data record.

14. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer of a magnetic resonance (MR) apparatus comprising an MR data acquisition scanner having multiple radio-frequency (RF) reception coils, said programming instructions causing said computer to operate said MR scanner to execute a measurement sequence comprising: a) an excitation phase; b) an evolution phase; c) a detection phase comprising: c1) readout of a first echo signal of a first image data record with said multiple RF coils while a first read gradient is applied, the first echo signal occurring at a first echo time, c2) application of at least one shift gradient in a slice-selection direction, and c3) readout of at least one further echo signal of a further image data record with said multiple RF coils while a further read gradient is applied, the at least one further echo signal occurring at a respective further echo time different from the first echo time, wherein c4) the shift gradient is positioned to cause a shift of at least one further image data record generated from the further echo signal or signals relative to the first image data record generated from first echo signals.

15. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner; a computer configured to operate the MR data acquisition scanner so as to acquire MR measurement signals representing at least two image data records respectively from at least two slices of a subject by executing a measurement sequence comprising: a) an excitation phase; b) an evolution phase; c) a detection phase comprising: c1) readout of a first echo signal of a first image data record with said multiple RF coils while a first read gradient is applied, the first echo signal occurring at a first echo time, c2) application of at least one shift gradient in a slice-selection direction, and c3) readout of at least one further echo signal of a further image data record with said multiple RF coils while a further read gradient is applied, the at least one further echo signal occurring at a respective further echo time different from the first echo time, wherein c4) the shift gradient is positioned to cause a shift of at least one further image data record generated from the further echo signal or signals relative to the first image data record generated from first echo signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates a magnetic resonance system in accordance with the invention.

(2) FIG. 2 shows a measurement in accordance with the invention sequence in a first embodiment.

(3) FIG. 3 shows a first k-space sampling pattern for the measurement sequence according to FIG. 2.

(4) FIG. 4 shows a second k-space sampling pattern for the measurement sequence according to FIG. 2.

(5) FIG. 5 shows a third k-space sampling pattern for the measurement sequence according to FIG. 2.

(6) FIG. 6 shows a measurement sequence in accordance with the invention in a second embodiment.

(7) FIG. 7 shows a first k-space sampling pattern for the measurement sequence according to FIG. 6.

(8) FIG. 8 shows a second k-space sampling pattern for the measurement sequence according to FIG. 6.

(9) FIG. 9 shows a third k-space sampling pattern for the measurement sequence according to FIG. 6.

(10) FIG. 10 shows a measurement sequence in accordance with the invention in a third embodiment.

(11) FIG. 11 shows a first k-space sampling pattern for the measurement sequence according to FIG. 10.

(12) FIG. 12 shows a second k-space sampling pattern for the measurement sequence according to FIG. 10.

(13) FIG. 13 shows a third k-space sampling pattern for the measurement sequence according to FIG. 10.

(14) FIG. 14 shows a measurement sequence in accordance with the invention in a fourth embodiment.

(15) FIG. 15 shows a first k-space sampling pattern for the measurement sequence according to FIG. 14.

(16) FIG. 16 shows a second k-space sampling pattern for the measurement sequence according to FIG. 14.

(17) FIG. 17 shows a third k-space sampling pattern for the measurement sequence according to FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(18) FIG. 1 shows a magnetic resonance system 1. This has, as the major components shown, a magnetic resonance scanner 2 and a control computer 3.

(19) The control computer 3 has a non-transitory data medium 4 with program code 5 stored thereon. Measurement sequences are executed according to the program code 5.

(20) A transmitter coil arrangement 6 is situated in the magnetic resonance scanner 2. The transmitter coil arrangement 6 is usually embodied as a body coil 7. Thus, it is formed by a single coil.

(21) A receiver coil arrangement is also present. This is fashioned as a coil array 8 with coils 9, 10, 11 and 12. To help differentiate between them, the transmitter coil arrangement 6 is represented by dashed lines.

(22) The coil array 8 is used only for reading out the measurement signal. The coils 9, 10, 11 and 12 of the coil array 8 acquire the measurement signals simultaneously.

(23) For clarity, further known components of the magnetic resonance system 1, such as gradient coils and a patient table, are not shown.

(24) FIG. 2 shows a sequence diagram 13 of an FSE measurement sequence 14 relating to a first embodiment of the invention. By means of the measurement sequence 14, two slices can be measured simultaneously.

(25) To this end, at least the excitation pulse 15 is a dual-band pulse. The excitation pulse 15 turns the magnetization through 90° out of the longitudinal direction into the transverse plane. The refocusing pulse 16, on the other hand, has a flip angle of 180°.

(26) The gradient echo train 17 generates an echo signal train 18 with three echo signals 19, 20 and 21. The middle echo signal 20 is simultaneously a gradient echo and a spin echo, as is customary in spin-echo-based sequences. This is generated by the sequence of excitation pulse 15 and refocusing pulse 16 or refocusing pulses 16. How many refocusing pulses 16 precede a spin echo will depend on which echo signal train of the echo train of an FSE is under consideration.

(27) The middle echo signal 20 is that signal whose middle defines the echo time TE.

(28) Δt.sub.in-opp designates the time interval between an “in phase” and an “opposed phase” arrangement of water and fat protons.

(29) Each of the echo signals 19, 20 and 21 has signals from two slices. More slices can also be acquired simultaneously, the excitation pulse 15 must have a corresponding number of bands.

(30) The read gradients 22, 23 and 24 of the gradient train 17 are set such that in the case of the echo signal 19 water protons and fat protons from the examination area are arranged out of phase, in the case of the echo signal 20 in phase and in the case of the echo signal 21 out of phase again.

(31) The echo signals 19 and 20 can therefore be used to implement a 2-point Dixon method. Separate water and fat images can thus be calculated from the echo signals 19 and 20.

(32) The echo signals 19, 20 and 21 are acquired with the coil array 8. As a result, the slices excited with the excitation pulse 15 fashioned as a dual-band pulse can also be unfolded. The method described is consequently a method for parallel imaging. In contrast to imaging with a single detection coil, fewer phase-encoding steps are then used. The phase-encoding steps are abbreviated N.sub.pe in the sequence diagram 13. N.sub.pe stands here for “number of phase-encoding steps”. N.sub.E designates the number of echoes of the echo train of the measurement sequence 14. The number of excitation cycles is therefore given by N.sub.pe/N.sub.E.

(33) In addition to the read gradients 22, 23 and 24, the measurement sequence 14 also has in the read direction G.sub.R a read-dephasing gradient 25. Its moment is normally half as large as the moment generated by one of the read gradients 22, 23 or 24.

(34) The read gradients 22, 23 and 24 are bipolar, i.e., their polarity alternates.

(35) A phase-encoding gradient 26 and a phase-rewind gradient 28 are applied in the phase direction G.sub.P. The phase-rewind gradient 28 offsets the phase-encoding gradient 26 such that the total phase in the phase direction between two excitation pulses 15 is equal to zero. This is known from FSE measurement sequences.

(36) In the slice direction G.sub.S, there is also a shift gradient 32 in addition to the slice-selection gradients 29 and 30 and the slice-rephasing gradient 31.

(37) In addition, the refocusing pulse 15 is surrounded by spoiler gradients 27.

(38) The shift gradient 32 occurs after the second echo signal 20. To be more precise, it occurs simultaneously with the end gradient ramp 33 of the second read gradient 23 and with the start gradient ramp 34 of the third read gradient 24. As a result, the shift gradient 32 can be incorporated in the measurement sequence 14 without shifting the times of the echo signals 19, 20 and 21.

(39) The shift gradient 32 causes at this point a shift of the images which are determined from the echo signals 21 relative to the images which are calculated from the echo signals 19 and 20.

(40) The shift gradient 32 is not applied in every detection phase 35, but only in every second detection phase.

(41) The shift gradient 32 follows gradients 55 in the slice-selection direction G.sub.S. The gradients 55 are blipped CAIPIRINHA gradients and provide improved unfolding in multislice experiments in parallel imaging. A gradient 55 has in most cases the same moment in terms of magnitude as the shift gradient 32, and either the same or an opposite polarity. In terms of magnitude, the gradient 55 does not necessarily have to be equal in size to the shift gradient 32; that is the case when FOV shift factors not equal to 2 are to be used. The gradients 55 are respectively arranged in a positive and a negative direction and are applied simultaneously with the spoiler gradients 27. In practice, the gradient moment of the Gradient 55 is added to or subtracted from that of the spoiler gradient 27 and a single gradient is applied.

(42) If the shift gradient 32 is applied, either the first or the second gradient 55 can be omitted. The applied gradient 55 can be applied like the shift gradient, i.e. only in every second excitation cycle.

(43) The interaction of the gradient 55 and of the shift gradient 32 gives rise to the sampling pattern shown in the figures described below.

(44) FIGS. 3 to 5 show the sampling of the k-spaces for the echo signals 19, 20 and 21. In all the figures of this kind described below, the acquired k-space lines are shown filled black and the omitted k-space lines are shown filled white.

(45) FIG. 3 shows the sampling of the k-space for the echo signal 19. As also in the Figures below, the axis 36 shows the k.sub.x direction, the axis 37 the k.sub.y direction and the axis 38 the k.sub.z direction. The acquired k-space lines 39 are shown circle-shaped in cross section, as in the chosen representation they proceed into the page. The circles 40 filled white show omitted k-space lines.

(46) FIG. 4 shows the acquired k-space lines 41 of the echo signal 20. These are arranged identically to FIG. 3.

(47) The recorded k-space lines 42 of the echo signal 21 in FIG. 5, are shifted in the z-direction. This is caused by the shift gradient, as the remaining signal encoding stays the same.

(48) FIG. 6 shows a sequence diagram 43 of a measurement sequence 44. The measurement sequence is largely identical to the measurement sequence 14 in FIG. 2. The statements regarding identical elements apply likewise to these elements in FIG. 6.

(49) The only difference in relation to FIG. 2 is that the shift gradient 45 is applied directly after the first echo signal 19 and is thus being applied at the same time as the end gradient ramp 46 of the first read gradient 22 and the start gradient ramp 47 of the second read gradient 23.

(50) The shift gradient 45 consequently acts upon the two echo signals 20 and 21.

(51) FIGS. 7 to 9 show the sampling of respective k-spaces relating to the echo signals 19, 20 and 21 for the measurement sequence 44 in FIG. 6.

(52) FIG. 7 is identical to FIG. 3, as the encoding with regard to the echo signal 19 has not changed.

(53) FIG. 8 shows a difference from FIG. 4, which both relate to the echo signal 20. In FIG. 8, the sampling is as in FIG. 5. This stems from the fact that the shift gradient 45 influences both the echo signal 20 and the echo signal 21.

(54) Accordingly, FIGS. 5 and 9 are also identical, as the shift gradient 32 or 45 acts upon the echo signal 21 both in the measurement sequence 14 and in the measurement sequence 44.

(55) FIG. 10 shows a sequence diagram 48 of a measurement sequence 49. The measurement sequence 49 differs from the measurement sequences 14 and 44 in that two shift gradients 45 and 50 are used. In the embodiment according to FIG. 10, the shift gradient 50 counteracts the effect of the shift gradient 45 with respect to the echo signal 21. However, the shift gradient 50 can alternatively also be chosen such that it causes a further shift of the echo signal 21 both relative to the echo signal 19 and relative to the echo signal 20. The shift gradients 45 and 50 have the same gradient moment when applied over all the detection phases.

(56) In addition, a further optional embodiment available is a phase-shift gradient 51. This is fashioned such that it shifts the k-space in the k.sub.y direction by a line.

(57) Consequently, in the embodiment according to FIG. 10, the shift gradient 45 effectively acts only upon the echo signal 20. Also, the phase-shift gradient 51 acts in the first detection phase only upon the echo signal 21.

(58) The phase-rewind gradient 28 is then adapted by the gradient moment such that the overall gradient moment in the phase direction is zero again.

(59) FIGS. 11 to 13 show the sampling of the k-spaces relating to the echo signals 19, 20 and 21 for the measurement sequence 49 in FIG. 10.

(60) Whereas in FIGS. 11 and 12 the shift gradient 51 is not apparent, in FIG. 13 the first k.sub.y line is omitted. This is due to the shift gradient 51.

(61) The shift gradient 45, on the other hand, shows effects only in FIG. 12, since, as described, its effect with respect to the echo signal 21 which is shown in FIG. 13 is counteracted by the shift gradient 50.

(62) FIG. 14 shows a sequence diagram 52 of a measurement sequence 53. This has essentially the same components as the measurement sequences 14, 44 and 49. In contrast to measurement sequence 49, the second shift gradient 54 in the slice direction G.sub.S is variable. Similarly, the shift gradient 51 can basically be used variably, i.e. it can have different gradient moments in different detection phases.

(63) This results in the samplings shown in FIGS. 15 to 17 for measurement sequence 53. The sampling of the echo signal 19 in FIG. 15 corresponds to that of FIG. 11. The sampling of the echo signal 20 in FIG. 16 corresponds to that of FIG. 12.

(64) However, the sampling of the echo signal 21 proceeds differently from the previous samplings. For example, a sampling as shown in FIG. 17 can occur. The echo signals 19 and 21 can supplement one another such that a complete k-space is sampled. This supplementary data can be used, for example, as calibration data for a GRAPPA reconstruction.

(65) The measurement sequences 14, 44, 49 and 53 all show a FSE sequence with a gradient echo train 17, the first and third echo signals 19 and 21 being recorded out of phase and the second echo signals 20 being recorded in phase. Depending on the gradient echo train, a dual-band pulse is used as the excitation pulse in order to record two measurement slices simultaneously.

(66) The application of the shift gradients 32, 45, 50, 51 and 54 can also be applied in other sequences which have at least one gradient echo train with two echoes.

(67) 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.