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

Abstract

In a method and apparatus for recording a magnetic resonance dataset with a number of reception coils, wherein the measurement signals of the magnetic resonance dataset contain measurement signals from at least two slices, the measurement signals are recorded segmented by the measurement signals being recorded in a first area of k-space with a first scanning density and in a second area of k-space with a second scanning density.

Claims

1. A method for recording a magnetic resonance (MR) dataset, comprising: operating an MR data acquisition scanner comprising a plurality of radio-frequency (RF) reception coils to acquire MR signals originating from at least two simultaneously excited slices of a subject situated in the MR data acquisition scanner; and operating said MR data acquisition scanner to acquire said MR signals as a segmented measurement by (i) entering said MR signals into a first area of a memory organized as k-space with a first scanning density, and (ii) entering said MR signals into a second area of said memory organized as k-space with a second scanning density, to form said MR dataset.

2. A method as claimed in claim 1, further comprising: entering said MR signals into a third area of said memory organized as k-space with said first scanning density.

3. A method as claimed in claim 2, comprising: in each segment associated with the acquired segmented measurement of MR signals, acquiring said MR signals of said second area before acquiring at least one of the MR signals of said first area and acquiring the MR signals of said third area.

4. A method as claimed in claim 2, comprising: in each segment associated with the acquired segmented measurement of MR signals, acquiring the MR signals of said second area after acquiring the MR signals of said first area and before acquiring the MR signals of said third area.

5. A method as claimed in claim 2, comprising: in each segment associated with the acquired segmented measurement of MR signals, acquiring the MR signals of the second area after acquiring the MR signals of the first area and after acquiring the MR signals of said third area.

6. A method as claimed in claim 2, comprising: acquiring some of the MR signals of the second area between acquisition of the MR signals of the first area and the acquisition of MR signals of the third area; and acquiring a part of the MR signals of the second area before acquiring the MR signals of the first area and/or after acquiring the MR signals of the third area.

7. A method as claimed in claim 1, wherein said second area comprises a middle of k-space.

8. A method as claimed in claim 1, wherein said second scanning density is higher than said first scanning density.

9. A method as claimed in claim 1, comprising: acquiring said MR signals of said second area after executing at least two segments associated with the acquired segmented measurement of MR signals, and using said MR signals of said second area as a complete set of calibration data.

10. A method as claimed in claim 9, comprising: acquiring said MR signals of the second area after two segments associated with the acquired segmented measurement of MR signals.

11. A method as claimed in claim 1, comprising: dividing the acquisition of said MR data into at least three segments.

12. A method as claimed in claim 1, comprising: filling k-space in said second area after executing each segment associated with the acquired segmented measurement of MR signals.

13. A method as claimed in claim 1, comprising: filling said second area of k-space at least twice.

14. A method as claimed in claim 1, wherein operating the MR data acquisition scanner comprises acquiring the MR signals originating from the at least two simultaneously excited slices of the subject in accordance with a simultaneous multislice (SMS) imaging technique.

15. A method as claimed in claim 1, wherein each of the first scanning density and the second scanning density is constant in a first k-space direction but varies in at least a second k-space direction.

16. A method as claimed in claim 15, wherein the first k-space direction is a k.sub.x-direction, and wherein each of the first scanning density and the second scanning density varies in a second k-space direction and a third k-space direction that includes a k.sub.y-direction and a k.sub.z-direction, respectively.

17. A magnetic resonance (MR) apparatus for recording a MR dataset, comprising: an MR data acquisition scanner comprising a plurality of radio-frequency (RF) reception coils; a controller configured to operate said MR data acquisition scanner, to acquire MR signals originating from at least two simultaneously excited slices of a subject situated in the MR data acquisition scanner; and said controller being configured to operate said MR data acquisition scanner to acquire said MR signals as a segmented measurement by (i) entering said MR signals into a first area of a memory organized as k-space with a first scanning density, and (ii) entering said MR signals into a second area of said memory organized as k-space with a second scanning density, to form said MR dataset.

18. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a controller of a magnetic resonance (MR) apparatus comprising an MR data acquisition scanner comprising a plurality of radio-frequency (RF) reception coils, said programming instructions causing said controller to record a MR dataset by: operating said MR data acquisition scanner to acquire MR signals originating from at least two simultaneously excited slices of a subject situated in the MR data acquisition scanner; and operating said MR data acquisition scanner to acquire said MR signals as a segmented measurement by (i) entering said MR signals into a first area of a memory organized as k-space with a first scanning density, and (ii) entering said MR signals into a second area of said memory organized as k-space with a second scanning density, to form said MR dataset.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a magnetic resonance system.

(2) FIG. 2 shows a segmented measurement sequence.

(3) FIG. 3 shows a first k-space scanning scheme.

(4) FIG. 4 shows a second k-space scanning scheme.

(5) FIG. 5 shows a third k-space scanning scheme.

(6) FIG. 6 is a flowchart for recording a magnetic resonance dataset.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) FIG. 1 shows a magnetic resonance system 1. This essentially includes a magnetic resonance scanner 2 and a controller 3.

(8) Arranged in the magnetic resonance scanner 2 is a transmit coil arrangement 4. The transmit coil arrangement 4 can be embodied as a body coil. As an alternative, the transmit coil arrangement can be embodied as a coil array.

(9) A coil array with coils 6, 7, 8 and 9 is present as a reception coil arrangement 5. The coil array can of course also feature a different number of coils. Measurement signals can be recorded by parallel imaging with the coil array 5.

(10) In a further embodiment, the transmit coil arrangement 4 and the reception coil arrangement 5 can be formed by the same coils.

(11) However, in the embodiment shown, the coil array 5 is used only to read out the measurement signal.

(12) The controller 3 has a processor 10, a non-transitory data storage medium 11 located therein, computer program code 12 stored thereon. Measurement sequences can be embodied in the computer program code 12.

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

(14) FIG. 2 shows a sequence diagram 13 of an FSE measurement sequence for recording a magnetic resonance dataset. With the FSE measurement sequence two slices are measured at the same time, a blipped CAIPIRINHA method is used for the scanning of the k-space.

(15) The radio-frequency pulses and the acquisition window are plotted along the axis ACQ.

(16) The excitation pulse 14 is embodied as a dual band pulse. It has a flip angle of 90°. The refocusing pulse 15 on the other hand possesses a flip angle of 180°. The excitation pulse 14 and the refocusing pulse 15 create the echo signal 16.

(17) An echo train composed of N.sub.E echo signals 16 is created by applying the refocusing pulse 15 N.sub.E times.

(18) In the read direction G.sub.R the read dephasing gradient 17 and the read gradient 18 are activated.

(19) In the phase direction G.sub.P a phase encoding gradient 19 and a phase rewind gradient 20 are activated. The phase rewind gradient 20 compensates for the gradient moment of the phase encoding gradient 19, so that the overall phase in the phase direction between two refocusing pulses 15 is equal to zero.

(20) Since N.sub.E echo signals per echo train are recorded, only N.sub.pe/N.sub.E part experiments or segments or echo trains are executed. N.sub.pe in this case is the number of phase encoding steps.

(21) In the slice direction G.sub.S a slice selection gradient 21 is present at the same time as the excitation pulse 14. This is followed by a slice rephasing gradient 22.

(22) A slice selection gradient 23 is activated in parallel with the refocusing pulse 15. This is surrounded by crusher gradients 24. The crusher gradients 24 are intended to avoid the detection of unwanted echo signals from the refocusing pulse 15.

(23) The gradient blips 25 and 26 are applied after each second refocusing pulse and move the readout location in the k-space in k.sub.z-direction. The gradient blip 25 thus makes possible a change in k-space in the k.sub.z-direction and the gradient blip 26 insures that there is a return to the original k-space position. With more than two slices the gradient blips 25 and 26 are to be adapted accordingly from the gradient moment forwards.

(24) Depending on FOV shift and R factor as well as the k-space trajectory (i.e., the path in k-space along which acquired data are entered, this does not absolutely have to be the case with each second radio-frequency pulse. The gradient blips can thus also be applied after each third or fourth or fifth . . . refocusing pulse.

(25) The gradient blip 25 can also be applied earlier and for example be amalgamated with one of the crusher gradients 24.

(26) FIG. 3 shows a scanning scheme for a segmented measurement sequence. In FIG. 3 the axes 27 show the k.sub.x-direction in each case, the axes 28 the k.sub.y-direction and the axes 29 the k.sub.z-direction. The k-space rows lie in the k.sub.x-direction and are therefore shown as a point. The acquired k-space rows are shown filled with black and the omitted k-space rows filled with white.

(27) The segments 30, 31, 32 and 33 are shown only as an example. Any number of segments can naturally be acquired with the FSE measurement sequence.

(28) In the first area 34 the scanning density Δκ.sub.y1 of the acquired measurement signals 35 is obviously lower than the scanning density Δκ.sub.y2 of the acquired measurement signals 36 in the second area 37. The second area 37 is adjoined by a third area 38.

(29) Measurement signals are shown in the first area 34 and in the second area 37 merely by way of example, which in particular are intended to illustrate the scanning density. In particular the number of the “skipped” k-space rows 39 and the number of the acquired measurement signals 35 and 36 are not to be seen as absolute.

(30) However the change in the k-space in the k.sub.z-direction between the rows 40 and 41 between the recording of two measurement signals 35 and/or 36 is relevant. This is achieved by the gradient blips 25 and 26. The progress in the k.sub.y-direction on the other hand is brought about by the phase encoding gradient 19.

(31) If all segments 30 to 33 and if necessary subsequent segments are executed then in the first area 34, which lies at the edge of the k-space and extends in the direction of the middle of the k-space, overall a zigzag pattern is scanned. That means that in each of the lines 40 and 41 each second k.sub.y-space row is scanned. The scanned k-space rows in the form of the measurement signals 35 are offset by Δk.sub.y in relation to one another. In the first area a blipped CAIPIRINHA scanning is obtained in this way.

(32) In the second area, after four segments 30, 31, 32 and 33, a completely scanned second area 37 is obtained. This can be used for calibration in a GRAPPA reconstruction.

(33) The second area 37 lies symmetrically (more precisely, axis-symmetrically) with respect to the middle 42 of k-space in the k.sub.y-direction. In the k.sub.z-direction there is no axis symmetry. The middle 42 is deemed to be enclosed by the second area 37, if at least the adjoining k-space rows are scanned with a higher scanning density Δκ.sub.y2.

(34) In the third area 38 the same scanning density Δk.sub.y1 as in the first area 34 is used. The third area 38 is however merely indicated for reasons of space.

(35) In the scanning shown in FIG. 3 the measurement signals 36 recorded in the second area 37 are acquired in the middle of the measurement of a segment in each case. First, the outer k-space rows are recorded in the first area 34, then the scanning proceeds to the middle 42 of the k-space, acquires k-space rows 36 in the second area 37 there, and then moves outwardly again, in order to record the k-space rows of the third area 37.

(36) However the scanning density Δk.sub.y2 in the second area 37 can also be chosen higher or lower than shown. It can be chosen so that, after two segments 30 and 31 or 32 and 33 respectively, a completely scanned second area 37 is already present. As an alternative it can be chosen so that the complete scanning of the second area 37 is achieved after recording of all segments 30 to 33.

(37) In summary it is noted once again that after all segments 30, 31, 32 and 33 have been executed, the first area 34 and the third area 38 are scanned at too low frequencies, but the second area 37 is not.

(38) For completeness, it is noted that first all measurement signals 35 and 36 of the first segment 30 are recorded, then all measurement signals 35 and 36 of the second segment 31, subsequently all measurement signals 35 and 36 of the third segment 32 and so forth until the last segment, here the segment 33.

(39) FIG. 4 shows an alternate method of operation for recording measurement signals 43 for calibration. In this method the measurement signals 35 of the first area 34, the measurement signals 36 of the second area 37 and the measurement signals of the third area 38 are scanned with a first scanning density Δk.sub.y1. Thus the first segment 30 is initially scanned as with a known blipped CAIPIRINHA scanning. Subsequently thereto, in the second area 37, one or more additional measurement signals 43 are also recorded for calibration purposes. How many additional measurement signals 43 are recorded depends on the level of SNR that the measurement signal still has in respect of T.sub.2 decay. It is naturally to be considered whether the calibration data should already be complete after the recording of one or two segments or only after the recording of all segments.

(40) In a further alternate method of operation the additional measurement signals 43 can also be recorded at the beginning of the measurement of a segment.

(41) FIG. 5, in order to explain the change of the scanning density Δk.sub.y2, shows a procedure in which, already after the measurement of two segments 30 and 31 or 32 and 33, complete calibration data are present. In this case the scanning density is twice as high as in FIG. 3, other than this the figures match.

(42) In summary, starting from a blipped CAIPIRINHA scanning the procedure during segment measurement can thus be as follows, in order to obtain calibration data at the same time during the recording of an image dataset with two or more slices, as FIG. 6 shows.

(43) First, in step S1, a second area 37 is defined, which is to be scanned completely, i.e. not scanned at too low frequencies. This includes the direction and the number of the k-space rows. In FIGS. 3 to 5 there are six k-space rows in the k.sub.y-direction, and these are the middle k-space rows. Preferably there are at least 16 k-space rows, in particular in the k.sub.y-direction.

(44) The result produced automatically by this is that the outer fourteen k-space rows in the first area 34 and the outer fourteen k-space rows in the third area 38 are recorded scanned at too low frequencies, as provided for in blipped CAIPIRINHA. The number of the k-space rows in the first area 34 and the third area 38 is produced automatically from the overall number of the k-space rows and the number of the k-space rows in the second area 37.

(45) In the second step S2 the scanning density Δk.sub.y2 is sought. This is chosen so that a complete scanning of the second area is achieved after x segments, wherein x is a natural number between and including 1 and the number of the segments. Thus after each segment, always after two segments, always after four segments, and so on, or after the measurement of all segments a completely scanned second area 37 is obtained.

(46) In the third step S3 it is defined when the measurement signals 36 and 43 of the second area 37 will be recorded. There are five options here. Either a part of the measurement signals 36 of the second area 37 is recorded as part of the usual measurement sequence of a segment 30, 31, 32, 33 and additional measurement signals 43 at another point in time. This point in time can lie before or after the measurement of the measurement signals 35 and 36 of the segment. Thus two options are produced.

(47) As the third option, the additional measurement signals 43 are woven in during the measurement of the second area 37. Then the second area 37 can be acquired before or after or between the measurement of the first area 34 and third area 38.

(48) In two cases the recording of the measurement signals 36 and 43 in the second area 37 is thus interrupted, in three cases it is directly one after the other.

(49) In this case the process is always described in one segment 30, 31, 32 or 33. The recording of the segments 30, 31, 32 or 33 occurs one after the other.

(50) In step S4 a magnetic resonance dataset is recorded, wherein the k-space or the measurement signals 35, 36 and 43 are scanned as defined.

(51) 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.